Mounting a turning block on mast without base plate

jaschrumpf · #1 (**permalink**) 02-24-2010

I would like to mount turning blocks at the base of my mast (one block on each side) to run my halyards forward for single handing. You can see that something was mounted in that area previously -- the holes are 2" apart in a square. The same pattern is on the other side of the mast.

The blocks I've been looking at have mounting holes around 1 3/4" apart, so obviously I can't use but one of the existing holes. Should I just drill the other hole and not worry, or is there an alternative mount I can use -- perhaps I could fabricate a small aluminum plate with holes to match the existing ones and mount the block to the plate?

Whaddya think?

scraph · #2 (**permalink**) 02-24-2010

I would go with the aluminum mounting plate option, if it is an option. While it is not strictly necessary to do so ... and drilling an extra hole in close proximity won't buckle your mast or anything like that ... using an aluminum mounting plate would be more aesthetically pleasing and wouldn't require drilling any more holes.

I'd recommend riveting the mounting plate in place. On the backside of the mounting plate, have holes drill and countersunk for flat head machine screws to insert through. You could then have the threaded ends protruding out of the mount plate, slide the turning blocks over these "studs", and then secure with washer, lock washer, and cap screw (if studs are proper length).

Be sure to chamfer or otherwise smooth the outside edges of the plate to prevent chafing you might get if the halyard starts wind whipping into it.

deniseO30 · #3 (**permalink**) 02-24-2010

We just drilled through the roof and used forged eye bolts with fender washers. works fine, doesn't look any worse then anything else on the underside. did the same with the rope clutches and the winch. 2 yrs now no signs of stress.
Garhaurer and others make plates that are "universal"

sailingdog · #4 (**permalink**) 02-25-2010

If you want to mount the blocks to the mast, make a plate up that has the proper hole pattern for the blocks and that you can bolt to the existing holes in the mast.

jaschrumpf · #5 (**permalink**) 02-28-2010

If I decided to use one of the existing holes and drill a new one for the other side of the block, would it be best to use a stainless rivet or a self-tapping screw for the mounting? I know there are some issues with galling when using stainless and aluminum together, but the hole size would only be a #10, which I believe is 3/16". Would that be a problem?

My first thought is to go with the rivet, but I'm not sure if the head of the rivet gun would fit into the space where it would need to go.

mitiempo · #6 (**permalink**) 02-28-2010

You won't get galling with stainless and aluminum, you get galling with stainless and stainless. I would not use self tapping screws. Rivet or machine screw. Why wouldn't the rivet gun fit?

sailingdog · #7 (**permalink**) 02-28-2010

What you will get with stainless steel and aluminum is galvanic corrosion. However, you can minimize this by using tefgel or lanocote on the pop rivets and using plastic isolation washers between stainless steel hardware and the aluminum.

How thick the wall of the mast is will help determine if pop rivets or theading and machine screws make more sense. With thinner walled spars, using pop rivets is probably a better idea than tapping.

davidpm · #8 (**permalink**) 02-28-2010

Quote:

Originally Posted by scraph

You could then have the threaded ends protruding out of the mount plate, slide the turning blocks over these "studs", and then secure with washer, lock washer, and cap screw (if studs are proper length).

What do you do if the hidden head of this screw turns so you can't tighten the nut?

jaschrumpf · #9 (**permalink**) 03-01-2010

Quote:

Originally Posted by mitiempo

You won't get galling with stainless and aluminum, you get galling with stainless and stainless. I would not use self tapping screws. Rivet or machine screw. Why wouldn't the rivet gun fit?

I'm looking at using the Harken 300 series fairlead block (the 300-31), and the rivet gun head would have to fit between the cheeks, which don't look very wide. Also, the bottom rivet would have to be within a few inches of the deck, and with the cheek in the way the length of the gun's handle would preclude getting it down there -- unless I could put in the bottom rivet with the block pointing down, then pivot on the rivet [rhyme not intended] and put the top one in.

Would that be possible with a plastic isolation washer between the block and the mast to provide a bit of slippery friction?

breezetrees · #10 (**permalink**) 03-01-2010

I would consider mounting a pad eye or pad eyes to the existing holes, hopefully you could find something with the right spacing, there is much more variety in pad eyes. I don't know your loads but maybe something like this

Then shackle your turning blocks to the pad eye, so they can align themselves with your deck layout.