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Old 25 April 2015, 11:26   #21
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Thus the idea of installing it below the prop. The idea being; not to fly but reduce some of the drag from the hull's planing surfaces by producing enough lift to cancel out the weight of the outboard and one driver. More than that and one might start to encounter problems with the strength of the outboard bracket and bearings
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Old 25 April 2015, 13:39   #22
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I think your starting to go down the road of a hydrofoil boat. I'd suspect the lift and drag that the "wing" would create would be no more or less than the lift and drag that you've saved from the reduced wetted area of the hull.
However the Americas Cup sailing catamarans have a wing on the bottom of their keels but I think the concept there is to lift the hulls out of the water altogether.
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Old 26 April 2015, 09:14   #23
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Thus the idea of installing it below the prop. The idea being; not to fly but reduce some of the drag from the hull's planing surfaces by producing enough lift to cancel out the weight of the outboard and one driver. More than that and one might start to encounter problems with the strength of the outboard bracket and bearings
I've not given the physics too much thought, but surely if you add a new surface that provides lift sufficient to remove the existing planing surface you gain nothing? And until this new magical lift state is achieved have added drag? if you really want a faster boat there are two far easier (and probably cheaper) ways than trying to milk the last 0.1 knots in perfect conditions out of it... 1. Buy a bigger engine. 2. Buy a better boat
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Old 26 April 2015, 09:26   #24
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Quote:
Originally Posted by Poly View Post
I've not given the physics too much thought, but surely if you add a new surface that provides lift sufficient to remove the existing planing surface you gain nothing? And until this new magical lift state is achieved have added drag? if you really want a faster boat there are two far easier (and probably cheaper) ways than trying to milk the last 0.1 knots in perfect conditions out of it... 1. Buy a bigger engine. 2. Buy a better boat
Poly,

This is how a hydrofoil works. The foils generate more lift with less drag than the hull surface does. If they didn't there would nt be any hydrofoils!

I agree it's quite magical though
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Old 26 April 2015, 09:55   #25
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Quote:
Originally Posted by Poly View Post
I've not given the physics too much thought, but surely if you add a new surface that provides lift sufficient to remove the existing planing surface you gain nothing? And until this new magical lift state is achieved have added drag? if you really want a faster boat there are two far easier (and probably cheaper) ways than trying to milk the last 0.1 knots in perfect conditions out of it... 1. Buy a bigger engine. 2. Buy a better boat

I think you forgot to factor in the axial lift created by the vertical thrust from the forward moment. If you divide the quadrilateral bi-nomial co-efficient of thrust, by the friction co-efficient of the hull Mu (assuming the hull hasn't been modified with a Dilithium matrix) you will arrive at the lift produced for a given speed. Obviously this doesn't take into account any drag from the engine leg, so you would have to assume a notional loss, probably around 10%. If you plotted the curve you would be able to see the optimal size of hydrofoil required for a given wetted surface area of hull at a given speed. Naturally, curve will be logarithmic.
HTH


.....sh1t happens.......
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Old 26 April 2015, 09:58   #26
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Here's some speculation:

I suspect the main benefit of fins is to get the boat on the plane more cleanly, so the whole boat comes up fairly level rather than rearing up first. That would get you planing slower, but reduce the top speed. They'd have an effect at those speeds that are half-way between planing and not, when the deck angle is quite nose-up, but the rest of the time they'd be at a low angle of attack and not doing much at all. The only ones I've seen were fairly crude approximations of an aerofoil shape, and I doubt they're thick or asymmetric enough to produce much lift at near-zero angles of attack.

Engine trim angle would also affect them: they'd produce extra lift if you were trimmed all the way down, losing engine RPM and power; and negative lift if you were trimmed too far out, pulling the hull into the water and adding drag.

The only ones I've seen have been very thin, with an aerofoil-shaped upper surface and no lower surface, just little moulded spars adding stiffness and drag. In other words, fairly crude things, not something you'd use for fine tuning.

A well-designed hydrofoil would work, but I think as far as fins go, any concerns about over-stressing the outboard bracket are probably misplaced.
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Old 26 April 2015, 10:27   #27
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The lifting (planing surfaces) on the hull are very inefficient, approx 5 : 1 lift to drag ratio. The lift to drag ratio on a foil at 3-4 degrees ( I think) can be as good as 25 : 1 (including the strut). I'm not sure if one could actually get the foil to work in such a narrow angle of attack with most of the vessel supported (lifted) by the hull and unless its a flat calm the boat would be bobbing around too much for the foil.

Bit complicated .....but possible if one could actively(constantly) control the position of the outboard to the vertical and if the foil was attached to the leg....then the foil aswell
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Old 26 April 2015, 11:08   #28
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Poly,

This is how a hydrofoil works. The foils generate more lift with less drag than the hull surface does. If they didn't there would nt be any hydrofoils!

I agree it's quite magical though
Duncan - yeah I follow how a hydrofoil works and they are indeed magic. Like those aeroplane things that fly through the sky!

However, the benefit of a hydrofoil comes (as I understand it) not from removing the planing surface of a hull from the water, but by removing the non planing surfaces that cause drag.

Cristiananthony was suggesting, "reduce some of the drag from the hull's planing surfaces by producing enough lift to cancel out the weight of the outboard and one driver".

Does a hydrofoil bring any more benefit than simply having more (well designed) planing surface on the hull? Or is the benefit of lifting completely clear about ride comfort by missing the waves?

However if we are talking about the boat in Cristiananthony's profile - it looks like its simply underpowered (Gemini 550 + Yam 60HP 4/s)? and over-propped (25"). The cynic in me says if you could get 100HP performance from an F60, with the same fuel economy essentially just by modifying the shape of the gearbox then Yamaha would be doing it, patenting it and boasting about it.

If you really want to add a hydrofoil then I'd think the last place you'd want it is in the turbulent water directly under the prop. Hydrofoils have been around for over 50 years and haven't really caught on other than in some specialist applications, that tells you something about their practicality for everyday leisure boating.
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Old 26 April 2015, 11:26   #29
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Just read that you're using a 25p prop on a yam 60, that's massively over propped. WOT at 2700rpm is nowhere near right, check the engine manual, it will say the WOT operating range, usually between 5k and 6k. If it's running the same 13 spline, 1.85 ratio gearbox as the 50's then you don't want to be over about 17p propwise
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Old 26 April 2015, 21:30   #30
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I am over propped if i want max hp from the engine. Four reasons for the prop choice: 1) I enjoy the peace and relaxed pace of 800 rpm and 4 knots for daily commute(evening)
2) 180ml / km fuel consumption
3) to work "backwards" to reduce drag, meaning the addition of a foil to gain a higher economical cruising speed. An incentive.
4) the engine is a 2008 with 2400 hrs on it although according to the ECU 2000 of those hours were run below 2000 rpm.

I am aware that the map sensor may read a lower negative pressure ("vacuum")and add more fuel as it thinks Im accelerating or under heavy load but I have been able to lean the mixture with a motor controller before the high pressure pump to get around this.

I suspect that the lower vacuum in the manifold may contribute to overall efficiency in a similar way there are smaller pumping losses in a diesel.
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