The booms are going to be fitted after the model is painted.
Really??
Yes. I want to leave the hull as clean and accessible as possible for sanding and painting. I’m not very good with a paint brush and I’m an absolute menace with a spray can. I need all the help I can get! (I hope to get the hang of using an airbrush by the time I get round to painting this model but we will have to see). So how is that going to work without getting glue all over the boat and still make a good joint?
Cunning plan. The weight of the boat is carried by the top of the boom, so that is the bit I need to attach to the frame. The structural box (between the main beams) has a flat top and is deeper than is needed for the boom to fit. The bottom face slopes up towards the centre on both sides. Between the main beams and the hull, there is another box, 1-2 mm bigger than the boom all round. Slow-set epoxy is applied to the top face of the boom where it fits the box (with a smear on the sides too) and the boom is then slid into place across the bottom floor of the box so the adhesive is not wiped off. The ends of the boom are supported on level blocks and the hull allowed to drop onto the top of the boom, maybe with a bit of ballast for good measure. The whole lot will be trued up and checked then left to set. So far so good. The boom is attached where it needs to be attached but the joint is a little vulnerable. Adhesives are good in compression (that’s how it is being used here) and pure tension, good in shear (if the boom tries to slide out sideways) but much less strong in peel and cleavage. If I were to push down on one end of the boom while holding the hull still, the adhesive would be put under a peeling/cleavage load and that’s a likely fail. To get round this, after the epoxy has set, I will insert two glued-up wedges under the boom to make sure that this loading condition can’t happen. So that’s why the box floor slopes!
We still have the ‘oversize’ boxes open where the boom comes out through the skin. I will apply a little low-modulus silicone sealer to seal this area to keep the water out, though it’s not strictly necessary. The booms can flex all they like now and not transmit any significant force to the hull skin.
There you go, George, I
was listening!
Instead of sheathing the booms in place, I am going to make up some cuffs that slide over the tube. They will be pre-painted to and only attached to the hull by a little silicone. (If I glued them to the hull, I would ‘short circuit’ the silicone gap round the structural booms). I’m not going to try to blend these into the hull or sponsons, just make them a neat fit. Oh dear, that requires some skill so it will take a long time.
The sponsons will be fitted in a similar manner. In this case though, it’s the bottom face of the boom that gets attached to the sponson structure and there will be a wedge above. That wedge will have to be part of the ‘cuff’. Oh dear, oh dear. More accurate building needed. I have some reservations about the attachment of the sponsons; it’s quite a small joint to rely only on adhesive. I will make a judgement when I see the sponson structure. I could add a dowel down through a hole in the top of the sponson and patch the paint up afterwards if necessary. On the other hand, if there is a crunch, it would be better to have the sponson fall off rather than rip the whole side of the boat out.
The pictures below speak more than my thousand words.
I now have a lot of design work to do so please be patient. Eventually, I will get some good predictions of the weight and CG out of the solid model and then it's on to the drawings so that I can cut
real wood!