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Old 14 July 2009, 21:30   #1
Country: UK - England
Town: whitehaven
Boat name: jazz
Make: zodiac
Length: 5m +
Engine: outboard petrol 75
Join Date: Jul 2009
Posts: 1
i need some prop advice..

Hello reader.

Since a few weeks im the proud owner off a zodiac pro 550 with a mariner 75bp 4stroke powering it.
The current prop is a 15 pitch that runs at 5500 rpm at about 34 knotts on calm water.
I bought a 17 pitch prop on the pretence top end speed would be higher but the opposite happened i lost 1000 rpm and 7 to 8 knotts.
Could any 1 tell me what i should do propwise?
Or do i need to put a bigger engine on it to achive my goal?

thanks peter

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Old 15 July 2009, 02:52   #2
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Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
If you think of props like gears in a car - what you may have done is put too large a prop on. I would guess that you have reached the top speed for that setup. Of course you can experiment with different props with different characteristics around the same pitch, or move the engine up a bolt hole or two, but how much increase in speed do you want and how much are you prepared to sacrifice? You may lose acceleration and increase prop slip or reduce the handling of the boat to get 1 or 2 knots increase in speed.

Short answer yes get a bigger engine!

But ............ do not exceed the rating of the hull in terms of weight or power without careful thought and good advice.
Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 15 July 2009, 06:46   #3
Country: UK - Wales
Town: Here
Boat name: doggypaddle
Make: Avon 5.4 Searider
Length: 5m +
Engine: yamaha 80
Join Date: Jul 2007
Posts: 1,107
Originally Posted by greenleaf View Post

Could any 1 tell me what i should do propwise?

Put the 15 back on.....5500rpm at wide open throttle sounds about right, and 34 knots for that engine on that boat seems about right.
check the rpm range for your engine, or the band where the power is developed.
My thinking on this is if your rpm range is say 5000-5500 then for normal use you want to achieve 5500rpm with just one person on board and a full tank of fuel, that way as you load the boat up you come down into the "powerband"
If you carry lots of gear all the time you may want to achieve 5000rpm with all the gear on board and sacrifice the top speed unloaded in favour of loaded performance.
In short as cookee says you have run out of power with the 15" so theres no point going up a pitch, if you want to go faster get more power!
I am usually not as green as i am cabbage looking.
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