Hi fellas, this is a problem with grp hulls moulded in this fashion the type 7 was partucularly prone to it. This was exacerbated by the acual shape of the hull design.
This takes longer to write than actually do but here goes with a lengthy description of how best to deal with the problems.
I always recommend that hulls be taped down together especially initially when the boat remains unbuilt and just sitting around. The grp is perfectly straight and matching when it comes out of the mould and this 'bowing' takes place slowly and steadily thereafter as grp will go on curing even after the boat has been purchased. This continued curing process is a nutural effect of polyester resin.
There are various ways of dealing with bowing hulls.
One way is the heat treatment, this will allow the grp to become maleable as it will give up it's rigidity fairly easily. Use your heat gun but heat it gently and steadily, you can actually get the stuff so hot you can't touch it but take care not to get too close with the gun or you may cook a local area. heat it enough to make it plyable. If the upper half has spead outwards and upwards as can be found by placing it on a flat surface, try this:
Place a wooden block under the nose and tail of the moulding . Take the top off and slowly and gently heat the whole thing up till it gets really quite hot, stick it back onto the blocks and place a weight in the centre eg: big books or whatever, until it bows downwards. Allow it to cool likethis and offer it up again, you will find the fit is much better and if not correct, repeat the process and adjust the weight till you get it where you want it. I have found a degree of hysteresis in the material and it may need to be bowed a little futher than you actually need as it will spring back somewhat. You may now find that the sides of the hull have narrowed and the lower half is wider than the top half, thats because the lower hul has spread outwards and this can be cured by fixing some cross members in the hull just fore and aft of the dive unit location so they will hold the hull shape. That may also require a bit of the same heat treatment if it is really bad.
Once you are happy with the general alignment of the hull you may find it still has a tendancy to want to bow again over time and I recommend the folowing modification:
Ditch the idea of the front hull locating plates and the aft vertical post with the attaching nut. Fill in a small area in the upper and lower stern halves with resin, tape the hull halves together and drill a 4mm hole through the bottom half and up into your resin filled area in the top half. Get a piece of 4mm brass rod, cut it to suitable lenght and taper it at one end. When inserted into the lower hull it should protrude upwards into the hole in the upper hull and form a locating pin.
For all the bonding methods in the following, do not use epoxy resin, use only polyester resin and glassfibre mixture. Get hold of some Isopon P40 from the local car parts shop. Don't get P38 as that is simply bodyfiller and has no real strength.
At the front of the hull, fix two perspex plates in the hull one above the other in the upper and lower hull halves, begin by placing the lower one across the lower hull, bonding it in with resin and repeat the same for the upper hull half with this plate sighted immediately above the lower one. The same idea applies here, ie to form a locating pin but of course now they are internal, you cant drill through these as in the stern to ensure correct alignment so here's another idea for you: Drill a 4mm hole in the lower late. Find a short lenght of 4mm brass rod, stick it in your electric drill and whilst spinning, file it to a sharp point then insert it in the hole in the plate, if you used the correct sized drill it should be a nice tight fit. Gently lower the top half onto the boat, aligning the hulls as you go and finally gently press the top hull down onto the sharp pin as this will mark your upper plate. Whip the top off and drill the 4mm hole through the marking. When these pins are glued into their respective positions, the top will simple drop on and stay there.
Ok, that doesn't cure your bowing problem so here's the next move which should solve it permanently for you:
Amidships, and each side of the boat, we want to attach a strip of half inch wide brass, one sixteenth thickness material will do it. Cut a couple of bits around two inches (50mm) long and drill some small holes in the strips for the first inch, leaving one end blank. Bond these strips into the inside of the lower hull each side so leaving a short length of around 12-15mm sticking up. The holes you just drilled in the plates will aid the fixing process and help to lock the stips into your resin. When fully hardened, lower the upper hull half in place over your two locating pins and mark of the area where the new brass plates are. Tape the hull top down firmly to the lower half so there are no gaps and using a pilot drill, drill through the upper hull and through the bras plates (now they are fully cured of course).
If you are fortunate enough to own a tap and die set, the holes in the plates can be tapped M3 and countersunk screws used throuth the hull to engage with the threads in the pates. If unable to do this, solder a couple of nuts to inside of the brass plates and apply the screws in the same way.
Theoretically you should have cracked the problem by now but if the are any further issues with various areas of misalignment, deft and careful use of the heat gun should help to solve the problems there .
If I can be of any further assistance just ask chaps. I have been very busy and don't get on here as much as I used to but will keep a weather eye open.
If you want to discuss it directly, give me a call (01493 754007) during work hours.
Chris