Hydrofoils

[Deep Blue]

Is there any real benefits of having these fitted to outboards, and what if any ar the disadvantages?

[Martin]

I've got them on my Yamaha 90hp and I find that it lifts the stern and gives a better ride. I can't say that it makes us go any faster or whether it is any more economical, but it has changed the way our boat handles the rougher stuff.

[Colin F]

It certainly made a big difference to my boat, the weight distribution is very aft heavy, and the lift produced off them makes the boat plane a lot quicker, which is a big help when pulling skiers. It also increased the effect of trimming the engine in or out. I found the planing speed was noticably reduced,and also a reasonable saving in fuel (although how much exactly i don't know). If your boat is slow to get on the plane or you want to lower the planing speed to make it easier to tow kneeboards or other 'slower' items then give them a go

[Colin F]

As for disadvantages - when i fitted them, i caught part of the leg with the drill chuck and took a small chunk out which peeeed me off

Other than that it took a while to adjust them so the boat tracked straight (i got the ones with 'wing tips' on). Once they were set up though, no problems at all

[steedthrust]

Got to ask the noobie question.

Why dont engine manufacters fit them as standard? I too see lots of claims about them and wonder if the hype is true. If it is I would get some.


Steve B

[Colin F]

Originally Posted by steedthrust

Got to ask the noobie question.

Why dont engine manufacters fit them as standard? I too see lots of claims about them and wonder if the hype is true. If it is I would get some.


Steve B

A few reasons I suspect:
1) Not needed on a well set up and correctly balanced boat
2) Of no use for lower speed applications such as semi-displacement boats
3) If used on semi-displacement boat it could have the effect of making them run "nose down" slightly.
4) Then can cause handling problems if fitted to a well balanced boat, making the stern high caused by the lift - if they were built in by the manufacturer, they would need to be removable.

Also, I personally feel they are only of use with power trim, so you can control the amount of lift/trim effects.

They are of most use to stern heavy boats, or boats that struggle getting on the plane (ie towing my best mate on water skis- he's not the slimmest of people shall we say )

[Scary Des]

To add to the points above they must also increase friction and therefore increase consumption and reduce speed. Race boats try to have as little metal in the water as possible hence things like surface drives. Des

[Jackwabbit]

Hmmm,
These could be of use for our dive boat!
With the bottle rack behind the console we have to sit 2 people in the bow, & another 2 by the console just to get it on the plane.
Then if we stop quickly it does a good impression of a submarine.

[Colin F]

Originally Posted by Scary Des

To add to the points above they must also increase friction and therefore increase consumption and reduce speed. Race boats try to have as little metal in the water as possible hence things like surface drives. Des

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.

God, I must sound like I'm selling them!!!!

[jwalker]

Originally Posted by ribnwheels

1) Not needed on a well set up and correctly balanced boat

4) Then can cause handling problems if fitted to a well balanced boat, making the stern high caused by the lift - if they were built in by the manufacturer, they would need to be removable.

Also, I personally feel they are only of use with power trim, so you can control the amount of lift/trim effects.

They are of most use to stern heavy boats, or boats that struggle getting on the plane (ie towing my best mate on water skis- he's not the slimmest of people shall we say )

You state this as fact. Are you sure?

I personally found them most use for giving longitudinal stability on a single engine boat and both longitudinal and transverse stability on a twin engined boat.
They didn't reduce top speed nor did the boat plane at noticeably lower speed.

[Scary Des]

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

[Colin F]

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
Richard
Registered member


Country: UK


Join Date: Mar 2003
Posts: 1
Credibility: Doelfin Hydrofoil

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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.
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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

http://www.rib.net/forum/showthread....highlight=doel

http://www.rib.net/forum/showthread....highlight=doel

[Colin F]

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.

[John Kennett]

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!

John

[Colin F]

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

Colin

[Colin F]

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

colin

[Jelly]

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

[Jeff]

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

[Scary Des]

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

Colin
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

[Scary Des]

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