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Longshaft motor slipping on 16' Achilles

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  • Longshaft motor slipping on 16' Achilles

    Seems like just crickets here for a long while, but I'll give it a shot. I've got a 1983 SU-16 with a longshaft Nissan TLDI 40HP 2-stroke. Once you get the boat on a plane, the prop ventilates at anything above bare planing speed. The problem seems to be affected by weight and by weight distribution - it also was less of a problem in fresh water than salty, same loading - but it's not as simple as more weight = less problem. I've never had more than 3 people and a little gear in it, but it was as bad then as ever. I've actually had better luck riding alone in it. The ventilation plate is right about at the bottom of the transom - but this boat has large tubes and sits pretty high. My guess is that the boat is floaty enough that moving weight fore and aft and side to side levers the motor position around enough that overall weight doesn't directly correlate with the problem. The motor is mounted on a 1/8" thick thrust plate that wraps over the top of the transom, but I doubt if this elevates the motor more than 1/4" over just sitting straight on the wood. I'm using a 10.3 x 14 3-blade cupped aluminum prop, and have tried a 10.3 x 13 Michigan Wheel also. I haven't measured the shaft length exactly, but it's around 19-20" to the ventilation plate from the transom bracket.

    I'm debating what to do - I could cut the transom down by a bit, or I could look for a longshaft with a slightly longer shaft (how much variation is there among models?). I'd hate to give up the motor, it's sweet. Just wondering if anyone has any other thoughts about fixing this? Any chance propeller selection could make much of a difference? If I cut the transom, any suggestions what I should shoot for as far as ventilation plate position?

  • #2
    Re: Longshaft motor slipping on 16' Achilles

    Looks like cutting the transom is your best bet
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    • #3
      Re: Longshaft motor slipping on 16' Achilles

      I took a closer look this weekend, the ventilation plate is about 1" lower than the bottom. I'm going to try a bolt-on fin/wing and if that seems to help, I may work with that approach a bit to see if I can improve operation w/o introducing any handling issues.

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      • #4
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        Re: Longshaft motor slipping on 16' Achilles

        I just bought a killer Achilles 13 ft, 1985. Believe it or not, I got it for 125.00 bucks! It is in very good condition. The seller just climbed out of a divorce. I drove 1 1/2 hours one way to get it. Anyway, can anyone fill me in on the best way to condition the surface of the craft material? Also, I would like to know what is the max safe hoursepower I can strap on it. The owner did not know, but had a 10hp on it. Thanks.

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