Thread: Mutant goat idea.

Yes,

I like the weights of Oughtred, Graeme Byrnes, a bunch of John Welsford's stuff.

We are all antipodean!

The Northern hemisphere went from traditional timber construction, to fibreglass in one hit pretty well. There are a few plywood racing dinghies in the North, but they almost all take their weight cues from traditional construction.

Thank Goodness for John Spencer NZ and Jack Holt UK (huge numbers of Jack's boats built in Australia - and he wasn't allowed to join the UK yacht clubs because "he worked with his hands)... in spreading lightweight plywood designs and building methods

MIK

With regard to widening the gunnels... You could play around with the aesthetics of this a bit but you could keep weight down by only enlarging the gunnel in the area that you hike out on instead of the whole length. Also, for those of us trying to increase flotation, I wonder what a box beam gunnel might have to offer?. Allan

Well I suppose a quick and simple addition to an existing Goat, if gunnel bum was a problem, would be a strip of 4mm ply with 20x20 edgings. Just shape it to match the sheer, and have the edgings outside the gunwale and inside the inwale. One or two bolts down through the inwale spacers, or even a bit of shock cord, would suffice to hold it in place. You would whip up a port and starboard one pretty quickly and easily, any length you wanted, and they'd give a sitting area 40 mm wider with a continuous surface.

Originally Posted by Boatmik

Yes,

I like the weights of Oughtred, Graeme Byrnes, a bunch of John Welsford's stuff.

We are all antipodean!

The Northern hemisphere went from traditional timber construction, to fibreglass in one hit pretty well. There are a few plywood racing dinghies in the North, but they almost all take their weight cues from traditional construction.

Thank Goodness for John Spencer NZ and Jack Holt UK (huge numbers of Jack's boats built in Australia - and he wasn't allowed to join the UK yacht clubs because "he worked with his hands)... in spreading lightweight plywood designs and building methods

MIK

Haven't heard of Byrnes, but yes Welsford's stuff is great too. Very sensible bloke with a good eye.

Graham Byrnes designs under the name B&B Yacht Designs. His most well known boat is the Coresound series.

Joost

RE: Vivier weights-- they are Euro weights, all up, with all required equipment. They are not bare hull weights.

ILUR sails fast and rows effortlessly. They are incredible, seaworthy boats.

So what does the hull weigh?

Somewhere around 125kg to 145kg depending if you count floorboards, benches, flotation foam, etc.

Heavier than the Goat, obviously, but built for a very different mission. Stable, large, sleep-aboard cruiser with massive amounts of storage, that can easily take two people for long periods of time. Great sea-keeping ability as well. I would certainly take an ILUR into much gnarlier conditions than my GIS, and that's saying something, I think. It would be a bit slower overall, but much more comfortable. Two different boats, really, but none of Vivier's boats should be pooh-poohed. I have seen them up close, and they are really incredibly well thought out crafts.

On Vivier's brochure he states "weight without rig" 245kg. Ready to sail, 300kg.

It's made from 9mm ply

http://www.vivierboats.com/pdf/EN_ilur.pdf

Jesus that's heavy, and he reckons an extra 45 kg heavier in strip planking! Why on earth would strip-built be that heavy?

Strip Plank vs Ply hull weights.

Lapstrake you can probably build out of a little lighter ply because of the stiffening effect along the plank edges where the planks are doubled up. 8 times stiffer, probably easily doubling over what a single skin hull would be. (If laps are about 1/8 of the plank width - 18mm is lap width for glued 6mm if plank is 8 x 18mm wide = 128mm non doubled which means on average the laps more than double the hull stiffness.

Now opposed to strip plank - For a Goat sized boat things start to get a bit fragile if the planking is much less than 8mm - remembering you have to clean up the inside and the outside. So say you end up with something around an average of 7mm.

Well ... there is the weight of the Lapstrake boat and you still have to glass both sides.

Specific gravities
Cedar/Gaboon approx 0.4
Glass 2.5
Epoxy 1.1

This is a really good example of how much weight glass adds to a structure lots of appearances of the throwaway line that "glass doesn't add much weight".

Pick up an unglassed canoe and a glassed one and figure out which one you want to carry for a couple of hundred metres

Or just to realise that glass is six times more dense than the planking wood.

There is also a strange common response to cutting down the weight of existing strip planked boat designs. Almost universally everyone accepts the glass weight but decides to make the timber thinner.

1/ They are reducing the amount of the lightest component. The heaviest component is the best target for weight reduction.
2/ The effect of thinning the timber makes the hull thinner and the glass less effective at keeping the hull stiff. Go from 6mm planking to 4mm planking without changing the glass and the panel stiffness is now 16/36 of what it was for a small weight loss.

MIK

Sure, but my thinking is that strippers are regularly built as light as glued lap, so there's no need to add an extra 45 kg just because you're using strips. 45 kg is a lot of weight. You could damned near do the whole hull for that.

Oh and I think your structural analysis is a bit simplified.

The thing with glued lap is that although you do get increased stiffness at the laps, this really only applies to longitudinal stiffness. The laps do nothing for transverse stiffness. Then you also have to consider that the face grain of the planks will be running more or less longitudinally, meaning the transverse stiffness of the planks will be lower than a sandwich composed of an equivalent thickness strip between glass cloth.

Another way of looking at it, in terms of transverse stiffness, is that the stripper has the high modulus stuff (transverse glass fibres, in this case) on the outer face, while the clinker boat has the high modulus stuff (transverse wood veneers) set more towards the neutral axis.

With longitudinal stiffness yes, the laps will give some advantage. However strippers also have good longitudinal stiffness due to all the wood grain being longitudinal, and half the glass (again, high modulus material, on the outer faces) also being longitudinal.

The other thing is that ply often seems to have slightly greater density than the same volume of the same species as raw seasoned stock. I think WRC strips would be lighter, volume for volume, than gaboon ply, assuming you selected them for weight (some pieces of WRC can be surprisingly dense). Plus you can use paulownia for strips, and you can't get ply that light (unless you use poplar, which has quality and availability problems).

I'd like to see a proper (quantitative) engineering analysis on both alternatives.

Originally Posted by Boatmik

Specific gravities
Cedar/Gaboon approx 0.4
Glass 2.5
Epoxy 1.1

This is a really good example of how much weight glass adds to a structure lots of appearances of the throwaway line that "glass doesn't add much weight".

Pick up an unglassed canoe and a glassed one and figure out which one you want to carry for a couple of hundred metres

Or just to realise that glass is six times more dense than the planking wood.

The glass is, but in practice we'll be talking about the glass/resin composite, which has a lower density than the glass itself. Assuming a 50/50 layup, which is probably better than most amateurs will get, the average SG would be 1.8, which is only about 4 times the planking density.

There is also a strange common response to cutting down the weight of existing strip planked boat designs. Almost universally everyone accepts the glass weight but decides to make the timber thinner.

1/ They are reducing the amount of the lightest component. The heaviest component is the best target for weight reduction.
2/ The effect of thinning the timber makes the hull thinner and the glass less effective at keeping the hull stiff. Go from 6mm planking to 4mm planking without changing the glass and the panel stiffness is now 16/36 of what it was for a small weight loss.

MIK

Well, densest component, yes. However a 200 gsm glass sheathing will be under 0.5 mm thick if done properly. Two layers equal less than 1 mm, at roughly four times the density of the planking (depending on species) and at 1/6 or less of the thickness if using 6mm or greater finished strips. So on that basis the glass would not actually be the heaviest component, all things considered. It's still a substantial portion of the weight though. The best solution for weight reduction is obviously a less dense core (ie: paulownia).

Good points.But in general what you see in practice is that only canoes get normally built of 6mm strip. Even small dinghies go to 8mm strip. Rare to see much less.In the same size ranges 4 or 5mm ply is fine for canoes and 6mm can build a largish dinghy.To get localised stiffnesses you can add longitudinal and transverse stiffnesses because of the standard superimposition methodology. The doubling of thickness is worth a lot in any case. I didn't go into the geometric effect that in effect each two planks form a compound beam because their angle is fixed with a structural join dealing with all mechanical loads.This is a mechanism for both strip plank and glued lap boats which ply boats can't take advantage of. But even in that case surprisingly little extra stiffening is needed in a GIS other than the internal structural divisions.But either way it is a pain to build a 16ft dinghy with 6mm strips to match the GIS weight. The difference is the weight of the glass plus a bit for the planking thickness left after fairing.Best wishesMIK