"peter" wrote in message
oups.com...
Thanks all, the Screwfix one is exactly what I had in mind, I did look
there but didn't use the right search terms.
Does anyone have any experience with these, in particular how accurate
an indication it would give through a GRP boat hull?


I had a fight to locate it in the screwfix catalogue or the website !!

I haven't used one, but I expect the accuracy gets better as the material
gets thinner, so it should be great on GRP.
Some hulls transmit light so you might be able to see a good spotlight on
the other side, particularly at night.

mrcheerful