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Scratch Build project SM U-23 Class World war one U-boote. Zero Bubble model design.

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  • Hello all,

    After cleaning up the molds I was almost ready for putting down a gel coat and laying up the first parts. I could do any number of the molds. Coat the Hard shell molds in PVA and with the Silicon ones I could just go right ahead. However I decided to add another feature the main hull molds that would make fabrication of the main hull part trickier but would lend to a much better middle hull half. I wanted to put an inner flange along the top of the Middle hull mold. This would give the top hull mold a flat surface to sit on and would also lend some rigidity to the overall frame. To do this I decided to use some acrylic sheet that would bolt down to the top flange of both middle hull halves. I then took a texta and marked out the curve of the rim of the mold and then added an extra overhand rim of about 10 mm. This is where I would cut along the length. I then cut this out and drilled a series of holes along the main flange of the hull molds and after more release agent could bold these long strips of acrylic sheet to the flange. The Gel coat would be next and then would be brushed up underneath with a brush, in the video I bent a brush to get up underneath here. I now gel coat the two halves separately and then lay up some continuous strand before joining the two halves and putting a little more gel coat down the keel and then a conventional layup for the rest of the one part hull mold.

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    The Acrylic sheet ends before the stern hard shell glove mold. As the first boat out of the molds is mine I didn't mold the stern separately as I had to ship it nowhere. The white masking tape helps to avoid flash down the inside flanges of both halves.

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    Gel coat down. Nice and thick and 27 degrees.

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    You can see how the Gel coat has worked up under the rim of the acrylic sheet. I created a similar fitting for the bow. I wanted to mold a register lip into the back end of the forward lower bow section. To do this I created an acrylic boat that goes across the perpendicular flange at the end of the bow molds. This piece follows a profile just a little bit shallow of the overall hull bow profile. This is designed to create a slight 'dam' effect with the gel coat and a raised step with the weave that will when pulled away translate to a slightly inner register lip. That's the plan anyway....

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    Wrapping the resin and weave around the complex profile of the torpedo tube doors wasn't easy. Some kind of profile that I can press up against the mold might do. The register idea works however there is some dribble along the in side flange between the dam and then flange.


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    Several layers of 225 weave. Extending to the rear stern torpedo section.

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    Finally removing the lower bow section. It is a bit rough but nothing that filler and sanding won't fix.

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    My Horten Ho 229 front end looks like a Troll doll.

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    More next week.

    David H

    Comment


    • How did you create that flawless stern? Thick gelcoat and weave only?

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      • Hi Joerg,

        Thats right, a nice thick gel coat and weave.

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        • Merry Christmas !

          Enjoy.

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          Part three...


          https://www.youtube.com/watch?v=f18tME33iaw


          Thanks for Watching..

          David H

          Comment


          • David,
            Great video. I noticed you used CA for sealing the ballast tank, is it strong enough to resist the pressure?
            Make it simple, make strong, make it work!

            Comment


            • Thank you.

              A dry acrylic surface and cyano, absolutely. It’s effectively welded. Extremely strong.

              Dave.

              Comment


              • Getting dizzy just from watching! well done, impressive development and a nice guy sitting by the pool - ah nice video.

                Are the Ballast tank endcaps acrylic as well? have you testet shock resistance? what happens should you blow the ballast with bow down?

                You could finalize the lateral limberholes with a small rattail file

                Very nice model!

                Am staying warm ;-)

                Jörg

                Comment


                • Hi Joerg,

                  Thanks,

                  The whole ballast tank is acrylic. I have made several acrylic ballast tanks over the years. You just have to make sure they aren’t sitting in the sun. Expansion and contraction of different sections. Otherwise they work really well. Blow the ballast briefly in bow down trim and then as the boat comes to the surface and levels off, then blow the rest. About the work on the raised radio masts…

                  Dave.
                  Last edited by Davidh; 12-21-2021, 07:01 AM.

                  Comment


                  • Hello all,

                    I now had all the parts laid up and just needed to pull them out of the molds. The top mold needed a gel coat layer and then some weave along with strand around the edges and sides of the hull. This would reinforce the curve in the outer top hull where it bends down to meet the lower hull. Once this was set and hardened then I could pull it from the mold. This was pretty straightforward and when it came out of the silicon mold I was very happy that there were no air bubbles, "tripe" or wrinkling. Fantastic. There would be less post processing needed. The Aluminium 'L' reinforce-rs had also created a dead smooth and flat main deck section. No undulations!'

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                    The Blue tape a placed over the edge of the mold to pull the silicon mold outwards in order for the rim of the top mold to meet with the rim of the bottom. I didn't really need to , the fit was really close.

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                    The top mold part pulled from the mold. As mentioned the decks are all dead level and flat. There are virtually no air bubbles and no tripe. (wrinkling of the gel coat.)
                    Very happy with how its turned out. Probably the best top mold I have pulled on any kit.

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                    After removing the acrylic top flange templates, I could them unscrew the wingnuts from the underside attachment points and then start prying the two main hull molds apart. I then managed to get one half off and this is how the part turned out. Some minor wrinkling in various places but not a lot. The keel and then stern transom have turned out really well. The joint line between the stern torpedo section and the stern of the main hull is barely noticeable.

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                    The main lower hull out of the mold and after a clean up. The stern and bow sections have been cleaned up and are looking really nice. Still will need to get some filler onto the tripe areas.

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                    Stern section and then light reflecting shows a smooth series of decks.

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                    The rim or flange along the top of the lower hull has turned out well. It is thick enough that hen I sand it back level there is enough material underneath not to have any thin sections that create holes.

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                    There was plenty of sanding to get to this picture. The top mold was laid on my sanding board and moved back and forth to get a nice level surface. The same was done with the bottom mold upside down to get a nice level surface along the rim. It was at this point that I noticed a slight lopsided-ness at the amidships with the beam. I didn't notice it before, bummer however, you really have to look at it to really notice it. I think that I will be able to sand it out to extent to create a more gradual curve on the offending side.
                    The interface between the two pieces however, is really good especially this early on.

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                    The bottom bow section mold is pretty good considering it was my first hybrid Silicon / hard shell mold. There is a discrepancy line around the torpedo tubes where the two mediums interface. This will be sanded and filled out. Here you can see I have already drilled the holes for the forward Hydroplanes that fit towards the back of the bow section. The rearward lip is also visible. The next step will be creating the forward attachment point that will be an aluminium plate with a bolt hole and a hole in the main hull for the attachment.

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                    In the meantime I got busy working on the Conning tower. I had cast the parts for these much earlier and hadn't got around to working on them. I scrubbed back and smoothed any imperfections on the surface. Then some filler and more sanding and a tiny bot of work on the Dremel. I did the same with the conning tower deck casting and then glued this down. The sides of the deck won't be seen as they will be covered by the canvas covered frame pieces. These needed a bit or work of their own.

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                    Helm and Daughter compass? These pieces fit into small holes cast into the deck and the conning tower forward base. I drilled holes in the deck to make sure water could escape.


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                    A light coat of paint to simulate the wooden deck of the conning tower. Starting to look OK. I was really worried about how the molds for the frame and canvas covering were going to turn out but I am going to declare that they are my 'Piece de resistance'. They turned out better than I expected. A light spray of a canvas like shape of beige and faded red of the rescue flotation ring.

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                    More later.

                    David H

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                      Make it simple, make strong, make it work!

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                      • Hey Redboat, I'm with you,

                        I really don't get this whole Northern Hemisphere snow white Christmas thing. I went slope soaring at the beach yesterday, (Christmas day, for about an hour, the sand was soo hot ,30 degrees, dripping).

                        Hello all,

                        The conning tower is coming along. I sprayed the Canvas covering with a tan beige and then carefully painted the inside of the railing structure with a dull grey colour. Did this with a small brush. Once the two railings were done I then gave the final touch ups to the rest of the conning tower especially the areas around the tower deck and the raised housing for the Periscopes in the middle. The helm was painted a flat grey colour as there is very little information on the colours of this unit. I drilled out the recess and glued in place. Once these parts were done them I got around to gluing the sides onto the conning tower.


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                        I added some extra scribe lines along the sides of the conning tower and a small hatch at the bottom. I will be weathering the conning tower and giving her some rust here and there. I'm also going to add the periscopes later. One of them will be the antenna mast for the tip of the 2.4 Ghz I intend on using.

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                        I had to fill in a small gap between the two railing sections at the front. This was done with a fine bead of filler and some sanding. I also had to give some light filler around the base of the railing section to blend a little more with the sides of the conning tower. I have had a hard time with the drainage holes. However I have made a start on the drainage holes along the sides of the main deck. After working on the Conning tower I continued on with the main hull. As mentioned in the last write up there is actually a slight lop sided-ness in the hull amidships. One side of the hull approaches the widest point with a slightly more pointed curve. The other side, less so. You really have to look to notice however I have determined that with a little aggressive sanding on the 'pointy' side I can give a more gradual curve that comes very close to the one on the other side. As mentioned, you have to look hard. So as a result I thickened up the rim on the pointy side to give extra material and then sanded back a more gradual radii and it looks better.

                        The next step was to work out the joints between the top and bottom hull. As is my custom I decided once again on the "Z' cut arrangement. This would see the bow bottom section glued to the main top hull and then very stern upper section glued to the stern lower hull. This has worked well on all my boats. The main thing at the moment however is the sand the top and bottom hulls level with each other. So I got out my sanding board and sanded till I had a nice horizontal flat surface for both.

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                        I had to level the lower surface of the lower bow so that it would mate up the top bow. There was some room for gaps here as I would use fine weave and resin as glue. I just needed to make sure that the lower bow and main hull aligned up perfectly. This design is an unusual departure for me as I usually have the lower bow as a one piece mold. The way I have assembles the Aluminium attachment plate on previous models has been to make the cut in the forward lower section and the rest of the hull after the a plate has been attached. Here the cut has already been made and it means that there are more pieces that have to be aligned simultaneously.

                        In the mean time lots of filling, sanding, filling ,sanding......

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                        I created the aluminium attachment plate out of 3 mm thick 10 mm wide aluminium bar. Drilled a hole and tapped a thread. Then drilled some holes at the other end for some weave and resin to work its way through. Then resin'd in place. I would then have to work out how to align the bolt hole for the forward part of the main hull. Whiles setting this to dry I cut and started shaping the piece of buoyancy foam (from a surfboard blank supplier) that I would stick in the front section below the waterline. Drill a few drainage holes to let the trapped air upwards and be ready to glue into the lower bow section.


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                        Drilled the holes on either side for the forward planes placement. These planes will be fixed.

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                        Once that I had got the two main surfaces level I would need to tape them up and get them precisely whereI wanted them for the glue up of the lower bow to the top hull. This involved marking out where the drill hole would need to be for the main aluminium plate at the front and making sure that the lower hull and forward section met at the right spot without a gaping gap between them when the bolt is affixed. Lots of converging geometries needing to line up all at the same time.

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                        More next time. I hope every one has had a good Christmas. For those up North, keep warm, for those in my part of the world, crank up the Air conditioning.

                        David H









                        Comment


                        • A most ambitious project. And the quality of your work gets better and better.

                          David
                          Resident Luddite

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                          • Thank you David.

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                            • Hello all,

                              A new video.

                              https://www.youtube.com/watch?v=r2GNVSl0PUA

                              Comment


                              • Hello David very nice work and video.

                                Gantu
                                Regards Gantu

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