I have 2 boards that I would like to get closer to actual volume . One is a board I modified , so it started with a known quoted volume. The other is a custom , based on length width its a guess the volume, probably close.

Does math work ?

Yes, but you might end up running naked through the streets once you learn how.

There is only one way. And that is to forcefully sink it and measure how much water is displaced.

take it to the beach on a calm day, jump on it and add weights until it just sinks? also consider salt water is heavier etc...

Thats probably easiest. I Dont think the SG or density will matter for volume calcs. Difference in weight between fresh n salt only 0.025kg per litre. Sink away!

Divide the board into a series of rectangles. Calculate the volume of each rectangle, Add it up. That is your volume.

It will be less accurate if your board has lots of vee or concaves and stuff.

Put tape on the board down the centreline.
Mark regular intervals along the length. The more intervals you mark the more accurate the result will be.
Measure depth and width at each interval.
Calculate.

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forceten said..
I have 2 boards that I would like to get closer to actual volume . One is a board I modified , so it started with a known quoted volume. The other is a custom , based on length width its a guess the volume, probably close.

Does math work ?

Yep, maths always works.
Masking tape down the centre line of the board. mark out every 10cm.
Measure the max thickness and the width at each point.
Calculate the average width and average thickness of each section and using the averages, calculate the volume of each section, add them together and multiply by 0.848 (accounts for rail tuck, deck curve) and you've got the volume.
All easy using an Excel spread sheet.
Have checked this method against many manufacturers claims and it's accurate.
Even down to finding out why the 2007(?) Fanatic FSW 85 planed so early.
It was 93 litres, which was later confirmed by the German W/Surf mag which volume checks for its tests.

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forceten said..
>>>> so it started with a known quoted volume. >>>

Don't be so sure, there was a period, I think when boards first went wide, that they quoted, "equivalent/effective" volume. Which was always a lot more than actual. It did give a comparison of early planing ability, but completely wrong on flotation.

Weigh the blank

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Mark _australia said..
Weigh the blank

Well that's the way I try and do it, but I've had some funny results.

^^ me too, but I always weight the whole block first so I know the density, my 28kg was 27.8 which makes a difference on a kiteboard where we are dealing with a change in 50 - 80 g for different volumes. Its really easy to take off a couple litres too much when shaping

However it was in jest, I don't think forceten wants to de-skin the whole board

A number of years ago Sally and Sherriden (Gerro) built a tank with Mark Stone to satisfy their curiosity about actual board volumes. It turned out loads of diferent makes were very inaccurate but Marks calculated guessed volumes for his boards were very accurate so I would chat to him, he may still have the tank or will guess it Ha.

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powersloshin said..
take it to the beach on a calm day, jump on it and add weights until it just sinks? also consider salt water is heavier etc...

this... and dont forget to subtract the weight of the board.

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pommypair said..
A number of years ago Sally and Sherriden (Gerro) built a tank with Mark Stone to satisfy their curiosity about actual board volumes. It turned out loads of diferent makes were very inaccurate but Marks calculated guessed volumes for his boards were very accurate so I would chat to him, he may still have the tank or will guess it Ha.

So what happened to S and S, I haven't heard anything of them for years? Sheridan made great jump pads!

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decrepit said..

pommypair said..
A number of years ago Sally and Sherriden (Gerro) built a tank with Mark Stone to satisfy their curiosity about actual board volumes. It turned out loads of diferent makes were very inaccurate but Marks calculated guessed volumes for his boards were very accurate so I would chat to him, he may still have the tank or will guess it Ha.

So what happened to S and S, I haven't heard anything of them for years? Sheridan made great jump pads!

Were they a nice older couple who used to come out here every year from the UK (they also used to be pig farmers years ago?)?

That's them, about my age. They bought a nice property out chapman valley, white hills way.

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mathew said..

powersloshin said..
take it to the beach on a calm day, jump on it and add weights until it just sinks? also consider salt water is heavier etc...

this... and dont forget to subtract the weight of the board.

Hmmm. Maybe ADD the weight of the board to the weight it takes to sink it.

Reasoning: If it has 100L volume, but weighs 10KG, it will only float 90KG.. It still has 100L volume but only 90L (KG) of bouyancy

So do the manufacturers actually quote Bouyancy or actual Volume or just some made up number?

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So do the manufacturers actually quote Bouyancy or actual Volume or just some made up number?

Hard to say. Some are real numbers. Some its the width. Some its a magical relationship between width , length n style of board Bahaha!!!
Weird really. Generally A 530 mast is 530. A 170-210 boom is 170-210. A 42cm fin is 42. Why arent boards accurately measured, volume wise???? Youd think big manufacturers would have programs to calculate or even a tank to submerse board?? My great grandad couldve achieved this accurately in 1907!!
+1 for ADDING weight of board to volume.

Sinking the board seems simple enough. How then to measure the water displaced ?

Yes Sherriden did live Chapman Valley way but gave up sailing a few years ago and moved to the East Coast, he made the pads for the Stoney boards - our loss, good people.
I guess a tank that takes boards only has to have measurements of 1 ltr up the side from a set point, that point is when its easily full enough to cope with most board sizes? Back in the day the UK magazines used to check test board volumes for us this way. I think Stoney found one 90 ltr board about 15 ltrs out!

So a three kg lighter carbon version of the same board should effectively feel three litres bigger in floatiness .

Imax1, sure if its lighter for the same volume. It should float higher in water.

Forceten, go raid the local gym for weights. Use your body weight plus weights in swimming pool or shallow water. Not so easy balancing it all though.

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olskool said..
Forceten, go raid the local gym for weights. Use your body weight plus weights in swimming pool or shallow water. Not so easy balancing it all though.

I'd like to see that . Someone holding weights , balancing perfectly still , on a board with no floatation. It would have to be at least a three person affair . The floater would have to be perfectly dry. Any part of that person under the water would have to be taken into account , ( and there would be because the board is banana shaped , unless u do this with the board upside down ). Take in the salt water factor.

when i got my first board - BIC Dufour
i tried to calculate the volume based on measurement

joewindsurfer.blogspot.com/2008/10/sailboards.html

here is an exerpt
-------------------------
Nowadays that should NOT be necessary. Information is posted directly on the board.
Older boards or custom boards did not have data on the board.
Since my BIC Dufour was received before much internet and after it's popularity - so, NO info in the mags, i did a calculation of the volume at that time ...

It may be fairly obvious that volume is area and not weight, butt i wish to stress the obvious just the same...

Let me see if i can dig up my calculations :-)

First I broke the board down into sections and used the theory that a circle would reflect the shape of the board - this yielded a volume of 462 liters = obviously way OFF.

Next looked up the area of an oval which is length times width times 0.8
Again broke the board up into sections and came up with a figure of 157 liters. This is a little more reasonable, butt obviously low. It looks much bigger than my 160 liter short-board !!!

The closest approximation was using the simplest method. The board is 378 cm long, 68 cm wide and 11.5 cm thick. These values are all at the longest, widest and thickest points. If we assume the board was a block then 378 * 68 * 11.5 = 295.6 liters. Now we know it is not a block and if we assume it is more like an oval shape from the top, then 295.6 * 0.8 = 236.5 liters. We are already approaching more realistic numbers. We know that the board is also shaped. Since the board is NOT a lean mean machine, then perhaps we can apply the oval logic i.e. use a 0.8 factor. So 236.5 * 0.8 =189.2 liters. This number is only 5 % off the figures literature has given me - namely 200 liters.

So, i would simply use this method to estimate volume of a board in the future.
Length in cm * width in cm * thickness/height in cm * 0.8 (for oval) * 0.8 (for shaping) / 1000 (to give liters)

Will try this approach with the modern 160 short-board to test validity.

Length = 268 cm
Width = 79 cm
Height = 10 cm
So, 268 * 79 * 10 * 0.8 * 0.8 / 1000 = 135.5 for a 160 liter board
This is a larger margin of error 25/160 * 100 = 15.6 % error.
The problem here is there is a significant difference between a 135 and a 160 liter board.

How can we adjust for that and where is the error coming from ?

My original estimate for thickness was 13 cm and adjusted to 12. When i looked from the side, i put 10 cm.
If i put 12 cm as the thickness check out the results !!!
268 * 79 * 12 * 0.8 * 0.8 / 1000 = 162.6 liters
WOW - cannot get much closer than that...
2.6/160*100 = 1.6 % error ONLY
This shows that the thickness MUST be evaluated CAREFULLY...

when i got my first board - BIC Dufour
i tried to calculate the volume based on measurement

joewindsurfer.blogspot.com/2008/10/sailboards.html

here is an exerpt
-------------------------
Nowadays that should NOT be necessary. Information is posted directly on the board.
Older boards or custom boards did not have data on the board.
Since my BIC Dufour was received before much internet and after it's popularity - so, NO info in the mags, i did a calculation of the volume at that time ...

It may be fairly obvious that volume is area and not weight, butt i wish to stress the obvious just the same...

Let me see if i can dig up my calculations :-)

First I broke the board down into sections and used the theory that a circle would reflect the shape of the board - this yielded a volume of 462 liters = obviously way OFF.

Next looked up the area of an oval which is length times width times 0.8
Again broke the board up into sections and came up with a figure of 157 liters. This is a little more reasonable, butt obviously low. It looks much bigger than my 160 liter short-board !!!

The closest approximation was using the simplest method. The board is 378 cm long, 68 cm wide and 11.5 cm thick. These values are all at the longest, widest and thickest points. If we assume the board was a block then 378 * 68 * 11.5 = 295.6 liters. Now we know it is not a block and if we assume it is more like an oval shape from the top, then 295.6 * 0.8 = 236.5 liters. We are already approaching more realistic numbers. We know that the board is also shaped. Since the board is NOT a lean mean machine, then perhaps we can apply the oval logic i.e. use a 0.8 factor. So 236.5 * 0.8 =189.2 liters. This number is only 5 % off the figures literature has given me - namely 200 liters.

So, i would simply use this method to estimate volume of a board in the future.
Length in cm * width in cm * thickness/height in cm * 0.8 (for oval) * 0.8 (for shaping) / 1000 (to give liters)

Will try this approach with the modern 160 short-board to test validity.

Length = 268 cm
Width = 79 cm
Height = 10 cm
So, 268 * 79 * 10 * 0.8 * 0.8 / 1000 = 135.5 for a 160 liter board
This is a larger margin of error 25/160 * 100 = 15.6 % error.
The problem here is there is a significant difference between a 135 and a 160 liter board.

How can we adjust for that and where is the error coming from ?

My original estimate for thickness was 13 cm and adjusted to 12. When i looked from the side, i put 10 cm.
If i put 12 cm as the thickness check out the results !!!
268 * 79 * 12 * 0.8 * 0.8 / 1000 = 162.6 liters
WOW - cannot get much closer than that...
2.6/160*100 = 1.6 % error ONLY
This shows that the thickness MUST be evaluated CAREFULLY...

Select to expand quote

Imax1 said..
So a three kg lighter carbon version of the same board should effectively feel three litres bigger in floatiness .

Exactly, but some people seem to get WAAAYYYY too obsessive with the weight of their boards to the point where they are willing to pay $1000 more for a board that is half a Kilo lighter! (and then wonder why it doesnt last very long).

Thats almost the difference between having a big breakfast or not! (or expelling your used up breakfast - to put it in a polite way )

I don't think I have seen any 'high tech carbon' versions of the same board that have a 3KG difference from the standard version, but there could be some in the raceboard type sizes??

I dont think even the most sensitive sailor would be able to notice the difference of a couple of litres of bouyancy, especially in 120+ litre boards.

'Feel' though, is a very hard to quantify thing. One can definitely 'feel' the difference when sailing (planing) a board that is, say, 2KG lighter, but whether it makes anything but a psychological difference is one of the great conundrums of windsurfing IMHO.

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forceten said..
Sinking the board seems simple enough. How then to measure the water displaced ?

Or, perhaps easier, but potentially a little bit less accurate, is to simply measure the weight required to completely submerge it.

I litre of fresh water = I Kg

If it takes 85 KG of weights to just submerge the board, it has 85 Litres (Kg) of Bouyancy, but then add the weight of the board to that for it's actual volume in Litres.

So my 87 litre board that weighs 7KG has an effective 80kg of Bouyancy. Add another 10-15 kg of weight for the rig and more for all your sailing clothing (wetsuit, harness, helmet etc) and no wonder you need a board about 25L over your naked body weight to 'float'. So thats why my '87L' sinks Knee deep when I try to uphaul it despite my (naked) weight being 75 KG!

So girls, if your board doesn not quite float you, dont waste lots of money buying a larger one, just sail in your Bikini!

(If you do it in salt water the numbers will be slightly different.)

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forceten said..
Sinking the board seems simple enough. How then to measure the water displaced ?

You could overflow the tank, wait for it to finish dribbling, then catch and measure all the water displaced by he board. Or have marks on the side of the tank.

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Imax1 said..

olskool said..
Forceten, go raid the local gym for weights. Use your body weight plus weights in swimming pool or shallow water. Not so easy balancing it all though.

I'd like to see that . Someone holding weights , balancing perfectly still , on a board with no floatation. It would have to be at least a three person affair . The floater would have to be perfectly dry. Any part of that person under the water would have to be taken into account , ( and there would be because the board is banana shaped , unless u do this with the board upside down ). Take in the salt water factor.

True, it might be tricky to balance, and the deeper you submerge it, the more bouyancy it has because of the increasing water pressure, but that could be calculated and allowed for - not that it would matter much in the top few cm of depth.

(Add one 'atmosphere' of pressure for each 10m of water depth.)

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decrepit said..

forceten said..
Sinking the board seems simple enough. How then to measure the water displaced ?

You could overflow the tank, wait for it to finish dribbling, then catch and measure all the water displaced by he board. Or have marks on the side of the tank.

Or instead of catching the water , take the board out and measure how many liters to fill it again.