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Old 15 August 2013, 12:11   #1
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Explode the myth for me please

Looking at fitting a 1998 90 hp 2 stroke optimax to my Humber rib. This will be the max the hull can take. Currently on a 1993 60 hp Merc. I also have the option of a similar vintage Yamaha 60 hp. I am told the horse power figure on the opti is measured at the power head whilst the yam is measured at the prop.

Can anyone shed any light on this.


Cheers in advance


buzzin
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Old 15 August 2013, 12:13   #2
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If the info is true about the Opti what sort of HP will it output at the prop?
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Old 15 August 2013, 12:41   #3
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They are all rated at the prop. Sometime in the mid-1980s was the switchover from rating at the powerhead to the prop.
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Old 15 August 2013, 13:00   #4
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Yep, they're measured at different points due to age. Not too much in it so don't worry unduly.
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Old 15 August 2013, 13:08   #5
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What is the typical loss from power head to output at prop? Will it be less than 5%?
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Old 15 August 2013, 13:26   #6
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Quote:
Originally Posted by captnjack View Post
They are all rated at the prop. Sometime in the mid-1980s was the switchover from rating at the powerhead to the prop.
What he said.

It was about 1986 for the changeover from powerhead to prop rating for Merc/JohnRude, but the Yamahas were always rated at the prop anyway.
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Old 15 August 2013, 13:45   #7
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Thanks for putting me on the straight and narrow chaps.
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Old 15 August 2013, 13:56   #8
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What he said.

It was about 1986 for the changeover from powerhead to prop rating for Merc/JohnRude, but the Yamahas were always rated at the prop anyway.
I think chrysler/force was the first to change around 1984. Not that you'd want one of those ancient POS anyway.

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What is the typical loss from power head to output at prop? Will it be less than 5%?
More like ~20%
E.g. 140hp at the powerhead was about 110hp at the prop.
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Old 15 August 2013, 14:04   #9
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I think chrysler/force was the first to change around 1984. Not that you'd want one of those ancient POS anyway.



More like ~20%
E.g. 140hp at the powerhead was about 110hp at the prop.


Yes and that would have been with rubbish oil, transmission, bearings, prop design, dynamometers etc.
So the differencials would be much greater then than now.
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Old 15 August 2013, 15:32   #10
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I didn't think they made a 90hp optimax in 98?

I thought it was only 135-150 2.5 litre V6 and the 200-225 3.0 litre V6 back in 98 & the 3 cylinder optimax only came along much later?
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