Sunday, May 18, 2008

Mast steps

The where, when, why of deck stepped versus keel stepped:

From Paul France - NZ
Can anyone explain clearly why we carry masts down through the cabin to the keel. I look at my 1970 IOR One-Tonner and don't understand it. To my mind for that boat, the shorter the length of alloy extrusion you have to keep in column the better, but there's this mast going all the way down to the keel. So why don't we step the mast on the deck and have a strut below?

From Scuttlebutt:
While we weren't there that day when they decided that keel-stepped was a better idea, we believe that the issues mostly have to do with deck compression. There is a lot of load on the rigs, and rather than dealing with the structure needed to keep the decks from sagging (and thus loosening rig tension), the simpler solution was to run the mast down to the bottom. Also, there are boats that have hydraulic rams at the butt to tighten the rig by raising the mast up. Other advantages have to do with mast bend, as you can either induce bend or restrict it by either moving the mast butt fore and after or by adjusting the mast chalks. We suspect the mast butt is also better mounted down low with only vertical loading rather than at the deck with more varied loading demands.

From Paul France - NZ:
Thanks for that. I'll try to locate someone who was there on that day, or maybe a designer/engineer. If I get a better answer, I'll let you know. Cheers and be assured I get a lot of pleasure out of Scuttlebutt

Additional thoughts? Post them in the comments section.

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At 7:28 PM, Blogger John Riise said...

I suspect it has to do with the same reason most cars have engines in the front: that's where the 'power' was located in the horse and buggy days and in the early days it seemed unnatural to put it anywhere else. In the maritime equivalent, masts were stepped belowdecks for about 500 years before we achieved the technology to even try a deck step.

At 12:30 AM, Anonymous Anonymous said...

A principal reason for keel-stepping the mast is that the deck provides support so the end is better fixed. With the end fixity greatly increased compared to a deck-stepped version, the load that would cause buckling is much higher and so a thinner (lighter) mast section can be used.

At 1:49 AM, Anonymous Anonymous said...

There's also something about pin ended columns and those that have their lower sections held at two points, but it's been so long since my GCE O Level Physics that I can't remember any more. Any enlightenment from a more up to date scholar? DJ

At 6:52 AM, Anonymous Anonymous said...

Yes, the two comments above are correct: By grabbing the mast at the deck, but not cutting it and using a strut, you greatly increase the bending stiffness of the mast.

Imagine this: Hold a yardstick out by one hand, and you'll see that it is only so-so stiff. But if you then grab it with a second hand, about six inches away from the first, then the stiffness will be much greater.

The technical language for this is "clamped fixity" versus "pinned fixity".

Like everything in engineering, it's a tradeoff: Clamping the mast at the deck does result in an increase in the bending moment that the mast experiences right at that point, whereas a pinned end experiences no bending moment at the ends.

Chris McKesson, PE
Naval Architect

At 6:39 PM, Anonymous Anonymous said...

It must be something in the water down here, but I, too, find the conventional wisdom unconvincing, even as a design engineer. I think it's largely another case of the innate conservatism of seafarers.

Keel stepping problems include
1) the mast is longer
2) the bending moment referred to by Chris can become quite significant if the standing rigging stretches or adjustment drifts so as no longer to be in concert with the deckstep
3) unless the mast collar is really well engineered, and/or the tube is locally thickened, the column walls can be crippled by the 'support'
4) it's the classic source of water down below. If you *must* keel step your mast, at least plug it completely above deck level, with angled drain holes, and bring the wiring out to a plug as for a deck stepped mast !

For cruising yachts, I think it's plain wrong to keel step the rig. If the bottom panel of the rig is problematic, I'd rather build a decent tabernacle on deck, with struts (which can cleverly double as 'granny bars') if necessary, so the mast is effectively "built in" right up to the level of the gooseneck. If this thrust is not transferred to the tube, now we're seriously talking saving weight further up. I'd think hard about going one stage further and ending the tube there, (ie gooseneck-stepped) making the bottom panel shorter, and the mast easier to lower.


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