Strip-planking thickness for trimaran

I'm having a giant lump of red Cedar milled next week and I'm hoping for some feedback on planking thickness. it's a 29' folding trimaran that's to be used primarily for cruising, but the boat should be light.
The Cedar is just for strip-planking the underbody below the chine. My plan was to mill it at 1/2" and plane it to 7/16 (10 or 11 mm), or so. The spans are considerable, crossways spans shown, but the lengthways spans would be over 4 1/2' (1400mm) long if I don't do intermediate ribs. The skins would be Hexcel 7725 8.9 oz (300 gsm) 2x2 twill (Rutan) glass, which is stronger than normal 300 gsm glass.

Any thoughts? Has anyone seen failures of similar, or thinner strip planked hulls?
The design is not finished, so the drawing is not complete, but I'm having the Cedar milled in a week.

 

One of the first cedar strip boats built in NZ in the 70's or 80's was a 35' trimaran. 6mm cedar with 200 gsm glass inside and out. Still going strong afaik. Gary Baigent will probably know the name and details.
From memory, Adrenalin, the Gougeon's 12m/40' tri was 8mm cedar with 200 uni carbon one, or both sides. Only had bulkheads at the beams.
I have built a few 35 and 40' strip planked boats for my own use with 6-10mm cedar, helped build or sold materials to several others over the years.
Generally speaking, the older the design, the lighter the scantlings. There are several reasons for this, none of them sound, in my opinion.
Make sure you have enough frames to support the unglassed timber while you fair it.
You will need to sand or plane the planked hull pre glassing so planing the cedar before stripping is unnecessary. The finish from a decent resaw (wide blade bandsaw, not sure what it is called in USA) will be plenty good enough.

 

Russell. Agree with Rob the first Tennant 35 ft tri had 6 mm WRC with 200 gsm unidirectonal inside and out. It was doubled over the bottom.
Excalibur 1 a very hard raced tube cat 32 x 20 in NZ that weighs 1760 lbs and displaces 2700 lbs with 600 square foot of sail has 9 mm WRC with 280 gsm uni either side plus an additional 1 meter wide 300 gsm cloth over the keel line. The hull has 3 ring frames inside each hull and bunks are structural. (page 55 Multihull structure thoughts or type Excalibur 1 Dave Knaggs into google).
Turning Point a Grainger racing tube cat 32 x 20 foot that displaced 2800 lbs had 8 mm wrc with 200 gsm uni either side doubled over the bottom. Minimal internal structure but required maintenance.
A Hughes 30 racing tri 30 x 26 displacing 3500 lbs mainhull had 9 mm wrc with 300 gsm uni on either side double underwater. No idea of internal structure.
Woods Skua cat 30 x 18 cruiser racer displacing 3300 lbs has 10 mm wrc 450 gsm double bias inside and out.
Tennent Stinger racing tri 28 x22 displacing 2000 lbs had 6 mm wrc with 165 gsm kevlar either side doubled to waterline.
Prescott Firefly 8.5 28 x 19 weight 2300 lbs displacement 3600 lbs. has 315 gsm double bias outside 8 mm wrc 200 gsm uni inside.
Final with warning Grainger 40 foot tube racer cat Flat Chat 40 x 25 foot 5000 lbs displacement 10 mm wrc 300 gsm uni either side, it split its hull open repaired with more glass and is still racing.
Hope this helps.

 

I think you should wait to mill the lumber until the design is finished. The drawings are not necessary at this stage, only the structural calculations. Rob says he has built boats with 6-10 mm cedar. You should take into consideration, that 10mm is about 4.6 times stiffer than 6mm.

 

You have plenty of feedback on the thickness and layup. How lucky to get oldmulti to summarize his incredible thread for you.

I did a strip canoe years ago. I used one side rough 3/4" boards. I found that gluing was actually better on sawn edges and the rough sides.

Obviously, a large tree getting milled would have too much variability and require planing the planks to a common dimension, but if you do a good job of reripping; I'd prefer saw marks for the glue seams, unless you are moulding it all bead and cove.

Another thing to consider is grain direction. It is less important, but the wood will always behave better if somewhat quarter sawn. I always shrink wrapped my strips to keep them somewhat straighter as well. Nothing too fancy, weights can work as well.

One other thing to consider is it is a pain to have two near dimensions. So, for example, if you plane to 7/16", then the rerip should be 3/4" and not 1/2" because you will always be getting confused when laying them down. Staring at and turning the sides would become tiring.

I built my strips a bit undersized at 0.180". I regretted that decision because when I prefaired the hull I got very nervous on curves. So, I like your plan to mill at 1/2" and plane to 11mm. Think a lot of those dimensions oldmulti shared could end up pretty light at corners unless they are fairing very little before glass.

I also started the planks in strategic locations so I could continue to plank nontop. I had to use intermediate clamping because my strips were so thin. So, I'd glue a strip, staple, intermediate clamp, then go the other side, then go the other side of the boat. It was quite a headache. Perhaps at 7/16"; you have some other means of keeping them in alignment or won't need to between stations, but it is something to consider with care. Stations too far apart will result in strips misbehaving and slowing the job way down. Strip planking is time consuming enough without intermediate clamping. I still remember gluing in four different places, then removing the first set of intermediates and that same routine walking around the boat until midnite..

 

Thanks Oldmulti,
This is about what I was thinking the range was, mostly from reading your thread. I made Jzerro's ama strip-planked cedar and thick enough that it got quite heavy, so I don't want to do that. I'm thinking 10 mm finished thickness lets me mill at 1/2" and have plenty left to plane to the finished thickness.

 

Russell, from Ross Pelham re Tennant 47' cat Timeliner build.
"Pahia hull construction was 13mm cedar and 440 gm unidirectional on each side, sounds lightweight I know but was fine,I put an extra layer of 400 db below WL each side for beaching loads on skeg, not specified by Malcolm, total boat weight 5.9 t fitted out with half full tanks ,she had two queen berth 2 double and 2 swr /wc ,would be interesting what the displacement of a new boat build would be under today's spec!
Most people are shocked when i tell them the planks were 13mm but she is very strong with no panel bigger than 1m x 2m with furniture and foam glass ring frames elsewhere,Tennant knew his stuff and I built exactly to plan except cabin shape which i modernized, I built that one at Coomera in Peter Obriens shed in ,99" JFWIW re Tennant build.

 

Thanks Tony. Amazing what you can get away with with wood and a bit of glass.

 

OM,
Do you have any more details of Flat Chat's hull splitting? I would be surprised if it was caused by sailing loads. Much more likely due to insufficient reinforcement at high load points, probably the daggerboards or rudder.

Note that the catamaran laminates you described will see far higher loads than the bottom of the floats on a trailerable trimaran. The Tennant Stinger (6mm cedar) is more in the ball park of what would be optimal. The Tornado cat which won the gold medal in Los Angeles was 3mm cedar strips with very light glass (150 gsm from memory) both sides. Was arguably sailed harder/abused more than the boats mentioned above.

 

Rob, the person I spoke to in Hervey Bay who helped build the cat said the builder wanted a "fair" hull and didn't worry to much about how thin the strip planking was sanded down to get the shape required. Result some areas may not have had full thickness WRC. The cat was racing in, I think QLD, when the hull split near the turn of the bilge over a short distance. It could have been some faulty timber, a knot or excessively sanded down WRC, I do not know but result was a repair job done by Jamie Morris I think, who you know, and then it was raced by Jamie and partially owned by a variety of people who had great fun winning many races. Flat Chat is in NZ now. Flat Chat is a 37 foot Taeping design stretched 3 feet and the main beam was rolled flat according to Darren Drew. My records show a gold medal winning Tornado had 200 gsm uni either side of 3 mm WRC.

 

Here's a Flat Chat photo taken from my proa.

 

Thanks, OM. Interesting.
Re Flatchat: The cedar supplies fore and aft strength, very little sideways, so a split caused by the wood thickness is unlikely. Maybe more likely the side loads from the dagger were more than the glass could handle? I knew Jamie well, drove his Formula 40 cat for a season. Nice guy, great builder.
Re Tornado: 200 gsm uni back then was 90/10 or 80/20 woven cloth. From memory (it was a long time ago!) they pulled the weft fibres from the laminate after applying resin. Sounds unlikely, but Chris Timms (crew/builder) was a fun mix of anal and experimental, so anything was possible.

 

Maybe you will find this interesting, the layout of 50` tri, looks interesting how they did the layout, If you can´t read let me know I have typed this table and save it somewhere in my computer, I can look for it. This is public on the webpage of Owen Clarke Design.

They used 80kg/m3 P500 Corecell, 25mm Thick, quite different thickness from the other examples before with cedar.