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Thread: Longer shaft engine than needed. Issues?

  1. #1

    Default Longer shaft engine than needed. Issues?

    Sorry if this has been asked before. I am looking at converting to a higher HP engine on my boat. The only issue is that the engine I have my eye on is a long shaft and I have a regular on my boat right now. Is there any real issue on using this longer shafted engine other than loosing clearance below the boat. I'm assuming that having an extra 5" sticking out below the boat is not that big a deal. Am I way off in assuming this? If this has already be brought up in detail, could you please post a link to that topic? Thank ahead of time.

  2. #2
    Member Akgramps's Avatar
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    Default

    The motor needs to be set at the correct height to get good performance,
    You didnt say if it was a jet or prop? or what size outfit you are piecing together. Either way you can get a transom extension bolted or welded to get things set right.

  3. #3
    Member captaindd's Avatar
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    Default Shaft

    The longer shaft will work but it will cause more drag and take away some of the increase horse power that you have added.

  4. #4

    Default

    It's a prop and it's a 17ft powerboat. I would be jumping up to the boats max rated hp, from 70 to 115, so I don't think that extra drag would be huge. I would be up for trying an extension to the transom, but the whole reason I would be getting this engine is price. Adding an extension, unless it's almost free, would bite into those savings. It sounds like it would make better sense to get a different shaft on ebay. Thoughts?

  5. #5
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    Default Transom height

    You have a 20" and you now want to go to a 25" is that correct?

  6. #6

    Default

    Quote Originally Posted by cool_husker View Post
    You have a 20" and you now want to go to a 25" is that correct?
    Yup. What are you thinking?

  7. #7
    Moderator stid2677's Avatar
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    When I was a teenager, I got a good buy on a long shaft 20 HP but my Jon boat needed a short shaft. I used it anyway. It was very unstable on plane. The engine wanted to try to get on step with the engine cavitation plate. Made the entire rig a death trap. Alot more to it than just drag. The engine needs to run at the correct height. You could use a cheap mechanical jack plate. Many folks run a 25 inch shaft, most jets run them. Buy the motor and sell or trade for the one you need. IMHO the boat would be dangerous to run like that, mine was.

    Good Luck

    Steve

  8. #8
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    Default

    Buy a Jackplate, and mount it so that the cavitation plate (horizontal appendage above your propeller) is an inch or so above the keel line of the boat. With the motor spaced back however far the jackplate spaces it, you'll be able to raise the cav. plate above the keel line quite a ways without cavitating.

  9. #9
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    Default there's a reason outboards come in different shaft lengths

    It has been my experience that shaft lengths are very important to the performance of the boat. The placement of the outboards, i.e. height in which it is mounted relative to the hull/keel of the boat, is paramount to the performance of the boat. If mounted too shallow cavitation takes place, if mounted too deep, even an inch or two can affect performance, the boat often times will not handle correctly and can prove to be extremely dangerous especially in the turns. More times than not the dealers will mount an outboard alittle too deep to ensure that it does not cavitate, they don't want the boat to come back. Boat seems fine to the average person, especially when its new and we don't have prior experience with that particular setup. I have raised many outboards an inch or two and improved the performance significantly. I would encourage you to put match the shaft length to the boat.

    my .02 cents worth

  10. #10
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    Default Jack plate

    I think a jack plate (manuel) would be the cheapest route for you to go. Just get one with 5" of height and a 25" should work like a 20". I think someone had one for sale on the forum a while back.
    Good Luck,
    Tim

  11. #11

    Default

    Thanks for all the info guys.

  12. #12
    Member fishnngrinn's Avatar
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    Default

    I used to race hydroplane, even 1/4 inch height adjustment made a significant difference in performance where the goal was maximum speed.
    NRA Lifetime Member

  13. #13

    Default

    So I may have been misinformed. I measured from the top to the bottom most part of transom. It was 21"-22". Am I correct in assuming that I would need a 25" shaft for that sized transom? Or, does the 25" not take into account the lower unit as well (thus making the overall length from the bottom of the engine to the prop closer to 30")?

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