Outboard hydrofoil / fins

[Blankton]

I have a yam 310S with an aluminium floor and a 15hp 4 stroke engine. The engine is within the rating of the SIB, but I don't think handling is as good as it should be. The main problem is that it is very difficult to get on the plane, falls off the plane and kicks up a big wake.

I am thinking of trying a hydrofoil. I've read a lot written about these, such as they are "just papering over the cracks" , but on the balance of what I've read I think its worth a go.

I think my fundamental issue is the 51Kg hanging off the back of the boat. I can improve matters by playing with weight distribution, but if by myself, there's a limit to what I can stick up front.

One of the issues I haven't seen addressed is engine storage. Currently I lay the engine on its side to move it around. Some of the fin designs stick out like a pair of ears, and I think would be in danger of getting knocked off or just making the engine rock. I have seen this design, which is a bit more expensive, but I think would let me lay the engine down.

Has anyone used one of these? Are they as effective as the big sticky-out ear types?

[jeffstevens763@g]

Your 15 should put that boat on the plane fairly quick with two adults and a bit of kit have you got the correct prop?

[paintman]

Is your engine mounted at the right height?

You need to know what rpm you are achieving at WOT. If you haven't got a tacho then you'll need to rig something up & hopefully there will be someone on here that can tell you what you need to do.

At WOT with a normal balanced load is the rpm with the current prop more or less at the top of the manufacturer's rev range?
I think you've got a newish Yam 15hp 4 stroke? This link suggests a 2007 on Yamaha 15hp should be between 5000-6000rpm so nearer to 6k would be the target. (Check that that WOT figure is still correct for your engine)
https://www.rubexprops.com/boat-prop...e/?make=Yamaha

Are you at best trim setting, do things get better or worse if you change the trim angle?

When you say 'kicks up a big wake' do you mean a rooster tail?

[Blankton]

It's the manufacturer's prop that it came with. I understand that playing with the pitch can optimise top speed etc. I wasn't sure if it would help with this kind of problem.

Basically the bow lifts up really high and not a lot happens as revs increase. After 30 or 40 yards of a very high bow and big wake it will slowly climb onto the plane. I've tried moving the tilt pin, but can't fix it this way. The bottom setting seems to be best.

When on the plane and in flat seas I've got about 20kts, so this makes me think the prop is ok? Or is there more to it than that?

[Blankton]

Originally Posted by paintman

When you say 'kicks up a big wake' do you mean a rooster tail?

No rooster tale, it just feels like a big wake. Maybe I'm paranoid, but it seems like a lot of noise and fuss, for not very much performance.

It's a short shaft engine and a short shaft transom, although the plate is probably 1" below the bottom of the hull. I'm not sure anywhere makes a 13 or 14" shaft that is completely perfect for a small SIB?

[paintman]

Originally Posted by Blankton

It's the manufacturer's prop that it came with. I understand that playing with the pitch can optimise top speed etc. I wasn't sure if it would help with this kind of problem.

Can make a big difference to getting on the plane as well plus fuel consumption.
My own boat - 17' Dory Sportsman hard boat - came with a 17 pitch ally prop. Fast onto the plane, heavy on fuel & not particularly fast. Well over recommended WOT. Over-revving is not a good idea.
Several props tried (ebay) & I'm on a stainless steel 21p.

I could possibly go to a 23 but the 21 is fine with a couple of extra bodies on board as well. Solo I'm just under max WOT.
Tried a 25 but slow to plane, max engine rpm below minimum WOT & obviously labouring. Again, not a good idea.

I do have a permatrim fitted - much the same as your pic - & it did make a significant difference to the speed at which the boat comes onto & drops off the plane.
Improved fuel economy, no rooster tail of any description & less splash.
You do need to drill your cavitation plate.
I have no idea what affect it would have on the warranty of an engine still under manufacturer's warranty.
Or what affect it might have on the engine's value.
They're basically a big aluminium plate with a powder coat finish & I suspect wouldn't be difficult for an engineering company to make.

[beamishken]

Originally Posted by Blankton

No rooster tale, it just feels like a big wake. Maybe I'm paranoid, but it seems like a lot of noise and fuss, for not very much performance.

It's a short shaft engine and a short shaft transom, although the plate is probably 1" below the bottom of the hull. I'm not sure anywhere makes a 13 or 14" shaft that is completely perfect for a small SIB?

I'd try sitting the engine on a piece of wood so the clamps are higher up the transom. The av plate being 1" below the hull will be creating extra drag, you probably wont get it as high as it could possibly run but every little helps. Next I'd invest in a cheap tacho & check the max rpm flat out to see if you have the best prop.
I wouldn't totally discard the hydro foil idea but Id save it for after a little more testing first

[paintman]

Originally Posted by beamishken

I'd try sitting the engine on a piece of wood so the clamps are higher up the transom. The av plate being 1" below the hull will be creating extra drag, you probably wont get it as high as it could possibly run but every little helps. Next I'd invest in a cheap tacho & check the max rpm flat out to see if you have the best prop.
I wouldn't totally discard the hydro foil idea but Id save it for after a little more testing first

+1

[Limecc]

Originally Posted by paintman

+1

+2

[Limecc]

When you have the engine height correct a four blade prop will give you more stern lift. I've purchased one but not tried it out yet.

I'm reluctant to remove the stupid looking Doel fin fitted by the previous owner. If running at sub-planing speed on a river they are quite useful to improve fuel economy and reduce wake, while hardly affecting top end which you can't use. I'm not pushing a big wall of water.

It's made of a very robust semi flexible plastic, probably a better material, I don't think I could break it. They are cheap enough so no great loss if you don't like the effect. Weird how there's no definite transition point onto the plane.

I think the engine would transport and store on it's side ok if it were light enough to do that. It would settle on it's side rotated left slightly.

[Old seahorse]

I’ve recently had a permatrim installed on my Suzuki 6hp, which is fitted to my 3 metre takacat. Before it was fitted, the wake from the outboard was excessive in volume, and masses of upward thrust. Progress was disappointing with so much ventilation/ cavitation. Catamarans with single, centrally located engines[ as apposed to twins fitted to rigid hulled cats] apparently are prone to this, with so much air being funnelled between the hulls under the raised deck.
Fitting the permatrim has improved performance, with less effort, and levelled the boat fore /aft. (though takacat doesn’t appear to suffer unduly from bow lift). Outboard is still being run in, but I anticipate improved top speed as well.
Stowing the engine on its side hasn’t been an issue.
My previous boat ( 4.5 metre rigid hulled boat) had difficulty planing from new. Recognising that it was probably slightly underpowered to plane if carrying more than 2 crew, I had a doel fin fitted. That gave the stern plenty of lift, made it easier to plane at lower speeds, and improved handling and top speed. Lasted 10 years, with no issues.
I have no experience of stowing an outboard fitted with doel fin, but might be less stable, due to shape.

[willk]

Originally Posted by Blankton

The main problem is that it is very difficult to get on the plane, falls off the plane and kicks up a big wake.

I am thinking of trying a hydrofoil. I've read a lot written about these, such as they are "just papering over the cracks" , but on the balance of what I've read I think its worth a go.

I think my fundamental issue is the 51Kg hanging

A hydrofoil might well be the solution - you have a shortish boat which doesn't help the transition to planing as weight tends to be close to the stern. Have you tried a tiller extension? - that would clarify the weight distribution question as you could sit forward and then slide back once on the plane. I used to do this in my 2.9m SIB - even a couple of feet forward made a huge difference in planing speed. The "big wake" sounds like the wall of water you're trying to climb to get on the plane.

[Blankton]

A lot of this advice will have to wait until next time I have the boat out, which won't be til next spring.

I do have a tiller extension and it does make a big difference, but I find it a bit unweildy sitting so far forward.

[Limecc]

Originally Posted by Blankton

A lot of this advice will have to wait until next time I have the boat out, which won't be til next spring.

I do have a tiller extension and it does make a big difference, but I find it a bit unweildy sitting so far forward.

Getting ready and setting the right engine height is a nice relaxing winter job then. Needs to be done regardless, my Excel 390 needed the engine lifting 25mm to get the cav plate the required 50mm above the bottom of the hull (50mm was specified in the Suzuki manual)

[Blankton]

Unfortunately the boat is in storage for the winter, so taking a further look will have to wait. I have gone over most of this before though and was 90% made up to try one.

Obviously fins and hydrofoils are a bit "marmite". One of my observations is that I often see people saying:
"if Yamaha /Suzuki / Mercury / mariner thought the engine needed it, they'd have built it into the original design".

That may be true, but so far I've been recommended to buy a new prop and build up my transom. Both of which are changing the original design of the boat / engine.


My guess would be, that as they are making thousands of these a year, that end up on all types of boats, they just make a "one size fits all, best fit" I don't expect the entire Yamaha design team were designing the motor just on the off chance I brought a SIB.

[Steve509926]

My advice, for what it's worth, would be to hold off fitting a hydrofoil and/or buying a new prop until your outboard is at the correct height on the transom, which can't be done unless you test it on the water.
There are a lot of threads on here with regard to ob/cav plate height and how to test it. Use your OB manual as a guide, remembering that OB are primarily made for hard hulled craft not soft inflatable boats.
It took me 2 weekends on the water (different locations lake/sea) to get the ob/cav plate to a height which I was happy with,. That said, the SIB still has to be trimmed correctly depending on who and what is on board. If I'm solo, all of the kit I carry is towards the bow.
Good luck in your quest.

[paintman]

Don't forget that most boats come with a manufacturer's recommendation of engine power range & they have no idea what engine the end user will be fitting or what performance requirements they may have.
It's the fine tuning to try & match engine to boat & your normal usage that's the 'fun' part.

For storing & moving it around off the boat consider finding a used sack barrow off ebay or gumtree, bolt a bit of wood to it & fit the engine to the wood.

[Orwell boy]

Fitted one of these ,no drill jobs ,well worth the extra not to have to drill ,not sure what it does for me but if i decide its not needed i can remove without trace and flog on ebay ,holds on with anode bolt backed up by a couple of grub screws on sides ,this is large but i think there is a smaller availiable
cant seem to upload pics at moment ,for some reason my computer needs a couple of days to catch up with newly uploaded pics
think its callled starfire
https://www.force4.co.uk/item/Force-...YaAieZEALw_wcB

[paintman]

I did a quick google for more info on the Starfire & some reviewers said that theirs fell off in use. Attached using the grub screws.
May have been fitting error as the claims are that they fell off not long after installation.
Appears they can also be drilled & bolted.

[smallribber]

I cant help thinking that the sheer weight of a 15 HP 4 stroke on the back of a 3.1 m rib is the issue here. The fin might help to lift it when you accelerate but I feel that more weight up front is what is needed

I would not mess about with the prop as if you have the factory one fitted that should be fine.

When you get a chance try a bag of somthing heavy in the bow simply as a test. If this helps you will know what the issue is then you just need to work out how to cure it.

Also if you know someone with a lighter say 9.8 or 15 hp 2 stroke see if it is possible to give that a go.

A 15 hp 4 stroke is a big heavy lump on a 3.1 m rib