gmane.culture.transportation.humanpowered.boats

hpv | 19 Jul 18:49

Displacement hydrofoils

Most hydrofoil designs have relatively thin bladed foils for "flying"
mode and auxiliary floats for "floating" mode.

Does anyone know of any designs that have fatter foils that make
flying more of a cross between the two modes? If so, how is the
overall performance?

Could this be an option for a long distance hydrofoil design that
wouldn't require superhuman strength and endurance?

Michael Lampi

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Larry Smith | 19 Jul 20:28

Re: Displacement hydrofoils

>Most hydrofoil designs have relatively thin bladed foils for "flying"
>mode and auxiliary floats for "floating" mode.
>
>Does anyone know of any designs that have fatter foils that make
>flying more of a cross between the two modes? If so, how is the
>overall performance?
>
>Could this be an option for a long distance hydrofoil design that
>wouldn't require superhuman strength and endurance?
>
>Michael Lampi

Now there's an interesting question I, for one, hadn't thought of.

Thanks for posting it Mike! 

A pair (fore/aft) of wings that provided enough volume/buoyancy to 
contribute significantly to the flotation, and enough lift when 
underway to finish raising the main hull from the water?

Wonder what the net L/D would be, and how skin-friction areas would compare?

Be interested in further discussion.

Larry

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Rick Willoughby | 20 Jul 02:59

re: Displacement hydrofoils

Mike
I did the numbers on this.  It depends on the power level you want to  
cruise at.  I looked at it very closely for Greg's record attempt.  I  
determined that if he could sustain 150W it would be borderline  
compared with an optimum purely displacement boat.  In reality he  
averages around 120W so it is not beneficial.

If you wanted to hold something like 200W then you would be in the  
area where foils would be a proposition.  You would design for about  
4m/s.  For power levels above 200W you would certainly be better off  
with foils.

If you just want to fly it is possible to do it with lower power and  
big foils but it is not as efficient as a displacement boat.

The things that have to be taken into account that are not  
immediately apparent are wave making and strut drag.  The wave drag  
on the foil can only be neglected if the foil is placed say 3 times  
chord LENGTH below the surface.  You can get equations for the wave  
drag on foils.  Now the deeper you go with the foil to get away from  
wave making the more strut drag you get.  So there is an optimum.

I can do reasonable design on the whole thing if you want to have a  
go at something.  I would use one large foil near the CoG and a  
smaller one well separated to give stability.

The extent of wave drag is clearly visible here:
http://poisson.me.dal.ca/~dp_04_5/images/testing/successful-hydrofoil- 
test-march-14-2005.wmv
The main foil is about 10" long so needs to be say 30" below the

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