Mast Section sizing

[lastconch]

I am looking for a new mast for my Irwin 34 citation Original mast was 195mm/120mm don't know the Ixx Iyy for that section. Would The charleston spar 330 section at 180/120 I 758/332 be as strong in fore and aft or stronger? What range of I's should I look for?

[Tartan34C]

The I stands for moment of inertia of the section and it’s given for the long and short axis of the spar section or column etc. It is a measurement of how strong (stiff) the spar will be. The people that can answer your question won’t without more information and even then probably won’t for all the usual reasons. I don’t want to be unfriendly but offering engineering advice with only half the needed information will only get you a half ass answer.

If you are building your own mast you are starting out without enough information and might want to step back and either study up a bit or select a different plan of action. If you are having a qualified shop do the job they can advise you as to what is a suitable section. If the shop can’t answer the question then they are, at least in my opinion, an unsuitable shop or things have changed since I was in the business.
Good luck and all the best,
Robert Gainer

[Gramp34]

Dave Gerr's book "The Nature of Boats" gives rules of thumb for mast sections in Chapter 39. The formulas are 'old school', and fairly conservative. They'll give you a mast and rigging heavier than necessary.

To get more precise you need to know the righting moment of your boat. If you do, Chapter 11 of "Principles of Yacht Design" by Lars Larsson & Rolf Eliasson has the calculations.

Maybe I'll start an argument here by suggesting that a heavier mast reduces static stability but increases dynamic stability of a boat.

Good luck,

Tim

[JohnRPollard]

Originally Posted by Gramp34

Maybe I'll start an argument here by suggesting that a heavier mast reduces static stability but increases dynamic stability of a boat. Good luck, Tim

You'll get no argument here!! The extreme example is the awkward motion of a sailboat with its mast unstepped. But you don't have to go to such an extreme to make a convincing argument. Our Melges 24 had a lightweight carbon fibre mast that we could step/unstep on the trailer. That boat was very stiff initially and had a snappy/jerky feel underway. Our Dana 24 had a mast like a bridge girder. It had less initial stability but firmed up nicely after 15-20 degrees of heel or so, and a far more comfortable/gentle roll motion.

This might be a fitting topic for a separate thread. Perhaps the professionals could quantify this effect?

[Tartan34C]

Tim,
I don’t think you will start an argument. Some, myself included will just say that anything that raises the center of gravity reduces all forms of stability. But reducing stability increases the period of roll and that makes for a more comfortable boat.

Dave Gerr's book "The Nature of Boats" is great but in the next paragraph after explaining his rule of thumb he says, to paraphrase because I don’t have my copy handy, "a sparmaker or navel architect will calculate the required moment of inertia based on the boats stability but this rule of thumb will get you into the ballpark for the average boat in average conditions." I think that if you do want to work it out for yourself try “Skene’s Elements of Yacht Design.” It is easer to read and has a very nice simple method that works well for everything except the very modern high tech boats and then you need to look into "Principles of Yacht Design" by Lars Larsson & Rolf Eliasson. But do it your self design has one problem. If you don’t know what you’re doing you can make mistakes without seeing a problem until the rig comes down. I think the best thing is to talk to a spar shop and avoid problems.
All the best,
Robert Gainer

[deniseO30]

I saw a beautiful tapered mast on a Pearson 31! all the science aside it sure looks "right"