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Thread: Transom modification

  1. #1

    Default Transom modification

    I'm thinking of buying an OB jet boat. Looking to buy a motor with a prop and jet foot to be able to switch out. If the boat does not have a transom capable of handling a large motor (225HP) how difficult, expensive, etc... would it be to upgrade the transom to take a larger motor?

    My thoughts for use is hunting in rivers where a jet foot is needed, running PWS and Resurrection Bay for deep sea and dipnetting on the Kenai.

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    Member Akgramps's Avatar
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    Quote Originally Posted by Wildwood_Hunter View Post
    I'm thinking of buying an OB jet boat. Looking to buy a motor with a prop and jet foot to be able to switch out. If the boat does not have a transom capable of handling a large motor (225HP) how difficult, expensive, etc... would it be to upgrade the transom to take a larger motor?

    My thoughts for use is hunting in rivers where a jet foot is needed, running PWS and Resurrection Bay for deep sea and dipnetting on the Kenai.
    Just a bit confused....? Do you have a particular boat in mind? Are you going to buy a new boat (just the hull) and rig it your self?
    “Nothing worth doing is easy”
    TR

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    You may just add a hydraulic jacking plate and make it work for prop and jet.

  4. #4

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    Not planning on buying new, and haven't picked a manufacturer yet. Thinking about repowering the motor if it is not large enough. As I understand, most river boats (Northriver, Wolly, Hewes, etc...) have a max HP rating on the transom and it is usually around 180-200. So if I buy a boat with a 200 rated transom and want to put a 225 on there for a little more power, will that adversely affect things or can I "beef" up the transom?

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    If you buy a boat with a 200 rated transom, it will probably handle your 225 just fine. Most 200's and 225's these days are the same motor or nearly identical weight, just a different computer. I think you will also find a pretty small benifit to having a 225 over a 200 in most cases. With the prop on, you may never use that last 25hp and with the Jet odds are the 200 and 225 use the same pump and impeller so you won't get much increase out of that set-up.
    With that being said, if you buy a used boat it would still be a good idea to take a close look at the transom before hanging a V6 on it. You may want to beef it up anyway. Hanging a 600# motor on, especially off a jack plate could add quite a bit of stress to a previously abused area of a boat.
    Casey
    Yamaha Dealer
    Petersburg, AK

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    There is a formula for determining the max horsepower on a boat. This is only a guide.

    Step One - Compute your Factor

    A. Length of Boat x Transom Width (measure in feet: 14, 15.5,16, etc).



    Step Two - Compare your factor to H.P. rating

    A. 0-35 = 3 hp
    B. 36-39 = 5 hp
    C. 40-42 = 7.5 hp
    D. 43-45 = 10 hp
    E. 46-52 = 15 hp



    Step Three - If your factor is over 52 (Choose only A or B)

    A. With Remote Steering (steering wheel) and at least 20" transom height

    Maximum HP = (Your factor x 2) - 90

    Example: 19' 6" long , 7 feet wide = (19.5 x 7 x 2) - 90 = 183 (Round up in multiples of 5. This boat's max is 185 hp).


    B. With Tiller Steering (hand or extension) or less than 20" transom height (Choose only 1 or 2)

    1. Flat bottom, or hard chined boats (hull has hard angles)

    Max HP = (Your factor x 0.5) - 15

    2. Soft chine, rounded hulls

    Max HP = (Your factor x 0.8) - 25

    This guide was found at NewBoatBuilders, which was ultimately used from "Title 33-Code of Federal Regulations-Subpart 183.53".

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    Quote Originally Posted by Wildwood_Hunter View Post
    Not planning on buying new, and haven't picked a manufacturer yet. Thinking about repowering the motor if it is not large enough. As I understand, most river boats (Northriver, Wolly, Hewes, etc...) have a max HP rating on the transom and it is usually around 180-200. So if I buy a boat with a 200 rated transom and want to put a 225 on there for a little more power, will that adversely affect things or can I "beef" up the transom?
    A hydraulic jacking plate will allow you to rase and lower depending if you are using the jet or prop. If you install a jacking plate it will make your transom stronger and distribute the weight to a larger area of your tramsom.

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    Quote Originally Posted by MGH55 View Post
    A hydraulic jacking plate will allow you to rase and lower depending if you are using the jet or prop. If you install a jacking plate it will make your transom stronger and distribute the weight to a larger area of your tramsom.
    I have a power trans on my outfit, its a good lift, but don't see how it added any strength to the transom as it bolts to the same holes the motor would use.....?
    “Nothing worth doing is easy”
    TR

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    Quote Originally Posted by Akgramps View Post
    I have a power trans on my outfit, its a good lift, but don't see how it added any strength to the transom as it bolts to the same holes the motor would use.....?
    1/4in oversized backing plate outside bedded with light cote of 5200 and oversized plates inside as space allows, I have done this on wood glass and aluminum boats. On aluminum I like to weld the doubler plate, but you can just bed it.

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    Quote Originally Posted by MGH55 View Post
    1/4in oversized backing plate outside bedded with light cote of 5200 and oversized plates inside as space allows, I have done this on wood glass and aluminum boats. On aluminum I like to weld the doubler plate, but you can just bed it.
    Ya, good idea to have a stiff transom......especially with the setback of a lift, to be clear to the OP, the jacking plate or lift by itself does nothing to strengthen the transom....
    “Nothing worth doing is easy”
    TR

  11. #11

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    Thanks for all the info, especially the formula for HP!

  12. #12
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    I looked into getting a wooldridge XP or XL with the prop to jet conversion. The 20ft XL can take a 250 jet and a 150 prop. The limiting factor was not the weight of the motor but the thrust I would assume. The math is fuzzy but the performance loss with the jet is obvious. A boat that could take a 225 HP jet reduced down to under 150 is losing a lot of potential on a very expensive boat. The prop would make the boat a work horse and the little jet would make it a dog. On the other hand, if you had a boat that was less performance minded, say a smaller john boat in the 90 HP jet range the difference might be less. You still would give up a lot with the jet but it might be worth it. It depends on what you want to use the jet for. Hopefully not taking a moose out and a bunch of gear. Maybe a quick trip to the deshka? Or, a long haul like the Yukon on the prop then swap to the jet for zipping around the sloughs?

    I was thinking of doing the swap out thing with my previous boat, a 17' wooldridge alaskan with a 115 jet. I could not see the fuel savings of buying a $3000 lower unit, so I kept it a jet. I sold it and bought an inboard 20' extra Plus. I miss the little nimble boat though. Again, it depends on what you are wanting to do.

    Good idea asking around. See who is doing it and why they went that route. Ask about performance and if the loss of jet was worth it.

    Here is a thread I found helpful when I went through it. Best of luck.

    http://forums.outdoorsdirectory.com/...ight=jack+lift

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