How fast are beachcat hulls ACTUALLY CAPABLE of going with lots of motor power on the back?

Supposedly beachcats are displacement boats and therefore have a maximum hull speed, since they are not capable of really planing.

We have a Hobie 18 that has been converted into a powerboat (no mast) on it. We put a fiberglass "pod" on it that is fastened to the main and aft beams. The outboard motor mounts on the aft end of the "pod," and the front end of the pod has a console with steering wheel and controls for the motor.

Originally, we had an 18-horse motor on it, and the boat would go just under 25 mph (verified by radar gun). That motor got stolen, so now we have a 30-horse motor on the back, and Rick thinks the boat goes about 30 mph (on flat water).

This is including the "pod," which weighs about 150 pounds, and the motor, which probably weighs over 100 pounds, plus a 200-pound person on the boat.

So what I am curious about is whether this means the hulls are capable of going that fast (even with at least 450 pounds of "crew" weight) and, therefore, they could potentially go that fast under sail if they had enough sail area, and with the center of effort positioned correctly.

Obviously, with a heavy outboard motor on the back, the sterns squat down, and the bows are up and there is no tendency to pitchpole.

I just think that if you can prove the potential hull speed of a sailboat by using a motor, you can then figure out how to design a sail plan with enough equivalent power to get it to that hull speed without pitchpoling. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Am I wrong?

No disrespect, but I think you are, unless you are designing for a very narrow wind range. It's a fact of life that if you want to be powered up in light air, you'll be struggling in high wind, and vice versa...

I've been clocked by the Michigan State police going 90 pulling a beach cat. Very expensive time trial! I believe I have additional undocumented runs around 95.

Obviously, I am not explaining this well.

There have been past threads on this forum, sometimes hotly debated, about how fast a beach cat can go. Some have said that the top potential speed is slightly under 25 mph (or maybe it was knots, I can't remember). There have been anecdotal reports about going as fast as 30 mph. People have been trying for a long time to get gps readings that confirm speeds into the mid and high 20's, but most are just brief spurts, like when surfing on a wave.

Some have said that the limiting factor is that most catamarans have displacement (rather than planing) hulls.

So the question is, what is the upper limit of the potential hull speed?

In almost all cases of beach cats going very fast, the thing that ultimately stops them is the bows digging in and causing a pitchpole.

Therefore, based on speeds with outboard engines, it seems obvious to me that, even under sail alone, if you can get enough weight at the back of the boat, keeping the bows up out of the water, you would be able to achieve much higher speeds than have so far been recorded.

Like, when it is blowing about 25-30 knots, strap some 100-pound bags of sand on the back of the boat, rake the mast back, put up a spinnaker (to lift the bows) and see how much faster you can go.

Also, I wonder how much faster it could go if you leave the mast off and use a kitesail. <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

1997 Bermuda World Tornado Championship, during an event called the Bacardi Blast sprint. A timed run between gates that were 500 feet apart.

The Record stands at 27 MPH or 23.5 knots average over the 500 foot course. Completed in 12.53 seconds.

I'm sure you have all seen it, but here is the short video of the event.

http://www.youtube.com/watch?v=do7nWUgijZs

Yes, that is one of the events I was trying to remember. Do you know what the wind was for that?

I have never heard what the wind speed was, but by judging the water in the background and considering it is a protected section of water, I would venture a guess of 18-20 knots. This could be a good discussion on its own (how much wind is in this picture?) That would be good practice for goobers like me to judge conditions. I know that my normal stomping grounds will start to white cap at around 12-13 knots if the wind is against the tide. But I have seen it blowing over 20 knots with no white caps and light chop when going with the tide. I have almost put my behind in a sling a couple of times trying to figure out the conditions. Other than erring on the side of safety or knowing the conditions will get worse, I could have had an unscheduled appointment with rescuers. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

modern hull designs will plane.

Some of the older ones would as well (see warbird's Tigershark project.)

Let me caveat this message with the fact that I am not any kind of engineering expert. I just read a little... plus I stayed at a Holiday Inn Express once.


I would assume that most catamarans plane since if they were displacement, they would be tied down by their theoretical hull speed. According to the Annapolis Book of Seamanship, hull speed is calculated as:

Hull speed = 1.34X (square root of waterline length in feet)

That would put the hull speed of a 20 foot beachcat at right around 6 knots. So anytime you are sailing over 6 knots, you are probably planing to some degree. Right?

-Rob

No; Cats, because they have long slender hulls do not obey of the hull speed rule.

Is this really part of the "Test"?

Okay, Pete, since you jumped in on this: If you put a 30-horse outboard on the back of your Tiki, how fast could she go? Or a 50-horse? Or a 70-horse?

This formula is derived from Froude's law. Froude's law is derived from how the wave created by the bow interacts with the wave created by the stern, as well as how the waves are related to the boat speed. If these two waves interfere contructively, you will get a big increase in wave drag.

The Froude's number is given by:

Fn= Boat velocity/ "the root of" (g*length in waterline)

Typical Froude's numbers:

Type of boat------------length(m)------speed (kts)------Fn

Oil carrier-------------300------------16,5-------------0,16
Catamaran(transporter)--35-------------35---------------0,97

As one can see the catamaran has a relatively very high Froudes number. These wessels are designed to carry people fast and safely between islands etc, and so that requires high speed, but also a not too big vessel that easily can be handled in port. The answer: Long, slender hulls in the water line. These hulls creates alot less wave drag as they "pierce" through the water. The slenderness however means loss of stability, hence why one have to have more than one hull.

When asked of what the max speed for a high performance beach cat was, my hydrodynamic prof. simply said: "there's no such thing as a theoretical max speed. If you have enough power, you can always go faster."

That been said, personally, I think the Tornado blast contest already mentioned here gives a pretty good estimate. Tornados are wide and have alot of power. In addition, the record set in the speed contest, was done by a highly skilled crew.

Expecting Wouter to fill in here as well.

There is always a limit at some point.

Why don't you post this question on boatdesign.net to see what answers that you get there as well?

It wouldn't matter. Max for that boat is around 12 knots, after that the rooster tail would just get bigger.

I'll bet posting that on Sailing Anarchy, with someone like Wouter to keep the pot stirred, would be quite entertaining.
:::Wicked Evil Grin::::: <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

Here is an image of the first run of the boat. Tiger Shark 18 foot Ron GIven design.
I sailed for two hours just checking everything that was wrong with the rig and boat (which was plenty).
The boat sailed faster than my Nacra 12sq and NEVER tried to pitch. The Nacra would have been a hand full at that stage and I would have been constantly altering trim with the bow an inch or two from the surface. My Hydra 16 would also have been pitching.
When I got onto this boat I stepped over the bows slopping sand spots on both hulls. The sand was still there on return to the beach.
The boat planed up wind and down.
most noticeable difference in driving the boat is a much lighter and more stable feel.

My reasoning is very simple here.
The TS was designed on the Paper Tiger basic platform and stretched to be a
30 foot twin trap rig.
WHile the hulls my want to pane thay cannot in the twin man guise as there is not enough boyancy to get it going as the power drives the hulls down into the water.
I have cut the rig down by more than 5 feet. I have square topped the old sail and taken some amount off the foot length. I have raked the rig back a lot.
I am at present one man up and so the power quickly pushes the hulls up and out of the water.

I have ben working getting my H17 together this last month as it is my daily ride and can now go back to this project.
Whirlwind are making new sails for it and I have to get into heavier weather.
Again, my stated goal is 25 knot runs over the distance the Tornadoes were running.

I guess in this case the bottleneck will become the structural integrity in high temperature/high pressure conditions.

If you provide progressively higher power and manage to keep the boat stable and in one piece (vibration, ressonance, etc.), the ultimate limit should be when the hull colapses and burns (or vice versa) due to the difficulty to dissipate the heat caused by hull-water friction. It shoud happen at a few hundred knots speed.

If this problem is solved, maybe with ceramic foils, about the same thing will happen at a higher speed but with hull and rig friction with air. I believe this is the actual limit for a space shuttle. Now we are talking a few thousand knots.

Sailboats are far from both limits.

Warbird,
The photo didn't fully upload.
-Rob

my under standing of hull speed is just that there is an energy "hill" to climb when the bow and stern waves meet and interact

traditionally most boats lacked the power to climb that hill and get to the other side

big engines on light boats changed that by lifting them into a plane

beachcats can also get over that hill with their huge sail area but they don't have to plane to do so

some experimentation with rocket bottles on an old hobie 16 in dead flat water would no doubt lead to boat speeds of
approaching 100knots
would it mean much to us sailors?

probably not

Speed is only limited by power.

Maximum velocity is acheived when power = drag

In general, drag increases as the square of the velocity. To double your velocity, you need to quintuple the power. This is true for ships, however, small boats behave differently.

Drag on the hull can be segregated into three components:
Skin Friction, Form and Wave-making.

At low speeds, skin friction and form drag dominate. At moderate speeds, depending on the fineness of the form, wave-making drag begins to dominate. As velocity approches "hull speed" (approximately 1.2* sqrt(waterline length) - varies depending on the fineness of the hull), wave-making drag is the dominant force. This is the point just prior to a hull planing and is analogous to the sound barrier in air.

Once a hull "breaks the sound barrier" and begins to plane, wave-making drag drops away and becomes almost negligible. Most motor boats have steps and strakes in the hull to facilitate separation of the water from the hull and achieve planing at a lower speed and reduce skin friction at higher speeds.

As speed increases, air resistance begins to exert more and more influence and becomes a major component of total drag (hydroplanes flipping over).

Like the old question – “How much wood would a woodchuck chuck, -----“
It’s like the fighter pilots analogy – “given enough thrust, a house brick will fly like an F16 (plane that is, not cat)”

Sorry, the net is on the blink here.
I think that as wee set weight out to the side for righting moment we push the wave piercing hull further into the water and create more stress.
The Tiger Shark as I have it leaves almost no hull in the water once up to about 12 knots and from then if I can keep the hull down it just keeps accelerating and planing higher and creating less drag.

I am including this shot also.
Note the halyard at ring of bridle as a speed indicator.
At this speed my Nacra would be creating an arc of spray from the bow to the stern. The Tiger is cruising at no fuss.
The bow is of absolutely no consequence, only trim for speed and I need to pray for more wind, set the trap up and use it.

That boat is NOT (fully) planing. There is none of the telltail spray pushed out to the sides of the hull. There maybe some hydrodynamic forces in the vertical direction but the wake does not look any difference then from any other lightweigth fast beachcat.

There is also alot of errornous comments presented in this thread, but I'm not going to correct them.

Wouter

That is very unusual of you.

I just fell off my chair. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I think practicality takes over before theory applies. In the case of my Tiki, the decking isn't strong enough to take the weight of a large motor. Also, fuel consumption goes up very quickly. At 4.5 knots, the Tiki will get about 30 mpg (the noise is also more tolerable) at 6 knots the fuel efficiency is less and the racket will make you crazy.

I don't see that testing sailboat hulls under power demonstrates anything useful.

>>>"I guess in this case the bottleneck will become the structural integrity in high temperature/high pressure conditions"

I agree, I think that the structural integrity of the hull will be the biggest limiting factor of speed for your average Hobie 18. You can continue to add more power (well, assuming the boat has enough flotation to support the motor), but eventually the forces generated by the flowing water will crush the hull. Just like with wind, the force of the flowing water increases as a function of velocity squared, so if you double the velocity, you quadruple the forces on the hull.


>>"the ultimate limit should be when the hull colapses and burns (or vice versa) due to the difficulty to dissipate the heat caused by hull-water friction. It shoud happen at a few hundred knots speed."

I seriously doubt the hull is going to catch fire/burn as a result of hydraulic frictional forces. Fiberglass hulls regularly travel through water at high speed- the water acts as a coolant. Again, the hull collapsing will probably be the main issue. Just a guess here, but I would think you will start to have structural issues around 50knots, maybe even sooner- especially in a turning situation.

sm

This is all very well studied already (using a Hobie 16 and rocket propulsion):


Results in a peer reviewed engineering publication and some more technical details in
scientific paper

Mary

To answer your question exactly, it depends.

To actually answer in a simple fashion that is probably wrong but gives you an answer that you can actually use:

Cats, for the most part, follow the same rules as other displacment hull boats. The very simplified way to calculate maximum speed is:

Maximum speed(in knots) = square root (waterline length(in feet)) times a constant

The constant is related to the ratio of the waterline length and the waterline width. For big heavy boats the constant is fairly easy to calculate, it's 1.2-1.5. For little boats it's not so easy. Little boats move a lot with respect to the water and lift from the hulls and sails come into play. My guess is the constant for TheMightyHobie18 is ~5.5. Which means at 25 kts, your boat is trying to plane.

I would be careful at speeds above 22 kts. On many cats, the stern trys to plane first and the bow digs in. With all the weight you have on the stern, I don't really know what will happen.

So what I am wondering is: If you have enough wind to potentially go over 25 knots on a beach cat, say a Tornado, but you are limited because of the boat's tendency to pitchpole at a certain point, can you break that speed barrier by putting a lot more weight on the back of the boat to keep the bows up? Obviously, two crew members on the back is not enough to do that.

Please... I'm no scientist or anything close, but it's not "Maximim" speed. It's hull speed or wave speed or some other term, but not MAXIMUM speed. Hull speed is not a synonym for maximum speed. Saying maximum speed just confuses these threads. Hull speed may be the max speed for the current configuration or the realistic max speed, but it's certainly far from the actual maximum speed.

That is all. Carry on <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I think (and thats the operative here!) that the answer is NO ONE KNOWS. Like Newtons theory, with poetic license, However fast a cat can go in theory, someone will have another theory as to why it won't. <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Just My Own Theory,

Clayton

Thank you for clarifying that. I always thought that "hull speed" was a speed beyond which that hull could not possibly go, even with more power added.

Has anyone ever strapped a rocket onto a cat and checked that speed?

This guy is ready!!!!

Adding weight presents the problem of needing to add "horsepower" to get the additional weight up to speed.
Then of course you have the problem with the additional weight causing the boat to sit lower in the water so you have to add even more sail area.... its a viscious circle that results in something that looks a lot like....................... Playstation.

It seem like I heard that the initial design of Playstation had a tendancy to pitchople, so they added like 25 feet to the bows to add bouyancy. Soooooo, My guess is that rather than adding weight to the back of the boat the more effective solution is to add bouyancy to the front of the boat. which lengthens the waterline which changes the calculations of the "hull speed limit"
O Gawd, my head is beginning to hurt. <img src="http://www.catsailor.com/forums/images/graemlins/crazy.gif" alt="" />

Stephen

Well, I don't believe any of this. I am going to test my theory this summer on my Wave. I figure if I tie about 12 gallon jugs of water to my aft beam, I should be able to radically increase my speed on the beam reaches and deep reaches in heavy air. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I sure can't eat enough to gain 100 pounds myself. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Fill them with Beer, Bourbon, Whisky Scotch, Pina Colada Mix, Marguerita Mix and Rum and Tequila and you will be the most popular girl on the race course. For a whole new reason besided your stunning good looks and your sterling personality. Heck, I'd probably follow you around, even if you Don't laugh at my jokes.

Speaking of Jokes......

An older man approached an attractive younger woman at a
shopping mall.

"Excuse me; I can't seem to find my wife. Can you talk to me for a couple of minutes?"

The woman, feeling a bit of compassion for the old fellow, said, "Of course, sir. Do you know where your wife might be?"

"I have no idea, but every time I talk to a woman with a chest like yours, she seems to appear out of nowhere."

I have devised a special diet to help with your goals… 1 pizza for breakfast, 1 pizza for lunch and an unsensible meal for dinner (like 12 bigmac’s, or 12 whoopers)

Thanks for the advice.
Breakfast and lunch sound okay, but I don't think I have ever been able to eat more than five Big Macs at one meal.

LOL!!!!

what's a "whooper" is that like a yooper from Michigan's Upper Peninsula?...or is it a whopper from BK? <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

Wouter, this is VERY specially for you.

The boat is only sailing at about 12/15 knots in these images. So no, it is not throwing spray out the side. It did on occasion on that day but no photo. I include a photo of my Nacra sailing in less wind and going slower as you can see by my being inboard. Check the lee hulls of the two boats.
I have been checking my theories out with a proven boat designer and Olympic sailor. Oddly enough I take his opinion far more seriously than yours.

I make this pledge to you.

This boat will travel comfortably at 20 knots and will be tracked at 25knots over a measured distance or I will crash and burn trying.

I do not care what you think about what is erroneous and what is not.
There is a point at which all of your computer "virtual" world becomes nothing more than the twittering of a "knowitall".
I have made my own less than scientific observations and will put my actions where my mouth is.
You however will wait and watch for the opportunity to pull someone down.

This is one of those impossible to kill falsehoods that is debunked about once of twice a year. We've been over this topic about 10 times the last 7 years so do a search and learn what is really true.

Don't believe 80% of the sailing books out there. They all got it all wrong.

Wouter

You could be talking to Glenn Ashby for all I care. That boat is not planing in those pictures in the scientific sense and that is final.

And I don't care what you think of me.

And why does everybody convince himself that I'm a computer sailor ? Hell, I have 2 sailboats (1 homemade), 6 landyachts (partly homemade), 1 kite buggy, a quiver of stunt kites (home-made) and 1 surfboard. And I've been sailing since I'm 12 and have worked as a sailing instructor for 4 years. Even got a engineering grade in this field.

If you don't like me, then that is fine, but don't convinced yourself that I'm some knucklehead who doesn't know what he is talking about. You only make a fool of yourself.

Wouter

to delay pitchpoling at high speeds, (where the sail is trying to go faster than the hulls and the cat trips over the bows), you could add weight to the rear of the hulls

BUT, because of the problems mentioned already it would be better to simply cant the rig back as speeds increase, thereby increasing the distance between the bows and the sail's center of effort

which is exactly what windsurfers do already...

Ain't it the truth. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

THE ANSWER IS 42 (Not 42 knots or 42 MPH or 42 KMS just 42) everyone SHOULD know that!!! - it is the answer to everything - (watch out for the mice!)

Can't wait for your book, when do you plan to publish?

Can we have a catsailor poll/vote for the name?

We'll need potential titles for a poll/vote. I'll go first.

"Steve Fossett and Larry Ellison Can Eat My Shorts"

"The Wouter Identity"

"True Lies II"

"Born to be Riled"

"Wouter Wonka and the Reality Factory"

"Mien Truth"

I pick #1 <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

In my case, the answer is not very damn fast until it warms up.

AERO-HYDRODYNAMICS OF SAILING, C.A. Marchaj;
SEAWORTHINESS: THE FORGOTTEN FACTOR, C.A. Marchaj;

HIGH PERFORMANCE SAILING, Frank Bethwaite

www.arvelgentry.com Arvel Gentry's website (Marchaj references him.)

Once you understand the principles, you can work it out for yourself.

Ah yes.... But the fun really starts once you MISunderstand the principles and work it out for everyone else <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

The head over heels moment is a product of the height of the center of effort(pushing point) above the drag (of the hulls) of the sails and the force of the sails. As such, the position of the sails fore and aft is irrelevant.
Windsurfers tilt the rig for various other reasons:
Their arms arn't long enough (the can only get their weight aft by pulling the sail with them)
They would rather have lift than de-power, so canting it over makes sense.
Canting it over lowers the center of effort

Wel let's see... Last year in the Hogs Breath over a measured 82 actual miles on long running tacks before jibeing to short running tacks we had a Marstrom 20 complete the course in 4 hrs 38 Mins and 6 seconds. Add 15%, my guess would be more, to the distance for the jibes and we see it traveled 94 miles in 278 minutes and 6 seonds. That means the average speed was over 20 miles per hour. This year if the forecast holds, we will see similar conditions with the winds at 13 - 14 mph out of the east. I will look for the first boat on Smathers Beach, Key West, around 3PM. If you do not like the 15% choose your own, In any way it is fast. Without the fudge factor, it means over 17 mph, average, for the same trip. <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" /> Some say they saw over 25 at times on the GPS.
Have fun with this, The Nacras will be close by and maybe ahead.

The heavy-ish mast I have raked back sits back past the rear beam at the tip. This does provide a little lift and a lower centre of effort which is not so irrelevant but it pitches weight back meaning I have to get out less to provide righting moment and so makes the boat more stable at speed.
Cutting the rig down by 5 feet ( much lower center of effort) also means I have to drag far less useless metal and rag through the air as if you check the boats such as Tornadoes trying for these speeds it is clear they have far too much power and are too busy dumping in gusts to settle to boat properly.

Darryl,
I saw this thread once and concluded SEP.

How could you see the thread and recognize it as such then? The math used here is very powerful though.

If you needed more weight while getting up to speed you could have a scoop to fill a bladder on the tramp just by putting a pipe facing forward which would force water into the bladder then you could let it out when you slowed down. You'd need good baffles in the bladder or the cat would roll over when you turned.
regards

Warbird: Do you have any GPS tracks from any of these 20-25kts sessions?
I am really interested in how those runs where built up because 25kts is a lot of speed (especially without a spi),
I have done 28kts on my windsurfer and cant image doing that on a "simple" beachcat.


(btw. I found 1 of your bookname suggestions a little disturbing, and its spelled "Mein" not "Mien". http://www.learntospell.com

Those books and articles are part of the 20% that got it (mostly) right.


Wouter

Forget about adding weight to the rear of the boats or any piping boys. Just add T-foils to your rudders and give then a downward angle of attack and watch those baby's provide many kg of downforce much like a F1 race car when the boat speed picks up.

But what the hell do I know right ?

Wouter

The boat has only been out twice since I changed the rig etc for speed runs.
It has not been out in more than 15 knot winds and has not travelled faster than that in its new guise.
I have sailed the boat at twenty with its high rig and they are known as an extremely fast reaching cat. Well faster than an H 16 for example.
The spinny is not my option at these speeds nor is it needed. There is heaps of power enough in 25/30 knot winds in this rig and indeed it may not need the jib.
I am happy with the results so far and so I have just ordered new sails for it from Whirlwind and as I set the boat up and run it I will prvide GPS tracks etc. The 25 knots over a measured distance is my aim. It has not been achieved so I may well fall flat on my face with these ideas.

But, obvously, I do not think so.

I understand that cats are racing up and down wind courses with no reaching.
Something about reaching being a parade.
Perhaps if we got them to go fast enough on a reach it might be worth it again.

Nice rig change on the PT. PTs go like stink off the breeze when it is fresh, especially for a 14 footer. I would watch those loads on that tiny front beam though.

Build a 18 or 20 foot version with 10 foot beams (cut and shut 2 old PTs for some fun even), reduce the rig size and dont be affraid to run it flat and I am sure you will get some impresive speed out of it.

As for no kite, a cat will sail the quickest on a 2 sail reach without kite due to less drag. Kite does not make you go faster..... Just increases VMG down wind.

Your eyes deceive you. That'a a Tiger Shark and it's already 18'.

Going faster than hull speed without planing is called "forced mode" and cats are doing that any time they have their transoms depressed but not submerged, indicating that they are making a longer hole in the water than their hull length, and they are sailing 'up' out of this hole. Conventional lead mines don't have the power:weight to start climbing out of the hole the way cats do, and 'hull speed' is the point at which one enters forced mode.

--Glenn

Spot on Glenn !

Wouter

As stated this is a foam core 18' version of the Pater Tiger and it is already a fast boat...I am not widening the boat as this will induce pitch not lessen it.
It would give me more righting moment with two "weights" strung out the side but this would push the lee hull down into the water more creating drag and the whole idea is to eliminate as much drag as possible by having the boat plane rather than be 'force moded".
The boat feels like a far less stressed platform than say the Taipan or the Nacra and that has a direct affect on my attitude in that it "feels" a lot safer at speed.

Who says that the drag associated to "forced mode displacement" is higher then the drag incurred by planing ?

Or if you will, are Tornado catamarans faster or slower then 18 foot skiffs while carrying less sail area ?

Without some dependable data we're all (me incl.) just guessing here and that isn't science.

Wouter

I am going to go out on a limb here Wouter and say that a hull of equal dimension, shape etc being forced through water has more drag than that same hull planing over the top of the water.

That is why that hull traveling through water needs more power to make it travel....hence the word "force" in the description "force moded".

In regards to the T traveling faster than the skiff it may be that the huge rig and wings and extra crew etc of the skiff create MORE DRAG via windage than the T.

Really, is that why we always rake the I20 crazy back for distance races and when we thing we're going to have big weather? Hu?... I never knew

[sarcasm off]

moving the weight and the CE aft helps survivability in big air.

Sorry Warbird but subjectively "observed reasoning" without empirical testing will invariably lead a person to incorrect conclusions, particularly when applying “low” speed hydrodynamics with aerodynamics interacting on an object at the boundary layer between the two – it is just not as simple as it “may” seem without applying the correct maths AND actual objective testing any/all theoretical results.
(Slightly off subject but question for Wouter – are you by any chance familiar with the “theory” and the maths that is being applied to research being conducted into “hyper-cavitation” for submerged “missiles” (formally called torpedoes) at present by the military?)

what about the 1 where the torpedo forces out steam? from it's skin so that it effectively eliminates almost all of the skin drag! kind of like air hockey i guess

cut from the bbc webpage
"The new missile, called Hoot, or Whale, could be deployed on Iranian ships in the oil-rich area, which is home to the US 5th Fleet.

No warship can escape from this missile because of its high speed

It is said to travel at 360km/h (233mph), three to four times faster than most conventional torpedos."

Don't be sorry Darryl, I understand what you are saying but we are not talking "missile science" here as I think (without checking google) the weapons you're are talking about are trying to create a "cushion" of air around the submerged projectile so the hydro drag does not exist and the missiles can travel mach 2 under water.
This science has been worked on by the Russians for two decades and they "may or may not" have it.
Like the scram jet engine it might finally come to fruition but not in an obvious way for the beach cat back yard.
So a truly erroneous avenue for what I am doing which is simply setting up a beach cat that planes easily and is predictable sailing above 20 knots.
In that area, as I suggest to Wouter I think a planing hull will go faster , easier than a submerged, wave piercing hull. I think that because I think the planing hull wil have less drag than the semi-submerged hull.
I am happy to be wrong about that and I will provide results here.

I spent a lot of time around science when working in the Auckland Museum. What I observed was that science bends and scientists are often have to back track. When they do they cover their butts by pretending they really knew what the problem was all along and just had to find some "empirical proof".
Science has its drawbacks as sometimes it is about what you don't know you don't know like Einstein not believing in the expanding universe theory and fighting until he died against it even the genius can be wrong.

You never got back to me about the foiled rudders by the way.

Not that it helps when you try to apply the results back to sailing, but since an engines thrust is likely below the center of drag you shouldn't be speed limited by the bows pitching down.

It would be better to have Jacob's Ladder problem, controlling lift.
http://www.cobrakite.com/jaclad.html

John

I'm aware of it and have read some articles on it but I'm not familiar enough with it to be knowledgeable about the math and physics applied.

But I always did feel that it bore strong resemblence with a ship hull design of the earlier 20th century. Here a normal but rather box like cargo ship hull had wide trenche all along its keelline that was filled with air and kept under pressure. This meant that a very large portion of the "wetted area" wias in fact between this aircushion and the water which lead to a lower drag overall. This idea was tested by never fully persued.

Wouter

Isn't that the same principle that makes a hovercraft and Erkanoplan work (although less extreme).

As an premise for further development and research that is a tenable position even though it is not the obvious one to take.





This is just nonsense. A planing hull also needs more power in order to travel faster, in fact you totally misapply the word power here. It is physically possible (and actually does happen in some cases) were the driving force descreases when travelling faster but the required power rises. This is because "POWER" is the product of "driving force" and "speed". If the speed increases more then driving force decreases then you get into this situation. Several dinghies that transition from displacement sailing to planing actually go through such a situation. So I propose that we'll only using the driving force and no longer the concept of power.

With respect to "forced mode" We must be careful to not read more into a given word then there is too it.




Or is may just be that long slender hulls on a lightweight platform in forced mode lead to lower drag at 20-30 knots speeds then a planing hull of similar length.


Wouter

this thread all seems a bit pointless ,

why not look at powerboat catamarans that allready exist and find the answer, feking fast !!!!, surely these big bouys and smaller ones can give an idea of what can be done if you strap enough hp onto two hulls ,(the thread started with engines) yon funny looking sailing cat above hulls dont look too unsimilar to those on the smaller powerboats .

its intresting to note that as far as i know all catamaran HP racing (motor )cats use planing hulls . and are probably more akin to low flying air craft using forced air under the bridge deck to provide lift .

Nigel Irens has developped high speed wave piercing muultihulls for long distance stuff (trying to keep up with orma 60s who are under sail ) haven't heard of any on smaller fomula racing cats though , would by struggle at corners <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />


PS I always find stuffing the luff exstension down the mast after hoisting effective and helps sail shape at the bottom of sail , though I don't think I'm alone in this tinking <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

I don't think so and I'll explain why.

With the Hovercraft and Ekranoplan the air cushion has a higher pressure as the surrounding medium. As such it is more akin to planing. This earlier 20th century CARGO ship has a air pocket that was at the same pressure as its surroundings. Basically the first derive lift from a dynamically created difference in pressure above and below the craft while this cargo ship was still fully in displacement mode. In fact that cargo ship design would also work if the trench was fully closed off and there were no pumps. The pumps were only there to keep the trench filled when air is lost due to rolling of the ship and other leaks.

Basically the Hovercraft and Ekranoplan completely dependent on the aircushion in order to remain levitated while this cargo ship could even work when the trench was filled with water. In the latter case it would just have more hull drug. So with the cargoship it was just a means to lower skin friction drag and nothing more. And that is why it could carry lots of cargo.

But yes, there are also strong similarities, all four (incl. planing) aim to put as little surface area into contact with the much denser water as possible in order to lower overall drag.

Wouter

That is just the question isn't it.

Powerboat cats are all designed the plane

Sailboat cats (like ORMA 60's) are designed to sail in forced displacement mode.

Only exception the latter rule is the Yves Parlier "seaplane" catamaran "Médiatis Région Aquitaine"

This cat has been pushed hard for 4 years now and not proven to be a winner.

Also of interest is this little bit of info as contained in a press release :




Source : http://www.ybw.com/auto/newsdesk/20040104160707ywnews.html

So indeed it is not a black and white situation. With each design you need to look at the specs and operating point to determine which of the two (or three) paths available is the best. For racing sail powered catamaran the paths most favoured are forced mode displacement and indeed foiling (L'hydropthere, longshot, Techiques Avancee).

A while ago I had discussed planing hulls with some A-cat designer and he replied that they had tried during the 80's to make it work, but failed to beat the normal hull designs. One issue was to keep the hull planing smoothly. Typically the boat would drop off the plane when heeling or pitching due to a gust. This could well be one of the more important reasons why powerboats are all planing designs were sailing cats aren't. It is easy to keep a powerboat "flat" or tilted at an optimal angle.

Wouter

An interesting thing about our Hobie 18 powerboat is that because it is NOT a planing boat, it is able to accelerate to its top speed very rapidly. Makes it better for pulling water skiers, for instance, because you don't have to wait for the boat to get on a plane. (Obviously, the ultimate top speed will not be as high as for a planing boat with the same amount of power.)

Maybe the same thing is true of planing multihull sailboats that it takes more power (and wind) to get them up on a plane in the first place.

Like Richard Roake's Hardcore 16, which seemed to be a dog in light air (compared to displacement cats), but very fast when the wind picked up and it could get up on a plane. Sort of like a sailboard, I guess.

Has anyone ever tried injecting air around the hull(s) of a catamaran when approaching top speed to create cavitation? What would be the probable result?

Power based boats are generally using the tunnel to create lift and less friction. Again, I think this is not realistic regarding sailing beach cats.

Well this is exactly what I am interested in Mary.
My experiment is in the size of the boat to the crew weight also.
My boat is almost as fast as the full rigged two up boat in light so is not exactly a dog but it is aimed at heavy air.
Are there any photos of the Hurricane as I can find none on google.

Are you being serious?, That is a phenomenal boat. It is the fastest catamaran ever sailed solo in 24 hours, no other solo catamaran even comes close to that boat.

Gareth

I think this is what some animals do to allow them to swim faster, a while ago I saw a piece on Discovery about a Blue Marlin who ejected some type of oily fluid to lessen waterresistance.
Also there is a powerboat that is based on dolphin skin.
More info here: http://www.physorg.com/news68812337.html

A while ago I had discussed planing hulls with some A-cat designer and he replied that they had tried during the 80's to make it work, but failed to beat the normal hull designs. One issue was to keep the hull planing smoothly. Typically the boat would drop off the plane when heeling or pitching due to a gust. This could well be one of the more important reasons why powerboats are all planing designs were sailing cats aren't. It is easy to keep a powerboat "flat" or tilted at an optimal angle.

Wouter [/quote]

I have certainly experienced this affect. I have presumed it was because I have cut the rig down to almost a masthead and I was overreacting to hull lift. It has helped to make much smaller adjustments in mainsheet. But when it does drop it feels like time has stopped.
Ideally I am trying to keep both hulls in the water and this is a learning curve also as I only want outright speed and am spending no time on general performance. The boat is so far much easier to keep on an even keel than say my Hydra which has less sail area and is heavier.

Warbird: From where are you doing your speed sessions?
I have spent some time in the BoI (Opua/Pahia area) but dont remember are places with flat enough water or high enough winds (then again, it was a few years ago).

I forgot another exception. Yellow pages and Macquary Innovation. These are planing contraptions for an all out speed record. But they are very different to a normal catamaran and mostly in the sense that they are very wide and don't have long hulls at all. Yellow pages and the other have three hulls which are all pretty small and the leeward ones are in line with a significant gap of water between them. These will remain "flat" very well due to this overall configuration, but makes it very different to a normal cat like the Tiger Shark.

Wouter

We must not forgot another things as well. The way a hull rides the waves and chop. Typically wind powered craft need alot of wind to propel them to high speeds and such conditions have a very strong correlation to a pretty uneven watersurface.

It would not be the first time that a sailing catamaran outpaces a powerboat in a blow because planing in such conditions is very violant. This also leads to the question of how the rig will behaved when it is shaken about very violantly because of the planing surfaces trying to follow confused seas.

Actually I have this experience with landyachts. Sometimes the beach is not flat but has small wave sand structure. It is very hard to keep the sail drawing in such conditions. Even so much that it pays alot to just take a much longer route around such an area, simply because of the ability to reach much higher speeds when the craft and rig operating in a smooth manner due to traveling on a smooth surface.

Of course waterskiing stops with even a very little wave like disturbance of the water surface.

Wouter

And still, it doesn't hold the outright record for 24 hour solo run.

Actually there are 3 forced mode displacement boats and skippers that in front of it in the record listing. One of these being a 60ft tri. Note how "Médiatis Région Aquitaine" is a 60 ft cat build to the same rules as the 60 foot tris.

Basically, you statement is only "true" because you choose to include the condition that any challenger must be a "solo catamaran". If you had used ""solo multihull" then your statement would not even have been in the least bit "truthful".

Situation is very much the same for fully crewed 24 hour records even in the 60ft or shorter class.

Nice try, but definately a smoke and mirrors kind of counter argument.


Wouter

If you meant to refer to the Hardcore, I cannot find any pictures of it, either.

The only time we ever saw it was when I think they had two of them at a Red Lobster Regatta in Sanford, FL back in the late 1980's or early 1990's(?) Excellent sailors were sailing both of them (one an Australian, and I can't remember his name, I think maybe it was Brett Dryland?)

Anyway, they did not do well, and after that debut, nothing was heard about them. The Hardcore is still listed in the Portsmouth ratings for old boats, but it obviously had a provisional rating, since it never raced enough to get any good data.

Randy Smyth won the Worrell 1000 one year on another Roake planing design, but he said it was a pretty rough ride. Rick saw him from shore and said the boat was skipping across the waves like a sailboard, but perfectly level, while the other boats were going up and down over the waves.

Warbird,

Overall width may really help you here also.

It could be an idea to just remove the trapeze and make the boat wider and only hike off it.

If you are really after planing then have the luff hull take part of the planing. This will make the platform very rigid in its side to side attitude and allowing you to keep optimal planing angle better.

Again this assumes that you can get proper planing to work at all.

Wouter

after almost 6 months of winter snow out at the lake this week i finally got my nacra 5.2 back in the (cold) water, surrounding volcanoes still covered in snow

today had the most breeze and as i had the gps on board i tried to better my previous top speed of 16knots, set the end of last year

still a noob and without a proper trap harness i've been making do with an old windsurfer harness and just sliding my butt off the side of the hull. all winter i've been reading "catamaran racing for the 90's" so was sitting just behind the dagger to reduce transom drag

winds picked up, aimed for the reach and off we went, played the main to keep the windward hull just kissing, wind blew a little more, speed picked up another level, leeward hull starts kicking up big spray and suddenly the whole platform starts tipping forward! leeward nose almost completely under and i slide back to the rear beam as fast as i can

everything pretty stable in that position and a couple of minutes we are across the lake and out of the main breeze

gps says top speed 15.5knots and later trackpoint data downloaded to show the points either side

23/04/2008 12:46:07 0.2 nm 0:00:41 14 kt 98° true
23/04/2008 12:46:48 0.1 nm 0:00:29 15 kt 109° true
23/04/2008 12:47:17 0.2 nm 0:01:01 13 kt 113° true

looks like it isn't going to be easy for me to get much quicker. going to need a proper trap harness, foot straps, nonslip tape, more time on the water and slightly bigger balls;o)

another plug for GPSaction replay, very cool cranking the speed up and watching the boat zip around the track with each section colored according to speed, pic attached

I hadn't heard of the Hardcore before and also haven't found anything on google. If RetiredGeek is around maybe he can share some history.

However while poking around on google, I did find the following, which might also be of interest (as a historical note).

First of all, my earlier post was more or less a brief summary of a few pages in my Textbook about Hydrodynamics on foils and ships (I'm a student at the Department of Marine Technology here in Norway, civ.ing degree), so that text is not false.

Erice, I wouldn't be too suprised of your top speed not being higher. Or, put in other words, 16+ knots on a Nacra 5.2 is fast! My dad and I sail a Taipan 5.7, which really is a fast cat (rated faster than F18s for instance), and we have never logged a better top speed than 19.7 knots. We are both certain that that particular blast isn't our fastest ever, so we consider our top speed as being just above 20 knots. This was done when a big puff(I'm guessing around 10 m/s) hit the boat, me out on the trap, dad inside on the helm, no spi up. No doubt, a better and more experienced crew could probably yank a couple of knots more out of the boat, but not a whole lot.

As for the 18ft skiffs, according to Frank Beathwait those monsters topped out at more than 30 knots in big winds and sailing with small rigs. There's a spectacular picture of a skiff screaming along in crazy conditions in his book. Around a course a Tornado would outsail them, as for sheer top speed however, I think the skiffs are way faster.

I assume you mean the Hurricane 6.5? What a monster!

I've seen them, sailed around with them, but never sailed one!

A quick google for "Hurricane 6.5 catamaran" came up with:

this

We even have a gallery on Catsailor here

This one is Reg White helming the rack boat:



This one is Rob White helming the ladder boat:



Here is a pic of a yellow one:



Some old stuff in german I think

I think he was referring to the Hardcore Hurricane, which was apparently a 16' boat (listed here), but there may be a connection to the Hurricane 6.5... Here's an article that indicates that the Hurricane 6.5 was aka the Roake 21 (and links to those photos here at catsailor). I guess this is the one Mary was referring to.

Edit: Indeed, one of the other images confirms this connection...

http://www.catsailor.com/gallery/displayimage.php?album=81&pos=8

Wouter,

My argument is not "smoke and mirrors", I am merely pointing out that "Mediatis-Region Aquitane" is a very succesful boat, contrary to what you seem to be trying to indicate in your post.

Regardless of the fact that there are three trimarans that have since beaten Yves Parlier's 24 hr solo record, I still can't see how you can maintain that this boat is not a winner, unless you are so desperate to never admit that you have made a stupid statement.

For people who are not familiar with this 60ft catamaran and who may take Wouter's statement at face value:-

As far as the 24 hr record is concerned, the boat is the fastest solo catmaran ever, it is the fastest fully crewed catamaran under 100ft and has proved itself to be up there with the best of the trimarans regardless of size. I will leave it to each individual to decide wether that makes it a "winner" or not.

Gareth

Ok, much more important!!!

What... is the Airspeed Velocity of an Unladen Swallow?

Well the Laden Swallow airspeed can be calculated by using the pi r round theory, thus the Unladen Swallow is calculated by subtracting the square root of the volume of recently ingested food times 1.23174 divided by its inverse factor to get EXACTLY what we think it will acheive (optimally). <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" />

Of course this assumes the bird hasn't had sex in a while which could make it much faster given its looking for a mate!

JMO,

Clayton
BS Engineer (and we know what the BS stands for)

African or European?

Is that one of those "Like you or love you....swallow vs spit" arguements?
Airspeed of her spit of an unladen swallow? Sorry for the triple XXX but Andrew brought it up. <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

Dave, swallow (the bird, not the act of)



and now for something completely different

and now for a different, something completely different

The Hardcore 16 was the Hurricane 500, but I'm not sure if it received the hull(skeg enlargement and reduced rocker) and rig changes of the 500. Great boat but too much volume low down for singlehanding in light winds. It was not a 'dog' as often quoted but did not have enough 'Bite' in the water without either a crew of two to load up the leeward hull or enough wind to generate lift from the skegs . Using a hurricane sport jib I had to slow down once to let the safety boat catch up. I had previously clocked the safety boat at 25.8 mph on gps and my hurricane 5.9 at 22.9 mph around the bouys. Pity it was not built with daggers or given a proper rig. John Pierce apparently tried to buy the moulds, but white formula wouldn't sell. So JP made the stealth.

Cheshirecatman