Keel stepped masts - chock at the coachroof or not?

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Independence

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I've had different opinions on whether a keel stepped mast should be chocked at the coachroof. So far I've been advised:

1) It's not necessary and down to personal preference
2) If you do, use something that has a bit of give in it so as not to create a hard spot in the mast set up (me currently)
3) Used soft wooden chocks to add additional support

What do other people do out there?

Thanks
 
MUST be supported at the deck.... and not with soft wood either.

Normal practice is for hardwood, or more recently, very hard rubber wedges.

The are also kits to fill with a very hard setting rubber to a ring mounted on deck.

You need a hard point at the deck, as this is effectively the bottom point for any flex... if the mast can move at deck level, it will eventually damage the coachroof, or the bits around the base of the mast anyway (called the partners in days of wooden craft)... at least it will leak through tye mast boot eventually, or pant and at worst it will break things.

No support at the deck is asking for trouble!
 
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Independence said:
I've had different opinions on whether a keel stepped mast should be chocked at the coachroof. So far I've been advised:

1) It's not necessary and down to personal preference
2) If you do, use something that has a bit of give in it so as not to create a hard spot in the mast set up (me currently)
3) Used soft wooden chocks to add additional support

What do other people do out there?

Thanks
Click to expand...

1) I disagree (have you ever seen a aluminium mast that have been grinding against the deck ring (not a pretty sight)

2 & 3)
When we bought the boat it the mast where fixed at the deck ring with wooden wedges like these.
http://www.benns.se/pub_images/large/i36072000001_1701.jpg

The problem with the wood wedges is that they tend to work loose.

My rigger sold me rubber plates like this, have one bent around the front part of the mast and one around the aft part.
Select plates with tight fit, we insert the front aft rubber plate first, use rope and winch to compress this plate and slide the front one in.
Used this for 5 years now - never worked loose.
http://www.benns.se/pub_images/original/K9008_1453.jpg
 
When using rubber chocks it's a good idea to run a mammoth jubilee clip round them below decks. If there is any movement between the mast and the partners the chocks don't fall out.
 
Our mastmaker explained the reason for supporting (firmly) at the coachroof as follows:

Take 1 metre bendy piece of wood - something like 6mm dowel. Stand it vertically on one end on the ground and press down on the top to see how much rigidity it has before bending.

Then take a 1.2 metre piece of the same wood, and push the bottom 20cm into the ground so 1 metre is above ground as before. Now press down on the top to assess the rigidity and you will find the above ground section is much more rigid, because of the way the below-ground 20cm is held rigid.

The below ground bit corresponds to the section of mast from keel to coachroof. Indeed I believe she said that most keel stepped masts are designed to be supported at the coachroof, indeed if they are not, then the length of the lowest unsupported section is longer than designed and therefore weaker in columnar compression.

It seemed to make sense to me, hopefully some engineers on here will elaborate or refute as appropriate.
 
dovekie said:
Our mastmaker explained the reason for supporting (firmly) at the coachroof as follows:

Take 1 metre bendy piece of wood - something like 6mm dowel. Stand it vertically on one end on the ground and press down on the top to see how much rigidity it has before bending.

Then take a 1.2 metre piece of the same wood, and push the bottom 20cm into the ground so 1 metre is above ground as before. Now press down on the top to assess the rigidity and you will find the above ground section is much more rigid, because of the way the below-ground 20cm is held rigid.

The below ground bit corresponds to the section of mast from keel to coachroof. Indeed I believe she said that most keel stepped masts are designed to be supported at the coachroof, indeed if they are not, then the length of the lowest unsupported section is longer than designed and therefore weaker in columnar compression.

It seemed to make sense to me, hopefully some engineers on here will elaborate or refute as appropriate.
Click to expand...

So why doesn't my deck stepped mast also need supporting 8 feet (or whatever) above its base?
 
The main reason for a keel stepped mast is to take the compressive loads down to the keel rather than onto the deck.
If the shroud plates also continue down to the keel as in my boat you effectively triangulate and isolate the lateral loads and reduce loading on the hull.
A good thing.

However one of the main lateral loads on the mast, (rather than compressive) is by the main boom.
On a deck stepped mast the boom thrust is only two or three feet higher than the mast base.
On the keel stepped mast the boom thrust will be around eight or nine feet higher than the base.
Therefore the boom thrust will be much more likely to bend the keel stepped mast if unsupported than the deck stepped mast.
 
LittleSister said:
So why doesn't my deck stepped mast also need supporting 8 feet (or whatever) above its base?
Click to expand...

Phoenix of Hamble said:
urm.... because it's 8 feet shorter than an equivalent keel stepped mast!
Click to expand...

The designer would have known that, and could position the stays and spreaders, or alter the mast section, accordingly. I am just struggling to understand the advantage of a deck stepped mast if you have it fixed at deck level. If you don't then you can tune the rig more, if you do fix it, then you may as well have either a deck stepped mast (with bulkhead or compression post taking up less room inside), or an unstayed mast, surely? I've no axe to grind - just trying to understand it.
 
Javelin said:
The main reason for a keel stepped mast is to take the compressive loads down to the keel rather than onto the deck.
Click to expand...

But the compressive loads aren't taken by the deck, they're taken by a compression post, ring beam or bulkhead on every deck-stepped mast boat I can remember seeing.

Javelin said:
However one of the main lateral loads on the mast, (rather than compressive) is by the main boom.
On a deck stepped mast the boom thrust is only two or three feet higher than the mast base.
On the keel stepped mast the boom thrust will be around eight or nine feet higher than the base.
Therefore the boom thrust will be much more likely to bend the keel stepped mast if unsupported than the deck stepped mast.
Click to expand...

So why have a keel stepped mast?
 
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..... secondly, if rigging fails, then you have a second point of support, unlike a deck stepped mast.... albeit one that carries risk with it.

Any straight column supported at 3 points is more robust than one supported at two.... just basic engineering principles.

Saying all that, nowadays, with modern materials, the advantage is lessened significantly.
 
The static compressive loads are significant.
On my 3/4 tonner the static compression loading is in the region of 5 tons.
From experience many of the compression struts I've seen are just not substantial enough given that the dynamic compression loads can increase four fold or more when crashing off a wave.

Yes sure if your boat is built substantially enough to carry these loads then no problem.
However I suggest it would weigh far more than a keel stepped rig.
 
I find it amazing that people can be so adament about this question. I suspect that they have their own particular boat and mast in mind. I suspect they have in mind a solid mast head rig mast. Where perhaps it is correct to support the mast at the deck. I wonder though just how strong the deck is in taking sideways or fore and aft force anyway.
I can't help thinking that a modern bendy mast on a fractional rig should not be chocked at the deck. The bend of the mast is intended to be over the whole length of the mast and any chocking would as OP suggests provide an undesirable hard spot in the bend. The mast in this case is still supported in column by the stays aft swept spreaders jumper struts etc.
Regarding end force on the boom. This should not be so great but is often be magnified by boom vang force.
Ultimately therefor the rig designer should be the authority on wisdom of chocking at the deck. It all depends... good luck olewill
 
We have a piece of that rubber chock stuff 1/3 around the leading edge and folding hardwood wedges around the rest, forming a wooden ring. Sealed in with builders foam and a piece of inner tube secured with jubilee clips on the outside, to keep it water tight.

No pumping of the mast at any time, and we've been through pretty ****ty weather over the last 7 years!
 
LittleSister said:
So why have a keel stepped mast?
Click to expand...


The structural engineers tell us that a simple mast is much stiffer if supported at three points. This bonus can either be taken as a bigger safety factor (perhaps for offshore sailing) or reduced mast section and less weight in the rig (perhaps for racing).
I suspect all the other considerations are no more than interesting side lights on the matter

If the mast is not chocked at the deck I think the OP will create a deck stepped mast - but 6 foot longer. It probably will not fall down but it was designed to work like that.

This applies to masthead rig, I have nowt to add on keel stepped fractional masts, are there any? It would be interesting.
 
LittleSister said:
So why have a keel stepped mast?
Click to expand...

This post explains it
doug748 said:
The structural engineers tell us that a simple mast is much stiffer if supported at three points. This bonus can either be taken as a bigger safety factor (perhaps for offshore sailing) or reduced mast section and less weight in the rig (perhaps for racing).
I suspect all the other considerations are no more than interesting side lights on the matter

If the mast is not chocked at the deck I think the OP will create a deck stepped mast - but 6 foot longer. It probably will not fall down but it was designed to work like that.

This applies to masthead rig, I have nowt to add on keel stepped fractional masts, are there any? It would be interesting.
Click to expand...

The same things for a fractional mast, the same physics apply.
 
knuterikt said:
This post explains it
Click to expand...

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!).
 
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