Thread: Crash and Burn in a PDR

Howdy,

A good summary of the two PDR building approaches by Rick.

then Woodeneye

Originally Posted by woodeneye

This reminds me of something that I've been meaning to ask MIK for a long time. Before I built Hakuna Matata, I read in a boat building book that was discussing the use of plywood in racing dinghy constructions, that the 2 outer layers of 3 and 5 ply carry most of the loads. I can understand that this is quite possible, especially when the ply is made to bend so that the innermost (concave) ply is under compression and the outermost (convex) is under tension.

Is this correct MIK?

It is true when there is a bending load on the ply sheet only. For example water pressure or wave impact is trying to bow the bottom of the PDRacer upwards. So the outside is in compression and the inside is in tension.

In the middle there is neither tension or compression as it is a smooth transition from one to the other through the thickness of the ply - so there has to be point where there is zero with maximum loads reached on the inside and outside faces of the ply.

However you also have to think of grain orientation. If the grain is in the direction of the compression or tension then it can handle that load very well. However if the tension and compression loads on the outside faces of the ply are across the grain it is not well oriented to handle the load ... so the next layer in, which will be at the right angle will be better oriented and take most of the crossways load if it can.

However this is why three ply sometimes starts to crack along the grain where it glues to inflexible structure that lines up with the grain direction - it is just asking too much of the middle veneer to resist the bending - it is in the middle where it cannot resist the load in that direction so ply bends a lot with the badly oriented outside layers get too much stress cross grain resulting in cracks. You can avoid it by either using 5 ply, or adding a layer of light glass (that way the middle veneer will not be at the neutral axis any more and its grain can be opposed by the glass fibre.

However on a macro basis it becomes confusing really quickly. For example for boats with stays the mast will push the bottom of the boat down and the stays will pull the ends up ... so the deck will be in compression and the bottom of the boat will be in tension and the mast below the hounds in strong compression.

The rule with engineering loads is you can superimpose them ... like ...
Load 1 - for a conventionally rigged boat if you calculated the compression load from standing rigging in the deck and the tension loading in the hull. So lets focus on the tension load in the hull bottom.
Load 2 - Then you can look at water pressure that loads the inside of the hull in tension and the outside in an equal amount of compression.

Inside ply veneer gets the tension from the rigging load plus the tension from water pressure.
Outside veneer gets the tension from the rigging load plus the compression from the water pressure.

This might look quite bad as it might indicate the inside will fail first. However wood is smart and is much stronger in tension than compression. Cool eh!

It is sort of beams inside beams inside beams.

I am sure you are completely confused now! Sorry If I haven't explained it well - ancillary questions?

MIK

No MIK, it makes absolute sense! Thanks for such detail.

This has been on my mind for a while since my brother sent me some updated pics of a Paper Tiger under construction. It is built using 3 ply, but the stay attachments also intrigued me. For the stay attachments they use carbon tape epoxied to the ply so that a loop is formed above the deck! For the massive loads the stays must carry, it looked very light-on to me as the load is attached to one layer of ply. I'll see if I can find those pics.

Here they are. That loop is epoxy coated carbon tape wrapped around a ss thimble and the frame is carbon/foam.

Attachment 143742

Attachment 143743

Originally Posted by Boatmik

Howdy,


I think it would be quite possible to have it all fixed within a couple of days and be back on the water. But I don't really want to suggest that too loudly as it would be great for Rick to get some sailing in on the Mk3. I really want to know how it feels with the off centreboard compared to the centreboard on the Mk2. The Mk3 has a little bit more weight too.

Best wishes
MIK

Okay, you have to be careful what you wish for, it might come true! I put the Oz MkIII on the water for the first time today. Not a breath of wind almost all afternoon. Glassy calm, flags limp, and me, bobbing around in the lake.
I used the rig and foils from my other boat and put them to good use on this one. The offset daggerboard worked fine, but in the really light winds (1-2 knots, maybe.) it was a bit better on the port tack than starboard. (The board is more on the port side) Maybe it was my imagination, or maybe the fact that the bad tack is to starboard for the lug rig.
Anyways, the boat was a hundred times more comfortable to use with the clear cockpit sole and no framing. I could lounge on the floor, using the airtanks as a backrest and stretch my feet out as far as I wanted. As a matter of fact, in the really light airs, I layed down on the floor, using a couple of life vests as a pillow, and steering with my elbow resting on the bottom and hand on the tiller. Eyes just above the coaming.
The small, open locker under the fore-deck was a good spot to put my bailer, lunch hook and rode, a couple of water bottles and my safety kit. Everything stayed put, nothing under-foot or hidden behind a hatch, and still organized. I am considering installing a couple of 'holsters' in the airbox sides for a pair of oars. 6.5' oars would fit just fine.

Here are a couple of pics. Sorry they are from afar, and not much action, but they give a bit of an idea.

Rick Landreville.