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Thread: Deck Camber?

  1. #1
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    Default Deck Camber?

    My copy of Skene's Elements of Yacht Design says to use 1/4 to 3/8 inch of camber per foot of beam, and it gives the geometric layout. So I did a 4' template with 1 1/16" camber (the light colored wood) and another 8' template with 2 3/16" camber (my bad measurement, but close, on the meranti)) however the two templates don't come anywhere near the same curvature, as one can seen in the photo. I am fine if the relationship is non-linear, but if so, how can Skene say 1/4" to the foot? When I lay a straight edge on the more curved template I get a bit over 1/8" camber to a foot. What is going on?

    Ken

    IMG_4270.jpg

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    Default Re: Deck Camber?

    Watching with interest. I'd have ended up with the same things you got...with the same question.

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    Default Re: Deck Camber?

    I can't find it but there was an interesting article in WoodenBoat a decade or so back. The problem is that to get the deck to fit smoothly to each deck beam, the actual curve of each deck beam is different. Some boats have small enough differences that a little plane work will solve. Others not.

    A method that allows you to loft the upper curve of each deck beam is to draw the stations. Start with the center station where the height of the camber is the most. You mark a line one side to the other and mark the camber elevation at the center. Two long battens are then joined essentially making the upper sides of a shallow triangle.

    If you slide this structure bringing the apex to each beam end, the apex will describe the arc you need for the camber at that spot. Use the same joined battens at each station to get the arc for those deck beams. If you make the intervening deck beams from the station closer to the center station, they will be close enough to plane the fit.

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    Default Re: Deck Camber?

    You will not get same curvature. If you are creating a circular camber the relationship between the radius of curvature to the beam and rise will be:
    If Radius = R, Rise = r and Beam =b the identity is R=((b/2)^2 +r^2)/(2r). Doubling the beam doubles the radius of the curve and flattens it.
    Last edited by Peerie Maa; 03-04-2019 at 07:03 AM.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Deck Camber?

    There are a number of design issues that must be taken into consideration depending on the type of boat being built and the materials in use. If plywood is being used as decking and there is a fore deck aft deck or cabin top, the apex of the crown should form a straight line for and aft. This allows a simple bend of the ply. Some boats (ussualy small boats) have deck crown that follows the sweep of the shear. This causes a compound curve that is extremely difficult to cover with plywood. If that's the case then planking, cold molding or some other method would be required.

    If you have a fore deck on this boat I suggest taking a measurement of the last full deck beam where it meets the cabin front or combing. Determine the actual vertical rise at the center line that the design calls for, or whatever you feel is correct. After establishing that point you can work forward and measure each beam separately if required by running a string to the bow and use a straight edge across the boat.
    Last edited by navydog; 03-04-2019 at 09:58 AM.

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    Default Re: Deck Camber?

    Found an illustration of the method I like. Depending on how you set up for the build, you can do this right on the station molds once they are set up.

    https://www.youtube.com/watch?v=OiLZUR1Go-I

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    Default Re: Deck Camber?

    Yes the avatar depicts me; yes I drew the comic boat pic, it's a joke on the pop song I'm not a robot by Marina and the diamonds

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    Default Re: Deck Camber?

    I think I must have read the article in WB that Ian cites when I lofted this thing, BC I lofted the deck line at each station and now no single master curve fits all the arcs I drew. That lofting was in 2012, and I can't quite remember what I was thinking. I would take that old lofting for want of a better plan, except that as I look at this thing roughed out I now see my strong sheer sending water from fore and aft toward 'mid-ships, and I want to dump this water overboard instead of into my open cockpit. The immediate goal is to create some template so I can plane a consistent, fair curve onto the top of my planking, my sheer clamp, and deck beams on both sides of the boat. I can't use the method with two sticks because it requires a shop 3x the width of the boat, and I barely have room to walk around the thing :-) Thanks also to Peerie Maa for comforting me that I wasn't loosing it. "Keep those cards and letters coming." Photo attached, the white stick standing in as my cockpit edge is cheap plastic trim from the big-box store pretending to be a batten.

    IMG_4273.jpg

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    Default Re: Deck Camber?

    I assume this boat will have a coaming to prevent water on the decks from entering the cockpit. The transition from a full deck to the side decks is where you need to keep the shear sweep and camber fair. Lay a wide board or piece of plywood down on the corners and check how it contacts the carlin using the crown of the forward deck. Do the same in the stern.
    Last edited by navydog; 03-04-2019 at 10:29 AM.

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    Default Re: Deck Camber?

    Quote Originally Posted by navydog View Post
    There are a number of design issues that must be taken into consideration depending on the type of boat being built and the materials in use. If plywood is being used as decking and there is a fore deck aft deck or cabin top, the apex of the crown should form a straight line for and aft. This allows a simple bend of the ply.
    That is only half of the story. To avoid any compound curve the crown has to follow a straight line as you say, but the shear will be defined by the half breadth at each beam and the round down from the common beam camber template at that half breadth.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Deck Camber?

    Theoretically you shouldn't be able to compound plywood, but in practice it's done all the time.

    Jim



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    Default Re: Deck Camber?

    Getting back to the original question, a curve of 1/4" per foot is exactly that. Whether the beam is shorter or longer shouldn't matter. They are all arcs cut from the same radius. In practice, as I learned bending pipe as an apprentice electrician and again in building my house, apparent variations from theory are usually traceable back to imprecision in either measuring, in marking, or in hewing to the made marks. In my work, anyway.

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    Default Re: Deck Camber?


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    Default Re: Deck Camber?

    And here's how I did it.

    First a string was stretched to describe the centerline. Then the width (W) and the height (H) was measured and both were used in this equation to determine the radius, each of which was then drawn out on the shop floor, which you don't have, but there's a way somewhere, perhaps plywood out in the driveway or on the living room floor.

    To calculate the radius

    Given an arc or segment with known width and height: The formula for the radius is:
    H
    2


    +
    W
    2


    8
    H




    where:
    W is the length of the chord defining the base of the arc
    H is the height measured at the midpoint of the arc's base.
    The beam in the centerline is also blocking, placed there to facilitate adding cleats and for stiffness in general.

    Here's a calculator to save you doing the math.

    https://www.mathopenref.com/arcradius.html

    The side decks are so narrow I just laminated a few pieces into a solid base for the decking.

    Be careful which side of the line you mount the beams on, you will need to fair the upper edges. In my case I wanted the fore and aft faces to be plumb so I started forward adding a beam then 2 blocks then a beam, etc. etc. on the aft side of the line.

    You can make a nice longboard for fairing by cutting a belt sander belt and gluing it to a board with Titebond.

  15. #15
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    Default Re: Deck Camber?

    I design most all my boats using the traditional deck camber layout (see #7 above). This gives a parabolic camber, not one that is part of a circular arc and is better suited to yacht decks and coachroofs. It has never produced a surface that you can't lay ply on OK, regardless of the sheer. Ply will conform to a somewhat non-developable surface.

    camber.jpg

    The illustration shows a beam of length L with a rise of A – A being 9% of L in this case (about 1" in 1'). The basic process to create the camber curve is as follows:

    1. Strike an arc, radius A
    2. Divide the arc into equal segments (4 in this example)
    3. Divide the base of the arc into the same number of equal segments
    4. Join the base segments to the arc segments – lines B. C & D in this example
    5. Divide the beam base L into equal segments (in this case 8 – 4 each side)
    6. Set out the lengths B, C & D each side
    7. Join E-D-C-B-A-B-C-D-E in a curve – which will be the beam camber shape



    This works for any camber. The 9% used in the example is high by traditional standards but not on many modern yachts.

    You don't get a master camber pattern with this method - you just set out each beam individually according to the beam full width (L above).

    Cheers -- George
    Last edited by debenriver; 03-04-2019 at 12:22 PM.
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    Default Re: Deck Camber?

    Quote Originally Posted by chollapete View Post
    Getting back to the original question, a curve of 1/4" per foot is exactly that. Whether the beam is shorter or longer shouldn't matter. They are all arcs cut from the same radius. In practice, as I learned bending pipe as an apprentice electrician and again in building my house, apparent variations from theory are usually traceable back to imprecision in either measuring, in marking, or in hewing to the made marks. In my work, anyway.
    Not according to the maths. Post #4
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Deck Camber?

    Quote Originally Posted by Jim Ledger View Post
    Theoretically you shouldn't be able to compound plywood, but in practice it's done all the time.

    Jim
    Jim,
    I wouldn't consider from the picture that there is a compound curve in the crown of your aft deck. The side decks are in reality flat planes in a twist.

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    Default Re: Deck Camber?

    Quote Originally Posted by Gib Etheridge View Post
    And here's how I did it.

    .
    Gib,
    Your boat is coming along nicely and looks great.

  19. #19
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    Default Re: Deck Camber?

    Quote Originally Posted by navydog View Post
    Gib,
    Your boat is coming along nicely and looks great.
    That one was years ago. Thanks though.

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    Default Re: Deck Camber?

    Actually, the curve used by all of the designers I have worked with and for have used the curve shown in #15 by A C Grayling here. This is a portion of and elipse or section of a cone and is the most strength supportive curve to be used for deck camber. It also is, visually more pleasing to the eye than a plane arc taken from an unimaginative circle. Also, bear in mind that as the deck progresses in each directed away from the midships section, as the ends of the pattern are shortened the height of the arc will be reduced. And the deck line can become hollow, when viewed in sheer. Even though rise of sheer towards the ends of the deck can compensate for this, it will often result in a centerline that sags. it is better to loft each beam individually to a desired pre-determined crown hight in relation to the sheer at the rails when viewed in the sheer profile plan.
    Jay
    Last edited by Jay Greer; 03-04-2019 at 01:47 PM.

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    Default Re: Deck Camber?

    Quote Originally Posted by debenriver View Post
    I design most all my boats using the traditional deck camber layout (see #7 above). This gives a parabolic camber, not one that is part of a circular arc and is better suited to yacht decks and coachroofs. It has never produced a surface that you can't lay ply on OK, regardless of the sheer. Ply will conform to a somewhat non-developable surface.

    camber.jpg

    The illustration shows a beam of length L with a rise of A A being 9% of L in this case (about 1" in 1'). The basic process to create the camber curve is as follows:

    1. Strike an arc, radius A
    2. Divide the arc into equal segments (4 in this example)
    3. Divide the base of the arc into the same number of equal segments
    4. Join the base segments to the arc segments lines B. C & D in this example
    5. Divide the beam base L into equal segments (in this case 8 4 each side)
    6. Set out the lengths B, C & D each side
    7. Join E-D-C-B-A-B-C-D-E in a curve which will be the beam camber shape



    This works for any camber. The 9% used in the example is high by traditional standards but not on many modern yachts.

    You don't get a master camber pattern with this method - you just set out each beam individually according to the beam full width (L above).

    Cheers -- George

    Holy cow! that is 9.6" crown in an 8' beam! I have been fretting whether 2.5" in 8' is appropriate

    Ken

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    Default Re: Deck Camber?

    Quote Originally Posted by Jim Ledger View Post
    Theoretically you shouldn't be able to compound plywood, but in practice it's done all the time.

    Jim


    Can you say what thickness plywood you are using, and the approximate Port / Stbd curvature?

    Ken

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    Default Re: Deck Camber?

    Thanks debenriver, I was going to translate the text in my book, but I realized I had to run for my train so didn't have time, and just thought a picture was better than no picture, and I find the picture quite self-explainatory.
    The ratio the author suggests is between 1:20 to 1:30, meaning the top of the curve should be 1" for a 20"-30" beam, which I get to be about 3-5%. But the books focus is on traditional, or classic, boats.

    /Mats
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    Default Re: Deck Camber?

    I considered using the parabolic profile but didn't think it would matter or even be noticeble in such narrow decks (as unimaginative as it is ).

    Not having the room to swing an arc one advantage to the parabolic method will be that you can lay it out on the bench.

    I don't know if it really does produce a parabolic curve, just need to call it something.

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    Default Re: Deck Camber?

    Here's a tangent (figuratively speaking): I have been planning on two layers of 6mm marine plywood to spring the curves of the deck. The spans of the side decks will be about 1' wide between sheer and carlin, by 16" long between deck beams. Does this sound good? If so, what would you slather between the two layers of plywood? I have been going through epoxy by the gallon and wonder if there its some kind of appropriate but cheaper waterproof mastic cement (underlayment, carpet goo?) to bond the two layers of plywood together. The reason I don't have construction drawings is that this boat is the "Western Lakes Mackinaw" from Chapelle (ASSC) and there are no construction scantlings, and even if there were, I am building strip-composite, so I need to invent everything but the hull shape. :-)

    Ken

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    Default Re: Deck Camber?

    Quote Originally Posted by kbowen View Post
    Can you say what thickness plywood you are using, and the approximate Port / Stbd curvature?

    Ken
    There are two layers of three eighths plywood epoxy glued together.

    I forget the actual amount of camber that I used but it was on the high side, five inches over twelve feet, perhaps, enough to get it to look nice and plump. The sheer is the typical Cape cat sheer, which is more springy than most sheers you're likely to find. If that doesn't add up to compound curvature I'll have to go back and check my figures.

    Jim

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    Default Re: Deck Camber?

    1/2" by 12" on frames on 16" centers ought to be quite rugged, in my opinion anyway.

    I'd keep on using the epoxy, thickened and spread with a 1/8" notched trowel.

    Also, I would make up some lapstrake clamps for clamping the 2 layers together between the frames for the side decks with longer clamps for the fore and aft decks, if that will even work. An alternative for the fore and aft decks would be to gently jamb a bunch of spring posts under the inner layer then weight the top layer.

    Epoxy starts to go soft at 150 F. You might want to keep the decks a light color.

  28. #28
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    Default Re: Deck Camber?

    Quote Originally Posted by kbowen View Post
    Holy cow! that is 9.6" crown in an 8' beam! I have been fretting whether 2.5" in 8' is appropriate

    Ken
    Well it's actually 8.64" in an 8' beam (9% not 10%). " (5.2%) to " i(6.25%) in a foot were quite common cambers on classic carvel boats in the 30' to 50' range that we built in the 1960's and 70's.

    With more demand for headroom on smaller boats, increasing the camber is one way of achieving some of the increase needed without making the freeboard and coachroof coamings unduly high. 9% is not that uncommon. It also enables shallower deck beams, which again improves headroom, and a thinner ply on the deck which saves weight above the dwl. I usually do 7.5% to 8% on decks and 8.5% to 9% on coachroofs – maybe 9.5% on a doghouse roof. It's common (mostly for aesthetic reasons) to make the coachroof camber a little more than the deck camber and often a doghouse or deck saloon camber a little more than the main coachroof.

    If you make the sidedeck camber the same as the foredeck and aft deck, using the whole beam length, then the sidedecks will fair in nicely to them.

    2.5" in 8' is pretty shallow in today's terms - though perhaps appropriate for a traditional build. It will look pretty near flat to the eye.

    The camber produced by the traditional method I described, being parabolic in nature (it will never return on itself like a circle of ellipse), is generally a flatter curve than a circular or elliptical curve will be – that is the rise on the middle coordinates (B, C & D in the image) is less that would be the case with a circular camber - and is generally a more pleasing to the eye and to walk on.

    The graphical method of setting it out has been used for hundreds of years by boatbuilders and loftsmen.

    Cheers -- George
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    Default Re: Deck Camber?

    As Jay noted, you can't take your master curve no matter how you come up with it and just shorten it; you will get hollow. What some of the old timers working on small boats did after they established their 'master curve' was set up straight edges or stiff battens that ran to the bow and worked out heights from these, basically 3-D lofting.
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    Default Re: Deck Camber?

    Here's a deck beam and three templates set up in the bow of a catboat. There's a straightedge hitting the top center of each one. Each one is a different curve.





    Here's the same deckbeam and patterns superimposed on each other. The need to increase the crown as the beam narrows becomes obvious.



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    Default Re: Deck Camber?

    Quote Originally Posted by Ian McColgin View Post
    Found an illustration of the method I like. Depending on how you set up for the build, you can do this right on the station molds once they are set up.

    https://www.youtube.com/watch?v=OiLZUR1Go-I
    Thanks for that Ian, I was having trouble visualising your description.
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    Default Re: Deck Camber?

    Quote Originally Posted by Jim Ledger View Post
    Here's a deck beam and three templates set up in the bow of a catboat. There's a straightedge hitting the top center of each one. Each one is a different curve.





    Here's the same deckbeam and patterns superimposed on each other. The need to increase the crown as the beam narrows becomes obvious.
    This is a perfect example of maintaining a straight line the length of crown apex.

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    Default Re: Deck Camber?

    There is language regarding deck camber in both the Lightning plans and in the Class Specs.

    The Specs say "The deck may be curved to a radius of not less than a 20' arc." [emphasis added]. In the plans, it says, "Note: Deck camber to be laid down as arcs of a circle of 20'0" radius."

    The note in the plans was straightforward enough, but I found the language of the specs confusing.

    So I asked Dave Nickels, one of the big Lightning builders, how I should loft the deck. He said that, to avoid a slight "hollow" in the forward deck, the deck CL should go in a straight line up to the stem. He showed me the method in the Youtube video from #6, above. I tried that method but wasn't confident with my result.

    Once I was using Rhino, I just drew a straight line from the stem backward, and lofted my deck beams so their CL heights created a straight line to the stem.

    So each of my deck beams has a different arc, and there is much more camber as the deck goes forward.

    I have since looked at Lightnings with a 20 ft deck camber everywhere, and I couldn't see the "hollow" that I went to such lengths to avoid. So, as with so many things on my Lightning, it seemed like a good idea at the time, but I don't know if all of the fussing was worth it.

    It was one of a long string of dubious decisions that turned my Lightning into an 8-year project.

    But, by hook or by crook, we are going sailing this Summer!

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    Default Re: Deck Camber?

    Actually using the traditional parabolic beam camber doesn't necessarily give a straight centreline at all – it all really depends on the sheer (or the line of the top of the coamings) and the plan view shape (and thus the beam length). More often than not applying a constant percentage crop (rise) - or amount in the foot - gives a slightly convex centreline, rather than straight. Ply will still lay on it however.

    I do have a program (written by me!) that allows me to create a surface where I can specify the heights of the centreline and create variable cambers from it. With this I can create any shape centreline that I want. Sometimes for example I use it to have a centreline that sweeps with the sheer to a certain extent. Whatever, it always seems to produce a surface that is intrinsically fair and which ply will lay over easily.

    Of course, builders of my designs don't have to work out their beam cambers - each beam has it's Table of Offset so that the shape and bevel is known.

    Cheers -- George
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    Default Re: Deck Camber?

    Quote Originally Posted by Ian McColgin View Post
    Found an illustration of the method I like. Depending on how you set up for the build, you can do this right on the station molds once they are set up.

    https://www.youtube.com/watch?v=OiLZUR1Go-I
    I used this method to set up a shaper jig. Using the jig it was easy to make every beam with a different radius to create a conical shape for foredeck, aftdeck and coachroof. The deck looks good but the sheerline is determined by the cone of the deck and the width of the hull. This is not always the same sweepinf line you see in the profile and needs to be hidden by a rubstrake or a lapping bulwark like in my case.

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