Tool to indicate where a drilled hole will exit?

Magnet and compass?

Look in screwfix: thruwall detector 15433

mrcheerful

screwfix sell a thruwall locator p/n 15433-76, page 499 sorry don't know how to munge their URLs

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 wasn't meaning to be flippant when I suggested a magnet and a compass, this should be all you need for a GRP boat hull and you've probably got them both in the house somewhere. However, I can understand the desire to buy a new gadget and this is a good excuse :-)

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

Yep! ... My wife was horrified when she opened a small cubby hole in the boat and discerned the sun through the hull! Unless the pigment is black, the resin should be quite translucent.

yes, that's fine for the topsides, but below the waterline (where you tend to drill holes for water intakes!) sunlight is not going to penetrate the antifouling. Also, the lay-up is heavier nearer the keel.

White LED torch

You might get away with a laser pointer, then, or even a little white LED torch.

Yes, the Thruwall Locator does exactly that, £37.95 from Mills

You use Blu-tak to stick the transmitter to the point where you intend to drill and move the locator up and down and sideways on the other side to pin-point where the drill will come out. Of course you'll still need a cable and pipe detector to make sure there's no surpises in store before you drill.

Put a 1/8th drill bit in a drill ... drill hole through material .... go to other side ... where drill is sticking through is where the hole will exit :-)

And if the drill is sticking through a fuel line you just wrap some tape around it and drill somewhere else I suppose.

No the lighted match you use to look for thr hole will take care of that