Mast Design and Heel Loads

Hi

Great website and forum.........even for a structural engineering wishing he had done naval architecture instead!

An easy question for the experts, how do you design the masts? is it simply a fixed cantilever with free end at head or pinned at base and head?

I am presuming that it is not fixed at base and free at head as the bending moment is massive for existing alu mast (432KNm! for 12.7m mast)

Trying to figure out base loads in heel section at base of mast that is 12.7m in height with max compression of 60KN.

Any guidance or weblinks would be helpful.

Apologies for my static structural mind above and dont worry about heckling me on here!

Cheers

 

An easy question? There are Naval Arquitects that spend years learning to design nothing but spars.

 

There are masts with stays and masts without, and ones with headsails and ones without, and combinations of the above. Compression loads vary--- stayed masts have a lot of compression at the step and unstayed masts without a headsail can have almost none other than their own weight.
Compression loads will vary on the same mast and same conditions depending on the shroud and stay angles. The whole boat's displacement and shape also come into play, with stiffer hulls and heavier hulls adding to rig loads, and hence to compression loads.
Since you are probably asking about a stayed mast, diameter or section can be decreased by adding spreaders, struts, and diamond stays, so rigging has to be considered along with mast size.
Masts with their rigging can look complicated but once a designer knows how long a section can stand free without staying at a given compression and what kind of loads a given hull is likely to exert on a rig, the rest can be very simple.
That's the real challenge, to analyze your particular hull and sailplan, Then analyze what conditions you're likely to encounter (offshore, lake sailing, daysailing, and local conditions).

 

You can refer to Principles of Yacht Design by Larsson & Eliasson for guidance. Also, Skene's Elements of Yacht Design by Francis Kinney is useful for starting points and understanding the engineering.

Eric

 

Designing the mast is the end part of the calculations. The loads are the critical part.

 

What if you put a spinnaker on a freestanding mast, does it need a back-stay? and now it's in compression...... Or is this just not done, the only example I can think of is the MX-Ray but that's 13ft long.

 

You can do it either way.

 

Are you refering to a "sailing" mast or a "ship-type" mast?

 

You can fly spinnakers on a free-standing mast. I always advise rigging a running backstay just to keep the mast from pumping. Technically, the loads are usually light with a spinnaker, plus you are sailing off the wind where the apparent wind is greatly reduced. So the mast should be able to handle the loads easily. However, there are always some nincompoops that do silly things, plus the best of us can get caught out in bad conditions, and an unrestrained free-standing mast with spinnaker flying can pump pretty hard, inducing high cyclic loads in the mast wall. With a running backstay rigged, the mast is restrained from pumping. Attached is a photo of Project Amazon, an older open class 60 cat ketch, flying a spinnaker off her forward mast. She had running backstays, but in this particular shot, they are not set.

Eric

 

All unstayed masts I am familiar with that also carry a spinnaker do have running backstays available on each side. Otherwise the mast would have to be designed for this load and would be way overdone when not flying the spinnaker.

 

the mast are calculated against buckling. there is no need to predict the load from the sails. it is already known from the stability of the boat.

i recommend, sailing rigs and spars by Matthew Sheahan.

regards
ulas

 

"there is no way to predict the load from the sails"
What are you talking about? That is basic engineering. You have a maximum righting moment and that is the maximum load on the rig.
Also, there is a misconception about spinakers putting a higher load than a main and jib upwind. Calculate the load to heel a boat over 25 degrees versus the hull resistance for forward motion and you'll see the difference.

 

The question has been asked often. There is a thread that discusses the topic:
http://www.boatdesign.net/forums/sailboats/mast-compression-loads-21474.html
You should get something useful from there.

Rick W

 

Thanks for all the help, and link above to other forum questions.

Very helpful and tempted to get the Principles of Yacht Design book.

Loads of good info on this forum, thanks.

.......wonder if i picked the more boring engineering field with static buildings (with some windy days and earthquake movements)......or is the grass always greener?

cheers

 

You will work around more reputable people on buildings than on boats