I've read of a similar encounter in France in 1944...M4 Sherman encounters a German 15.0cm SFH18 crew with a HE shell loaded...15.0cm fires, hits the glacis of the Sherman, and with it not being sealed, the cuncussion killed the crew stone dead. The author was a repair section commander for the US 3AD, so I took his word on it.
With that said, it is my impression of WP rounds that it is a bursting charge, not a true explosive payload, with the aim to scatter the WP material for maximum effect.
I would think that charge on a 4.2" or 120mm mortar round to be on the scale of a standard grenade, maybe two, three tops.
It depends on the thickness of the sheetmetal and condition of the corrugated material here in particular. It's been a while since I was at my Grandparents and saw the stuff up close.
I would assume you would have some penetration and concussive effects on the crew. It depends on how sadistic (if the vessel is friendly) or generous (if an enemy vessel and you wish to reward the gunner) you want to be.
I would probably put in enough havoc to drive the vessel off (if enemy) or require the players to stop, come along shore and get sand or something to snuff out the WP if they don't have something like that aboard.
I know corrugated siding isn't very thick...maybe .125" or so, but if taken off something common (like a barn or equipment shed) and you have a good supply, it could be layered and/or spaced, increasing the protection.
You also get benefits from the shape, what you put underneath it (if anything), and any reinforcing structure/support structure.
If I was trying to roof over a USN LCM or a barge, that's what I'd do...probably run a layer of metal. a layer of timber...4-6" thick, then a layer of sheemetal. If you are getting the materials from a farm or farms, there should be plenty of both to do an LCM.
Yes, that probably means getting materials from a couple of good sized equipment sheds (big enough to store tractors and combines) and a barn or two.
I hope that helps-
Dave
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