Fins or foils

S

Scary_Des

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Hello (hope this is the right place for this)

I am interested in people’s opinion or experiences of the effects of fins or foils that are attached to the anti-ventilation plate on an outboard.

I have heard anecdotal evidence that that they can cause instability in a small boat (5.5Mtr) especially when turning or when working in heavy weather conditions. Any help would be really appreciated.

Des
 
I have used these on two different craft with different results.

I have a 24' aluminum power catamaran that is relatively heavy and, of course, has relatively little planing area compared to a monohull. I use the SE Sport model hydrofoil on my twin Johnson outboards, and it has made a great improvement in planing characteristics. I can now plane easily with heavier loads, and in following seas, both of which used to be problem areas. I don't experience the cavitation problems sometimes experienced with monohulls, because the catamaran turns flat and doesn't "lean". I also don't experience any instability at max speed (about 38 knots), again probably due to the fact that even a planing catamaran rides a little deeper and doesn't skip across the water quite like a monohull.

I noticed a slight improvement in fuel economy (easily visible because I use a twinscan fuel totalizer), but I think the the improvement is due primarily to the fact that I send much less time laboring to get on plane.

I also tried a foil on my little flat bottom aluminum boat with a 50 hp outboard, and the results were nothing short of scary. The boat is light to begin with, and prior to adding the foil, the waterline was about 4-5' forward of the transom when planing. After adding the foil, there was so much additional lift at the stern that there was practically no hull at all left in the water. Even in calm water, the boat was overly sensitive the the controls, and in my opinion, unsafe. I removed the foil. In case you were wondering, I only tried it because a neighbor found it wouldn't fit his motor and gave it to me.

Hope this helps,

Jeff D.
 
Thanks Jeff that is helpful.

The specific boats I am interested in are deep V RIBS of less than 5.5 mtrs(17’) this type of boat is become very common in the UK and are generally chosen for their sea kindly ride and good heavy weather handling characteristic.

A RIB will lean happily in a turn but what worries me is that with fins fitted that lean will be increased or become unpredictable.
From what I have seen in the UK these fins are not specifically designed for a particular boat and if you consider that two boats of the same length could be completely different weights how can one product meet both needs :confused:

I think that what Jeff’s reply highlights is that on larger more stable boats the effect can be mild to beneficial but on smaller boat the effect is dangerous :eek:
Des
 
These fins give you lift at the motor. A lighter, faster boat probably has most of the boat out of the water anyway. Why would you want to lift the stern? I had a heavy built 15 foot trihull, like a whaler, with a 70 HP outboard. I tried the fin. It pushed the nose over and nocked about 4 MPH off the top speed. It wasn't "dangerous" but certainly didn't help one bit. Luckily they were easily removed :)
 
woodboat said:
........ Why would you want to lift the stern...........
Click to expand...
Again this is something I don’t understand :confused: but a lot of people are putting together RIBS of about 17’ long with anything up to 150 hp outboard on the back and then loading the transom area with dive equipment, spare fuel and people. My view is that they should buy bigger boats or expect less from the one they have. But what seams to happen is that people fit fins and think they have solved the problem while in fact creating another:eek:
Des
 
Ive never seen them used on large engines that you guys are talking about. However they are very common around here.If you had for instance a small yacht tender of 2.5 mter and a 8 hp yanamar that would get on the plane with you in it but not with the wife. Yu can,, A leave the wife at home,,B get a smaller wife (popular these days) Or C, fit fins. They work very well under these circumstances.
 
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