ideal spreader attachment

[Douglas]

Ahoy bm , my Forespar mast fabricated around 1985 has spreader bases welded right to the mast wall .

FWIW , author Nigel Calder says this , in his second edition of Boatowner's Mechanical and Electrical Manual ,,,, page 484 , " ......... any welds around the spar produce a week section prone to buckling. Notable in this respect are welded spreader sockets, especially where the sockets wrap a good way around the spar. "

As for mast wall compression at the spreader bases , are you thinking about Z spar's through bar spreader base system ?

[brm3]

yep. you got it. I've seen the bars and the sleeved bolt option. My mast is oval so compression tubes would be challenging. I'm trying to either copy a solid design or come up with something to fit the bill. My wall thickness is 3/16" but I want to be safe with the loading at the point of compression on the wall.
Thanks

[Jim Fulton]

I would expect that a rigid connection between the spreader and the mast like that pictured would increase the likelihood of metal fatigue at the joint due to vibration. All of the spreader bases that I have seen (or paid attention to) have some allowance for the spreader to move, at least a little. Am I off base here?

[brm3]

I guess it would depend on the amount of deflection resulting in flex inducing fatigue. I'm still curious if the side compression of the mast is really a problem. Nigels book raises concern. Is this opinion or fact? Does anyone know? I know how stories can be mutated to unreasonable point.
Thanks

[Robbie.g]

The load is spread over a large area, and using a band has made the spreader load land on double the thickness of material and more than quadruple the spreader cross section surface area.
Spreaders should not move, any movement would in itself, cause fatigue and wear. Many carbon rigs have the mast and spreaders etc glued n glassed. No movement there.

Possibly the 'deflection resulting in flex inducing fatigue' in you post, is an opinion from a bygone day.
Regards

[Ian McColgin]

Since many boats of "a by-gone day" are still sailing, any look at flex or not should start with understanding the engineering of the boat and rig. One might also recall that while there are times when you might get away with a rigid spreader to mast attachment, there are very very few (if any) times when an intelligently engineered flexable connection will do anything but good.

G'luck

[Brion Toss]

Let's see, first a rigid connection is not necessarily a bad thing, assuming that the lead of the shroud is fair over the tip of the spreader; you don't want that shroud to be pulling that spreader tip significantly fore or aft. This can be an issue with bendy masts, but even here the spreader angle can be set for the fairest leads when the loads are highest.
Many spreaders that have "some allowance for the spreader to move" are just originally-rigid setups that have become deformed from unfair loading. There are spreaders intentionally set up to swing, but these have their own issues, especially the tendency of the spreaders to slam around on the leeward side, imposing very high fatigue loads on the components, including the mast. It can be done well, but it must be done carefully, and for most boats it just isn't worth the effort.
Welding on aluminum spars is not straightforward. It is a tricky job, requiring a high degree of skill and knowledge. Many, many rig failures are directly attributable to weld failure. Re the assembly in the picture, I have no way, from here, of knowing if the right stick was used, that penetration was complete, that surfaces didn't deform, etc. The thickness of the weld looks quite, um generous. In general I prefer no welds on a mast, partly because it is so difficult to find someone who does it well, and partly because there are usually better approaches.
For instance, the through-bar, with spreaders socketed over, its ends, is strong and simple.
Compression from the spreaders is usually not an issue, given the relatively low magnitude relative to bearing surface.
Fair leads,
Brion Toss

[brm3]

Thanks all for your input. I'll just chew on this a while. It sounds to me if I can keep my stay angle bisect properly and keep the spreader from walking down I should be ok.

[Robbie.g]

Just to be clear the spreader assembly in the picture has the welding on to a separate band.
You quite correct that welding directly onto a spar should be done carefully and with the correct materials and techniques, as the heat applied in the welding process can adversely affect the performance of the spar.
The need for a solid spreader attachment is much more important 'now days' as many rigs have a designed degree of sweep in the spreaders, and therefore the load path is somewhat less straight forward [for want of a better term] than a more 'traditional' inline spreader rig.
More of a horses for courses kind of thing, i guess.
Good discussion.
Regards