Hydrofoils

Is there a particular reason why hydrofoils are not that common in boat design?

[Willallison]

I'll take a punt at this one - someone more knowledgeable than I could probably add a few points, but here goes...

1. Deep draft - not so good for cruising boats
2. Foils in a monohull, by necessity stick out under the boat, so are prone to damage. The exception is mutlihulls, where foils can be located between the hulls. Indeed they are becoming more and more common on both recreational and commercial multi's
3. I would think that to be most efficient, foils would need to be designed around operation at fairly specific speeds - at all others a great proportion of their advantages would be lost.
4. The old favourite - cost. Complexity = $$

[tspeer]

I think you have to look at the strengths & weakness of hydrofoils. In terms of pure speed, subcavitating hydrofoils have a top end near 40 kt. Planing boats can hit these same speeds, so there's no speed advantage in flat water unless you go to supercavitating hydrofoils. And even then, the bottom of a hydroplane is essentially a fully ventilated hydrofoil operating at the surface.

Where hydrofoils excel is in rough water. By lifting the hull clear of the waves, they reduce the slamming and smooth out the ride. So if the mission calls for going fast in waves, hydrofoils are a good choice. A good example of how hydrofoils can improve the seaworthiness of a conventional hull is Harry Larson's Talaria - see http://home1.gte.net/hlarsen0/indexIndmnt.htm (and http://home1.gte.net/hlarsen0/).

At lower speeds, the drag of a hydrofoil climbs while the drag of a displacement hull drops as the speed goes down. So there's some crossover speed below which it doesn't make sense to try to fly - you can go faster for the same power without the foils. But at the higher speeds, it's possible to reduce the drag compared to a planing hull and cruise more efficiently because of the greater span of the hydrofoils compared to the width of the hull. Depending on the takeoff speed, you could optimize the hull for low speeds - say by going to a long, narrow displacement hull - while using hydrofoils to be efficient at high speeds. The hydrofoils can be retracted completely out of the water, allowing the hull to change gears and be more efficient than a conventional planing hull across the whole speed range.

Against these advantages are the difficulty of engineering a hydrofoil system compared to designing a planing hull. It's not enough to draw some lines that look pretty much like other boats and have a reasonable expectation that it will perform the same way. You have to design the hydrofoil section and planform specifically for the mission requirements, and deal with the stability and dynamics of the boat in a seaway. And in waves, the currents due to the orbital motion of the water can affect the hydrofoil as much as the difference in height of the water surface. A fully submerged hydrofoil needs to have a feedback control system of some sort, and this means an electronic system with sensors, computers and actuators, or at least a mechanical feedback system like Sam Bradfield's wands on the Rave (http://members.aol.com/HYDROSAIL/). This kind of control system engineering is not something that most designers are used to doing. It takes mathematical models and test data to get it right.

In service, the hydrofoil would have to operated more like an airplane, with attention paid to having the right speed for the conditions and not exceeding the design payload. I would expect you'd have to pay more attention to the numbers than most boat operators are used to. and then there's the susceptibility of hydrofoils to collision with debris in the water. Hit something with a planing hull and it's just a glacing blow, while everything is head-on with a hydrofoil. I've heard of hydrofoils being dropped off the foils for hitting something as simple as a beer can that wrapped itself around the foil and caused a huge increase in drag.

So on the whole, I think the reason you don't see many hydrofoils is it's a lot of effort to gain seaworthiness and efficiency when most owners would be just as content to put in bigger engines and choose not to go slamming through the rough stuff. Given the right requirements, though, a hydrofoil could be the right choice. I think that if you wanted to cover long distances quickly at high speed in a boat under, say, 40 ft, the hydrofoil would hard to beat.

Cheers,

[yipster]

software for numerical calculation of wig craft, hydrofoil and planning boats at http://www.se-technology.com/ but dont think its free.

[Nomad]

Also hard to trailer for inexperienced boaters, not good for fishing craft, and (if no one said it already) not good fuel economy and handeling at low speeds, etc.

[yipster]

...

i know from a small (and older) hydrofoil i once drove that at low speed it -how do you say it- zigzagged, but once on speed its hard to beat! but not the easyest "boat" indeed.

to my surprise i got an e-mail back from the university of rostock that under their service button the software for numerical calculation of wig craft, hydrofoil and planning boats is free to download as working demo, i did and it looks very good...

yipster
ps: very nice orca's you'r doing Nomad!

[yipster]

forgot the link for the download, its under the service button www.argo-group.de