Building a foiler - getting started

A short while back I came across pictures of Lorsail’s foilers and decided to try and build one. I started with some old plans for the Snapdragon Mini 40 and Doug’s information on this site has been most usful. So far, I’ve made the main hull and two amahs - planked in balsa and glassed. I’m currently looking at using helicopter blades for the struts and the foils. Anybody interested - let me know.

Hey Paul,

Sounds like a fun project.

Have you done any calculations to determine the foil sizes needed? You will need larger foils in the lower speed range (for lift-off) than you will need at the higher speeds. I also believe that the positions of the foils for optimal lift distribution requires the outboard foils to be further forward on the amas than Doug put them.

How do you plan on controlling the altitude? The system that Doug uses is effective, but based on what I have seen could be improved upon. I would be interested to see if you have any clever ideas in this regard. Some full size boats use forward facing skimmers rather than the trailing arm system that Doug uses. The problem with that is nosediving under the waves. If you are using helo blades for foils, do you intend to incorporate a flap into those foils? Another option would be to change the angle of attack of the entire foil. This could be done and would be mechanically easier, but you will need to be very careful with the pivot point that the foil pivots around. If you have it to far aft, then the foil will want to hinge forward to a higher angle of attack and it will not respond to skimmer pressure. If you put the pivot too far forward then there will be too much pressure on the skimmer which will lead to excessive drag.

I also think that Doug was on the right track with his retractable foils. I think it would be pretty easy to swing the foils inward to get them out of the water for light winds. leaving one “wing tip” in the water would provide a “daggerboard” type device. but this is only recommended if you can ensure that the foil is at the correct angle (not toed in or toed out). Since these foils create lateral resistance when flying, it would also be nice to have the foils “lock” into the down position tso that you were not holding them there. Otherwise the servo would need to be strong enough to do that and you would be using up battery power…

How soon do you think the base boat (without the foils) will be done. I would get out and sail it a few times before attempting the foils…

  • Will

Will Gorgen

Sounds like fun! I’ve been wanting to build a model of the Hobie Trifoiler for quite some time.

sounds very interesting. i found a website that shows a mini40 foiler. it’s in french but i suppose it could be translated if desired. i’m thinking about doing the same with my old mini40. do you have a webpage so we can see your progress??

That French foiler is very interesting.

He is using a variation on the V foil. V foils on the surface appear to have some advantages. They automatically adjust their lift. If you have too much foil area under the water, then the extra lift will cause your boat to rise which will reduce the foil area until equilibrium is achieved. But when you dig a little deeper you will see that while this idea works for some applications, it is the wrong way to go for a foiler.

It appears that he can actively control the angle of attack of the two foils. This will allow him to adjust the windward foil to pull down on the windward hull while the leeward foil pushes up on the leeward hull. This is a great way to react the heeling moment of the sails, but this combination (V foil with variable angle of attack) may cause some problems. Here’s the scenario:

You are sailing on a beam rach or even beating. The heeling moment generated by the sail is reacted by the leeward foil pushing up and the windward foil pulling down. You have adjusted the windward foil so that it is at a negative angle of attack to generate the downward lift force. Now you get hit by a puff. The heeling moment increases and the downward lift on the windward foil is not enough to keep the windward ama from rising. What happens? As the windward hull rises, the amount of foil area under the water DECREASES. This results in even less downward lift and the ama will begin to rise even faster. At some point, the windward foil leaves the water and you are headed for a capsize. Your only chance is to react quickly enough to the puff to command more downforce from the windward foil.

I suppose this is no less stable than a non-foiler multihull. But it does not have the inherent roll stability of an altitude sensing system like the F3 has.

The other problem with V foils is that the lifting surface pierces the water’s surface. This leads to ventilation of the foils. You can see that he has added a lot of fences to his foils to try and prevent this. But, again, a fully submerged T foil does not have this problem.

More info on V foil ventalation here:

and here:

Getting back to the french foiler, you will notice that his foils are positioned well forward of the F3 foils positions. I believe that this can result in slightly lower drag by balancing the forces better between the main foils and the rudder foil. The problem with this is that the forward shift in the lateral resistance is going to cause an increase in the windward helm. But the overall drag story (and thus the speed of the boat) should be better with a better balance between the foils.

I would stay away from V foils if I were you. I would stick with a T foil system. I would also try to achieve automatic control rather than manual control of the foils. But I would take a look at positioning your foils a bit further forward.

  • Will

Will Gorgen

Thanks for the interest, folks, and the advice. I am busy doing my calculations! Doug’s method of control looks the most promising and I’ll go down this path for now. I got some good advice on foil aspect ratio, and will probably get some 1.75 to 2 inch helicopter blades. I can cut a flap into the rear section (yes, mylar hinge. I’m not sure about trying the base boat out as the amahs won’t have sufficient bouyancy woithout the lift from the foils.

I don’t have a website, but promise to post a few pics as I progress.

A few years ago, I modified a Mini40 by sticking some aluminium J foils on it. It worked well enough in high winds but was a real dog in light weaher - the boat was too heavy and the foils were too small. It would not pitch-pole, but in strong gusts would lift right out of the water, skip a few feet, and continue sailing.