Foil for 40hp mariner

[Hpd]

Hi I’ve got a 2008 40hp mariner engine which I think would benefit from Doel fins /foil. Just wondering what’s the recommended one or size to go for?

A quick google pops up with a variety of generic looking ones for ‘up to 50hp’

Any recommendations would be great!

[Nos4r2]

IMO no rib benefits from doelfins unless it's massively underpowered.
Doelfins are a bit marmite. IMO They do have their place (if the boat is underpowered and overloaded for one, where the boat will never reach any real speed and is struggling to plane) but in my experience at speeds over 25 knots and/or on a motor with power trim, they're bloody dangerous, with tiny trim adjustments causing major and unpredictable fluctuations as the stern is forced down and the tubes are forced into the water.

What makes you think your setup would benefit?

[Hpd]

Ah okay thanks for advice. Everything I’ve read so far points to them being the answer to a stern heavy setup.

My 4.5m rib has a bench rather than a jockey seat and the bow is high until it planes so thought a foil might help. I think the 40hp is about right so I wouldn’t say it’s underpowered.

Will have a think on this one

[Nos4r2]

Originally Posted by Hpd

Ah okay thanks for advice. Everything I’ve read so far points to them being the answer to a stern heavy setup.

My 4.5m rib has a bench rather than a jockey seat and the bow is high until it planes so thought a foil might help. I think the 40hp is about right so I wouldn’t say it’s underpowered.

Will have a think on this one

What's the max HP for the boat and does it have power trim?

[Hpd]

Max is 60hp and yeah it has power trim

[Nos4r2]

Originally Posted by Hpd

Max is 60hp and yeah it has power trim

I'd be tempted to grab a bigger motor instead. Should be able to sell a good 40 for the same ballpark as a 60 so wouldn't cost much.

The whole power trim and fins thing...In my experience they act like hydroplanes on a submarine when you trim out. It feels horrible and I suspect puts quite a lot of stress through the engine and the boat..

[paintman]

At low speed fins/foils may not make any difference.
Will only show an effect once the boat is travelling fast enough for them to provide lift.
So you will still be bow high until that happens.

That said, might get you onto & keep you on the plane at lower speeds.
I've got a permatrim on my Mercury & it does exactly that.
It's basically a large rectangular piece of aluminium bolted to the cavitation plate. www.permatrims.com
I've had no handling issues, but it's a dory not a rib so YMMV.

[Peter_C]

I had a Stingray fin on my 4.2m SIB with a 40hp 2 stroke and LOVED it! It would plane at around 12mph and come up almost flat with a manual trim motor. When loaded with 8 steel 100cf scuba tanks and gear for 4 divers it got on plane in a reasonable amount of time.

Fast forward to my current RIB which is a 4.5m rear seat setup and I am undecided. I will say when trimmed down and running close to full throttle it pivots on the bow which is a very uncomfortable feeling. I took it off and plan to run more testing without it.

Personally I would never run a Doelfin as they are two piece units and do not have the lift nor strength of other brands.