downwards load at mast base, how much is it?

[CLP]

Apologies if I have missed something in the discussion about compression in the mast when the boat is on its beam ends, but I thought that with the mast horizontal, the compression load in the mast will be W/2sin theta, (assuming for the moment no initial rig tension at all, and a single shroud to the top of the mast)

Where:
W = weight of mast
theta = angle of stay at top of mast

So for a mast of say 150 kg and theta of 10 degrees, the compression will be 431.9 kg (nearly 3 times the actual mast weight). The smaller theta gets the bigger the compression gets.

Add to this the resolved mast componants of rig tensions and then the inertia loads, and the compression force can get quite large.

 

[bikedaft]

its a proctor mast, who went out of business a while ago, sparcare took over from them. sparcare were most helpful, i could get a sleeve/collar to put over area, but this hides the problem, so not keen. also potential difficulties getting mast through deck hole etc. the mast is an "8254 section" which has not been made for 15-20 years apparently. (i had hoped to get a short section of another mast and join it for simplicity). the mast alum is apparently "6082", 5mm thick.

its been a very interesting thread, thanks to everyone for the replies, always good to know the "why" even if i might struggle to explain it to someone else!

cheers

 

[mikemonty]

Apologies if I have missed something in the discussion about compression in the mast when the boat is on its beam ends, but I thought that with the mast horizontal, the compression load in the mast will be W/2sin theta, (assuming for the moment no initial rig tension at all, and a single shroud to the top of the mast)

Where:
W = weight of mast
theta = angle of stay at top of mast

So for a mast of say 150 kg and theta of 10 degrees, the compression will be 431.9 kg (nearly 3 times the actual mast weight). The smaller theta gets the bigger the compression gets.

Add to this the resolved mast componants of rig tensions and then the inertia loads, and the compression force can get quite large.

It has been said - however it's buried so deep that I'm not surprised you missed it.

 

[Avocet]

Don't get the force due to pre-load tension and force due to gravity mixed up - they can be treated, and should be treated, as separate entities.
Although the tension due to pre-load on the lower shroud disappears, (assuming the lower shroud goes slack) its because of the sagging of the mast and yes, the tension in the upper shroud will increase - but that's the gravity component affecting it.
The compressive force on the mast due to pre-load - using your 2.8 tonnes per shroud is 2.8*cos10 per shroud - which works out about 2.75 Tonnes. This is applicable only to the upper shroud - the lower one has gone slack and cannot "transmit" tension load.
This is in addition to the 5.66 tonnes compressive load due to gravity so the total is about 8.5 tonnes.

I have to say I only used 2 tonnes as a mast weight for simplicity!

Bear in mind that a real mast is a complex system where the lower section is a compressive strut supported by the lower shrouds (because it can "pivot" at the mast base) and the upper section is a cantilever (because it is "fixed" at the hounds) - being a beam which will bend a bit in a controlled fashion to give you mast pre-bend for tuning purposes.
So the above is over-simplified, but the principles are sound.
Or at least they appear to be to me, after the best part of a bottle of very nice red!

I think we're all converging on the same thing. - or pretty close to it! The "2.8-ish tonnes" already had the Cos 10 applied to it, so you'd be applying it twice to get the 2.75. HOWEVER, I'm only looking at this from the perspective of a simple, idealised, uniform (and infinitely rigid!) mast with only one cap shroud on each side and no spreaders. I completely agree that the lower shrouds change everything. In my example, the "leeward" shroud is assumed to reduce its tension and there are no lower shrouds on either side of the mast. It has just struck me that some of the confusion might arise because of me using the term "lower" shroud, meaning the one on the bottom when the boat is on its beam ends - I've now changed to "leeward" in the hope to avoide the confusion!

So, what I'm saying is that the rig has some initial tension on each of its two cap shrouds. When you lie the boat on its side, the tension on the "windward" shroud goes up and the tension in the "leeward" one goes down by about the same amount - leaving the overall compressive load down the mast the same (that being only a function of the angle between the shroud and the mast and the weight of the mast). I think Mikemonty is saying the same thing but with a different mast weight.

 

[mikemonty]

I see what you are saying Avocet,
But I still think it is a mistake to mix gravitational forces with a balanced and coupled tension in the rigging.
If we accept that the purpose of the rigging is to hold the mast at 90 degrees to the deck:
put the boat on its beam end with no shrouds
fit the windward shroud and adjust the length/tension until the mast is horizontal (90 degrees to the deck)
fit the leeward shroud and adjust it until the shroud has no slack but is not yet under appreciable tension. (we are assuming inelastic shrouds here)
You must accept that the tension in the upper shroud is due entirely to gravity - the lower shroud is not pulling anything.
Put the boat upright (the shrouds are now JUST holding the mast up) and apply tension to each shroud such that the mast remains vertical - assuming the shrouds are the same length the tension in each has to be the same. There is now a compressive force due to shroud tension through the mast INDEPENDANT of gravity.
Put the boat back on its beam ends - (the force due to shroud tension adjusts per your understanding, and)* the total force due to shroud tension remains the same - however the gravitational force changes - increasing independantly of shroud tension. to the same value as on the untensioned condition and so you have a gravitational force and a shroud tension force running down the mast at the same time but independant of each other. The shroud tension force - as I said some time back and in agreement with you - not really dependant on the heel of the boat and the gravitational force absolutely dependant on heel.
All - of course - ignoring the much more important dynamic forces.

Edit - *I'm retracting this bit the force due to shroud tension adjusts per your understanding - I still maintain the shroud pre-tension forces should not be considered part of the same system as the gravitational system - the only thing they have in common is the structural members they are "transmitted" through

 

[john_morris_uk]

So, what I'm saying is that the rig has some initial tension on each of its two cap shrouds. When you lie the boat on its side, the tension on the "windward" shroud goes up and the tension in the "leeward" one goes down by about the same amount - leaving the overall compressive load down the mast the same (that being only a function of the angle between the shroud and the mast and the weight of the mast).

I have come back to this thread and read it through and your comment above interests me. I think I see where there is a misunderstanding.

Your explanation doesn't tell the whole story as it ignores the reason that the boat is heeling! What you need to add is the weight of wind in the sails to your speculations about the tension in the rig (and therefore the compression load at the mast base). The boat doesn't heel without reason, and the heeling force of the wind (balanced by the righting moment of the form stability of the hull and the counterweight of the keel) adds to the load in the rig and adds to the compression load at the mast foot.

 

[mikemonty]

“When you discover that you are riding a dead horse,
the best strategy is to dismount.”

Get off! Get off the horse now and walk away!
Don't worry about the carcase - We'll get the council round to remove it later...

 

[Norman_E]

Bikedaft, I think this thread has become bogged down in discussion of theoretical loads, whereas you just need a practical solution. There is a very good suggestion in one post that if you can take up all the bottle-screws sufficiently, you do not need to add a spacer.

If you have corrosion at the bottom of the mast then a possible cause is electrolytic corrosion due to seawater in the bilges and contact between the aluminium and the stainless steel that you mention. I think you need to consider very carefully how much needs to be cut off, and just remove the bare minimum. If you can adjust the rigging so that no spacer is required, then that is a good way to go, but if you have to have a spacer then consider tufnol, as it is a good insulator. Just one caveat, is your mast electrically connected through to the keel or to a grounding plate as a lightning conductor? If so I suggest you ensure that electrical continuity is maintained.

 

[bikedaft]

yes corrosion must be due to seawater. i need to take off between 30-40mm to remove all corrosion (took off 20mm and base plate last night)

no i cant take this up on bottle screws, there are internal halyards in the mast, the entrance hole is too close to the mast partners already. will probably get a new mast heel plate made up, but with a plate thicker by 30-40mm. or an alum block with tongue and groove machined.

the lightning conductor has been disconnected at some point, from one of the chainplates. there is some s/s at the mast step, i do not know if this is connected electrically to the lead keel, but i take your point. if it was a simple pad i would probably use tufnol "bear brand" but its not,

cheers for your help

 

[Avocet]

I have come back to this thread and read it through and your comment above interests me. I think I see where there is a misunderstanding.

Your explanation doesn't tell the whole story as it ignores the reason that the boat is heeling! What you need to add is the weight of wind in the sails to your speculations about the tension in the rig (and therefore the compression load at the mast base). The boat doesn't heel without reason, and the heeling force of the wind (balanced by the righting moment of the form stability of the hull and the counterweight of the keel) adds to the load in the rig and adds to the compression load at the mast foot.

No, I understand that. I'm still trying to get my head round Mikemonty's post! It's not that those loads aren't relevant, of course they are, it's just that I'm trying to get the mast loads sorted out first before I look at the other loads. However, I'm keen not to annoy everyone excessively with this discussion! I find it interesting, but I can appreciate that others might not!

 

[Avocet]

I see what you are saying Avocet,
But I still think it is a mistake to mix gravitational forces with a balanced and coupled tension in the rigging.
If we accept that the purpose of the rigging is to hold the mast at 90 degrees to the deck:
put the boat on its beam end with no shrouds
fit the windward shroud and adjust the length/tension until the mast is horizontal (90 degrees to the deck)
fit the leeward shroud and adjust it until the shroud has no slack but is not yet under appreciable tension. (we are assuming inelastic shrouds here)
You must accept that the tension in the upper shroud is due entirely to gravity - the lower shroud is not pulling anything.

Yes, I accept that, happily! The only thing I would say is that the "inelastic" assumption might be taking simplifaction a bit far. I think we can probably assume the shrouds are inelastic, but not the hull and deck.

Put the boat upright (the shrouds are now JUST holding the mast up) and apply tension to each shroud such that the mast remains vertical - assuming the shrouds are the same length the tension in each has to be the same. There is now a compressive force due to shroud tension through the mast INDEPENDANT of gravity.

This is where the "inelastic" bit becomes a problem. I think that if everything really was inelastic, the tension in each shroud would drop to zero and the only compresive load at the mast foot would be the weight of the mast. In reality, I think that there wil have been a small amount of stretch in the "windward" shroud and a bit of deformation of the hull round the chainplate. As the boat somes up vertical, these will relax, so I'd expect to see a small tension in both shrouds, equal, and the mast leaning very slightly towards the side of the boat that was uppermost.

Put the boat back on its beam ends - (the force due to shroud tension adjusts per your understanding, and)* the total force due to shroud tension remains the same - however the gravitational force changes - increasing independantly of shroud tension. to the same value as on the untensioned condition and so you have a gravitational force and a shroud tension force running down the mast at the same time but independant of each other. The shroud tension force - as I said some time back and in agreement with you - not really dependant on the heel of the boat and the gravitational force absolutely dependant on heel.
All - of course - ignoring the much more important dynamic forces.

Edit - *I'm retracting this bit the force due to shroud tension adjusts per your understanding - I still maintain the shroud pre-tension forces should not be considered part of the same system as the gravitational system - the only thing they have in common is the structural members they are "transmitted" through

Ok, this bit lost me!

I've a feeling I can just about picture what you're saying, but am not sure. In the meantime, see if this makes any sense....

You get a bolt and you use it to fasten a hook to a wall. You tighten the bolt until it is JUST about to yield. If you hang your coat on the hook, does the bolt then yield due to the extra tension caused by the weight of the coat?

My feeling is that when you preload the shrouds, (with the boat upright), there won't be any more tension induced in the "windward" shroud as the boat heels UNTIL, the gravitational load from the weight of the mast exceeds the initial tension in the shroud.

...but I admit I'm not at all sure!!!

NOTE, by "windward", I' not talkign about loads from the wind on the rig (that comes later!) I'm just talking about the one that's on top, but not wanting to confuse the argument by using the terms "upper" or "lower" when talking about shrouds!

 

[CLP]

Apologies if I have missed something in the discussion about compression in the mast when the boat is on its beam ends, but I thought that with the mast horizontal, the compression load in the mast will be W/2sin theta, (assuming for the moment no initial rig tension at all, and a single shroud to the top of the mast)

Where:
W = weight of mast
theta = angle of stay at top of mast

So for a mast of say 150 kg and theta of 10 degrees, the compression will be 431.9 kg (nearly 3 times the actual mast weight). The smaller theta gets the bigger the compression gets.

Add to this the resolved mast componants of rig tensions and then the inertia loads, and the compression force can get quite large.

The above is from my post from further back in this lively ongoing debate. The situation described above is at the 90 deg heel scenario but the calculation at any angle of heel is similar in approach with the static forces diagram changing as the lever arm of the mast's centre of gravity moves further away from the centreline of the boat. The mast compression load must increase as the boat heels over, and the windward shroud tension increases, while the leeward shroud tension decreases. If the leeward shoud tension gets to zero then you get a slack leeward shroud. The initial pretension simply tries to make up for some of the stretch in the windward shroud which happens as the boat heels and reduces the likilihood of slack leeward shrouds. If you imagine having shrouds which had no stretch under load at all, then you wouldn't need any pretension.

An analogy to understanding that there has to be a mast compression load at 90 deg heel is to imagine you are abseiling down a cliff keeping your body horizontal. There has to be a force through your feet pushing against the cliff. And the smaller the angle between your body and the rope the more the force will be through your feet and the tighter you'll have to hold the rope.

 

[johndove]

They use to say that that the main chain plate on one side of the yacht should take the wight of the yacht.

ie. You could lift a the boat out of the water on shroud fittings on one side.

So the downward force (plus rig) should be the same or more as the upward force on the stays on a normal yacht.

On most yachts the lee stays are slack (ish)

Unless you have extra load for a racing rig and the mast is tightened with prebend so would be a lot more!

 

[Lakesailor]

Oh for God's Sake!
If it had an aluminium mast foot. Use some bloody aluminium.

The quasi-technical discussions on here could launch a probe to Mars.

 

[john_morris_uk]

Oh for God's Sake!
If it had an aluminium mast foot. Use some bloody aluminium.

The quasi-technical discussions on here could launch a probe to Mars.

Nothing like a bid of Fred Thrift to get the hackles rising in some quarters....? I thought that the discussion on mast loads was relevant if getting a bit tedious at times...

 

[mikemonty]

Avocet I can't quite put my finger on the source of your impression - though I accept it as your understanding.
I drafted this response and got to several hundred words and elaborate scenarios substituting shrouds with pairs of spring balances until I realised that what I'd put down was too long and boring even for me - and since I can't be bothered (sorry) to figure out how to post a diagram you can either accept my assurance regarding treating shroud pre-tension and mast weight as separate or ignore it.

I make no apologies to anyone who seems irritated by this digression from the OP - it is of interest to me and others and forms part of an understanding of the workings of a sailing boat.

Apart from which - I answered the OP succinctly in my first post on the subject.
This is simply background - is that not how a conversation works? Or do some believe that this forum should be some form of Oracle that delivers answers and no more? (RHETORICAL QUESTION!)

 

[Avocet]

Cheers Mike, (and others)! The only way I can think of doing diagrams would be to draw them, scan them, save them as a JPEG and put them on some sort of site like Photobucket, then put the reference to them in this thread as if they were a photo. HOWEVER, I can completely understand that being too much of a pain in the backside!

I'm NOT convinced that I'm right, but intuitively, I feel that the reading on a load cell stuffed under the mast heel won't change anything like as much as many seem to think it will when the boat is being sailed. My belief is that a big part of the art (science!) of designing a rig would be to ensure that the stays aren't subject to big cyclic loadings - and that can only happen if the static preload is such that whatever loads occur when sailing don't get much bigger than the initial preload. I accept that leeward shrouds on most boats slacken demonstrably when close-hauled, so clearly, rigs can cope with a REDUCTION in load to below the initial static preload, but I'm not sure about increases in load above that.

However, this debate seems to be causing a certain amount of distress to other forumites. (Quite why they should feel compelled to repeatedly return to it in order to confirm just precisely HOW miffed they are is beyond me!), but I am happy to desist for the time being and will ponder the matter further. If any great enlightenment strikes me, I shall return!