Outboard hydrofoil / fins

[Blankton]

Originally Posted by 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

Yes, this is exactly the problem. If I have a passenger up front, it's better. If by myself I put fuel and anchor up front and it helps to compensate. I am also quite heavy which doesn't help!

A recent example was that I was out all day and the fuel tank was light by the time I returned. I'd also got a bit of water in the boat (which sloshed to the stern when accelerating). The combination of a few kilos of water in the boat and a few less kilos of fuel made it very hard to get the boat planing.

My hope was that an aerofoil might increase the "window" where the boat will quickly plane, without it being so sensitive to issues such as the fuel tank being below half full or getting some rain in the boat.

With hindsight I can now see the benefit of sticking with a nice reliable, light, 2 stroke engine. Unfortunately I now have a nearly new 4 stroke, which I would loose loads of depreciation on if I sold it to buy a 2 stroke. In fact, looking at Prices, a decent second hand 2 stroke would probably cost me more than I could sell my 2019 4 stroke for!

The idea of a hydrofoil was that for somewhere between £20-£100 I might see a reasonable improvement, without the much bigger cost of buying a new engine.

[jeffstevens763@g]

looking at the weights two stroke at 33kg yam 4 stroke 51 kg 18kg difference not going to make that much difference i had a 15 Johnson two stroke on a 3.1 zodiac solid and it flew with my 19 stone in it plus kit only a comparison.

[Steve509926]

Originally Posted by Blankton

A recent example was that I was out all day and the fuel tank was light by the time I returned. I'd also got a bit of water in the boat (which sloshed to the stern when accelerating). The combination of a few kilos of water in the boat and a few less kilos of fuel made it very hard to get the boat planing.

I'm with Jeff re:the weight differential between a 2 stroke and a 4 stroke, it's shouldn't be enough to cause such a big problem.
I would still be of the opinion that something else is the main cause if 10kg of fuel is having such a dramatic effect on the performance of your SIB.
I'd still try altering the height of you OB on the transom and the trim of the OB in relation to the SIB before I'd be spending money on a hydrofoil or a new prop.

[Steve509926]

Originally Posted by Blankton

A recent example was that I was out all day and the fuel tank was light by the time I returned. I'd also got a bit of water in the boat (which sloshed to the stern when accelerating). The combination of a few kilos of water in the boat and a few less kilos of fuel made it very hard to get the boat planing.

I'm with Jeff re:the weight differential between a 2 stroke and a 4 stroke, it's shouldn't be enough to cause such a big problem.
I would still be of the opinion that something else is the main cause if 10kg of fuel is having such a dramatic effect on the performance of your SIB.
I'd still try altering the height of you OB on the transom and the trim of the OB in relation to the SIB before I'd be spending money on a hydrofoil or a new prop.

[Blankton]

Originally Posted by jeffstevens763@g

looking at the weights two stroke at 33kg yam 4 stroke 51 kg 18kg difference not going to make that much difference i had a 15 Johnson two stroke on a 3.1 zodiac solid and it flew with my 19 stone in it plus kit only a comparison.

I think the issue is where that weight is. I've already found that the 20kg difference between a full and empty fuel tank in the bow is enough to have a significant effect. 20kg hanging off the end of the boat, I suspect, has an even greater effect than 20kg within the confines of the boat such as fuel, passengers etc. due to the fact that it is cantilevered out over the end of the transom, so would be generating an increased 'moment'.

[Searider]

Originally Posted by Limecc

+2

A SIB is "Heavily Loaded" so the diagram posted shows that 1"-2" below the bottom line is correct. (300kg and 15hp is a pretty low power to weight ratio).
Playing with the height of a 15hp motor on a SIB isn't going to make a massive difference.

Is the fuel tank in the bow and strapped down?
Do you carry an anchor? If so, put this in the bow.
If you're on your own in the boat it's the nature of this kind of boat that you will get a massive bow up trim before the boat comes on to the plane. Try accellerating a little more gently.
Top speed does sound about right but check the RPM. A Tinytach copy is only about £10 on ebay last time I looked.
A lot of people will use a tiller extension to enable them to sit on the thwart when single handed.

I've never used fins so can't comment on those.

[Steve509926]

Originally Posted by Blankton

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

If you don't like using a tiller ext and you can't get any more weight in the bow, try using a collapsible camping water carrier. Fill it with water when you're solo giving you the extra weight in the bow.

[Fenlander]

Sorry to disagree with some re weight but I think Blankton knows exactly where his problem lies as detailed in his post #21.

That Yamaha 310s is quite a stubby SIB and in my experience the shorter they are relative to width (for example his Yamaha is over 2 feet shorter than my Bombard yet only 3 inches narrower) the longer and more dramatic the bow-up transition period... to the point where they can be hard to get on the plane at all if loaded with a stern bias and constant dropping off the plane is an issue if throttling back for a moment when working a sea.

And to add to this I've found each couple of extra kg weight right on the transom adds to the problem. The weight of that 51kg Yamaha above my 35kg 15hp 2-stroke is very close to carrying a full 25l extra tank of fuel right at the transom and that's going to greatly increase the bow up attitude on transition.

I don't get on with foils and often have said they are only of use to correct a setup that's less than ideal when everything else has been tried. This may be such a circumstance.

[paintman]

Originally Posted by Blankton

With hindsight I can now see the benefit of sticking with a nice reliable, light, 2 stroke engine. Unfortunately I now have a nearly new 4 stroke, which I would loose loads of depreciation on if I sold it to buy a 2 stroke. In fact, looking at Prices, a decent second hand 2 stroke would probably cost me more than I could sell my 2019 4 stroke for!

Stick with your 4 stroke. You have a nice quiet engine.
There's no reason why you can't fit a trim plate if that's what you want to do but there are other things you can do first that don't involve making holes in the engine.
Many years ago I had a 12' Heron dinghy hull (without rags & poles) with a 4hp Johnson 2-stroke. If I sat in the stern solo above half throttle the bow was out of the water altogether! Cured by borrowing a 56lb Avery scale weight & putting it as far forward as possible.

Your main testing issue is much the same as mine, we're a long way from the sea in any direction & the local big bodies of water (Rutland, Pitsford & Thornton) won't let you have a play.
Might be worth having a chat with the local marinas - GUC, Rivers Soar & Trent - to see what they would charge to let you launch & what permits (Canals & Rivers Trust for example) you may need. Full throttle running might be a problem.

I don't know if permatrim still supply direct or whether you would have to go through one of their stockists.
Don't forget you would need to pay p&p plus import charges which will be collected from you on the doorstep by the deliverer.
ETA On ebay!

[Blankton]

Originally Posted by Fenlander

Sorry to disagree with some re weight but I think Blankton knows exactly where his problem lies as detailed in his post #21.

That Yamaha 310s is quite a stubby SIB and in my experience the shorter they are relative to width (for example his Yamaha is over 2 feet shorter than my Bombard yet only 3 inches narrower) the longer and more dramatic the bow-up transition period... to the point where they can be hard to get on the plane at all if loaded with a stern bias and constant dropping off the plane is an issue if throttling back for a moment when working a sea.

And to add to this I've found each couple of extra kg weight right on the transom adds to the problem. The weight of that 51kg Yamaha above my 35kg 15hp 2-stroke is very close to carrying a full 25l extra tank of fuel right at the transom and that's going to greatly increase the bow up attitude on transition.

I don't get on with foils and often have said they are only of use to correct a setup that's less than ideal when everything else has been tried. This may be such a circumstance.

That was pretty much the conclusion I'd come to over this summer, hence starting the thread! Its good to hear that you agree with my logic.

Originally Posted by paintman

Stick with your 4 stroke. You have a nice quiet engine.
There's no reason why you can't fit a trim plate if that's what you want to do but there are other things you can do first that don't involve making holes in the engine.
Many years ago I had a 12' Heron dinghy hull (without rags & poles) with a 4hp Johnson 2-stroke. If I sat in the stern solo above half throttle the bow was out of the water altogether! Cured by borrowing a 56lb Avery scale weight & putting it as far forward as possible.

Your main testing issue is much the same as mine, we're a long way from the sea in any direction & the local big bodies of water (Rutland, Pitsford & Thornton) won't let you have a play.
Might be worth having a chat with the local marinas - GUC, Rivers Soar & Trent - to see what they would charge to let you launch & what permits (Canals & Rivers Trust for example) you may need.

I don't know if permatrim still supply direct or whether you would have to go through one of their stockists.
Don't forget you would need to pay p&p plus import charges which will be collected from you on the doorstep by the deliverer.
ETA On ebay!

I'll take a look at those permatrims. They look simlar to the oz hydrafoil product I put the photo of in my first post - no big sticky out bits to get in the way when transporting the engine.

[jeffstevens763@g]

id be looking at dropping a pitch personally i run two props a 10 & 11 the difference is night and day especially in the chop the 10 keeps me on the plane for longer and up on the plane quicker ok you drop a knot or two but it makes for a better ride all round but as others say you need a tacho to see where you are no good spending money until you have all the numbers

[beamishken]

Before you spend money on props or hydrofoils spend a tenner on one of these & check your wot rpm & find out what's going on first
Anything else is a shot in the dark
https://www.ebay.co.uk/itm/293736843014

[smallribber]

I think you know the answer and in my view many of the posts on here are just adding confusion. Some people as usual are just reading the last post and not reading the rest.

So having read all the posts I would suggest the following:

1) Dont mess with different Props
2) You have already tried changing the trim angle on the outboard Pin
3) Dont mess about with the height of your motor
4) Just take some removable ballast and put it up front (water as has been suggested in a soft container that you can fill from the sea and empty again if needed). You could even rig a small pump and battery (Kept up front) to adjust in your experimenting.

Try some fins if you wish (as you are dying to do this - (get the ones that need no drilling) if it dont work they are cheap and it will have answered your question

Spend the tenner on the tiny tach if you wish as it will keep you happy looking at the numbers. In my view however you can generally hear if an engine is running correctly to the revs..... but no harm in having one

As Fenlander says it is the weight distribution that is the problem but you know that anyway.

I have several sibs and engines from 2.3 hp to 20 hp on them both 2 stroke and 4 stroke. I have read so much about props and transome height etc and never had any of these issues. The reason is that I hardly ever sib alone in the boat and the weight of the second person up front keeps the balance right.

[Poly]

Whereas I have also read all the thread and would say adding fins before trying to optimise the prop is a waste of time, and optimising a prop by “ear” rather than a tacho might work for someone really experienced but is probably a bit too subjective for Internet forum advice for normal people. Weight up front will help (if it’s properly inflated) but adding weight simply to overcome the problem is going to have downsides too. Complex solutions with pumping water in and out are a very strange way to avoid going and buying a smaller pitch prop to improve acceleration.

[Nos4r2]

In all honesty I absolutely despise hydrofoils in any shape or form on anything with a hard hull, having had multiple scary experiences with them at speed.

However, on a SIB there may be a place for them. You're usually not making the speeds I had the experiences at (20+ knots) and typically the helmsman in a SIB is heavier than the boat and engine, so the help in getting over the hump could be useful.

[Limecc]

Originally Posted by Nos4r2

In all honesty I absolutely despise hydrofoils in any shape or form on anything with a hard hull, having had multiple scary experiences with them at speed.

Can you elaborate please?

Although I like the low speed effect on my hard hulled boat I would not want unpredictability at speed.

Doel fin was already fitted when I purchased it so I was thinking of trying it without when I've raised the engine two holes and fitted the four blade prop.

[jeffstevens763@g]

Originally Posted by Nos4r2

In all honesty I absolutely despise hydrofoils in any shape or form on anything with a hard hull, having had multiple scary experiences with them at speed.

However, on a SIB there may be a place for them. You're usually not making the speeds I had the experiences at (20+ knots) and typically the helmsman in a SIB is heavier than the boat and engine, so the help in getting over the hump could be useful.

+ 1 used one once waste of time just masking the problem of the wrong prop. weight distribution is key but as weight drops [fuel use] it shouldn't make a dramatic difference, carrying 70 litres plus on my long trips doesn't make much difference to performance when 50 litres is used plus i don't need to move kit about to keep performance either. the key correct prop at optimum RPM and weight to allow the engine to perform plus all the other bits for fine tuning height, trim, etc

[Limecc]

Originally Posted by jeffstevens763@g

+ 1 used one once waste of time just masking the problem of the wrong prop. weight distribution is key but as weight drops [fuel use] it shouldn't make a dramatic difference, carrying 70 litres plus on my long trips doesn't make much difference to performance when 50 litres is used plus i don't need to move kit about to keep performance either. the key correct prop at optimum RPM and weight to allow the engine to perform plus all the other bits for fine tuning height, trim, etc

Even with the right prop and correct weight distribution at semi-displacement speed he will always push a wall of water with this size of boat because of high loading on a small footprint. Wrong distribution will be even worse. At low speed a hydrofoil will make a massive difference lowering the planing threshold and saving big time on fuel because less throttle is needed. Nobody's asked the OP yet but maybe for the majority of time spent on the water he's not at planing speed? My RIB used to plane at >10knots, not sure about the new boat with Doel fin, it just rises level and I can't really give a particular speed.

Truth is, the boat's a minimum of 1/2m too short for the OP and if buying over again he would probably make a different choice.

Originally Posted by smallribber

I think you know the answer and in my view many of the posts on here are just adding confusion.
1) Dont mess with different Props
2) You have already tried changing the trim angle on the outboard Pin
3) Dont mess about with the height of your motor

Smallribber's post basically says no need to set up and fine tune your boat which should always be a given if you want to get the best out of your gear. Getting it right could save a lot of frustration and effort trading up for something else.

[jeffstevens763@g]

well that's obvious at displacement speed limecc but he's on about at planning speed when he's overcome the displacement or trying to and keep it there. i just ask why don't the manufactures fit or make the cav plate the same as an hydrofoil i guess no need for one with the engine set up correctly. the only time i tried one was for a 4 metre Humber inflatable with three divers and a 25 two stroke Suzuki didn't do a thing so we just threw all the kit forward moved our weight forward until on the plane and move back to sort it.

[Limecc]

Originally Posted by jeffstevens763@g

well that's obvious at displacement speed limecc but he's on about at planning speed when he's overcome the displacement or trying to and keep it there. i just ask why don't the manufactures fit or make the cav plate the same as an hydrofoil i guess no need for one with the engine set up correctly. the only time i tried one was for a 4 metre Humber inflatable with three divers and a 25 two stroke Suzuki didn't do a thing so we just threw all the kit forward moved our weight forward until on the plane and move back to sort it.

Must have been quite a squeeze