Small DIY outboard hydrofoil boat
My first post here, thought I would make it thorough (long)Ö
I am working on designing and building a homemade outboard powered hydrofoil boat. I have an old 14ft alumacraft like the one in the attached picture to attach the foils to. I also have a 4hp, 10hp and 18hp outboards available to power this boat. I would like to reach about 20 knots for a top speed. I estimate the total weight will be about 650lbs for boat, motor, fuel and 2 passengers. The boat will be run on inland lakes in Minnesota, USA, and so large waves are not a concern; however I would like the boat to be able to handle wake from other boats without breaking the surface or crashing. I have thoroughly read Ray Vellingaís book (in addition to Hook, DuCane and any others I could find) and I am planning to use a similar foil setup, but on the boat instead of a surf board. I like the fully submerged foils for reducing the chance of ventilating and crashing.
I think I will make my own Shutt strut for the front. Iím considering making the joint between the feeler arm and the strut solid (ala Hook) and pivoting the entire strut instead of the linkage and a pivoting foil. This will also allow the system to pivot up and out of the water for beaching or trailering. What are the main consequences of this decision? I am also thinking about making my surface feeler quite long, about the wavelength of any waves I want it to "filter" out. The front foil will control the pitch of the craft. If needed the strut could be made to steer, although Iím wondering if it would be better to use roll and a little rudder (on the outboard) for directional control.
For the rear foils, I have not yet come up with a good plan for controlling roll. I am thinking that I will probably have two foils, one on each side of the outboard. However, maybe it would be best to have one large foil under the boat in front of the outboard. I am wondering what the consequences of having surface piercing foils in the rear would be. If the rear foils were to crash, but the front did not, what affect would that have on the boat? I like the simple design of the Upright hydrofoils for the rear. Otherwise, Iíll probably try to do an incidence controlled section like Ray did on his Hifybe.
One further question I have is about recent foil construction. I would prefer to keep costs down and build the foils myself. I do not have a lot of experience with composite layups, but I would be willing to incorporate this into the project if it seems like the best route. I am comfortable with TIG welding, and I see that some of the older foil designs have used weldments. I do have access to a 5 axis water jet cutter, which is really easy to use as a 2 axis cutter for plate/sheet. I was thinking of cutting a bunch of foil shaped ribs with a square hole at the ľ chord point and spacing them on a square shaft (spar). I have attached a model of this. Then I could fill the space between with foam. I could wrap this assembly with fiberglass or thin sheet metal. Would I be able to get the surface accuracy required if I wrap this in fiberglass without using a mold? I also may be able to use a 5-axis mill and machine some foam without the need for the ribs and just lay the fiberglass over this. I might need more than one layer though, and it would see that the more layers, the less accurate the surface will be. Also, speaking about foils, it seems that Tom Speerís H105 section will be good up to 20 knots before cavitation (depends on wingload I suppose). If I wanted a section that was capable of going a little faster, which ones should I be looking at?
As an additional consideration, if this application is going well, I would like to use these foils on a heavier boat. So my goal would be to design specify the area so the small boat would run at a low CL and the heavier boat would run at a higher CL on the same set of foils. I would be willing to accept a drag penalty on the lighter boat to make this possible.
Any additional feedback would be welcome as well.
If I was making a hydrofoil boat for my personal use, I believe I'd go for some surface-piercing wing setup. It would be simple, cheap, would have no moving parts which could break or jam due to corrosion or marine growth, would be self-regulating in vertical flight level and would give a heel stabilization thanks to dihedral angle. The backlash would be a drag increase due to reduction of aspect ratio - due to surface-piercing aerating tips. It's all about averaging the pros and cons, and taking a decision.
Apart the choice of the wing setup, you'll also have to resolve the problem of the length of the o/b motor leg. At some point, as your boat rises up from the water, your prop might end up working very close to the water surface and start aerating, reving up and losing thrust.
Ah, yes, I didn't comment on the motor height. I was planning to incorporate a "transom" into the rear wings that would lower the motor. It would run a bit deep under hull borne conditions, but hopefully not ventilate when foil borne.
As far as the moving parts goes, it's a compromise I'm willing to take. I feel pretty comfortable making sure the materials will work/last. I do have access to a small lathe and manual mill, so making the small brackets should be ok.
So now onto your design. One thing you didn't mention was what you are hoping to achieve by making the boat a hydrofoil. Are you doing just because you think hydrofoils are interesting or are you trying to improve the boat's performance in some way? I say this because a 14 foot aluminum boat with 18 hp and no hydrofoils should also be able to achieve 20 knots and handle moderate boat wakes, which begs the question... why?
I bought Ray Vellinga's book and have also read Hook's book. I am not familiar with DuCane. The Shutt strut looks simple and effective in calm water. Other than the Hifybe, it seems that it is used mostly on human powered hydrofoils that operate in glass flat conditions. I question how it would react in any sort of chop. For me the aesthetic problem of a long feeler in front of the boat puts it out of contention. It also will tend to generate spray that can go right into the boat, which seems like a small issue but can significantly detract from the boating experience IMO. If I was going to use fully submerged foils I think I would go with the trailing feeler style which seems much better to suited to waves while being less obtrusive. This boat is a good example.
As far as a crash on the rear foils, I think the boat would just squat down momentarily and quickly regain lift. I don't think it is nearly as catastrophic as a front foil sea-crash. Watch this video at 3:04 to see the rear foil lose lift.
Your proposed method of foil construction is interesting. For me, I think I will be going with welded aluminum, but my loads are going to be higher. I would make sure that you look at the max bending stress in your foil. From what you described, the fiberglass skin of the foil will see that stress so it will need to be designed accordingly. I can't offer much on the foil shape selection. I need to do some more research on this myself.
As far as the lower and higher CL go, for a fixed foil this will be adjusted by angle of attack or incidence control. You are limited in how high the CL can go, which goes back to foil loading. Obviously you will want to design for the heavier boat. The lighter boat should be alright. For a surface-piercing foil, CL doesn't change but the immersed foil area does. This brings up a point which I haven't seen mentioned in any of the books. It seems to me that a surface-piercing system has far greater potential for high speed than a fully submerged system. As speed increases, a fully submerged system will reduce its angle of attack, thereby reducing induced drag. Meanwhile the wetted surface area is unchanged. With a surface piercing system, the angle of attack is unchanged but the wetted surface decreases as the foil climbs out of the water. For any small boat at high speed, wetted surface friction is a far greater source of overall drag than induced drag. This means that as speed increases a surface piercing foil system should see considerably less drag than a comparable submerged foil system. Just something to consider.
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