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Laminating a new transom and stringers

Discussion in 'Marine' started by Jimbo570, Mar 18, 2019.

  1. Jimbo570

    Jimbo570 New Member

    Hello all, I’m about to start gathering materials for replacing my transom, which was rotten to the core. As I’m starting from scratch I have an idea to add additional strength across the back and up the internal panel to allow steps and auxiliary engine mounts to be fitted safely. Because I am using hardwood ply, would it give more strength to laminate the transom to its correct thickness with say several laminations of 12mm hardwood ply?
    I’m aiming for 50mm thickness or there abouts. As I am also replacing the stringers, could you suggest what materials (resins, CSM, BI cloth etc) would suit best for both applications please?

    I have been told to go with the following, 450g-900g CSM, biaxial tape to tab in joints, polyester resin, chopped glass and silica for radius joints.
  2. Martin @ EC Tech

    Martin @ EC Tech Tech Staff Member

    HI, For this type of project It would be best to have an experienced boat builder to look at the boat and advise according to your boat's structural requirements and engine size?.


    However for most small boats the lay-up is quite simple as the majority of the strength will come from the plywood. So if you are confident the plywood’s strength alone can handle it, you can do the following lay-up to waterproof and add a bit extra strength.

    If using polyester, rather than using a thickening agent in the resin , use freefix bonding paste to glue the transom in position and to create the fillets. Score the plywood on ether side with a knife to give it more of a key. Use 491PA resin thinned down with 7% styrene, catalyse and apply this as a primer and leave to cure. For tabbing the wood transom in place use chopped strand first then biaxle and then chopped strand again. If you apply biaxle with polyester resin and do not sandwich it between chopped strand it may de-laminate from the original fibreglass hull or deck as polyester does not have the best bonding properties for bonding smooth woven fibreglass to other materials such as old fibreglass etc.

    Once cured you can apply the fibreglass. For the main lay-up : - Use a layer of 450g chopped strand matting, then a biaxle or woven roven cloth and then finish with two layers of 450g chopped strand matting, surface tissue. Leave to cure then you can flowcoat to finish.


    The other alternative is to use epoxy rather than polyester. This will allow you to just use biaxle cloth instead of any chopped strand. You can just apply 2-3 layer of 450g biaxle, leave to cure , over coat with more epoxy then prime and paint with boat paint once it has been sanded down to a flat finish. For gluing and filleting you would add cab-o-sil to the epoxy to make a paste.

    In either case you will need to use epoxy to glue the plywood together for doing this :- coat both surfaces to be bonded with epoxy and leave to cure. do not coat the other surfaces with epoxy however if you intend to cover using polyester resin. Drill a series of 8mm holes in the boards then mix a batch of epoxy to a thickened peanut butter consistency and spread on coated side of one of the boards. Clamp the other board (coated side down) on top so that the epoxy spreads and squeezes from the holes. Use a scraper to remove any excess epoxy and leave to cure. once the boards are glues you can begin installation.

    Ensure before you start any of the above that the original hull/deck area is thoroughly sanded back and wiped with acetone to prevent poor adhesion to the original fibreglass that you are bonding the transom to.


    It would be best to thoroughly research by checking out info on repairing or replacing transoms online before you start just to ensure you are confident carrying out the work if you have never used the above materials, there are many videos online showing replacing and repairing transoms and will give you a better idea as to which way you want to approach this and how to use the materials.
  3. Jimbo570

    Jimbo570 New Member

    Excellent reply thank you.

    I have researched the work thoroughly but do take on board your comments. I was especially interested in the type of products you recommend for the work (which I shall order from you) as I understand the differences in polyester and epoxy resins in conjunction with CSM and the like. My main concern was what to use in with what is already in place to bond to, I have no idea what materials would have been used for the initial construction, however, as it is to be all but replaced would this have a bearing the new materials to be used?

    Thanks

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