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mariner2k
02-07-2014, 11:19 AM
I am going to be replacing a rotted main mast on a 45' ketch. The current mast is a box/oval mast with a single spreader rig. The replacement is the same height dimension. (45') The cap/pully and gooseneck measurements are perfect. However the replacement mast is a round mast and a bit less beefy. (about 8-9" diameter at it's thickest point) It was originally rigged with two spreaders.

The question is: Is it necessary to keep the double spreader arrangement from the replacement mast or can I rig it identical to the original box mast?

Once again the major difference between the two is one being a box/oval (rotted mast) and the other being round. Also between the two boats, the round mast had a 20' boom and a much larger mainsail, with about the same headsail area. The existing box mast has a 16' boom leaving a much smaller mainsail area. In other words the newer mast will be carrying much less sail area than it used to carry. Is there a trade off?

Rich Jones
02-07-2014, 04:20 PM
Not that I know anything about mast loads and stresses, but I'd go on the side of caution and rig the mast (the new round one) as it was designed. Why would you want to change it to one spreader? Do you have the spreaders or is that something you have to have made?

JimConlin
02-07-2014, 04:25 PM
Even if it is rigged as it was designed, you could still be creating a problem. Hire a qualified naval architect. There are lots of them in RI.

mariner2k
02-07-2014, 06:34 PM
From much of the research I have done it appears that the mast is rigged for the individual boat as well as its' own dimensions. For instance, the old boat (round mast) carried a staysail and running back stays. The new boat carries only a head sail and has no runners. There is a lot of calculations that go into it. It's definitely not as simple as putting up a stick with some shrouds. I do have all of the spreader and hardware to go with it. I agree Jim, I will also consult some professional help. I post here however, because this forum is a wealth of information. And over the years, (yep years), I have found many ideas and opinions to be quite helpful. I know I can use the new stick, but I don't want to find out underway that it's done wrong.

Roger Cumming
02-07-2014, 08:34 PM
Is the new mast solid or hollow? If hollow I doubt you can just attach your old hardware and spreaders to the new mast. Does the new mast have solid portions where the spreaders are to be located? How do you know you "can use the new stick"? Are its engineering properties equal to or greater than the old one? A naval architect could tell you pretty quickly and render an educated opinion.

mariner2k
02-07-2014, 09:59 PM
The new mast is mostly hollow. I do have the spreader hardware (rings) for the new mast for both spreaders, though I would have to change the spreader lengths themselves for the optimum angle. No bolts or screw need to be attached to the new mast. The new mast also used to carry around 30 percent more total sail than the rotted one. It actually came off the same size ketch. The cap will work on the new mast.
As far as the shrouds and stays, I can used soft eyes as needed, which it originally had, and they look cool. They will be some adjustment on length of course. Not to mention the partners. Brian Toss has good information on the way it should be rigged, but I do need to find out of the different riggings pertained to the boat/sail configuration or the mast size...or both.

Jamesh
02-08-2014, 05:00 AM
Hi
1) the new mast will have a lower second moment of area (less stiff)
2) it will have less sability due only having one rather than two spreaders.
Just because its off the same size boat dosent mean that it will be suitable the donor boat might have been very tender so the mast loads were less?
Ask a NA.
yours James

mariner2k
02-08-2014, 07:56 AM
I am leaning toward using two spreaders, but your point is well taken. The newer boat is heavier, but I do not know if , or how , that affects the load. Even under one sail it reaches it's healing point rather easily. The weight/ ballast distribution is much higher that in the older boat. Better safe than sorry.

Woxbox
02-08-2014, 08:50 AM
"A bit less beefy" may mean a lot. Stiffness increases with the cube of the thickness. If the new stick is just 1" thinner than the old one, it will be markedly more flexible. Also, mast and rigging work as a unit in three dimensions, so as stated by others, there's more to it than meets the eye. Some real world calculations from a NA will allow you to cruise in confidence.

mariner2k
02-08-2014, 10:03 AM
Yes I was a bit vague on that. I've been researching this for a few months. It seems that the more you research, you find the less you knew. The formulas are mind boggling and also fairly vague. So far....I am still confident I can used the new spar with the correct rigging. However this is still in the early planning stages. Until I get the old mast down, I haven't thrown out the possibility of rebuilding. I did rebuild the mizzen and it came out well, it depends on the extent of the damage, but it looks pretty sad.

Old Frog
02-08-2014, 11:08 AM
The current mast is a box/oval mast with a single spreader rig. Reads to be a hollow section mast.


the replacement mast is a round mast and a bit less beefy. In post #6 your say "mostly hollow". What does that mean?

Remember hollow wooden masts typically have have internal blocking for load transfer at attachments such as spreaders, tangs, etc. Simply changing double spreader rig to single spreader rig and vice versa, it not a good idea without considering all factors, loads etc. Also, remember that you may well be dealing with significant weight differences between the two masts. If heavier, it could have a deleterious effect on the boat's stability, etc. It reads that you may also be changing the sail plan with this project. That likely will alter the sail CE which in turn may adversely affect the boat's handling, trim, helm balance, etc. Rig & sails loads on a 45' ketch and stick of this size can be significant.

Jamesh in post #7 offers wise counsel: "Ask a NA"

Remember: When rigs fail & come down, those on deck & nearby maybe injured. In today's litigious society, there could be significant and life changing consequences if you or someone else is injured in a rig failure on your boat, even more so if litigation occurs and you do not have proper documentation/calculations for your alterations. The term negligence comes to mind. Not a good place to be... Do not put yourself there.

Again, Jamesh offers wise counsel: "Ask a NA"

mariner2k
02-08-2014, 11:19 AM
The spreaders would go exactly as designed on the newer mast with no added rigging to unknown areas. I would not be changing the sail plan at all, sorry if I indicated otherwise. The formulas for figuring loads etc. are beyond my capabilities., so I will have to consult a NA. In the background , my main concern is , of course for the safety of those on board. I will cut no corners. It will either work or it won't. These many words of caution do not fall on deaf ears. It can't just look pretty, it has to be safe.