Fitted Doel Fin to Mercury 25HP

Posted: Wed Feb 04, 2009 3:54 pm

Rabbi2 mentioned the doel fin in reply to my first post in this forum so I found out a bit about them and decided to fit one to my Mercury 25HP.
To fit the fin using the pre drilled holes meant drilling through the anode. I didn't fancy that so, with my brother's help, we drilled a fresh hole well clear of the anode using a wood spade bit to 'counter sink' the bolt and nut heads before drilling through with the correct size steel bit.
The first time we took the boat out last summer the turbulence from the prop was throwing tiny fish and sea 'fleas' over the transom. If this fin directs that turbulence astern I can well imagine the bow being forced down(so we might need to adjust the engine position)and the boat moving more quickly forwards. We shall see.

Posted: Wed Feb 04, 2009 4:13 pm

You should find that you can come up on the plane a lower revs thus saving fuel also it will give you berretr steering at low revs.
Cheers keith big grin

Posted: Wed Feb 04, 2009 5:57 pm

hi feenagh, looking at the dol fins that you have fitted i think you may find they are too big for your engine they come in two sizes and should fit snugly to the cavatation plate not hang over the end they look like the ones designed for z drives or engines over 100hp

Dave big grin

Posted: Wed Feb 04, 2009 5:57 pm

Feenagh, I have owned and operated boats for the best part of thirty years including the same hull as your craft. I have never experienced the the engine throwing anything clear of the water, or even seen anyone else suffering these antics.
1. That boat should be fitted with a long shaft engine. Is the engine you have fitted a long shaft?
2. If it definatly is long shaft have a word with someone experienced with boats to ensure it is mounted and trimmed to the hull.
The cavitation plate (the plate you have mounted the fins to) should be slightly below the lowest part of the hull with the engine leg in the driving position.

Regular Joined: Sep 14, 2008 Posts: 424 Location: Burnley

steady tomcat your coming up to 100 posts winking

Posted: Thu Feb 05, 2009 12:51 pm

Thanks for the replies and comments. The fin was sold as suitable for engines from 8-115HP but I agree it does look large. The engine, which is long shaft, has been trimmed in and out using the tilt pin to find the best position for performance. The cavitation plate sits about 1" below the hull.
Water has always slopped over the transom at speeds over 10 knots or so.
I hope I've done the right thing in fitting the fin and I'll soon find out.
Regards Paul

Posted: Thu Feb 05, 2009 12:57 pm

Can you photograph the boat so as to give us lads some idea of the problem Better still one from the front of the boat, one from the side and one from the rear with the engine in as near the driving position as possible. Cheers keith big grin