OK, this could be fun...
What do people do when mounting fittings singlehanded? A while back I was installing a bunch of cleats but luckily they were all the same hole base and I was able to 3D print a spanner that held both nuts at once. Now I need to take off some small pulleys and saddles to repaint the deck. They are all different hole centres and I no longer have access to the printer.
What innovative ways have you come up with? 
The problem was solved by the above method with a twist when the nut and vice grip started turning while tightening the bolt from above.
I taped the vice grip to the surface using duct tape, went above tightened the bolt, put the next bolt in, taped it, went below, undone the vice grip, threaded the next nut on the next bolt, tightening it by hand, put the grip on the nut, taped it on, went above, tightened the bolt, put the next bolt in, taped it, went below...etc etc etc...
until job finished. Strewth!
- Don't tighten the bolt , tighten the nut.
- Dremel (or insert fav cutting blade thingy) a slight cut with a steel cutting blade on the end of the bolt. Jig it with a piece of wood so you do multiple bolts at once. Cut on the thread end so you can use a screwdriver to hold the bolt from moving as you use a ring open end to tighten the nut.
Edit: If they're stainless bolts, take em to a metal-basher and get them to cut them for you. We used to do it all the time in a workshop environment. 
Select to expand quoteshaggybaxter said..
- Don't tighten the bolt , tighten the nut.
- Dremel (or insert fav cutting blade thingy) a slight cut with a steel cutting blade on the end of the bolt. Jig it with a piece of wood so you do multiple bolts at once. Cut on the thread end so you can use a screwdriver to hold the bolt from moving as you use a ring open end to tighten the nut.
Edit: If they're stainless bolts, take em to a metal-basher and get them to cut them for you. We used to do it all the time in a workshop environment.
Gotta agree, it was a big no no when using butyl to seal hull/ deck fittings holes.
tightening bolt will/ can break the sealant seating you have used.
More expensive option is to have children. They can also be especially handy when there's a need to gain access to small areas adults don't fit. I've found it also useful to plan a bit ahead and have a few at different intervals so you can choose which to use according to the need for strength or size. 
If you don't use nylocs you can use a variation of the technique above to install and remove. This only works when you can use a 30mm longer bolt than needed.
Insert bolt with goop and head inside. Lightly hold bolt with vice grips where it exits overhead - gently - (or hold with hands). Spin nut up to where the vice grips are. Then take vice grips off and place on end of bolt and spin nut up to washer. Use open ended spanner to tighten whilst you stop bolt from turning with vice grips.
I have used this method to bolt on fittings onto boom and masts on dinghies too. Light cotton down hole, out end or halyard slot, pull bolt through hole from inside and tighten up as above. Good for when you haven't got a doubler to tap into and rivets are not up to it. I do use nylocs on smaller diameter bolts.
slammin I also used that method. Hence I am a number of years away from upgrading my boat. Based on your profile picture it looks like it didn't work so well for you either
Select to expand quoteshaggybaxter said..
- Don't tighten the bolt , tighten the nut.
- Dremel (or insert fav cutting blade thingy) a slight cut with a steel cutting blade on the end of the bolt. Jig it with a piece of wood so you do multiple bolts at once. Cut on the thread end so you can use a screwdriver to hold the bolt from moving as you use a ring open end to tighten the nut.
Edit: If they're stainless bolts, take em to a metal-basher and get them to cut them for you. We used to do it all the time in a workshop environment.
Great idea!
Someone should invent bolts with Allen key holes on the threaded end, would make a fortune!!!
There great ideas, saves me dragging the misses out to the boat and having to put up with her winging
