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Old 21 December 2008, 10:42   #1
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Country: UK - England
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Mercury 4 stroke 30 HP

Hi all

I wonder if any of the experienced of you can help me out?

I have a 5 metre GRP fishing boat with a 4 stroke Mercury 30. The prop is stamped with '13'
Flat out with two big blokes & fishing gear/twin fuel tanks etc it will do 18.5 knots on the chartplotter, but it's only pulling 4500 rpm.
The motor is fully serviced & runs sweet.
Would a change of prop yield any more speed (allowing the motor to achieve it's peak power rpm of 5400?

Sorry it's not RIB related but I'm hoping the knowledge carries across?

Any advice gratefully recieved.

Regards

Dave
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Old 21 December 2008, 13:21   #2
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There must be more that 13 on the prop. The prop has two number diameter (in your case probably 13") by a pitch. The reader will need to know the pitch of the prop before they can give you advice. The type of boat or the all in weight of the boat may also assist them in this case. Basically you need to come down in pitch probably by approx 4" at a guess
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Old 22 December 2008, 00:12   #3
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Installing a tacho will be the way to go to play with prop pitces, a decrese in one size will give about + 200 more rpm's, so will asume you are 4 sizes off to match best wot top speed.

Happy Boating
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Old 22 December 2008, 08:26   #4
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The newer Merc props on the small four strokes only have the Pitch stammped on them. So that will be a 13". I would put a 10" pitch on it, rev the nuts of it and you will get better performance and better fuel economy out of it too.
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Old 24 December 2008, 20:17   #5
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Thanks for the replies guys, much appreciated.

I'll order a 10" pitch & see how it goes.

Where's best to buy a prop from?

Regards

Dave
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Old 25 December 2008, 03:45   #6
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Try Gregg at G B Watersports, 01733 322108. He will probably be your closest.
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Old 25 December 2008, 14:49   #7
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Thanks Ian.
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