I’ve read stuff about how to measure rocker in a shop with shaping/glassing rack, but is it possible to get accurate rocker measurements from a board out in the field? And by “out in the field” I mean that you’re at the beach with no shaping/glassing rack to rest the board on, etc. If it’s possible to do it with accuracy, can you please tell me how?
I can take the following with me: tape measure, calipers, 8’ & 4’ rulers, 8’ x 5" x 3/4" piece of plywood, string, masking tape, pencil, and notepad.
I’m not sure… but here could be my way of doing that:
take 2 sticks, put them in the sand deep enought so you can attach a string between them
level the sand under the string so it’s “horizontal”
attach a string, the more horizontal the better, the distance between the sticks is slightly longer than the board
put the board under the string, deck facing down
take all measures you want between the string and the board
take the thickness measures with the caliper
once back at home, if you report the “string” measures + the thickness measures done with calipers, you should get a fair enough estimation of the board.
Here a super duper drawing to illustrate my wandering:
Got a kerb?
Put the board, deck up, with the stringer on the kerb.
Lie in the gutter and measure the gap between board and kerb at regular distances (chalking inch marks on the kerb first would help).
As an aside, I got to thinking about measuring volume. I reckon you could do this pretty accurately using only a set of digital bathroom scales
(plus a couple of volunteers and a body of still water… say a pool or big trough).
using the scales, weigh the board in kilos.
weigh the bathroom scales! (also in kilos)
you, the board, your volunteers and the scales get into the still water.
stand the board on it's rail in the water (get your helpers to keep it vertical without pushing on it). You will see how far the board will sink under its own weight.
put the scales on the upper rail at the mid point. You will see how far the added weight of the scales sinks the board.
push down on the scales until the board is submerged to the centre stringer and note the reading on the scale.
do the math(s). Add the reading + weight of the board + weight of the scales, this will give you the weight of water that is displaced by half the board.
So double that figure to give the total displacement of the board in kilos.
The board's volume (in litres) is the displacement (kg) divided by the specific gravity of the liquid in the pool
(fresh water = 1, salt water = 1.025).</li>
Tape the string, nose to tail, on the deck side. That is your datum line. Then measure from the string to the deck, every 6, 8 or 12 inches, marking the location with a piece of tape. Record those distances. Next measure the thickness at each location, with calipers. Record those measurements too. On your plywood using one edge as the datum line, plot those points and distances. In this process you will have captured the deck rocker, bottom rocker, and the thickness distribution of the subject surfboard.
Look up Bill Barnsfield method of measuring rocker. You just needa long strieght edge and a tape mesure and a little help might be needed for a Longboard
Most of the replies center on measuring the rocker, which makes sense because that’s what you asked. Bill Thrailkill’s method is methodical, logical, and gives more key info than just the rocker, its worth remembering.
If on the other hand you just wanted to duplicate or record the rocker, I would go about it a little differently, using a carpentry technique called scribing. You don’t give the length of the board you are working with, but under 8’ would be easier than over 8’, since plywood and masonite come in 8’ lengths.
I would rip (cut lengthwise) a piece of plywood or masonite (could also use cardboard if you had a long piece, say from a refrigerator packing box or the like) about 10" wide by the length of the board. Then I would just guesstimate the rocker and mark it with a pencil or sharpie, get it as close as I can just by guessing, and cut along the line.
I’d take the piece with the concave curve with me to the beach, along with a marker, and a spacer about 2" or so (I’d bring a couple different sizes just to be safe, any rectangular object will work. Laying the board down bottom up, I’d place my pattern on top, fitting the imagined rocker to the real thing. Then taking my marker, and using the spacer (keeping it vertical), I just slide along the board marking a line the exact shape of the rocker.
A little bit of work up front, but once at the beach the whole operation would go quick and easy, and I would end up with the actual curve of the rocker, not just some reference dots to be connected later with a bent stick. (I think the o.p. did mention accuracy.)
You could do the same with the convex piece, on the deck side, to duplicate the curve of the deck.
A variation of the scribing technique that is used to make countertop templates would be a variaton of Huck’s method ,but instead of trying to cut the board to the approximate shape leave it straight,drive two stakes into the sand at either end of the board (bottom up) attach the long straight piece to them, then you take a bunch of shorter pieces (3 or 4 inches long) and tape them to the straight piece following along the profile of the board, making sure to overlap the pieces a little so they can be taped together for stability (putting tape on both sides of the little pieces keeps them from flapping around), this would make a pretty good representaion of the rocker without having to make sure it doesn’t shift as you measure , it can then be used as is as a template or can be taken home to measure in conditions that are a little more convenient…
How do you factor in the depth of a single concave? Would you measure the depth of the concave with a straightedge and then deduct the depth of the concave from the rocker measurement at that point? For example, let’s say that at 24" in front of the halfway point the rocker measured was 2" and that INCLUDED the 3/16" deep concave. Would you do this to get the rocker? 2" rocker - 3/16" deep concave = 1 13/16" rocker
BILL BARNFIELD SAID: “I think the reason people struggle with this is that they really don’t understand how simple it is to create the baseline by simply pressing on a straight stick at the center point. Level doesn’t matter. Balance doesn’t matter. Racks don’t matter. Length of stick doesn’t matter. Even gravity doesn’t matter. You could be in outer space and it would still work perfectly. Simply press on center with one hand, measure quickly and efficiently with the other… End of story.”
That was from an 8 year old thread so could someone please explain to me why the LENGTH OF THE STICK DOES NOT MATTER? If the board is 8’ long and the stick is only 3’ long then how can you possibly place the stick at the center point and measure the rest of the rocker past the point where the 3’ long stick ends? What am I missing here?
True, a three foot stick wouldnt be very useful, but a stick that is a bit more than half the length of the board would be.
You could do an 8’ board with a 5’ stick, you’d just have to do each end separately (a stick longer than the board, is a bit easier tho’).
Using the Barnfield method, you can get both hands free to measure by (after you have pushed the stick down on the centre), putting something under the stick to keep it in that position, a stone, piece of wood, pile of coins, whetever.
So the bottom line for using the Barnfield method is to hold the stick down at the center point with one hand and then prop up the end(s) of the stick and start measuring. Got it, thanks!
Would you factor in the depth of a single concave the way I previously described?
OK, here’s my guess. If you hold the ruler at arms length and measure the building height visually, then measure the door height with the ruler in the same place, you can get a scale. Say the door measures out at 1 inch. A standard commercial door is 7’, so your scale is 1:84. Then use that to scale the building from the measurement you came up with for the height of the building. Its an approximation, but should get you in the ballpark.
There’s probably a more accurate method using trig, but it doesn’t readily come to mind.
