Bolt on hydrofoil fins onto the anti cavitation plate of an outboard. Do they work?

[jason clarke]

We have been advised to by and fit some fins which bolt onto the plate above the prop on our 4 stroke 70 hp Suzuki engine which we have fitted to a 5.5m RIB, to help get it on the plane quicker and easier. To do this we'll have to drill holes through the original plate to bolt them on.
Has anyone fitted these and will they improve our performance, and will they weaken the original plate? Will they cause us any problems?

Cheers
Jason Clarke

[Andy Wade]

We have been advised to by and fit some fins which bolt onto the plate above the prop on our 4 stroke 70 hp Suzuki engine which we have fitted to a 5.5m RIB, to help get it on the plane quicker and easier. To do this we'll have to drill holes through the original plate to bolt them on.
Has anyone fitted these and will they improve our performance, and will they weaken the original plate? Will they cause us any problems?

Well, without wishing to state the obvious, why don't the engine manufacturers make them with larger plates in the first place?

Been there done that, we thought that they didn't make that much difference.
Eventually they cracked and fell apart too.
I wouldn't drill holes in any part of an engine (not anymore anyway).
Actually we got far better results by trimming the engine rake angle and loading the boat properly.
If you can't get the boat on the plane quickly enough then there are a few ways to remedy this.
Adjust the engine rake/tilt to help get it off the plane, trim back when on the plane.
Loading the boat correctly.
Lessening the weight carried. (sounds too obvious, I know)
Make sure you have the correct pitch prop for the task in hand.

You don't say whether you've tried these other things, so I've listed all I can think of off the top of my head.
(I don't want to sound patronising).

Can anybody think of anything else?

[Mike Halligan]

Frankly, I dunno, but I doubt it.

Before attempting surgery on your engine - which the manufacturers would have done if it was likely to help - think how does performance compare to other boat/engine combos of your experience.

You don't say what you're trying to carry. If getting onto the plane is a worry and staying there is not, do you have the right leg on the engine (does the prop completely clear the hull)? You've got 51kW on a 5.5m which doesn't seem a lot to me.

We have 66kW on a 5.3m. It feels comparatively slow to reach the plane with 4 poeple, 4 sets, 5 spare cylinders and an O2 kit. (It flies with just the 4 people!)

We also have 96kW on a 6.0m. That will labour with 8 people, 8 sets and 8 spare cylinders but gets on the plane even so. (It also flies with just the 8 people.)

The balance of engine, load and pitch is a tricky one to achieve, but a bit of reality seepage when considering load would probably help even more than trialling different pitch.

Looking back over our experience, 30 kW extra power shifts 4 divers, kit and spares in the bigger boat, you have 15kw less than our smaller boat. Do you have the grunt to shift 4 divers and their kit onto the plane if you're also carrying second cylinders?

Hope this helps,

Mike

[Neil Radcliffe]

Have a look on the seperate RIBnet forum - highly recommended for lots of useful RIB info - I'd suggest doing a search on 'hydrofoil' and also 'doelfin' which seems to be the market leader.

When I did this search a while ago there seeemed to be a range of opinion spread between 'no difference' and 'works well' - with not much if any on the more negative side. Ride quality through rough seas is another factor which generally seemed to benefit.

Propeller swapping is another alternative - but that seems even more of a black art - and at twice the price of a hydrofoil a potentially much more expensive one too.

One thing I never saw mentioned, but picked up from a prop / engine repair shop - if you do happen to have a lower-leg incident, the foils can contribute to significantly more damage e.g ripping off parts of the cavitation plate.

The reason I'll probably go for one on my RIB is in the hope of lowering the planing speed. At the moment there's a large gap between its max displacement speed and its min planing speed, and in rough water the latter can be just too uncomfortable. But that's probably hoping for the impossible!

Why not have a word with Steel Developments - they are both propeller experts and also sell the doelfin - and have a good reputation for sensible advice.

Neil

[NeilB]

In our equipment shed are two pairs of these fins. The anti-cavitation plates of both 75hp engines have holes drilled in them and welds where cracks have been repaired. The fins where removed before I joined the club and may not have caused the damage but you would have to have OJ's lawers to persuade me.

[jason clarke]

Thanks everyone this has helped. We are trying to make the best out of a bad situation.We we're given bad advice and I don't think we'll deal with this boat manufacturer again. The engine is great just under powered.
Cheers
Jason

[Simon Brookes]

We're going through a very similar scenario and are looking at lifting foils. To answer an earlier point about AV plate size, the reason they don't automatically fit foils if they are so good is quite simple; firstly they cause extra drag that isnt needed on correctly matched engine and hull, secondly they act to lift the rear of the boat this is what we want but isnt what a displacement application requires this would push the bow down and cause ploughing.

One reccomendation was to correctly prop the boat pitchwise and to go for a 4 bladed prop as this gives more torque but less top speed. I dont know yet why all dive boats dont use a 4 bladed prop but will find out.

[nickjaxe]

We're going through a very similar scenario and are looking at lifting foils. To answer an earlier point about AV plate size, the reason they don't automatically fit foils if they are so good is quite simple; firstly they cause extra drag that isnt needed on correctly matched engine and hull, secondly they act to lift the rear of the boat this is what we want but isnt what a displacement application requires this would push the bow down and cause ploughing.

One reccomendation was to correctly prop the boat pitchwise and to go for a 4 bladed prop as this gives more torque but less top speed. I dont know yet why all dive boats dont use a 4 bladed prop but will find out.

Have you a rev counter on your boat, without one its gess work, if you are not getting full revs normally around 5500rpm with the normal load on the boat the engine will not give full power, as the other guys say most of it is to do with the correct pitch prop, but with out a rev counter your in the dark,as for the fins my mate put them on a dory, they made it very very twitchy at speed, not very nice, I would think your 70 should be up to it, we have a 5.1mtr with a 65 suzuki with 5 up + kit it goes very well.

Nick.

[Si Brookes]

We have a rev counter thats the reason for concern when loaded and in slight to mderate swell (nothing too big) we are only getting 4400 to 4700 and this is producing a low 12kts SOG, all at full throttle so fuel is just being emptied through the engine. If anyone else has a Delta with a Yam 100hp 4 stroke I'd be interested to hear of there set up.
When conditions are nice and flat and we leave a crew member off it improves a lot. But there is no power reserve and response is slow it labours up to the speed and is easily knocked off the plane.

[nickjaxe]

We have a rev counter thats the reason for concern when loaded and in slight to mderate swell (nothing too big) we are only getting 4400 to 4700 and this is producing a low 12kts SOG, all at full throttle so fuel is just being emptied through the engine. If anyone else has a Delta with a Yam 100hp 4 stroke I'd be interested to hear of there set up.
When conditions are nice and flat and we leave a crew member off it improves a lot. But there is no power reserve and response is slow it labours up to the speed and is easily knocked off the plane.

4500 to 4700 is no good, you need to get it up to 5500rpm, no wonder it wont go, sounds like you need a finer pitch prop, but do make sure your engine does not have any other probs that are limiting the revs such as a faulty low oil warning sys ect.

Nick in cheshire.

[Simon Brookes]

Pretty sure its the pitch of the prop, likely candidates are 13" or 15" 4 bladers Steel Developments reccomended the 13" which is as low as you can go to recoup the near 1000RPM we are lacking. But am a bit loath to fork out on spec and get it wrong, not that I do ofcourse!!

May still have to fit the foils to counter the extreme stern down aspect.

[nickjaxe]

Pretty sure its the pitch of the prop, likely candidates are 13" or 15" 4 bladers Steel Developments reccomended the 13" which is as low as you can go to recoup the near 1000RPM we are lacking. But am a bit loath to fork out on spec and get it wrong, not that I do ofcourse!!

May still have to fit the foils to counter the extreme stern down aspect.

Simon I dont think there would be much differance in price between the foil or a new prop and it does sound like you need a new one keep your old one as a spare, some of the dealers will let you change a prop if the one they give you is not quite right anyway one worth his salt will know what pitch you need by telling what revs you are getting now, I dont see the need to change the dia by the way just go down in pitch but keep the same dia as the engine came with, when they say go down an inch the dont mean 15 to 14" dia but pitch which is the angle of the blades.

Nick.