Planing Angle and Trim

[Christiananthony]

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

[Last Tango]

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.

[Poly]

Originally Posted by Christiananthony

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

[Searider]

Originally Posted by Poly

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

[Pikey Dave]

Originally Posted by Poly

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.......

[JamesF]

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.

[Christiananthony]

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

[Poly]

Originally Posted by Searider

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.

[Roflhat]

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

[Christiananthony]

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.

[JamesF]

Sorry if this seems terse, but it sounds like you're trying to have your cake and eat it.

If you want more speed, choose the right prop for that. If you want something else, choose one for that instead. Since variable pitch props for outboards are at best very unusual, why not carry two?

Or you can fiddle with tiny things in the hope of making a tiny difference.

[Pikey Dave]

Originally Posted by JamesF

Sorry if this seems terse, but it sounds like you're trying to have your cake and eat it.

If you want more speed, choose the right prop for that. If you want something else, choose one for that instead. Since variable pitch props for outboards are at best very unusual, why not carry two?

Or you can fiddle with tiny things in the hope of making a tiny difference.

+1, if I didn't know any better I'd say Jeepster was back😎


.....sh1t happens.......

[Poly]

Originally Posted by Pikey Dave

+1, if I didn't know any better I'd say Jeepster was back😎


.....sh1t happens.......

He's not suggested drilling holes in the hull to make it lighter yet...

[Pikey Dave]

Originally Posted by Poly

He's not suggested drilling holes in the hull to make it lighter yet...

Give it time, probably still doing the calcs

[Christiananthony]

Quite correct yes carry a few props. I have two, a 11'' P and the 25P. Might try a 21 or 18". I wonder how much Royal Mail would be to Hong Kong. One thing I learnt when I was younger and working on cars was that if the max hp is in 5500-6000 rpm then the engine is highly tuned, meaning radical cams etc and will never be super fuel efficient. I suppose I could de-tune it....but heading for a diesel I think. The small TDIs compare now in power density to the most advanced outboards. Safer too.

[Roflhat]

cars are four stroke, with a 2 stroke the power band can be wider, having a low revving 2 stroke is not suitable for small craft applications, unless you want a very very top speed.
I'm not sure what you're trying to do but I think you're going about it the wrong way...

[9D280]

Okay, I haven't had time to digest al lthis buit would I be right in thinking the following summary:

- You want a low RPM quiet engine for your daily commute with one aboard.
- You want it to do decent performance outside these times wth much extra weight aboard
- You have been doing a lot of maths to try & achieve this.

OK. I have a 1984 45m hull, transom only rated to 110Kg / 60Hp. Reason for this - it's made from 1/2" ply. The modern version of my hull is rated for something like 75 Hp / 150 Kg. - Why? - it;s 50% thicker, and the hull has a lot more reinforcing round the transom = much heavier. I have a Clamshell for a reason - it weighs 83Kg. My entire boat incl engine & full tanks clocks in about the same weight as the modern hull, and guess what - I can do 30 knots and drink 0.8l/NM just like the higher powered modern one.

if you want a quet 4 knot commute and a performance rib I think your best bet would be to buy a second boat with no transom (i.e a sweeping counter) and power it with either a small 4- stroke or an electric. Then re- tune your 60 to be the engine it was designed to be and put up with the noise when at full power, coz you are ging to need that power to plane.


Lift the hull out - you need buckets of power for that (do the lift - wing calcs or just have a quick look at hydrofoil boats & the size of the foils. Ignoring that, even if you do have this magic wing, a) will your vent plate be able to support the lift? and b) if you are lifting the hull out wil lthe engine's tilt bearing be able to support it?

I reckon lightening the boat and arranging the weight would be better use of your time, and If you can make this happen my suggestion would be to patent your findings before posting back with the results!

[Last Tango]

Kind of got to agree with D9280. although I'd probably go for a 3m rib with a 15HP. you'd get more noise on the commute but you'd only have to put up with it for a quarter of the time. If the water you're using it on isn't sheltered enough for that, your hydrofoils ain't going to work anyway.

[jyasaki]

Originally Posted by Christiananthony

Quite correct yes carry a few props. I have two, a 11'' P and the 25P. Might try a 21 or 18". I wonder how much Royal Mail would be to Hong Kong.

Aren't most props made in China anyway? Why get it delivered from the UK?

jky

[apshutt]

Good article in POWER BOAT and RIB magazine on angles of outboards and hulls. issue 126 May 2015 out now.