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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction
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  #1  
Old 02-21-2008
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Mast specs....

Okay, I have a pretty good idea of what the general answers are, but am curious about specifics. I've seen quite a few mast specs recently, and am curious as to the determining factors that establish mast construction and strength requirements. Specifically, I have seen boats of approximate parity in size (overall, and rig) that have different masts. Even on RigRite, they have cruiser masts and high performance masts. The cruiser masts tend to be larger in overall diameter. The high performance units tend to be smaller in diameter, and many have variable wall thickness. For example, take a mast, essentially a flattened oval shape 8 inches by 6 inches, continuous wall thickness .18 inches. Compare it to 6.5 inches by 4.5 inches, essentially a teardrop shaped mast with the narrower part forward. This mast design has .25 inch ribs internally that taper down to a minimum wall thickness of .18 inches. They run lengthwise, forward and to either side. The differences in weight per foot are less than .2 lbs, the performance mast being slightly lighter. Ideas on strength, fatigue resistance? Does ship weight factor in, or mainly the rig stresses? Anything? The masts in question are from a Bristol 32 (cruiser mast) and C&C 29 (high performance). Aside from a shorter boom, the other rig specs are within a few inches. Granted, the Bristols tended to be overbuilt, but is it that much stronger than the performance C&C mast design? It seems to me they'd be pretty close.
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Old 02-21-2008
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High-performance masts are smaller in diameter but take a lot more rigging to keep them upright and in column. The reason they're so much smaller is that they're designed that way to help keep the air flow over the sail better since most monohull racing rules forbid the use of rotating wing masts. The high-performance masts will suffer more from fatigue issues, since they're often lighter extrusions, but that's generally okay, since the rig on a racing boat is generally considered a consumable as part of the cost of racing.
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Yeah, makes sense when looking at cruisers vs real competition boats, but the C&C 29, well, I mean it's not really a race boat, per se. I mean, not in the real competition sense....it's pretty much a sporty cruiser, and not disposable in that sense. So, do you think the thicker inner ribs help compensate in strength for a smaller diameter? The minimum wall thickness is the same for both masts.
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Old 02-21-2008
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When I worked in naval architect offices, we usually ran calculations for masts based on empirical formulas that have been around for a long time. Stability played a role but if I remember the formula correctly there was a simple surrogate formula that added increased load factor to the formula that compensated for higher stability. While today there are sophisticated computer programs to size spars and rigging, They really would not be necessary if you are simply replacing the spar on your 1960's era Seabreeze.

If I remember correctly Skene's had a pretty simple chapter on spar design.

As to making a decision between different spars, assuming that you are not changing your shroud attachment points or your spreader positions, the key numbers that you will want to pay attention to are the axial area of the spar, and its moment of inertia. Masts, especially in masthead rigs like yours, take enormous compressive loads and the axial area is the determinant of the axial load that the spar can take if in column. They also are subject to bending forces, and the moment of inertia refers to the stiffness of the spar, and so its likelihood of remaining in column.

For the same rig geometry, smaller dimensional spars are not usually lighter than bigger sectional spars since it takes more wall thickness to achieve the same necessary stiffness. Weight savings can only be acheived with smaller dimensional spars when the rig geometry is adjusted to reduce panel length so that the required stiffness can be reduced and perhaps spar tapering can be employed.

If you want to wipe out your beer supply and wear out your thinking chair you can also use the formulas from German Llyods:
http://www.germanlloyd.org/infoServi...h/abschn01.pdf

Whatever you do, you are probably looking for a pretty conservative design. If worse came to worse, you should be able to determine the area and I (moment of inertia) of you old spar by looking up its dimensions and wall thickness at Dwyer or Kenyon, since most of those old spar shapes are still in production and use those numbers to make a judgement call on which spar section best suits your goals. (BTW within the constraints of conservatism likely to be employed in designing a spar for a cruising boat, fatigue should not be an issue since the safety fators are much higher than the sophisticated formulas used with high performance oriented computer aided design programs.)

Respectfully,
Jeff

Last edited by Jeff_H; 02-21-2008 at 03:46 PM.
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1960's era? So what ya sayin' Jeff? Huh? Huh? Huh!!! Wanna start somethin' punk? Come on. Come on. Start somethin'!!!!


Kiiiiidddinnnnng. ;-)

I'm not looking to re-invent the mast. Curiosity. I just get into these situations where I have limited resources to figure these things out. With all the net has to offer, I haven't found something helpful that says, this is mast design, and this is why it's done this way. The link you posted looks to help fill in some blanks. As you mentioned, (and I'm in the ballpark with this), the rigs tend to be conservative and understressed (relatively speaking) on cruisers. I just have that voracious need to understand why something was done, why it changed on other newer designs, what's good and bad, how said changes affect things. etc. I see the different designs and start wondering....why? Thanks.
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I think that aluminum spar design evolved from wooden spars at a time when the underbody design of boats were the limiting factor in their pointing abililty more than their rig design. Narrow spreader widths and narrow shroud bases would not have particularly helped the pointing ability of the boats of the day.

By the same token, the racing rules of that era over-penalized fractional rigs making the masthead rig the dominant rig of the era when spars were trasitioning from wood to aluminum.

Comparatively speaking, masthead rigs operate at very much higher compressive load and that places a premium on stiff spars to take those loads without buckling. With the wide shroud bases and need for large cross sectional areas, and given the aluminum alloys available in the 1960's, a deck stepped mast with a two panel- single spreader configuration, with fore and aft lowers to reduce pumping was a very good choice.

But as underbodies became more efficient, allowing boats to point higher, rigs began to evolve as well. And with the advent of lower stretch sail cloths it became easier to design around higher aspect sail plans and expect more reliable shape holding. But that then placed a premium on somewhat contradictory objectives, precision control of spar bending and headstay tension, combined with narrow spreader/shroud base widths.

If the designers had stuck with two panel design but had narrored the spreader/shroud base, the spar would have been subject to either a lot more weight, bigger stiffer sections and/or a lot more flexure which makes mastbend/headstay sag control far more difficult.

The logical solution was multi-panel design, which through the miracle of continuity reduces moment and deflection while permitting a narrow rig width, albeit at the price of complexity.

I come back to one of my frequent stated sentiments that good boat design produces a design that works well as a system. Your rig works well as a system with your boat's underbody.

If you could reduce the weight of your rig, your boat would be more stable and could carry more sail into a higher breeze which would improve the windspeed range of your larger headsails. That increased range could result in a small performance gain in changeable conditions.

Coupled with low stretch sail cloth, a lighter rig could also improve windward ability at that windspeed range where sail cloth stretch and leeway due to heel come into play. Taking advantage of the stability increase from a ligher mast, you might be able to increase the length of the mast a little resulting in a better light air performance and perhaps reduce the overlap on jib for increased ease of handling and efficiency.

But in the big picture, the gains would be small compared to the gains that would occur on a boat with a more efficient underbody where there could be big gains from a more efficient rig.

Respectfully,
Jeff
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Yeah, but every little bit helps. If it were a fringe benefit from a rig change, instead of an actual goal of the swap, so much the better. My main concern is the strength issue, but from everything I've seen so far, the newer, slightly smaller design is up to the task, perhaps more so than the original.
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Old 02-22-2008
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You need to run the numbers but, but in a general sense there are a wide range of choices that should be structurally sound. You should be able to taper the top of the spar which would be one of the bigger gains that you might achieve with a multi-spreader design. I should also point out that many of the better spar builders have pretty sophisticated design programs and will include the design of a suitable spar in the price of the spar.

Jeff
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On the spar in question, I contacted Southshore Yachts in Canada. They handle a lot of the C&C stuff, and they said the spar was a C&C design. I was asking about the step, etc, but after a couple polite exchanges, they ended with, "Cheers." A polite kiss off. Anyway, I've seen similar versions on other sites, but none quite like this one. I know C&C was big on pushing the line in design and materials use. Agreed, it'll take more digging to learn the rest. Considering that it was C&C and so much was changing in boating designs, I have a good feeling about it in general. Any online C&C you know of that might reveal some info? I haven't seen much so far.
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Maybe I am making a mistake here, but from your nom-de-net I have always assumed that you had a Seafarer Seabreeze. Is that not the case?

Back in the 1960's and 1970's there were relatively few spar extruders. At the time that most American and Canadian spars came from the company that became Kenyon http://www.rigrite.com/Spars/Kenyon_...ast%20Sections and from Dwyer http://www.dwyermast.com/families.as...cat1Name=Masts. Both were mast fabricators as well as selling extrusions to other rigging shops.

Around here companies like Atlantic Spars and Rigging http://www.atlanticspars.com/mrig/default.asp and Chesapeake Spars http://www.chesapeakerigging.com/main.html do projects like your pretty routinely.

Jeff

Last edited by Jeff_H; 02-22-2008 at 11:53 AM.
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