finding CLR while boat is in water?

Discussion in 'Sailboats' started by francis k, Oct 26, 2006.

  1. francis k
    Joined: Oct 2006
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    francis k Junior Member

    I have a 30 foot,9ton,5foot draft, 9.5 foot beam,full keel(traditional style-no cutaway),flat transom stern, no overhangs to speak of- about 28 foot LWL. cutter stepped on deck, 39 foot mast. Flush deck
    The boat was homebuilt (designer and builder unknown)...The hull is beautiful, but The mast is stepped dead in the center. I can not find one example of an amidships stepped mast anywhere. the underbody of the hull is not unusual. I have only had it out in winds up to 15 kts. once and sure enough the weather helm was bad. Can I find the CLR without pulling the boat out. My guess(I am not an expert) is I will have to move the mast.
    A lot of people have told me it makes sense for this or that reason but nobody can show me one legitimate design of a sloop or cutter with this configuration. How do I find the CLR, or what do I need to check. sorry I don't have any pictures yet.
     
  2. RHough
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    RHough Retro Dude

    Did you get a photo directly from the side? A quick and dirty test would be to cut out the hull profile and balance it on the edge of a ruler.

    I wouldn't go moving the mast until I knew for sure that it needs moving. I'd play with sail combinations, trim (both hull and sail), and mast rake before I'd try moving the mast.

    Was the boat supposed to have a bowsprit? How long?

    If the mast is placed as designed, and the boat is hard mouthed, the design is flawed.

    Are the chainplates for the upper shrouds in line with the center of the mast step? Does the boat have single or double lowers? All of this stuff would have to get moved if you move the mast. How about compression post? How much of the interior will you have to rebuild to move that?

    Is a boat of unknown pedigree and unknown build quality worth re-engineering the rig?

    Always try the simple stuff first ... moving the mast is not on the simple list. :(
     
  3. yokebutt
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    yokebutt Boatbuilder

    Francis,

    Knowing the CLR (wich is NOT the center of area!) won't really do you much good. Start with raking the mast further aft and retuning it to fit, if that doesn't do it you should talk to a NA before making major changes.

    Excessive heel angle will give you a lot of weather helm too.

    Yoke.
     
  4. francis k
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    francis k Junior Member

    I guess my point is this. If my boat, with a typical full hull can have the mast dead on center, then all calculations and methods of determining the lead are useless, if you can just stick it in the center anyway. I have the only sailboat that I know of with a mast dead in the center. numero uno. I have posted on numerous forums for someone to produce just one example of a ligitimate design with an amidships stepped mast . so far no one can. The boata requires the second reef point in only 15 kts. wind. I doubt raking will correct that. I
     
  5. RHough
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    RHough Retro Dude

    Raking the mast aft will just add to weather helm.

    For a quick check on the effect of moving the mast, Sail the boat with the main reefed and your largest headsail(s). That will move the centre of effort forward and should reduce weather helm.

    Before you go too far, when you say "I have only had it out in winds up to 15 kts. once and sure enough the weather helm was bad." Do you mean that the effort to steer was high? Or do you mean that the boat wanted to turn into the wind very strongly and it took many degrees of rudder to hold it straight? High effort with low rudder angle would be a rudder design problem (poorly balanced). High rudder angle (with high or low effort required) would be a sailplan, sail trim, hull trim, or design problem.

    If the sail plan reaches from stem to stern (30 ft) and to the top of the 39 foot mast, you have about 580 sq/ft of sail area. For a SA/D of only 12.53. At 9 tons the D/L is over 400. The boat is under canvased by modern standards. It should take a stiff breeze before the boat needs to be reefed. If you have to put two reefs in at 15 knots with that tiny sail plan, there is something amiss. There is no way a 9 ton boat with a 28 ft waterline should be that tender.

    If you are determined to get the boat sailing properly, you should talk to a NA and have them take a look at the lines and structure of the boat.

    You may have answered your own question with a combination of "no legitimate design" and "designer and builder unknown".
     
  6. PAR
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    PAR Yacht Designer/Builder

    Yes, you can have the mast in the middle of the boat. Most cutters have the mast placed very near the midship location. Though it is uncommon to have the stick this far aft, it is certainly feasible, of course the underwater area configuration would need to qualify the placement.

    It's also not uncommon to reef in winds building over 15 knots. Many boats, having a high SA/D, require early reefing to remain on their feet, where they do their best.

    The is no such thing as a "typical full hull". Underwater area, appendages and their coordination with the rig are application specific. Not even two boats of the same design and construction will be the same, each requiring a different "tune" to get things right as far as balance. Even though you don't have a resource in the original designer, a reasonable picture can be produced by any component designer, willing to take on the task. This will involve hauling the boat, careful measurements and some effort on both parts, but the result will be a balanced boat.

    Adjusting the rig, may eliminate your problems, including the early reefing. Most of this work can be done at the dock and some of it while underway (final tuning). I'm also in Florida, if you care to drop me an email (click on my name), I could offer some local addresses for the help you will need.

    There are a number of methods to address weather helm, without resorting to redesigning the rig (which may not need it), on the other hand, this wouldn't be the first boat to have to get it's stick moved forward.
     
  7. RHough
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    RHough Retro Dude

    I agree, but this boat can't have a high SA/D unless it has a bowsprit or the main is gaff rigged. With the 39 foot mast in the centre of the boat, the foretriangle is 39 x 15 x .5 = 292 sq. ft. Assume the main is also 292 sq. ft. and you get 584 sq. ft. total area. SA/D = 12.5 is very low, even for a cruiser.

    If the SA/D was 16+ a single reef at 15 knts, sounds right to me. Having to double reef at that speed does not sound right.

    I think we need more information to go any further.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Ditto, much information about the boat will be required to make any reasonable analysis. 9 tons sounds pretty heavy, but if she has full ends and the beam is carried down to the LWL, etc., etc., etc. it quickly adds up. A profile of her afloat and on the hard would go a long way to clearing things up.
     
  9. francis k
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    francis k Junior Member

    I know you are right that I need to get a picture of the boat out of the water to get the answers I am looking for. I am trying to make this assessment without the cost of pulling her. I don't have much invested yet.
    Also, the main is only 200 sq.ft. And yes, the beam is carried down to the waterline. The underside looks very much like the westsail32 , but with a transom. She is documented 10.44 net(which means nothing).But must be at least 9 tons. With only 200 sq. ft.main needing two reefs seems ridiculous. I can't expect anyone to help me make an assessment without at least some pictures. There is supposed to be 15-20 kt. wind the next two days. Maybe I can get a better IDEA THANKS
     
  10. yokebutt
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    yokebutt Boatbuilder

    Tipping it forward was what I was trying to mean.

    Yoke.
     
  11. jonsailor
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    jonsailor Boat designer/builder

    Answer to your question

    Everyone is evading the question and yes, you are going about it the right way and yes, you may have to rip the whole mast out to re-configure the correct balance.
    To find the CLR...easy.
    Get some one with a dinghy and outboard and on a calm day, lash the tiller in the middle and attach a rope amidships on the vessal somewhere and gently pull her sideways. Keep shifting the rope for and aft, until you find the exact point the it will tow dead square. You dont need a lot of pull and you certainly do not want the boat to get any weigh up but it will work. It must be very calm and infact you can do it by rowing.
    Some designers like myself, do not use all the rudder plan for the balance as I believe it should be used to turn the yacht, not balance the yacht so you may have to tickle your CLR a bit aft to allow for some of the result.
    Good luck and I am glad to see you take a bit of a calculating approach:p
     
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  12. francis k
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    francis k Junior Member

    I knew what you meant yoke, I didn't consider raking the mast. I only carry 200sq. ft. of mainsail with 9 ton displacement and I need 2 reefs to ease the weatherhelm in 15 kts.?? I need to get pictures to show how ridiculous the mast looks in the center. Not to mention the compression post is right up near the galley breaking up the whole cabin space. I found enough other oddities in this boat to lead me to believe he would have screwed with the mast placement. There's no telling what else he did. For instance: fairing compound very thick behind the chainplates. chainplates not aligned. water and fuel tanks located under cockpit seats which, because this is a flush deck, puts them about 17 inches below the deck. The bilge is full of lead bricks. No chain locker. No cleats at mast. 8 inch cleats on bow of 9 ton boat. On and on....Sorry if I wasted everyones time. But I found my answer. Get rid of this boat. Thanks.
     
  13. feetup
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    feetup seeker

    Although I have not attempted this with a boat as large as yours, you might try this. If you watch your tide and current tables and find a time with slack where you can tie up beam on to an expected current. Make a bridle from headsail winch, forward to a chock near the stem and back to the other winch with a stout line tied off from the middle of the yoke (looks like a "Y") to a mooring bouy or pile. When the current starts to flow you can adjust the yoke fore and aft with the winches till she lies beam on to the current without wanting to head up or fall off. The single line to mooring (the leg of the "Y") will point to (or close to) the CLR. The flatter the bridle the more sensative the experiment will be. You will have to work quickly if there is very much current expected, and be sure that it is easy to cast off the aft leg of the bridle if the current gets too strong. This will not take either keel or rudder lift into account for you are working with a totally stalled keel and rudder (which should be lashed amidships), but this will be less of a factor with a full keel than a fin keel and spade rudder, since the lift of these is actually a vector, involving angle of attack and leeway or slippage.
     

  14. francis k
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    francis k Junior Member

    thanks feetup. I thought of that type of procedure, but wasn't sure how to do it. My idea was to fasten a 3 or 4 foot cable along the side of the hull near the estimated position of the CLR and pull with a motorboat while moving the towline along the cable until the hull is perpendicular to the motorboat.. I like your idea better since I don't have a motorboat. I realize it would be only an approximation.
     
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