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-   -   What is a Deck-stepped mast and keel-stepped mast (http://www.sailnet.com/forums/general-discussion-sailing-related/60795-what-deck-stepped-mast-keel-stepped-mast.html)

djr3203 12-24-2009 03:49 AM

What is a Deck-stepped mast and keel-stepped mast
 
I have heard references to deck-stepped mast boats and keel-stepped mast boats. Can anyone explain what that means? Thanks.

JomsViking 12-24-2009 04:30 AM

Basically what it says.
The Deck stepped mast is standing on the deck, and the deck is supported from below.
The keel stepped mast goes through the cabin and its foot stands on the keel.

There's a discussion here on SN http://www.sailnet.com/forums/gear-m...ped-masts.html

CaptainForce 12-24-2009 06:58 AM

The mast "step" is the location where the bottom of the mast is secured. If this step is on the deck, the weight of the mast is usually supported by a "compression post" or weight bearing pole under the deck and transferring this force to the keel. A keel steped mast passes through a hole in the deck and continues to the keel without a compression post. 'take care and joy, Aythya crew

Jeff_H 12-24-2009 08:18 AM

As has been explained the mast step is the method of attaching the bottom of the mast to the boat.A keel stepped mast typically sits on a reinforced structure in the bilge of the boat. The mast passses through a hole in the cabin top or deck and is continuous from the bilge to the top of the mast. A deck stepped mast is stepped in the cabin top or deck, and there is a supporting structure below the deck that keeps it from piercing the deck. The following is the draft of an article that I wrote for a different purpose but which talks about the relative metits of each.

Deck vs. keel stepped masts

While I personally strongly prefer a deck-stepped mast over a keel stepped mast, I completely understand that 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 opinion 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 design approaches.
<O:p
I would like to start this discussion with the structural 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). But because of the continuous attachment of mainsails and the point loadings of intermediate control lines like vangs, pole lifts and the like, 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.
<O:p
One of the goals in designing a mast is to create a configuration that, within reason, reduces the size of the bending moments within a mast. In a general sense, the greater the number of panels (segments between shrouds and other supports) the smaller the moments within the mast would 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 (adding more panels above the deck) and to a lesser extent as moment connections at the deck are being employed on deck mounted mast steps.
<O:p
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 the deck is not engineered for side loads it 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 superimposing loads into the deck.
<O:p</O:p
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 with 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.
<O:p
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 (they must if they are going to reduce the moments in the mast as mentioned above). This added side load when combined with the multiple panels above deck greatly reduces the structural advantages of a keel-stepped mast to next to zero assuming that a deck-stepped mast is properly engineered, and of course that is a big ‘if’!
<O:p
There are several things that I consider critical to engineering a deck stepped mast properly. Primary is having a properly engineered jack post below the mast to take the vertical loads of 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. My preference would also be to design the jack post, mast foot and mast step to create a moment connection between the base of the mast and the top of the jack post, so that the jack post could still act as an extra mast panel. Then the deck and interior structure need to be designed to distribute 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 elements are obviously more complex to engineer and build properly than simply having a fat spot on the keel for the mast step to land on.
<O:p
<O:p</O:p
My biggest objections to keel stepped masts are to the purely practical. Keel stepped masts with internal halyards mean that there is always water in the bilge. This water comes in at halyard boxes and other openings in the mast and there really is nothing you can do will stop that. Second, it is way harder to step and un-step 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 for offshore use advocates point out that if you lose a keel stepped mast you are more likely to end up with a bigger stump after the mast fails. I am not sure that that is always the case. For example with a deck stepped mast there are cases where you are able to tow the rig as a drogue until things quiet down enough to rig a jury 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. .
<O:p
As mentioned above, 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. If the mast buckles it can be unbolted and jettisoned, or kept up partially by the moment connection at its base. My current 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.

JomsViking 12-24-2009 08:31 AM

Jeff,
While I won't argue with the rest of your analysis (You rule), I do disagree that it is harder to step and unstep a keel-stepped mast. Having stepped and unstepped a large number of masts, I strongly prefer keel-stepped masts, because you have better control over it, as soon as it is inside the boat. It is also faster to rig these because of that, as you dont have to worry too much about the standing rigging.
For smaller boats, like H-Boats its easier and faster with a deck-stepped mast, though.
Could be me :)

Respectfully
/Joms

Quote:

Originally Posted by Jeff_H (Post 554013)
My biggest objections to keel stepped masts are to the purely practical. Keel stepped masts with internal halyards mean that there is always water in the bilge. This water comes in at halyard boxes and other openings in the mast and there really is nothing you can do will stop that. Second, it is way harder to step and un-step a keel-stepped mast making the boat more subject to damage in the process.


Tempest 12-24-2009 09:27 AM

Great discussion of the pro's and con's!

I was one of those, who blindly assumed that a keel stepped mast was a stronger, better design. Recently, I have read a few similar articles that have exposed the fallacy of those assumptions. While, I'm not prepared to convert my rig, or sell my boat; I can attest to the " water in the bilge"
Last year, I stepped my mast to pull the baseplate blast it and paint it, (among other things) I also did the same with the foot of the mast. Luckily, it was timely. If ignored long enough, I could see where it might have become neccessary to cut the base of the mast, and build a new base plate. I wonder how often that happens?

Another interesting point: I've never suffered a dis-masting, and hopefully never will, but my instincts would tell me to cut the rig away completely, (bolt cutters), or IF remotely feasable secure it and lash it to the deck.

Towing it as a drogue, would have never occurred to me. ( visions of a torpedo). I'd be interested in what technique would be employed to do that.
You're obviously running down wind at this point, you'd have to get a long line/warp attached...

If the stick is on deck why not lash it there...and deploy a drogue..
.if it's in the water...keeping it around while trying to attach it seems very dicey.......(although the whole situation is)

This is not criticism, but very educational for me.

cormeum 12-24-2009 10:00 AM

I strongly prefer a keel stepped mast for offshore work.
The bigger stump reason is the most common one but the real reason is that the mast is better supported in the first place.
A keel stepped mast has independent support from the step to the partners to limit movement and takes some loading off the shrouds rather than the end supported column of a deck stepped mast. I've been in the situation of losing the windward shroud and with keel stepping it's possible to pop the sheets and switch tacks to save the stick which we did.

If the mast were deck stepped it would have gone over the side- game over. (BTDT too)

Also a "drogue" on the side of a boat in big seas would probably be bad news as it'd bring the boat beam on to the waves- so hanging on to the rigging most likely wouldn't be a good option.

If the weather is bad enough that significant water is coming in through the sheave boxes, trust me, you'll be otherwise occupied.

Jeff_H 12-24-2009 10:27 AM

Tempest,
My comment about using the jetisonned rig as a drougue was not well explained. My thought on this was that a spare anchor rode would be tied to the wrecked spar as a way to control it and as a safety measure while the rig was being separated from the boat. Once the standing and running rigging was detached from the boat, the anchor rode could be eased a few hundred feet to allow the abandoned rig to be used a sea anchor/drogue. And once things quieted down, the rig could then be winched in and salvaged as possible.


Cormeum,

The reason that I favor a moment connection at the deck (as described above) for a deck stepped mast is the mast will behave like a keel stepped mast in structural terms of being a fixed end condition and so the stump length, or the ability to keep the mast up if a shroud parts, in theory would be the same as a keel stepped mast. But with a moment connected, deck stepped mast, offshore if the upper portion of the broken mast were beating the boat to death it can be unbolted and jettisoned.

Respectfully,
Jeff

flatracker 12-24-2009 10:34 AM

Keel stepped for me
 
Last summer (2008) I had a cotter pin come loose and the clevis pin was next, which totally disconnected the forestay. At the time I was a few miles offshore, flying a 150 close hauled in 10 to 15 knots. With that much sail flogging around, and the forestay gone, I firmly believe I would have lost the rig with a deck stepped mast. Before that I had never given much thought to the deck/keel option. Now I don't think I want anything but a keel stepped mast. To end let me remind everyone, check your rigging regularly, including those lowly cotter pins. Even if you do not loose the rig, you do not want a shroud to fail, anytime, anywhere.

cormeum 12-24-2009 11:01 AM

Quote:

Originally Posted by Jeff_H (Post 554039)
Tempest,



Cormeum,

The reason that I favor a moment connection at the deck (as described above) for a deck stepped mast is the mast will behave like a keel stepped mast in structural terms of being a fixed end condition and so the stump length, or the ability to keep the mast up if a shroud parts, in theory would be the same as a keel stepped mast. But with a moment connected, deck stepped mast, offshore if the upper portion of the broken mast were beating the boat to death it can be unbolted and jettisoned.

Respectfully,
Jeff

Jeff-
I think I see where you're going- you're advocating a rigid attachment at the tabernacle? The only issue I see there is that the full leverage of the stick will be applied to a comparatively small base plate which would probably involve a tear out of the tabernacle if a shroud should fail. I imagine it could be engineered to take such a transient transverse load, it'd just have to be larger than the load to failure of the (unsupported) stick.

With modern extrusions, the wall thickness is small enough that the stick usually tears through under failure, but in the event that it doesn't, I agree that it'd be nice to be able to throw it all away without having to saw through the extrusion. Usually if the failure is mid column without a break, like your scenario, it's best to draw the failed portion toward the boat (halyard to a sheet winch for example) and secure it until the sea state allows for cutting it away.


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