So if foils are the way to go....

Ok so if we say that foils are the way to build a rocketship of a multione whats the best way to do it? As i understand Doug’s foilers use a system similar to the Rave for altitude control, then you have the system used on the Hobie Trifoiler with like a ski out front. Also windsurfing has thrown up a couple of posiblities, the miller hydrofoil board with a canard out front to trim the main lifting foil on the fin could be adapted somehow to a multi if it was thought about, pivoting floats maybe?

If its not blowing it sucks!

There are a number of ways to design hydrofoils into a boat. If you have the time, read the chronicles of a boat I helped to build. While not a sailboat, the lessons learned here are applicable:

Hydrofoil Basics:

Based on that project, I believe that the T foil arrangement that Doug uses on the F3 is the right direction. There are a couple of things I would do differently if I were starting from scratch.

  1. While the flap system is mechanically easy to accomplish, it is hydrodynamically not as efficient as it could be. I would try to design a system where the entire horizontal foil actuates. This would allow you to design a slightly smaller foil as it would be more efficient at the size limiting conditions (low speed takeoff). This is maybe where you were headed with the pivoting floats idea? I think you will need about 15 degrees of total travel and I don’t think you could get that much movement out of pivoting floats (they are simply too ling).

  2. The balance of the boat on the F3 is such that the rear foil is underutilized. A few years back, I began a design on paper for a hydrofoil and I took into account all the forces and moments (statically) acting on the boat. Then I set an optimizer loose on the foil placement. I found that the two side outboard foils should be much further forward than Doug has placed them. The forward drive of the rig is such that the rear foil on the F3 is always near neutral or pulling down. If the outboard foils were further forward, then the rear foil would be lifting up and the outboard foils would have less work to do (and thus less drag). This arrangement would also reduce the tendancy to pitchpole.

  3. Forward skimmers (which we used on the canards for Decavitator) work well in really flat water, but they tended to trip in waves. (See the schematics of configs II, III, IV and V here: ) The Trifoiler has used very large skimmers in an effort to overcome this problem, and I think the drag from those skimmers is quite high. I like the idea of the wands, but think you could reduce the drag a bit if you went to a following skimmer using the same linkage system that Doug uses with his wands.

  4. The biggest problem with the F3 hydrofoils is that when the boat is not flying, the wands are commanding takeoff lift from the foils. This means the flaps are at full deflection. This causes a lot of drag and this is the main reason why Doug is toying with retractable foils. Retractable foils are a great idea, but I think another step would make the overall system even better. When you deploy the foils, it would be nice to have them initially at neutral lift and then allow the skipper to command the takeoff lift. I think this could be built into the deploymentmechanism such that the wands would be deployed after the foils.

This would be particularly useful for the rudder foil. Since this foil would not be retracted normally (you need to keep the rudder in the water), you want to neutralize the lift on this foil in light air. It would still be nice to hve this foil available to lift down to prevent pitchpoles (although the risk of pitchpole is low in light winds). So for this foil, you would want to design a system that would change the length of the linkage between the foil and the wand such that when the hull was in the water, the wand would be commanding zero lift from the foil.

Those are my ideas… I’d love to see some of those pictures of the F3 if it is OK with Doug…

  • Will

Will Gorgen

I posted a lot of information on the multiONE site about areas, weight and the altitude control system for a foiler of the F3/X3 type-I’ll double check whats there and update if necessary.VERY IMPORTANT: there follows some discussion and theory that YOU don’t need to know to make a succesfful foiler.And as soon as my retractable system sails I will make that public as well-a foiler without a retractable system cannot be competitive in light air! So if you find it convoluted or hard to understand don’t be discouraged or give up!!! I can and will tell anyone who asks what size foils and where to put them on a multiONE or F48.

One thing I’ve learned since the first boats is that the wand needs to be flat on the side hitting the water in order to prevent oscillation and to allow the end of the wand to sort of plane; the entry on the current wands is more parallel with the water than on the early wand types. It is important that wand HEIGHT is easily adjustable and in fact it may be beneficial to have radio controlled wand adjustment so the boat can fly low upwind and high downwind.(Vectored Lift™ -replaces 3D Sailing™ which has been sold.You control the lift vectors while you sail in three dimensions!!
Making the wands sort of plane tends to “mash” waves reducing their effect on the system.Previous incarnations of wands could skip and the new design tends to keep the wand firmly planted in the water.A word on foil design: the foils on the F3 were designed by me as an experiment.On the full size Rave sailing in choppy water is like driving a truck over a bumpy road because the wands esp. before the new type wands would respond to each little wave and the full flap foil would tend to cause the boat to jump up and down! The Rave has a full length flap on each foil allowing the flap to develop maximum control power from the foil(theoretically). But it seemed to be too responsive so my idea was to take the same basic flap area and concentrate it in the center of the foil hence the unique shape of my foils. This allowed there to be a solid flap hinge since there is no bending of the foil in way of the hinge(full length flaps on thin foils tend to cause hinge problems as the foil bends under load). It also allowed the outer portions of the foil to sort of act to slow down rapid foil up and down motion caused by the wands. David Hollom will be looking at this design scientifically to see how much it can be improved.
Skimmer type sensors seem to me to be too much affected by
surface waves.“Radar on a chip” is being looked at to replace wands…
I’m interested in the concept of an all moving foil but there are serious drawbacks for models.The response will be much greater than a partial flap foil and that may be a good or bad thing depending on how altitude is controlled. But mechanically if you don’t move the whole vertical fin like Hobie does(bad idea on models imho) then you have to have a hinge at the joint between the vertical fin and the foil.For models this may be very hard to make strong enough.
On the new X3 the main foils are being made retractable and it is being done so that they can be retracted or deployed in less than a second to take advantage of gusts-transitional conditions where the steady state wind is not strong enough to foil in. A foil system like this MAY be able to be controlled manually(via radio) and that would eliminate the drag of the wands. But because the wand on the F3/X3 is only moving a flap it can be made relatively small so manual control is not a priority(and woud be MUCH more difficult to sail with) EXCEPT that a small servo can be used to overide the wand to allow these boats to JUMP–which has already happened accidently and will be a real rush to do intentionally!
Will is right about the rear foil of the X3 remaining after the main foils are retracted but the rear foil does not use a wand and is set at ZERO degrees. I’m not sure that I would agree that the rear foil is underutilized since the F3 has absolutely NO tendency to pitchpole primarily because of the rear foil…The flap on the rear foil is never adjusted but could be.
Because the rear foil acts as an endplate on the rudder it is possible to have a smaller area to the rudder than would be possible without the foil; this can make up a little for leaving the foil in the water when retracting the main foils.
The F3 and X3 are designed with 80% of the weight of the boat on them with 20% allowed for the rudder T-foil.In practice it is more like 100% plus on the main foil and download on the rudder foil but this changeover happens automaticlly.Since the mainfoils are supported by vertical foils that ALSO provide the lateral resistance for the boat their position is relatively far forward requiring a raked rig and smallish jib. This rig position would NOT be suitable for a “normal” trimaran-it would be too far forward. If one was to move the hydrofoils -and therefore the vertical fins even further forward in an attempt to load the rear foil more I’m afraid the rudder fin area would have to be increased to compensate or the rig would go to far forward-even for a hydrofoil. Also keep in mind that this method of designing foil placement was pioneered by Dr. Sam Bradfield and has the advantage of being “tried and true”.I don’t want to discourage experiments but if you want to get a foiler going right off the bat this system works.
And please Dick post the pix!

Doug Lord
High Technology Sailing/Racing


I am suprised that the rudder foil does not have a wand. It seems like without that, you are in a very meta-stable situation - basically balancing on the main foils with very little assistance from the rudder to add stability. Of sourse under this configuration, the main foils are correctly placed. but it seems to me that you are carrying around a lot of extra drag without any benefit. If you balanced the load so that the rudder foil was carrying about 25% of the boat weight, you would reduce the lift of the other foils and since lift and drag are proportional, you could reduce the drag - perhaps significantly.

In order to do that, you need to move them forward and as you point out, that would change the balance of the lateral forces significantly. But you are also contemplating retracting those foils altogether, so do you have plans to compensate with some other form of lateral resistance when the foils are retracted? The lateral surface area of the rudder foil is already quite large, so you probably could change the balance quite a bit before you would have to increase it. And if you get the boat flying at the right height, the usable surface area on the rudder will double.

Another idea would be to extend the wings frward of the vertical fin by about 1/2 to 3/4 of their chord. Of course this might tend to catch more weeds, but it would also help the ventilation concerns…

From the sounds of you description of the bumpy ride on the rave, it sounds like the gain of the system is wrong. I like your idea of angling the wands back (more parallel to the water) and adding a planing skimmer to them rather than having them straight up and down. This is what I had in mind when I suggested the “following skimmer”. By adjusting both the sweep back angle and the length of the wand, you can independently adjust the gain of the system and the altitude. I think this is a benefit for the skipper to be able to tune those two aspects seperately.

Instead of flaps, I had in mind that the entire foil would pivot to create the changes in angle of attack. Flaps - and especially partial flaps - generate a lot of drag for a given change in lift compared to just changing the angle of attack of an unflapped foil. The canards that we had on Decavitator worked on this principle. It was relatively easy to actuate, but requires getting the pivot point right. We actuated ours with a leading skimmer such that the skimmer and the foil were locked together in a rigid structure. With the wands, you would need to employ a cam of some sort, but that should be easy to do with a bell crank or some such thing. Do you use a spring on the F3 that the wand has to fight against? A similar thing could be used if you actuated the entire foil.

I think it is great to have a working boat - the F3 - that we can use as a starting point. It makes it a lot easier to tweak once the basic system is functioning. I hope, someday soon, to buy an X3 and use that as a starting point for some of these experiements. Of course, every good America’s cup skipper will tell you that you need a trial horse boat to compare to, so maybe I’ll need to buy 2!!!

  • Will

Will Gorgen

If you took an f3/X3 and set it down so that the front foils were on one scale and the rear foil was on another scale 80% of the total weight would be on the front scale and 20% of the total weight would be on the aft scale. This means that the CG of the boat at rest is aft of the center of lift of the front foils.The front foils are set at an angle of incidence to the flight waterline of +2.5 degrees; the rear foil referenced the same way is set at zero degrees.
When the boat is flying if you move the flap on the rear foil via a servo the pitch angle of the boat changes minutely but nothing else happens!
The boat is extremely stable in pitch as well side to side and more so the harder the wind blows.
I would suggest that anyone interested in foils start with a system like this because we know it works very well. No question that, over time, improvements will be made.
Oh, and I think Will had a great idea: everybody that wants to learn and experiment with foils should buy(or build) TWO X3’s- that way I’ll be happy and you’ll learn a lot! Or two TX-1’s the multiONE version of the retractable foil X3…
One other thing: Will, you mentioned about the lateral resistance required when the foils are retracted: the way the system works the outboard portion of the foil on each side is still partially in the water to provide lateral resistance-or MAY be- if it tests well. But the first couple of boats will have a daggerboard trunk in the main hull with a board activated by the retract servo just in case.
This board will only be used ,if at all in winds under about 5-7mph when the boat will foil.
As best as I can determine there are no ventilation concerns with these models at all so I would make no design decisions based on the possibility of ventilation on a model foiler with the POSSIBLE exception of a surface piercing foiler(like Joes in the picture which never flew by the way).
Foils offer us ,the multi fanatics, the chance to provide the sailing public with high speed little multihulls that can be sailed with the same level of skill(depth perception may need to be higher and reflexes a little quicker) as a monohull only much,much faster with virtually no danger of capsize or pitchpole.When you can take a small multi out sailing in 22mph of wind without the slightest concern of flipping your fun level rises expoentially! And that impresses everyone that sails…

Doug Lord
High Technology Sailing/Racing

Hey Doug,

I can see from the videos, that the boat is stable. I am sort of surprised, but as they say, the proof is in the pudding…

I did not realize before this thread got rolling that you had done this. I sort of assumed that you had altitude control on all 3 foils. It seems like a good comprimise if it works: you make the boat a little simpler (one less thing to tune)and you reduce the drag when not foiling. There may be a little less stability for pitchpole as you cannot actively compensate with down lift to a pitchpole event, but that does not seem to be a problem. The extra drag you get from loading you main foils so high is probably quite small.

Still, I’d like to experiment…

I like the idea of leaving the tip of the foil in the water. But it seems to me that you need to get the retract pivot axis right in order to make that happen. Otherwise the two foils will “toe in” by that 2.5 degrees and fight each other which would be slow. It may be simpler to have a centerboard in a trunk or even just a fixed low depth keel that would be out of the water when foiling.

The centerboard would fix some of the weed problems, too. If you caught weeds, you would probably slow down enough to stop foiling. So you retract the foils which might allow the weeds on the main foil to drop off. If you get weeds on the centerboard when sailing at low speed. You retract it for a moment to clear the weeds. So the only foil where you still have a problem is the rudder. Not sure how you would fix that, but there might be a way…

Do you have any idea what your aparent wind angle is when you are foiling? Are you in any way limited by the induced drag angle on the sail planform you have chosen? For example, if you look at iceboats or E-scows which have a very low Beta angle, their sails are very flat (less that 5% camber in light wind and maybe 2% or 3% when they are fully powered up). This allows them to sail tighter aparent wind angles. This may also be an area worth looking into…

  • Will

Will Gorgen

Weeds are not a “hydrofoil”
problem ; they are a vertical fin problem with any boat having vertical fins–I don’t see a solution except to try to sail on weed free water.
The 2.5 degree problem is THE problem for using the retracted foil as lateral resistance. There are possible solutions in how the mount is attached and in the rotation of the foils.For instance, if the mount is angled up 2.5 degrees while being parallel to the boats centerline and the vertical fin is attached at 90 degrees to the axis of rotation of the mount AND the foil attached at 90 degrees to the leading edge of the vertical fin then when the hydrofoil is retracted so that it is vertical it will be parallel to the centerline of the boat(thats one illustration: it can be done using the angled mount and a vertical instead of angled forward fin). The difference is between just rotating the foils 90 degrees or having to rotate them 130 degrees-and of course having the daggerboard.I think the biggest question is whether or not the foils will provide enough lite air lateral resistance effectively enough to be able to eliminate the need for a daggerboard or other solution.I’m afraid a non foiling centerboard would be too low aspect but it may be worth looking into.
I’m not 100% positive of apparent wind angles but when the rig is set up right it seems to work very well.
Beam limitations on the F48 require that the same size mainfoils used on the F3 be used on the X3 to try to achieve very early takeoff(5-7mph to start foiling). I’m going to try a rudder T-foil with 50% of the area of a single main foil(F3 single main equals rudder foil area) just to reduce wetted surface a little.

Doug Lord
High Technology Sailing/Racing

I think the foil area reduction on the rudder foil is a good idea. However, if you are not controlling the altitude of the rudder with a flap, it will take twice the change in angle of attack to achieve the same change in lift for a foil half the size. At some point, you will not have enough range of altitude in the back to handle the range of lifts needed by that foil. That is why I think you should consider adding a wand to the rudder…

As far as the weed issue goes, yes, all boats have issues with weeds. But, since weeds that are still growing tend to grow straight up, a vertical fin will not snag them whereas a T-foil would. Any floating weeds are going to be snagged by any boat with a vertical fin and I would argue that the T-foil is no better or no worse than a bulbed keel for retaining the weeds, but you have 3 of them instead of 1 1/2 (a rudder behind a keel is less likely to snare the weeds if the keel gets them first).

Based on the pictures that Dick posted, perhaps I will name my X3 the “Millfoiler”… Just Kidding.

  • Will

Will Gorgen

Fin keels with bulbs have similar characteristics to vertical fins with hydrofoils and I know from experience that you can back up a fin keel boat and clear the keel ; its not as easy as using a spinnaker to back up but maybe the foiler could be cleared that way to.Also, in my experience I’ve never seen a weed catch on the hydrofoil here-on the fin yes because the weeds you catch are floating on the surface and each hydrofoil has a little angle that apparently allows weeds to slide off.
But the fact remains any boat with vertical fins can be slowed and or stopped by weeds; it is not a phenomenon any more particular to a hydrofoil than to a fin keel and bulb or other type multihull with vertical fins–ALL of them have to stop and back up at the very least.

Doug Lord
High Technology Sailing/Racing