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Old 05 May 2004, 12:36   #11
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Quote:
Originally Posted by ribnwheels
The foils themselves do have a frictional component, but where you benefit is the fact they lift the hull if you use the power trim, reducing the wetted area and consequently the amount of friction. My avon will plane at about 9 knots with these on. If you imagine the wetted area when you are running at full speed, engine trimmed out, then imagine it even if it just lifted by 1", the wetted area is a lot smaller, consequently less friction, increased fuel economy and theoretical top speed.
Although they might reduce the wetted area they increase drag, in effect a poor trade off of one for the other, your boat weighs the same regardless so either you are supported by the hull or the Hydrofoils but with the Hydrofoils the force exerted on the top of the Hydrofoils is of no benefit from a planing point of view.

I accept that with some problems such as poor balance they can be useful but generally what they do is overcome a design deficiency in a specific boat; they are not a panacea for all boats

Des
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Old 05 May 2004, 15:54   #12
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JW.

Post from Doel fin thread ref an example of fitted to a previously well set up boat. My minimum planing speed reduced by a considerable margin. Not sure on effect of top speed as i didn't have a gps to check it against hence why i said theoretical. see also links at bottom of post

25 March 2003, 19:34 #23
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--------------------------------------------------------------------------------

I'd like to add a cautionary note about the use of Doel Fins. I use a 4m Avon Searider with a Mariner 40HP 2 stroke when teaching sailing to a local Scout group. When we got the new engine last year I took the boat out for a "test drive" starting slow and building up until I was doing everything I could think of to get the boat to misbehave. The boat was perfectly composed throughout. Later on last year against my advice Doel fin was fitted. I have allways thought that this makes the boat plane to high and too early casuing the boat to feel as if it is riding high in the water and is unstable.
A few weeks ago I was driving the boat in a force 3 wind with a following sea of around 1 ft with the throttle at half way and the boat just comming onto the plane. In the middle of a shallow turn the boat was caught by a wave and rolled over.
I am convinced the fin is to blame. I have since had emails fron several people saying that fins cause instability in following seas and just below planing speeds. If someone would like to give another explanation for the incident then I would like to hear it.
I don't think the fins are necessarily a bad idea I just think they arn't a good idea for every boat. If your boat has problems then try them but if your boat works then I think they cause problems.
--------------------------------------------------------------------------

These links show many examples of reduced planing speed and a couple of increased top end. people talking from experience of using a boat with and without them

More about Doel fins

Doelfin Hydrofoil
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Old 05 May 2004, 16:02   #13
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Quote:
Originally Posted by Scary Des
Although they might reduce the wetted area they increase drag, in effect a poor trade off of one for the other, your boat weighs the same regardless so either you are supported by the hull or the Hydrofoils but with the Hydrofoils the force exerted on the top of the Hydrofoils is of no benefit from a planing point of view.

Des
Des

To give a query then, how do boats with 2 hydrofoils go faster when running on only the foils eg. the ferry from southampton to IOW? another example, I have an airchair, in the rare event i manage to get going on it how come it will glide for a lot further while the ski is out the water and only the foil is in it? Very small drag coefficient off the foils in both cases I think. could be wrong

Also the the force exerted on the top of the foils is of use when planing as it lifts the stern and keeps the nose down IMHO of course.
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Old 05 May 2004, 16:46   #14
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Quote:
Originally Posted by ribnwheels
how do boats with 2 hydrofoils go faster when running on only the foils
Because the hull lifts completely out of tha water. This is a rather different effect to fitting a foil on an outboard!

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Old 05 May 2004, 17:39   #15
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Quote:
Originally Posted by John Kennett
Because the hull lifts completely out of tha water. This is a rather different effect to fitting a foil on an outboard!

John
True enough john, which backs up my point that the drag is less off a foil than of wetted surface area. (ie wetted area drag to zero, max drag off foil still gives higher speed) It was a response to the reply that your boat weighs the same regardless so either you are supported by the hull or the Hydrofoils, showing the lower drag off a foil

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Old 05 May 2004, 18:39   #16
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One further point worth making is the effect they have will be different depending on the size of boat, although the foils do come in 2 sizes. eg the effect on my 4m outboard powered boat, will be a lot greater than on for example a 10m diesel inboard boat

I'll wait for the replies

only joking

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Old 06 May 2004, 04:26   #17
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There was a thread a year or two ago in which Andy Gee the previous owner of my boat talked about fitting fins, he said that it planed alot quicker and at lower RPM's.
Hope it helps
James
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Old 06 May 2004, 05:08   #18
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Not wanting ( or being qualified) to get into the technical aspects of hydrofoil benefits - from my experience they do have value in certain set ups.
My toy is rather short at 5.5 in relation to beam and the layout (wide cruising console, under-deck fuel tanks, bench comode seating, 125 hp engine, and bottle rack mounted aft of seat) makes it somewhat stern heavy - even with anchors/ ropes and weightbelts in the bow locker.
With four up and four dive cylinders, getting up on the plane was a relatively slow process and irrespective of trim position I did get a pendulum effect in minor swells.
I added hydrofoils (after plucking up courage to drill the anti cavitation plate) and have been really pleased with the change. Planes easily, and maintains this at lower speeds, and the ride is much more stable.
I am sure that these would not suit every rig ( especially the big boys ) but works for me
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Old 06 May 2004, 07:12   #19
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Quote:
Originally Posted by ribnwheels
True enough john, which backs up my point that the drag is less off a foil than of wetted surface area. (ie wetted area drag to zero, max drag off foil still gives higher speed) It was a response to the reply that your boat weighs the same regardless so either you are supported by the hull or the Hydrofoils, showing the lower drag off a foil

Colin
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John is right, this is an entirely different effect and was used on the now defunct IOW hydrofoils for comfort reasons and not to get onto the plan.

Des
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Old 06 May 2004, 07:29   #20
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Quote:
Originally Posted by ribnwheels

25 March 2003, 19:34
Richard

Doelfin Hydrofoil

I'd like to add a cautionary note about the use of Doel Fins. I use a 4m Avon Searider with a Mariner 40HP 2 stroke when teaching sailing to a local Scout group. When we got the new engine last year I took the boat out for a "test drive" starting slow and building up until I was doing everything I could think of to get the boat to misbehave. The boat was perfectly composed throughout. Later on last year against my advice Doel fin was fitted. I have allways thought that this makes the boat plane to high and too early casuing the boat to feel as if it is riding high in the water and is unstable.
A few weeks ago I was driving the boat in a force 3 wind with a following sea of around 1 ft with the throttle at half way and the boat just comming onto the plane. In the middle of a shallow turn the boat was caught by a wave and rolled over.
I am convinced the fin is to blame. I have since had emails fron several people saying that fins cause instability in following seas and just below planing speeds. If someone would like to give another explanation for the incident then I would like to hear it.
The deep V hull is designed for stability, anything that compromises this, in my view, is a bad thing and these comments above point to the potential problems that these fins can cause.

Jeff
Not criticizing your set -up but is it within the manufactures specification? Having a boat too stern heavy is not good.
Des
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