Foil design for my motor trimaran

[Michael Browne]

Hi there,
I was looking some advice for a motor trimaran that I have. I have a 55hp inboard diesel with shaft drive it does 16knots but I feel like the outriggers and bow are digging in too much. I want to get the boat to cruise at 20 knots. I was considering trying to put a foil between the front of each Outrigger to the centre hull to give me a bit of lift. Pictures of the boat for reference are attached.

I don’t want the boat to be fully out of the water I just want a bit of lift. Is this possible with one front wing. How would you go about designing it and what should it be made out of?

 

[bajansailor]

You certainly have a very interesting trimaran Michael - did you design and build her yourself?
If somebody else designed / built her, have you consulted them for an opinion yet?

Re your current cruising speed of 16 knots, and ref your diesel engine, are you achieving the maximum revs with the existing propeller?
Did she have a clean bottom when you achieved 16 knots?
I am sorry, but I don't know anything about foil design - just trying to think of other ways that your speed could possibly be improved.

 

[Michael Browne]

Yes bottom is clean and smooth plus I have had the prop worked which gave me an extra knot of speed I have considered altering the gearbox to a lower ratio as I can’t increase propeller diameter. But thought of the foil in the way that Thunder Child 2 has done

 

[Heimfried]

I didn't know Thunder Child 2 and looked first time at their website. The foils are designed by Hysucat and there are three of them. Additional to the main foil there are two short foils near the aft end of the boat. I don't think you will be able to build a stable operating boat with only one foil. I recommend seaching the web for Hysucat or read Ray Vellinga's book "Hydrofoils Design Build Fly".

 

[Heimfried]

Where is at your boat the longitudinal center of gravity located, where the front of the outriggers?

 

[Squidly-Diddly]

how about some video of that thing at speed and in various size waves.

I don't know foils either but might help see what the issues are.

 

[Michael Browne]

Actually organised to do exactly that tomorrow To get a better understanding myself

 

[Squidly-Diddly]

Actually organised to do exactly that tomorrow To get a better understanding myself

I'd try to make it as "data dense" as possible. Set a couple of cell phones to capture RPMs vs KNTS and use a Movie Making "Action!" device to sinc time. (doesn't need to be like shown, just needs to be recognizable). That way people will be able to see boat in action and know RPMs and KNTS. Also verbally note to onboard cell phone recorders if adjusting trim and weight, etc. They use those Clapper Boards in old days to sync audio with video, since those two systems were completely different operations.

 

[Mr Efficiency]

That's an interesting idea. I certainly am no foil expert, but I have made and fitted one to a box-tunnelled boat, and it did work, alone. It was more or less, at the longitudinal centre of gravity. I would be inclined to install a bracket or track that it could be moved back and forth along, to find the best spot for it. Not on the move, but in between trial runs. Also allow for adjustment of the angle of attack, ideally.

 

[Michael Browne]

That's an interesting idea. I certainly am no foil expert, but I have made and fitted one to a box-tunnelled boat, and it did work, alone. It was more or less, at the longitudinal centre of gravity. I would be inclined to install a bracket or track that it could be moved back and forth along, to find the best spot for it. Not on the move, but in between trial runs. Also allow for adjustment of the angle of attack, ideally.

Hi thanks for the reply. That’s great that it has worked and a good idea on the back and forth I had thought of being able to change angle of attack but not back and forth but it does make sense. What material did you make it out of? And shape? Would you have any pictures?

 

[Mr Efficiency]

Sadly I don't have any pics, the tunnel was 30 inches wide, it was made out of GRP, (unidirectional mainly) and made in a simple mould made of sheet metal bent to a shape that "looked right" Not a great deal of science involved, but it did lift the boat noticeably, as demonstrated by the way the boat banked into turns somewhat more than previously. The width of it was probably 5-6 inches, but it was a long time ago. The maximum thickness would have been maybe 3/4" give or take, and that about a third of the way from the leading edge. A slightly "bodgie" production, but it looked pretty professionally made ! It was not going to be pushed beyond 30 knots too often, so the niceties of a slimmer foil probably would have been redundant. It could not have been causing much turbulence behind it, because there was a big outboard about 6 feet further aft, and higher up, and I never noticed any problems. The idea was to get the boat out of the water a bit more, be drier, and turn less flat, and all that was accomplished, whether there was any performance gain, I can't really say, you'd assume at least some, but that wasn't the principal objective.

 

[bajansailor]

Here is an article in Professional Boatbuilder about foils on a small power cat - there might be some similarities here that you could perhaps use or adapt to your trimaran?
The Efficiency of a Foiling Powercat - Professional BoatBuilder Magazine https://www.proboat.com/2020/03/foiling-powercat/

 

[Mr Efficiency]

Stability might be more a concern with a tri, than a cat, when foil assisted.

 

[W9GFO]

I don't think a foil where you describe placing it would work, it would be constantly ventilated.

From the picture but it looks like the prop shaft is at quite a high angle. A significant portion of the thrust is going towards lifting the stern and pushing the bow down - instead of propelling forward.

I don't know how you would go about it in a cost effective manner but I think getting the thrust line more horizontal would be the most effective solution.