Forces on mast by sails yet again

Evaluation Of Axial Compression Force On A Mast

Just to add a further technical contribution on the issue (with a resulting compression load value !!...)which I believe being of good interest to many keen speculators on this subject......

Cheers...

YADES ​

 

"It is well known that higher aspect sails produce greater lift when close hauled." "What's the Ideal Sailing Rig?" Michael Kasten, Kasten Marine Design

Yes. Think schooner, as in Bluenose, America.

 

There are limits, though. Too high and thin (too high an aspect ratio) and they will stall at low "angle of attack". In other words, they will be great when pointed well up, but not efficient at other points further off the wind. Not versatile. I think AR of 3 -4 is considered best for all around use.

 

I hate these compromises. One would think that after all these years and years of sailing experience you guys would have come up with some kind of golden rule to what works and what doesn't. Instead everyone blunders ahead and this theory and then this formula... I think I mentioned this before.

Eh YADES !! The pdf you uploaded is just about spot on with my size cat. Same mast length, same boat size, couldn't ask for a better calculated example Only thing I don't like is the 6000kg mast loading That is quite a bit of loading, considering that is the weight of 6 toyota carolla 1600's eh !

 

Fanie
The lack of exact definition is because anything to do with fluids and dynamics is basically indeterminate. Thats why we use static loads and then a factor of safety to allow for indeterminate loads. The FOS is then refined through observation. It is only with dead static loads that we can design absolute to specific numbers and know it will last.


The most up to date rule from any society for rig design is from Germanisher Lloyd, (it is a public domain document) attached. As the knowledge and particularly materials and design move on the newer guides are more applicable.

GL as underwriters suggest that a vessels rig be designed to this spec. So they are putting their rule where their money is, as to speak

Hopefully it will help you along with the others contributions.
Cheers

 

Mast Compression Load - reply to Fanie..

I'm pleased to read on your reply that the example is just fitting yr situation....... That' s because I made some ......realistic assumptions..... as stated on the enclosed paper, for the evaluation of mcl, based on the infos gathered from your previous threads...........However, bear in mind reason for such a high compression load is mainly due to a no-spreaders rig arrangement and also due to a deck-stepped mast.....

 

Fanie
If you want to reduce mast compression then reduce displacement. If you get it to displace one toyota corolla rather than 6 then you only need to allow for one corolla of compression.

Poor design results in failures and why you see boats set to sea and then make a quick retreat as things start to fall apart.

Yades result is close to my original number provided in one of the very earlier posts on this thread, #11. My calculation did not have any safety factor apart from a small one based on my mostly conservative simplifying assumptions.

Rick W.

 

Fanie you want to have your cake and eat it too. And you want the icing to be thicker and you want candles too

Quote: "won't a few long thin vertical sails give you more windward speed than one big sail ?"

The way I see it, one big sail (long and thin) will give more windward speed than several smaller ones, but will also give you more heeling force and bigger rigging loads.

 

Sorry Fanie, I haven't been keeping up with all your threads. Have you decided on a boat length yet?

 

Righting force dictates rig size.

Hi Fanie,

Am not a expert but have a 57' heavy offshore cutter. The righing moment of the hull is the firts thing to look at. Walk the docks and you can see fast differents in size of rigs.

Look at it this way. your rig should be strong enough to witstand a knockdown. A tender vessel will lay over a lot easier than a stiff vessel thus a tender vessel needs a lot les rig than a stiffer one. so you start with getting the righting moment than calculate forces. just doing the math on sail area is sitting you up for disaster.

good luck

John Van Strien
Sv Western Grace

 

Thanks for your comments, all is taken in consideration. I still have some things to work through before making any final decissions on boat length, sail or sails sizes and so on. So many compromizes... in the 'old' days things were much more simple, not even to mention time. Since this is an expensive exercise I'd like to do it more or less right the first time.

Thanks for the goog luck John, I'll take it

 

be carefull

Fanie,

As you are starting out be carefull. building a boat is very expensive even when building a proven design. If you go and build a unproven design you set your self up for failure. having come to a large building process can only recommend buying a baot or finshing a well done project. have seen many go down with the project that is financial.

John

 

If you are trying to understand the relationship between wind speed and pressure, it is calculated with Martin's formula, which is the square of the wind speed in miles per hour x .004 = pounds per square foot.

 

A very rough rule is the sails will produce 1 lb/sq/ft of pressure at 17K.

FF