How to estimate waterline change with bracketed outboards?

Discussion in 'Boat Design' started by Michael Gray, Jul 9, 2020.

  1. Michael Gray
    Joined: Jul 2020
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    Location: USA

    Michael Gray New Member

    I am refitting an outboard boat and I want to place the engines aft on a bracket to close the transom. Is there a quick and dirty set of calculations I can use to estimate what the force will be on the stern? For instance, if I have the waterline marked when the boat had 1300# of outboard on the transom, and I want to move them further aft by 30", can I calculate how much weight I could put in the hull, at the transom, to simulate where the water line will be? My thinking is, add the appropriate weight, float the hull, observe waterline change, then move some items around and forward to compensate and maintain previous waterline.
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If you want to actually do a test instead of the calculations, it can be simpler and more accurate. However, that is only to see where the static waterline will be. Simply clamp a couple of board to the deck and add the same weight as the engines at the location you want them. However, moving weights forward may not be possible. 1300 lbs is quite a lot. One thing to consider, is that if you have weights moved to both ends, the boat will pitch less (be stiffer), which will change the handling.
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The dynamic effects and the hydrostatic effects, will be different. It depends on the boat involved. A smallish boat will be more affected, because the engines are a bigger proportion of the total weight of the boat, and moving them 30" on a shorter boat, will also have a greater effect, than on a longer boat. Also, if the boat is a bit bouncy underway, and needs motors trimmed down, or tabs, that doesn't augur well for moving them aft. Also, a boat with generous waterline beam aft, is going to be less affected than a narrower hull. If the concern is that the boat will be trimmed down at the stern at rest, then it mightn't be easy to get it sitting as before, but that also depends on what buoyancy the new installation adds, with say, pods, rather than frame brackets. Then you have to consider how much buoyancy you can get, without the pods affecting the performance on plane, it is not unknown for water to climb up the reverse slope, and create a downforce, that is something you don't want. So it is a complex matter, and one way to short circuit the process, would be to copy an installation, that has proved successful on the same boat hull, by an earlier conversion, by someone else.
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