How many masts are practical in a 20-30' blue water cruising boat?

Discussion in 'Hydrodynamics and Aerodynamics' started by misanthropicexplore, Jul 6, 2018.

  1. misanthropicexplore
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    misanthropicexplore Junior Member

    If it were a single master, we'd maybe have a 240 sq ft of sail.

    So what happens if we design it as a 2 120's? Or 3 80's, or 4 60's?

    We lose the airspeed higher up, so that's a penalty, but we gain a lower center of pressure, and can use less ballast, so that's a gain.

    We lose simplicity in the sense we have multiple masts/lines, but gain simplicity because we can use simpler individual rigs (Like a dinghy rigs).

    We add more weight with more parts, but decrease the weight of reinforcement for the masts since the hull can take more of the lighter, more distributed loads.

    We lose speed running, but should gain speed on broad reach, due to a higher point of sail.

    Obviously there is limit to this, but a lot of cruisers swear by 2 masts. Why not 3 or even 4 and use dingy rigs?

    Is it really just as simple as lower speed due to a lower sail or is there more?
  2. JanAF
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    JanAF Junior Member

    Designing for the same sail area with, say 4, masts as with one, would (still) require quite tall masts, and lot's of standing rigging. Some simple sketches would show that.

    Masts and rigs are dimensioned to take knock-down winds, essentially based on the maximum righting moment of the boat. With multiple masts, you'd better make each end every mast strong enough to take knock-down winds, for cases where only that one mast was carrying sails. The wind resistance of such a rig would most likely degrade the upwind performance, in addition to weight and cost.

  3. fastsailing
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    fastsailing Senior Member

    misanthropicexplore, in my opinion you are mixing 2 separate issues. The trade off between mast height and sailarea for a given righting moment = heeling moment is one thing, and the number of masts have nothing to do with it. You can select the former to be what you want no matter what the number of masts is.
    Therefore you should do one comparison between different amount of masts for the same mast heights to compare windage and weight while keeping aerodynamic efficiency the same as well as total lift force in any given wind conditions.
    The number of masts might have an effect on the size of a single mast if you do not make it strong enough to carry all the heeling moment on its own. In that case releasing sheets when a gust hits, but not doing it at the same rate for sails in all the masts would result the mast without sheet release to become overloaded and brake up. When doing it properly you will found out multiple masts with the same height of the single mast they are replacing have far more windage (both mast and its rigging) and are also heavier. Look up how buckling- and bending- and torque- strength scale up with size to understand that. Any weight saving in the hull won't compensate weight increase in the masts & rigging, and weight high up is far more important *) even in the case total weight would be the same.
    *) for righting moment as well as moment of inertia in pitch and heel which have an effect on seakeeping and motion comfort in addition of affecting max allowable heeling moment.

    Then make another comparison for trade off between mast height and sailarea to optimize that for the points of sail you are interested in, or all of them with adequate weight factors.
  4. misanthropicexplore
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    misanthropicexplore Junior Member

    Everything you're saying is correct, but it's puzzling to me, maybe I'm just looking at it from the wrong perspective.

    You have a boat, and design it with one mast. You make the mast as high as money and safety will allow for that particular boat, to catch highest winds and carry the largest sail.

    Keeping the same basic hull and ballast you decide design it for two masts. You keep the mast height the same, and double the aspect ration while halving the sail area per mast....but why would you do that? Why would you take a boat that worked fine with 1 mast and design it for 2? Why would you bother to add a second mast if you don't use it as an opportunity build a wider, lower sail plan, with smaller easier to handle sails, and lower tension loads, then possibly because of physically lower healing moment, less ballast to offset the additional weight of an extra mast?
  5. MurphyLaw
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    MurphyLaw Senior Member

    If you have a keel underneath each mast and you have 2 masts then the torque from one mast cancels the other out. Big benefit with 2 keels in the water, not with just one keel.
  6. Blueknarr
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    Blueknarr Senior Member

    That same reason quad- planes don't work. As airfoils are added, they start to negatively interfere with each other.

    Most efficient one last two sails. 30 footers are not large enough to warrant dividing sails for handling purposes.
  7. CT249
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    CT249 Senior Member

    You wouldn't do it because it doesn't work. Have you ever seen an airliner with ten wings of 20ft span, one after the other? No? So why not?

    Perhaps the other way to look at is to question why it would work? Okay, so you have a wider, lower sailplan - so what? Aerodynamically, what's going to happen? I'm not an aerodynamics expert, but as I understand it basically an airstream will encounter the foremost sail, and will be turned by it. That turning will develop forward thrust.

    So what happens then? How does the next mast deal with this turned flow? How will it develop an angle of attack, after the first mast has already turned the airflow so that it is already basically running closer along the centreline of the boat? How will the second mast develop lift from an airstream that is often thrown into turbulence by the first mast?

    If the second mast is struggling with a higher angle of attack and dirty wind, how will the third mast fair?

    Is it easier to handle four small sails than one big sail? Why? Why is it easier to physically pull on (say) four sheets each developing X power than one sheet developing X x 3 power? You still probably need to uncleat the sheet, wind it in on a winch and cleat it off again, however with a multiple mast rig every time you adjust one sail it will affect the airflow over the rest of them, so all the other sails need adjustment. Is it easier to use a $3000 winch to adjust one sail than to use three $2000 winches to adjust three smaller sails?

    Where will the sails sheet to? Do you have three travellers instead of one, with three traveller cars, three sets of traveller pulleys, three sets of traveller cleats and line? Do you have three jib cars, with three sets of tracks, three sets of holes to drill and seal, three sets of bolts to fit? How do you lead all three sets of sheets to one place where they can be trimmed?

    Will the multiple masts have lower heeling moment? Why? You now have three spars, all of which must have walls thick enough to withstand buckling loads. You have three times the stays, three times the halyards, and more sail area. Where is the evidence that this is all going to create a lower c of g than one taller mast? And in a rising wind, how easily and quickly can you depower three rigs? How much time and cost does it take to build and handle three systems to reef or dowse sails rather then one? How much do such systems weigh?

    Arguably, there's two ways of looking at these issues. One is to ask what would happen if designers created a different design. The other is to ask why designers created the design they did create. As far as I know, and I've done a lot of talking to designers, you learn a lot more by assuming existing designs work well and trying to puzzle out why they work well, than by assuming existing designs are crap and trying to work out why they are crap.

    Having been lucky enough to talk to many great designers, it seems to me that good designers tend to say "John Smith created a really good boat" and then provide specific information about why it went well in specific conditions, and what tradeoffs were involved. They then use that positive information to create even faster boats. They are not blind to the design's problems, but they are generally positive. The people who imply that all current designs are crap tend to just stay on the internet.
    Last edited: Jul 21, 2018
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  8. sharpii2
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    sharpii2 Senior Member

    The only time multiple masts (more than two) makes sense is when the boat happens to be a very narrow monohull (1:5 Beam Length ratio or less). And this is when said monohull cannot have a deep, heavy ballast keel. In this case, the maximum number of masts should not be more than four. At the turn of the 20th century, schooners were the last sailing vessels built in large numbers. Most had two or three masts, but a good number had four. A few five masted ones were built and even fewer six mast ones were. As far as I know, only one seven mast one was built. It proved very disappointing.

    The first and formost price of having multiple masts is windward performance. This is especially true if the sails on this vessel are of more or less equal size and similar shape. Vessels with this type of rig have trouble doing better than a close reach.

    Vessels rigged with dipping lugsails or Chinese lugsails will often have three masts. This is when they don't use jibs. And most often two of the three sails are smaller than the largest sail.
  9. Dolfiman
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    Dolfiman Senior Member

    More than two masts seems really not relevant for that size of boat (< 30 ft).

    But small boats with 2 simple masts without rig is a market explored by "alternative" builders like M.E.R., yet more oriented day sailer than blue water able to cross an ocean but perhaps some ones could be reinforced into blue water configuration. An approach made possible thanks to carbon masts, their flexibility is used for the sail design and made them resistant without rig, + possibility to reduce the surface by rotating (although the sail efficiency is then not optimal for upwind sailing). Here their book :
  10. Dolfiman
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    Dolfiman Senior Member

  11. sharpii2
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    sharpii2 Senior Member

    Up until the invention of self-steering windvanes, two masted rigs were quite popular for blue water cruisers. Typically they were ketches and yawls, with both masts well secured with shrouds and stays. With the rig spread out over three sails, it was easier to get the boat to stay on course.
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