Swept back Spreaders and Pre-Bend

[stephenmh]

Brion

thanks so much for the explanation, it is very much appreciated. I have been struggling with this problem for years. It takes everything just to keep the mast straight and without any reverse bend in the mast.

So, I'm assuming that any play with the spreader rake is not good. The spreaders should be rigid to the base/mast? Catalina has improved the design of the base over the years, I might need to reach out to them and see if I can upgrade the spreader base.

I attached a line drawing of the Capri. you will notice that the chainplates are aft of the mast and she was designed with only one set of lowers.

In reference to the numbers (also see sketch), I follow the tuning recommendations from Catalina and check all of my numbers with a loos gauge.




Cheers
Stephen

[Brion Toss]

Aha,
I had forgotten that it was an aft-lowers-only rig, though this makes sense with swept-back spreaders; rigs like this with forward lowers -- and they exist -- end up with overloaded aft lowers.
In your case, it appeared that the sweep on the spreaders was less than planned, because the forward edge was what was being crushed, right? If that's the case, it might be because the upper shroud chainplates were too far forward. And if so, that would mean the the spreaders were exerting less forward push than planned, and that in turn would mean that the aft lowers meet with too little resistance. Result: with any kind of tune on those lowers, the mast inverts.
I recently struggled with this on a friend's Halberg 34, and ended up keeping the lowers slacker than I liked, in order to keep the mast right. Seems to be working.
Anyway, another possibility is that the spreader base is simply cut at the wrong angle, and/or that the tenon is positioned incorrectly, and/or that the primitive bases are going to give you grief regardless. In any event, if you aren't going to move the upper shroud chainplates, then you need to get the spreaders to line up with their bases, for fixed-position spreaders, or fabricate swinging spreaders, which is a whole other can of variables.
What diameter wire are we dealing with here? The numbers on the sketch would be right for 3/16".
Fair leads,
Brion Toss

[Ian McColgin]

Just as an aside, many rigs allow for considerable flex in the direction normal to the stay, whether this is spreaders or a dolphin striker.

The spreader needs to bisect the angle that it creates in the stay to avoid slipping up or down, but under tension it will automaticly find the fore and aft angle that allows for the shortest distance. If you've a flexable joint (such as I always install on the striker for a bobstay since without the flex, the joint will sooner or later fail due to anchor or mooring rode side stresses) you can see this for yourself by just pushing on the spreader and feeling the resistance.

Or set up a line between two trees and push it out of straight with a stick from the ground. You'll see how this 'spreader' will find its angle and will be stabile then so long as the end away from the stay is past a line between the two ends of the stay. Simple force geometry.

In the swept rig, you absolutely must have some room for the spreader to swing aft as you increase stay tension and bend the mast.

On many traditional rigs, spreaders that are rigid in the fore and aft will fail at the joint due to flexing stresses that happen as the unloaded lee stay takes a strain when tacking.

G'luck.

[stephenmh]

Brion/Ian,

You are correct, the stays are 3/16 and I'm not able to make any adjustments to the location of the upper chainplates.

Since the mast is down for the winter, I'm unable to site up the shrouds but I was able to locate a side picture (and I highlighted the stays for better visiblity). Although it appears that the upper's are fairly straight from deck to masthead, it's now easier for me to visualize the problem that you (both) pointed out and how I might be having a problem. If I correctly understand Ian, once tension is applied on the shrouds, the shrouds will force the spreaders to "find" its angle, either fore or aft and by the signs on the spreader base, it appears that the spreaders are being forced forward resulting in the spreaders not being able to introduce pre-bend.

Just to recap:
Even though I'm following Catalina's spec's on loading to the tee, I'm thinking that I have too much tension on the lowers which is not allowing the mast to flex and it's forcing the spreaders forward. This should be a easy correction/fix.

It now appears that I do have a problem with the spreader base from the inboard edges, to the wallowing of the clevis pin holes from years of wear. Fixing the pin holes should be straight forward but how do I ever determine the proper angle of spreaders? Or by Ian's comments, should I expect to see some swing (of the spreaders) to the aft as I tighten the shrouds but try to control any forward movement? It just seems like a delicate balancing act with swept back spreaders.

[Ian McColgin]

Brion is far more scientific and knowledgable in tuning that I am, but on boats like this with a not particularly extreme spreader rake, I tune by first getting the head stay and back stay about right, then using the upper to center the truck. For this stage the lowers are slack. With swept spreaders, besides getting the truck centered I try to induce whatever mast bend I want. Only when that's done do I tension the lowers.

G'luck

[Brion Toss]

Hi again,
It might be only the perspective, but it looks from the photograph that the chainplates are much further forward than in the sailplan. Wouldn't be the first time something like this has happened. It would account for the spreader root damage, assuming the roots and spreaders were built for a greater aft sweep. It would also account for your inability to bend the mast, as the sweep would have too small a forward vector to make much difference. On the other hand, the lowers would still have enough sweep to bend the middle of the mast aft.
A quick Web search doesn't reveal any Capri's with chainplates further aft than yours, however. With the rig as-is, I'm with Ian: tune from the top down, and be moderate in your takeup of the lowers.
Fair leads,
Brion Toss

[stephenmh]

Brion & Ian

Thanks for all your help!!!! It was very informative.

Happy Holidays
Steve