Lock Crowther 1962 Kraken 25 trimaran build and rebuild log

400 will be good - lighter would be better. No! Don't put DB on the bulkheads - only over the fillet and about 3-4cm onto the hull skin and bulkheads. Epoxy coat the rest of the bulkhead.

cheers

Phil

 

I've retrieved the mast from storage. It all looks pretty good to be honest I'm wondering how best to inspect the fittings and whether it's a good idea to re rivet the hardware. I'm thinking of swapping some of the standing rigging shrouds/forestay and trapeze wires for synthetic. I've heard that it's best to leave the wires for the spreaders?

 

guessing spreader is stainless? looks clean


Barry

 

I've been poking around in the main crossbeam of the trimaran lately. It was rebuilt at one stage after a breakage and is now excessively heavy. Opening the fairing it was easy to see why as oversized hardwood sawn bulkheads are fitted.

I'm seriously considering building a new one as all those heavy hardwood nailers are unnecessary and heavy and can be replaced by a simple plywood bulkhead with an epoxy fillet and double bias. My question is since all of the strength is in the rear truss of the beam. Is it acceptable to rebuild the fairing in 3mm ply? I'm thinking of maybe marking the larger unsupported areas of ply as "no step" or am I getting excessively concerned and bonkers about weight?

 

Ask yourself "what would Loch do ?
I guess it depends who is going to sail the boat? Is it going to be thrashed or is it a demonstration (museum) piece ? I think it's great to be weight conscious but don't make the boat unusable.

 

If you want to go nuts, use 1/8th aircraft ply with the bend lengthways, and wrap two layers around the leading edge.

Tortured ply is stiffer, à la Tornado et al.

 

It's going to be thrashed around the course but looking at Gary's ultra light trimarans has me examining every area of the boat to see where I can save weight without compromising the structure and the beam fairings do seem overbuilt for this application. Plans nominate 3/16 ply (approx 4.8mm) which seems excessive. Out of interest the original failure was in the truss when the glue failed on one of the ply doublers in the rear of the beam so not relating to the fairing portion of the beam.

 

Lock would have said,"build new beams, as light as possible, just as shown in the plans".
That would mean use Spruce for the top and bottom members and also the intercostals. Ply is OK for the webs providing it is a good quality ply with no voids or knots. The skins are OK too at 3mm since they are simply streamlining and don't take much part in resisting the stress reversals on the beams.

 

Thanks Paddy, I was thinking along those lines but nice to have some confirmation that I'm not going crazy. I'm looking forward to getting a lot done on the boat in January it's my quiet month at work and I don't want to be boatless for another Southern Ocean Multihull Regatta.

Cheers.

 

Might be hard to get Spruce

Gday Owen

It might be quite hard to get the small amount of spruce you will need for this job. IIRC we had to import spruce for a job 16 years ago.

You can get clear western red cedar easily in Australia. WRC and spruce have these numbers

Cedar - Modulus of rupture - 50MPa. E = 7.66 GPa
Spruce - MOR - 71 MPa. E = 9.69 GPa

So the two important properties have to be beefed up if you go to cedar.

MOR factor = 71/50 = 1.42
E factor = 9.69/7,66 = 1.27

So I would increase the thickness of the cedar by 1.4 times (or whatever comes close to that and use cedar)

Remember that Lock designed this before epoxy. As such the shear strength of the glue joins will be far, far better than anything made with resorcinol or heaven forbid animal and casein glues.

Also ply and wood is a wonderful material for beams. I do wonder when people say ply is old fashioned. What ply and timber beams do is give you time. If the beam is going to fail it will usually do so with plenty of warning - years in fact. Many a multi has had a small crack develop in a ply beam that can then be fixed, often with no more than a few pieces of strategically applied uni.

Any new beams will be way stronger than the originals because of the glue.

cheers

Phil

 

Hi Phil,
The beams were built (or rebuilt) with resorcinol the float hulls with a casein based glue and the difference is stark. All the glue bonds in the floats have completely failed but the beams are fine.

I'll do some comparative pricing and see what I find in terms of material substitution I don't have the plans in front of me but I think Lock called for clear Oregon in the beam truss.

 

Cool - Oregon is often used in beams

Apart from the fact that there really is no such thing as Oregon. We have it in Aus and NZ but if you go to the US where it is sourced then no-one will know what you mean. Yellow Pine, White Pine, Sitka Spruce etc.

I think when we say Oregon we usually mean Douglas Fir. Still a good material for tops and bottoms.

Just a quick check. Did Lock make the top flange slightly deeper than the bottom? Wood handles tension better than compression.

 

The originals have douglas fir. The plan dimensions are 2 7/8th" x 7/8th" top and bottom, free from flaws. The lower flange also has a series of holes drilled in it to accept the net attachments. I wont be doing that I'll attach a glassed on pvc tube with cutouts to take the edge of the tramp

 

Corley do you know about a business called marine timbers. In mordialloc or moorabbin i think . They stock clear oregon for boat building.

 

Thanks Whitepointer, My last batch of douglas fir/Oregon came from Just Oregon out in Dandenong. They were good to deal with their prices were pretty reasonable and the timber clear and of very high quality. I've dealt with Marine Timbers/Southern Sails in the past but find their prices to be at the higher end for timber/ply however for other boatbuilding supplies they are competitive.