Keel stepped masts - chock at the coachroof or not?

[LittleSister]

Compression posts are designed to reinforce the deck and stopped it being compressed, not take all the load..... and none of the deck stepped approach are as robust as a mast section.....

Why would our stainless steel compression post be less robust than a mast section? And if it were, why not use, say, a bit of mast section for the same job?

 

[doug748]

But it doesn't, because I was asking 'So why have a keel stepped mast?' .......

Purge your mind of all that went before.

Here is a quote attributed to Sparkman and Stephens, in another thread:

"The strength of a mast relates to the strength (measured as stiffness) of the mast section and the length of unsupported mast sections. Having 2 sets of spreaders reduces these section lengths and means that for the same strength the mast does not need to be as large or heavy. This translates into less heeling, less pitching and more speed. The disadvantage is the added cost of the extra set of shrouds and spreaders. As more spreaders are added there is a diminishing return and for the S&S 34 two pairs are optimal. The strength of the twin spreader and original single spreader rig is about the same, both have ample strength. Having the mast keel stepped adds about 35% to its structural strength, and therefore reserves of safety. Although it is possible to step the mast on the deck, this is not recommended and requires an extra heavy mast section to give the same safety margins as a keel stepped mast.."

Thanks to Rebecca and Steve.

 

[Phoenix of Hamble]

Why would our stainless steel compression post be less robust than a mast section? And if it were, why not use, say, a bit of mast section for the same job?

urm.... confused arguments you say?

If you use a section of mast, then you've got a keel stepped mast, but cut at deck level!!!!!!

By the way.... your stainless steel post isn't solid.... its a tube.... so unless its got a particularly heavy wall section, its not as strong as a mast section.

 

[LittleSister]

If you use a section of mast, then you've got a keel stepped mast, but cut at deck level!!!!!!

You missed the joke, then?

By the way.... your stainless steel post isn't solid.... its a tube.... so unless its got a particularly heavy wall section, its not as strong as a mast section.

It doesn't need to be as rigid as the mast section as it's only 6 foot long, not 32', and has no sideways loads on it, but it appears strong enough to take the downforce of the mast. There's certainly not much else holding the mast up! The deck, apart from keeping the rain off, is, I suspect, mainly just supporting the mast base laterally (and, along with the chainplates and mainsheet track, dragging the boat through the water when under sail!).

 

[William_H]

. Having the mast keel stepped adds about 35% to its structural strength, and therefore reserves of safety. Although it is possible to step the mast on the deck, this is not recommended and requires an extra heavy mast section to give the same safety margins as a keel stepped mast.."

Thanks to Rebecca and Steve.[/QUOTE]

Sorry I find this hard to believe. Just because it is "attributed to S and S " doesn't make it correct. Or at least that is the way I read
it and understand the situation. A mast is supported by the 2 or 3 sets of stays not by the end that is between the keel and deck.
Indeed I can't see any difference between a keel stepped mast firmly fixed to the deck and a deck stepped mast. Certainly not 35% stronger.
Any mast I have seen break breaks about the middle of the mast where one set of stay supports have failed while another holds the mast. Or perhaps where a force has hit middle of the mast.
All this of course unless we are talking an unstayed mast!! olewill very confused but also very doubtful

 

[Bajansailor]

Just a few thoughts, with a lot of simplification (altho' pedants will probably still correct me on something).

The formula for the buckling load of a perfectly straight column (it could be a mast, a fence post, anything, made of any material) is (pi*pi*E*I)/(L*L)
Where pi is 22/7 (or 3.142.......), I is the inertia of the section and E is the modulus.

Or in other words the buckling load is directly proportional to the inertia of the section times the modulus of the material.
And indirectly proportional to the square of the length of the unsupported section (eg between the base, or partners, and the first set of spreaders).

Lets say the first set of spreaders is 4m above deck.
For a deck stepped mast, L *L = 16.
If we only have 2m of headroom down below, and we have a keel-stepped mast which is NOT supported by partners where it comes through the cabin top, then the effective unsupported length would then be 6m.
Hence L*L = 36.

Thus the buckling load for the keel stepped mast (not supported by partners) is less than half what the deck stepped mast is (all else being equal re E, I and the mast being perfectly straight, and ignoring the side forces created at the gooseneck by the boom).
Hence why the partners are so useful!

I cant remember offhand what the formula for the buckling load for a bent strut is (it is more complex, and involves the amount of deflection from absolutely straight), but it is significantly less than for a straight strut.
You can see for yourself this difference if you try to compress a perfectly straight timber dowel, and then try again after putting some 'pre-bend' into it.
Hence why fractional rigs that are forced into banana shapes have to be relatively stronger than ordinary straight masts.

(Yes, I know there are lots of simplifications in the description above, but these are just to try and make it easier to understand).

 

[jwilson]

Why would our stainless steel compression post be less robust than a mast section? And if it were, why not use, say, a bit of mast section for the same job?

I see lots of surveys on boats, and a surprising number find some flaw in the mast support setup on yachts with deck stepped masts. Often it is compression of the deck material itself, particularly with balsa sandwick decks where some ply is inserted under the mast base instead of balsa, and damp has got in. Other times it is bilgewater getting at the base of timber mast support posts. There is no reason not to engineer things properly and have a sound support setup, but yacht builders seem to fail to do so remarkably often, hence the number of older boats with hefty new S/S posts fitted under the mast (often way stronger in compression than the mast).

Despite this I personally prefer deck-stepped: mainly because I have never met a keel-stepped mast that did not cause constant wet bilges. Also with most modern boats the base of the mast is nowhere near the keel - it is several feet forward of the front of the keel. Keel-stepped masts are a relic of long keels and wooden spars.

 

[doug748]

. Having the mast keel stepped adds about 35% to its structural strength, and therefore reserves of safety. Although it is possible to step the mast on the deck, this is not recommended and requires an extra heavy mast section to give the same safety margins as a keel stepped mast.."

Thanks to Rebecca and Steve.

Sorry I find this hard to believe. Just because it is "attributed to S and S " doesn't make it correct. Or at least that is the way I read
it and understand the situation. ...Indeed I can't see any difference between a keel stepped mast firmly fixed to the deck and a deck stepped mast. Certainly not 35% stronger.
[/QUOTE]



I find it difficult to believe as well but it is fair to say that my impact on the world of yacht design has yet to be fully felt.


I have tracked down the quotation and am fairly sure it did not come direct from Sparkman and Stephens:

http://www.ss34.org/ss34/The new S&S 34/frequently_asked_questions.htm#Q6

There seems to be a lot of subjective, value loaded stuff being posted. The strength of keel stepped masts is a matter of Physics and so is a different type of question.

 

[Quandary]

I am not going to enter in to this discussion beyond saying that while I have studied structural engineering at university it was not my primary subject but some of the opinions given here appear to be dangerous. Rig design is a straightforward engineering problem and has straightforward engineering solutions. Arbitarily deciding that parts of the rig are redundant and can be omitted because one does not have an understanding of their function and then recommending that others do the same is just irresponsible. I know forum advice can be dangerous, this is a prime example.
Before anyone decides to dispense with major components of their rig, seek advice from an expert.

I can recommend a book 'Principles of Yacht Design' by Larsson and Eliasson, Adlard Coles Nautical, only £39.99 given to me by a structural engineer in the hope of stopping me asking stupid questions.

 

[pmagowan]

Well my mast is keel stepped a nd sits in a shoe that holds the bottom end still. It was the normal stays and was unchocked when I bought it. When I took it home to renovate and removed the mast there was significant rubbing on the aft part of the mast as it came through the steel deck fitting. It had obviously not been supported properly by the stays and was getting wear. I have chocked it with hard rubber after adjusting the rigging. It has likely reduced its strength but I do not think it is that significant. I did think of getting someone to rivet in a sleave.

 

[LittleSister]

Well my mast is keel stepped a nd sits in a shoe that holds the bottom end still. It was the normal stays and was unchocked when I bought it. When I took it home to renovate and removed the mast there was significant rubbing on the aft part of the mast as it came through the steel deck fitting. It had obviously not been supported properly by the stays and was getting wear. ....

Sounds like it was not so much not properly supported by them, as perhaps they were the wrong length (or adjustment) and were canting the mast too far aft. Shorten forestay and lengthen backstay (etc.) a little?

 

[knuterikt]

But it doesn't, because I was asking 'So why have a keel stepped mast?' in response to Javelin, who was saying that a keel stepped mast needed supporting at deck level because the boom was about 3 times further from the foot of the mast than with a deck stepped mast (and following Pheonix of Hamble who was also saying it was necessary because of the extra c8ft length of a keel stepped mast.

So if a keel stepped mast needs extra support (because it's longer and the pressure of the boom is further from the foot) then the 'extra support' available with a keel stepped mast is not an advantage as such, but a necessary (it is claimed above) requirement resulting from the mechanical disadvantage of the keel stepped mast. You don't need that extra stiffness with a keel-stepped mast.

I'm not against keel-stepped masts, I hasten to add. Just curious about their merits (and amused by the confused arguments being put forward in their favour!).

There are obvious advantages and disadvantages of both keel stepped and deck stepped masts - here are some..
Deck stepped
- No hole in the deck where water can come in
- On some boats the structure to take compression can be made for a better interior design

Keel stepped
- For the same boat you (might) get a lighter mast so you get a stiffer or lighter boat
- A keel stepped mast is easier to trim (bend)

For a given mast height (above water/deck) and boat design the designer can chose between both depending on preferences & priorities.
It's the normally the boats righting moment at 30 degrees heel that is used in in he formulas used to sizes mast/rig.
Since a keel stepped mast (normally) is supported at to points keel & deck the designer can chose at thinner/lighter mast section than with a deck stepped mast,

The normal way to select a mast section is to calculate the load for the selected configuration and then select the closest larger standard section from the spar maker.
So in some cases you will probably end up with the same section for both alternatives.

 

[knuterikt]

Sounds like it was not so much not properly supported by them, as perhaps they were the wrong length (or adjustment) and were canting the mast too far aft. Shorten forestay and lengthen backstay (etc.) a little?

If this mast was designed to have support at deck level (as most keel stepped masts are) this is the wrong medicine.

 

[knuterikt]

Reading the instructions from a spar maker?

There are several spar makers, I checked Selden is writing as the have a lot of documentation on there web site
They can be found here http://selden.se/frameset.cfm?id=6740&randnum=952456176

Keel-stepped masts
..
• Lower the mast onto the T-base. Ensure the heel is securely located fore-and-aft and in-line.
• Attach the shrouds, forestay and backstay. Tighten them by hand until the mast is held fairly steady in the deck ring.
• Take off the lifting strop and leave the crane.
• Insert the aft rubber mast wedges and lubricate the forward fixing wedge with soapy water. Tighten the nut of the forward fixing wedge in the deck ring, so that the mast is held securely.
..

source http://selden.se/_download.cfm?id=6597&download=4492009&filename=595-540-E.pdf HINTS AND ADVICE on rigging and tuning of your Seldén mast" page (22,23,24) section about rigging.
(Bolded by me)
I'm wondering why they make a point on using the deck wedges?