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Old 20 April 2006, 05:38   #1
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Country: UK - England
Town: Felixstowe
Boat name: WhaleOilBeefHooked
Make: Ribeye
Length: 5m +
Engine: Yamaha F80
Join Date: Nov 2004
Posts: 66
Prop Pitch Yam 80HP 4-Str

Myself and a mate have nearly identical boats and both have the Yam 4-stroke 80hp.

I have a 19K Pitch and he has a 21K Pitch.

Yam handbook suggests WOT range 5000-6000rpm

Depending on boat load & trim I get between 5300-5700rpm WOT

Similarly he ranges from 4700rpm-5000rpm WOT
(Obviously he has better top speed and I accelerate out the hole faster)

Question: Am I revving too high (wearing engine) or is he revving too low? (straining engine)
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Old 20 April 2006, 06:08   #2
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I would say you are about right - you are smack in the middle of the recommended WOT range.

I doubt your friend is straining his engine at only a few hundred rpm lower but he will be slightly off the rated 80hp if his WOT rpm does not fall between the recommendations of the manufacturer.

The WOT recommendations are usually the rpm range at which the engine is producing its rated hp any lower and you will have sacrificed some hp.

by my calculations
5700rpm with a 19" prop should give just over 36 knots
5000rpm with a 21" prop should give just under 36 knots

are you sure he has the edge on speed - if so are you sure the boats are equally loaded when doing the comparison and are you sure of the 5000 max rpm on your friends boat.

IMO - from the figures you've given me - 19" sounds the perfect prop.
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Old 20 April 2006, 07:07   #3
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Country: UK - England
Town: Felixstowe
Boat name: WhaleOilBeefHooked
Make: Ribeye
Length: 5m +
Engine: Yamaha F80
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Correction (Ahem!)

Checked with my mate and his Max RPM is in fact 5300 (lightly loaded)

Your calculations are spookily correct - My best speed ever is 36knts

I guess Gary must be getting 38-39 (I'm sure your about to tell me )

Cheers
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Old 20 April 2006, 07:15   #4
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Yes - just under 38 knots at 5300.

nothing spooky about it really, its a simple calculation - I put the maths into a program which you can download from a link in the thread below. you will need to have installed the microsoft .net framework v2.0 though to run it.

Free Prop Calculator
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Old 20 April 2006, 08:22   #5
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Town: Clitheroe
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We must have a very slow hull then, same engine and with supplied prop (unmarked) we got 30knots, at about 5300- 5400rpm, 2 people on bord.

At the 10 hour service had the dealer change the prop now it does 25knots at 5000rpm, same load, this prop is also unmarked and both are white. We were hopeing to get a bit more speed.

Should we change back to the orgininal prop or go smaller than that? The engine has always struggled to pick up from a standing or slow start, before its on the plane. More so with the new one.

Any reccomendations its a Zodiac pro 12.

Thanks
Alex
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Old 20 April 2006, 08:48   #6
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Country: UK - England
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Boat name: WhaleOilBeefHooked
Make: Ribeye
Length: 5m +
Engine: Yamaha F80
Join Date: Nov 2004
Posts: 66
What electronics do you use to calculate your speed?

Could be faulty or inaccurate?
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Old 20 April 2006, 08:52   #7
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GPS, garmin 178c
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