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  #1  
Old 02-20-2002
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Deck stepped Mast-Always Bad ?

I was checking out Cheoy Lee 44s on the net. The specs sounded pretty good ,however they appear to have deck stepped mast and yet I see them referred to as good bluewater boats. What is up with that? Yes I know about the Cheoy Lee Web site but didn''t find any mast info.

M.M
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Old 02-20-2002
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Deck stepped Mast-Always Bad ?

This is another one of those, "there is no one right answer" questions. I personally strongly prefer a deck-stepped mast over a keel stepped mast but once again this is an area where opinions can differ widely. There is no right answer here. There is a contingent that thinks that the only proper way to step a mast is on the keel. There is a logic to that but it is a logic that can be engineered around and which comes out of a historical context that is less relevant with modern materials.

If you look at this as a structural engineering problem, the base of a mast has a vertical and horizontal thrust to it that tries to push it down through the bottom of the boat and also sideward off of the mast step. In normal conditions the down load is several times greater than the side load. Beyond the loads imparted to the boat, there is also the issue of the loads that happen internally in a mast. When you look at the structure of a mast, spreaders and rigging,it is really a truss standing on end but it does not completely act as truss because in classic engineering terms, the components of a truss are not supposed to have bending loads on them. Ideally, the loads in the mast are primarily axial (acting along the length of the mast) rather than in bending (acting perpendicular to the long axis of the mast). Of course, masts actually do have fairly large bending loads imparted into them. The two most often cited reasons for keel stepped masts being considered stronger is the way that the bending loads (moments) are distributed within the mast itself and the way that the mast imparts its loads into the boat.

If the goal of designing a mast is to reduce bending moments within a mast, the greater the number of panels (segments between shrouds and other supports) the smaller the moments tend to be. In the days when single spreader rigs were most common a keel-stepped mast added one extra panel, the segment between the mast partners at the deck and the keel. With only three panels this extra panel was very important.

The need for this extra panel has became less significant as bigger boats have routinely gone to multiple spreader rigs and moment connections at the deck mounted mast steps.

In terms of the way that the mast imparts its loads into the boat, masts are generally located in the area of the cabin trunk and because of the shape of the cabin (i.e. the deck folds up at the cabin side and horizontal again at the coach roof), this area (if not engineered for side loads) is more prone to lateral flexing than would be the keel. One idea behind a keel-stepped mast being stronger is that a keel stepped the mast is not supposed loads are put loads into the deck.

In reality, this ideal is rarely accomplished for a number of reasons. First of all, if the mast is not tied to the deck or the deck tied to the keel near the mast, either with a tie rod or a tie from the mast to the deck and a connection from the mast to the keel, the downward force of the mast working in opposition to the upward loads of the shrouds can pull the hull together like a bow and arrow, lifting the deck and separating the joint between bulkheads and the deck. You sometimes see this type of separated bulkheads on inexpensive or early fiberglass boats with keel stepped masts.

Not only do keel stepped masts impart vertical loads into the deck (through the ties mentioned above) but they also typically end up imparting side loads as well (if they are going to reduce the moments in the mast as mentioned above they have to). This somewhat reduces the structural advantages of a keel-stepped mast to next to zero assuming that a deck-stepped mast is properly engineered, and that is a big if!

There are several things that I consider critical to engineering a deck stepped mast properly. Primary is having a jack post below the mast. A jack post is a vertical member that carries the vertical loads of the mast to the keel. My preference is to have an aluminum jack post rather than a wooden one but a wooden post can work as well. The other issue is the distribution of the side loads. Ideally there should be a bulkhead or ring frame adjacent to the mast that can take the side loads and distribute them into the hull. These are obviously more complex to do than simply having a fat spot on the keel for the mast step to land on.


My objections to keel stepped masts are to the purely practical. Keel stepped masts mean that there is always water in the bilge. This water comes in at halyard boxes and other openings in the mast and nothing you can do will stop that. Second, it is way harder to step and unstep a keel-stepped mast making the boat more subject to damage in the process.

Beyond that if you loose a mast (I have lost two in my life) it is better in my opinion to loose a deck stepped mast because a keel-stepped mast is more likely to damage the deck when it fails and a deck-stepped mast is easier to clear away. The keel stepped mast advocates point out that you are more likely to end up with a bigger stump after the mast fails. I am not sure that that is the case if you are able to tow the rig as a drougue until things quiet down enough to rig a jurry rig. With a keel stepped mast, I am not sure what you do when the boat is being beaten to death by the upper portion of a mast that has buckled 20 feet off the deck at the spreaders.

My preferred set up is a deck stepped mast that has a welded flange on its bottom that is through bolted through the deck into the top flange of a structural aluminum jack post so that it transfers moment through the deck. My new boat has a keel stepped mast. If I go offshore it is my intent to pull the mast and have it modified to that arrangement if I ever go offshore with her.

Regards
Jeff
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Old 02-20-2002
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Deck stepped Mast-Always Bad ?

Jeff
Thank you,thank you ,thank you. What a good explanation. Now I have a better notion of the physics of the thing and what to consider and to ask. Really that was very helpful.

M.M
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Deck stepped Mast-Always Bad ?

Jeff,

I agree. I would take a deck stepped mast any day over a keel stepped mast.

It gets very frustrating trying to keep water out of the boat when the rain water comes dripping down the inside of the mast every time it rains hard.

Then there’s the chore of keeping the mast boot waterproof.

A few years back I saw a 40’ sailboat that had a deck stepped mast which was rigged to be taken down and raised at any time! The owners kept the boat in Newport harbor in California, which (apparently) has a low fixed bridge. To sail outside the harbor, they lowered the mast a bit, went under the bridge, and then raised it and the sails.

I have a friend who designed and built a square stainless steel channel structure, in several pieces, which takes the load of the deck stepped mast and transfers it to the keel. The boat is only 30’ long and a load-transfer post in the cabin right under the mast would have hogged too much room.

He says it bolts together in several pieces and works great.

Bob….
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Old 02-20-2002
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Deck stepped Mast-Always Bad ?

Jeff: we have a deck stepped mast on T.P.III
I call the brace between the mast and the
keel,a Sampson post.Ours is stainless steel and the flange under the mast base is about
12" in dia. larger than the base of the mast.
Rather than just mounting the mast to the deck,we have a tabernacle which allows us to raise and lower the mast a will.It my opinon
that if it wasn''t for the cost factor all masts would be deck stepped!!
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Old 02-22-2002
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Deck stepped Mast-Always Bad ?

I also agree. And if you DO loose your rig
you are probably left with a complete rig(minus a shroud or stay).
Dennis L.
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Old 01-25-2008
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Running the mast thru the deck to the keel, does the same thing as multiple spreaders you can use a lighter section for the mast. Weight aloft is not good for stability but some times in reducing weight to improve performance, designers create a structurally deficient mast for anything but racing, where sponsors can pay the cost of replacement.
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Old 01-25-2008
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The bottom line is a keel stepped mast with no shrouds still stands up. It starts out stronger before anything else is added. As for the water in the bilge, who cares. I prefer Keel stepped but would take either id I liked the boat.

Gaz
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Old 01-26-2008
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Old 01-26-2008
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