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="" />