Masthead revisited

[ianhlnd]

Having recovered the stolen parts from the masthead, with the help of a rigger and a machine shop was able to piece everything back together, and weld in the broken pieces and add reinforcements to beef it up.

I've found that this arrangement (less main furling bracket) is pretty common for early 80's boats with wood masts. Surprisingly, the steel used is only 1/8 inch thickness. Seemingly weak for a heavy cruiser.

Side brackets were added along with beefing up the area that was broken. The main cause of the failure appears to have been constant torqueing of the furling main to the masthead bracket. Twisting in and out eventually tore the bracket off. Well, that and 4 gales in a row.

This failure followed only a few weeks after having the rigging inspected.

[sailingdog]

Ian-

I'd be pretty pissed at the rigger who did the inspection.

[Tartan34C]

Can you post a picture of the fitting taken from the side? I think you may still have a design defect which is what lead to the first failure. Adding metal but keeping a poor design will only increase the time before failure and not prevent failure. It looks like the fitting was converted to handle the roller furling main and the original conversion was poorly designed.
All the best,
Robert Gainer

[ianhlnd]

Hi Robert,

Here's a horizontal view, from the initial 1/8 inch steel I've added 1/4 inch over the top, welded at the sides and ends, I think the original refit was probably about 20 yrs ago based on the design of the furler. All bearings on the furling mech. work smoothly. Appreciate any suggestions. The masthead has been mounted, can pull it back down at the next port of call.

[Tartan34C]

Thanks for the picture. The short answer is that you have a bad design. The tang is binning pulled in a few directions by things that will vary in tension as everything moves. This means the strap will flex to line up with the greatest strain. It’s the flexing that will fatigue the metal and cause a break. The simple and old fastened solution is to stack as many tangs as needed and each one is bent to line up with the strain from its own wire. The top tang of course goes with the wire pointing at the mizzen and the second one is below that and is bent to line up with the backstay and a third one is on a crane and bent straight down for the main. Each one will stay pointed at the wire and not flex enough to crack.

The brace you added simply adds a stress riser (hard spot) and that’s where it will break in the future. By the way if you use tangs and MS jaws you can do this without doublers so the weight is much less then your design even with added tangs.
Good luck and all the best,
Robert Gainer

[ianhlnd]

Thanks Robert,

If I understand you right, all the tangs have to move independently and line up with the direction of pull?

I think that's what's happening, here's another photo of a close up of the pieces. the backstays swivel as does the main tang (both two pieces) off a 1/2 inch pin. the only thing fixed at the masthead is the triatic. and forestay. Upper shrouds are thru bolted to brackets to the vertical straps on the side of the mast.

Am I right about this? or maybe not understanding you?

[sailingdog]

Umm... so all those loads are on a single 1/2" pin?? That sounds awfully small for a 45' boat. Sounds like a pretty piss poor piece of engineering IMHO.

[Tartan34C]

The fact that the split backstay can pivot side to side doesn’t change the line of pull up and down. The triatic pulls at a different angle then the backstay so the strap will move up and down as the pull from each wire varies during a sail. And if you had a single backstay at the masthead that split further down you save a lot of weight and windage up high. You could then have a single tang and use one MS jaw instead of two marine eyes between triangles for the backstay. But this is a design choice and each designer has his own preference.

You now have a line of weld across the strap just above the backstay plates. This is a real stress riser because it’s a hard spot and the metal was changed during the heating that goes along with welding. You also have a large downward pull from the main so as the sail fills and collapse when the boat rolls for instance the tang bobs up and down as the tension in each change because the backstay pulls back and the main pulls down. Did I explain this in a way that makes sense?

On a separate subject a 1/2 inch pin for a MS standard eye is sized for 1/4 inch wire. What size is your backstay?
All the best,
Robert Gainer

[ianhlnd]

Ok, starting to be clear. Sorry, a little dense when it comes to this stuff.

both backstays are 1/4 inch.

[Tartan34C]

I have a 34 foot, 12,000 pound boat with a single 1/4 inch backstay and a 1/2 inch pin. I calculated it out and am very comfortable with my rig offshore. You have a much larger boat and can put twice the strain on the 1/2 pin because you have two 1/4 inch wires attached to it. You feel that’s strong enough? It would make me nervous. Got to run now so if you need anything else please feel free to e-mail but I think you need to reconsider your truck and make some changes.
Good luck and all the best,
Robert Gainer