Is it better or worse to step the mast on the cabin top?

[djn]

Hi All, just got back from a trip up into Canada to work on the new boat and have a newbie type question. Before I ask it, I have to say that this is one of the prettiest and well maintained boats...much less wood boats, that I've seen. We are absolutly tickeled about her. Anyway, when the boat was built in Norway in 1958, the square hollow mast was stepped on the keel, then along the way, one of the POs had a yard cut off the bottom of the mast and permenently afix it between the keel and top of the coach roof, so now the mast steps on the coach roof. It was all very professional and the standing rigging looks to be half again to big for this size boat. So is that mod OK? Can it cause any problems in the future? Thanks for your thoughts. Cheers.

[outofthenorm]

Should be fine as long as it was done well - the cabin-top step should be stoutly fastened with a strong backing plate. Most of the loads on the system should be compressive, with a tiny bit of twist (torque) at times, so as long as there is no movement or flex vertically, I'd say it's a good setup. Any pics of the boat?

- Norm

[JimD]

The mod is ok if it was done with due diligence to the fact that a keel stepped mast is stronger, bends differently, and requires less support in the way of shrouds and stays because it is supported by the boat at two points - the keel, and the point where it passes through the deck or cabin. A tabernacle stepped mast such as you now have does not have this advantage and relies more heavily on the stays. Some cabin or deck stepped masts have elaborate tabernacles that provide much and sometimes all the support the mast requires but it doesn't sound like yours does so probably when the mod was made the shrouds and stays were beefed up. That's the prevailing wisdom, more or less.

[djn]

Hi Norm, here is a link to the L&A forum where I posted pics and a write up. The pictures do not do this boat justice. There are sooooo many little extras and cool features and the beauti of the joinery does not come through either. The reason I am asking about the step is that my wife and I want to sail it to Europe in three years and I want to make sure the boat will take it. Cheers.

http://72.32.109.245/forum/viewtopic.php?t=5586&sid=f0790b6530f83300d8032c9c4201ec6b

[outofthenorm]

Very nice.

http://i9.photobucket.com/albums/a60/djnagle/atanchor2.jpg

[John B]

looks like a well loved and well sorted boat:D
The Robb Lion class boats ( at least, the one I've been on, Siandra) have deck stepped masts without compression posts, just a hefty ring frame. There's plenty of precedent for it but as said above , only if the conversion has been done in a professional way. I notice mine can set up a shake in the rig when I flake off the sail to depower that I've never experienced with a keel stepped mast ,but thats a mizzen on a ketch. Just check out the step and bulkhead/ post and watch for movement as or after a sail.

[rbgarr]

I'm wondering whether the chainplates would be sized differently for a keel stepped versus deck stepped mast.

[Bob Cleek]

I'd be a whole lot more conservative about telling somebody who plans to "sail to Europe" with a boat designed for a keel stepped mast that's had it stuck on the cabin top that it "sounds fine" if it's been done "properly." Maybe it has, and then, maybe it hasn't. One thing's for sure, the above comments regarding the RIGGING have NOTHING to do with the ultimately correct answer, so forget that.

First, let's get some basic parameters out of the way. By your own definition, you plan to sail the North Atlantic. Okay, maybe just the Atlantic. In a 35 foot boat, it don't make much difference either way. In for a penny, in for a pound. So, we aren't talking about a weekend cruiser here. We are talking about an ocean cruising boat that should be able to take whatever the ocean can dish out.

Second, the boat was designed to have its mast stepped on the deck. It was built in 1958. By that time, deck stepped masts were not completely uncommon. Thus, it is pretty likely that with a hull shape like hers which put space below at a premium, they WOULD have stepped the mast on deck IF SHE WERE DESIGNED FOR THAT. Odds are, the designer had a GOOD REASON for stepping the mast on the keel.

In any event, I would hazard to say that there is NO 36' boat designed for a keel stepped mast that has a deck and cabin engineered to step a mast on them. There is no reason they ever would go to the trouble and expense. That said, unless the boat has obviously been PROPERLY engineered and modified, which means much stronger cabin top, sides, deck framing, etc., etc., or a well engineered metal ring support running from the keel, up the sides and across the deck and cabin top (pretty near impossible to install after the fact, BTW), or a support post beneath the heel, which apparently it does not, I would NOT consider this a suitable vessel for long distance ocean cruising.

Note that the first crossing of the Atlantic, from east to west, in a vessel with any sort of deck stepped mast was the 25' Vertue XXXV in about 1952. At that time, it was thought impossible for a boat with a deck stepped mast to survive such a voyage. They carried it off with a very clever steel yoke that girdled the boat beneath the mast heel. It was, effectively, by the transfer of stresses, a keel stepped mast with a big hole in it the size of the boat, for all intents and purposes. This worked because the relatively small size of the boat allowed for strength limits that could be carried off by the yoke. The bigger the boat, the bigger the yoke, and as the distances of the span increase, the need to increase the strength of the yoke increase exponentially, making this "solution" less feasible in larger boats.

Now, since nobody seems to have really explained it here yet, the problem to be evaluated is not the strength of the rigging at all. Rather, the real question is whether the cabin top and deck framing can withstand the COMPRESSION LOAD that is generated by the mast heel when sailing. Without proper engineering and design, few can and none were ever intended to.

Absent some pretty unusual retrofitting approved by a naval architect, if it were me, I'd think seriously about putting the heel of the mast back on the keel where it started before I took off across the Pond! Ask yourself this, "Do I think she would take a roll-over and come back up with all standing?" "Could she belly flop straight down off a big wave without that mast heel punching right through the cabin top like a big arrow shot out of a giant bow?"

[Paul Pless]

Cleek, you just couldn't explain it without throwing in a Vertue reference, could you?;) :D

[John B]

din't get any Choy lee reference in though did he.:D
A very good and suitably crusty post to make the point.

Bob, Siandra ,The Lion I mentioned is about that size,( 34? 35 36 ft?) I was very surprised to discover the missing mast down below. Took a while to notice it truth to tell but it did seem a very big boat for its specs LOL.

[Bob Cleek]

Naw, Paul, I guess not! LOL I didn't say that they thought Barton and Riordan might as well be sailing off the edge of the flat earth, though. Folks back then apparently thought they were totally insane to attempt the crossing with that new fangled deck stepped mast. I Think that feature more than any other was what made them famous.

Lousy picture, but the best I could find of XXXV. You sort of have to guess where the yoke is...

http://homepages.rya-online.net/vertueowners/UsedPages/XXXVdiagram.JPG

[djn]

Hi Cleek, thank you for your insight. Even thought the bottom portion of the origanil mast is now the permenent compression post, I think I will have a NA take a look at it before we venture out of the Great Lakes. I would hate to see this beautiful wood mast become firewood half way across the pond. Cheers.

[Bruce Hooke]

Cleek,

Most of your analysis seems to be predicated on the idea that this is a deck stepped mast without a compression post between the coach roof and the keel. In this case it sounds like there is just such a compression post.

That said, having a naval architect check it over would certainly not be a bad idea...

[outofthenorm]

Hmmm. Yet another diatribe based on a faulty reading of the original post. If there were no compression strut, Mr Cleek, your post would have made sense. But there is, isn't there? Your tone was offensive.

- Norm

[John B]

Aww thats just BC.
There is a bit more to it than the compression post though Norm.( which I'm sure you know) The new step on the cabin top has to be 'located' very well doesn't it. Partners and tie rods are important in a normal keel stepped mast but there is a different dynamic going on with the deck so a good extra hard look( which we were advocating on the basis that it should be a 'professional quality' conversion) plus some consultation wouldn't hurt. ( given the transat plans)

I'm amazed by how much more movement there is in my deck stepped mast.. amazed at what the 'cost' of the bury( lack of) is.

[py]

A solid mast will transfer compression straight down to the keel. A deck stepped mast with a compression post will transfer compression every which way, it will try to act liike a hinge. My question would be whther the cabin sides, knees etc were built or have been beefed up to handle those stresses. I suspect not. I'd scarf a new bottom end onto the mast so that the off watch can sleep.

[outofthenorm]

Yeah John, I know. :rolleyes: It's interesting about the movement of the mast stepped on deck. I've had both, and that's my experience as well.

djn, my sense has always been that so long as the compression load is properly transferred to strong structure (like with a proper compression post or ring frame) there is actually less lateral load on the cabin with deck stepped than with keel stepped. In a keel-stepped system the partners act as a fulcrum for the leverage of the mast, but with a deck-stepped mast, the point where the mast sits on the cabin-top or deck is actually a "hinge" in the system, with very small range of movement, but a hinge nonetheless. This hinge relieves stress at the partners and transfers the load to the standing rig. The movement is in effect a re-direction of energy that would otherwise be carried laterally through the partners and the deck or cabin top around it. I'm assuming that the step itself is strong enough to keep things where they belong.

Could be completely wrong of course, so I'd be interested to hear a NA's take on it.

[Don Z.]

Cleek,

Most of your analysis seems to be predicated on the idea that this is a deck stepped mast without a compression post between the coach roof and the keel. In this case it sounds like there is just such a compression post.

That said, having a naval architect check it over would certainly not be a bad idea...

OK, I'll weigh in on Bob's side. He still has enough Whuffie to pay for it...

I've re-read this a couple of times. Went over to the linked page. Saw this photo, but can't really tell from it: http://i9.photobucket.com/albums/a60/djnagle/dinette.jpg Did a search using Safari 3.0's new search feature. I may have missed it too, but based on what I just saw, the first time a compression post was mentioned was after Bob's second post. So that might be why his analysis was predicated on the idea of a deck stepped mast without a compression post...

[JimD]

From the original post:


one of the POs had a yard cut off the bottom of the mast and permenently afix it between the keel and top of the coach roof, so now the mast steps on the coach roof.

Sounds like a compression post;)

[Andrew Craig-Bennett]

This is quite a common modification. It is one that I have done myself, on a smaller boat. My present boat is 37ft and the stresses on her mast are much larger, but that is partly because she is gaff rigged.

The stresses in the mast will be a little different, because the mast is no longer a pinned end column; I think it would be wise to have a naval architect look at it.

[Don Z.]

From the original post:



Sounds like a compression post;)

I guess so... but I didn't read it that way at first. The antecedent of it was a little unclear...

[Wild Wassa]

Something just doesn't sound right, what have I missed?

When the mast is sitting on a keel it is sitting on a girder. When it is modded to being stepped on the coach house/deck it is sitting on what? ... something without a compression post, or sitting over a bulkhead?

How are you going to allow for this lack of real support? No one wants a mast driven through the roof?

Warren.

[S/V Laura Ellen]

...one of the POs had a yard cut off the bottom of the mast and permenently afix it between the keel and top of the coach roof,...

The bottom of the mast was cut off and installed between the keel and the coach roof to serve as a compression post. Although the post could have been clearer, it is quite apparent what djn was saying.
I'm not sure what benefit this alteration serves.

[djn]

Hi Allan, the reason the PO did this mod, and this is still hard for me to comprehend, is to eliminate the scraping that occorred to the bottom section of the mast when it was slip in through the coachroof. That being said, with that attitude, you can imagen how beautiful the joinery is below decks.....the pictures do not show it, but it is beautiful below and above deck. Cheers.

[Wild Wassa]

Laura, evident to you but not to me. I missunderstood the term "permanently afix it ..." To me it meant should it be permanently fixed. "Afix it" isn't a term that I have heard before. Nor can I find the word "afix" in a dictionary.

Is the term a Canadian coloquialism?

Warren.

[S/V Laura Ellen]

Laura, evident to you but not to me. I missunderstood the term "permanently afix it ..." To me it meant should it be permanently fixed. "Afix it" isn't a term that I have heard before. Nor can I find the word "afix" in a dictionary.

Is the term Canadian slang or French/Canadian?

Warren.

First my name is Allan (my boat's name is Laura Ellen).
Second, affix is not a uniquely Canadian or French Canadian term, but is still in somewhat common use today in North America. (i.e. Don't forget to affix your stamp to the envelope)

Affix
To secure to something; attach: affix a label to a package.
To impute; attribute: affix blame to him.
To place at the end; append: affix a postscript to a letter.
Grammar. To add as an affix.

[Wild Wassa]

Thankyou Skipper of S/V Laura Ellen, your answer is what I originally though.

"To secure" ... isn't yet secured.
"To impute" ... hasn't been changed yet.
"To place at the end" ... not in place at the end, and meaning still 'to' do.

The English speaking countries are seperated/separated by a common language.

Cheers.

Warren.

[Bob Cleek]

Yep, I wrote with the assumption that there was no compression post below the mast. I'm still not sure! LOL Fact is, I couldn't comprehend why anyone would cut a mast like that because there really isn't any purpose to it that I can see. Somebody must have had their reasons, I suppose. No dispute with the subsequent commentary, though.

[djn]

Hi Cleek, the reason the PO did this mod, and this is still hard for me to comprehend, is to eliminate the scraping that occorred to the bottom section of the mast when it was slip in through the coachroof. That being said, with that attitude, you can imagen how beautiful the joinery is below decks.....the pictures do not show it, but it is beautiful below and above deck. Cheers.

[paladin]

Just looking at your photo it would seem that the spreader arrangement might not fit a deck stepped mast. You can't just add a compression post below decks and keep the same rigging arrangement, sorta as Cleek put it...the post wiggles differently...:D

[djn]

Hi Paladin, I'd heard that before, but can not find a reference. I am actively looking for an NA to do an analysis on the boat. If I have to replace the mast (please nooooo), it is still a great boat. Cheers.

[John B]

That has got to be the most peculiar reason I've ever heard to cut a mast. I presumed it was for some area specific maintenance factor.. a gantry that could only lift x long ,or something.
djn, you won't have to replace the mast even if you're advised to go back to the original configuration so don't get too worried.
Scarf jointing into spars is pretty easy, especially if its just a heel.

[djn]

Hi Brian, no, he did not fortify any part of the deck, coachroof, or chainplates except for making the top of the compression post solid to the keel, bulkheads, and the coachroof (which includes the structure under the coachroof).

[donald branscom]

This is quite a common modification. It is one that I have done myself, on a smaller boat. My present boat is 37ft and the stresses on her mast are much larger, but that is partly because she is gaff rigged.

The stresses in the mast will be a little different, because the mast is no longer a pinned end column; I think it would be wise to have a naval architect look at it.

"Have a naval architect look at it." Yeah..... well I have had
naval architects make drawings that did not work at all in real life.
IF you want one of those real life stories just ask.
Better have an experienced ocean crossing person look at it.
As far as keel stepped versus deck stepped the deck stepped has some advantages as far as the ultimate safety of the boat and crew.
a properly supported deckstepped mast is easy to do the math on.
Just use Euhler's formula and Machinery's handbook for a PIN to PIN beam with a load being applied. With the keel stepped it gets more complicated to figure because it is not a pin to pin because it is supported near one end and ends up having a "s" shaped curve.
A square beam mast is 16% stronger than a round beam. Figure that in too.

The safety of the deck stepped mast is (and it hardly EVER happens),
is that if a wave did sweep the boat exerting the whole 77,000 lbs. per sq. ft. load it would clean the decks off which has hap[pened BUT the hull would be intact. Where as the keel stepped mast can break loose INSIDE the boat and cause damage or injury.It has happened.

Then you have a hole in the deck that can leak depending where it breaks.
A deck stepped mast is a SHORTER stronger. beam than a keel stepped beam.

As far as that rigging info goes just make sure that on the intermediate stays that they go to the spreaders on a singlespreader rig INSTEAD of the intermediate's going to about half way between the top of the mast and the spreaders like some production boats I have seen.
That way you have inner forestay,two intermediate's and two lowers and two babystays all going to the spreaders. VERY tough.