|Topic Review (Newest First)|
|10-03-2007 01:54 PM|
I believe that you are mistaken. The set up that I described above (a moment connected base plate that is moment connected to a structural kingpost) when properly engineered has been used fairly routinely. My former boat, a Laser 28 for example had just that set up (although in its case the base plate also included a hinge allowing the mast step to act a tabernacle as well.)
Farr's office thinks that this can be done reasonably easily on my current 38 foot boat, which they designed. Except for minor rotational loads, which should be taken out with some minor deck reinforcing where the partners used to be, a properly designed baseplate and a stiff enough kingpost, the loads exerted on the deck are no different than they would be for a keel stepped spar that has engineered to include partners as a panel point in the design. That design would be a simple thru bolted connection and would not include a tabernacle.
|10-03-2007 01:39 PM|
It would be nice if you didn't revive dead threads, in the case of this one it been gone for over four years. It is considered poor net etiquette to do so. Granted, you're a relative N00b...but still... please check the dates on threads before replying.
|10-03-2007 01:29 PM|
You can't bolt a dynamic(mast and running rigging) assembly to a fixed(boat hull and deck) structure.This deal needs a toggle of some sort.It can't be fixed tight, it will smash itself.A deck stepped mast that can be lowered and raised is called a tabernackle.It can be a rewarding set up if you got the balls.
|08-17-2004 06:32 AM|
which 38ft boat?
With that many people on board a 38 footer could get a little cramped. What is your budget?
With regards ro Keel vs Deckstepped 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 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.
|08-17-2004 05:50 AM|
which 38ft boat?
As far as specifics about my sailing needs go:
I will be sailing with my family which will be 2-4 people at any time, and we are all in excellent physical condition.
I have been around boats, both power and sail, for around 40 years, including 4 years in the USCG. Up until now I have sailed 20-25 ft. such as a cape dory and am looking for something bigger to do some more serious sailing. Ideally I want a cruising boat that doesn''t ignore performance.
Also, I was wondering what some of the advantages and disadvantages of a keel stepped mast, versus a deck stepped mast. Is either particularily preferable?
|08-16-2004 04:37 AM|
which 38ft boat?
I know of numerous original Tartan 37''s that have done Atlantic Circles with no apparent problems, with one exception all of them with centerboards. But I don''t think that recommends them as any cruising is going to find you living aboard where a more spacious layout, fewer berths, more storage, and a more sensible keel choice (insofar as the early 37''s) would be preferable.
Your budget seems to be sizeable if you''re looking at a new Tartan. European boats like a Vilm, Hallberg-Rassy, Najad, Comfortina, Alubat and many more might be worth reviewing, if only to expand the baseline when comparing boats. I think the U.S. market is extraordinarily narrow in choices for folks who don''t look very far beyond their copy of CW and the U.S. boat shows.
Good luck on the shopping!
|08-16-2004 04:35 AM|
which 38ft boat?
To really provide a meaningful answer, I would think that it would be helpful to have more information about you and your plans. For example;
-How many people will be on board?
-What is your budget?
-How experienced are you as a sailor?
-What sort of physical shape are you in?
-Why are you looking at new boats?
-What is your taste in boats?
and that type of thing....
In a general sense, the sailing venues that you are considering permit and rewards pretty deep draft boats. Within the range of posibilities there are boats that run the gamit from high performance offshore boats to more conservative boats that are better suited for the kind of sailing that you are considering. At the higher perforamce and ease of handling end of things there are boats like the Aerodyne 38 that were designed to be a high performance offshore cruiser and at the low performance end of things are tried and true offshore boats like the Pacific Seacraft 37. To give a quick list of examples in between these extremes are boats like:
-Beneteau First 40.7 (better suited to cruising New England than the Atlaantic Circle)
-Dehler 38 (with some mods)
-Dehler 39 (with additional tankage and storage mods
-Hallberg Rassey 39 (If you can find one without teak decks)
-J-44 (at the extreme end in terms of a singlehander but a great boat for someone who is physically fit.)
-Oyster Lightwave 395 (These boats began life in one of two ways. Some where set up as performance cruisers and others as out and out race boats. The performance cruisers have proven to be good performance oriented long distance, short-handed boats.)
-Valiant 37 (Esprit)
-Wauquez Centurian 38
-Wauquiez Centurion C 37 S
I am sure with more time I could come up with a more complete list but this should give you some ideas.
|08-15-2004 02:27 PM|
which 38ft boat?
My plans are to do some coastal cruising for a couple of years and then some bluewater sailing. Such as a trip from New England to Bermuda, and then a trip across the Atlantic, or perhaps the Atlantic Circle.
You mentioned "more modern offshore oriented designs".
Please list some of the designs that you would consider.
Also, talking to the Tartan factory, they indicated that one could sail the 37'' on bluewater trips. but I have noticed that I would have to get creative with finding storage space on this boat.
One final item, I am looking mostly at 37 to 38'' due to budget, If I had more money I would consider a bigger boat, which of course, would help with more speed and more storage space.
Please let me know what you think.
|08-15-2004 06:05 AM|
which 38ft boat?
I believe that you are mistaken that the Island Packet and the Hans Christians have better inherent stability in heavy air when trimmed properly. Both of these boats have a lot of form stability that comes from their wide beam, a lot of weight for their length, and a comparatively small sail area relative to thier displacement. That gives the illustion of a lot of stability, but does not result in a lot of actual stability. Both of these boats exhibit a lot of drag relative to their stability, and as such need to carry a lot more sail area in heavy going than a more easily driven hull form and at that point their relatively low stability in relation to the amount of sail that needs to carried to overcome their drag,means a comparatively higher heel angle and helm loads. Neither are especially good heavy air designs relative to more modern offshore oriented designs.
Island Packet had a achieved a relatively high angle of positive stability by raising the height of their freeboard and increasing the cabin volumes on their boats. This comes at the price of carrying a their weight much higher than ideal and results in a fairly high VCG (vertical center of gravity). A high vertical center of gravity reduces stability and increases the tendancy to roll quicker (somewhat offset by the boat''s comapratively high inertia) and through a wider roll angle(somewhat aggrevated by the boat''s comapratively high inertia). The Hans Christians with their heavy decks and interiors also have very high vertical centers of gravity and have a significantly lower ballast ratio, but also lack the higher freeboard that Island Packet uses to increase their limit of positive stability. With their wide beams both of these boats require a lot of energy to re-right once overturned.
One other point, in your post you say "the IP and HC have somewhat better inherent stability in heavy air when trimmed properly." My experience with IP''s is that they have really poor deck hardware making proper trim very difficult or imposible in heavier going. This problem is even worse on the inmast furling boats where luff tension cannot be controled with the mainsail partially furled. I know that IP uses a lot of name brand hardware, but the deck layouts are such that controlling twist in the main or making changes in jib sheet lead are extremely diffiicult and the problem is only exacerbated by the use of comparatively high friction Ronstan solid sheave blocks and mid boom sheeting. Most quality builders have long since gone over to the more expensive, longer lived, and lower friction roller and ball bearing blocks.
With regards to the Hans Christain, a 1987 model is 17 years old and is likely to need a major overhaul and update if it is going to be doing a lot of offshore work. I also would discourage you from buying a HC that still has its teak decks. That should be a deal killer on any boat intended for offshore long distance cruising.
On the other hand, a boat like the Tartan, while it has a lot of stability relative to its drag, also has a more generous sail plan and so might need to be reefed sooner as the wind builds than the other two boats. That means better lighter air sailing ability, less motoring but a little more work. If your nom''d net ''bluewaterdreamer'' means anything then I would suggest that the Tartan, while it is a really nice boat, is really more of a coastal cruiser than an offshore cruiser lacking the kinds of seaberths and large bulk stroage located low and central in the boat that is so important for distance offshore cruising.
I guess at the heart of this discussion is the question, where do you sail and where do you intend to sail. If your plan is to stay coastal for a while then perhaps the Tartan is a good boat. If your plan is a mixture of coastal and offshore, then perhaps you should keep looking.
|08-15-2004 03:52 AM|
which 38ft boat?
What are your sailing plans? Do you intend to live aboard or make long passages? My preference, based solely on performance, would be the Tartan. I prefer a boat that has reasonably good windward performance, which the Tartan has compared to your other choices. The Tartan also requires a lot less maintenance to keep looking good.
That said, the IP and HC have somewhat better inherent stability in heavy air when trimmed properly. Still, I would choose the Tartan.
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