stays needed for a mast with in-line spreaders

[srah1953]

What sort of stays are needed for a mast with in-line spreaders and compared to swept back spreaders? Looking at a few boats near me, the most obvious thing I can see is that they seem to have some type of babystay. Do masts need to be more sturdily built for in-line spreaders?
Thanks

 

[knuterikt]

Mast profile is designed to have a slight curve with the top bending aft. If the mast profile is curved with the top pointing forward it's called mast inversion and can lead to mast buckling and collapsing.
On mast with inline spreaders the babystay and aft stay is used to give the mast a proper pre bend.
On masts with swept spreaders the spreaders will push the mast forward to give the same effect.

You can read more here https://www.seldenmast.com/files/1571039872/595-540-E.pdf

 

[johnalison]

Masts need to be protected from bending the wrong way, as well as being kept straight when viewed from fore and aft. I have had straight crosstrees, single or double on each of my boats and the lowers have varied.

My current boat achieves its desired forward bow with twin lowers on each side, the forward ones doing this job. On my last boat with single spreaders, the two lowers were aft and there was a single baby stay. This stay clearly does a lot of work as it was the only bit of rigging to fail in the twelve years we had her.

 

[srah1953]

Thank you both

 

[William_H]

Mast and stay design is complicated. Essentially it depends on the loads on the rig boat size and type versus the inherent strength of the mast in dimensions particularly fore and aft dimensions but obviously also total length between stay points.
So often with a dinghy of low rig load we will see single shrouds about 3/4 height with spreaders. This rig usually has shrouds with chain plate aft of abeam the mast to provide support aft in lieu of a back stay. Hence the spreaders sweep aft to match the shrouds. That rig is quite ok given the relatively large cross section dimensions of the mast and relatively short mast and distance to hounds. (where shrouds meet the mast).
Now small catamarans have a much greater rig load but tend to have the same shroud arrangement but with a larger cross section dimensioned mast. (still ok)
However when we get to a smallish keel boat say 21 ft we see rigs either so called mast head where we have cap shrouds abeam the mast intermediate shrouds going to chain plates aft of abeam the mast witha an intermediate or baby forstay. So the mast top is located by the cap shrouds forstay and backstay while the middle of the mast is located forward by the inner (baby or intermediate ) forstay and the 2 aft located intermediate stays. The spreaders are there to improve the angle of approach of the cap shrouds to the hounds. ie to help take side loads on mast top.
Now the fractional rig boat will have the back stay with forstay about 3/4 height (varies) to hounds where side stays also attach. Side stays (cap shrouds) go to chain plates aft of abeam the mast. From a point about half way from base to hounds spreaders are fitted which are swept aft. This sweep aft is not just to meet the shrouds but actually they are swept further aft to provide some push forward of the middle of the mast against the aft mounted intermediate stays. The end effect is to have the middle located firmly for and aft and sideways with as siad some middle forward shape. On keel boats and bigger boats this is necessary because the mast section is relatively light and small and mast is tall. All this to handle huge loads especially on the forstay with large genoa jib. Bigger boats with more stays and spreaders. ol'will

 

[johnalison]

As you say, it's complicated. Not all fractional rigs have swept spreaders, though. My Selden rig has unswept double spreaders and 19/20th rig. A friend with a different 34ft boat had a similar rig but with swept spreaders, also Selden, which suffered mast inversion when quite new. Additional forward lowers were installed.

 

[Lightwave395]

Back in the day, most boats with fractional rigs and inline spreaders had running backstays and checkstays too which held the rig up. My masthead rig has checks too but just for 'tweaking' along with a babystay

 

[RJJ]

I would expect your mast manufacturer

What sort of stays are needed for a mast with in-line spreaders and compared to swept back spreaders? Looking at a few boats near me, the most obvious thing I can see is that they seem to have some type of babystay. Do masts need to be more sturdily built for in-line spreaders?
Thanks

I would expect your mast manufacturer got it right. The mast, spreaders, deck, mast heel and chainplates are a system; you don't necessarily replicate what's seen on other boats as part of a different system.

It's quite normal with in-line spreaders to have either a babystay or to have fore/aft lower shrouds reaching the lower spreaders. You may also see a socking great telegraph pole-sized mast with no additional support, especially if keel-stepped.

It's like the "debate" about whether keel-stepped masts are sturdier. the best explanation I heard was that on a keel-stepped mast, the deck acts as a set of spreaders. The mast can therefore be lighter. Then there are other trade-offs with both deck-stepped and keel-stepped masts, but (assuming the whole system is designed properly) no particular reason why one should be intrinsically stronger than the other.

 

[srah1953]

I would expect your mast manufacturer got it right. ..........

What annoys me about the swept back spreaders is that it significantly reduces the extent to which the mainsail can be let out without rubbing against the spreaders and therefore affects your downwind angle. I was thinking that if I came to change the boat (in the 35 -40 ft size) should I seek out one with in-line spreaders? Assuming of course that that option was available. It may be the case that a mast with swept-back spreaders allows for more adjustment of mainsail shape - not that I tend to bother that much.
Thank you for all the comments.

 

[TLouth7]

Sonatas have square spreaders and no forward lowers or babystay. I believe it makes rig tuning interesting...

I think the angle of the spreaders is a minor consideration when choosing between rigs; I would care much more about whether it was masthead or fractional, or 140% vs 110% genoa.

 

[RJJ]

What annoys me about the swept back spreaders is that it significantly reduces the extent to which the mainsail can be let out without rubbing against the spreaders and therefore affects your downwind angle. I was thinking that if I came to change the boat (in the 35 -40 ft size) should I seek out one with in-line spreaders? Assuming of course that that option was available. It may be the case that a mast with swept-back spreaders allows for more adjustment of mainsail shape - not that I tend to bother that much.
Thank you for all the comments.

Yes, with in-line spreaders you can typically let the main out further when running.

You may find the mast is, as mentioned, a telegraph pole which will limit your adjustment options. That may well not trouble you, however I would point out that the ability to add a bit of pre-bend can make all the difference to a slightly tired mainsail.

Or, you may find such a boat comes with a babystay and checkstays (sometimes erroneously dubbed as running backstays, which is the correct term only if they align with the forestay). Those add some complexity back, although typically you can sail without them. The babystay is a pain if you ever want to gybe a spinnaker, but again, many of us just don't do it.

 

[davesparkes]

Assuming that spreaders with any kind of substantial sweep-back are only fitted to fractionally-rigged boats, the sweep-back helps with supporting forestay tension. If you have a fractional rig with inline spreaders then you will require running backstays to support the mast fore-and-aft and also to maintain forestay tension.

Some sportboats with big roach to the mainsail leech delete the backstay and rely instead on the sweep-back of the cap shrouds to support the mast. These boats often have asymmetrics and hence sail angles downwind so not being able to ease the main really far doesn't affect them.

When I had a Sonata the fashion was to push the mast step as far back as class rules would allow, which reduced the already marginal sweep-back on the spreaders; running backstays were soon allowed under class rules to help support the mast.

My thoughts are that an amount of sweep-back is desirable if you want to avoid running backstays, but unless you sail angles downwind then too large a sweep-back would be a pain!

Cheers,
Dave