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 :cool:

[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.,,,,,

:cool:

[brian eiland]

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/showpost.php?p=34894&postcount=38

[yades]

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

[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?
:cool:

[brian eiland]

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]

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......




:cool:

[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.....:D

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......

:cool:

[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 (http://marina42.net/cgi-bin/p/m42p-custom.cgi?d=passage-yachts-inc&id=615)...
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 (http://pdf.nauticexpo.com/pdf/selden/hints-and-advice/Show/21696-6227-_52.html)
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?

[RHough]

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

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?

The number of spreaders and shrouds does not change the compression load on a straight mast. The compression load is a function of righting moment and shroud base.

I have read that the B&R rig has a narrow shroud base ... this is not correct, or the rig is not a B&R. B&R rigs use small, non-overlapping jibs. Inboard shrouds (that serve to increase compression loads) are not required. Since the shrouds can be at the gunwale, the shroud base is wide, not narrow.

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?

One of the goals of a B&R rig is get a pre-bent mast without adding the pre-bend load to the deck compression load.

One reason that a smaller section can be used is the B&R rig creates shorter panels in the rig.

Another reason is the pre-bend. Think of a bow. Unstrung, the bow wobbles and flexes. When the bow is strung and pre-bent it does not wobble.

It is a slight miss-representation of the B&R to say that it reduces the mast step compression loads compared to a conventional rig with a straight mast. The lack of a backstay does not reduce the compression load, since the 30 deg aft sweep of the spreaders provides the forestay tension. There should be no net reduction in compression load.

An interesting idea would be to build a B&R rig with a rotating mast ...

[yades]

I have enclosed the spreadsheet for the Main mast compression and rigging loads for an aluminum built 36,50 m LOA ketch. Trust it could be of somewhat interest.;)
Any remarks and/or suggestions for improvements are more then welcomed.
:cool:

[brian eiland]

...I have read that the B&R rig has a narrow shroud base ... this is not correct, or the rig is not a B&R. B&R rigs use small, non-overlapping jibs. Inboard shrouds (that serve to increase compression loads) are not required. Since the shrouds can be at the gunwale, the shroud base is wide, not narrow.
I agree with you here, the B&R rig does NOT have a narrower shroud base. It makes you wonder why several web site references discussing the B&R rig repeat this error? Maybe it’s the same phenomena that kept promulgating the incorrect explanation for the ‘slot effect’ for so many years??

I brought this ‘narrow base’ subject up for another reason. I’ll begin by quoting Chris Mitchell of AES, Applied Engr Services of NZ (http://www.aes.net.nz/), “Rigs with No Backstays”;
Rigs with no backstays are not so common. Most small dinghys under 20 feet have no backstays and no runners and no checkstays. They do however normally have some 30 to 35 degrees of spreader sweep. At larger sizes engineering requirements do not work out terrible well. The sidestays have to compensate by generating the fore/aft loads also, generally from a fairly small staying base in a fore/aft sense. Sailing up wind the rig can manage without the sidestays being enormous, given 30 degrees of spreader sweep and chainplates on the gunwhales.

Now if I take the drawing (attached) for a B&R rig from this site
http://pdf.nauticexpo.com/pdf/selden/hints-and-advice/21696-6227-_52.html
…and I search for this ‘effective fore/aft base’, I come up with this attached sketch impressed upon the original drawing. In a fore/aft sense the ‘backstaying portion’ of the shroud is working at a 12 degree angle measured with respect to the mast….pretty shallow for a backstay.

How did I arrive at this 12 degree angle? Looking at the upper spreader zone, a spreader raked at 30 degrees (normally quoted for the B&R rig) results in an actual spreader length of dimension “A”, and thus the effective aft reach of dimension “C”. Viewing this from the side view, we see an effective aft angle of 12 degrees.

This shallow backstay angle was always a concern of mine when evaluating the conventional 3-point staying arrangement of many multihull rigs. Here we often see a 3 legged staying arrangement of one forestay and two ‘shroud-backstays’ splayed out at 120 degrees to each other. Interestingly we might even draw an analogue between the multihull rig and a ‘spreaderless B&R rig’ as you might imagine from a portion of the description at this website:
http://kobernus.com/hunter260/rigging/rigging.html
A three-legged stool is more stable than a four-legged one. The B & R rig has the same approach. To accomplish this, the rig utilizes 30 degree swept-back spreaders creating 120 degrees between each rigging point. This tripod arrangement is similar to the huge radio towers you see from the highways.

Both the 3 point multihull rig, and a B&R rig, are taxed with maintaining reasonable forestay tensions under the burden of shallow-angled backstays.

[brian eiland]

.....In a fore/aft sense the ‘backstaying portion’ of the shroud is working at a 12 degree angle measured with respect to the mast….pretty shallow for a backstay.
So shallow backstay angles are workable if we consider the number of B&R rigs and 3-point multihull rigs out on the water.

Rather interestingly these shallow backstay angles are very close to same magnitude as that shallow backstay on my aftmast rig. From a recent posting of mine:
http://boatdesign.net/forums/showpost.php?p=221559&postcount=110
.... let me point out a couple of critical items I see in my aftmast rig design:
2) Hounds Loading: Here is were I will experience some problems. As drawn at present There is only a 10 degree angle between the lower backstay and the mast....thus significantly higher compression loads to the mast. But if I attach this backstay to the front of the mast and run it to the very sterns of the vessel I get a 15 degree angle...much better. And if I chose a 6 degree rake for the mast rather than the 10 degrees shown, things change again...likely for the better. This is a portion of the 'stress mapping' I am seeking, and this is Chris' forte.

Now before someone jumps down my throat, let me say that I do realize that my shallow backstays are working against larger forestay angles and loads, and this puts them at a greater disadvantage The point I sought to make was that shallow-angled backstays by themselves should not a sole argument against my aftmast rig concept.

Here is an interesting B&R style rig on a Tennant catamaran. The adaptation of this concept combined with my mast-aft rig might produce an interesting arrangement. I’ll think about this, and maybe sketch it up in the future

[brian eiland]

As noted in the paper from AES, Geometry:Cap-Shroud/Chainplate (http://www.aes.net.nz/Rig%20Design%20Commentary.html)
Most sloop rigs which use rod rigging use a cap shroud angle of 10 degrees to the mast wall. This is normally considered a state of the art angle and it usually permits a reasonable compromise between sail sheeting and rig stiffness. Similarly, the chainplate angle for an overlaping 140% genoa on an inline spreader rig is normally about 13.5 degrees from the forestay (forward end of J). This defines the most common chainplate position. With the V1 shroud vertical and the cap shroud coming from the hounds at 10 degrees it then remains to obtain a modest spreader envelope to generate even spreader pokes on each spreader; kind of like joining the dots. For 110% genoa and swept spreaders on the gunwhale a whole different approach is required. However, it rarely pays off to have a cap shroud angle greater than 14 degrees. QUOTE]

I brought this subject up here because, as Chris points out:
[QUOTE]If the cap shroud angle that is too large this can actually lead to some torsional instablity which has been seen on carbon rigs. Carbon rigs have a fairly low shear modulus and are prone to excessive twisting, which can be annoying or unsettling in some cases.

It would be interesting to look at this subject of cap-shroud angles on the B&R rig and how they might affect the upper mast sections of these rigs, and particularly those that might be constructed of carbon fiber material.

From my sketch (http://www.boatdesign.net/forums/attachment.php?attachmentid=24490&d=1219615867) at posting #18 above note that the effective cap-shroud angle appears to be 18 degrees, while the actual angle the upper shroud makes with the mast tube is about 22 degrees. Both of these angles could result in significant torque loads.

[RHough]

I agree with you here, the B&R rig does NOT have a narrower shroud base. It makes you wonder why several web site references discussing the B&R rig repeat this error? Maybe it’s the same phenomena that kept promulgating the incorrect explanation for the ‘slot effect’ for so many years??

Both the 3 point multihull rig, and a B&R rig, are taxed with maintaining reasonable forestay tensions under the burden of shallow-angled backstays.

Yes, that Seldon mast tuning guide is one of the best. ;)

You are forgetting something ... a few things actually.

First hint, the B&R was designed for dinghies to reduce deck loading at the mast step an still have mast pre-bend adjustment.

Second hint, how do beach cats and dinghies maintain forestay tension?

Third hint, when is high forestay tension needed?

What you are missing is that the backstay or stays on a fractional rig dinghy or beach cat don't provide the forestay tension.

The mainsheet and the mainsail leach provide the forestay tension.

You cannot consider a B&R rig without considering the mainsail.

When high forestay tension is needed, is upwind in a breeze ... when the mainsheet is on hard. Off the wind you want the forestay tension to be reduced to power up the jib ... the main is eased and automatically reduces forestay tension.

Upwind in breeze, the unsupported mast above the hounds can flex and de-power the main without also reducing the forestay tension and powering up the jib just when you don't want it to.

Masthead rigs do all of this backwards. The backstay must be on hard and the mainsheet load adds to the forestay tension, since the main has no built in gust response, the sheet must be eased or the traveler let down. Either action reduces the forestay tension to some extent and powers up that masthead Genoa at just the wrong time. Off the wind the backstay tension has to come off to power up the Genoa. A masthead rig requires multiple adjustments to get the same results as a simple and natural mainsheet adjustment on a fractional rig, B&R or not.

I think the term B&R Rig is abused, it has come to describe any rig with reverse diagonals. At one time I think there was one builder that had a "B&R" with a conventional backstay! No way. If the rig does not have the same dynamic response as a true B&R it shouldn't be branded that way. ;)

[brian eiland]

First hint, the B&R was designed for dinghies to reduce deck loading at the mast step an still have mast pre-bend adjustment.
Is this really true? I do know this style sweep spreader rig is utilized on many small vessels, but I don't think the actual B&R rig was developed for them. Wasn't it an effoert for Warren Luhr's ocean racing boats??

Second hint, how do beach cats and dinghies maintain forestay tension?
I do know this having raced a lot of small cats.

What you are missing is that the backstay or stays on a fractional rig dinghy or beach cat don't provide the forestay tension. The mainsheet and the mainsail leach provide the forestay tension.

When high forestay tension is needed, is upwind in a breeze ... when the mainsheet is on hard. Off the wind you want the forestay tension to be reduced to power up the jib ... the main is eased and automatically reduces forestay tension.
But how about crusing catamarans, not just light weight vessels. They aren't sailed with their mainsheet strapped in this tight (In some cases they aren't sailed with their mainsails even hosted).

In many cases these cruising vessels are not sailed with their mains tightly loaded, but rather with the top of the main twisted off, and the mainsheet 'at-the-ready' to dump more wind from the top of the mainsail, particularly in heavier air.

I do understand where you are coming from with your answers, and I appreciate and welcome your viewpoints and input, but I believe you are more into the lightweight racing mode of rig design than I am trying to expound for more heavily loaded cruising vessels.....and that's another reason I've sought out a lower aspect, multi-element rig rather than the more efficient hi-aspect sloop or uni-rig.

In some cases rigging schemes that work for lightweight vessels don't always translate over into cruising vessels, and vice-versa....at least that's what I am realizing more and more everday.

[yades]

Thanks to Glenn Ashmore for pointing out the bag on the spreadsheet. :D
Have duly revised it and splash it again on the forum. Would be pleased to receive comments suggestion so as to ameliorate it, as done.
:cool:

[prof roberts]

Thank you Yades and Brian Eiland for your analysis. Yades, your PDF file is most helpful. I have designed and am building a 60 ft catamaran and in a year or so I will attempt to build a rotating carbon fibre wing mast with a 450 mm chord. My working number for mast compression is 100kN.

By the way there is an excellent 3D structural analysis program called Mutliframe. You can draw all the vectors in Acad and export a DXF file to Multiframe.

[MikeJohns]

And here for interest is GLloyds rig design method.

http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-7.html#post277211

http://www.boatdesign.net/forums/attachments/sailboats/32123d1243647285-sail-loading-rig-rig-loading-vessel-germanischer-lloyd-mast-rig-yacht-under-24m.pdf

[prof roberts]

thanks for that I will study it carefully

[prof roberts]

It is very generous of you Yades to publish all those lovely rig calculations on the spreadsheet. You use wind pressure of 14.29 kg/sq m at 27 knots. From Skene I have been using 5 kg/sq m at 14 knots and four times that for 28 knots. Have I been too conservative using 20kg / sq m.