Welcome to the Piper Boat Owners club Forum. The Forum for owners of Piper built Narrowboats and Barges and anyone interested in Piper Boats. The owners club is completely separate from Piper Boats. Please feel free to post a message to club members and web site readers.
Which specific boat are we talking about? There was some variation in thicknesses way back in the late 60's and very early 70's as there was a big variation in design then - from the little 16 foot cruisers but if we're talking the traditional David Piper design, flat bottoms and in cruiser and trad cabin styles, there wasn't ever a decrease.
So, unless you're talking of a very early boat, They were typically 8,6,4 65a grade msp. Some of the longer boats had 10mm bottoms as the customers choice - saving ballast but not doing very much else.
If you let me know the specific boat I can give you more details.