downwards load at mast base, how much is it?

[Avocet]

Interesting thread. For practical solution, I agree with Pyrojames - forget about rig tensions - just duplicate (or exceed) the compresive strength of the mast material and you can't go wrong! In any case, surely a lot of the "fudge factor for dynamic loadings" discussion is a red herring as we're always being told the shrouds and stays shouldn't go slack - so that means that whatever the dynamic loads are, they won't be as great as the static tensions? It works for engine cylinder head bolts - the forces trying to blow the head off the block vary considerably (can even go "negative" on over-run) but the tension induced by torquing the bolts up is always much bigger than any of the dynamic loads so it doesn't matter what they are!

OK, I know that leeward shrouds do go less tight (or even almost slack) on a hard beat but presumably the windward tension goes up by about the same amount so the overall compressive component of the load down the mast is about the same?

If you want to make up a packer, I've used these people before:

http://www.theplasticshop.co.uk/#1X0

and have been pleased with the service. PVC is probably the cheapest stuff to use. I'd just be wary of the mast foot sliding on it (or any plastic). You'd need some beefy positive location and not try to rely on friction.

 

[vyv_cox]

thanks everyone for replies.

the mast is about 12" x 9" oval section 5mm thick wall so will calculate area etc and go down that route,

cheers

ps the tufnol stuff sounds interesting

Another vote for Tufnol. I have used it a few times in industry, it is strong, tough and easily machined. My Yacht Leg sockets are backed by Tufnol pads about 20 mm thick, they have been a great success.

Be very careful about plastics in general. Materials like polyethylene, PVC, PTFE etc are what they say - plastic. Over time they will creep and lose shape and thickness. Tufnol is a composite using a thermoset resin that will not deform.

 

[Neil_Y]

I really hate to be pedantic, but it might help people to know that Tufnol is a plastic, and so is any composite formed by polymerisation.

A plastic is a polymer or non crystaline structure. Polyester, polyethelene, polyprop etc are all prone to softening and creep where as Tufnol which is a phenolic resin based composite hardens with heat and is a good plastic to use where you want a rtigid structure.

Tufnol is a thermoset resin and fibre composite. But Vyv iis right in highlighting the fact that many other plastics will creep under load and soften with heat.

Plastic covers the whole range, out of interest we are currently working with a spar manufacturer to develop a new type of phenolic resin composite for use in large mast base plates running at around 65ton loads requireing easy adjustment of mast base position to adjust rake.

For this job Tufnol would be ideal as a mast base.

 

[john_morris_uk]

Interesting thread. For practical solution, I agree with Pyrojames - forget about rig tensions - just duplicate (or exceed) the compresive strength of the mast material and you can't go wrong!

OK, I know that leeward shrouds do go less tight (or even almost slack) on a hard beat but presumably the windward tension goes up by about the same amount so the overall compressive component of the load down the mast is about the same?

I agree with the practical solution - mainly as its pragmatic and hopefully the original designed of the boat will have had some idea about calculating mast compression loads.

I don't agree with yours (and several others) suspect suggestions regarding the compression loads at the mast base.

Snowleopard described it up pretty well, but to summarise:

compression load is a combination of

Static:

The majority of the weight of mast and rig.
The static compression forces of the tension in the rig.
The righting moment of the boat at the angle of heel. (This has a maximum value at a particular angle for every hull shape. After all righting moment is a product of form stability or the hull and righting moment of the keel, with the maximum compression force usually being exerted when the boat is knocked down and the rig is perpendicular to the water.)

Dynamic

The forces from acceleration of the weight of the rig as the boat falls off a wave, a sudden gust hits the boat and the inertia of the boat prevents it instantly heeling etc etc.

All this means that the maximum compression force at the mast foot can be MUCH MUCH more than the tension in the rig!

Its also why fitting oversized rigging can have a negative effect on the rig - more weight aloft and more righting moment equals more compression...

 

[vyv_cox]

I really hate to be pedantic, but it might help people to know that Tufnol is a plastic, and so is any composite formed by polymerisation.

A plastic is a polymer or non crystaline structure. Polyester, polyethelene, polyprop etc are all prone to softening and creep where as Tufnol which is a phenolic resin based composite hardens with heat and is a good plastic to use where you want a rtigid structure.

Tufnol is a thermoset resin and fibre composite. But Vyv iis right in highlighting the fact that many other plastics will creep under load and soften with heat.

For this job Tufnol would be ideal as a mast base.

I could have said thermoplastic but I didn't want to be pedantic.

 

[Avocet]

I don't agree with yours (and several others) suspect suggestions regarding the compression loads at the mast base.

Snowleopard described it up pretty well, but to summarise:

compression load is a combination of

Static:

The majority of the weight of mast and rig.
The static compression forces of the tension in the rig.
The righting moment of the boat at the angle of heel. (This has a maximum value at a particular angle for every hull shape. After all righting moment is a product of form stability or the hull and righting moment of the keel, with the maximum compression force usually being exerted when the boat is knocked down and the rig is perpendicular to the water.)

Dynamic

The forces from acceleration of the weight of the rig as the boat falls off a wave, a sudden gust hits the boat and the inertia of the boat prevents it instantly heeling etc etc.

All this means that the maximum compression force at the mast foot can be MUCH MUCH more than the tension in the rig!

Its also why fitting oversized rigging can have a negative effect on the rig - more weight aloft and more righting moment equals more compression...

I'd still like to see some meaningful numbers on there before agreeing with you!

The force on the mast heel due to the weight of the rig will be, I think, almost insignificant - (a few hundred kilos maybe??) compared to the other forces. In any case as the boat heels, it will DECREASE (in fact, to zero when it's on its beam ends)!

The dynamic loads, I'm not sure about but falling off a wave would temporarily REDUCE the weight of the rig (in effect). I guess the worst "falling" loads would in fact be when it lands hard - perhaps the "slamming" that some boats do?

As for the compressive load in the mast being "much much more than the tension in the rig", the more I think about it, the more I feel that can't be true! In fact, I'm starting to feel that the compressive load in the mast can NEVER be anything OTHER than EQUAL (and opposite) to the (vertical components of) the tension in the rig! (plus a bit for weight). The compressive load in the mast is only a reaction to the tensile load in the rig, after all!

With regard to the "creep" problem, I admit that's true, but in reality is it likely to be a problem? A slab of UPVC the same area as the mast foot will have very small compressive streses in it (assuming it's a decent fit against the mast heel and whatever is underneath it). At the sort of temperatures its likely to encounter, wouln't it take a hundred years or so to do any significant creeping?

 

[mikemonty]

Imagine a boat on its side.
The mast is horizontal.
The COG of the mast is centre at the centre.
half the vertical vector of the weight is at each end.
half as shear (vertical) at the mast base the other half is being supported by the (upper) shroud. (one for simplicity)
the vertical vector is equal to half the weight of the mast, however the HORIZONTAL vector at the shroud is equal to the other side of the right angle triangle that is the mast/shroud/decklevel i.e MUCH bigger than the weight of the mast.
The shroud can only be in tension - so where is the balancing compressive force acting through? - the mast base.
Do it yourself - loop a string round the blunt end of a pencil and place the point against your leg, hold the end of the string against your leg an inch or two above the pencil and press down (vertically - not towards your leg!) on the pencil end - does it start to get painful?

 

[john_morris_uk]

I'd still like to see some meaningful numbers on there before agreeing with you!

The force on the mast heel due to the weight of the rig will be, I think, almost insignificant - (a few hundred kilos maybe??) compared to the other forces. In any case as the boat heels, it will DECREASE (in fact, to zero when it's on its beam ends)!

The dynamic loads, I'm not sure about but falling off a wave would temporarily REDUCE the weight of the rig (in effect). I guess the worst "falling" loads would in fact be when it lands hard - perhaps the "slamming" that some boats do?

As for the compressive load in the mast being "much much more than the tension in the rig", the more I think about it, the more I feel that can't be true! In fact, I'm starting to feel that the compressive load in the mast can NEVER be anything OTHER than EQUAL (and opposite) to the (vertical components of) the tension in the rig! (plus a bit for weight). The compressive load in the mast is only a reaction to the tensile load in the rig, after all!

NO! I wish I could draw a picture and put it on here, but hopefully someone with greater IT literacy will be along and draw a diagram to show the leverage of the rig on the boat when it is heeling - and the resultant compression forces at the mast base involved. Our keel weighs 4 tonnes or more and a proportion of the righting moment from that ends up going through the mast base when the boat heels.

On the wave business, at least understand that when the boat falls off a wave and lands on the bottom of a trough, the deceleration forces of the weight of the mast and rig multiply the weight of the mast many times and produce more mast compression than when the boat is static, (assuming that the boat is near upright.)

Compression force from weight decreases as the boat heels, but is overtaken very rapidly by the righting moment compression of the wind in the sails vs the weight of the keel etc.

The actual load at any given point is very hard to calculate completely accurately as it depends what the boat is doing and the load has a dynamic element. It can certainly end up as greater than the weight of the rig and the tension of the rigging.

 

[Avocet]

Imagine a boat on its side.
The mast is horizontal.
The COG of the mast is centre at the centre.
half the vertical vector of the weight is at each end.
half as shear (vertical) at the mast base the other half is being supported by the (upper) shroud. (one for simplicity)
the vertical vector is equal to half the weight of the mast, however the HORIZONTAL vector at the shroud is equal to the other side of the right angle triangle that is the mast/shroud/decklevel i.e MUCH bigger than the weight of the mast.
The shroud can only be in tension - so where is the balancing compressive force acting through? - the mast base.
Do it yourself - loop a string round the blunt end of a pencil and place the point against your leg, hold the end of the string against your leg an inch or two above the pencil and press down (vertically - not towards your leg!) on the pencil end - does it start to get painful?

Agreed - completely!

If there is (say) only one shroud from the masthead to the chainplate and it makes an angle of (say) 10 degrees with the mast, and the tension in the shroud is (say) 2 tons, then the compressive load induced in the mast will be 2 tons x Cos 10. (which is pretty close to 2 tons). If the angle gets smaller, the loads become more equal. If the angle ever got to 90 degrees, it would be zero compressive load. In practical terms, of course, it's more complex because the load is shared with the lower shrouds and spreaders make the angle more complicated.

But none of that changes anything I've said!

I still believe that the compressive load in the mast can never be more than the tensile loads in the rig PLUS a very small amount for the weight of the rig (which decreases with heel).

Put another way, if there were no shrouds, there would be no compressive load in the mast.

 

[Avocet]

NO! I wish I could draw a picture and put it on here, but hopefully someone with greater IT literacy will be along and draw a diagram to show the leverage of the rig on the boat when it is heeling - and the resultant compression forces at the mast base involved. Our keel weighs 4 tonnes or more and a proportion of the righting moment from that ends up going through the mast base when the boat heels.

Agreed, absolutely! What I'm saying is that whatever that force ends up being, it can only be the same as the tension in the rig (plus a bit for its own weight). What's more, I've a feeling that when the boat is upright with no heeeling moment, the initial tension in the rig will produce a compressive load in the mast of "x" tons. This will be shared between the port and starboard shrouds and the stays. As the boat heels, (say, to starboard), the tension in the port shrouds goes up, the tension in the starboard shrouds goes down and the tension in the back and forestays stays about the same. What I'm suggesting, however, is that the rig designer will have sized all the components to induce an initial tension in the rig (and therefore and equal and opposite compression in the mast) that is about the same as the "worst case" (a knock-down?) tha tthe design is supposed to be able to cope with, so the total net downward force in the mast won't be too much different to the load that's on it when it's tied up in the marina. (but having never designed a boat in my life, I'm happy to admit that I could be worng on that)!

If the owner decided to step the mast and do all the bottle screws up finger-tight, THEN, I would expect the compressive load in the mast to rise ENORMOUSLY when heeled.

On the wave business, at least understand that when the boat falls off a wave and lands on the bottom of a trough, the deceleration forces of the weight of the mast and rig multiply the weight of the mast many times and produce more mast compression than when the boat is static, (assuming that the boat is near upright.).

Again, I'm happy to admit to being wrong here, but I'd be amazed if that factor was more than 2 or maybe 3.

[/QUOTE]

 

[john_morris_uk]

What I'm suggesting, however, is that the rig designer will have sized all the components to induce an initial tension in the rig (and therefore and equal and opposite compression in the mast) that is about the same as the "worst case" (a knock-down?) tha tthe design is supposed to be able to cope with, so the total net downward force in the mast won't be too much different to the load that's on it when it's tied up in the marina. (but having never designed a boat in my life, I'm happy to admit that I could be worng on that!)

I think where we disagree is that I don't believe that the tension that you put in the rigging when the boat is static is anywhere near the tensions that exist when the boat is heeled. Even if the rigging never goes slack to the point of wobbling about, when you heel, the forces rise significantly. Looking at the geometry of our boat I suspect that the maximum compression load might rise to several tonnes. I certainly don't pre-stress the shrouds to that degree.

I remember reading an article on the subject by Andrew Simpson in PBO some time ago, where he went over the calculations that he had done when he was designing Shindig's mast compression post. He explained it rather better than I have!

 

[Deleted member 36384]

I think where we disagree is that I don't believe that the tension that you put in the rigging when the boat is static is anywhere near the tensions that exist when the boat is heeled. Even if the rigging never goes slack to the point of wobbling about, when you heel, the forces rise significantly. Looking at the geometry of our boat I suspect that the maximum compression load might rise to several tonnes. I certainly don't pre-stress the shrouds to that degree.

I remember reading an article on the subject by Andrew Simpson in PBO some time ago, where he went over the calculations that he had done when he was designing Shindig's mast compression post. He explained it rather better than I have!

John Morris - you are correct. To help others understand consider this silly scenario.

1. Place your yachts keel into a giant vice and tighten.
2. Tip the vice over so that the yacht is at 45 degrees.
3. Put a giant finger tip on the top of the mast and pull the mast to the low side (leeward). Remember the geometry will not change.
4. Keep pulling until the lower (leeward) shroud goes slack.
5. This extra tension in the up hill shroud (windward) must go somewhere.
6. The sum of the forces about the axis equal zero (this is a fundamental principle). The axis in this case is the vertical mast axis.
7. Hence the tension on the mast caused by the giant finger must be a resultant component. One component is distributed as extra tension on the shroud and one as compression in the mast.

 

[jetwake]

I suspect the giant finger would rsult in chain plates ripping out and the mast snapping. If the shrouds went "ping" i'd suspect under speccing.

Anyway, changing from keel stepped to dack stepped seems to my thuggist eye to be a major diversion from both the designers idea and the buildres instruction sheet! Is this wise? Will it affect insurability?

David

 

[mikemonty]

The compressive force through the mast due to the shroud tension does not change irrespective if angle - what WILL change is the compressive force due to gravity - or dynamically due to accellerations (FAR too complex to describe here).
If the mast weighs 2 tonnes - perfectly upright, the compressive force due to weight is however much the mast weighs plus - as you say, the cosine 10 deg * the wire tension.
However, when the mast is horizontal the "preload" due to wire tensioning remains the same (no reason why it should change!) but half the weight of the mast (one tonne) is pulling down at the masthead.
The tension that this creates in the shroud INDEPENDANT OF PRE-TENSION will be 1/(sin 10) = 5.75 tonnes.
By simple trig, the compressive component is the root of 5.75 squared minus 1 = 5.66 tonnes.
To this can be added the compressive component of the rig tension that you previously provided.

Your belief is wrong because you have not accounted for the manner in which the supporting shroud is - irrespective of pre-tension - forcing the mast to "swing in" towards the mast foot under its own weight and the leverage applied by the acute angle at which the shrouds meet the mast.

 

[snowleopard]

compression load is a combination of

Static:

The majority of the weight of mast and rig....

You can leave that bit out because, in extremis, the rig will be near enough horizontal so not contributing to thrust on the foot.

OTOH in a multihull the max righting moment is just as the weather hull leaves the water so the weight of the rig is included.

 

[mikemonty]

You have fallen into the same trap as Avocet - by your model of what is happening the weight of the mast at the masthead is being supported by a force acting purely vertically upwards - the shroud would have to be hanging from a skyhook right above the masthead for this to happen - or else be made completely rigid with respect to the hull and act as a cantilever. In reality there has to be a horizontal component to match the vertical component of the force and the total force has to be in line with the shroud - which means that there will be a thrust horizontally through the mast foot.

What you and Avocet are proposing implies, for example, that a boy on a swing could stop the swing at any point in its arc and jump off - leaving it "hanging" to one side of vertical.

If you are interested (probably not by now!) I showed an example here...
http://www.ybw.com/forums/showthread.php?p=2302728#post2302728

 

[Lakesailor]

Stick a block of wood or alloy in and look again after 3 months. It'll either be OK or crushed a bit.

 

[Avocet]

The compressive force through the mast due to the shroud tension does not change irrespective if angle - what WILL change is the compressive force due to gravity - or dynamically due to accellerations (FAR too complex to describe here).

Agree on both points.

If the mast weighs 2 tonnes - perfectly upright, the compressive force due to weight is however much the mast weighs plus - as you say, the cosine 10 deg * the wire tension.

Agree (although 2 tonnes is one hell of a mast)!

However, when the mast is horizontal the "preload" due to wire tensioning remains the same (no reason why it should change!) but half the weight of the mast (one tonne) is pulling down at the masthead.

Agreed (subject to a few assumptions)! The FULL weight of the mast is acting vertically downwards through it's centre of gravity. If it is a deck-stepped mast, AND we make the assumption that it's uniformly sectioned rather than tapered, AND we forget about radar reflectors, nav, lights, boom fittings, etc. then 2 tonnes acts downwards through the centre of gravity and the centre of gravity will be half way up.

The tension that this creates in the shroud INDEPENDANT OF PRE-TENSION will be 1/(sin 10) = 5.75 tonnes.
By simple trig, the compressive component is the root of 5.75 squared minus 1 = 5.66 tonnes. To this can be added the compressive component of the rig tension that you previously provided./QUOTE]

I think this is where we start to diverge. I agree with the sums, so the tension in that shroud times Cos 10 will be the 5.66 tonnes, which must be equal to the compressive load down the mast. However, why should you simply add the initial rig tension to that? The initial rig tension will come from two shrouds (one on each side) and (I believe) that they would each be tightened to an initial tension of about 2.8-ish tonnes. When the boat heels, the tension comes off one and increases on the other, the net compressive force down the mast stays about the same.


Your belief is wrong because you have not accounted for the manner in which the supporting shroud is - irrespective of pre-tension - forcing the mast to "swing in" towards the mast foot under its own weight and the leverage applied by the acute angle at which the shrouds meet the mast.

 

[mikemonty]

I think this is where we start to diverge. I agree with the sums, so the tension in that shroud times Cos 10 will be the 5.66 tonnes, which must be equal to the compressive load down the mast. However, why should you simply add the initial rig tension to that? The initial rig tension will come from two shrouds (one on each side) and (I believe) that they would each be tightened to an initial tension of about 2.8-ish tonnes. When the boat heels, the tension comes off one and increases on the other, the net compressive force down the mast stays about the same.

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!

 

[grumpy_o_g]

Couldn't Z-spars or somebody like that help you out?