Yamaha T8 on light boat

Discussion in 'Outboards' started by biglar, Mar 9, 2016.

  1. biglar
    Joined: Mar 2016
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    Location: Silver City, New Mexico

    biglar New Member

    I have a high thrust Yamaha T8 kicker on my 19 ft and will soon be moving to Florida and buying a jon boat. I'll be using the 19 offshore in Gulf of Mexico, so want to keep the T8 - it pushes the 19 to about 5 - 6 mph at 1/2 throttle. More gas makes no difference and is harder on the motor.

    An 8 hp would be a good enuf choice for fooling around in the swamps in a 12 - 14 ft jon boat, but don't want to buy another 8 hp when I already own one. Even prop dealers don't have an answer for me -

    What would be a suitable prop to give better performance on the lighter boat ?? The stock prop is 11 3/4" dia X 5 3/4" pitch and I don't see too many options online. I know the lower unit is slightly larger and it is geared lower, but with the right prop it seems to me that 8 hp is 8 hp as long as it's applied correctly. Correct ?? Thanks Lars.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    So it seems you want to press this engine in to the role of planing a small, light boat. It might work if you get a different prop, and the boat is lightly loaded. Check the prop selection available, to see the pitch range. You might need something like 11". If they go that high in that category of props !
     
  3. biglar
    Joined: Mar 2016
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    biglar New Member

    Thanks for the quick reply.....you're efficient. :)

    I've checked and all I can find is....uh....9" or 9 1/2" dia (can't remember) with pitches of 7" , 8", etc. One dealer said he had no idea - I'd hafta experiment. At over $100 per prop and me living on SS, that isn't an option.

    Another dealer said it'd take a 7", 8" or 9" pitch or maybe more; still with 9" dia. He was a great help, too.

    I'm flabbergasted that I seem to be the only one who's ever wanted to do this. Maybe the rest of the population are all rich and don't mind buying duplicate motors ?? Wonder where I went wrong ?? :-( I'd hoped that someone else on the forum might have already been there, done that.

    I "think" that loading the engine to its max rpm with the correct prop should give the same thrust as the F model with its smaller prop. With its bigger gearcase mine will have a little more drag, but it should still go. Maybe the larger dia. prop would have less slippage ?? (Actually, it may just surprise me with the stock prop and a light load. We'll see. Hah - isn't there a formula that will calculate pitch vs RPM to come up with theoretical distance covered, minus slippage ?? I'll ponder on that)

    In the boat will be 185# me, my 44# dog, fishing rod and lunch. Should be a moderate load for it.
     
  4. biglar
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    biglar New Member

    OK, following that line of thought - for what it's worth I ran some numbers thru the calculator. Figuring 5,000 rpm thru 2.86 (I think that's close) gears would give 1750 prop rpm. With a 10" pitch prop that worked out to a bit over 16 mph, figuring no slippage, so possibly a '"real" speed of 14 or 15 mph.. To my little pea brain, that sounds reasonable for 8 hp on a light boat - but would be marginal for planing.

    What think you ??
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I think it may be 2.92 to 1, so close enough, 5000 rpm is pushing it hard though, OK for short periods. 14 mph you should be planing comfortably, the question is whether the motor can spin out to the 5000 rpm.
     

  6. biglar
    Joined: Mar 2016
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    Location: Silver City, New Mexico

    biglar New Member

    I've given this a lot of thought all day and think I'm going to reconsider. I've always been a get going and get 'er done kind of guy and really appreciate performance. That little 8 hp does a dandy job on the big boat and would be marginal at best on a jon boat....and I'd really prefer something a bit more stable and versatile than a jon boat.

    Years ago I had a light 13 ft plywood skiff with a 20 hp Mercury outboard. That thing was almost scary and was a hoot to run. IIRC, it'd just about hit 30 mph and felt like I was flying in it. I think I'm going to look for something similar when I get to FL.

    Thanks for the thoughts. Lars.
     
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