Mast & rigging calcs

I'm having some troubles with some mast & standing rigging calcs.
The boat is a 50ft monohull, triple spreader, masthead rig.
Simplicity is the order of the day here. I would prefer to run a furling, self-tacking jib . Main will be boom furling. Spreaders are not swept back.
The RM @30 degrees is 14100 kg.m
longest transverse panel is 664cm
longest longitudinal panel is 2077cm
1/2 beam at the chainplates is 1.37m (in order to allow a genoa to be sheeted in to 14 degrees off the CL)

The result of all this is that Ixx = 2109 cm^4 and Ill = 9863 cm^4

Which leaves me with a required aluminium mast section bigger than any that appear on manufacturers catalogues!

Further, the standing rigging (lowers) has a required breaking strength of around 22200kg!

Now, I can run an inner forestay (though I'd prefer not to) &/or I can move the chainplates out to the gunwale (which means a sheeting angle of around 20 degrees).

Anyone got any out-of-left-field suggestions?

 

Will,

I checked your numbers quickly with the formulas in Skene's which is simple to calculate and conservative, and come out with Itt = 1686 cm^4 and Ill = 5267 cm^4. So I would recommend that you check your math and the procedure to make sure all is correct.

However, I also noticed two other things. Your righting moment for a 50' boat is really huge, so you must have a wide hull and/or deep keel. To support a big righting moment, you need a big mast. The other thing is that your shroud base by comparison is very narrow, and the narrower the shroud base, the higher the compression load in the mast. To reduce mast size, you should look to ways of modifying the rig that reduce the compression load. That means changing the staying arrangement, and/or reducing righting moment. It is as simple as that, after double-checking the math.

Hope that helps.

Eric

 

Probably a wrong calculation in Ixx, should be less (half?, maybe program devides 1.37 by 2).

Ill is more or less correct, you can't have an in-line spreader rig without runners and babystay. Chainplates at the gunwhale won't change that.

Breaking strength of lower V looks o.k. for a 50'

Arthur

 

Thanks guys -
I used Skenes to do the calcs, and ran a known set of numbers thru to check my inputs - all was correct.... so not sure why I wind up with such a vastly different number(s) to Eric...

Anyway, I have completely re-engineered the rig to incorporate swept-back spreaders. This allowed a widening at the chainplates to the full beam of the boat and solved almost all my problems - I now have a sensible sized mast! My intial chainplte beam was narrow to allow a 14 degree sheeting angle for a genoa....

Incidentally - you're right about the RM - the boat is not really what I'd call beamy - it's fat!


Thanks again for the help!

 

I'm curious Will what the GM is--I'm with Eric you have alot of righting moment--If the boat is too stiff then it will be very uncomfortable at sea. even dangerous to the crew. A simple way of evaluating a sailboats stiffness is to use the Dellenbaugh Method:

(57.3 x SailArea x HealingArm) / Displacement x GM ( lbs. and ft. )

the value is close to the expected heel angle in AWS of 25ft/sec (approx 15kts)

 

Without having the figures in front of me, I did the dallenbaugh check and the boat wound up a little on the stiff side of normal - though not excessively so. I'll be sure to check it again though! Thanks

 

Will is this the same boat with the displacement of 16 tonne with the GZ of .76 metre

 

Yes it is - but the GZ I gave you was incorrect - it is in fact 0.92m (off the top of my head....)

 

I've got another problem now....
I'd be thoroughly lost with you guys....

Having used Skenes for the mast and rigging calcs, the only mehtod that I have for calculating the spreader sizes gives only the compression that the spreaders must resist. Of course all the maufacturers tables give spreader sizes in terms of inertia(s), same as the mast.
Can anyone point me towards a method (preferably metric) for calculating the spreader inertias....

 

Priciples of Yacht Design has a good method for determing spreader size, and it is in SI.

 

scantling mast for sail boat

there are several method to scantling mast for me the best is finits elements but if you use the BV for pleasure vessel you have a complete methode to calculate mast and all fitout the LRS has a methode too,i send a little excel sheet to evaluate the load i don't translate all but it's easy to understand

 

Doug - yes I have the Principles method and it may be the easiest to for me to incorporate - for some reason I find the methodolgy(s) in that book hard to follow...

French44 - wow! I was planning to post my spreadsheet once it was complete, so that others might get some use out of it too (I still will..) But yours is WAY better. Very kind of you to post it.

 

to willallison
in addition to my last reply ,for all calculation about mast by the BV you can find the rules book with reference NR381 DNP R00F i have the 1993 edition may be there is one more recent (i don't have electronic version only paper) ,to complete you find in the PIERRE GUTELLE books more methode and calculation ,there are three books and you can find them on "loisirs nautiques " web site ,if you need i can scan THE BV RULES but the book is in french so if you have in trouble wiyh the translation send me a mail ! otherwise i asked in boat design forum a question about wave impact on anchor mast if you have a opinion do not hesitate.

 

4 ans plus tard...

Bonjour french44,

Je suis très intéressé par ce BV rules, si c'est encore possible, pourrais-tu me l'envoyer, sinon me dire où je peux me le procurer.

merci d'avance