Mast swap. What do I need to know?

[andypag]

Hi, I'm looking at a 41ft Lagoon with a cracked mast that needs replacing. I've been told about a used mast that's more than long enough (18.5m) but it's from a keel stepped monohul.

Can I trim it to length, and strip the fittings and spreaders from the old mast and fit them to the new mast? Or is it more complicated than that?



I think the cross secton will likely be a bit different so I might have to have a new foot fabricated, but happy to absorb that cost. Just want to know if I'm missing anything else from a design point of view...



Weight? Strenth? flexibility?... ?


Thanks

[chris95040]

Originally Posted by andypag

Hi, I'm looking at a 41ft Lagoon with a cracked mast that needs replacing. I've been told about a used mast that's more than long enough (18.5m) but it's from a keel stepped monohul.

Can I trim it to length, and strip the fittings and spreaders from the old mast and fit them to the new mast? Or is it more complicated than that?



I think the cross secton will likely be a bit different so I might have to have a new foot fabricated, but happy to absorb that cost. Just want to know if I'm missing anything else from a design point of view...



Weight? Strenth? flexibility?... ?


Thanks

You need to talk to a naval architect or a very competent rigger. In about 2 billable hours they’ll answer if you can use that mast or not.

The most important thing is the stiffness (“Ixx” and “Iyy”) in the fore/aft and port/stbd directions. This is a function of the cross sectional geometry of the mast.

The required stiffness for a mast is a function of the righting moment of the boat (since that’s what the mast is pushing against), the shroud and stay locations, number of spreaders, the sail plan, etc.

Since a multihull has a gigantic righting moment compared to a monohull, it’s unlikely a mast from a mono will be of sufficient stiffness. (Especially a keel-stepped one, due to pinning at the deck these are generally allowed to be a less stiff mast than if it were deck-stepped all else being equal.)

Buckling failure is what you should be worried about. Pay an expert and you’ll learn a bunch from them!

[psk125]

You may also want to check the luff groove. It would be a shame to step a new mast and then find out you needed new sails, or had to add a battslide track.

[Manateeman]

Hi, Captain Chris nailed this question exactly.
I don’t know anything else to add unless the question involves carbon fiber.
Then Chris is Twice as correct.
Happy trails to you,
Mark, a high five to Chris manatee

[barnakiel]

It can be attempted given the extrusions are similar in specs - alloy grade, wall thickness, diameter, etc.


Look at spreaders and hardware layout (winches, stoppers, backing plates, etc.) - you do not want to drill one hundred new holes in it and plug 100 old ones.



You want a pro rigger handle this for you too.


barnakiel

[andypag]

Originally Posted by chris95040

The most important thing is the stiffness (“Ixx” and “Iyy”) in the fore/aft and port/stbd directions. This is a function of the cross sectional geometry of the mast.

Thanks Chris, that's very helpful. You've drilled down to the heart of my question: If the alloy grade, footprint length and width, and wall thickness are the same, does the shape of the mast really make that much of a difference to the stiffness? I assume that when calculating published Ixx and Iyy specs the manufacturers assume an oval shape extrusion, and aren't getting into finite element analysis of the precise extrusion shape.

[Cheechako]

How badly cracked? Sailboat masts are beefed up, sleeved, modded all the time. Especially in the racing community.
I had the furling mast on my 47' mono reinforced. They put an outer sleeve on it, used structural epoxy bonding and aircraft rivets all over the sleeve.

[barnakiel]

Positive. At times a mast can be saved by welding, riveting, etc. Not all cracks are fatal.


Broken masts are sleeved and reused on a regular basis.



barnakiel

[Cheechako]

Originally Posted by barnakiel

Positive. At times a mast can be saved by welding, riveting, etc. Not all cracks are fatal.

Broken masts are sleeved and reused on a regular basis.

barnakiel

And many new masts are sleeved to start with on larger boats!

[Stewie12]

I would go for either repairing the existing mast or buying a new one rather than trying, and maybe failing, to cut the mast to size. As other have suggested there is a lot more to it. In fact too much.

[meirriba]

Cat masts have a 'diamond' shrouds. These keep the mast in form and create pre-bend. Find a cat near you and look at it.
IMHO a mono mast would not fit.