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Down rating Larger/older motor :-)

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Here is a thought

You have a boat rated for a 25-30 horse motor

What I am going to do is find an older 40-50 horse (that's not too heavy)

And Prop it as a 30 horse would be propped. 13 pitch then run it up to 28 mph

Sure the motor could push the boat to almost 34 mph but an older motor that may have a weak cylinder or low compression will last for many years loafing along at lower output ...

Kind of like a race horse doing children's rides ...

It's all about Cheap & Good ;-)
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· BBA Counselor
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This makes little sense to me. Why put the wrong prop on a motor so it's over revving, and try to run it slower? You'll end up with a motor that won't performed in it's intended range well and have a short life. If you want an old 30hp, just go get one.
 

· Lowcountry Degen
2021 Conchfish 17.8
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How do you plan to limit RPM?

I think a better solution is to buy the lowest hp version of a given motor design. So in your case, maybe use the older Yamaha 2 stroke 40 as an example. Same block as the 50, but different carbs and (I think) exhaust manifold.

In theory, that 40 should last much longer than a comparable 50. In practice, however, it probably has more to do with maintenance and usage than power output, though. Once you get to the ragged edge of performance things might change, but most of these engines are so overbuilt that power output really isn't the deciding factor when it comes to service life.
 

· I Love microskiff.com!
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In terms of coast guard ratings do they rate that horsepower number at the prop? The flywheel? If it's the prop, what if you put on a jet lower unit? Effectively you're putting less power to the water, does the CG go by that number?
 

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Discussion Starter · #15 ·
With zero prop slip you'd be looking at sub 30mph with a 13 pitch at 5500rpm on most older outboards. Where are you getting 34mph?

The boat hit 32 with an 11 x 12 (3 blade) The next prop will be an 11 x 13 (4 Blade) The motor hit 32 with an 11 x 16 but was NOt winding out But I will see ... The boat is a Tunnel Hull so propping is very different ...
 

· BBA Counselor
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In terms of coast guard ratings do they rate that horsepower number at the prop? The flywheel? If it's the prop, what if you put on a jet lower unit? Effectively you're putting less power to the water, does the CG go by that number?
Since the early 80's I believe they have been rated at the prop.
 

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the older motors are already downrated. the manufacturers claimed hp at the flywheel. later the feds required rating at the prop shaft. at least a 5hp difference, maybe more. I have a 1970's merc 4 cyl 50 and a 2014 hatsu 50 TLDI, definitely a difference.
 

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Discussion Starter · #19 ·
Funny you should say that ... I have "9.9" 2 stroke and 9.8 4 stroke tohatsu the 4 stroke is a DOG !!!

The 2 Stroke is a "Scalded Dog" !!!

the older motors are already downrated. the manufacturers claimed hp at the flywheel. later the feds required rating at the prop shaft. at least a 5hp difference, maybe more. I have a 1970's merc 4 cyl 50 and a 2014 hatsu 50 TLDI, definitely a difference.
 
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