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-   -   Deck vs. Keel Stepped Masts (http://www.sailnet.com/forums/gear-maintenance/2455-deck-vs-keel-stepped-masts.html)

Envirella 10-22-2001 07:23 AM

Deck vs. Keel Stepped Masts
 
I''m at the beginning stages of buying a boat that can stand up to some serious cruising (eventually). I''ve always been of the opinion that keel stepped masts are the best for cruising as they are less prone do dismasting in heavy weather. In my search however, I have found some very sea worthly boats with deck stepped tabernacle mounted masts. Does anyone have an opinion regarding this issue?

VIEXILE 10-22-2001 08:28 AM

Deck vs. Keel Stepped Masts
 
Opinions are like sphincters . . . anyway, forestay lets go on keel stepped, it might remain standing. If it kicks out of the keel shoe, you have over 6'' of mast waving around belowdecks, possibly driving holes in the boat. Used to be a big concern for me, with a deck stepped mast. It seems like I ascertained the loss in strength/utility when comparing the deck stepped to keel stepped was quite marginal - only about 10%. This was loose-mounted simply on a shoe. Add a through-bolted tabernacle mechanically attached to your compression post (like the retrofit the guy that still maintains the Westsail website custom builds) and I''ll bet the differences are negligible.

WillS 10-25-2001 08:02 AM

Deck vs. Keel Stepped Masts
 
Our boat (1985)has a keel stepped mast that is wet if there is a quart of water in the bilge.
We bought it last year and I had to trim 1/8 inch from the bottom to square it off. I also cleaned up the corrosion and sealed with 2 part epoxy paint.
When the mast was stepped last spring, the paint at the base was scratched...resulting in some additional corrosion this season, although nothing like before. Looks like we will be pulling the mast at a minimum of every other year.
If you decide to go with a keel stepped mast, try to get one that is spaced up a bit and easier to keep dry.

rdt123 10-26-2001 02:02 PM

Deck vs. Keel Stepped Masts
 
our cheoy lee 44 - perry has a deck stepped mast with a compression colum passing the load to the keel. I believe that if we were to be dismasted the sucker would do less damage outside the boat than it would inside the boat. And of course carry something to cut away the shrouds - quickly.

timer 11-06-2001 07:12 PM

Deck vs. Keel Stepped Masts
 
My boat is keel stepped but the mast butt does not contact water and after 34 years is still fine - this is a serious problem it seems with many boats. If the rigging fails, most masts when they start to bend - thats it they just keep on going. Most emergency procedures are to cut rigging and lash parallel to boat. I, if given the choice prefer the keel step as the drive is right from the bottom of the boat and in a cruiser thats a beautiful thing. Whatever your choice hope you enjoy the boat - the most important thing Good luck Chris

Jeff_H 11-08-2001 07:19 AM

Deck vs. Keel Stepped Masts
 
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.

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 advoactes 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.

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 jack post. The boat that I am buying has a keel stepped mast. It is my intent to pull this mast and have it modified to that arrangement.


FtLMale 06-29-2007 05:47 PM

re:Deck vs. Keel Stepped Masts
 
Jeff H,
I read with particular interest your comments on the thread "Deck vs. Keel Stepped Masts". I have an Islander 37 sloop whose mast was crushed on the bottom 3 feet by an errant boat hoist operator. You mentioned that your next boat would be a keel stepped mast which you would modify to a deck stepped mast. I would be very interested in what modification you undertook to accomplish this. To quote you "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 jack post. The boat that I am buying has a keel stepped mast. It is my intent to pull this mast and have it modified to that arrangement."
I am strongly considering the same modification, although using a fiberglass structural jack post to prevent rust, rot and other assorted maladies with a stainless steel flange on top and bottom of the cabin for connection. I am interested especially on how much side loading I should anticipate on the deck to gauge the structural additions I should add. Any input you have would be appreciated.

sailingdog 06-29-2007 06:18 PM

You'd really need to talk to a naval architect or marine engineer to get a good idea of how to go about making this modification to your boat. I don't know whether JeffH ever had the boat modified to that end or not.

Rickm505 06-30-2007 11:17 AM

On smaller boats, one of the advantages to deck or cabintop stepped masts is that the mast and chainplates can be designed so the mast can be lowered and raised without a crane. My boat utilizes a tabernacle and is rigged in this manor. The chainplates are extended off the deck to match the height of the swivel bolt in the tabernacle.

I would imagine this advantage disappears as the boat length increases much over 30' as that dang mast is heavy!!

Rick in Florida

CapnHand 06-30-2007 12:48 PM

Fiberglass (GRP) is very strong in tension but not so strong in compression. It's not the best choice for a compression post.

The advice of a marine acrchitect / engineer should be sought. Modifications to the deck, chainplates and standing rigging would likely be required. If your only reason for doing this is cost, it may be cheaper to replace the mast.


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