hi there,
does anyone know what is meant when they say the hulls are canted to a sertain angle and whats the advantage of having the hulls canted? also on the inter 17 they say on the specification that they have high aspect daggerboards (4.75:1) and high aspected rudders (3.1:1), whats the advantge of having high aspected foils and how would a designer work out the ratio for the foils?
I would be very greatful if anyone could help me with my questions
regards Tom

Hi,

Canted Hulls mean that the Hulls will be vertical when you have a little heel on to fly the windward Hull

Aspect ration is a calculation based on width / length of the plate into the eater. Longer paltes are more efficient.

The Inter 17s (among others) use canted hulls so that the leeward hull and board and rudder are perpendicular to the water when the windward hull is flying just above the water (the ideal configuration). This positions the hulls for lowest wetted surface (lowest drag) and the lifting foils for their best performance in resisting leeway.

Higher aspect ratio foils are more efficient than lower aspect ratio foils, in general. Aspect ratio is length over width of the segment that is under water in design position, in this application (I think).

I hope this helps.

Hi Sparky,
The jury is still out on your canted hull position. There is some advantage in having the daggerboard and rudder not in the vertical plane sailing to windward and flying a hull. When the daggerboard and rudder are perpendicular to the main beams and the boat is flying a hull, the foils in the water are generating side force and a lift force. This lift makes the hull float higher out of the water and reduces the hull wetted area. If the board is vertical when the boat is flying a hull, there is no lift force from the board. The tilted sail plan, hull flying, produces side force and forward force and "down force". There is no centerboard upward force to counter the sail down force with canted hulls.
What really needs to be done is a test. Put a boat together with a canted hull on one side and an uncanted hull on the other side. Use a GPS to record boat speed and direction data. Sail on both tacks, upwind, downwind and in different wind strengths. Repeat the test several times. Analyse the data and then we know for sure if there is a benefit to canted hulls like you describe.
Also there is this argument: If there is a benefit to canted hulls with hull flying, then there is a penality to sailing a canted hull boat when not flying the hull. In a race if you fly the hull to windward and sail with both hulls in the water downwind, what is the gain, loss? We need a test!
Foil aspect ratio in an areo text book is span divided by average chord. On a centerboard or rudder this means depth below the hull divided by average width. Higher aspect foils are said to be more efficient. A better term is higher aspect ratio foils have a higher lift to drag ratio than lower aspect ratio foils. WHY??? In the equation for calculating induced drag, drag due to generating lift, the foil's aspect ratio is in the denominator of the equation. Therefore as aspect ratio goes up, induced drag goes down. What is the disadvantage to higher aspect ratio foils? The bending moment is higher in the longer foil, the higher aspect ratio foil. Therefore it must be built with stronger materials like carbon and this makes them more expensive. So again, higher performance costs more.
Bill

drag induced by the "tip vortex" effect is less for high aspect than for low
Darryl

If hulls are "cantered" they, and the foils, should be "toed" in to reduces drag when sailed flat.
Darryl

All the questions concerning "raked" boards, twin rudders, cantered hulls etc, have been answered by the appropriate experimentation years ago, in particular (but not exclusively) for their adaptation and use (of twin dagger boards & rudders)
on mono hulls sailing in restricted depth inland lakes, rivers etc, in Holland, Germany etc.
Darryl

Thanks for the explanation, Bill. The canted hull question is interesting and the thought of the sailplan forces driving the canted hull deeper into the water is interesting. Seems that it would have higher drag than non-canted hull design. Would that lead us to cant the hulls in the opposite direction resulting in less drag?

Cogito had canted boards that were asymmetric and canted inward to lift the hull, probably to reduce wetted surface area and drag. This design required that the windward board needed to be retracted when going to weather, which is probably not practical on most catamarans. I wonder, which of Cogito's boards (if any) should be down going downwind?

Yo all,

Nice thread here.

Now you have me wondering,...why am I lifting my d-board as the wind increases?

The I-17'ers actually have a formula ( board lift vs. wind strenght) that we use,........and it works!

Why?

( example - hull beginning to lift * board up 6 inches
15 mph wind * 10-12 inches up
20 mph wind * 16 -ish inches up
25 mph and up* 24 inches up and find a way to go home

regards,

Bruce
I-17
St. Croix

Hey Les- congrats on the A2

Thanks, Bruce!

I always sailed with my I-17 boards up at least 8", because it just seemed that if the board was long enough for the I-20, it must be too long for the I-17. Whether in light air or heavy, the other I-17s did no better than I did and they all had their boards all the way down. I think Performance knows that they don't need the boards that long for the I-17, but tooling cost for different boards for the I-17 and the I-20 doesn't make any sense to them, and since they are one-design, they are not optimizing the I-17.

Hi Sparky,
The wind force from a mast/sail/rig leaning to leeward at the top tends to push any hull deeper into the water, canted or not.
You don't have to cant the hull to cant the board. You can have canted boards and vertical hulls but as noted on Cogito, the windward board is always raised. On Cogito each board is the proper size to generate the side force to balance the sail side force. This results in a very high aspect ratio board which has a higher lift to drag ratio, which means less drag, than two boards half as deep.
Cogito will always use the leeward board to generate lift because of the asymmetric shape, Again the asymmetric board shape has a higher lift to drag ratio than a symmetric shape but only when loaded from the correct/proper direction. The flatter side of a foil is the high pressure side and the more curved side is the low pressure side, just like an airplane wing.
Bill

Hi Darryl,
Where's the book? Where's the imformation documented? It is much easier and faster to read about what someone else did years ago than work it out myself.
Bill

Hello Bruce and Sparky,
Since the centerboard force opposes the side force from the sail, centerboard area is directly related to sail area. If the I17 had the same sail area as the I20, then the same boards would be right for both boats. If the I17 sail area is, for example, 20% less than the I20 sail area, then 20% less board area is required to sail towindward on the I17.
Also on other points of sail where the boat goes faster than to windward, board area can be reduced because the lift capability of the board area increases as the square of the boat speed due to the dynamic pressure of the water. For example, if a boat sails to windward at 10mph but can reach at 20mph, then at 20mph only 1/4th as much board area is needed as when sailing to windward at 10mph.
Bill

Ahh you know this Brobu !

We've been through all this when we tuned out your boat. Theory and reality are somethings two completely different things.

Problem with theory is that it is very easy to stare yourself blind on one aspect of the situation (higher theoretical lift to drag ratio) only to completely miss the full picture that may limits any gain of that particular point you're looking at (longer boards make boat trip happy, nervous and limits raw power by a system of linked fulcrums and leverages)

Wouter

Hi Bruce,
The points Wouter brings forward are correct for a "righting moment limited" situation. As long as you can hold your boat down and are not overpowered, higher board aspect ratio is less drag. The daggerboard is a small contributor to overturning moment, like 1/8th or 1/10th as important as the sail plan. A lower aspect ratio sail plan can also be faster than a high aspect ratio sail plan in "righting moment limited situations". When you are overpowered, get the CE down and the CLR up but the CE lever arm is a bigger driver.
Bill

In addition to bills comments I would like to add that apparently there is another phenomenon at work and nobody has figured yet what it exactly is.

some describe it as "tripping", A long daggerboard (long is closely related to high aspect of course) tends to introduce nervous behaviour of the boat in trapezing conditions or gusty conditions. triggered be various thing of whcih increase in windforce is just one in my opinion the boat quickly raise its windward hull necessitating the skipper to dump the main or head up violantly. This behaviour can be so nervous and unpredictabel that a boat skipper combination is all over the place. Raising the daggerboards a little calms the boat down very quickly.

There is another report that indicated that in lights winds the smaller daggerboard isn't at a disadvantage to a longer one. This came from a design that was identical in every respect except the daggerboard size and aspect ratio. Boyer A-cats also don't seem to loose as much or even anything in both ends of the spectrum (big wind- light wind). In medium winds the difference was noticable with the bigger boards being a little faster.

Someone needs to take out some time and try to discover why these thing are happening. I have a few thoughts but no time or resources to test them. But one thing is for sure the theory suggest significant differences all over the wind spectrum but in reality this is not always reflected.

A few things that I thought of ;

-1- in light winds the boat moves relatively faster (as ratio of windspeed) than at higher speeds. This is mostly because wave making drag and other forms are far less significant at the speeds. So less sailforce and relatively more speed. This would allow the boards to be smaller than the ideal size at medium wind ranges. The difference could be something like a factor of 1.25 to 1.75

-2- At high winds control of the boat and smooth sailing is far more important than power. It must be compared to a racers that full breaks and than fully accellerates at each corner instead of a driver maximizing is cornerspeed by carefull steering and achieving the biggest corner radius. The latter will be faster around the corner. Same with boats I think.

-3- Overly high aspected board may have a to small (or narrow) band of optimal performance. Especially in gusty conditions the onslaughts of sailforces and waves may shake the board off it optimal working point often enough. It is well understood in wing design that short stubby wings give improved control and more dependable lift in violent manouvres. Besides nothing is more draggy than a ventilating or cavitating foil. The whole flow is disrupted for a realtively long time. A short board or wing quickly establishes attached flow again in addition to having a higher threshold for disruption to occur. I refer to the Inter rudders at this point. Many sailors have experienced that it is easy to stall them (or have them ventilate) especially with rudders you wnat sharp and large lift forces without "complaints".

-4- Shorter low aspect boards are in general very much stiffer than the longer ones. Also high aspected boards are more sensitive to disturbances and also to imperfections on the boards surface. I think that a low aspect baord can be made critically smaller with a more ellipical leading edge (milder entry). For example the inter boards are so narrow that there is a relatively short leading edge section that has to build up to full width in probably 1/2 or 1.3 of say the boyer Taipan/ Mossie boards.

I'm not saying that these are explanations but in aspect like these the answers to why should be found.

It is very possible that a combination of they above or a selection of them with several other explanations conspire to loose much of the theoretical and laboratorium established gains. Even to such an extend that there is hardly any difference between the two schools of thought. It wouldn't be the first time something like this happened.

One of the great examples are the America cups keels. One keel was absolutely superior in theory, computer models and lab test (under carefully regulated conditions) In real life sailing is was a monster to sail. Bad behaviour all alround as the it would fall off its intended and advantagious optimal working point all the time and transition into a draggy dirupted flow mode. The keel was not forgiving enough of the constant changing heel of the boat and the associated angular rotation of it. Do not forget that these rotations introduce all kinds of 3 dimensional turbulance and currents not to mention continiously changing flows ALONG the keel or blade.

Theory was established on lab test. Designing more than often includes adapting the theory to real life conditions which are more than not continiously changing.

So the jury is still out on this one

Regards,

Wouter

Hi Bill, I can't remember which books (and there are many references to this effect in quite a few books) but from memory, the most comprehensive explanations are from the 70's to 80's for adaptations of "lee" boards as well as twin centre boards (and rudders) on shallow draft sailing mono hulls and sailing barges, sailing and working on the lakes and canals of Holland and Germany in particular, but also for pleasure craft in the Florida region. It would seem that to greatly reduce drag (and still have a similar efficiency to a more conventional single board) from having both boards working at the same time, they had to be set up "cantered outwards and toed in so that they "pointed" to a point approximately one third of the length of the hull, forward of the bow, and the rudders set up to point to that same point. This is a very "approximate" description as I am relying on memories that date back some 30 odd years. The idea to have twin boards recieved some popularity for a short time for "pleasure boats" as it had some obvious advantages over deeper foils, ie to be able to sail and cruise in shallower waters (as the actual depth of the cantered boards were not much deeper than the keel line of the boat, but due to the angle of the board, there was still a respectable area of board in effect), to be able to "beach your boat in tidal harbours and when the hull was sitting on the mud the boat would still be in an upright attitude instead of laying on its side (popular in a lot of European harbours), and for working, sailing barges it was, of course essential.
The adeptation for these "twin" boards on a mono is different from that of a multi hull only in the number of hulls, but the principles for both are the same.
Darryl

Bill, Wouter and all,

Again, a nice thread.

Bill, thanks for the input, interesting.

C. Ogletree of Ulman designed a 'small' main for me a year ago, 150 sf instead of stock 160 sf., in the 16 + mph (plus 3 ft waves)range, the I-17 is very fast, powerful and stable.

So, for me, in this region, reducing sailplan was one of the keys.

And as Wouter, John of Stealth, the Aussie T4.9 sailors and the F 16 forum taught me,...those boards must be adjusted, as the wind increases, in conjunction with the mast rotation/travellor/downhaul....they taught me to consider all of these as a part of a formula and adjust them all until the boat gets 'happy'

..but those boards,...if they do not come up,...its up ,down,up,down,up,down, up, down, up, down,up....

I am a adjunct Professor at UVI for the last 5 years down here ( this way I can sail all the time) and I am very methodical about testing this stuff,.....there is alot of physics to these boats,.for sure!

So, you all keep posting the ideas, we will go out and test it!

Hey Les, did you part with the I-17? You are going to miss it!

regards,

Bruce

Hi Bruce,

You are right...this is a great thread! Lots of interesting stuff to read, much from those deserving a lot of respect.

Yes, I sold the I-17R and yes, I am missing it. It has been a tough decision whether to buy a new I-17R or to get the NACRA A2. My dealer, The Cathouse, supports both and I need dealer support for all the things I break, so I never would have bought an A-Class without NACRA supporting it. The toughest part is to go back to a single-handed boat that has no spinnaker. It adds so much to the enjoyment of racing when you get to the weather mark, turn down and put up the chute, heat it up and fly downwind! Two more sailers in our club just bought I-17Rs and they both have lots of I-20 experience. I expect to see them having lots of fun with their boats! We also have a growing A-Class fleet, and it should be fun to sail with them. Usually, our NACRA 5.5 Uni fleet, I-17R fleet and the A-Class all start together and it makes for some interesting racing! Our club, Catamaran Racing Association of Michigan, seems to be split nearly 50 - 50 between single-handed boats and sloops. If all the single-handed skippers could end up on one type of boat, that would be ideal! (I am a dreamer!)

Summing it up, I am missing sailing the I-17R, a truly GREAT boat, and looking forward to learning to sail the NACRA A2! Thanks for asking! Always look forward to your posts, Bruce! Best of luck and Lots of Fun sailing your I-17!

Les,
Tell us about the N2

Is it a 'round' bottom boat ?

I see the sf is 150,....like all A's,....this will be a challange for you.

Upwind, on a heel,..does the downwind hull have its' bows submerged ,..or not?

When is your A2 scheduled to arrive in Michigan?

regards,

Bruce

ps,..Les, your posts to me about 1 1/2 years ago really helped also,....I still see a I17R in your future.