Moving rear beam forward on small trimaran.

Discussion in 'Fiberglass and Composite Boat Building' started by revintage, Mar 2, 2020.

  1. revintage
    Joined: Nov 2016
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    Location: Sweden

    revintage Senior Member

    I am working on a A-cat hull to be used as centerhull on a small trimaran I am building. The project is presented in this thread:
    Windknife trifoiler build

    I have realized the rear beam pocket is far to much to the rear. Instead of adding new beam fittings on top of the deck, my idea is to cut out the original beam pocket including reinforcements and mount it from the underside a little further forward. This should be doable as the hull is ca 1.5cm wider if moving the pocket 0.5m forward where a smaller slot is cut.

    Except for fitting it against top and sides with thickened epoxy after making a snug fit between inside of hull and the beam pocket, I will add glass or carbon fiber down the inside of the hull side and also add a new bulkhead. Building up new surfaces where the original pocket was, should not be a big problem.

    Added some images and a cross section sketch.

    To simplified or to complicated?

    IMG_5682 (redigerad).JPG IMG_5679 (redigerad).JPG IMG_5680 (redigerad).JPG IMG_5684 (redigerad).JPG beam pocket.jpg
    Last edited: Mar 2, 2020
  2. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Cut the pieces and will now make the beampocket fit under the forward slot.

    cut out.jpg
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Thank you Rev for the update photo.
    Are you still planning on bonding the beam pocket into the forward slot from the underside?
    I am thinking it might be easier / stronger to have the lip around the beam pocket sitting on and bonded to the deck?
    You could still add bonding laminates (as per the sketch in the previous post) - but it looks like you will have to do this by feel only, via the aperture that was created when you removed the beam pocket initially?
    I guess you will still have to do it by feel if you offer up the beam pocket from the underside. If you do this, will you use temporary screws from above to hold it in place?
    Re the photo of the transverse hull frame in your previous post (2nd last photo - I assume that this is a frame?), was this in way of the original beam pocket? If yes, are you going to laminate in another frame in way of where the 'new' beam pocket is attached?

  4. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Hi Bajansailor,
    The idea is to make a MDF board jig going from above the main beam pocket, using the four beam bolts to align everything correctly and to pull the rear pocket up against the underside of the deck. Plenty of time to adjust as I use slow curing epoxy.
    I guess what you call "frame" is what I call "bulkhead"?
    Will off course make a new frame/bulkhead and fit it in way of the beam pocket as before. As I see it, this is the important piece as to lead the forces down the hull sides.
    I am used to do repairs by feel, but I will have to take up another hole in front of the forward slot for epoxying the frame/bulkhead in place.
    The deck will be like a Swiss cheese, so it will be a lot of work left to cover the holes, when the important bits are in place;).

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