Small Surface Piercing Trimaran Foiler

Hi Gary. It's a small world! You did well to work out I'm from Nelson.

There are some really interesting boats from contributors to this thread and I particularly like the video of Doug's Broomstick foiler when he appears to have removed the amas!

My next job is to replace the rudder and rudder foil and I wonder if any of you have comments of the foil size and aspect ratio for the rudder foil. I think for a small boat it is good to keep the rudder foil reasonably substantial for stability. What is everyone's experiences with rudder foils?

Cheers,
Craig

 

There are lots of variable that affect the horizontal foil attached to a rudder (using airplane jargon it is a rudder stabilizer and if you have flaps they are elevators).

The biggest variable is the range of expected design loads. I could see loads from 10% to 50% as possible with 20% to 30% as more reasonable. At some speed on some headings you will probably transition through 0% and go negative.

The moth is the most well developed small foiler and they are still playing with ratios, but it would be hard to argue with looking at the latest moth rudder foils and just scaling based on total boat weight as a good starting point. Your boat balance does not strike me as drastically different than a moth.

If I were to consider one variation it would be to back off on aspect ratio a little bit for "homebuilt" vs. "factory built" practicalities. With a lower aspect ratio, you can probably afford to go "thinner" on section thickness than you would with a higher aspect ratio.

If you are pushing for performance, many will point out that a good planform is a big deal and worth the extra effort.

 

Dr. Bradfields rule of thumb was 80% of the load on the main foils, 20% on the rudder foil. Foil area on the rudder t-foil was such that it had lower loading per sq.ft. than did the main foils at takeoff.
On the Rave and Osprey, the two main foils and rudder foil were all the same size. The mainfoils carried 40% of the load each while the rudder foil carried 20%(at takeoff). Of course, on a Bradfield fully submerged foiler the main foil loading increased a lot after takeoff since the mainfoils provided all the RM for the boat. At maximum RM, the foil loading on the lee foil is twice(or nearly twice) what it is at takeoff and the loading on the windward foil is nearly the same as it was on takeoff but pulling down instead of lifting up. On the Raves that had adjustable angle of incidence on the windward foil, the down force was equal to the lift at takeoff. Since the main foils were set up with a +2.5 degree angle of incidence, the downforce was a bit less than the lift at takeoff in boats not equipped with adjustable angle of incidence.
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Craig, there is an excellent book by Ray Vellinga called "Hydrofoils Design Build Fly",ISBN: 9780982236116 available at Amazon. Lots of good stuff including a chapter on the Moth with lots of data.
Eight Moths are listed-all eight were designed with the nominal mainfoil load at 75%, rudder 25%. However, that can be changed a lot by crew movement to facillitate a quick takeoff. Mainfoil areas are all around 1.02sq.ft-to 1.19 sq.ft.. Rudder foil areas around .83 sq.ft..
Rudder foil loading about half of the main foil loading,per sq.ft..
Main foil aspect ratios for the Moth-7.1-10.2/1 (average: 8.25/1) (Note: recent Moth foils have gone up in aspect ratio)
Rudder foil aspect ratios from 5 to 9.6/1 (average: 6.7/1).
Rave main and rudder foil aspect ratio=6.75/1 .
Bradfield Osprey main and rudder foil aspect ratio-10/1 .
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Below-Bradfield Osprey and main and rudder foils. Note Foils on the Rave and Osprey are retractable allowing for beach launching:

 

Sailing Without Amas

Hi Craig,

I wrote about sailing without the amas after I 1st tried it back in 2008. Here's the link to that thread:http://www.boatdesign.net/forums/mu...ng-hydrofoil-trimaran-without-amas-23293.html

Since then, I've probably sailed without them more often than with them; I very much prefer how the boat performs & feels that way. However, you've got to keep the boat moving or things can get pretty dicey (as the video shows). You can't stop to rest, take notes, remove weeds, etc. After only an hour or so, I'm usually pretty exhausted. So it's nice to be able put them back on easily.

I also have some comments about aft foils, but I'm digging through my photos to try to find some to illustrate the points I'll be making.

Keep up the good work. Seeing your boat has inspired me to try to get Broomstick back out on the water sometime soon.

Doug Halsey

 

Rudder foil

Thanks for your comments everyone.

I think your ideas to look closer at the Moths is really helpful.
I wrote a velocity prediction program when I started the project (so I have the basic maths sorted), but I haven't included the dynamic effects of gusts and waves and a few other elements. I should really update it at some stage - actually rewrite it, but my day job gets in the way!

Its the variable loads/gusts/waves and lapses in sailing skills that I'm looking into now. I've managed to get hold of Ray's book and some of the comments in there on aspect ratio and the Moth data are useful. The aspect ratio of my existing rudder foil is a bit small (approximately 3). It was build with carbon wrapped around a wire cut polystyrene core with a wooden stringer. For my main foil I built molds and in general they have turned out better. I'll make molds for the new rudder.

The photos of Osprey are interesting and I like the flap adjustment mechanism.

I've attached a photo of Kotuku on the beach. You can see the front foils retracted (almost). The rudder foil retracts like Osprey, but is removed in the photo because it doesn't retract fully - something I'm going to fix on the next iteration.

 

Rudder Foils

Hi Craig,

I'd be careful not to make the rudder foil too big. My VPP calculations for Broomstick (in a wide variety of cases) show the aft foil to be very lightly loaded (typically CL<0.1) & the induced drag to be very small compared to the profile drag. If that's the case, increasing the aspect ratio would be good only if you do it by reducing the area, not by increasing the span. Of course, VPP codes depend on numerous assumptions & approximations, but as far as I know, nothing in my experience sailing Broomstick contradicts this.

Also, at the small lift coefficients involved, having a flap on the rudder foil is not really necessary; it only increases the profile drag. However, it is important to be able to adjust the aft-foil lift while you are sailing & you may find it more convenient to adjust a flap than to change the incidence of the whole foil (which I do on Broomstick).

But, Kotuku & Broomstick are different boats, so ...

 

Hi Doug

Thanks for your comments - some useful pointers. My rudder foil is indeed lightly loaded, and definitely negatively loaded when the power comes on!

Cheers,
Craig

 

More Comments About Rudder Foils

Hi Craig,
I'm quoting myself from post #30 of the Training Wheels thread, asking about the foiler NF3 :

The 1st photo I'm attaching shows Broomstick's original aft foil on the kick-up rudder. In addition to being too shallow, the up/down control lines were totally inadequate, leading to some interesting rides, like in the 2nd photo. The stiffness of the rudderhead was also poor, as you can see in photo #3.

The 4th photo shows the boat sailing with the "improved" aft foil that was still too shallow. Boat speed at that instant was slightly over 25 knots, but it was flying too high, leading to the situation in photo #5 (which at least was useful for comparing the foil depths).

My next (& current) aft foil was about the same depth as the tips of the main foils. This allows me to use a wider range of main-hull pitch angles, like the bow-down low-flying mode in the 6th photo. Using this foil in 2010, I set my personal best top-speed of 28.7 knots.

Some of these comments may seem obvious, but maybe by pointing them out, I'll help you avoid some of my mistakes.

 

Hi Doug

Great photos!

No problem with the obvious comments (in hindsight everything is obvious) as they help confirm what I have been doing. I've been flying solo with my design so far, so its good to get some confirmation and to stop me going off track. My rudder foil depth is the same as the main foil tips. Like your design, my VPP has predicted max speed when the bow is down and low flying - not too surprising. I think it is time for me to get cracking with pitch or flap control of my rudder foil.

I'll have to say one thing. Now that I've sailed on foils I think I've become addicted!!!

Cheers,
Craig

 

This makes no sense since the mechanism used in part was negative lift on the weather foil. Now its true that just prior to negative lift kicking in your loadings are probably more like 70% on the leeward foil and 3% on the rudder foil as the weather foil goes neutral, but that's no more variance than you would see coming off the top of a 2' wave.

 

I should have given photo credits to Terry Curtiss, a friend & photographer, who has been to the lake with me several times, taken hundreds of photos each time & been more than reasonable about what he charged me. All of those photos were taken from the shore.

 

From the photos, you seriously need flatter sails :--)

 

Oh, Really ???

My first impulse on reading this was to simply ignore it, but I guess the smiley at the end convinced me that maybe you weren't just being negative.

My second impulse was to be defensive about it. After all, part of my credo is that foilers don't have to be high-tech & expensive; lots of times you can just "use what you've got." The mast, sails, original rudder & other miscellaneous gear were all things that had been cluttering my garage (actually garages in 3 different houses) since 1973, so they were almost 30 years old when I built Broomstick) & are over 40 years old now. Since I'm not a racer (anymore, at least) & my main interest is learning whatever I can & testing out some of my own ideas about foils, I figure the sails will do just fine.

My third impulse now is to look at the photos & see if I think you are right.

I'll bet your opinion was formed mostly from looking at photo #2 (the one with the bow way up in the air.) However, in that photo I'm using a different main than in all the other photos. That one was a Hobie-14 main that I tried for 1 season (2007), thinking I needed more sail area. And yes, it definitely was way too full!

It could also be that the poor set of the jib is contributing to your impression. It never has enough forestay tension to keep its luff from sagging excessively (partly because of the wide staying angle.) Its leads are set very wide in photo #3 (as an experiment one day when I was focusing on reaching). And it's actually luffing in the last photo (because this is a very shifty, gusty lake & I don't have enough hands or quick-enough reactions to keep a sloop rig trimmed optimally, while also steering the boat. In some other photos (not shown), you can see a seriously stretched leech.

Photo #1 shows the boat at anchor & in photo #3 the view of the main is mostly blocked by the jib. So, that really leaves only photos # 4 & #6 by which to judge the main.

In photo #4, I had just been hit by a pretty good puff. I see lots of mast-bend & twist in the sail, but the camber doesn't look excessive to me. The top 3 battens look almost perfectly straight!

Photo #6 has probably the best view of the main, set in a pretty representative way. In spite of the glare that makes it harder to judge, the main looks pretty flat to me.

In view of all this, would you still say I need flatter sails ?

 

Looking at the wind on water, there were some hard gusts and it looks like you were depowering, easing main sheet - and that allowed some twist and leech falloff - hence BB's criticism ... and if hardcore BB was substituted for you, the foiler would have buried and arsed over probably.
But I have a little comment too; get rid of the headsail, go una and increase main area to compensate, then your problems of distorting headsail shape (because of Broomstick's wide staying rig base), will disappear. A main is much easier to keep in shape, imo, plus better aero efficiency.

 

Gary : I'm sure I would have hit that tree, if not for the jib !