21 ft Cat advise

Discussion in 'Powerboats' started by Flight Risk, Dec 13, 2023.

  1. Flight Risk
    Joined: Dec 2023
    Posts: 2
    Likes: 0, Points: 1
    Location: Florida

    Flight Risk New Member


    Hello gentlemen, this is my first post here but I lurk occasionally. I recently have acquired a new to me boat and in the pursuit of never leaving anything alone, I’ve been thinking of ways to make it better. It’s a 21 ft cat that is a copy of a 21 Skater. These boats were designed in a time of 2 stroke outboards that weighed 200lbs or more less than the L6 Verado it’s currently got hanging off of it. Some of them however, less than 10 total, got twins. So even with lite motors, we are talking about 800 lbs minimum.

    So for my question, I’ve been considering the idea of removing the transom notches on the boat but want some opinions on how that would affect the handling and overall attitude of the boat. My understanding of the notches was to help get water to motors mounted behind the sponsons, but in this case, no motors there. They also shorten the running surface a bit which I feel would be beneficial to have more at that point to help lift the hull up. They would also add a bit of flotation to the boat.

    The idea would be to fill them up to the transom and just match the running surface all the way to the transom.

    Thoughts? Ideas?
  2. BlueBell
    Joined: May 2017
    Posts: 2,638
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    Leave them.

    Or at least drive around and gather some performance data.
    Then, if you just can't resist, fill them in.
    Let us know what happens.
    Ad Hoc likes this.
  3. Flight Risk
    Joined: Dec 2023
    Posts: 2
    Likes: 0, Points: 1
    Location: Florida

    Flight Risk New Member

    I have had the boat out several times and a 21 skater is well documented as to what it can do with different power and setups. This same boat with a 400r went 108 with the previous owner.

    I’m more interested in understanding why they added the notches in the first place. What benefit would they offer for a single engine application?
  4. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    You could install trim tabs in the notches.
  5. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    They look suspiciously like a recess for trim tabs. Given their "exact" nature and location.

    Or do as Bluebell suggests.
    bajansailor, BlueBell and BMcF like this.

  6. BMcF
    Joined: Mar 2007
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    Location: Maryland

    BMcF Senior Member

    We tested a 16' free-running scale model of a 72' high speed (30 knots design speed) catamaran yacht, with and without transom trim tabs. Similar assymetric planing hulls as the ones in OP. The trim tabs provided a big improvement in the time and power required to get the craft on plane and reduced the minimum planing speed significantly. As the yacht's displacement grew over time, as so often happens, it reached the point where it could not even get on plane without a lot of trim tab lift.

    You can't see it in this picture, but the outboard engine on the scale model is mounted on a thrust measurement plate so we had accurate measure of the drag for various speeds and trim tab settings, in addition measuring speed and trim.

    I included a picture of a tab installed on the yacht. The trim tabs were actively controlled to dampen pitch and roll motions as an added benefit to helping plane out the boat.

    Attached Files:

    Last edited: Dec 14, 2023
    Ad Hoc and bajansailor like this.
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