mast design

[CaptainDaniel]

I will soon need a 65 foot mast for my 52 foot sailboat. This will be Bermuda rigged. Can anyone help me find more info on how to build such a long mast? Will birds-mouth method work?

Where can I find info on how thick the mast needs to be? I do have woodworking skills, but am not sure where to get the right info for the mast dimensions. Any help is appreciated. If you need more info, just ask.

[MikeJohns]

The section is dictated by the mast material, the maximum righting moment of the vessel and the number of spreaders and the shroud and stay angles.

Even within a bermudan rig there can still be a considerable variation.

Is this a re-fit sans detailed plans and info, or or a new vessel with all details available?

[Eric Sponberg]

If you can find a copy of Skene's Elements of Yacht Design by Francis Kinney (out of print, but copies can be found), there are a few design methods in there that you can use to design a wood stayed mast. However, mast design is an important engineering problem, and if you are not used to doing engineering calculations, it is best to have a professional do it for you, or at least check over your work.

Mike is correct in his statement of what mast design is dependent on, and there can be wide variation in the results, depending on your choices. That is why a profession eye on your calculations would be a minimum requirement.

Eric

[CaptainDaniel]

The boat never had a mast. It's a custom-built. What would I look for to find a professional? A mast engineer or boat designer? I know I can't afford to buy a mast. I have to build or get help building.

[Eric Sponberg]

A naval architect (who holds a college degree) and who specializes in sailing yacht design is the best choice. Yacht designers (who hold a yacht design school certificate) and who design sailboats would also be OK. If you were to buy a mast from a mast builder, they will do the engineering and design for you.

Alternatively, you can scrounge boatyards to see if there are any spare masts lying around. The University of Rhode Island in Kingston, RI, has a boat donation program where they take in boats and resell them--they sometimes have a number of extra masts available. We just had a few hurricanes come through the Gulf of Mexico, and there are lots of masts available now. If you do find one, it is best to have a professional check over the adequacy of the design for your particular boat. Recently, I checked over a Hunter 41 mast for a fellow who is rebuilding a Cheoy Lee 38, and the mast section and design fortunately was a pretty good fit.

Eric

[Petros]

An optimized mast design is a complex analysis with many variables. If there is something unique about the design of your boat or rig then it is best to have a marine engineer or navel architect do the calculations. But that is not always necessary.

If it is a fairly conventional design, there are good basic formulas and "rules of thumb" design tables that work quite well for conventional designs that you can do yourself. It would not harm anything to go a bit stronger if in doubt, but be careful you do not add too much weight.

Find a good boat design text books and it should have tables and design charts that will do just fine. It should have simple formulas and "rules of thumb" that have been used for many years (even hundreds of years), developed from lots and lots of observed data of successful and failed designs. "Principles of Yacht Design" by Larson & Eliason has easy to use tables. It is available off this site at: http://books.boatdesign.net/

Another option if your boat is of conventional design is to copy the rig off of something about the same size as you need. You can even buy a used rig about the same size and adapt it to fit, the tricky part would be to make sure your anchor points and mast step are strong enough (this is where an engineer or NA can help you if in doubt).

Or see if you can find the original plans, either from the builder of the hull or from the designer of your boat. Or try this list if you know the name and model of the design. The plans should have the mast and rig specs, and show you how to build it.

Building a large wood mast is routinely done, if you can not find a good book on building wood boats with instructions on building masts, you can always find a boat yard that builds wooden masts and poke around, ask a few questions and see how they build them.

I would not use birds mouth joints personally, unless you are wrapping it with fiberglass. The joints should be near where the spreaders are. If I was doing it I would use long scarf joints (like 15:1) on smaller segments of the mast cross section. And then stagger these joints with the multiple sections of these smaller segments. It would take some easy to make jigs to hold all the segments together as you build-up the mast and allow the adhesive to cure. With a mast that big you will need a very long shop space to make the jig and build it up.

Good luck.