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  #1  
Old 05-12-2006
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Deck Stepped Mast or Keel Steeped Mast???

Good day to the sailing world!

It's been some time since I've requested any other opinons, I've have a great delima an asking help with information on this subject. This is regarding a Deck Stepped Mask, or a Keel Stepped Mask. Should a Keel Stepped Mask give you far more strength than a Deck Stepped Mast?

Does a Keel stepped Mask add more overall weight to ones boat?

Wouldn't one want to consider a Keel Stepped Mask than a Deck Stepped Mast and why?

I've check out a lot of Deck Stepped Mast Boats, that are very strong and good sailing boats, why would I want a protenial hazard? Or is this not such a harzard!

Thanks for you help!
Ted Gladden
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Old 05-12-2006
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tbeargladd there are two schools of thought on this sub. a deck stepped mast reduces the holes in the hull/ cabin top by one. so if you get rolled there is one less potential leak/ water ingress while rolled. however more than likely you will loose the stick, which will make a jury rig extremely difficult. also there is a compressin post in side where the mast would be which is normal, but the design must be such that it will adiquately support the mast rigging and sail loads.
with a keel stepped mast there is the hole but if properly secured will leak but not alarmingly. also if rolled usually the mast will break at the spreaders if at all leaving a stump to get you home so i've read about. neither is a good senario as both can hole the hull until you clear the debris/ mast and rigging and let it sink
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Old 05-12-2006
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You'd really need an engineering report to find out how strong any mast setup is. Star masts are stepped through the deck, and they break all the time. Moody masts are often deck-stepped, but don't seem to fail much. some long-haul sailors like keel stepped masts because the partners may provide some support if a shroud or stay parts, and keep the spar up long enough to take the load off it. Racers may think a longer keel-stepped mast provides more leverage for bending it. Others like deck-stepped masts so they don't interfere with interior arrangements, or provide a conduit for rain to hit the mast and end up in the bilge. Both setups seem to work. It's a question of what works for you.
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Old 05-12-2006
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Keel or Deck

First of all good day, to all replier's and hopefully more collaboraters.

What a great amount of opposition here. I love it! Some very interesting facts above for me to consider and hopefully helps to assist any and everyone else who finds this a useful subject. More question below!

If a keel stepped mast corrodes at the keel how would a survery be able to diaganose if their is structural problems with the mast and keel?

Is their a life expectancy of a mast before it would need to be replaced and if so what is the expectancy? Like the standing rigging, suppose to be good for up to ten years! True/False?

If a deck stepped mast doesn't have a compression post and the mast has concave the cabin top, would that area on the cabin top have to be cut out and replaced or would the entire cabin top have to be replace or some type of support would be just enough to give the sunken portion of cabin top some lift to its normal stature?

When I see a "rubber boot" (I hope that is the correct terminology) at the bottom of the mast is that what protects the Keel Stepped Mast from leaks into the interior or does this assist the Deck Stepped Mast from water softining the cabin top?

As many boats as their are that are Deck Stepped the amount over shadows the amout that are Keel Stepped, other words I see more Deck stepped than I have seen Keel Stepped. Does that have any affect of the value of the boat?

Thanks everyone for your words of wisdom is truely appreciated.

Ted Gladden
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Old 05-13-2006
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Boat Design Forum => Similar thread ...

http://boatdesign.net/forums/showthread.php?t=10203
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Old 05-13-2006
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Both types of masts have pros and cons. A properly designed deck-stepped mast is preferable IMHO. There are too many possible problems with having the mast go through the cabintop, such as delamination of the cabintop, water leaks, etc.

Also, a properly designed deck-stepped mast can often be designed to be unstepped without a crane. This is very useful for boats that are travelling in areas without full marine service facilities.
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Old 05-13-2006
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This is from a draft of an earlier article that I had written for a different venue:

"I personally strongly prefer a properly designed 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 very respectible logic to that position but it is a logic that can be engineered around and which comes out of a historical context that is less relevant with more modern designs and materials.

To start with the basics, the base of a mast has a vertical and horizontal thrust to it that tries to push the base of the mast downward 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 within a mast. When you look at the structure of a mast it is really a truss standing on end, although it does not completely act as truss because the components of a truss are not supposed to have bending loads on them and the individual panels of the mast are exposed to bending as well as axial loadings. 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 which 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."

Jeff


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Old 05-13-2006
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Keel steped masts

Just my two cents, I just bought a 37 foot O Day sailboat without a survey and on taking it out on my maiden voyage, I lost the fore stay due to improperly installed swage fittings and a cracked deck fitting. Because the mast is keel steped nothing happened and I was able to spill wind enough to get to the boat yard 20 miles away. Had the mast been deck stepped I believe it would have come down on top of us. Just my opinion.
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Old 05-13-2006
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If there had been a moment connection at the mast step, or a forward lower shoud, you would have had an equal chance of keeping your mast whether the mast was keel or deck stepped. But with a normal compression deck stepped mast and no forward lowers you are right that your circumstance would have been a lot more perrilous.

Jeff
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Old 06-13-2006
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Thanks to everyone, that was vital information very in dept and helpful! Their is still a choice to be made. This will take some time to figure out Thank's again for all your support!

tbeargladd
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