|Topic Review (Newest First)|
|09-24-2007 11:58 PM|
|wnor||I have owned a factory-finished 1980 Southern Cross 31 for 11 years, doing the usual coastal New England sailing things. Sundry thoughts: Like every boat, she is a collection of compromises, but fairly effective ones in my view. She is almost ridiculously over-engineered - you may not get there fast, but you are going to get there. The rig, in particular, is comically massive. Though I love the boat, there's no doubt she is a pig in close quarters. Backing down, you are just along for the ride and have little directional control. On the other hand, the boat sails delightfully in heavy weather, and, with a little tweaking, will beat hands-off better than I can sail her. She pops over following seas like a cork. New sails, particularly a nice full-batten main with generous roach, have made light-air performance reasonable. Stowage is quite limited. Some have reported cracking of the welds in the pintle/gudgeon assemblies; I've inspected mine in detail and noticed no problem. Compression post is fine, and I have not heard of problems with it. I had to overhaul the engine and thought at first that pulling the engine from the tight compartment was going to be impossible, but suspending the engine from a horizontal pipe levered with a chain fall worked fine. But getting to the stuffing box requires gymnastics - if I gain one more ounce I will not fit in the engine compartment. Water stowage is limited (45 gallons). The largest holding tank I could figure out how to install was 6 gallons; just a toy, really. I'm seriously considering installing an Airhead to prevent being forced out of anchorages to dump sewage (I just refuse to violate those laws). Since these are old boats, the majority currently on the market are probably neglected wrecks, but a good one is a wonderful boat, in my view.|
|09-15-2007 10:04 PM|
|southerncross31||There was water intrusion around one of my thru-hull bolts. The airex was a little soft for about 1/8 inch, then looked fine. There was no evidence of water migration beyond that first 1/8th inch. It is a very hard material. Not what you would think of as foam really. It was nearly impossible to remove...i snapped several allen bits with my drill trying to scrape out a 1" area around the thu-hull holes! There also seems to be a lot less condensation inside the boat and it really mutes outside noises. Compared to the hull on my Pearson 26, it seems like a bank vault When you come crashing over waves beating into the wind, it doesn't pound at all!!!!|
|09-15-2007 12:31 PM|
Are J Boats (the 34C or 35C) airex core or balsa?
|09-14-2007 11:07 PM|
Nothing, unlike less expensive foams normally used on power boats and airex is a cross-linked closed cell foam so the water stays pretty much within that localized area where it got access. There is a slight exception to that in areas with a large number of freeze thaw cycles, where the melting and refreezing water can cause a delamination between the foam and the skin. Unlike Balsa this is a pretty easy repair accomplished by drilling holes, draining the area, drying it out and injecting with resin.
The nice thing about airex is that it has a little bit of memory which greatly increases impact resistance over a non-cored hull.
|09-14-2007 09:27 PM|
|GBurton||sc 31- if water gets into the airex core, what happens?|
|09-13-2007 03:55 PM|
The SC31 has 4, 1/4 inch,19 strand shrouds on either side, a split backstay, the headstay and the inner stay for the staysail. Of all of the deck stepped masts i have seen it has by far the most support. The mast itself is not very tall (~45 ft off the water) and it is very rugged. If i loose a wire for some reason it might stand a chance if i can get the sails down. If for some reason it goes overboard I have 2 hacksaws and a wirecutter onboard to cut it away.
I feel the same way as Jeff, the last thing i would want was a 200 lb 20ft peice of aluminum swinging around poking holes in my boat. If it's going over all is better than nothing!
On the other hand, I think the bottom of my compression post is soft. It is glassed to the keel so it is hard to see but there is definate compression going on...like the bathroom door won't shut in heavy wind! I think I am going to take the mast back off this winter and jack up the cabin top, then cut out the lower section of the compression post and replace it with either metal stock or a fixed jack. Any ideas? All's good though, it isn't getting much worse so i think i can sail out the season. I was out yesterday with a reef in the main and the staysail in 20-25knts! I brought a powerboater friend who had NEVER sailed. He was hooked after the first upwind tack
|09-12-2007 01:59 PM|
This is from another venue but it reflects some of the thinking on keel vs deck 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 must 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 of course that is a moderately 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. Done right 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 mostly 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 there is nothing that 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. I am not sure what you do when the boat is being beaten to death by the upper portion of a keel stepped 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 in such a way that there is a moment connection and yet the mast stub can be unbolted and jetisonned if it risked sinking the boat. 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 intend offshore with her.
|09-12-2007 12:04 PM|
Like I said, the pros and cons are open to debate, and I am not claiming that one approach is markedly superior to the other. But the corollary to your argument above is that there have been instances where keel stepped masts have torn massive, gaping holes in the coachroof during a rollover. And the smaller the boat, the more susceptible to rollovers in extreme conditions. All things considered, I'd rather have a well-designed deck-stepped mast in this size range. I think the primary reason you see some keel stepped masts in this size range is that it is simply cheaper to build them that way.
|09-12-2007 11:23 AM|
JeffH - thanks for the detail on the capsize figures.. am aware that it doesnt deal with weight placement, or depth of ballast.. but what else is there?? Im an engineer and want just the facts .. not anecdotal data! sigh
JohnR - with respect to KS masts, I think its my conservative sailing nature that likes that one more level of safety. A KSM provides that. Sure a deck stepped can be strung up tight and strong, but a failure of any one of those shrouds or stays and your in deep doo doo.. the same failure with a KSM, presents a problem, but having that solid member bolted to the keel is at least reassuring, if not more..
and there are some smaller bluewaterish boats that go with them.. Bayfield 29, Tarton 30, Bristol 29.9, so there must be some design factors that play into it.. but your right - been chatting with an SC31 owner and he said for sure to look hard at the stepping plate area and the base of the compression post for deterioration..
Thanks for all the insight..
|09-12-2007 10:02 AM|
DB27513 asked: <
There are many, many, solid blue-water boats in this size range that have deck-stepped masts. There will always be debate about what is the better approach, but properly executed there is nothing inherantly wrong with a deck-stepped mast on a boat of this size. There are eve some advantages to deck-stepped, and it is a fairly straightforward process to design sufficient support via a compression post that transfers the loads to the keel.
As boats get larger, it eventually becomes easier to just step the mast on the keel. So speaking very generally you will often see boats in the sub-35 foot range with deck-stepped masts, in the mid-upper 30's it can go either way, and above 40 usually it's keel-stepped (again - generally speaking). In the realm of "blue-water" boats, a good example that I'm familiar with is the Pacific Seacraft line, where all their models under 40 feet (Flicka 20, PSC 25, Dana 24, Orion 27, Mariah 31, Crealock 31 & PH32, Crealock 34, Crealock 37) all have deck-stepped masts. The Crealock 40 & PH40, as well as the Crealock 44, have keel-stepped masts. All were built to be blue-water boats (even if many are considered small for that purpose by today's standards).
I wouldn't lose sleep over the deck-stepped mast. But, someone pointed out that some of these boats were kit or home-built, so be sure that the compression post arrangement was properly designed and executed.
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