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Go Back   SailNet Community > On Board > Gear & Maintenance > Is one better than the other?
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Thread: Is one better than the other? Reply to Thread
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Topic Review (Newest First)
07-11-2002 06:08 PM
Jeff_H
Is one better than the other?

I would guess that the spars on a Coronado might have been either Kenyon or Dwyer both of which are still in business and both have websites with thier various products.

Dwyer
http://www.dwyermast.com/dwyer.htm

Kenyon
http://www.rigrite.com/kenyon/kenyon.html#replacement spars
http://www.kenyonmarine.com/marine/medit.html

Jeff
07-11-2002 03:40 PM
MGauthier
Is one better than the other?

Ok VIEXILE, do you happen to have an address to this person in california? We just got "Desidrata" a Coronado 25 and the mast step has a crack in it, and we are not sure where to get a replacement, any info you can pass on will be greatly appreciated. thanks, Marty & Liz
07-09-2002 05:44 AM
VIEXILE
Is one better than the other?

I can''t remember the text, but a few years ago in my ardor to "bluewater" my boat, I had concerns about deck-stepped masts. I finally found some data that indicated an approximately 10%, as I recall, loss in the column strength due to deck stepping without additional connection from spar to compression post. The guy at former Westsail in California builds a tabernacle for around $250 that can be mechanically attached to deck and compression post to overcome most, if not all, of the strength loss. A friend who dismasted in the "Pigs on Water" (debauchery at its finest) race in Coral Bay emphasized his delight with deck stepping, since, he felt, a keel-stepped mast might''ve holed his hull. I think he was distracted by the all-girl crew coming up on him doing the necessary and permissible distraction technique of, for lack of a better term, "lifting their canvas."
07-08-2002 04:16 PM
gandydancer
Is one better than the other?

Very interesting!! At least either setup is a strong setup. To me it seemed as though the keel-stepped mast was a much stronger setup. But it is good to know that either type stepping is relatively safe and secure. Thanks to both of you!!
07-08-2002 03:00 PM
Jeff_H
Is one better than the other?

Hi Denr,

I did not get the point that your were making when you said, "Think of a keel stepped mast as having one more point of support compared to one that is deck stepped."

With a moment connection on a deck stepped mast, you are equally likely as a keel stepped mast to keep the mast if a shroud parts, or a mast stub if the mast does fail but if you need to jetison it you have half chance of doing that quickly.

Changing over to a deck stepped mast with a moment connection is pretty easy to accomplish the way that my boat was constructed and so is a very real and not terribly expensive option but you are right could be substantially more difficult on other designs and on a boat that does not need its deck painted anyway.

Respectfully,
Jeff
07-08-2002 11:28 AM
Denr
Is one better than the other?

Thanks for all of the minutia Jeff.

I prefer keel stepped masts because the rigging load is more evenly distributed through out the entire structure of the boat. A keel stepped mast, if properly engineered, will have a tie rod connecting the mast and partner that will adequately handle the lifting force imposed on the partner (and cabin top) by the blocks at the base of the mast. Think of a keel stepped mast as having one more point of support compared to one that is deck stepped. I was dismasted once as well, (I didn''t actually loose it I knew where it was, in the water) and it was decked stepped! If you loose a shroud with a keel stepped mast, in all likelihood the mast stays standing, with a deck stepped mast she’s most likely going overboard.

It is shear (pardon the pun) nonsense to hack off six feet of your mast and mount the mast on the deck and then re-engineer the bulkheads and compression post to take the loads required. Someone has way too much time on thier hands!

Yeah, a little water gets into the bilge from the exit plates however that might only be a problem in a hurricane.
07-08-2002 08:48 AM
Jeff_H
Is one better than the other?

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 one 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 bending 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 often to 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 to be putting side or vertical 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 strongest objections are to keel stepped masts are 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 at the step for a deck stepped mast is that mast has a welded flange on its bottom that is through bolted through the deck into a moment connected 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.

Jeff
07-06-2002 06:36 AM
gandydancer
Is one better than the other?

As I am new to sailing and looking for a boat, one major difference I see in some used boats is that some masts are deck-stepped and that some are keel-stepped. Is one way better than the other? One sailor who was trying to sell me his boat said that deckstepped boats are made better, meaning stronger, when masts are deckstepped. His reasoning seemed logical but then again, I''m new. You can just about tell me anything and I wouldn''t know better. Can any of you set me straight on this new sailor''s question???

 
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