Mast Design and Heel Loads

[navalarchwanabe]

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

[gonzo]

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

[alan white]

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).

[Eric Sponberg]

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

[gonzo]

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

[idkfa]

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.

[gonzo]

You can do it either way.

[Ad Hoc]

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

[Eric Sponberg]

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 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

[tom28571]

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.

[urisvan]

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

[gonzo]

"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.

[Guest625101138]

...

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

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

[navalarchwanabe]

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

[gonzo]

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

[navalarchwanabe]

don't know about that!

Steptoe and Son are regular clients of mine! (so near true it's scary!)

[tom28571]

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.

Gonzo, You know that is a half truth:o Shock loads from a spinnaker can be much larger off the wind than a main and jib upwind. The boat can heel further, the mast can bend more, both relieving shock loads upwind. Longitudinal hull stability is much greater than righting moment and the mast has to take almost the whole shock load. Also, the spinnaker usually exerts the highest load on the mast on a reach. I saw a Freedom loose its mast while gaining on me in a race. They were late in setting the new running back after a jibe.

[gonzo]

Maybe it is too much of a blanket statement. However, do you think a boat knocked down has less of a shock load?

[Joakim]

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.

I did some comparison in my own VPP for a 33 footer. 10 m/s beat resulted in about 1 kN hull resistance and almost 4 kN side force at the heel of 19 degrees. 15 m/s 140 TWA resulted to hull resistance of almost 8 kN and a side force of a bit over 2 kN at the heel of 13 degrees. 20 m/s TWA 170 resulted to hull resistance of 13 kN and side force of 1 kN at heel of 3 degrees.

In all the cases part of the loads is carried by sheets, but at some point the forces on mast must be higher than on beat.

So I didn't see the difference, but I do know, that masts are dimensioned based on righting moment. Righting moment limits the forces on beat and on knock downs also the inertia is important. What limits the forces on downwind? Or are masts just so much stronger in for-aft direction, that this is not important?

[Eric Sponberg]

All masts, whether free-standing or stayed, are designed to the boat's transverse maximum righting moment because the engineering determination of that load is so easy to calculate and it results in the predominant load on the mast and rig. It is immaterial what the actual wind speed, wind pressure distribution and boat's heel angle are. A boat is a free-floating body, and the heeling moment is always equal and opposite to the righting moment, so we go with the righting moment as the load to greatly simplify the problem.

In the case of carrying a spinnaker downwind on a free-standing rig, the potential exists for forward tripping of the boat stern over bow, but this condition is so unstable and transitory that it quickly reduces to a bad broach as the boat suddenly rounds up into the wind. The bending stress in the fore/aft direction rarely, if ever, has time to increase to a suddenly high and momentary level. It reduces quickly to a high transverse load as the boat broaches. Therefore, the fore/ast situation is usually ignored in the engineering of the mast. As I stated earlier, it is simply prudent to set a running backstay when flying a spinnaker to keep the mast stable and from pumping too much which could increase cyclic loading beyond normal levels. If I know a spinnaker is going to be carried, I will usually check that situation just in case.

Tom's note above about a Freedom losing a mast on flying a spinnaker is interesting as I would not have expected that since the Freedom masts were usually built pretty heavy. There was a situation back in the late 80s or early 90s were a batch of bad masts when through the TPI factory, and a number of them broke in relatively benign conditions. I can't remember which model of Freedom it was, perhaps the Freedom 35, and I wonder if that was the situation there.

Eric

[Velsia]

Skene's Elements of Yacht Design by Francis Kinney
Eric

Surely by Norman Skenes?

[gonzo]

The revised Kinney version has a rather obsolete chapter on fiberglass.

[tom28571]

Maybe it is too much of a blanket statement. However, do you think a boat knocked down has less of a shock load?

Probably yes Gonz, I regularly sail on boat with unstayed masts (2) and an asymmetric spinnaker on a sprit. This boat is unballasted and the spinnaker load is Way more than the working sail load. That is an unfair example but I do think that the spinnaker exerts a higher amplitude shock load than working sails on a just about any boat. A spinnaker that collapses and fills can be pretty impressive.

[gonzo]

How are you measuring the load?

[idkfa]

In the normal course of sailing; transverse loading greater than longitudinal but how about in the extreme case?

[gonzo]

From the water looks like they run aground full speed.

[Eric Sponberg]

Surely by Norman Skenes?

Yes, the original older version was by Norman Skene and originally published in 1927, updated in 1935 and 1938. Francis Kinney of the Sparkman & Stephens design office rewrote and updated it considerably in 1962 and 1973. My copy is of the 8th edition of that last date. There were other printings later, one of which is famous for the bad printing and production errors of the book itself. This book has long been out of print, and the only thing that has appeared recently is a version of one of Norman Skene's earlier works. Kinney's version is greatly expanded from Skene's original.

Eric

[gonzo]

It includes the Herrshoff scantling method

[Paul B]

From the water looks like they run aground full speed.

Nope.

Due to tidal conditions there was a large standing wave created, and the angle of the shot doesn't show it. The boat shown barreled bow on into the standing wave at a high speed and you see the result. The bowman was actually at the bow pulpit while this photo was taken.

I have seen a similar standing wave when sailing in Raccoon Strait in SF Bay. It had a near vertical face of maybe 4 feet or more.

[tom28571]

How are you measuring the load?

Just ruminating, like everyone else:D