Mast Compression Loads

[yades]

As a matter of technical interest would perhaps be useful starting a new thread entirely dedicate to the evaluation of mast compression loads.
As with regard to this issue, would like to propose on the initial stage what I have inserted in the motorsailer forum.....

Adds-on and comments on the subject...are more then welcomed

Cheers

YADES

[catsketcher]

Hello Yades

Thank you for your information. I am quite interested in total mast compression loads in catamarans.

cheers

Phil Thompson

[tspeer]

I haven't gone through your calculations in detail, but I think your approach is quite reasonable, at least through the calculation of the compression load. I presume that the coefficients (K factors) are based on buckling of the mast with the calculated loads. Dynamic loads can also be important, and you should include some allowance for them as well. I don't know if they've been factored into the coefficients, either.

It is true that multihulls require a stronger rig for their length or displacement than a monohull. Multihulls also have a higher roll radius of gyration, and a faster natural frequency in roll. I would expect this to increase the dynamic loads compared to a monohull, too.

[yades]

Just to clear matters out....the factor of safety in all computational procedures...(Skene's, BV, DNV and so and so forth..) are obviously considering the dynamic loads applied on mast. Furtermore the procedure shown on the previous thread enclosed paper (.pdf file) is not considering rigging loads derived from Knock-down by main+genoa and is not considering rig arrangement with spreaders, where loads evaluation is slightly more complex. Will elaborate a further technical script on the subject and will splash it on this forum soonest.
Meanwhile any other elaborations on this issue would be welcomed.,,,,,

[brian eiland]

Originally Posted by yades

Meanwhile any other elaborations on this issue would be welcomed.,,,,,

You might have a look thru some of these discussions, there is quite a lot of good material here:
Sail Loading on Rig, Rig Loading on Vessel
http://boatdesign.net/forums/showthread.php?t=2293

Are you interested in a 'rig mapping' exercise ??
http://boatdesign.net/forums/showpos...4&postcount=38

[yades]

Thks yr tip will go through and revert on this forum....

[yades]

Thanks again...as you said a lot of good material....(I've just gone through......it took quite sometime though...but v.m. of interest !).

Few comments though.....

First..
I share your statement which I red on one of yr posts....
"....I don't pretend to be any kind of an expert in these engineering/computer structural analyses. I would just like to get a clearer picture of how the sails actually transmit their forces to the vessel; at what points, and in what path(s)"...??[/i]
and it is right to try getting matters clear enough on the issue....

Second..
as suggested by S. Ditmore ....."At http://www.boatdesign.net/forums/sho...354#post112354 Guillermo posted a paper. Appendix G of that paper (page 95) details spar & rigging calculations. How do people feel about what they see there?".......
The spar and rig calculations coming out from that Sot'on Institute project (...it reminds me of good old times....) bears a procedure involving, besides the usual forces acting transversally on a rig, (and generated by a heeling moment), also the longitudinal forces acting on the rig (and generated by the driving force). However, it does not seem necessary the calculation of longitudinal forces, as, the longitudinal inertia could easly be determined in fairly good manner (and giving sound results) by applying to Euler's formula an adequate FS which has been established in years of what is reputed beeing the ........"best practice"...

Third..
Yes...I am interested in a 'rig mapping' exercise. Let me have, if you like it, the proposal....as I didn't see it on C. Mitchell address you pointed it out.

Forth..
Nevertheless I will splash on this forum the main mast and rig calculation for a 36m alu ketch rigged (3 sets of spreaders and built by Abeking and Raasmussen) which clearly and simply shows the computational procedure used for compression loads and shrouds tension calculation based on the following steps:
1. Knock down by main + genoa (assumed)
2. max RM at 30° (assumed)=HM= F(Wind Pressure * Sail Area) * H(distance CE to CLR)
3. determination of the WP [kg/m^2]
4. determination of the main sail load per unit luff
5. determination of the genoa load on mast head
6. determination of indirect loads acting on the rig
7. determination of shrouds tension
8. determination of mast panels compression loads
9. determination of spreaders loads
10. determination of both tranverse inertia (Ixx) and longitud. inertia (Iyy) of main mast section
(as seen....the procedure involves the wind pressure...acting on sails...!).

Besides the calculation which I will enclosed ...and time allowing...I will try to enclose a spreadsheet for the above procedure....let's see....

It would the be nice to share some others computational procedures and relative results regarding a job already performed by a good willing fellow...nice if shared in this forum....but perhaps I'm asking to much....am I?

[brian eiland]

Originally Posted by yades

Yes...I am interested in a 'rig mapping' exercise. Let me have, if you like it, the proposal....as I didn't see it on C. Mitchell address you pointed it out

I'll be back to you in a day or so. I'll send you a private message or email.
Brian

[brian eiland]

Originally Posted by yades

Third..
Yes...I am interested in a 'rig mapping' exercise. Let me have, if you like it, the proposal....as I didn't see it on C. Mitchell address you pointed it out.

Hello Yades,
Please send me a private email so I might forward a proposal to you. Send it to info@runningtideyachts.com if you would

[catsketcher]

Hello Yades

Thanks again for the mast compression stuff. When I worked out the loads using the Skenes formula I came up with a greater second moments of area required athwartships than fore and aft. This is because the factor is less for fore and aft than sideways. Should this not be the other way around?

cheers

Phil Thompson

[yades]

I've enclosed herein mast sections and dimensions extracted from Whale Spars Catalogue as well as the details referred to section WS307. It clearly shows all relevant characteristics as well as the inertias Ixx on the xx axis (running along the centre line...fore-and-aft...so to speak..) and the Iyy running transversally, along the yy axis (...athwartships...as you mentioned).
As shown, Iyy is more or less 2 or 3 times, the inertia on Ixx (in all cases !!).

Trust this clears matters further......

[catsketcher]

I don't mean to be obtuse and I very much value your work. It has helped me a huge amount in my design but I seem to find an inconsistency between your own method and the Skene's C factor.

In your own method you do end up with the Ixx less than Iyy (as it should be) However when I use the Skene formula 1.422 C P L 2 I come to a conundrum.
By having a greater C factor (single spreader deck stepped - 16.2 ) for the athwartships than the 8.9 C factor for longitudinal I get more I required athwartships than longitudinal.

It is probably just a matter of definitions but I wold like to properly understand my misconceptions.

cheers

Phil Thompson

[yades]

thanks yr punctual comment.........

as a matter of fact i have extracted the table under question from the book....."Sailing Yacht Design - theory; edited by Claughton, Wellicome & Shenoi" - mast and rigging design pag. 205".
It seems like the decimal place regarding the values of coefficient "C" is somewhat misplaced for the units used. I have revised carefully the table on the doc previously enclosed (considering the same subdivision used on the book and updating with data partially extracted from other papers - namely from E.G.Van De Stadt coefficients used for alu masts only) and now it seems being ok!
Check it out.....and see if it works satisfactorily.....

Will also revert shortly on this thread with the complete loading and inertias computational procedure performed with a spreadsheet, for a 36.50m loa ketch main alu mast three spreaders rig arrangement, assuming knock down from main sail + fore triangle......

[MAINSTAY]

This ExcelSS was posted in Aft-mast thread for other purposes. But it calculates the compression in a masthead rig due to forestay, backstay and halyards using only I, J, L(modified), forestay tension and halyard tensions. The bugger is determining the tensions.

Ignore the two rigs on the right.
Larry Modes

[brian eiland]

Please help clarify a few points about the B & R rig

...from one website...

The Bergstrom & Ridder:
The B&R rig takes the swept back spreader a step further. The angle on this rig is a massive 30 degrees. The idea behind the rig was to contain rigging loads as much as possible within the mast structure itself and avoid loading the deck and hull. This allowed builders to make lighter boats and not have to reinforce deck and hull structures for strength, reducing construction costs. This also allows builders to use a lighter mast section, reducing the cost of the rig. More recent developments of the rig includes reinforcements by incorporating rigging struts between chainplate and the gooseneck. The purpose being to distribute the compression loads on the mast reducing the amount of reinforcement the deck needs to take the download pressure of the mast.


The big advantage of this rig is that it allows more roach in the leech of the mainsail increasing sail area for better downwind performance. The swept back spreaders also give a great amount of fore and aft support to the mast, eliminating the need for a backstay.

The downside to having spreaders swept back in excess of 25 degrees is that it creates too narrow of a shroud base, which means the mast gets less lateral support and the shrouds have to pull harder to keep the mast standing. This creates enormous load pressures.

Are there not some conflicting statements here?
1) With more spreaders, isn't the summation compression loading to the mast base increased?
2) With a narrower shroud base, and the shrouds having to double for backstays, isn't the compression loading to the mast increased?

________________________________________________________

....from this site

Reverse diagonals (RD) are used to induce pre-bend
compression, adding rigidity to the mast section. This
negates the need for a baby stay or inner forestay.
• Runners and backstay are not usually fitted as the
spreader sweep angle allows the cap shrouds to provide
the necessary longitudinal support.
• Inner forestays and baby stays are never used.
• Sometimes fitted with fixed struts which stay the lower
part of the mast.
The absence of a backstay reduces the mast compression on this type of rig in comparison with conventional rigs.
This, along with any fixed struts, means that the mast profile is often relatively small both athwartships and fore-and-aft.

Advocates of the B & R rig maintain that its “better aerodynamics” make it suitable for racing, and the leisure sailor benefits from avoiding trimming the rig while sailing. The lack of backstay and runners means that there are no adjustments to be made at sea. The foredeck is free from baby stay and inner forestay, and this makes tacking easier.

Most of the trimming of the rig must be done before the mast is stepped on the boat.

My reading is that with the multiple spreaders and reverse diagonals, it is a goal to contain much of the mast compression loads within the mast section itself...even those that might arise from the more conventional use of backstays. Normally this would predispose that the mast section needs to be larger and heavier than normal to absorb this increased compression loads. Diamond staying by the reverse diagonals must be particularly high.

But they say they can use a smaller mast section, and a lighter mast section.??

I can see where the use of multiple spreaders would allow it to stay in column more easily, and thus allow for a smaller mast section, but wouldn't the increased compression loading call for a heavier mast section?