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Thread: Why the speed difference?

  1. #1

    Default Why the speed difference?

    Old 28hp Johnson ,heavy battery, floor boards, transom, and kicker = 24knots

    Naked Boat,121 less pounds and new manual start 30hp 2-stroke =17 knots

    Why the difference in speed with less weight? Both motors were deemed Long Shafts. Perhaps my shaft is not long enough for my transom still. I am very confused.

    Thanks for you input!

  2. #2
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    slower speed could be caused by having the prop too deep as well. We raised a long shaft motor on a zodiac, and the speed almost doubled, because the boat needs a short shaft.

  3. #3
    Member Tolman24's Avatar
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    What about prop size? What are your RPMs and are they in the correct range? Just a thought.

  4. #4
    Member Bullelkklr's Avatar
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    Default how old

    How old is the 28horse?

    Old outboards were rated at the prop. New outboards I believe are rated at the Crankshaft.

    Your old 28 could be actually more horse than the new 30.

  5. #5
    Member northriver21's Avatar
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    Quote Originally Posted by Bullelkklr View Post
    How old is the 28horse?

    Old outboards were rated at the prop. New outboards I believe are rated at the Crankshaft.

    Your old 28 could be actually more horse than the new 30.
    I'm pretty sure it's the other way around.

  6. #6
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    Yes, the older ones were rated at the crankshaft, in the lat 1980's they switched to prop hp. I had an old 1970's Johnson and switched to an late 80's version of the same 55hp and the newer one gave me 3mph faster up river.

  7. #7

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    Could it be that those items you removed were actually helping provide some trim to the boat that helped it plane better? I know trimming my boat I can increase my speed several knots. Just a thought

  8. #8
    Member atvalaska's Avatar
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    Post prop pitch,how high or low, trim rite?etc......

    WHEN IN DOUBT> THROTTLE OUT.......

  9. #9

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    Does the rpm's sound like there hitting peak when at full throttle? Do you seem to have cavitation issues? Are you using the same fuel system or the motor come with new lines and tank? Some questions to think about as well. Sounds odd you get less speed out of a lighter set up when power is the same. Perhaps a balance issue of weight but don't know if your fighting it to keep the bow down getting on step.

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