Attaching fittings to curved mast section.

[Davidhoy]

I am installing some rope clutches for the halyards and topping lift, and am trying to figure out the best way to attach them to the curved surface of the spar. My plan is to drill and tap M8x1.25 threads into the aluminium spar, then use M8 stainless pan head machine screws, coated with Tefgel, to actually attach the clutches to the mast, but the surface where they will be install is curved. What is the best way to handle the curve so that the loads are properly transferred?

Also, any recommendations for cleaning light surface corrosion on the spars, and treating them to protect against future corrosion?

Thanks,
David.

[SVTatia]

Easy to make one of these. Cut to size, drill then bend the lips to the desired angle using a vise.

[Suijin]

I would consider making wedges, or even a fully shaped foot out of G10. Would probably be fine using Starboard as the point is to stabilize them; the force is longitudinal to the clutch axis. Or a shaped metal platform.

As for cleaning corrosion, there's the full-on proper way and the faster way. The full on way requires grinding back entirely to good shiny smooth metal, every last pit taken out, acid etching, treating with alodyne, then repainting. Then seat fixtures with lots of Tef-gel or even sheets of cut plastic to insulate them from the spar. This will give you the best long term performance.

The easy fix is grind down as best you can, clean, spray with aluminum primer and then topcoat, then Tef-gel the fixtures and fasteners. The corrosion will come back sooner or later but either one might be later enough for you.

[paulajayne]

Originally Posted by SVTatia

Easy to make one of these. Cut to size, drill then bend the lips to the desired angle using a vise.

+1

Paula

[Cheechako]

Clutches are not too wide, so not too big a deal but I always put them on the flat part of the mast. The load is up/down the mast, not in the direction of the curve (if that makes sense) so it shouldn't matter much though. Aren't your halyard exit slots on the side of the mast?
Use coarse thread in aluminum.

[sailjumanji]

I've made bases from plastic by taping sandpaper onto the mast where i want clutch to do, and then moving plastic base up and down on the sandpaper to get correct shape cut in by sandpaper. Takes some patience but base can almost be shaped to perfect contour.

[JPA Cate]

Originally Posted by sailjumanji

I've made bases from plastic by taping sandpaper onto the mast where i want clutch to do, and then moving plastic base up and down on the sandpaper to get correct shape cut in by sandpaper. Takes some patience but base can almost be shaped to perfect contour.

^^^^^^^^^^
The above, it will also work with teak pads.

Ann

[roverhi]

Single line rope clutches fasten to curved masts just fine. The load is all in shear on the fasteners so the relatively narrow surface contact on the mast is not an issue. If you were going to use a double clutch it would be a more difficult. Have used washers as spacers to fit multi line clutches on curved surfaces, works fine.

[Davidhoy]

Thanks, everyone! The clutches are Lewmar DC2 doubles, and due to where the halyards exit the mast, on the side but towards the front of the section, they will need some form of additional support. I like the idea of taping sandpaper to the mast and then sanding the pad down to the perfect curvature. The bent metal plate idea is not as appealing, as this would have the clutches standing a lot prouder of the mast than I'd like - I'd like to keep the profile as low as possible.

One thought I had was to use a contour gauge to measure and model the curve of the mast section in some CAD software, then create a 3D model of the pad and print it using ABS or nylon.

[Davidhoy]

Originally Posted by Cheechako

Clutches are not too wide, so not too big a deal but I always put them on the flat part of the mast. The load is up/down the mast, not in the direction of the curve (if that makes sense) so it shouldn't matter much though. Aren't your halyard exit slots on the side of the mast?
Use coarse thread in aluminum.

Planning on M8x1.25, which is coarse thread :-). My mast section does not have a flat part, and to properly align with the halyard exits I will have to mount the clutches on a more curved section.

-David

[Halien]

Rivnuts set in lanolin or Tef-Gel are more secure than tapping the mast.

[foufou]

You might use epoxy and cabosil filler . Mylar tape on mast, with thickened epoxy consistency of peanut butter make a curved shim by pressing a flat surface wrapped in mylar tape onto the uncured epoxy bog. A few hours later, presto you have the right curved shim

[Robert Tilbury]

Originally Posted by foufou

You might use epoxy and cabosil filler . Mylar tape on mast, with thickened epoxy consistency of peanut butter make a curved shim by pressing a flat surface wrapped in mylar tape onto the uncured epoxy bog. A few hours later, presto you have the right curved shim

foufou ,
That is the coolest and neatest idea i have seen ,,, and no hangups with sharp edges ,,,

[Davidhoy]

Originally Posted by foufou

You might use epoxy and cabosil filler . Mylar tape on mast, with thickened epoxy consistency of peanut butter make a curved shim by pressing a flat surface wrapped in mylar tape onto the uncured epoxy bog. A few hours later, presto you have the right curved shim

Great suggestion! I think I’ll embed some fiberglass mat to add some extra strength. I already have the epoxy and fiberglass, just need to get some cabosil.