Mast Compression Loads

[RHough]

Originally Posted by brian eiland

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.

[brian eiland]

Originally Posted by RHough

...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, “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...-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]

Originally Posted by 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/showpos...&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
[quote]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:

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 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]

Originally Posted by brian eiland

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]

Originally Posted by RHough

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.
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.

[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.

Sail Loading on Rig, Rig Loading on Vessel

http://www.boatdesign.net/forums/att...-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.