OB brackets, I/O conversion to OB, pitch, buoyancy & weight - a designer may be needed?

Discussion in 'Powerboats' started by Catboy, May 21, 2022.

  1. Catboy
    Joined: Mar 2009
    Posts: 22
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    Location: usa

    Catboy Junior Member

    I am considering converting my Trophy 2152 WA from a I/O to an OB. I have been reading that many boats perform better with a transom bracket extension and an OB hanging off that bracket. My current boat has an I/O and while I love the boat and it meets my needs it's a bit stern heavy. I typically use the trim tabs to get the boat on a plane and the drive trim must always be all the way down. It works and I've learned to use what I have to best advantage, but I am imagining an improvement.

    I know that brackets can provide buoyancy but if put the OB weight further aft than the I/O is now I'm at a loss to figure out if the overall boat trim (pitch) will be improved. Also, doesn't putting an OB further aft with a bracket effectively change the waterline length? Will that help the pitch, or not? It depends, I think.

    I'm wondering at what point I'd be better off extending the boat length (say 24") and hanging the OB off that new transom as opposed to using a bracket, although a bracket would obviously save a lot of work. I suppose if the bracket had enough flotation it might off set the weight. 1 cubic foot = 64 pounds of flotation. So a 500 lb. OB would need +- 8 cubic feet to offset it's weight.

    Am I looking at this right? Anyone out there with a heap of knowledge or experience weighing in would be greatly appreciated. Or referring me to a designer that might be able to help would also be greatly appreciated.

    Or links to similar discussions? Thanks!!!
  2. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Whether the bracket provides buoyancy or not depends on the type of bracket. If it's just a tubular frame type bracket like a Gil Bracket, it doesn't add Buoyancy. On the other hand they don't weigh as much as the box type brackets that add buoyancy.

    Adding a 2 foot extension adds a whole new complication. Now you need a stress analysis to determine how much strengthening of the hull you need. I can tell you a story of a well known manufacturer who did just that. It caused stress cracks and transoms falling off, and they had to repair or replace 250 boats. This is not a good idea if you don't know what you're doing. It's not as simple as just adding 2 feet to the boat.

    How it will change the trim depends on a lot of factors. The first is determining how much the current engine and sterndrive weigh and what is the current center of gravity and center of buoyancy of the boat. Next is the weight of the outboard, how far aft it will be and how it will change the center of gravity and center of buoyancy. If the outboard weighs a lot less than your current engine/sterndrive combo, it may not change much at all. If the O/B weighs the same or more it will definitely shift everything back. This will change the trim, lowering the stern and raising the bow.
    fallguy likes this.
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