Catamaran Hull Extensions - for outboards

Discussion in 'Boat Design' started by powercat247, Apr 11, 2022.

  1. powercat247
    Joined: Apr 2022
    Posts: 2
    Likes: 0, Points: 1
    Location: USA

    powercat247 New Member

    I've seen quite a few catamarans get refreshed with hull extensions (the seawind 1000 series especially comes to mind here) with owners mostly really happy with the results.

    I'm curious if the same paradigm of adding transom extensions could be applied to turn a sailing cat into more of a motorsailer (or, indeed, even a powercat).

    Most of the "transom extension reports" I've read have *not* needed to support the weight or propulsion of outboard engines, but I'm curious how difficult such a thing might be to accomplish. There are a few relatively lightweight 30-35' catamaran designs that appear to my untrained eye like they could be candidates for "transom extension, but sturdy, and flattened for extra buoyancy and anti-squatting"

    Has anyone attempted this, or have any thoughts on it?
     
  2. jehardiman
    Joined: Aug 2004
    Posts: 3,266
    Likes: 699, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Hello, welcome to the forum.
    Lengthening the sailing waterline almost always improves a "small" sailing vessel. There are several reasons for this; some associated with wave making and some associated with seakeeping.
    Using a transom extension, bracket extension, or jacking plate for an outboard also has several reasons, mostly not associated with why and how you would extend the sailing length.
    Sailing cats and outboard power cats have (or should have) very different hull forms aft based on the intended use. What transom modifications you would choose for a sailing cat would not be those you would choose for a power cat.
    So... just what are you thinking you can accomplish? Remember that the MacGregor 26M/26X tried to straddle the line between sail and power but never really did either well.
     
    bajansailor likes this.
  3. Ad Hoc
    Joined: Oct 2008
    Posts: 7,285
    Likes: 1,175, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    As JEH notes, it is all about the hull shape, and the shape for its target speed.
    A very brief simplified explanation can be found HERE
     
    bajansailor and jehardiman like this.
  4. powercat247
    Joined: Apr 2022
    Posts: 2
    Likes: 0, Points: 1
    Location: USA

    powercat247 New Member

    Thank you both for your input! When I see sailing cats with minimal rocker, the lines look so similar to the almost totally rocker-less illustration Ad Hoc linked to above.

    So it *looks* to my untrained eye as though one could add hull extensions to, say, a Seawind 1000 (or a Maine Cat 30, a TomCat 9.7, PDQ 32, etc.), but instead of following the curve of the hull up and out of the water, introduce a flatter aft section, thereby making a similar profile to a Stealth 36e (Power - Asia Catamarans http://www.asiacatamarans.com/stealth-designs/power/) or a Woods Skoota.

    I was imagining (and, I admit, hoping!) that this might result in a boat that could achieve speeds in the low to mid teens with relatively modest power. Sounds like I might be barking up the wrong tree though!
     
  5. fallguy
    Joined: Dec 2016
    Posts: 6,063
    Likes: 1,212, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    The problem I see is that a sailboat transom generally exits above the water.

    Forcing it to exit below the waterline requires the aft section to drop below the lines of the boat. This would create, not rocker, but hook, which is evil to all planing boats.

    Unless a hull had significant rocker or was already underwater at the transom; it is wholly impractical to make anything other than a displacement cruiser afaik.

    I'm just a boat lover. Take me with a grain of salt. Corrections welcome.
     

  6. jehardiman
    Joined: Aug 2004
    Posts: 3,266
    Likes: 699, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    If it was a mono, I would say look at the aft diagonals as to how the transom should be extended. As a cat though, you should look at the aft buttocks. As Fallguy implies, don't change the aft buttocks unless the cat under power has significant squat. Unfortunately, many high SA/disp sailing cats (and monos) have this problem because of the vertical lever difference between the sailing force and the propeller force (i.e. the high center of driving force offsets the hulls tendency to squat at speed). One needs to tread lightly here, or otherwise spend significant money in testing, to ensure that the gains made by one powering method don't cripple the other. As I have stated before, the best place for the engine in my Catalina 22 is between the V-berths as doubling the HP is not nearly as effective as lifting the transom out of the water and improving the run.
     
    fallguy likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.