Hull vane...hydrofoil for a semi displacement cruiser

[Howsounder]

I have a cape Dory 28 cruiser with 165 diesel HP and am using 137.5 HP at 11 knots at 2800 of a 3600 rpm redline. The wake only cleans up really nicely at 11 knots, and the boat is on a very sloppy plane at 8.5 knots 2500 rpm where the HP draw is still 118 Hp. At 1300 rpm I get 6 knots, so I already have quite a bit of propeller slip at 8.5 knots due to that wallowing bow high barely planing…..plane……… I wanna fix that, and gain efficiency

I want to cruise at 8.5 knots, but get the bow down and that nice clean wake using a hull vane or hydrofoil, mounted under the swim grid. (Search “hull vane” if curious on Utube, very cool) I will build it from stainless steel, and have a 1.5” pipe connection on each sides vertical strut, so I can keel cool the boat as well sending coolant down port side, through the foil, and back on the starboard strut. (Keeping the in and out of the coolant through hull connections above the waterline)

The only thing stopping me is a foil design. I know I want it about 80” wide as that is the limit of my existing mounting points. It needs to start to “get to work” at 7 ish knots, and will never see more than 12 knots. So far my guess work is 82” long, 18” chord length and about 1.5 - 2” height at the tallest prt of the foil section.

Can anyone here point me to someone that has an idea on basic foil design so I can avoid a crappy inefficient foil build? Or even foil extrusion suppliers in aluminum?

thanks Steve

 

[Robert Biegler]

So far my guess work is 82” long, 18” chord length and about 1.5 - 2” height at the tallest part of the foil section.

Can anyone here point me to someone that has an idea on basic foil design so I can avoid a crappy inefficient foil build? Or even foil extrusion suppliers in aluminum?

www.windknife.com http://www.windknife.com/ offers two standard foil extrusions with the required span, though shorter chord. They used to offer a nose section that you could extend into foils of various thickness and chord (Duckworks - Extruded Aluminium Hydrofoils https://www.duckworksmagazine.com/15/howto/windknife/index.htm), but I can't find that on their home page now. You would have to ask whether they still make those.

I suppose you would want a longer span, and bend the foil up by 90 degrees on either side, so that you get steady flow of your coolant through the foil. Cutting the ends at 45 degrees and welding on vertical sections would be simpler, but I don't know whether that would slow the flow at the corners enough to create a problem.

 

[Howsounder]

Thanks for this. I would need a third support to span the distance I need with this foil. The chord is a bit short, and I think I want an asymmetrical foil too.

 

[Ad Hoc]

Hi Howsounder,

Welcome to the forum.

The biggest issue you will have to overcome is that of the L/D ratio.
What I mean by that is, can you get Lift - yes! But, there is also the associated parasitic drag.

Reason is that in your foil needs to be 'structurally' sound, i.e. it needs to be thick enough to structurally support the load applied to it and not crack/fatigue.
So whilst you can get the lift you want, to satisfy the structural loads, you then need to increase its thickness, or use more exotic materials or a combination of the two.

When you run the numbers, the amount of lift you gain, is generally offset by the drag - owing to the thickness.....a classic zero sum gain.

Does this mean it can't be done...no, as it can be done.
But it takes more than just selecting a foil and dumping under the stern.

Some drawings will help.

 

[Howsounder]

Thanks for your thoughts. The drag issue has been proven out with positive results

 

[Howsounder]

I am afraid that so far at least, I am a bit of a “dump it under the hull type”

on a 25 foot Bertram, I powered with only 150 Hp outboard on a pod, I still was not really on a decent plane until 18-20 knots, so going slower in rough water was a pain as you would constantly be going too slow, or too fast, and then have to trim up to stay at a constant speed. I added a 30” wide outboard foil, in the form of an aluminum plate. The dam boat stepped up on plane at 9 knots, and held any speed with authority. This a classic case of the drag being fully worth the price. On a 14 foot jet boat where I removed the 140 hp jet and installed a 25 hp outboard, I used a Davis planing plate, with doel fins attached on each side, and further, two sets of doel fins mounted on vertical masts on each side of the boat, that little bugger flew along at 22 knots with only 25 HP. Another case of all that drag paying for itself and more. In my present endeavour, I seek to, as in the Bertram, lower the speed in which a healthy plane is achieved. (In the case of the jet boat, get it to plane at all!) so I visualize the 80” foil and ask myself how much Hp would be required to drag just it through the water at the desired speed of just 8.5 knots, and conclude….not much. Then I ask myself if the boat was on a healthy plane at that speed, instead of being bow up, *** down, trying to climb out of its own hole……would there be enough efficiency gained that the drag would pay for itself with dividends? My experience says it smells like it. The “Hull Vane” videos on utube seem to show up to a 27% increase in efficiency. (Yes at a given speed) I have a sense that even an 80” chunk of 16” wide flat plate with a very moderate angle of attack would get this thing on a efficient plane at 8.5 knots, so I thank you for challenging me to think on this further. The foil will give it the strength I need to span 80” but the idea that a chunk of plate would work, makes me think a more moderate foil height is the way to go. Where 2” high was my gut, 1.5” is probably a better guess. Perhaps I will just take the ratio of an existing foil being used and apply that to my 16-18” width and see what that number is?

 

[DCockey]

Have you considered trim tabs or interceptors, both of which can be adjusted to suit the speed?

 

[Howsounder]

Hello, and thanks for your reply. Yes I did consider trim tabs. The boat has a round belly, so anything even custom, would tot match the curve of the boat, and would require me drilling holes in the boat below the waterline, and I just don’t want to drill holes in the boat. I did consider simply buying a big sheet of stainless, and glueing it to the bottom with sikoflex, and having about 18 “ glued, to 18” of protruding “tab” pulling it off would be quite a chore, and require hauling the boat. It would offer another 18” of waterline length and trick the boat into thinking it’s a foot and a half longer, all with very little weight or drag. It would also send the centre of weight forward, at least when at speed. Nothing for a log to smash into either, and supports could go up to above the waterline where the swim grid could take some load too. Glueing the thing to the bottom seems a bit radical, but I suspect you could dangle the boat from it once that stuff has cured, and being wrong will only send some 1/16” stainless to the bottom.
The solution I am playing with here allows me true lift from the foil, and adjustment through simple addition of washers, not only with angle of attack, but port and starboard by adjusting one side more than the other. I am not going to rush into this, as you and other thoughtful folk will poke holes in weak ideas, and as above suggest possibly better ones. A small hinged tab at the end of a big boat wide tab could offer some adjustability though……….and I like not worrying about a log strike. (My boating area has an eye watering amount of logs from our logging industry)

 

[Alan Cattelliot]

I have a cape Dory 28 cruiser with 165 diesel HP and am using 137.5 HP at 11 knots at 2800 of a 3600 rpm redline. The wake only cleans up really nicely at 11 knots, and the boat is on a very sloppy plane at 8.5 knots 2500 rpm where the HP draw is still 118 Hp. At 1300 rpm I get 6 knots, so I already have quite a bit of propeller slip at 8.5 knots due to that wallowing bow high barely planing…..plane……… I wanna fix that, and gain efficiency

I want to cruise at 8.5 knots, but get the bow down.
thanks Steve

Hi Steve. I may be able to draw some numbers, but I have to check some figures. According to CDSOA, Inc. -- CD28 Power http://www.capedory.org/specs/cd28pwr.htm,
the displacement of the cruiser version is given around 8,000 lbs. The recommended power is 275-hp. Chrysler V-8 gas / 200-bhp Volvo turbodiesel. I've checked with my own formulas, and get a planning speed around 14knots with a chine beam slightly greater than what you probably have. So it seems to me that your boat is currently underpowered. Right/Wrong ?

This boat is announced to be a semi-displacement type. If you are indeed underpowered, then @8.5 kts, you must be on the "hump" of the drag curve of the hull. At this speed, you've got the bow down. Do you confirm ?

In these conditions, if you are willing to clean the wake, the foil would have to push down at the transom, rather than lift up, like in the hull vane example. So your foil would be mounted upside down. But in this case, you won't have the wave suction effect achieved by the hull vane. Are you aware of this ?

 

[Howsounder]

Thanks for your thoughts. The wake begins to clean up at 8.5, but is steep, and not a clean wake. Flat water coming off the stern, but steep, so frothy water spilling back towards the stern. Boat is bow high at 8.5, with that big sucking steep stern wave. By 11 knots the wake is perfect, oily, slick with the stern wave well back from the boat. My efforts are to get that happening at 8.5 knots

 

[Alan Cattelliot]

Ok, I get it. I thought that you mainly want the boat trim to be corrected... So forget about the inverted foil pushing down. We can think at the "vane" effect in order to try to solve your problem. Do you have any idea (even rough) of the immersed depth of the transom at 11 kts ?

 

[Howsounder]

The hull at 11 knots is not immersed at all, water is spilling off quite flat, clean as a whistle. Stern wave well back. The stern is only about 4-6” below the waterline when static. Even at 8.5 the stern is not immersed, but does have a steep wall of water going up to the steep stern wave only 2.5 feet back. I can send you a video of the stern wave at 11 knots. By pm maybe or email etc