Mast Compression Post Question

Discussion in 'Sailboats' started by Jetboy, Apr 23, 2015.

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I'm sorry what do you mean? You made a statement about anticipating, i.e. inferring a guess. Just because you don't understand how to do it, don't assume others who are trained do not.

See, there you go assuming again. The only approximations are the loads to be applied. These approximations become more and more accurate with each vessel made from in-service experience. But the calculations for the design is simple beam theory - once the laods are obtained. The theory any engineering/naval architecture students learns at university. Petros has given you one of the most common formula used.

Again, YOU can't. Don't assume those trained and educated to do so cannot.

Another assumption. So before FEM, how was this calculated?...simple beam theory and successfully for decades. How is it calculated today..simple beam theory. One does not need FEM. It can be used but is not essential.

That requires an open and enquiring mind. All you have done is demonstrate a closed preconceived mind set.

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Mr EfficiencySenior Member

Jetboy becoming a whipping boy ?

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JetboySenior Member

I have a very hard time believing anyone can accurately calculate the loads. I know you can't do the calculations by hand in a lifetime. I don't believe any cad model has been developed that can do a comprehensive FEM of the system dynamically. I'm still open to be convinced but I don't think you or anyone else actually calculates these loads. I think you use approximations. Simply saying you do means little to me. Explain how you go about our and convince me. Otherwise I'm inclined to believe is s lot of ego stroking hot air.

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Mr EfficiencySenior Member

Another satisfied customer ! At least it didn't cost you a consulting fee.

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JetboySenior Member

Nope. Just calling out some bull ****. I'm not sure at this point whether he actually believes that simplified approximations are "calculating" the actual loads. If that's the case I'd hate to relieve him of his delusions. It seems as though he's quite proud of them.

I was just curious about what generally is used for this particular component. That's been answered by a few folks. I'm not sure why he came to this thread just to stroke his ego and add nothing of value. But since he did am happy to point out how nonsensical his delusions of precision really are.

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Mr EfficiencySenior Member

I'm sure you'll have more lively discussions !

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DCockeySenior Member

In addition to the compression loads from the rigging, a mast with a sail and/or boom attached also has a side load from the sail and/or boom. That load may need to be included in calculations.

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TANSLSenior Member

I think the "compression loads from the rigging" are quite small compared to the bending stresses caused by sails. It is the latter that must be studied thoroughly.

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gggGuest...

Compression loads may be decidedly non trivial in racing craft. I used to run 600lbs down the checks and 500lbs down the shrouds on a 12ft racing dinghy with a double spreader rig. Only in my wildest dreams could sail induced forces approach those sorts of levels..

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AlikSenior Member

As rule of thumb, for cruiser-racer sailboat the compression load on the mast is about 85% of displacement.

Stayed mast and under mast pillar are designed for compression buckling, i.e. stiffness defined by Euler formula. In that formula, stiffens of section is used that is a product of modulus of elasticity and moment of inertia. As pillar is short, usually its section is much smaller.

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TANSLSenior Member

I am a layman in this issue and not clear to me how a mast works, and why. Do you tell me that a mast (not talking about the pillar under mast, if any, nor the cables) works primarily in compression?, no bending from sails or that it is negligible?

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gggGuest...

The drive force from the rig of the boat I mentioned above has been calculated, reaching under spinnaker, as around 100kg drive, 80kg heeling and 60kg lift. So its by no means insignificant, but much less than the compression loads from the rig tension. But that's a high performance racing craft. Other styles of boat may use much lower rig tension - indeed would simply disintegrate under those sorts of loads. If you think of a similar boat with an unstayed mast (which wouldn't have a compression post of course!) then compression forces will be minimal by comparison, but drive forces in the same sort of order. So really I think you need to treat each case separately and be very wary of generalising.

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TANSLSenior Member

Yes, of course racing sailboats must be something very special. Like a Formula 1 car, a normal designer will not be able to design one of these wild horses.
However, it is not logical to think that the distribution of the forces produced by the sails has the effect, through the rigging, of important compression efforts on the mast. As important I mean more critical than bending ones.

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DCockeySenior Member

Any stayed mast will have a compression force exerted by the rigging, due to the rigging not being perpendicular to the mast. This compression force will greater than the lateral force the mast exerts against the rigging because the angle between the mast and stay is less than 45 degrees. See attached simple vector diagram.

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15. Joined: Sep 2011
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TANSLSenior Member

It is clear and, frankly, it was very easy to understand. It was a slip. Thank you all, really.

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