WW2 mini sub build

I have now cleaned up the 2 halves, using one of the handles that takes a hacksaw blade, using a full size hacksaw I found I couldn't get close to the line, but a blade in a handle is OK, though the blade does bend if it snags as youre pushing.


I held the 2 halves together using masking tape, though I know that some suggest putting them back into the mould to hold the halves together. I then used a lot of resin internally followed by pre-wetted 50mm woven tape which was very fiddly right up in the bow and stern areas. I did do the taping in several lengths especially at the ends, to make things a bit easier, but in general not too bad.


When I took of the masking tape I wasn't entierly supprised to find voids in the joints in places as I knew the 2 halves didn't have that good a fit, not sure if that was because the hull haves had warped or just poor cutting to the line.

I wanted to fit the prop & shaft before making the stern fins to ensure that the rudder & 'plane hinges cleared the prop, but I had decide to make a stern bearing rather than use a commercial prop shaft assembly, as I wanted to give plenty of bearing surface and I didn't need the shaft sealing and wasn't sure about the brass-resin bonding. The bearing was based on an M12 x 30mm bolt bored for a 5mm shaft (I used the shaft from a standard prop shaft, just not the outer). I machined 4 countersink dimples on the threaded part which when embedded in resin should stop the bearing rotating, even if the fixing becomes loose.  I had to ease the bore a bit as well, as either the shaft was a tad oversize or the drill a tad under.


The shaft is a standard 6" x M5 with a 70mm dia 4 bladed prop, about the biggest I could get. Eventually it will need a collar on the inside to take the reversing thrust.
I fixed the bearing to the hull using resin loaded with glass micro-balloons (basically look like a white powder) added after the resin & catalyst were mixed, with enough added to make a thicker paste that would stay in place a bit better.
I used 2 batches, the first to fill in the gaps between the hull halves (on the outside) and the 2nd to 'pour' into the hull to retain the bearing. I did use masking tape again to hold the bearing in place, with an inner block to keep the shaft straight, but it leaked so I had to hold the bearing by hand with the hull on end untill it cured sufficiently to stay in place - tedious and a bit messy.
Once rubbery I could cut off most the excess with a sharp craft knife, including freeing the shaft from the bearing that had stuck due to the leakage.
Mixing small batches of resin (50ml & 25ml) even with a syringe to add catalyst seems a bit hit and miss, but I had no problems with the resin not curing using 1% (by volume) catalyst in an unheated garage.

I'm quite pleased with the way the resin has filled the end of the hull, though where I poured it it is a bit obvious.


It's now starting to look like something nautical!

I might add an inboard shaft bearing for additional support, and if I was doing it again I would think about recssing the bearing into the hull as it looks a bit ugly due to the gap between the end of the hull and the actual propeller. I could have used the supplied outer tube, but it is so smooth I was woried about it bonding and there wasn't enough material to go machining lumps out of it. Maybe solder a tappered perforated metal plate next time as it would obviate the need for an additional inboard bearing.

The next job is to tidyup the outer before starting on the fins.

I now have what looks like an X-class hull, at least from a distance!
The middle 'lid' section was much like the two hulll halves, again required quite a lot of trimming, a hack-saw-blade-in-a-handle thingy from B&Q helped here.



The trouble is that after trimming the joint is far from perfect. I used masking tape along the edges to mark the horizontal mating line, but the 'lid' is now slightly narrower than the hull, and also taller, I can't trim down the height as it would make the width mismatch worse, so I'm stuck with what I have I think, bit of a shame. I'm not sure why, the obvious reason is that I cut the hull plug into hull and lid before adding the surface skim, so it is possible that the hull ended up with more surface skim (wider) that the lid. I don't think the moulds warped as they seem fairly rigid (2 layers CSM) nor the hull warped (3 layesr CSM) but the end result will have to do.
Not that obvious is that there was an air void under the gelcoat in the mould so I need some filler just on the break of the upper hull, and I hadn't noticed that some stray fibres form the hull haves had wafted into the lid mould so are visible on the surface at the moment. It would have been better to have finished the hull halves before waxing the lid mould as I could have cleaned the mould then. As it was I was waxing all 3 moulds at the same time (many times!)

You can also see the rough blank for the upper rudder fin, these are all ply and I intend to reinforce the joint with brass pins and resin/filler paste inside the hull. They are way over scale in terms of thickness but as a first build I am worried about them breaking off if I go too thin.

Obviously once finished I will need to prime/paint the entire hull, I was hoping that only the fins would need it but there is too much surface 'texture' & filler paste for that.

The rudder & 'plane hinges will be short brass tubes for a 2mm steel rod acting as the hinge. These tubes will have 'U' shaped brass strips soldered to then that can be fixed to the ply fins.

I'm thinking of coating the ply fins initially in a coat of thinned resin (acetone?) to allow resin to soak into the ply, anybody tried that?

I also need to add some small stops fixed to the hull inner to support the lid and provide some degree of locking in place, then on to the WTC supports.

Trim the hatch for height both fore and aft. there are ways of fixing the mismatch in width.

first, apply heat (hair dryer or heat gun) until the material is pliable, then use formers to change its shape... a few wooden bulkheads inside the hatch should work. you will probably have to bend it past the proper shape, it will spring back a little bit after it cools off.

second, put an inner lip on the hull, forcing the hatch to match the shape of the hull when it is on the hull.

Thanks to Tim for advice on getting the hatch to fit better, and it does!
I ended up using a grinding wheel to trim of the GRP hach, there wasn't enough to use a saw, and using a file would be a long job, but going easy with a grinding wheel sorted the height, followed by use of a hairdryer (our spare, of course!) with 2 plywoord templates for expanding the width.
It took a while to get the GRP to any warmth and I didn't want to overheat, but it seemed that with a small hairdryer the problem was too little heat, as the hatch never got much more than hand hot.
The templates were basically rectangular sheets that fitted horizonatlly inside the hatch with the idea to heat on the curved section not the rectangular upper casing. These could be forced into place as there was enough spring in the GRP, then held in place whilst applying heat.
I had to make 2 sets of bulkheads as the first didn't fit well enough even for me.

I also made the WTC bulkheads, this time from two laminations of 2mm plastic sheet cut to shape and then bonded using plasweld solvent based glue. The advantage of laminating is that the 2mm flexes so can be trial fitted even when oversize so can be gradually fitted to the inside of the hull.
The resulstant 4mm bulkheads seem rigid and strong enough.

The issue now is that when I trial fitted with the actual WTC the prop shafts don't align well enough. Some mismatch is OK as I need a middle section so UJs will take up any small misalignment, but I need to lower the WTC by about 15mm, due to that miscalculation.



I did try as an experiment using plasweld to glue a section of the plastic sheet to a dummy piece of GRP, but possibly due to the gaps (inside of hull is very uneven) it didn't work, so its some kind of epoxy glue to bond the bulkheads in place.

Dropping the WTC reduces that all important meta-centric height. The offset of drive shaft and prop shaft can be corrected several ways. A simple dog-bone connector is one way.