Forums: The Deben 5-Tonner: Mast - keel stepped New Topic Post Reply
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Posted: Nov 15th, 2011 at 01:35   |   Subject: Mast - keel stepped
Good evening George;

I thought I would write me reply to your mast comments in another topic just in case someone sometime, might want an easier way of finding it.

you wrote:
"The mast doesn't absolutely have to be keel stepped. A gaff rig just gets much better support from a keel stepped spar, because there isn't a standing backstay – and as the boat gets larger, this becomes more evident.

A deck stepped mast generally puts more compression loads into the spar and the structures. So a deck stepped mast usually needs about 35% higher moments of inertia than a keel stepped spar.

If we can arrange for a keel stepped mast, then I think we would have a nicer, more stable rig. Specially if you are going to set a topsail.

But if it's going to ruin the interior, then we can arrange for a deck stepped spar. And especially if you are building a cold mounded hull and structure, we can arrange to take the compressive loads more easily.

If you are sticking pretty much to the internal arrangements as shown, then we might need to make the WC compartment a little larger, so that the mast could be accommodated in the forward corner – either inside the WC compartment, or possibly outside it, with a curved corner partition."

I like the keel stepped mast for the "extra" support it gets, plus the look as well I have to admit. You mentioned that if flying a topsail, a keel stepped mast is better. We that clinches it as, I simply love the look and the idea of being able to fly a topsail.

I am not set on whether the head needs to be in the same location or not yet. If so, making the mast a part of the WC structure is a nice idea. It does make getting into the forward berths tighter and thus might have to adjust the sink/cooker space some. Or maybe not have a stardboard sleeping berth and switch the seating with the kitchen area. Open to other layout ideas for sure.

Also on my wish list would be to have a chart "table" area if possible; probably where the electronics are also located. Maybe at the aft end of the port bench/berth. I know there are space limitations for sure, but while I am scheming..... =)

Greg H
Posted: Nov 15th, 2011 at 12:07   |   Subject: Mast - keel stepped
The thing about a keel stepped mast is that it can be considered a fixed end column whereas a deck step mast is a pin-ended column (Euler). The resistance to buckling under compression is greater for a fixed end column, hence the spar can usually be smaller (or to be accurate, have smaller moments of inertia).

With a bermudian rig, virtually all the loads from the sails can be converted into compressive loads in the spar, so life gets pretty simple. By contrast, a single masted gaff rig which can't easily have a standing backstay has a more complex situation because not all the loads can be converted to compression – there are bending loads as well.

My original design for the Deben 5-tonner showed the mast deck stepped (OK – stepped in a tabernacle, but this is not an effective fixed end).

We support the mast with the aft and forward shrouds, which run from the hounds. But because the angle they make with the mast is quite narrow fore-&-aft, the compressive loads get quite high. I am happy with this on a cold-moulded structure – not quite so happy on a traditional carvel structure.

And, as with all gaff rigs, it is not possible to properly secure the top of the mast, except by running backstays and the tension in the leech of the mainsail and topsail, transmitted through the mainsheet – plus of course the stiffness of the mast itself.

A keel stepped mast does provide substantially more support for the mast in all respects, reducing the compressive loads and providing better support for the bending loads

I've also been discussing some elements of this in the technical article on unstayed masts – which I haven't had time to finish yet!

But as you say it does give us a few problems in the interior layout. I shall be interested to see what you are thinking about this.

By the way, the new lines and offsets are on the side so you can download them now and throw away the old ones!



Posted: Dec 11th, 2011 at 18:13   |   Subject: Mast & Corner of head
Good afternoon George;

I was looking at the measurement numbers from the interior layout pdf you have on the main deben-5 page for the forward head bulkhead and the "longintudinal structures" pdf from the download page for the centerline of the mast. The mast is forward of the bulkhead but depending on the mast's diameter(s) it may or may not impinge upon the bulkhead. The centerline for the mast and the edge of the head bulkhead at the top of the coach roof is about 80mm and at the step for the mast, about 100mm (due to rake of mast of course).

Do you have the mast dimensions/drawing for the keel stepped, mast for use with the headsail?

Posted: Dec 16th, 2011 at 11:47   |   Subject: Mast keel stepped
Oops – I posted my reply as a new topic! Sorry about that.