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Old 14 April 2007, 16:33   #1
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Prop size

Help needed
Boat 3 up 3/4 to full tank
1st Prop standard 23" black S/S Yamaha W.O.T 6200 rpm 43 Kts
2nd Prop 22" Yamaha polished S/S pro plus as above went to rev cut out 43 knt
3rd Prop 24" Yamaha Polished S/S pro plus as above W.O.T 5500rpm 45 kts
cavitation plate sits 3" above bottom V of hull.
my last rib was 1"&1/2 above.
Dose any body has any experience i thought i should get a bit more speed
but the bigger prop at lower rpm is faster ? any thoughts please
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Old 14 April 2007, 16:59   #2
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Take it conditions were the same all round ref wind etc, also on same run & tide, ... and was speed taken from GPS ? since running with and against tide in some areas makes a hec of a difference. 5 Knots of tide against you for example could explain your lack of apparent speed gain (sorry Roy I only suggest without knowledge of your experience) Also how long is 8m+
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Old 14 April 2007, 18:50   #3
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Dose any body has any experience i thought i should get a bit more speed
but the bigger prop at lower rpm is faster ? any thoughts please
I don't have any Scorp experience but your figures sound about right to me unless Scorps are built pretty light. 250hp isn't a lot of power for an 8.5mtr boat

You mention the height of the motor, is there a prop holding problem?
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Old 14 April 2007, 19:23   #4
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If your prop is gripping in a corner and not ventilating at top speed then I would guess that outboard hight is OK.

Agree with above....Big RIB and 250hp sounds like 45 knots is about right

You might like to re-prop for different circumstances like i used to. ie: smaller prop for heavy running and poor sea state. Larger prop for lighter running and smooth sea state.
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Old 15 April 2007, 03:38   #5
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I have an 8.5 Scorp with 275hp and get around 49 kn with a 19 inch prop, the 21 gives me 50 kn+ but seems a little sluggish at cruising speed acceleration. My 8.1 stepped hull Scorpion which was a pretty light boat and ran a 225 Opti was capable of 53 kn so looking at things I guess that you wot top speed is about right. The interesting things is that the 8.5 is so much smoother that in the real world it is a quicker boat over a distance with varying sea conditions but the way it delivers the speed is so relaxed it feels slow. For me the 8.5 is a much better boat than my 8.1 was.

However the easy way for you to hit 50 kn+ is buy one of the new 300 hp engines that are out there!!!! Oh and you could buy me one at the same time if you are feeling generous.
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Old 15 April 2007, 06:07   #6
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havin similar experiences , bigger pitch went faster at 300 rpm less untill I had the usual 3 people and gear on board and a bit of chop then it was slower .
Only positive thing is I went from a 17 to a 19 so if i can find an 18 Ill be sorted .

What I need now is quick release re usable split pin for the prop nut as Iam changing them a lot.
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Old 15 April 2007, 16:59   #7
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Thanks for the help
Yep same conditions inside langton harbor. (very naughty i know)
Boats 8.1mt
Ollie i tried the 8.5 mt and it dose feel better but i keep my boat at home and the 8.1 is as big and i can get into the farm its also a big rig when towing and launching.
maybe i am expecting to much
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Old 16 April 2007, 14:50   #8
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Hi Roy

Did not realise its a 8.1, does seem a little slow to me. Is she a heavy boat. A frame/radar etc? Have you had any other engine fitters look at the setup?
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Old 17 April 2007, 17:30   #9
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Have asked BHG of Lymington Apparently UK biggest Yamaha engine supplier
he said that when they set up cobra ribs they put the cav plate in line with the v of the hull, Scorpion say this should make more drag = slower top end etc and they put it much further up.
i am going to try a 23" pro S/S should give a bit more but looks like i will never see 50 kts . To be fair the tests have all been done with literary a full tank of fuel 400 lts and 3 to 4 people so am i wanting to much.
I,ll post the speed with the new prop
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Old 17 April 2007, 18:15   #10
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To be fair the tests have all been done with literary a full tank of fuel 400 lts and 3 to 4 people so am i wanting to much.
Roy, that's over half a ton of load. Depending on the weight of the folk, it may be well over.

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