low tide

The problem with exterior grade ply (WBP) over marine ply is that you cannot see the voids, they are not apparent until the sheet is cut. The top veneer is rarely a single sheet on exterior ply, which can also cause problems when cutting for something as accurate as a hull section. If it is a single veneer in order to keep the cost down the internal layers are often poor.

Remember that trhe timber merchant wants to sell you what he has in stock.  They will have to order in marine ply and their margins on it are small due to the quantities ordered.

Several Mirror dinghys have been home built using WBP, you will recognise them as the rotten lumps in the corner of the dinghy park or the ones with bubbles on the ply where it is delaminating.

I think that the 6' skiff is an excellent and bold design. I do think however that if you compromise the materials you use, (and I know that you are only going to use a certain design for a short period) you will not be able to properly test a design since it will be unable to take the loads properly.

I don't intend to be arguementative on this issue, and I appologise if I appear so I  would just hate to see something like this fall at the first hurdle or get a reputation for something that falls apart as soon as it gets going, or that people waste their money on unsuitable materials.

"Several Mirror dinghys have been home built using WBP, you will recognise them as the rotten lumps in the corner of the dinghy park or the ones with bubbles on the ply where it is delaminating."

Yes, but they have been there many years. I've built 7 1/2 foot canoes from WBP 4 mm ply and they are still going many years later. And I've just bought some 3mm Robbins Elite marine ply for a deck at £20something a sheet, and 3 mm is far more likely to be used. £100 for the hull easy, but fittings, rig etc, unless donated off the dead Mirror, will cost a bomb.