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Old 15 September 2016, 14:10   #1
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Country: UK - England
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Prop size on a sib

Hi I've just got a new to me zodiac zoom 310sr solid grp hull with a 10 Hp mariner 2 stroke, my question is what prop are others using with a similar boat ? Mine has a 9 X 9 prop it goes ok but I thought it would go a little betterClick image for larger version

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Old 15 September 2016, 14:34   #2
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Country: UK - England
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Load the boat with what you would be carrying in its normal use.
Look up the engine manufacturer's spec for your engine. You are looking for the wide open throttle (WOT) rev range.
Run the boat at WOT. Try trimming the engine in or out for best result. Move yourself in the boat - all the weight at the stern is likely to adversely affect the ability to get on the plane & a tiller extension can make a big difference. See if the rpm is within the range specified. Once you've got the figures you'll get better advice as to what - if anything - you need to do prop-wise.
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Old 15 September 2016, 15:19   #3
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I've tried the trim and moving around in the boat but just don't think it's getting near max revs from the sound of it but I don't have a rev counter so it's hard to tell, just wondering what prop others were using with a similar boat
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Old 16 September 2016, 03:32   #4
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Is it really a small RIB with GRP hull/transom... of a SIB with GRP floor and inflated keel?

How many folks and what else are you carrying when it doesn't seem to go well? In truth unless you have a GPS speed and OB revs it's hard to advise. How about buying an Ebay 10 rev counter to get an idea of what's going on?

9" is the standard prop for these motors and I've found it copes well on pretty well all smallcraft and average loads. I'd be guessing if yours is a true small RIB you should be seeing around 17kts... perhaps nearer 15kts if it is a soft bottom SIB.
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Old 16 September 2016, 08:29   #5
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It has a grp hull and a folding transom so not totally rigid. I will try and get a rev counter and see if I'm getting max revs
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Old 16 September 2016, 14:02   #6
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All good advice above. Agree get a cheap tach and check revs at WOT.
Just to give an idea, our alumin floor / inflatable keel SP350 Zoom bowls along one up at 18knots/21mph with a 9.8 Tohatsu running the stock 8.5" dia. x 8.5" pitch prop. Even two up will do17knots/19mph on a good day.
Yours being a rigid hull should better this.
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Old 16 September 2016, 14:19   #7
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I will get a cheap rev counter and give it a try when I can get back on the water, anyone got any recommendations on a rev / tac ?
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Old 16 September 2016, 14:27   #8
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Have a look at this thread....

Best Tiny Tach type
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Old 24 September 2016, 17:50   #9
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I have a Zodiac 285s and Tohatsu 9.8 hp.
With the standard prop l get 18-20mph at WOT as above.
RPM is 5500 (WOT range is given as 5-6,000)

I tried a higher pitch prop as the boat is so small. The max speed was the same, but at 5,000rpm.

I am not sure if this is better? It takes longer to get on the plane with two on board (with the standard prop there's no delay at all) but is running 500rpm lower at any given planing speed.

I've currently got the original prop back on.
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