Sheered waterline.

Discussion in 'Boat Design' started by michael pierzga, Dec 10, 2010.

  1. michael pierzga
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    michael pierzga Senior Member

    Local double ended 30ft motor launch was recently launched after a total re plank , refit. They have established floatation marks and the boat is back out for painting. Is there a simple formula for developing a sheered waterline and boot top dimensions. ? This double ended is moderatly fine in the ends.
     
  2. JLIMA
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    JLIMA crazed throttleman

    The easiest way I know of is to put the boat so that the intended waterline is level then take a garden hose with clear tubing stuck in both ends or a long length of just the clear tubing fill it with water and have a buddy hold one end so that the water is just at the intended point and all you have to do is hold the hose such as the water doesn't spill out marking the line as you go around the boat. I repeat the process for the boot top as well ... I also like to make a solid or nearly solid line with a sharpie or other marker then tape and color in the lines so to speak .....Unless your talking about something else and I'm not understanding either way i hope this helps somebody
     
  3. michael pierzga
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    michael pierzga Senior Member

    The water level is great for drawing a level straight line. I looking for Something else. A Sheered waterline is an arc. Higher in the bow and stern with the lowest part near the middle of the boat. Its done as an optical illusion to hide bow down or stern down trim . With a little skiff you simply float her...mark the level floatation. Then you depress the bow with a weight..say 25 liter of water on the bow...to sink it...then mark the bow sunk waterline...then repeat in the stern with your 25 liter jug remembering to mark ...the axis of rotation... near the middle of the boat. After the three marks bow stern, middle... you mess around with a long batten and draw a long acr as the waterline.

    For bigger boats there is a formula to define this arc and a formula to design an attrative boot width.
     
  4. BATAAN
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    BATAAN Senior Member

    Not in Skene's or LFHerreshoff? Wooden shipbuilders of the 1840s used a stretched rope's catenary to establish the sheerline on a 140 packet ship (per Laughlin McKay) so I imagine using a stretched (not too tight) small rope would hang nicely with a few masking tape helpers until it looked right, then measure and transfer to the other side. This will give the mathematical result in 3d without the math!
     
  5. Tad
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    Tad Boat Designer

    There is no simple formula.....this is art.....the boottop should be curved top and bottom (not a straight line in profile). The lower edge is curved less than the the top.....the amount depends on the look of the boat, more sheer equals more curve in the boottop. The low point (in profile) of each curve is not midships but some place aft of that, usually about station #7, but not always.......

    In a 30' boat I would start the lower edge with about 3" above DWL at the stern, 2.5" at #7, and 4" at the stem, and make the width about 3.5" at the stern, 2.75" at the low point, and perhaps 4.5" at the stem.....It's hard to judge without seeing the boat.

    A rough rule of thumb is that the boottop width (height in profile) is equal to the distance above DWL. But you need to be careful about this aft if the boat has a wide stern with lots of horizontal (modern sailing boat) width. Then the line that's narrow in profile becomes huge in plan.......

    I drew the example below about 22 years ago, today I would give her more perk forward....but it's not bad. Note she is rather wide thus the boottop height aft is the absolute minimum.....but there is certainly taper.

    Wardrop46.jpg
     
  6. michael pierzga
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    michael pierzga Senior Member

    Looks about right. And as you say , its only a guess and its art. . Luckily paint is not permanent , a boat gets painted many times during its service life. Always room for a tweak next time. Nothing worse than a bow or stern down grassy waterline.

    I will pass on your thoughts to the guys working in the paint shed. Thanks.
     
  7. michael pierzga
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    michael pierzga Senior Member


    Im a sailor Baatan...Ive got no access to books on design when on the boat, sitting at the nav station , surfing the net. Its the life !!!. Always many questionthat I cant answer. I know the " formula " is out there , since Ive seen it on plans from Sparkman and Stephens for a 65 footer I once sailed. .

    The catanry effect is what fisherman boat builders use. Very effective. Fishing boats are always the best example simple ship building elegance.

    The problem is were is the centre of the catenary curve on a double ender ? fine ends ? Hmm...I dont know. Shame I wasn't around when they floated the boat or I would have sat my fat *** on the ends of the boat to simulate..." DEAD WEIGHT ? !!!!!

    No easy answer. Suspect that the local painter guys will indeed use catenary sag to mark out the waterline , boot top.
     
  8. tom28571
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    tom28571 Senior Member

    Here are the ratios I use. Stern = 1, midships = .6 to .75 and 1.5 at the stem. These are vertical projection measurements. If the size of the boat calls for a midships boot top of 2.5 to 3" then the stern would be 4" and the stem would be 6". The sheer of the boat, as well as the transverse shape along the waterline, will dictate this to some degree and, in the end, it is all about what looks nice to you.

    Actual width of the boot top at any point would depend on the flare, flam or whatever the transverse shape is at that point. I make the bottom of the stripe in one plane with the stem higher than the stern, just enough so the eye sees the boat is not down by the bow under any normal loading condition. Tad likes the arced bottom but that is more difficult to do well and the straight line looks just as good to me.
     
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  9. PAR
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    PAR Yacht Designer/Builder

    There are several formulas, some are highly prized secrets. On a double ender I suspect you'll have to alter the formula a bit to compensate for the pointed stern, but nothing unmanageable.

    As Tad has pointed out, it's an art and a dieing one too.

    The most common "formula" uses fractions of the LWL as the heights. Typically the LWL is divided into 10 equal stations, then the bottom and top of the boot stripe is ticked off at each "station", then a batten is sprung. Naturally, the guy eyeballing the batten then makes adjustments, to suit the particular craft he has before him.

    Starting at the bow (of course) the top of the boot would be (at each station): 1-.25, 2-.226, 3-.205, 4-.187, 5-.16, 6-.153, 7-.15, 8-.153, 9-.163 and lastly .18
    The bottom of the boot would be: 1-.125, 2-.113, 3-.103, 4-.086, 5-.08, 6-.077, 7-.075, 8-.077, 9-.082 & 10-.09
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    I wonder if a laser level would work here. I mean the type with a motor that spins, not the el-cheapo type with an optical grid which doesn't generate a truly straight line. It could be operated at a slight angle to the horizontal, beamwise, to create the waterline that the boat would have when heeled. Don't pick a day that is too bright of course ...
     
  11. PAR
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    PAR Yacht Designer/Builder

    A laser is what I use to make waterlines, but it's not especially useful making curves. The problem isn't the heeled waterline it's one of looks, to compensate for an optical illusion and to help hide trim issues. If a dead straight waterline is painted (I had a client force me to do this once, when he saw the laser, so I got to charge him twice for a boot stripe!) it will look like it droops at the ends as a result of the narrowing of the boat ends being further from your eyes. The compensation is a sheered LWL and/or boot stripe. The amount is a mater if taste, boat type and experience. This is an easy thing to make look bad.
     
  12. mark775

    mark775 Guest

    Par and especially Tad, thanks. Michael, true, "always room for a tweak next time." The hanging string, I don't care for - hard to transfer that hanging string to a curved in the horizontal plane surface. I bought a lazer level for this once and went back to the water level because of the difficulty seeing in different lights, and the fact that some fool with a pencil seemed always in the way of the lazer when I was trying to mark. Hanging fine line tape, as far as you can see, is then walked around from stern to stem and allowed to touch at the marks, which you have altered from horizontal by eyeball, or as I now know, by specific amounts. If you are taping in front of you, at no more than arm's width or whatever, it cannot be as fair as walking the tape out. Battons? Naw - perhaps hot glued on for scribing a line previously located. They don't scribe lines in boats much anymore. with a perfect scribed line and a steady hand, one can paint without tape and allow the loaded brush to fill the scribe. Lynn Senour scribed mine...wrong and there's nothing worse than getting paint to not bleed under tape over an incorrect scribe line. Great guy tho, with a lot of great ideas. I'm sorry he's gone.
     
  13. michael pierzga
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    michael pierzga Senior Member

    I see the guys in the workshop now. They have struck a chaulk line on the workshop floor and are lofting a sheered line on the floor. Looks nice...follows the launches deck sheer with a bit extra in the ends. .

    Incredibly Difficult job...many, many people required...three guys on their hands and knees with long battens and ten other guys standing around with cups of coffee talking football and giving live commentary. Even the shipyard gaurd dog took time off from growling at the UPS man delivering a box of stuff to closley scrutized the arc . . Very technical. They tape the water line today, then roll the launch out in the sunshine tommorow. Probably going to need at least two dozen guys with coffee cups positioned at various standoff angles, backed up by space based lasers and an old man with a walking stick pointing things out, to get it right.
     
  14. Tad
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    Tad Boat Designer

    I'll add a few more comments as the boottop line is screwed up more often than not.......very few get it right....;)

    I see so many piggy looking boats whose looks could be improved enormously by simply working on the boottop.....usually it's the boottop is too low or too small, or too straight, or non-existent.....making it higher (above waterline) and wider is rarely wrong.....especially on modern high-freeboard hulls. Really high freeboard hulls can benefit from making the boottop twice (or more) as wide (high) as normal, then adding a sheerstrake in the same colour transforms the boat. This is why formulas don't work.....

    On flat sheered powerboats I start aft with the boottop parallel top and bottom and to the waterline, then gradually curve it up to the stem, again more curve in the top than the bottom edge....like this.......

    Talaria44.jpg

    That's the way it was originally designed....in those days we provided a table of offsets for the boottop top and bottom edges....I still do that on occasion.....Today Hinckley use two narrow dead straight parallel white lines.....which just illustrates where the company has gone........

    As for those who do get it.......Gary Mull was a master at boottops, if you look at his drawings the boottop appears too curved...but he was compensating for perspective and they come out perfect IRL. German Frers also always got it right back when he was still doing the drawing....today the help have followed everyone else to a bunch a straight lines easily done (afterthought) in CAD.......

    Going through the S&S book The Best of The Best I find that, like almost everyone else, they got it wrong (on the drawing) more often than not. Funny thing is both Mull and Frers trained at S&S, as did Dave Pedrick, another guy who did get it right in most drawings I've seen.

    LF Herreshoff's drawings show some development in the drawing of boottops, in his early work there is just a sheered top to the anti-fouling.....then in later years, a thin parallel paint line (also sheered) at the top of the anti-foul.....I haven't come across a sheered boottop by him.......
     

  15. michael pierzga
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    michael pierzga Senior Member

    Motorboats must be particularly difficult to get esthetically pleasing, one pointy end , one square end. I just walked past the Superyacht quay...every big motoryacht that I could see was stern down with a grassy waterline aft. Even the fishing boats look better.

    . Sailing yachts are easier, they look good when the sheer is exaggerated a bit in the ends..
     

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