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Old 30 April 2015, 16:57   #1
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engine hp

Been asking about getting my SIB on plane in the inflatable forum. Had some great answers and help there but it got me to thinking.
Why are boat outboard so weedy per cc. For example, my first motorbike was a Yamaha fs1e which was 50cc and made 4.8hp I the had various 125's which were restricted to maximum of 12hp (although they mostly made around 10hp) but were really de-restricted to give 16hp or so.
Now looking at most 6hp outboard motors (2 stroke) they seem to be around the 100cc mark.
So why is it that we only get 6hp from a 100cc motor.
Any thoughts?
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Old 30 April 2015, 17:40   #2
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They've got to last longer and develop the power at lower revs with a lot more torque.
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Old 30 April 2015, 17:41   #3
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HP isn't directly related to cubic capacity,for instance,a Tohatsu 20hp 4 stroke is the same cc as the 15hp and the 9.8hp carburettor jetting is what determines the HP in this case.
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Old 30 April 2015, 17:45   #4
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Last longer? Not so sure. My 50cc Bike was 25years old when I got rid and still going strong. It made 4.8hp at about 5500rpm iirc. It had done 33000miles. Not sure how many hours that equates to but it a lot and 90% of those hours were spent at WOT and full revs!
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Old 30 April 2015, 17:54   #5
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Yes, I understand it's not directly related to cc but there does Seem to be a generally lower power output per cc for boat engines than bike engines.
Just makes me wonder why there isn't more of an aftermarket tuning market for outboard motors. Mine is a 6hp 100cc which I know should be able to make 10hp really without stressing it
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Old 30 April 2015, 17:55   #6
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I bet it had been rebuilt a few times in those 33k miles though.
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Old 30 April 2015, 18:25   #7
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Yes but unlike your bike the outboard only has one gear remember.
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Old 30 April 2015, 18:25   #8
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Is there a bigger HP model of the outboard you have,? If so check through the parts catologue and see what's different.Thats all I done and got 30% extra power by changing carb jets.
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Old 30 April 2015, 18:34   #9
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My son rides a kx85 which is 29bhp @ something like 14000rpm but you don't dare go past the recommended 20 hours for a new piston or your paying for a barrel too
People have tried bike engines in boats thinking they have great power to weight ratio but they just don't work as it's lots of torque you need for a boat.
In boat engines there realy is no substitute for cc ' s hence the popularity of big lazy American v8 ' s as inboards
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Old 30 April 2015, 18:38   #10
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Is there a bigger HP model of the outboard you have,? If so check through the parts catologue and see what's different.Thats all I done and got 30% extra power by changing carb jets.
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