Foil Assisted Cruising Cat

[Richard Atkin]

I want the following type of boat:

25 to 30 ft trailerable, beachable, hydrofoil assisted sailing cat (not tri) with at least 700 kg load capacity and cabins in hulls.

I want the hydrofoil function for efficient sailing in light to moderate winds. I'm not concerned about top speed - just want to go cruising with friends.

So far I haven't found anything on the net that looks suitable. Any tips would be greatly appreciated.

- Richard

[Richard Atkin]

I like the idea of buying a Wharram Tiki 30 and getting a professional boat builder to fit a foil-assist system onto it. I like the feel of the roofless open cockpit (modern cruising yacht designers seem to prefer the 'trapped in' fishing boat feeling) and roomy hull accomodation on the tiki 30.

It has deep 'V' canoe shaped hulls which i guess would be ideal for extremely light winds when water displacement is the only option, and maybe the V shape would help to make a smoother ride when the foils are working???

[Doug Lord]

Fitting a ' foil assist system' is a major design exercise FIRST before you ever talk to a builder. Be prepared for a large expenditure to get a state of the art system working productively on a cruising boat. It can be done but probably will require a design from scratch to maximize the value of such a system.

[Richard Atkin]

Yeah I was thinking that would probably be the case. Thanks Doug.

[tspeer]

...I want the hydrofoil function for efficient sailing in light to moderate winds. I'm not concerned about top speed - just want to go cruising with friends....

I think you have the wrong solution to your problem. Light to moderate winds and low speeds are precisely where hydrofoils are at their worst. If you want to improve performance under these conditions, I suggest you try a deeper board.

[lazeyjack]

Yeah I was thinking that would probably be the case. Thanks Doug.

well right there in NZ you have a guy who builds nothing but foil cats, bladerunner boats Kumeu, G Shine is his name, , nope he does not do sailing boats, but he is very interesting to talk to. they have a website

[Richard Atkin]

Tom, I read somewhere that foils will lift a hull to some extent (depends on the type of boat ofcourse) enough to break even and neutralise their own drag, even at slow speeds....like 2 or 3 knots. Any speed increase after that and the foils become profitable. But I'm no expert, so I appreciate your comment about trying a deeper board. I can see I need to take my time and keep gathering info.

Thanks lazeyjack, I'll check out that website

[Trevlyns]

Tom, I appreciate your expertise in this area and indeed, have your website book-marked. I’m thinking of incorporating ‘winglets’ horizontally on the bottom of the leeward hull of a Proa. My reasoning is the aerofoil section (longer dimension over the top) will create vertical lift to assist in countering the heeling force generated by the sail. Does this sound feasible? Your comments would be greatly be appreciated.
Take care

[tspeer]

...I’m thinking of incorporating ‘winglets’ horizontally on the bottom of the leeward hull of a Proa. My reasoning is the aerofoil section (longer dimension over the top) will create vertical lift to assist in countering the heeling force generated by the sail. Does this sound feasible? ...

First of all, the longer dimension over the top has nothing to do with lift. But that's a discussion you can find elsewhere on this board.

The sum of the vertical force from the winglets and the buoyancy of the hull will remain the same, because they have to add up to the weight of the boat & crew plus any download from the sail. Either one will provide the same heeling moment because the distance from the centerline is the same. So the question really should be, "Which produces the least drag - additional displacement of the hull or lift from the winglets?" The winglets have to buy their way onto the boat by reducing the net drag.

The winglets will add wetted area. If the buoyancy of the hull is less (because of the lift from the winglets), then it will have less wetted area. So first of all, the reduction of wetted area of the hull due to a reduction in buoyancy has to be greater than the added wetted area of the winglets used to lift the hull. It should be straightforward to figure out how much less buoyancy the hull must have after the winglets are added to get back to the original wetted area of the hull at full displacement without the winglets. If you can't produce that amount of lift out of the winglets, then you have no hope of breaking even on performance.

If the form drag and wave drag of the hull is reduced more than the induced drag due to the winglets add, then you have a fighting chance at improving the performance. But the winglets are operating near the surface, and the induced drag there is double what it would be if the winglets were producing the same lift while deeply submerged. And the drag due to lift is inversely proportional to the square of the span of the winglets, so if they are narrow they will be very draggy. They need to be more like a high aspect ratio hydrofoil to be efficient at lifting.

Without running any numbers, I'm skeptical that you can make your strakes pencil out. I suspect you'll find you have to make the winglets very heavily loaded to get the wetted area reduction you need, and the drag due to lift on the winglets will be worse than the unmodified hull - at least for low and moderate speeds.

I would bet on adding a deep board as a far more efffective means of improving the performance of your proa. A long, narrow displacement hull is very hard to beat for low drag over a wide speed range. But it's not the most efficient way of generating the side force necessary to resist the loading from the rig.

[tspeer]

Tom, I read somewhere that foils will lift a hull to some extent (depends on the type of boat ofcourse) enough to break even and neutralise their own drag, even at slow speeds....like 2 or 3 knots. Any speed increase after that and the foils become profitable. ...

No way. The drag due to lift goes down with the square of the speed. So if you reduce the buoyancy by a given amount, the drag due to lift at 2 kt will be four times what it is at 4 kt and 16 times what it is at 8 kt. If you size a foil to produce that lift at 2 kt, it will have a tremendous amount of wetted area that will reduce performance as the speed increases.

I don't see any way vertically lifting foils will be any help at those kinds of speeds. Buoyancy is the most efficient way of supporting the craft then.

[Richard Atkin]

Tom, you have just made things very simple for me. I didn't realise that foils have a very limited speed range and that the drag increases exponentially....so after telling me that, it's pretty clear that I should just forget about foils. After all, I just want to cruise.
Thanks very much Tom.

[sail]

No way. The drag due to lift goes down with the square of the speed. So if you reduce the buoyancy by a given amount, the drag due to lift at 2 kt will be four times what it is at 4 kt and 16 times what it is at 8 kt. If you size a foil to produce that lift at 2 kt, it will have a tremendous amount of wetted area that will reduce performance as the speed increases.

I don't see any way vertically lifting foils will be any help at those kinds of speeds. Buoyancy is the most efficient way of supporting the craft then.
I just read the interesting comment you wrote in 2007 (long time ago !!)
I wanted to use hydrofoils for a 7 m cat but I wonder if a speed of 15-20 Knots could overcome the hydrofoils' drag and increased wetted area?

[sail]

I just read the interesting comment you wrote in 2007 (long time ago !!)
I wanted to use hydrofoils for a 7 m cat but I wonder if a speed of 15-20 Knots could overcome the hydrofoils' drag and increased wetted area?
Hi Tom
I forgot to tell you the hydrofoil cases would be "hanging down" from a longitudinal beam supporting the cross beams and the folding system.The foils could be neatly retracted above WL at low speed or light wind.

[tspeer]

I just read the interesting comment you wrote in 2007 (long time ago !!)
I wanted to use hydrofoils for a 7 m cat but I wonder if a speed of 15-20 Knots could overcome the hydrofoils' drag and increased wetted area?

Possibly. It would depend on the details of the foil design. The problem could be getting to 15 kt in the first place, with the added drag of the foils.

[powerabout]

what happens to a lifting foil when you hit something...
game over?

[sail]

The foils could be lifted at low speed and lowered at a "reasonable" one..that would need to be defined.
The biggest problem is probably the "heavy" 650-700 kg displacement for a 7 m cat. Wave breaking foils are probably for very powerful sailboats or motor boats.
If cruising sailboat cats have no foils there must be a reason. Foils are probably like shoes.They don't fit any sailboat and must be custom designed by experts for a narrow speed range. Since I am not an expert in this field..I will probably end up with a standard dagger board..

[tspeer]

what happens to a lifting foil when you hit something...
game over?

Not necessarily game over, but it's not going to be good, either.

Foils are really good strainers. Every impact with a foil is a head-on impact instead of a glancing blow along the side. And the foils sweep out a large area compared to a narrow hull. That's why I think hydrofoils make sense for a passage-maker, but not necessarily for a coastal cruiser or daysailer. Too much junk in the water.

For a cruiser, the foils need to be designed to break away cleanly. In this regard the foiler can be safer than a conventional boat. In the event of impact with the proverbial semi-submerged container, the foiler can pass over the container and land on the far side. The foil may be severely damaged, but the hulls would be intact. It would be a good idea to attach an anchor rode to each foil when foiling, ready to run freely, so the foil can be retrieved after foil shears off and the boat comes to a stop.