Deck Camber

[Hesp]

A month or two ago Woodenboat had an article about deckbeam camber. I don't have it to hand now, but I expect you've read it. In it the author told us that the conventional way of using beams of constant camber produces unfair decks. There must be a lot of unfair decks out there.

Coincidently, Classic Boat magazine had a short article on deck camber at about the same time. The author, John Perryman, says "It is common practice for a designer to quote a standard beam camber, say 3/8" per foot of max beam and for the builder to make a camber pattern or 'crop mould' with all the beams made to it. However, to do so is to ignore (or be unaware of) the possibility of an extremely unattractive deck shape and appearance."

I think the Woodenboat author was not quite as reserved.

I had heard of this effect before, but not seen a deck suffering from it. To see just how bad using constant camber beams could be I cranked up the 3D CAD and did some drawings. Then forgot about it.

Today I saw a post on the boatbuilding board asking about deck camber and it reminded me of the drawings I had done. I've put them on the web, you can see them here:

http://members.aol.com/Deckcamber/

It seems to me that even with a flatish sheer, constant camber beams of normal camber produce a fair sheer. If the hull had pinched in ends it might produce an unfair deck, but I think that's a hull problem rather than a deck problem. In this case the use of varying camber beams would be to overcome an existing problem rather than prevent a problem happening.

Having said that, the drawings show that the geometry will produce an unfair deck.

So what to do? If you're sawing out deckbeams I don't suppose it's that difficult to saw out beams of different camber, but laminating....... I think I'll carry on using constant camber beams, but bear the following in mind:

i) On a flat sheer use a low deck camber, particularly on a hull with 'pinched in' or straight ends.

ii) On hulls with a more pronounced sheer use a moderate camber. This looks better to me anyway.

Anybody else got any thoughts on this.

John

[WFK]

As you get closer to the stem with your beams, it'll definatley produce a flat spot if you don't increase the camber on your beams. Obviously, you can't change the sheer line, so a slight increase to your pattern is what's required. It's more of an eye thing to see what looks right.

[Hesp]

Originally posted by WFK:
As you get closer to the stem with your beams, it'll definatley produce a flat spot if you don't increase the camber on your beams.Well this prompts another question. If 3/4" marine ply is use for the deck, as is common, is the area 9" (say) from the edge of each sheet going to remain flat, as its impossible to introduce curvature into these edge areas? I mean at the deck edge, and the edges where sheets join. (This spells trouble for those who want to use an elitical shaped beam with greater curvature at the ends).

At the bow where there is little width, will the deck be dead flat because the ply is to stiff to bend?

John

[ 05-01-2002, 09:50 AM: Message edited by: Hesp ]

[WFK]

John, When I was first shown this "flat spot" it was taken care of in the first 2 or 3 beams and we aren't talking about a huge amount in the change of camber. As far as your plywood being to stiff, we always used two layers to make up the total thickness, and your seam would start out close to 4' from the stem well beyond any change in your original camber. One other point is that with two layers, you have a much stiffer deck as well as a better transition from seam to seam.

[Ian McColgin]

I thought the article's points were well taken. I also had the experience of making constant camber, being told I'd done it wrong and had to plane and trim and shim, and eventually observed that many did just that and never tumbled to the actual problem. Wooden boats were made fare by hard work after the fact.

One additional wrinkle I've used to make up for the fact that I have a very poor eye. I'v stretched a centerline string and twiddle with both tension and judiciously wrapping parts of it with another twine to move the center of the parabolic curve aft a bit. When I could, I'd make a full length batten and move some light weights around on it. It's worth making a bunch of battens and laying out the deck like 1/3 wood and 2/3 air so you can view it from different angles. Don't you hate it when your darling has an unattractive bump when heeled over. Lots of squint and fiddle to make it look right.

If you've tight curves and are making a plywood deck, use multiple layers of 1/4".

G'luck

[Paul Jeffrey]

Interesting discussion here on cambering a deck. I designed and built an 18 ft racing sloop out of strip plank several years ago. As time went by I decided to redo the deck, so a couple of years ago I designed a new deck using a constant camber. The deck beams where laminated for strength and light weight and set in the hull every foot. The fairness of the camber is good to the eye. But as mentioned at the bow which is fairly narrow it does flatten out some. This is not a problem as far as I can see, at least by eye. You just run out of enough bow width to be able to generate noticable camber.

The camber of the hull was base on the arc of a 22' radius at amidships. The hull does have some sheer rocker and it may be considered to be alot for a boat it's size. From station 0 to amidships the sheer drops 4.25" then rises 1.75" to station 10. It is a ten station boat with a plumb bow. As in the drawings on the link above the increased rocker of the sheer does increase the fairness of the deck camber.

I do not have any photo's of the boat to post at this time but will later this summer when I get back to working on that hull again. I had to put the project aside for the past couple of years. But I am clearing out some room in the shop [selling a couple of boats] and will be able to work on it again. I will then take some photo's and put up a web page on the project.

But over all so far the deck frames seem to look good on the boat and I am pleased with the results so far. But as always I will not know truely until the entire deck is planked. But the constant camber does seem to be working.

[chesterm]

Just finishing up a 23 foot mahogany triple. the constant camber forward would have resulted in an unpleasing flat foredeck. we raised the camber until it looked right and built it that way. A lot of deck camber just looks good. regards, mitchell in CT

[Art Read]

"WoodenBoat" has an uncany knack for publishing "how-to" articles a few issues after I already tackled the project in question. I had already "roughed" out all my deck beams with one master pattern as I had read in all the "standard" texts. It worked fairly well over most of the length, but I definatly noticed the "flattening out" phenomenom up at the bow. A little fairing with a stiff batten and recutting the first two beams seemed to smooth things out somewhat. If I squint at it at just the right angle, I can still see some unfairness at a few spots. But only enough to aggravate me, knowing about it, not enough to really stand out. (At least not to my eye...) Actually , the worst spot is at the transition from the foredeck to the side decks. But I suspect that was just my own blunder in shaping the cockpit carlins. I'm pretending it's "supposed" to look like that... I think on a smallish boat, Bob Cleek was spot on in that other "Deck beam" thread. Shape your beams so they "look" right and they probably will be. Do a lot of "checking" with battens and "sighting" along the various angles before laying the deck and it should go on without undue trouble. Mathmaticaly derived arcs are all well and good, and probably much more critical on larger vessels, but if you're as bad a wood butcher as I am, you're going to be doing a lot planing and sanding anyway to get 'em to look right once installed. ;)

http://www.imagestation.com/picture/sraid16/pc687b2c0cfee46ccea6a65fec3b21b5d/fdf3df84.jpg

[ 05-02-2002, 12:46 PM: Message edited by: Art Read ]

[videoguy]

Art how in the world could you call yourself a woodbutcher after doing work like that. Darn good job I say!