Why bigger holes?

[Otter]

We're installing a new windlass and we actually read the instructions first!!! It comes with 8mm bolts but clearly says drill 10mm holes for the bolts. I can't understand this, surely you'd want the bolts to sit snug in the deck not potentially shift back and forth? Any idea why they say oversized holes? Is it to prevent cracking of the gel coat?

 

[RunAgroundHard]

It’s to allow for misalignment and to ensure the bolts sit aligned correctly in the winch base. The security of the winch comes from the clamping force, not the bolt closeness of fit in the hole. If you can drill accurately, no harm will be done with smaller holes.

Use thick, 3 to 5 mm thick, large OD washers under the nut, not thin penny washers.. Penny washers just distort.

 

[mjcp]

Might also be to allow for some sealing of the through holes - if the deck is cored, don't want water into the core!

 

[AngusMcDoon]

Might also be to allow for some sealing of the through holes - if the deck is cored, don't want water into the core!

Layup below winch positions is usually solid laminate.

 

[penberth3]

We're installing a new windlass and we actually read the instructions first!!! It comes with 8mm bolts but clearly says drill 10mm holes for the bolts. I can't understand this, surely you'd want the bolts to sit snug in the deck not potentially shift back and forth? Any idea why they say oversized holes? Is it to prevent cracking of the gel coat?

That is not "oversize". Standard practice for steel work etc. is to have the holes +2mm.

 

[Boater Sam]

You will struggle to get several 8mm bolts through 8mm holes, you need a bit of tolerance.
I would use 9mm but mark out very accurately.

 

[Poignard]

When I fitted a windlass I used the base as a template for drilling the holes.

 

[mjcp]

Layup below winch positions is usually solid laminate.

But the OP is fitting a windlass, not a winch ;-)