Annie wrote:

To me, this 7 metre variant is very clever - a designer's dream.  But the builder is going to end up with the interior the designer has decided s/he wants.  In fact, there is absolutely no flexibility at all.  So why would you build it?  Why not go out and find an existing boat that is almost what you want and remodel it?  It would take little less time, be cheaper and, in the meantime, you could be sailing.

And what is all this business about saving time?  You don't build a boat to save time, you build a boat because you want to and because the journey is a large part of the pleasure.  The concept is brilliant, no doubt about that, but it is a concept much more suited to a small factory producing several boats than one person building alone.

Ya, that was my first thought, "how do I visualize the interior?" However, do please note that this can be done one off. That is, the cost of cutting is $x per minute no matter if it is one off or twenty. So it is still possible to customize the interior. The hull shape is pretty much set but the bulkhead position does not have to be every 100 cm (or whatever the designer specs). It is possible to shift a bulkhead for or aft by quite a bit to fit the interior rather than having to build the interior later around preset bulkheads. So rather than getting what the designer wants, you can actually get something closer to what you want with this method. Also a lot of the gluing can be done with more elbow space before the hull is formed.

The problem I see is still having to decide before the first cut is made. I think maybe something in between where some of the major stuff can be prebuilt in. Then the interior can be finished after. I would give examples here but I don't have the experience of anything more than day and overnight where it is easy to make do. I am more interested in seeing the full size finished out and the time it takes.

I think if I were to use this method of building I would have to set up some sheets of plywood and add chairs, benches, boxes and tables to help me figure out a layout before cutting. Because the cutting can be per sheet, it may be worth while to cut in smaller batches. Cut a few sheets of bulkheads that I know, set them up (right side up), put one over top if needed and try things out for space. Move things around. Even those bulkheads already cut could probably be moved a bit with minimum cutting or adding at that stage. Then the rest of the bulkheads and longitudinal bits could be cut and assembled... if you have space right side up (or maybe dry fit right way up) and the walk through continue and more interior things could be done at this stage.

Perhaps the thing to do is start with a boat of similar size and "I want this size but set up this way". Really it just means knowing what _you_ want before you start building and I think that is the hard part.

As for speed of build, building is fun, but being on the water is still what it is all about.

Anyway, I am looking at a glass boat/hull for free (24 foot-ish by the picture) to change the rig on... so taking your advice ;)

Dry trial fit of the topsides.

Starboard quarter, with a gap (though the port quarter is OK) - topsides could be a bit longer.

Both topsides could be longer - a trimming allowance could be added, also a tab extending forwards with a hole for a spanish windlass to haul the two sides together.

Gary wrote:

To me the idea of a CNC kit of parts is a somewhat different experience - one tries to get the CNC to do as much as possible in order to rule out misalignment or idiot mistakes. Any tabs dowels or whatever that speeds this process are fair game to me.

Although I've managed to design out most of the chances of misalignment, there's still one: the bulkheads at stn 1 and stn 3 and the transom need to be truly aligned with no twist around the longitudinal axis, so centrelines should be marked and checked as being truly vertical and parallel with each other. The main chance of error is when making the join between deck stringers and berth fronts; it will help to wedge the transom in place dry, and check for verticality when gluing this join. A little bit of twist will make it a lot more difficult to fit the hull panels in one piece, and is an argument for fitting them in two or three pieces. I think I may have a little bit of twist, which has affected the fit of the topsides.

Personally I would still be looking to assemble the bottom panels as single pieces I do not see a 7 metre strip of 9mm ply being too much of a physical problem to get roughly in situ, but juggling it into precisely the correct location could easily consume inordinate amounts of time.

I've done a trial assembly of the topsides, and the tabs at the sheerline are working well to support the weight and induce curvature. I'm altering the tab and slot in the middle of the panel so that it acts as a hook to hang the panel on, but allows a little bit of rotation in case of errors in the setup.

I do think David's upside down and inside out technique is a novel approach, one that I believe speeds things up.

It certainly does.

I do fancy taping inside things with gravity assist however so I think turning the hull before internal taping would be my preference if physically possible.

Absolutely. Only sufficient initial filleting to keep the hull in shape without damage during turnover, while the hull is upside down, then completion of filleting and taping after turnover. If there are chine logs, the hull will have a lot more strength during that manoeuvre.

David wrote:

I have joined plywood panels with scarf joints, and also two layers of double bias tape on the inside to take the place of a conventional butt block, (butt plate?), and butt blocks. In fitting the hull panels of my new catamaran I have been using butt plates which are notched and glued into the stringers and chine log prior to fitting the panels. It has been very quick and easy for my small 6 meter boat and this has been the way I have done all the topside panels, fitting one panel at a time. I am thinking though that for the bottom panels I will put the joins in a different location than in the top panels, and rather than using the plywood butt plates i will use two layers of wide double bias cloth on the inside, if I can get a clean even join in the ply panels as I install them, and I can hopefully leave the taping of the inside of the joints until I have turned the hulls upright.

So in the case of the 7m SibLim, if I were building I would be happy to go with butt blocks, or fiberglass taped joins on the plywood panels. Making scarf joints in plywood does require care and a level of woodworking expertise, and unless they are done well I do not think they would be as strong as a joint using butt blocks. 

Regarding the length of panels which can be successfully glued in one operation; I am finding that in our New Zealand summer temperatures, and using slow hardener, it is a bit of a challenge to get a 2.4 metre long panel glued up and fastened, and cleaned up before the glue starts going off. I am doing this single handed so with a second person mixing the glue while the first person was applying the glue, things might go a little better. But having said that I am building the catamaran in a site where it gets full sun, next time I will choose a location in the shade. It is not all bad though, with the glue curing so fast I can move onto the next process very quickly.

BTW for more info about how Rm yachts put their plywood boats together this link is worth following.

http://no-frills-sailing.com/at-rm-yachts-of-la-rochelle/

• They build their internal structure on a permanent jig, which will give good accuracy but is not sensible for a one-off build.
• I see some very large puzzle joints in the bulkheads.
• The parts are laser cut, not with a rotating bit.
• I see no sign of tabs and through slots, but some grooves to locate edges, and possibly some holes for dowels.
• 20mm radius colloidal silica fillets to join everything on both sides.
• Between three days and a week, depending on size, to fabricate a hull (!!!)
• Then they lift the hull off the jig and support it on oil drums while all the fillets and biax taping are finished on the inside.
• Thank goodness I don't need a massive galvanised frame to support the keel and shroud loadings.
• They leave quite a gap between hull panels, fill it and then round it off and glass over with biax tape on the outside. When the hull is turned over, they finish the join with more filleting and biax tape.
• Hull panels are scarphed, not puzzle jointed.
• I think there's glass sheathing inside and out as standard, kevlar as an option outside.
• Foam-cored GRP deck.

I think there are two options for me: either framing on the bulkheads and a filleted and taped join between hull panels, with the whole panels added as one piece; or a chine log at the upper chine and fillet and tape at the lower chine, in which case butt-joined single sheets are feasible. I think I'm going to need the chine log, as there are some inaccessible areas where it's too difficult to fillet and tape inside. I think I need the topsides to be added as one piece, or I'd also need a sheer clamp, but possibly the edges of the bottom and topsides can be faired in line with the bulkheads and then individual sheets of the middle panels can be cut oversize at the upper and lower edges, butt joined and trimmed after gluing on to those bevelled edges.

Anyway, I'll press on and add the bottom and bow transom, as there is no doubt what I've to do there. At full size, the bottom can be 12mm, butt joined at substantial floors just forward of stn 3 and just aft of stn 6. Then two layers of 6mm form the forefoot and bow transom (with possibly a third layer of 6mm overall, though I'm not sure that's necessary, as the deadwood and ballast will do a great deal to stiffen the bottom).

BTW for more info about how Rm yachts put their plywood boats together this link is worth following.

http://no-frills-sailing.com/at-rm-yachts-of-la-rochelle/