Converting inboard to outboard

Discussion in 'Boat Design' started by RobCEW, Mar 16, 2002.

  1. RobCEW
    Joined: Mar 2002
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    Location: Plymouth, MN

    RobCEW New Member

    I am seriously considering converting my 23' inboard utility to outboard power. This is a 1958 May Craft, built in NJ. The construction is conventional... (MDO) plywood over oak frames with much solid and veneer mahogany throughout, as was typical of that era. The engine is a Chris Craft 431H (Lincoln 430 cid) rated at 275 hp driving a Paragon reverse gear. This power package weighs about 1200 lbs. and obviously sits on the designed CG.

    For a variety of reasons I'd like to pull the 430 and build or buy an outboard bracket capable of 200 hp. My first concern is the load/trim/handling aspect. The boat has an 8' beam and the transom waterline is 86" with a 5 degree deadrise. The hull weight is about 1700 lbs.

    I'm wondering if there is a formula ( a rule of thumb will do) regarding the mid point beam and transom beam ratio relating to how much weight I can hang on the rear end of this boat.

    Now...if this seems realistic to anyone at all... I'd sure like to be directed to a site that would explain the beefing-up process for the transom before installing the bracket. I only know of three companies that build something like I'm looking for; Porta, Gil and Stainless.

    Oh yeah, now that I've got that big OB hanging out there, what about my trailer tongue weight? Nevermind - I'll worry about that later!

    Meanwhile, thanks for any tips or observations on my project.

    Rob W.
  2. David Dobbs
    Joined: Mar 2002
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    David Dobbs Junior Member

    I wonder how the existing heavy inboard is located in relation to the vertical center of gravity?

    In addition to the fore-aft trim, moving the VCG might affect handling in a large way, esp. since the existing engine is a large part of this boat's weight and moving to outboards will presumably raise the VCG. I wonder how the inboard is currently located relative to the VCG.
  3. Guest

    Guest Guest

    Armstrong makes similar brackets.

    Land & Sea also makes a nice hydraulic outborad bracket, but not the type of full transom bracket as the others.
  4. Evolution Yacht
    Joined: Feb 2002
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    Location: Eastern Long Island

    Evolution Yacht Junior Member

    Hi Rob Usualy when designing a boat large weights such as engines and tanks are placed in locations to create the proper ft.lbs. so the center of gravity is over the center of boyancy or in a spot to create a proper trim. I think if you remove that 1200lb and hang 600 to 700lb off the back you may have to readjust some weight.
  5. RobCEW
    Joined: Mar 2002
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    Location: Plymouth, MN

    RobCEW New Member

    Thankyou all for the observations. I had given some thought to the VCG along with the fore and aft trim. My thinking was that moving tanks and batteries forward, as well as a possible cabin or hard top would resolve some trim issues. As it is now, there is seating for eight in front of the engine and a porta-pottie under the deck.

    I would hope something on the order of an Armstrong bracket would provide more stern bouyancy as well as lateral stability.

    I suppose it sounds like I'm trying to convince myself (as well as you guys) that I'm doing the right thing. It's just that this boat looks to be just a big version of plywood OB runabouts I've had and built, only with a little more transom beam in proportion.

    If anyone knows of a source dealing with the designing/building/installing of brackets of this sort, I'd like to know about it.

    Thanks again - Rob W.
  6. Gary Langone
    Joined: Mar 2002
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    Gary Langone New Member

    If you don't mind my asking, what is the objective of going to outboard power rather than just installing a nice new inboard?

    I won't go into the "don't mess up a fine old boat by trying to make it something it isn't" line I usually use when someone wants to hack into a nice old boat, but I'm curious what you want to gain. Speed? What is the boat's speed now and what do you want it to be? Reliability? A newer engine could serve this purpose, and while you would lose some authenticity, you would still have authenticity of design/function. Redundancy (if twin OB's)?

    It certainly will change the way the boat sits at anchor and runs at lower speeds, as it will no longer be as level. In the extreme if you add a huge heavy outboard it could change the boat from a nice slicer as slow speeds to a pounder, depending of course on the design of the bottom.

    Also I could be wrong, but I doubt that the buoyancy added by the setback bracket will make up for the additional setback it adds. I'm sure the bracket won't help the fore-aft balance, but of course it's main purpose is to set the OB's back for better performance and keep you from having to cut into the transom or sacrifice cockpit space. (BTW, I don't think beefing up the transom will be any problem at all with a little common sense).
  7. ARW3A
    Joined: Jan 2004
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    Location: MA

    ARW3A New Member

    I am considering the same modification on a 25' 1970 Trojan Marlin CC. The reasons for such a project are:
    1. More deck space.
    2. Twin outboards would provide redundancy for offshore fishing.
    3. I'm re-doing the deck anyway.
    4. Better handling. There is nothing more frustrating than docking a 25' boat with a single inboard plus, the thing is all but useless in reverse.

    The obvious problems involve trim and transom strengthening. I think I can solve the trim issue by using a full flotation bracket and installing a 120 gallon fuel tank further forward. The hard part is strengthening the transom because the live wells are in the way. I got to destroy them so that I can get full access.

    BTW, one option I've seen for converting inboards to outbaords is lengthening the hull at the transom by a max. of 10%. Seems like a lot of work but maybe it's the best solution. If one did this how far should the glass be lapped into the extg. boat?

  8. I am tinking of doing thesame thing to my 1979 24 foot Grady White W/A as robCEW is doing, by removing a 350 chevy engine and an old OMC outdrive then installing 29inch stainless marine truss bracket to hang a 225 or 250 hp outboard on. does anybody have any clues to any problems I might incur with this installation??

    Harry E.Music
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