How Do You Buy a Used Mast?

[betwys1]

Originally Posted by GRIT

I've, just this spring, assembled my mast and rigging myself. My previous boat was a heavy Corbin 39, and the rig on my new boat is humbling, as to it's complexity and robustness. The new boat weighs 66% of what the Corbin 39 did, and yet the rig seems twice the size. (Engineered by Selden)

Cheers.
Paul.

This mast-maker offers a few tips - about monel pop rivets and machine screws for example.

http://www.seldenmast.com/files/1587.../595-540-E.pdf

Finding a mast of similar or slightly larger major/minor diameters, of similar or slightly greater wall thickness, and similar length from any old source is just the start of the fun! Through bolting or multiple machine screws into the mast wall are just some of the choices. Selden reminds us that protection from electrolytic corrosion is always important.

[andypag]

Here's my opinion after thinking about this for some time. Engineer by training, but been a long time since I used any static mechanics theory.



Even given that the cross section shapes might be different, if the overall footprint of the mast width and length are the same, and the wall thickness is the same, then I think it's a safe assumption that the Ixx and Iyy are going to be close enough.



A different Ixx may demand a different tension in the diamond shrouds on a cat, or forestay/backstay on a mono to get the same bow in the mast. And the different Iyy will require a different tension in the shrouds and cap shrouds to get the same lateral rigidity. If there was a significant difference you might risk getting s-ing of the mast.


But given that you can tune a rig for racing (actually do people do this on cats?) by tightening up the backstay, that tells you there's some leeway in the tensions that the rig can work under, so that gives you an idea of the tollerance range on the tensions and the flexibility in a working mast.



My subjective feel is that even a different shaped profile extrusion mast of the same dimensions and wall thickness is not going to give significantly different Ixx and Iyy. (FWIW I suspect that the figures are all calculated on an approximated oval shape and no one is doing finite element analysis required to calculate the effect of the track - but who knows)



The Ixx and Iyy are not a measure of breaking strain, but of moment of inertia (not quite springiness but that's a good mental aproximation). There's undoubtedly an indirect correlation but it's not linear, I'd guess that in this context even if your moment of inertia is a bit different the fatiguing strain will not be dramatically different. So if you get it wrong and you might get marginally sub-optimal performance - you won't be able to get your sails flat as flat when they are a few years old, but I can't see you snapping a mast.



I'll caveat that with the fact that a used mast will have a load of old holes in it from previous fittings. Try and recycle the existing halyard openings, but the tangs must mirror your original rig plan. Not sure that epoxy on the holes would really make much of structural difference. Obviously good to keep the water out though.


I'm looking at a 40f cat with a crack in the mast, so like you I have the model mast, but am wondering what specs I need to look for when looking for a used mast.


Weight aloft of an over sized mast on a cat is a significant strain on the rigging, so I don't think over engineering with a bigger wall thickness is the right answer, but maybe some dynema shrouds could compensate for that.



Happy to hear feedback on these thoughts.

[Manateeman]

Wow . There is so much wrong information here I don’t know where to start.
A mast on a catamaran vs a monohull is as different as the hulls are different.
The chances of finding a used mast are very small. I’m converting a cat main spar to a mizzenmast on my mono. Just got really lucky to find a section that had higher moments, ideal length, thick wall, almost new. I had to check all the calculations via Principles of Yacht Design and then have two naval architects review it and one looked over the spar. Had to build a new aluminum I beam spar trailer...bolted, not welded, because over 53’ the truckers will not do it. Closing off unneeded exit holes...on Boat Design net this was a hot topic as to the best method. No one could agree. I need a new thread to explain how and why I did mine. I had to fabricate new spreader attachment plates, strip the mast of everything, rewire, new primer and paint, new heel plate, longer spreaders...not counting the hours on my TIG machine. I did save money. LOL
I love building boats. Mast building is hard work. Maybe you can find someone who is converting from aluminum to carbon or an ex race boat.
It’s just simple. There is a lot of engineering to do...with cats...more so than monohulls. Fast cat...that’s me. I had a wooden mast break while I was at the spreaders. Tarzan dive into the harbor. I was 18. A poorly designed or calculated or built mast, does not make a cute hat. Please have some one do all the math. Talk to the spar builders. Consider buying new extrusions.
Happy trails to you.
Mark, not a pole vaulting manatee.

[Chotu]

Originally Posted by Manateeman

Wow . There is so much wrong information here I don’t know where to start.
A mast on a catamaran vs a monohull is as different as the hulls are different.
The chances of finding a used mast are very small. I’m converting a cat main spar to a mizzenmast on my mono. Just got really lucky to find a section that had higher moments, ideal length, thick wall, almost new. I had to check all the calculations via Principles of Yacht Design and then have two naval architects review it and one looked over the spar. Had to build a new aluminum I beam spar trailer...bolted, not welded, because over 53’ the truckers will not do it. Closing off unneeded exit holes...on Boat Design net this was a hot topic as to the best method. No one could agree. I need a new thread to explain how and why I did mine. I had to fabricate new spreader attachment plates, strip the mast of everything, rewire, new primer and paint, new heel plate, longer spreaders...not counting the hours on my TIG machine. I did save money. LOL
I love building boats. Mast building is hard work. Maybe you can find someone who is converting from aluminum to carbon or an ex race boat.
It’s just simple. There is a lot of engineering to do...with cats...more so than monohulls. Fast cat...that’s me. I had a wooden mast break while I was at the spreaders. Tarzan dive into the harbor. I was 18. A poorly designed or calculated or built mast, does not make a cute hat. Please have some one do all the math. Talk to the spar builders. Consider buying new extrusions.
Happy trails to you.
Mark, not a pole vaulting manatee.

Mark,

I have plans for the rig of course that came with the design as well as an exact Selden model that works and is installed aboard Grit's cat, which is basically a sistership.

Is it really impossible to find a used mast that fits these already defined dimensions?

[Manateeman]

Hi. It is possible. I didn’t see you have a sistership so you have the spar plan and therefore it should have the moments of inertia..the x and y.
I’d try finding new extrusions. Finding someone to build the mast head, heel and spreaders is easy. You can do a lot of the labor to save money.
Try carbon spar builders. Call yards that have race boats. Try the boat design net. People who build boats close to your design. They will at least tell you the oem price. They might order for you. A splice especially at the spreaders, is not unheard of and strong. One of the big spar ompanies in the USA was selling new aluminum extrusions because of the demand for carbon. The race guys even turn in perfectly good carbon spars. You got to call and call.
They need length and dimensions and moments. Weight per foot also helps.
I apologize for lecturing. I don’t want any fellow boatbuilders to have a problem down the line. The fast catamaran builders might help especially the ocean race guys. I looked into a mast in Italy and sea transport. Beat a lot of USA stuff on price. Big spar builders always have scratch and bends. Again splices are done on new spars especially because they only ship in 40’ containers.
I’ll make some calls if you message me with the details.
Happy trails to you.
Mark, a sometimes grumpy manatee

[andypag]

@chotu - I found this spec sheet. Might be helpful to you. It has a table showing specs for a C and F profile mast. Maybe you can work back from the specs you have and then forward from used masts you find.

http://www.seldenmast.com/en/product...10-11.pdf.html

[GRIT]

Thanks andypag.

Nice write up. Most informative post on this thread yet.

Cheers.
Paul.

[andypag]

Originally Posted by GRIT

Thanks andypag.

Nice write up. Most informative post on this thread yet.

Cheers.
Paul.

Thanks, I'm hoping for some peer review as it's been a long time since I studied this stuff.


Currently looking up aluminium extrusion manufacturers local to my boat to see if they have the capacity to make one off/Short run extrusions.


I suspect that the cost of the extrusion is probably less than a couple of thousand dollars.

[andypag]

Originally Posted by andypag

@chotu - I found this spec sheet. Might be helpful to you. It has a table showing specs for a C and F profile mast. Maybe you can work back from the specs you have and then forward from used masts you find.

http://www.seldenmast.com/en/product...10-11.pdf.html

And here's z-Spars info@ https://www.zsparsuk.com/mast-sectio...al-information

For the same sizes they seem to have the same inertias. I think that the footprint size dictates the wall thickness too. So you don't get a choice of wall thicknesses for a given footprint size.



This stuff isn't standard but it's more homogenous that I first thought.

[acem]

Good luck with your project.

I recently bought a used mast for my Catalina 27 that was damaged in a tornado.

My old mast was bent, kinked, and the entire marina turned upside down. Luckily all 4 sailboats were dismantled but otherwise ok, the 150 plus power boats didn't fair so well. My sailboat broke loose from the dock or it would have sank.

Anyway, I looked for a used mast for some time. I found many used masts at Don's in Clearwater FL, probably 100. I ended up buying one here in Arkansas with rigging and a mainsail. Freshwater boat parts are in much better condition than from saltwater.

Luckily I found a factory mast that will be easy to install. Since you probably won't have that opportunity you get to find something compatible. As a former engineer I recommend that in addition to cross section you make sure the alloy has similar strength properties. I don't know what alloys are used in masts but there could be alot of variation.

Thx-Ace

[andypag]

Originally Posted by andypag

And here's z-Spars info@ https://www.zsparsuk.com/mast-sectio...al-information

For the same sizes they seem to have the same inertias. I think that the footprint size dictates the wall thickness too. So you don't get a choice of wall thicknesses for a given footprint size.



This stuff isn't standard but it's more homogenous that I first thought.

https://maxspar.com/aluminiummasts.html

[acem]

I was searching on the Houston craigslist and this came up. Several mast from 18 to 56 ft. I thought it might be helpful. Thx-Ace

https://houston.craigslist.org/for/d...170685932.html