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Old 12 May 2003, 13:31   #1
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Country: UK - England
Town: Tisted Hampshire
Boat name: Hot Lemon
Make: Scorpion
Length: 8m +
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More about Doel fins

A member kindly donated a set of fins (Thanks Mark) duly fitted and have now tested with interesting result .
Now getting 5950 rpm at WOT b4 it was 5650 and rev limit warning is now coming on . 42 mph in flat water .Does this mean I need to change the prop or should I remove the Fins ?
All opinions awaited !
PS Towed Outanabout new 7m Humber back home with no drama fab boat and now very jealous ,he's away for a few day so might borrow it b4 he gets back.
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Old 12 May 2003, 19:06   #2
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Had my fins off Friday in plymouth, and could not beleive it!

Prop. was cavitating all the time, had no grip, a real struggle to get onto the plane...

Reason for removing them was to try in conjunction with new 19" prop for sking - when all is fine but with my much larget 24" prop the boat is undrivable.

Re-fitted that evening and all behaving well again. Swear by them now!

As for your situation i guess you could go up in prop size... but surely there must be a disadvantage of this new found speed from nowhere??? My boat is not too good in tight turns - could this be the pay-off?
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Old 13 May 2003, 03:15   #3
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Country: UK - Wales
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Make: Ring Powercraft 685
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Fitted them to my little Humber, which makes her a lot more driveable in a confused sea. She responds better to trim, grips well on turns, doesn't porpoise, and planes at lower speed. I could also do with going up a pitch on the prop I think.

Has also cut down on the prop slip under hard acceleration, though my personal feeling is that this problem is a function of the blade shape of OMC's SST. Still not got to the bottom of that one - any ideas?
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Old 13 May 2003, 03:22   #4
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I have a doelfin fitted and noticed t'other day that I could stay on the plane at 14kts (albeit against a 2 kt tide) in a 7.5m boat! For some reason I found this tremendously impressive. I am not sure that a doelfin in itself should make that much difference to the RPM you are able to pull and hence the pitch of prop you can run?
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Old 13 May 2003, 04:48   #5
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Agree entirely, but, as I said, I haven't got to the bottom of the slipping problem yet!

When I bought the Humber, the o/b was one hole higher, which meant superb top end on flat water, but no scope to trim in rougher stuff. Then, prop slip on acceleration, and cavitation in turns was drastic to say the least. I took advice from Humber and Jets Marivent which resulted in a change from a the standard Al prop to their SST stainless, albeit of the same pitch and blade profile ( tapering considerably to the tips). I also went down a hole.

This improved handling, but not prop slip. She will also still run to over 6000rpm flat out (too high). The fins have helped some more, but I still feel that I need more "grip" on the water.

I asked "Steel Developments" who have suggested a 4 blade prop, possible one pitch coarser.

Any ideas?

The Doelfins are brilliantr for towing kids on inflatable toys too by the way!! But beware of imitations, they rarely work as well.
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Old 14 May 2003, 16:43   #6
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A nice new shiney pair of DoelFins arrived this morning from Steel Dev. to be fitted and tested this w/e.

Hopefully they will stop the boat trying to go into orbit when going up on the plane [Scurvey crew keeps complaining that it spills his coffee ]

Or maybe I should just get rid of the A-frame & boxes
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Old 16 May 2003, 07:08   #7
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A Frame & Boxes

Hi Ribald,

I would like to stake my claim for the Boxes would look very nice on my A frame

Have fun on Saturday.
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