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formula for a sheered waterline.
junior
Posted: Sunday, August 15, 2010 8:01 AM
Joined: 14/01/2009
Posts: 1026


Friend of mine is refitting a classic canoe sterned tender/ launch . Goes in the water at the end of the month to be trim ballasted and have the load waterline marked. He asked me for the formula used to develop a sheered waterline and boot top. I used to have an S&S supplied formula but it seems to have disappeared into my mountain of junk. A quick google search was unsuccessful. Does anyone have the formula for a sheered waterline ?
Chief
Posted: Sunday, August 15, 2010 4:23 PM
Joined: 02/06/2008
Posts: 341


"... a sheered waterline?"

I this a very flexible boat? No girder strength? Plan to sag a lot?

Except when it's moving or otherwise disturbed, water lies flat. A waterline is a straight line. Why would a waterline ever have sheer?

Unless your friend has a good hull design program and a really accurate set of lines it sounds like a waste of effort since he is going to dip the boat and mark the actual waterline anyway.

junior
Posted: Sunday, August 15, 2010 6:34 PM
Joined: 14/01/2009
Posts: 1026


Professionally designed yachts have sheered waterlines, boot stripes. Higher in the bow and stern with the low point about two thirds aft of the stem. http://img716.imageshack.us/img716/2576/sheeredwaterline001.jpg A sheered waterline creates an optical illusion drawing your eye away from out of trim floatation as the yacht shifts fore and aft on its center of buoyancy. A yacht looks clumsy when its waterline is flat.....drop 75 meters of anchor chain, the bow rises , the yacht rests on her stern and grows grass on her boot aft. The slightest change in ballast...half full tanks, cases of Budweiser , over packed stewardess's, grandma sitting forward etc. sets her off her lines and makes her look bow or stern down . Like an out of level "see' saw". Particularly noticeable on a vessel with fine ends that easily deflect. You could form your own sheered curve with a ten meter long batten deflected on the loft floor to create a table of offsets , but its a lot of work. The formula tells you how much to offset the painted waterline above the load waterline at each station, for a vessels length and type ie. blunt ends, fine ends . http://img21.imageshack.us/img21/3386/sheeredwaterline012.jpg
Chief
Posted: Sunday, August 15, 2010 8:09 PM
Joined: 02/06/2008
Posts: 341


Thanks for the info, Junior, that is the first time I have heard of a formula for doing that.  To tell the truth, I always thought the appearance was an optical illusion or my bifocals bending the world.

I think I would use a "water level" to set the fore and aft points then a slack wire to fair the line to the low point. Let me know if you find that formula, the idea of calculating a visual impression is interesting. But I guess architects do it with building designs.



junior
Posted: Sunday, August 15, 2010 8:19 PM
Joined: 14/01/2009
Posts: 1026


Its possible that Im doing my google search with the wrong term.. "sheered waterline"....so much boat talk is local slang.
Chief
Posted: Sunday, August 15, 2010 10:10 PM
Joined: 02/06/2008
Posts: 341


Word on the street is your answer is here:

 http://www.woodenboatstore.com/WoodenBoat-Issue-198/productinfo/205-198/

It looks like you can download it for a couple of dollars. 


Google  sheer   waterline   boot top   and you will get quite a few discussions on how other folks have been reduced to babbling fools attempting to get it right.  Who knows, maybe progressive bifocals are the answer, they all look curved to me ... 

kiwi74
Posted: Wednesday, August 18, 2010 6:37 PM
Joined: 20/07/2009
Posts: 8


Junior is right here, and a great explanation. With the older style wooden launches there is usually a lot of fore/aft curve in the hull due to the beam, therefore they need a slight curvature on the sheer line and boot stripe to project the illusion of a straight line. I am not sure of the formula but it has something to do with the top view orthographic plan projection at the point of start and finish of the stripe. If I remember correctly the bow and stern are almost at the same level with a slight curve downwards. With most yachts nowadays however having a completely different shape the yacht is set level in the workshop and the waterline level set from the bottom of the keel. Then a laser level is set up (Spinning laser so you see the red line projected on the hull) and chalk is used at each frame station to mark the waterline. After this is done if you stand back and look it will give an illusion of shape, whereas it is actually a straight line. But what do I know, I could be talking Greek as it has been years since I was building boats..... Good luck and if you find something let us know!

 
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