Best Placement of Outboards on 24ft Power Catamaran

Discussion in 'Boat Design' started by Brent AZ, Aug 22, 2010.

  1. Brent AZ
    Joined: Aug 2010
    Posts: 5
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    Location: AZ, USA

    Brent AZ Junior Member

    I am restoring a 1988 24ft power catamaran that came to me with twin outboard engines. The engines where mounted on hydraulic jack plates and the jack plates where mounted very close together near the centerline of the boat. This placement of the engines would have the propellers running in the less disturbed water that runs down the center tunnel of the hull. The jack plates allow the engines to be lifted to allow the props to be raised up out of the water as much as needed to allow for the greatest speed with the least amount of hydrodynamic drag on the lower units of the engines.

    This boat is a true tunnel hull with no center pod located in the tunnel area. Here is a photo of the boat with the original narrow spaced engine position.

    And here is the way most power catamarans have their engines mounted widely spaced.

    At first thought, the original position of my engines (closer together) appears to be the best design because this places the props in the least disturbed water for best propulsion. I can lower the engines to have more propeller in the water for best push at start and then raise the engines to reduce drag at higher speeds. But almost all of the 24-34ft power catamarans that I have seen have the outboard engines placed so that the engines/props are almost inline with the centerline of each sponson so that the prop is located just behind the widest portion of the sponson instead of close to the centerline of the boat. I don’t know why this is the way they do it. Maybe it was learned from years of racing trial and error? At first thought, directly behind the sponson would provide the most disturbed water for the propellers to run in and I would think this would cause losses in propulsion efficiency.

    I now have the opportunity to change the location of the engines if it is better to space them further apart versus putting them back to the original location near the centerline of the boat.

    So which option is the better choice and why?
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    The open space or exit of the tunnel is not necessarily the area of least turbulence. Cats and tunnels suffer from wave interference of the opposing hulls, so under some conditions the maximum turbulence may be found at the middle exit. I have confidence that the racer dudes have pretty much optimized the location for the props.

    The above remarks are not to suggest that you relocate your engines. There is much to consider. The spacing between the hulls, the total displacement of the boat with occupants, the anticipated speed, and a few other variables.

    If your boat works well then why not leave it as it is? If it does not work as you would reasonably expect, then perhaps change might be in order. Explore the multihull section of the forum. Lots of good stuff there.
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