# deck concaves - depth, width and impact on flex...

OK so as more folks are doing parimeter stringers more are doing deck concaves.

I’m wonder how deep?

How Wide?

and how this effects flex?

The deck concave would results in thinning the core (which would increase flex), but introduces additional compound curves (which would decrease flex/stiffen).

My current thought is 1/4" deck concave, thus thinning my planned net finish thickness for my 6’8" from 2.5" to 2.25" thick, but due to the lack of discussion about deck concaves this depth has no data to support it.

how do you measure the thickness of a concave decked board? is a board that says 2" thick measured after the concave or before the concaved is added?

the last egg board posted said 1 7/8"… meaning in the middle of the concave or at the thickest point on the board? just trying to grasp how thin that board really is…

4est-- somewhere on sways there is a dis. of the concave deck thing complete with an animaton of the theory of the way it helpes the board move (i am no longer using the buzz word flex)

as for concave depth… I would say if you placed a straight edge across the concave how deep is the cavity?

as for the flex, I’m refuring to adding or reducing stiffness, not discussion about dynamic shape. I know a while ago Bert talked about his convace deck enabling the bottom contours to change with flex creating vee and concave…

I missed the other discussion you talked about, and was not able to find much usefull in the archives (with only a quick look).

I’ve always thought the boards thickness was measured at its thickest point. On a concave board its no longer in the middle of the board. Is this accurate?

Yep Dan, thats how I would measure the thickness of my boards. As you move the callipers across the board, they register the thickest point ie the ‘peak’ of the rail.

Put a straight edge across the board and measure the concave depth.

On the boards I have built with concave decks, they seem to have a ‘natural’ width, when I shape the rail taper into the foam/balsa rails, prior to vaccing the deck on each board has its own high point determined by rail thickness and taper.

I then shape the concave to suit.

My super thin flexy 13’9" has a 1/4" of concave across the deck, and it still flexes heaps.

I imagine that when the concave becomes really extreme (like rolling the board into a half circle) it would affect the flex a lot but at 1/4" there’sno way that it makes a noticeable change in the flexibility

Flex adds thickness in the cross-section, this creates a moment to resist bending…how large it is will depend on how thick the skin is, and how deep the concave (or vee) is.

A 1/4" concave on a glass/epoxy board, with 0.02" thick glass, will change flex a lot. The same concave on a board with 0.25" thick hollow wood panels will have less of an impact.

If you are doing glass/epoxy, make one either way. Bet you can tell the difference. If you are making 19 foot long hollow wood boards, bet you can’t tell.

Same is true of hull concaves, btw.

Quote:

Flex adds thickness in the cross-section

Hi Blakestah, I assume that you meant that “concave adds thickness in the cross section”

With long wooden boards, if I add a 1/4 of an inch to the thickness I can definitely notice the board stiffen up, not so with the concave, not sure why.

The concave in my case is introduced by bending the entire board, which will increase thickness in the cross section (see picture at the bottom, below)

4est was talking about creating concave by reducing the thickness in the middle of the cross section, which will not add thickness in the cross section (see picture in the middle, below)

Quote:
Quote:

Flex adds thickness in the cross-section

Hi Blakestah, I assume that you meant that “concave adds thickness in the cross section”

Of the skin.

Not the board.

Take a piece of paper and bend it.

Then take the same piece of paper, form a concave, and bend it.

Notice a difference?

The points on the paper on the “inside” and “outside” of the curve you are trying to bend are lengthened and shortened by the depth of the concave and the bend you apply. Much as they would be in a beam.

Which is why if you really want to dial in flex, I’d recommend

1. perimeter stringers

2. flat cross-section

Play with the stringer width and glass schedule first to put it in the right ballpark. Then cautiously introduce concaves, but check board flex in a controlled way…it will decrease.

Yes, which is why my boards have perimeter stringers !

Please note also that the way in which I introduced deck concave DOES add ‘thickness’ in cross section to the entire board, not just to the deck skin, whereas the hollowing out of the deck as 4est describes only adds ‘thickness’ to the deck skin.

Thus one would expect that creating deck concave by bending the entire board and thus effectively thickening the entire board as I have done would reduce flex more than if the board is thinned as 4est described to create a concave deck.

However it’s still the case that with my boards, adding a quarter inch to the overall thickness decreases flex markedly, whereas the deck concave has made no noticeable difference at all to the flex.

hey 4 est

i agree with tom and others

board thickness seems to be the single most imortant factor wrgtf

in sandwhich construction

i have no template over 2 inches thick in a shortboard

i intend to make my boards under 7 ft ,2 inches thick or less

and get my float from a bit more width

so for a high performance thruster

it would be say

19 1/2 or 19 3/4

2 inches thick or less(with concave maybe)

an 8 ft

2 1/4 thick or less

and benny and Danb would know numbers for longer boards

as far im aware deck concave seems to reduce panel stiffness while still alowing stiffnes and volume in the rails.

Bert alluded to the fact that it allows for bottom Morphs as well