Mast Design and Heel Loads

don't know about that!

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

 

Gonzo, You know that is a half truth 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.

 

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

 

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?

 

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

 

Surely by Norman Skenes?

 

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

 

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.

 

How are you measuring the load?

 

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

 

From the water looks like they run aground full speed.

 

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

 

It includes the Herrshoff scantling method

 

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.

 

Just ruminating, like everyone else