Outboard engine fins

[catalac08]

Hi
The sellers of these retro-fitted fins like Doel fins and other makes claim that some of the prop thrust is diverted to give some "lift" and improve thrust and claim better speed and fuel economy. My boat (cat) squats under power and I planned to modify the transom angle to reduce this squatting but got bogged down doing rigging , engine, sails etc so finished major jobs now to do some sailing. I am wondering whether to fit some fins but am not sure whether this would put excessive loading on the cavitation plate when the boat is bouncing a little.

So questions to peeps with experience of these:-

do they actually give any lift-and would this be measurable on a 3T boat.?
do they improve economy?
any speed benefits?
any experience of cavitation plate breaking or suffering under the undesigned for strain?

thanks
keith

 

[whisper]

I've used them on lightweight SD motorboats and they give lift, improve top speed (slightly), decrease time to get on the plane, improve fuel consumption. The only downside was an increased tendency to broach when being chased by a following sea.
Sorry but I've no idea if at your size and assumed displacement speed, they will be useful or not.

 

[Csail]

Hi
The sellers of these retro-fitted fins like Doel fins and other makes claim that some of the prop thrust is diverted to give some "lift" and improve thrust and claim better speed and fuel economy. My boat (cat) squats under power and I planned to modify the transom angle to reduce this squatting but got bogged down doing rigging , engine, sails etc so finished major jobs now to do some sailing. I am wondering whether to fit some fins but am not sure whether this would put excessive loading on the cavitation plate when the boat is bouncing a little.

So questions to peeps with experience of these:-

do they actually give any lift-and would this be measurable on a 3T boat.?
do they improve economy?
any speed benefits?
any experience of cavitation plate breaking or suffering under the undesigned for strain?

thanks
keith

Main bits i noticed was quicker to get on the plane,much more stable.And if you do fit them take the spanners with you as they always need slight trim.Also they stop dolphoning effect.

 

[AndieMac]

Probably not a benefit (afaik) for a displacement yacht but....

I never heard of them causing cav. plate damage, they are generally best suited to weighty, outdrive powered planing hulls, where, in my books the Stingray is the best all round performer, as the whale tail is located well aft and the surface area is large enough.

Operational sensitivity is increased and the boat becomes more responsive at cruising speed, but entirely understandable when you think about what's been added to such a crucial area.
Same with running down-sea, the drive position needs tweaky adjustments to set the leg height correctly for the given wave conditions.

Examples where a high top-sided boat (7 to 8 metres) under acceleration from D to P would lay on one chine (lateral instability) until levelled by a trim tab, would, with the addition of a proper foil, come up square and sit flat.