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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > what mast?
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Topic Review (Newest First)
03-14-2004 09:19 PM
WHOOSH
what mast?

JR, there''s a lot of content wrapped up in Jeff''s and Jon''s replies but the reality of your circumstances - buying a 25'' sailboat - will probably present you with only one option: deck stepped mast. It would be a totally acceptable choice for you, assuming the boat was properly engineered.

I share the same preference Jeff does re: deck vs. keel stepped (altho'' I sail offshore with the alleged ''better'' keel-stepped mast) but, for a small boat which has simplicity as one of it''s key benefits, I would think a deck stepped mast would be far preferable: easily raised/lowered by you with some inexpensive line, rings and tackle and no deck penetration to worry about re: weather when unstepped. Can''t imagine why you would want to change ''simple'' for more complicated (and most likely a bit more expensive).

Jack
03-14-2004 08:18 PM
jkumin
what mast?

I''m surprised this question didn''t generate a little more discussion, as conventional wisdom favors keel stepped rigs for off-shore work. A couple comments:

Jeff feels keel stepped rigs guarantee water in the bilges as it comes in through halyard boxes etc. Not necessarily - you can get a water stop in the mast with a drain hole just above deck level. Isomat used to do it with a glob of foam. Hall Spars does it with a sloped aluminum plate. Jeff - you should try this, you''ll like it. Dry bilges are nice. The advantage of deck stepped masts is they eliminate sealing around the outside of the mast, although Spar-tite has made this simpler.

The main reason for tie rods connecting the deck to the mast or the step with a keel-stepped rig is a force Jeff may not have mentioned - upward pull of lines led aft such as halyards. Crank hard on a halyard led to a winch on the cabintop and you are exerting a powerful upward force that can lift the deck right off the bulkheads. This isn''t a problem with deck stepped masts as the mast keeps the deck from lifting.

Mast are placed in enormous compression to off-set shroud tension. If you run the numbers this is very much more than any sideways kick. The limiting factor for most masts is buckling from this compression - popping out of column. This is governed by Euler''s formula. The big advantage of keel stepped masts is the added resistance to this buckling. A simple analogy is push down on a thin dowel with one end on the floor. It pops out of column easily. Take the same diameter dowel and clamp the bottom in a vise and push down. Even with the same length exposed it is much more resistant to this buckling. That''s why a keel stepped mast can be a lighter section for less weight aloft, always a good thing. Or if the same section it can be that much more resistant to getting out of column, which can lead to failure if uncontrolled.

There are other ways of getting this stiffness. Some of open class boats use a tripod setup to brace the mast above decks and Hunter has experimented with this. Jeff''s idea of using a base plate through-bolted to a matching plate on the jack post has merit, although it seems rarely done. This would be like a column on a moment resisting frame as used for buildings. It would require careful engineering to avoid localized buckling of the thin mast wall at the plate.

As for dis-masting, unfortunately I have been there with both. The deck-stepped mast popped out of column and folded, something that seems to happen from time to time. It was blowing about 90 at the time and the reefed main had blown out of the bolt rope. Without the stabilizing aft pull of a reefed main the center of the mast popped forward when we fell off a wave and stopped. The keel-stepped rig came down from a chainplate failure. It left 6'' or so of stump. They were both a pain to clean up.

Which brings me to the main point. There is no simple right answer. Both setups can work fine -if they are properly engineered, maintained, and sailed.
03-13-2004 05:56 PM
Jeff_H
what mast?

Properly designed a deck stepped mast can work quite well in most applications. 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 and rig design.

To start with the basics, 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 it is really a truss standing on end but it does not completely act as truss because 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 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. This has become 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 with 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). 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 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. 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. My new boat has a keel stepped mast. It is my intent to pull this mast and have it modified to that arrangement if I ever go offshore with her.

Respectfully,
Jeff
03-13-2004 05:23 PM
jrwright
what mast?

I am looking at a 25'' boat and know the answer to most of the questions, but the more input the better.

I some boats have a mast that bolts directly to the deck. . . others go down through the cabin.

Is the one that bolts to the deck a problem?

Physics would seem to dictate it would be a problem.
Is it?

Thanks for everyones help

 
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