(strip)planking a Wharram TEHINI ?

Hi to all on this forum.
I`m considering building a Tehini with planks instead of plywood.
The reasons for me would be mainly that here in Seychelles. marine ply is rare to find and rarely used as well. It would have to be ordered for me separately with all the cost, import duty, Vat etc.
Good local tropical wood would be by far more cheaper.
Secondly, finished clear and varnished this cat will be a beauty without a doubt.
Personally i got some bad experience with (marine)plywood on some boats i was repairing here.
Think it is the constant humidity , but ply don`t last very long out here.
Therefor i was thinking and hoping i would build the whole design (bulkheads, backbone etc,) with solid wood instead of ply. (don`t like to work with it either !)

Now i think that the classic designs of Wharram are build ply on frame, tightly fit joints.
The Tama Moana of the same designer is build in strip plank over bulkhead. Strips are 15mm thick and advised width is 35mm to 45-50mm.
But here is what i don`t know.: can the design be build with planks like 15-20mm thick and 100 to 200mm wide, epoxy and glassed outside and inside? Would i need to change the design, or would some extra vertical ribs and heavier glassing on the inside be enough ? The straight V-shape hulls of the Tehini are advised to be build with 15mm marine ply. So i don`t think there will be any really tight curve in the design, considering the flexibility of 15mm ply. But i can always cut a 100mm in half or smaller to manage a more curvy part.
I know that planking a cat of 51 foot is quite more work than with ply that`s why i want to build with wider planks than strip planks glueing and fastening with Raptor nails/staples. also i can get here workers for reasonable money and i myself can work full-time on this project. Apart from all that it will be most rewarding when finished with the wood visible.
I`ve been reading and following this forum quite some time now and noticed that there are many people here with good knowledge and willing to help people like me to make rational decisions about designs and material use.
Therefor i would be grateful for anybody`s: Advise, remarks, critics, questions etc.
Wish u all a good day, Thanks
Islander63

 

As you strip plank the hull the planks have to not only go around the hull which is less of an issue with the Wharram, but they also have to be able to follow the shear, as you plank up towards the keel more bend has to be found as the midsections have more circumference than the ends. This is where your 100mm planks may have issues.
You can spile each plank like the traditional builders or another way is to use a circular saw and recut the
line flat so the next plank will lie fair with no twist. Of course if you cut your planks to 50mm you can go a lot further before twist is an issue.
Finished clear and varnished in the tropics is a going to look great until it doesn’t.
UV, last time I checked even top shelf varnishes don’t last anywhere near as long as a solid colour and 2 x 50’ hulls every few years will put you in an early grave.
Interior, knock yourself out !
Hope this helps.

 

The lamination schedule will be wrong.

100mm planking is subject to a lot of twist and bend, the reason for making planks narrower is to avoid issues like cupping that can occur in a plank later..etc

 

Thanks Redreuben for the reply/advice. I thought already that some areas will be difficult for the wider planks to follow the sheer etc.
But as you also wrote i can always cut my plank smaller, only on those areas where it`s needed. And yes as you pointed out, epoxy`s clearness will suffer quickly under UV. But as far as i`ve heard Polyutherane Varnish with UV protection will be able to prevent this yellowing of Epoxy, but you have to put 5 to 6 coats and freshen the coats up after some time again. Indeed a big job on a 51' cat ! I am planning to live on board, so the maintenance could be spread over days, but it is still a lot of work.

What i was more afraid of is what Fallguy mentioned. Cupping, twisting and maybe even splitting or cracking. I only wondered if that could not be avoided by using a stronger glass fiber on the inside, as well as vertical ribs in between the frame ?

 

Over 100mm you are well into carvel territory, stabilizing the wood is difficult. A design intended for plywood is easiest converted to solid wood by cold molding. Two layers of 7-8mm solid wood is easy, even if some spiling is necessary. Usually done double diagonal, you can also go Ashcroft, with both layers in the same direction, wich speeds up planking.
This type of construction has glass only on the outside, since the inside stringers make glassing difficult.

A varnished exterior in the tropics is a high maintenance item. If you want it to look perfect expect yearly recoating with a complete stripdown and refinish every few years. The only thing keeping the epoxy healthy are the UV blockers in the varnish, so it's extremely important that the varnish is fresh. Paint is much better overall, so my advice is paint the outside (keep some bright trim if you like) and varnish the inside.

 

I didn't want to tell you bright is crazy, but unless you can pay for a month of varnish work every year and staff it reliably; forget it. Two part polyurethane paints can last almost 10 years, depending on where..it'd be wild to varnish.

 

How the wood behaves will largely depend on the suitability of the species to the task
The position in the log from which the board is cut, you want quarter sawn.
And the drying time of the timber, air dried is best, something like a year per 25mm ? Local knowledge is best.
Also keep in mind that every kilo over the designed weight you go you lose a kilo of payload. Many self builders forget this and add thickness here extra frames there an extra layer of glass so their boat is superior.
It’s not.

 

And I think that Rumars is being rather generous here - I have seen large professionally crewed yachts in the Caribbean where they are sanding and recoating the varnish every couple of months, and they will strip down and refinish every year.

Rumars also notes that cold moulding would be much more suitable for wider strips of timber being used - and if you are going down this route, then there are so many much 'better' designs than the Wharram Tehini that are available now.
Yes, the 'traditional' Wharram designs (like Tehini) were the rock stars amongst the home built catamaran community in the 70's, but I don't think that many folk on here would argue with me when I offer an opinion now that their time has passed.
Have a look at Richard Woods' designs - he started off working for James Wharram in the 70's, and sailed transatlantic with him on the original Tehini, before branching out on his own - and he has never looked back.
www.sailingcatamarans.com

 

The thing about cold moulding with veneer is the guy is in the Seychelles and can’t buy plywood, what hope of getting veneers and at what cost !​

 

Another really important thing to remember is if you encapsulate wood with a high enough emc, it will rot from within. This is especially important when using solid wood or strip planking in the tropics. Air dried timber in a coastal climate may not be dry enough to encapsulate. Sorry, I can't advise the emc% to shoot for, but I can tell you that above 10% is not good and in a coastal environment, you'll not likely stay below 15 out of climate controls.

 

Would it be feasible for him to rip up the locally available timber into suitable sizes for cold moulding?

@Islander63 what kind of timber can you typically get in the Seychelles?
And will it be possible to dry the timber sufficiently there, given the relatively high humidity?

 

Waste is not a problem, the lumber is always cheaper. Let's say a 15mm (~5/8) sheet of marine fir ply is costing you 100$. Go to the big box store, buy 12 pieces of 8ft 2x4 fir to get the same area, that's 60$. One 2x4 ripped with a tablesaw and planed yields 3 pieces of 8mm (5/16) veneer. Add the epoxy and you still don't come to 100$, and this is just a crap comparison. In reality the difference is even greater because you wouldn't buy 2x4's from the big box store.
There are many places where labor is cheap enough that it's not worth sawing your own veneers. The problem is usually finding someone willing to deal with the small quantities involved.

 

A clear-finish/varnished strip-planked Tehini anchored in a tropical bay would be a beautiful thing to look at, and a nightmare to build/maintain/live on. You'd want to build the beams better than Wharram did, the original Tehini rotted several sets of beams in a few years.

 

OK, Okay, guys i`ve heard enough...
You all have convinced me it was a bad idea, maybe a bit over romantic.
So actually it`s not only with the wider planks i will encounter problems, but with wood in general. We have all year round min. 70% to 85% humidity sea-climate.
I don`t think we have drying kilns for wood here.
I guess the humidity here would not affect proper marine plywood, if kept dry ?
Yes i followed most of the Wharram discussions on this forum, and i am aware of most pros- and -cons of their designs. I have been looking for quite some time now for a design that would fit my "needs" Richard woods Flica ; Mike Waller CC40 , Bernd Kohler KD105.
All designs are either too small or too much enclosed space or having keels or huge dagger boards. The Bern Kohler KD105 comes the closest, but with only 34 foot it is lacking (open) deck space. I asked Bernd if his design could be stretched to 40+foot, but didn`t get the idea he would be all to keen on that proposal.
That is my dilemma, searching for the right design for me. Think i should be considering a custom design, but i don`t know if that would be affordable, and not more than all the material together. A good design is priceless, i agree but should be in proportions.

Again thank you all for trying to prevent me going the wrong way and ending up with tears.
Regards islander63