Diminishing deck beams

Hello all,

I am making a mill creek 16.5 and have noticed that some mill creeks I have seen tend to lose the deck camber, in my eyes too abruptly 2' or so from the Bow  and Stern. Is there a way of preveventing this ? I am considering installing additional deck beams diminishing from bulkheads fore and aft.  Does this bother anyone else or just me?

Fred


7 replies:

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RE: Diminishing deck beams

I did something similar on my plans-built LT-17 and Ches. 14.  I used scraps from the laminated beams (which is just forward of the cockpit) to form an additional beam about two feet abaft the stem.  In the LT-17, I also installed a 4 mm. beam about two feet forward of the stern; this beam was aligned vertically, and was about 1 1/2" deep.  To spread the load at the point of contact with the deck, I made a web about 3/4" wide of 3 mm and placed it between the beam and the deck, forming something rather like a curving "T" shaped beam.  I was happy with the results.

RE: Diminishing deck beams

HI Jim ,

good to know.I think two extra beams will do it, thanks

RE: Diminishing deck beams

I am looking out the window at my MC 16.5, perched on top of my car.  That foredeck has a bit of compound curvature (camber plus sheer).  I don't know for certain, but possibly if you force holding camber as you go forward you will not be able to pull the plywood down to the sheer.  You could end up with a nasty buckle someplace.  I recommend you put your extra deck beams in temporarily with hot-melt glue and then dry fit the foredeck.  Be sure you can pull it down to the sheer all the way forward and that everything is fair.  If you can, then great - glue the beams in to stay.  If not, then either modify the beams until you can or get rid of them and follow the original design. 

PS:  You will find a slight hump where the deck meats the coaming just aft of the forward end of the coaming.

Good luck and happy building.

Paul G

RE: Diminishing deck beams

I understand Paul G's concern, but I don't think you need to worry. Because the curvature of your added deck beams will match the curvature of the standard deck beams, you'll not be changing the radius -- and hence, the amount of curvature -- required of the deck from what is already required of it.

I do think Paul G's suggestion of test fitting is excellent. For just this reason, I built both my LT-17 and Ches. 14 using countersunk silicon bronze screws AND expoxy to attach the deck to the sheer clamps.  I made simple jigs to locate the screw holes uniformly.  I also used fabric clamps to help hold the deck in place while I drilled and drove the screws. If your deck doesn't "want" to be curved so sharply (a problem with the LT-17 foredeck) you can wet down the plywood as you test fit, and let it dry to the finished deck curvature.

After test fitting everything and "dry screwing" the deck down, I backed the screws out, applied the epoxy, and screwed it down again for real.  One note if you go this route: silicon bronze screws can easily be over-torqued to the point of snapping, so I drilled pilot holes almost the diameter of the screws, and lubricated the screw threads with bar (hand) soap when I drove them dry (the epoxy lubricates them just fine when driving wet).

My approach does mean filling the screw heads afterwards, but that's easy.  I should add that I paint my hulls and carry the paint up 3/4" onto the deck, i.e., over the screw heads, leaving the great bulk of my deck to be varnished.  Many CLC boats are finished this way, and are quite attractive.

RE: Diminishing deck beams

While the test fit will tell the tale, I do want to clarify the issue of compound curvature.  If you pull a string from the deck beam at centerline up to the bow, you will notice that it does not contact the forward bulkhead.  In order to contact the bulk,head you must pull it down at the point.  Replace the string with a batten bent to touch the bulkhead and you will see the curvature the sheer line must take for the deck to fit.  So, you have transverse curvature (the camber) and longitudinal curvature (the sheer).  Plywood does not like this.  That is why, when installed per design, the camber flattens out where there is no bulkhead or beanm to force it to stay.  If sheer was a straight line then the deck would easily maintain a constant camber and there would be no issue.  Hope this clears up the technicalities.  Now dry fit it all and lets hope you get what you are looking for.

Paul G

RE: Diminishing deck beams

Paul G

 I have fitted the deck after drawing the conclusion you mentioned ie. a Transverse curviture being formed on the fore deck. I now view it for what it is, but at the stern I have installed a second bulkhead to square of the aft locker (to stop the locker having a wedged end) this bulkhead acted like a deck beam i think the result was well worth the effort, will try to post a photo when I have finished the build.

Thanks again

Fred

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