# foil info

Discussion in 'Multihulls' started by quist, Sep 29, 2007.

1. Joined: Sep 2007
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### quistJunior Member

Are there any cruising multihulls that use foils?
I would like to find some info on whether foils can be used efficiently to increase speed without completely lifting the boat out of the water to fly.

How much lift can i get at 8 knots ?
How much foil is needed?
Is there a chart showing lift vs area of foil?
Chart showing velocity and lift?
Where can a novice find the correct calculations?

I appreciate you help,
Quist

2. ### Guest625101138Previous Member

I find the easiest way to play around with foils is to use JavaFoil:
http://www.mh-aerotools.de/airfoils/javafoil.htm
When you download the Applet it needs the Options page set for the water. The density in metric units is 1000kg, the kinematic viscosity is around 0.0000013 (temperature dependent) and the aspect is the width divided by the chord for a narrow foil. Speed of sound and Mach number do not need to be changed. The analysis page then gives you coefficients of lift and drag for the foil you select.

I find the modified NACA 4-digot gives a lot of scope to optimise.

This assumes that the foil is deep relative to its chord s there is now wave making. If the foil is close to the surface then you have to allow for wave drag. There is also support strut drag.

A foil will work at 8kts but it will have to be quite a large area.

About the best lift to drag ratio you can get is 35.

The way to determine lift or drag once you have the coefficients is:
Lift = 0.5 * Density * Speed^2 * foil plan area * CL
A value of CL for an efficient foil is say 0.6.

You will also need to determine the Reynolds number as CL and CD vary somewhat with Reynolds number.
Re# = Speed * Chord / kinematic viscosity
Rick W.

3. ### Doug LordGuest

cruising foils

You might find this thread of some help:
Foil Assisted Cruising Cat - Boat Design Forums
Tom Speer has said that for foil asist to work-that is, to have less drag than the boat w/o foil assist- the wetted surface reduction must be on the order of four times the area of one side of the hydrofoil.
You might also google the"Catri 26" trimaran (and do a search of this forum) which, as far as I know, is the closest thing anywhere to a cruising multihull utilizing foil assist. The reports on that boat are that in winds over 20kts it is very fast; in light to moderate wind not so much.
The ORMA 60 tri's use foil assist to reduce the wetted area of the leeward ama when flying the main hull and it works very well in most conditions. But in lighter air they can retract the foil and in heavy air the foil can develop too much lift-sometimes resulting in a crash.
The foil system you use has to be designed very carefully or it won't be worth the effort.
------
Also see: www.foils.org -the site of the International Hydrofoil Society-there is a wealth of information available there.

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### tspeerSenior Member

Only one, Williwaw, by Dave Keiper, and it was destroyed at anchor on October 15, 1977.

The Catri uses this approach.

But I think you may have it backwards. There is nothing magical about hydrofoils. Adding foils does not automatically make the boat faster, and may well make it slower. I believe foils are best thought of as making a fast boat more seaworthy.
As much as you want. It's only a matter of adding enough area.
That depends on your takeoff speed and takeoff weight. The question you really need to ask is, "How much foil can you stand?" The issue is drag, not lift. And stability. And height regulation.
Get AMV CD#1 from the International Hydrofoil Society. It has a whole design handbook on hydrofoils.
You can find a complete study on the design of a cruising hydrofoil here. At the time the paper was written, the design hadn't quite met all of its requirements, as the drag flying on foils was greater than the drag hull-borne. But it will give you an idea of some of the major considerations.

5. Joined: Sep 2007
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### quistJunior Member

thank you

WOW
Any word on how the Basiliscus project is comiong along?
There goal was different than mine, but there is alot of info there.

6. Joined: Feb 2002
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Location: Port Gamble, Washington, USA

### tspeerSenior Member

It's been on hold for the last 5 years. Building a new boat was basically not affordable, so now the emphasis has shifted to modifying an existing boat.

With regard to foil-stabilized boats, the Catri has some information on their design approach. With hydrofoils, you need to be very clear about the engineering requirements. You need to know the difference between stability and trim, what the sources of drag are, and how they change with speed and loading.

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