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Bobcat · 03-03-2018, 12:16

Are there any new direct replacement alloy extrusions for the older Kemp style ‘gold’ anodised egg shaped profiles found on 1970's cutter rigged ketch cruising catamarans (8m x 4.5m disp about 4 tons)?

Seems that the newer squarish profile Seldon masts (who took over Kemp) might not be suitable for cats. Sparcraft who manufacture oval profiles for catamarans look a better match but are much more elongated.

Are there other manufacturers of smaller catamaran masts? I am looking for a 29ft/8.9m main (‘egg’ major diam 5.5”/140mm & 4.25”/110mm minor diam) and a 23’ 10”/7.25m mizzen (‘egg’ major diam 4.5”/110mm & minor diam 3.5”/90mm).

I have not been able to find any suitable second hand masts. Your thoughts much appreciated.

chris95040 · 03-03-2018, 12:31

Quote:

Originally Posted by Bobcat

Are there any new direct replacement alloy extrusions for the older Kemp style ‘gold’ anodised egg shaped profiles found on 1970's cutter rigged ketch cruising catamarans (8m x 4.5m disp about 4 tons)?

Seems that the newer squarish profile Seldon masts (who took over Kemp) might not be suitable for cats. Sparcraft who manufacture oval profiles for catamarans look a better match but are much more elongated.

Are there other manufacturers of smaller catamaran masts? I am looking for a 29ft/8.9m main (‘egg’ major diam 5.5”/140mm & 4.25”/110mm minor diam) and a 23’ 10”/7.25m mizzen (‘egg’ major diam 4.5”/110mm & minor diam 3.5”/90mm).

I have not been able to find any suitable second hand masts. Your thoughts much appreciated.

The key numbers are the fore/aft stiffness (Iyy) and the side to side stiffness (Ixx).

Its trivial for a rigger to run the calculations to determine what stiffness your mast needs to be. Email Brion Toss, he could help you through the whole process.

Its even more trivial to just find a new mast that is close to the stiffness of what you've got. Check out the charts here from Skenes covering oval shaped aluminum masts, look up your stiffness based on wall thickness, major axis and minor axis:

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Bobcat · 03-03-2018, 15:32

Thanks for the feedback. Would a catamaran mast otherwise be any different to that of a monohull bearing in mind the broader beam and different standing rigging arrangement?

chris95040 · 03-03-2018, 15:46

Quote:

Originally Posted by Bobcat

Thanks for the feedback. Would a catamaran mast otherwise be any different to that of a monohull bearing in mind the broader beam and different standing rigging arrangement?

Well, that chart is just stiffness of an oval section, has nothing to do with what its used for. The assumption is someone designed your mast stiffness appropriately for your boat, you are just identifying the stiffness they chose by consulting that chart.

(BTW I didn't realize the forum would kill the resolution of that image, if you tell me the wall thickness of those masts referenced above, i can look them up on the chart. And your 'egg shaped' sections aren't actually perfectly oval I take it, so I'd just use the number as a rough ballpark.. )

But in answer to your question, yes, the fact that its a catamaran has a huge impact on mast design. A rigger uses the righting moment of the boat as one of the most important factors in deciding the mast stiffness required, after all, thats what its pushing against!

Bobcat · 04-03-2018, 04:30

That's really good of you I'll check the wall thickness asap. I can't get out yet as we are still snowed in but the weather looks better next week!

Yes, the profiles really are 'egg shaped' - nature knows best. However, the thickness varies round the profile ie thicker on the sides (minor diam) so that might make life a bit more complicated.

I should have thought that someone would have measured 'carry over' stock with an eye to the future when masts would need replacing. Maybe Selden has those stiffness figures for the old style masts, I'll have to check.

There are still a lot of old 1970's boats in use and if their masts haven't been replaced yet they may be soon ... if the boats have survived! Otherwise will just have to work out the righting movement but I have an article written by the designer, Bill O'Brien some years ago which might have that info since it was about catamaran stability.

chris95040 · 04-03-2018, 08:54

I think theres no need for 'carry over' stock because in the end its pretty trivial to find a suitable section being manufactured today.

If your thickness varies, though (and most modern-ish sections do) then this chart for simple oval sections is not going to make a great approximation.

Luckily it'd only take a few minutes of calculus for a mechanical engineer to calculate the stiffness of your egg shaped section, and I'm sure a competent rigger could get you a reasonable estimate too.

If I were you I'd head over to spartalk.com, give Brion your info, and for a few hundred bucks you'll have all the info you need to go spar shopping.

captlloyd · 04-03-2018, 21:53

Lots of used masts on eBay.

CaptSmith · 04-03-2018, 23:54

Spars Main Page RigRite.com

been here ?

Bobcat · 05-03-2018, 01:58

Quote:

Originally Posted by captlloyd

Lots of used masts on eBay.

Thanks for response ... but very difficult to find the right one even if you live nearby!

johncruise · 05-03-2018, 05:32

bobcat, may be Z-spars in Ipswich uk teleph. +44 1473 822130 or Eurospars Plymouth +44 1752 550550 can help you.

If you have the XX and YY numbers of your old mastr , it may be helpful, regards johncruise

chris95040 · 05-03-2018, 06:48

Quote:

Originally Posted by johncruise

bobcat, may be Z-spars in Ipswich uk teleph. +44 1473 822130 or Eurospars Plymouth +44 1752 550550 can help you.

If you have the XX and YY numbers of your old mastr , it may be helpful, regards johncruise

Nice tip.

OP, these are the two stiffness figures I referenced earlier.

Once you've got those numbers (either by figuring out what your present mast is, or by recalculating what it should be based on your rigging and hull geometry) you've just gotta go looking for a mast thats close to those numbers.