|Topic Review (Newest First)|
|09-09-2006 05:33 PM|
|devildad||What type of sailing are you planning to do? I believe this is more important than the fine points of enginering. Both types of masts have their benefits and neither is flawless. The C30 is an excellent boat for protected waters. Planning extended cruising, something more substantial would be a better choice.|
|09-15-2003 11:04 PM|
That''s a nice explanation of mast dynamics. I''m interested in your preference for a "moment connection" (by which I presume you mean what my engineer friends call end fixity, as opposed to pin fixity, if I remember right?) at the mast to deck connection. You describe the mast heel bolted thru the deck to the jack/compression post. In the event of a lost rig, this seems like a disaster for the deck as first the heel is ripped out of the deck, and then the compression post, too.
And is there any gain beside a marginally stiffer bottom mast panel? On a three panel, two spreader rig, the additional stiffness applies to the lower third of the lower panel, and is that really a benefit? It seems like an S-shape might result from a rig so supported, once substantial backstay is applied, that is the head goes aft, the middle forward and the bottom remains fixed at the initial rake. Is there any sail shape benefit to this? Or any other post-rig-loss benefit, presuming the heel can be made not to rip out of the deck?
I''m just curious what your thinking is...
|09-13-2003 07:26 PM|
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 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. That said most older boats and high production boats with deckstepped mast do not establish a moment connection at the deck and often count of minimal below deck support structure. Often this is adequate for coastal cruising but is not a very good system for rougher conditions.
I am not clear if your question was about deck stepped masts or about the Cat 30 but the following is a discussion of deck vs keel stepped masts. Starting with the basic engineering issues, 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 using normal engineering principle 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 the most common rig, 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 supposing 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 to keel stepped masts 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 jury rig. More significantly, 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.
The set up on the Catalina 30 would fall in that category or adequate but not as sturdy as it might be.
|09-13-2003 05:34 PM|
I"m interested in buying a boat in the 30 ft range and looking at the Catalina"s. I think the mast on these boats are "deck stepped". Is a "keel stepped" a better rig?