Mast rake

Discussion in 'Hydrodynamics and Aerodynamics' started by Steve W, Mar 14, 2015.

  1. Steve W
    Joined: Jul 2004
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    Location: Duluth, Minnesota

    Steve W Senior Member

    Is there any reason other than appearance for any particular amount of mast rake. Most boats have some mast rake, maybe 6" or a foot or more on a shorter mast of say, 30-40ft. Most designers draw the rig with a certain amount of rake, it just looks right to the eye, and then boatowners dick around with it trying to adjust the helm balance if there are problems but it actually take a lot of rake to move the CE very much. Ive seen J24s sailing with so much rake that the mainsheet is almost block to block upwind.
    I am in the process of re rigging my old Gemini 3000 cat with the taller double spreader rig off a 3200 and acquired a sail plan drawing from the sailmaker and see that there is absolutely no rake, its bolt upright which confirms what I have always suspected when looking at them, they just don't look right to the eye. As with many things in boat design a mast that is bolt upright visually looks like it is sloping forward which looks wrong to what we are used to, so I am planning to rake it and am curious as to what others have used, the mast is 38ft off the cabintop and ive drawn it at 12" and 18", either looks fine, I like it at 18" but there may be a conflict with the boom hitting the existing factory hard dodger which I also plan on extending down the road apiece so I will probably compromise at 12". Incidently the difference of the CE between bolt upright and 18" of rake is only about 5" and will likely make no difference to the balance so the only issue to me is appearance, might as well fix it while I can, these older Geminis need all the help they can get.

    Steve.
     
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Some messy trig is involved with mast rake calculations of course. But there is more than just moving the CE to whatever advantage. A raked mast will modify the tension on the shrouds for instance. If the mast has swept spreaders, then rake will influence mast bend. The whole thing becomes a can of worms when the boat has lowers, diamonds, backstays, etc.

    A tall mast can indeed move the CE a useful amount Five degrees of rake for a 40 foot mast will move the mast head aft about 3.5 feet. That could move the CE a foot or more. On a 65 foot boat that might not matter much. But on a shorter boat, a J24 for example, it could make a big difference in helm.

    I'm going to make a wild guess that tired old sails will be better with less rake than the boat needed when the sails were newer. I base that wild guess on the fact that tired sails usually move the effective CE aft.
     
  3. Steve W
    Joined: Jul 2004
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    Steve W Senior Member

    Yep, im only talking about about a foot of rake, not 3.5ft. No complicated rigging, just a double spreader masthead rig with inline spreaders. Im not concerned about any balance issue, im really just wondering what sort of rake is typical in a rig of this size, no rake at all is not typical.

    Steve.
     
  4. gonzo
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    gonzo Senior Member

    Rake may decrease turbulence when going upwind. As the boat heels, the air flow starts at the clew (boom) and goes up the sail. The more the boat heels, the more the angle changes. The air flow will have less than 90 degrees to the leading edge of the mast.
     
  5. Steve W
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    Steve W Senior Member

    You mat be right, I don't know but on my old Gemini im really only looking to make a not very attractive cat look a little better. The evolution of the "classic" ha ha,Gemini went from the Aristocat 30 in the UK with a single spreader masthead rig stepped on the bulkhead, when Tony Smith got hold of the molds for whatever reason he moved the mast aft by about 27" and put a compression post down thru the table and the Gemini was born, the next iteration was the 3200 with the major changes being changing the rig to a 3ft taller double spreader MH rig and moving it back to where it belongs on the bulkhead, he also added a wedge shaped piece into the deck mold aft, presumably to give more height in the aft lazarettes for the larger fuel tanks, what theis did though was give the boat a more nose down appearance which, coupled with no mast rake, to my eye at least looks odd, the 3rd and final model was the 3400 which kept these changes and added scoop transoms and kick up inboard rudders. With each new model it appears that the bridgedeck clearance diminished (based on actually measuring them all in a yard in Florida where they were all represented, to the scum line, not bottom paint line)
    Anyway, i bought my boat, a 3000, sans rig as a project and by sheer chance found the complete rig and sails from a 3200 that the owner was converting to a powercat so that is how the changes have come about and while im doing it its going to get some rake, probably 12. Im really just curious as to what other boats with this rig height have for rake. The last boat I did a rig change on was a J24 fractional rig on to a C&C24 formerly masthead and I used 12" on that boat with a 31ft mast.

    Steve.
     

  6. Ben G
    Joined: Jun 2013
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    Location: Brisbane

    Ben G Junior Member

    Good question.. I've wondered the same. I don't think there's much in it really. There are skiffs with more rake and less rake that go just as well.. there's also some tales about adding more rake to make them sail faster. These -overpowered- boats are pretty sensitive to rig setup, so you notice small differences quickly. Unlike a cruising cat :)

    A few practical and obvious things come to mind:
    Raking the mast more moves the CE aft and loads the rudder up more, so the side forces are shared by rudder and centreboard. The shared load gives less overall drag.
    Too much rake moves the CE too far aft and makes it hard to tack (or exit a tack, to be exact)
    More rake twists your jib leech more, for a given sheeting position
    If you want a bigger cockpit, and keep the existing centreboard position, you can move the mast step forward and keep the CE position close by adding more rake.
    Raking the mast tightens the luff of an asymmetric masthead kite, or brings its tack position further aft.
    A more raked rig (especially with a large assymetric kite) can give more bow lift. As can pushing the tack further out on the bow pole.

    Obviously if you're setting up from scratch, I reckon just make it look nice and you won't go too wrong!
     
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