Low Verses High Aspect Ratio Main Sail

Discussion in 'Boat Design' started by viking north, Feb 5, 2011.

  1. viking north
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    viking north VINLAND

    In planning ahead for my present build which will be sloop rigged, I have to make a decision. Sometime back i purchased a damaged 34ft. wooden sailboat.(Barkhouse motorsailer). It came with a new set of sails(Stevens sails, Chester) The marconi main's 16ft., foot was bolt rope mounted on a 17ft. boom and the 28 ft luff slide mounted on to the mast. Two reef points with 3 battens. Definately Low Aspect. My new build 27 1/2 ft overall length, 71/2 ft beam, Full keel(Brewer bite skeg) will have approx. the same height mast but in alum. not wood. I would much prefer a little higher aspect main, say with a 12ft foot.I would like to make use of this sail but have to make one of two choices. Recut the leach which effectively moves the pocket further aft(maybe not good). or set the boat up for it as it is, wear it out, then re position the mast and re rig for a higher aspect main.(not my preference) While this build will not be in the racing class by no means it still will be fairley good performance as a cruiser/motorsailer goes. I want to get the best out of her. Which lesser of the two evils should i choose, :confused:
    Tnx. Geo.

    A yacht is not defined by the vessel but by the care and love of her owner.
     
  2. messabout
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    messabout Senior Member

    If you intend to have similar area then the 12 foot foot will need 37+ feet of luff. That is pretty tall for a boat the size that you mention. The tall sail might (operative word might) go to windward a little better than the shorter sail but probably not better on other points. The shorter sail will need less ballast and thus a lighter ship which will tend to make it faster on all points.

    For general purpose I would use the sails that you have and watch some of the boats, with high aspect rigs, vanish in your rear view mirror.
     
  3. Landlubber
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    Landlubber Senior Member

    ...horses for courses...what do you want to use the boat for, cruising in "comfort", racing around the bouys, or something in between.....low aspect means far less mast height, leverage requirements, etc etc, so for sailing long term (cruising), i would go for the low.....
     
  4. PAR
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    PAR Yacht Designer/Builder

    The best thing you can do is talk with your sail maker. I know what he's going to say, but he may have some options for you. He'll know what you can "get away" with in regard to cutting up a sail.

    As far as the rig selection, you need to consider the hull and it's appendages in the equation. You might have a higher aspect rig, but will the hull and appendages be able to do much with them? Making huge changes to gain a couple of degrees of pointing ability isn't what a cruiser, particularly a motor sailor is all about. In most cases the hull just can't take advantage of the loftier rig.
     
  5. michael pierzga
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    michael pierzga Senior Member

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  6. viking north
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    viking north VINLAND

    In this case once again in reverse engineering i can see now i didn't go back far enought to give you guys enought proper info to really get into the subject, but at least now thanks to the education i have and am receiving on this forum i am able to see the error of my ways. First i should have determined if my new build can actually handle the origional sails area without being overburdened. That would take me all the way back to hull shape, Ballast to Disp., then on to Sail area to Disp. ratios. I might have a "given" hull to reverse engineer but from that point on it becomes standard forward engineering of the build.I have all my hull lines taken off but not compiled into standard drawings,(a designer friend now working on this using his computer program).I have keel info,ballast amount,and a final displacement goal, so maybe i can give you enought to get into a little more details.I really don't want to use a 17ft. boom, one reason,i want to attach the mainsheet to the aft. top of the wheelhouse as i have done on other builds.This keeps the cockpit clear with the back of the wheelhouse an ideal spot to hang the coiled up sheets. This attachment point is 8ft.back from the gooseneck and would be impossible on a17ft. boom. Looking over my rough drawings there is a possibility i can accomidate a 14ft. boom. PAR, you have possibly already answered that question on behalf of the sailmaker however if the 14ft. boom is feasible it might put a better light on the picture. Michael, good info, drive, twist, and set on a sail. I didn't realize how big a drop occured in wind pressure per given area occured from the masthead to boom, roughly 50%. Is the 8 deg. windshift typical of all sails regardless of mast height? Ok some specs. on my build and a couple of questions.
    Some Specs(surfboat to motorsailer conversion)
    Hull, Surfboat style lifeboat, LOA 27ft. 6in. Beam 7ft. 6in. Shape very similar to a Banger 37 scaled down, double ender fairely symetrical with max. waterline beam just aft. of midships. Low to med. deadrise with a full keel about to be attached.
    Displacement, completed build, 10,000lbs. min. empty, 12,000lbs loaded.
    Ballast, 3500 to 4000 lbs. encapsulated automotive wheel weights.
    Rig, 32ft, alum. mast deck stepped. Boom 14ft. Fore triangle approx. 180sq.ft.(mast approx. 12 ft back from stem head fitting.)

    Questions:

    #1 Is it feasible if i use an oversize boom extrusion, ovesized mainsheet tackle, and good strong attachments to the boom and wheelhouse aft, roof frame, that i could use a 14ft boom with the mainsheet attached 8ft. back from the gooseneck and 6ft. overhang back over the cockpit.In other words the attachment point is 6ft. in from the boom end on a 14ft. boom. I've done this before on past builds but the attachment point on the boom was only about 3ft. in from the end.
    #2 A 14ft. boom would require cutting 3 ft. off the leach at foot and gradually up the leach.This would still effectively move the pocket aft but if in the process i had the sailmaker enhanse the curvature of the leach as much as my backstays would allow and use full battens on the main would i still end up with a decent sail.?
    #3 I have in my boat fitting collection a new boom roller furling goose neck fitting, whats everyones opinion on this setup.?

    Thanks again for the info. Geo

    A yacht is not defined by the vessel but by the care and love of her owner.
     
    Last edited: Feb 10, 2011
  7. FAST FRED
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    FAST FRED Senior Member

    For most cruisers about 6-1 Aspect ratio is as much as its worth using.

    This is the Airodynamic aspect ratio , not the boom to hoist.


    Span (hoist) squared /divided by sail area .

    Full battens can improve the performance by getting more of the area higher, with no need to increase the area.

    FF
     
  8. viking north
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    viking north VINLAND

    Thanks Fred on the advice of the former posts and in order to maintain good stability with my shallower keel, CG, pretty well limits me to a low aspect ratio system which actually once pointed out to me makes sense. My problem was not knowing the dividing line between the two. I now assume that the closest to a sloop rigged heavy cruiser that would have a high aspect ratio sail would fall into the racer cruiser class, which certainly not you typical full keel heavy displacement vessel. De information is slowely penetrating thru capillary action into de rocks in de head. The more i learn the more i realize this boat engineering is a BIG game of trade offs, probably more critical than most other man made moving vehicles.Every majour decision of a design feature boxes you into a place of limited selections and from that thru the process of wise eliminations one ends up with a functioning product. Very challenging field, i think i'm getting hooked :)
    Ok so it's low aspect. The next question was can i modify my present sail without destroying it's drive, and possibly as PAR suggested, only a sail maker can answer that 100%, however you have answered one part of it, i can make up for some lost area thru leach curviture and full battens.I might be luckey and meet a sailmaker on the forum to give me a complete overview. The boom question would definately be one for the pro's. There must be a design out there that for reasons had to go with an attahment point this far up the boom. The last question of the boom roller furling, if an acceptiable system, would force me to attach at the end of the boom.Possibly future decisions made by another trade off and on and on it goes, consulting time is accumulating, capitalism is working :) :) :) .... Geo
     
  9. michael pierzga
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    michael pierzga Senior Member

    Gee Viking...modifying a sail is normally a fools game...you modify the shape and reinforcement patterns right out of the sail.
    Youre best advised to seek a used sail for your application. A big mainsail that you chop the foot off of, not chop the luff and leach were the shape and reinforcements are. The race boats go thru sails like they were toilet paper. Check out the sailing club in Halifax , the big sail makers or a friendly yacht broker. . Pay no more than one cent on the dollar for used raceboat sails. If the sail comes with a nice sail bag throw in an extra 10 bucks.
    My old sails...still plenty of life left..end up as boat winter covers because no one wants them and I have to PAY to have them disposed. My sailbags get turned into shipyard fender...rubbertire...covers or motorcycle covers.

    Also consider an entire race boat mast , rigging and sail package. This nice gear often gets thrown in the dump , the local shipyard has a stack of ex race boat rigging, piled up, burning in the sun. .The typical raceboat junk come from trailorable 30 footers that got smashed in a trailer accident with only the sailing gear left

    When choosing sail power go.... MAX. You can always thrown in a few reefs and back off the power. . The most enjoyable boats to sail are the ones with good performance in light wind. This means a powerful sail plan.
     
  10. viking north
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    viking north VINLAND

    Yes Michael, wish i had the money to do the 100% correct thing but might have to live with 75% efficiency for now. I am looking around for another main but they are not easy to find in good condition at the correct size. Regular guys wear them out then try to sell them.In the part of Canada that i live in money is tight, we live frugal.The lady who made the sail Michelle Stevens (Stevens Sails) is still very much in production so i will get her input. Thought i'd throw it out to the forum, maybe someone else has done the same and i wanted their feedback. Any ideas on the boom question. Is there a production boat out there that has it's main sheet attachment point 40% up the boom length?
    I am not an engineer but with a given triange of sail area 14ft. boom,(foot) 34 ft mast (30ft)luff that would give 210 sq.ft within the triangle (disregarding) the extra on the curviture on the leach)
    With a steady say 15kt. wind a certain pressure in lbs per sq. ft would be exerted on the sail resulting in an overall sail pressure of possibly tons. Assuming the boat maintains course, the boom then is the only moving part of the rig that is controlling this force.
    With the normal attachment point on the end of the boom there should be a way to calculate the bending force exerted on the booms mid section and the force exerted on the goose neck as well as on the main sheet gear and the attachment points.
    Ok taking it one step further as i want to do and move the mainsheet attachment up the boom 40% of it's length, what happens to the bending forces on the boom extrusion and the force on the mainsheet gear and attachment points.
    Logic tells me there will be less bending force on the boom along it's length as now the overall lenght is divided but logic also tell me there will be more force on the mainsheet and it's attachment point(the lever has gotten shorter)and resultant shock loads will effectively try to break the boom at the attachment point, as breaking a stick over your knee. So to offset this will i need a stronger boom.(thicker wall or bigger extrusion)
    What i need to know as i can't find any graphs in any books on this, is
    #1 What is the origional loads on the main sheet and attachment points and the % of increase by moving the attachment point 40% up the boom. From these info i can determine origional rope and pully size and attachment fittings and with the % of increase step them up to handle the loads.
    #2 If possible the % of increase in shock loading if significent enought to warrant an increase on boom strength.
    I realize we are now getting into stress engineering as on aircraft and i don't know if marine architects get into this level of engineering but maybe someone out there can refer me to a set of graphs especially on the mainsheet gear loads. The boom i can take an educated guess at, say 25% increase to be sure.
    This should open up some good info for me in particular and for all in general, a part of boatbuilding not often discussed( rig stresses). I want to stress this is not a thread for the fun of it, i am not wasting anyones time, I really need this info for the build as you will see in future photos of the build. Thanks everyone--Geo.

    A yacht is not defined by the vessel but by the love and care of her owner
     
  11. PAR
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    PAR Yacht Designer/Builder

    Tackle arrangements are strictly (and easily) basic engineering 101 and available in most any reference (number of falls, purchase angles, etc.). Wind pressure loads can be estimated with fairly good results (wind pressure coefficients). Loading issues are also fairly easy to track down, such as breaking and working strengths of the various elements. A rough rule of thumb is 250 sq. ft. of sail equals 5 HP in a 12 to 14 knots (14 - 16 MPH) of wind. Naturally, you'll want a reasonable safety margin. You're getting into several different areas of understanding, some simple geometry, some simple math, others, basic engineering and physic principles. None especially hard, unless you're dealing with generics and generalities, on a hypothetical what ever.

    Rather then educating you, it would be much simpler to just provide the data and have the members crunch the numbers. I know this doesn't help your understand so much, but the problems get solved, which is the goal I would suggest.
     
  12. viking north
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    viking north VINLAND

    mainsheet tackle size verses boom attachment location

    Thanks for responding PAR was hoping you would. Forget educating me, too old to warrant a return on inversment:). As Paul Simons song preaches a one trick pony is just fine but be a good one trick pony.(Mine being a builder.) Just lookin for some feedback on if my idea is feasible attaching 40% up the boom as i really would like to have the mainsheet attached to the wheelhouse tops last roof beam aft where the back wall acts as a super webb to keep it all strong.This keeps it in full view, easy access and the cockpit clear of tackle. If I could get some idea of the increased loading on the mainsheet gear or what the loading would be in comparison to a larger boat, i could simply use that boat tackle as my install. I could just increase evrything by 50% and install it but would that be enought? The numbers i listed in my prev. post re the boom, boom attachment point, sail luff, mast height, and so on are the actual specs. i am working with. I'll do a drawing and post it to give a clearer picture. Tnx. Geo.
     
  13. michael pierzga
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    michael pierzga Senior Member

    I think youre intuitive enough from messing around with boats to understand that when you move the mainsheet forward to mid boom you create bigger loads... several ways to overcome this...best to simply understand the relationship,.

    For instance a three point boom mainsheet attachment spreads load on the boom . A boom vang properly tensioned also helps. Leeding the mainsheet forward increases loading on the gooseneck .....if you dont want extra loads on the gooseneck...dont leed forward. To avoid overloading the gooseneck, dont dead end reefs on the gooseneck...use the mast wall or mast collar for the tack and boom walls for the leach

    spreading rigging loads is 90 percent common sense.

    And once again, be sure to check for discarded masts. Each evening I walk thru the shipyard and use two nice aluminum mast sections as a step bridge to clear a persistent shipyard puddle of water. These spars have been laid flat for years and will soon be sold as scrap aluminium .
     

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  14. viking north
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    viking north VINLAND

    Michael, a picture is worth a thousand words. It is reassuring that it is possible to do it and designers have actually done it. I like the idea of multible boom bales to spread the load. Do you know the make of this boat maybe it will give me some idea of sheet and block sizes. As suggested by PAR,I am going to do up a scaled drawing of my build that i can scan and post to give everyone and idea of what i am attempting to do and maybe from that everyone can get a better idea of mainsheet tackle size. Just great, thanks Geo.
    P.S. Stories of masts laying in the mud just breaks my heart, some got and some don't got.

    A yacht is not determined by the vessel but by the care and love of her owner
     

  15. CaptBill
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    CaptBill CaptBill

    I'm surprised nobody suggested a gaff rig yet.
     
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