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  #1  
Old 01-18-2007
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Mast types...? Through deck on deck...?

Hello,

On my sailboat I have a mast that is mounted to the deck, meaning it does not protrude throught the deck into the galley like I see on other boat designs.

What is the reason for this...?
What are the positives and negatives...?

Also what is the correct terminology for these 2 types of setup...?

Thanks...

ms
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Old 01-18-2007
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Other's will give you a better explaination of the pro's and con's, but they are called deck-stepped and keel-stepped. One advantage to the deck stepped mast is the ease of removal, especially for trailer-able boats.
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Old 01-18-2007
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Thanks now I can search more with the correct terms...

I have a 30ft Cruiser... I live in Miami and have sailed to Key largo many times and biscayne bay. I plan on sailing to Key West next week and Bimini to Nassau in March...

I recently changed a chainplate for a lower shroud that broke...the mast did not move...and I have an deck stepped design. Also changed all the lower shrouds...

thanks again...
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Old 01-18-2007
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This is copied from a piece that I wrote for a different venue.....

"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 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 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. "


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Old 01-18-2007
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PB is correct: Deck Stepped if on the cabin top.

I am not qualified to give a whole history of the two, but I guess you will see some strong opinions wither way. I will tell you my perception:

Most of the original masts were keel stepped. My perception is that keel stepped masts are probably safer and more adept to offshore work. Just looking at it, consider how much harder it would be to demast a keel stepped versus a deck stepped. If that base plate moves or you somehow lose your compression post, it is gone. However, like all things in boats, there are drawbacks. One of the key drawbacks for a keel stepped mast is its tendency to leak. Even with all the new contaptions, etc, my guess is that you will always be faced with replacing/tending the collar.

Deck stepped masts seem to have grown in popularity among many, many builders. The reality of demasting a boat that is deck stepped is pretty darn slim. Most likely the forces that would require that are going to take down a keel stepped anyway. Deck stepped masts do not leak (well, hopefully) and are becoming more accepted even with the old traditionalists.

I have no doubt that there are design reasons for keel stepping masts (even today), but I would imagine you will see a continuiing trend toward a DS mast for reasons stated above. Just a note: Catalina 400 and 380's are deck stepped, the Catalina 36 is keel stepped. It is not necessarily a sign of a cheaper made boat... just design trends.

A quick answer to the obvious reason for your post is: No. You have nothing to worry about with a correctly deck stepped mast and it is fine and safe.

- CD
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Old 01-18-2007
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Uhh, yeah, what Jeff said.

Now Jeff, I type pretty fast... don't you tell me you just typed all that.

- CD

PS I guess I have always been under the impression that keel stepped masts were safer and less likely to demast. I can see you argument about the vertical loads... but not sure I totally agree that demasting with a keel stepped versus a deck stepped is going to be that much better. My guess is that the deck stepped will come right through the top of the roof. Hope no one was standing there.

PSS I prefer deck stepped too.
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Old 01-18-2007
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The best way to settle this argument is to see how the best blue water sailboats handle this question. Shannon 43, IP, Valiant, etc. Hinkley, Alden, and others. What does Perry think?
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Old 01-18-2007
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Something I just came across from another forum.

They are great boats!! Cape Dorys in general are very well made. The only real flaw I can think of is that some of the Cape Dorys have deck-mounted masts, which can be an issue after years of use... Other than that...go for it.
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Old 01-18-2007
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I think an issue you didn’t cover in enough detail is how the type of step affects the rigidity of the mast and how that affects the choice of section. Euler's Column formula is used to determine the required moment of inertia of the section you need to handle the strain from the point loads of the stays and shrouds and the disturbed load from the mainsail. Part of the formula takes into account the type of ends the column has. A deck stepped mast is considered to be a pin end and a keel stepped mast is considered to be a fixed end. A keel stepped mast is 20% more efficient (ratio of effective length/length =.8) then a deck stepped one. This means for the same weight the keel stepped mast is not only stronger but is more rigid then the deck stepped one.

Either mast can be more then adequate for the job but a keel stepped mast is along the lines of a belt and suspenders approach to sailing. It is less likely to fail even if the lowers fail unlike a deck stepped mast which will almost certainly fail under the same conditions. I will not say that one is better then the other but a keel stepped mast does have its advantages and with some boats such as mine, a Tartan 34C the mast is not in the way because it’s in the head compartment.

You raise some interesting points but as you point out the final decision is largely based on factors that have nothing to do with the efficiency of a design and instead are based on emotion and habit. In short everybody has an opinion and there is no one “right” design for the job.
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Old 01-18-2007
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(EDITED Robert, I was writing when you posted!! Slowwwww writer, me!!)


The choice of mast set up depends mainly on the purpose of the boat.

They both good and bad. Depends. Many people want one set up other want the other set up. At the end of the day, the choice depends on what the boat is meant for. (opinions like allways, are just an extra. experience should dictate what you want on your boat)

For Cruising and small boats a deck step makes more sense, as it is easier to remove and transport. Also cruising boats normally do not require mast bend features, because they're not racing, and do not require built in mast tunings. Most prodctions use deck step to avoid the water ingression, and to make the mast smaller to ferry with boat when new.

As we move towards larger boats, mast transportation issues do not make a difference because the mast is already so big that 9 feet more do not make a difference, and you'll allways need a crane anyway.

I have a keel step mast for several reasons. One of them is because of the races. We needed a stiffer set up, to allow more mast control.

It allows better sail shape, specially on the lower end of the mast, because it allows to bend better to flaten the sail.

Initially we were going to have 3 sets of spreaders, but because it affects rating, we went for 2 sets and profitted the rigidity of the keel mast.

I can modify mast/shroud tension while racing, by using a special hydraulic jack installed under the mast, and can move the mast up or down according to the needs. So it needs to be bellow decks for this. We change tension when beating or downwind. It has a "locked" position so I can cruise with the family, too.

When deciding the inside arrangment of my boat, I moved the front bulkhead backwards and installed the mast in the front head shower, so all water that gets in is going directly to the shower drain. Solved a big problem here!! My mast boot is there for looks.

We needed the rigidity of the set up and my cabin was properly reinforced around the cabin hole for this purpose.

Also if still have some time to change tack if a shroud fails, before the mast colapses.


here are a few photos of the thru deck and mast installation.

Bellow the thru deck and on the bottom the mast base box, underneath it the hydraulic jack.





Mast installation









Last edited by Giulietta; 01-18-2007 at 11:27 AM.
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