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Author Topic:   HP vs HP or 2 stroke vs 4 stroke

jameso posted 06-24-2003 09:00 AM ET (US)
Good Morning Whaler fans.
I had posted a query in the performance lane concerning powering a 15 SS LTD with a 50 HP Honda. Usual stuff, props ect. Got a couple of replies and some email concerning performance.
My question is this;
I have had three 15s with 50 HP engines, all were two strokes. The two strokes generally run 6K at WOT. Would not the same prop size for a 50 HP 4 stroke be the same as a 50 HP 2 stroke? The Honda is rated at 5700-6000 RPM. I know torque curves are different between the strokes but is not the overall output the same?
Thanks for any thoughts. Jim Armstrong
Peter posted 06-24-2003 09:47 AM ET (US)
Jim,
Prop size very much depends upon the final gear ratio. For example, in my response to you in the other thread using the prop calculator, the calculator predicted a 14" pitch was the appropriate prop based on the gear ratio of 2.08:1(2.08 flywheel revolutions will produce 1 propeller revolution) . My Johnson 48 SPL (a 2-stroke 50hp motor) has a gear ratio of about 2.4:1 and therefore requires a taller pitch. In my case, I use an 11 3/4 by 17" pitch propeller. For these two motors running at the same crankcase speed, the Honda with the 2.08:1 ratio will have a faster prop shaft speed than the Johnson with the 2.4:1 ratio. Accordingly, the Honda needs a shorter prop and the Johnson a taller prop to achieve the predicted top speed of about 37 mph (use Clark's top speed predicting formula as explained in the Reference section) assuming both motors produce the same prop shaft horsepower output.

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Author	Topic: HP vs HP or 2 stroke vs 4 stroke
jameso	posted 06-24-2003 09:00 AM ET (US) Good Morning Whaler fans. I had posted a query in the performance lane concerning powering a 15 SS LTD with a 50 HP Honda. Usual stuff, props ect. Got a couple of replies and some email concerning performance. My question is this; I have had three 15s with 50 HP engines, all were two strokes. The two strokes generally run 6K at WOT. Would not the same prop size for a 50 HP 4 stroke be the same as a 50 HP 2 stroke? The Honda is rated at 5700-6000 RPM. I know torque curves are different between the strokes but is not the overall output the same? Thanks for any thoughts. Jim Armstrong
Peter	posted 06-24-2003 09:47 AM ET (US) Jim, Prop size very much depends upon the final gear ratio. For example, in my response to you in the other thread using the prop calculator, the calculator predicted a 14" pitch was the appropriate prop based on the gear ratio of 2.08:1(2.08 flywheel revolutions will produce 1 propeller revolution) . My Johnson 48 SPL (a 2-stroke 50hp motor) has a gear ratio of about 2.4:1 and therefore requires a taller pitch. In my case, I use an 11 3/4 by 17" pitch propeller. For these two motors running at the same crankcase speed, the Honda with the 2.08:1 ratio will have a faster prop shaft speed than the Johnson with the 2.4:1 ratio. Accordingly, the Honda needs a shorter prop and the Johnson a taller prop to achieve the predicted top speed of about 37 mph (use Clark's top speed predicting formula as explained in the Reference section) assuming both motors produce the same prop shaft horsepower output.