Basic Hydrofoil Question

[joz]

How much horsepower or what speed should you require to get hydrofoils to work?

Also are there any hydrofoils that don't rise the boat out of the water to be efficient?

Thanks.

[trouty]

Now theres a good Australian name for ya! ;) Why did I know he was from Melbourne eh? :D

OK - Foils...

I am guessing you mean a foil for a twin hulled cat style boat?

The foils can be manufactured / designed to work at varying speeds and vessel weights.

Like an aeroplane wing cross section, the amount of 'lift" depends upon the depth of the curvature of the top surface - the speed thru the water and the angle of attack...

The attached rear view pic of a 32 ft alloy fincat, clearly shows the Hysucat style inverted "V" (reverse boomerang) foil, as well as rear fins on which the sponsons ride at rear.

The foil is fitted at or near the LCG (longitudinal centre of gravity) and the vessel rides on said foil at that point - and on the two short fins at rear.

The whole idea of the foil - is to lift as much of the two hull sponsons clear of the water as possible, while retaining directional control and stability in maneouvering.

The reduced wetted surface area on the hull surfaces = less resistance and hence more top speed or reduced fuel consumption.

The foils are designed to give a perceived benefit of some kind, in either more load carrying capacity, more top speed or less fuel consumption etc.

You'll see if you look closely that the foil is supported with a central vertical strut off the cente of hull...to minimise flexing / add structural support / strength.

Imagine if you will - that say a lobster rope / and set of floats gets tangled around that centre inverted V and strut...

I wonder who's gonna be the fella diving under the boat to cut the rope free with a knife between his teeth?

I'll give you one tip - it won't be this little black duck!

Cheers!

[BOATMIK]

How much horsepower or what speed should you require to get hydrofoils to work?

Also are there any hydrofoils that don't rise the boat out of the water to be efficient?

Thanks.

Hi Joz,

I did meet someone who added foils to a canoe with a 2hp outboard and struggled to make it lift for months. He then borrowed a 3hp for it to go at around 15 knots - but this is back in the early days of guesstimation in the early 1980s!

There is a single seat kayak with foils available - so less than 1/2hp in that case.

The drag is reduced because the hull is lifted out of the water reducing drag from wetted surface and drag from wavemaking.

Google - drag wave making wetted surface froude
and you will dig up more info.

Michael

my boat pages (http://www.ozemail.com.au/~storerm)

[joz]

Thanks for that guys

[marshmat]

It's possible with a bit of calculus to tune a hydrofoil for just about any speed and weight you like. The math is tedious but not excessively hard; most fluid dynamics textbooks cover it.
Many boats have foils even though the hull stays in the water- Trouty's cat there is an example; one builder that does this a lot is Prout Yachts (they have a 70,000-lb 64-foot cat with foils, that does 46 knots on a pair of 1100hp diesels).