epoxy vs. Poly

[dougirwin13]

Hi Roy!

I hope you and yours had a good weekend - mine was on the busy side. But that's life :D

I am a bit pressed for time - also I think some of my responses have been a bit on the lengthy side :) So, in this case, I'll address the crux of your post, rather than a point by point response.

First an interesting reference from http://www.sas.org/E-Bulletin/2001-12-0 ... olumn.html :

"Once you think about the idea carefully a contradiction occurs. Galileo recognized this and exposed the fallacy for all to see. If a heavier object falls faster than a lighter object, and a lighter object is tied to a heavy one the heavy object should then slow down due to the action of the lighter object. On the other hand, the lighter object tied to the heavier object that creates a compound object that is heavier still than the heavy object, so it must fall faster. Whenever an idea generates two conclusions that are contradictory, that idea is incorrect."
"The only correct conclusion is that all objects fall equally fast. Based on the last few columns we can put this into more modern language, the acceleration due to gravity is a constant."

Galileo went on to PROVE his theory using inclined surfaces, carefully constructed spheres and intricate water timing devices. These experiments were performed in air, not vacuum. These experiements are fully supported by modern physicists using high speed photography, laser measurement, electronic timing and electromagnetic releases.

Secondly a scenario for you:
Consider two men of identical weight jumping out of an aeroplane, wearing identical parachutes. One deploys his chute immediately. The second experiences parachute failure (the cord snaps). Now the person with a deployed parachute has a massive surface area and the second does not. How will their rate of fall be affected?

If weight is the primary factor they will fall at the same rate. If surface are is the primary factor one of them is going to make a terrbile mess very quickly.

Look forward to reading your response.

-doug

[Roy Stewart]

Hi Roy!

Hi Doug

I am a bit pressed for time - also I think some of my responses have been a bit on the lengthy side So, in this case, I'll address the crux of your post, rather than a point by point response.

"Once you think about the idea carefully a contradiction occurs. Galileo recognized this and exposed the fallacy for all to see. If a heavier object falls faster than a lighter object, and a lighter object is tied to a heavy one the heavy object should then slow down due to the action of the lighter object. On the other hand, the lighter object tied to the heavier object that creates a compound object that is heavier still than the heavy object, so it must fall faster. Whenever an idea generates two conclusions that are contradictory, that idea is incorrect."

"The only correct conclusion is that all objects fall equally fast. Based on the last few columns we can put this into more modern language, the acceleration due to gravity is a constant."

The quote above is used to attack the argument that heavier objects will necessarily fall faster than lighter objects, not a theory that I have ever subscribed to . . . . . it isn't possible to judge how the light and heavy objects in the thought experiment will behave because there is no information regarding the density and shape of the objects.

Furthermore the conclusion (namely that all objects must fall at the same rate) is not established by the experiment. . . due to the fact that density and shape are not part of the experiment, there is not necessarily a contradiction at all. . . . taking density and shape into account, there are several possibilities, one of which is that the heavier object falls more slowly than the lighter object. . . . ( for example a heavy man with a parachute deployed falls more slowly than light man without the parachute) . . . . .the bottom line being that in any real case, the result of tying the objects together will depend upon the mass, density, shape and size of the objects. . . . and the result will not always be the same. The experiment attempts to graft two of the possible results of tying two falling objects together, but those two results, although they are both possible in seperate situations with different objects, are not possible for any two particular objects . . . .

Galileo went on to PROVE his theory using inclined surfaces, carefully constructed spheres and intricate water timing devices. These experiments were performed in air, not vacuum. These experiements are fully supported by modern physicists using high speed photography, laser measurement, electronic timing and electromagnetic releases.

Although these experiments are conducted in air they are designed to simulate a vacuum. . . thus the experiments say nothing about the effect of friction on rates of fall in spite of the fact that they are conducted in air.

Secondly a scenario for you:
Consider two men of identical weight jumping out of an aeroplane, wearing identical parachutes. One deploys his chute immediately. The second experiences parachute failure (the cord snaps). Now the person with a deployed parachute has a massive surface area and the second does not. How will their rate of fall be affected?

If weight is the primary factor they will fall at the same rate. If surface are is the primary factor one of them is going to make a terrbile mess very quickly.

Yes exactly. . . . differences in friction affect the rate of fall of objects of the same weight, that is obvious

It seems that you have decided that I am suggesting that heavier objects fall faster. .. . . I have never said that. . .. . what I said was that heavier objects have more gravitational potential energy (i.e more 'thrust'). . . . . . whether or not the heavier object falls faster than any other object depends also upon the amount of drag present. . . . . and drag is affected by surface area and shape. . . . so the density and shape of the objects must be known before their rates of fall can be calculated.

My most recent posts were describing how a relative improvement in the thrust/drag ratio of a falling object is achieved when an object is scaled up ( thus by definition keeping the same shape and density). .. . . . and the reason why this thrust/drag ratio improvement occurs is because the surface area of the object does not increase as at the same rate as the mass of the object does at it increases in size. . . . . . however it DOES NOT follow from this that I am saying that heavier objects must fall faster, as I have explained



Thankyou for the fascinating mind exeriment, it reminds me of an Ancient Greek 'proof'. . . . . . and is just as illogical.




Roy

[Phil]

id like to get back onto surfing speed here

Roy ive been thinking about your claims that your boards travel over 30mph and im still not convinced that it is possible (on the size of the waves you surf)

Going by all the videos ive seen of you surfing you have never really surfed anything over head high and its always been fairly mellow crumbling waves

So lets say hypothetically that your surfing a 3m (9.8ft) wave. According to experts on average depth a wave will break in 1.3 times the height of the wave. To get the speed of the wave when it breaks would be 3.13 x square route of depth according to airy wave theory the wave speed is only governed by the depth so

3m x 1.3 = 3.9m

So the speed of a 6m wave

3.13 x √ 3.9m
= 6.2 m/s
= 13.8 mph

so the maximum speed the wave travels at when it peaks and breaks would be roughly 13.8 mph, so as the wave hits the sandbar it refracts that allows you to stay on the face of the wave and travel down the line just staying ahead of were the wave is breaking but given that you boards are a lot longer your likely to be able to travel further ahead of the breaking part of the wave generating more speed so lets say you can travel 30% faster than the wave breaking before you drop out of the wave that’s 4.14mph giving you a top speed of 18mph a lot slower than your 30mph claim and that’s on a 10ft wave a lot bigger than anything ive seen you surf on your videos

so there are a lot of factors I haven’t included but I don’t think they are going to effect the speed by a great amount ± 10% maybe? At least not enough for you to hit 30mph, some one on a shorter board pumping it down the line or staying on the peak generating more speed would possibly go faster, but from what I’ve seen in your videos you just drop in and stay in trim with the wave cruising down the line

[Baratacus]

id like to get back onto surfing speed here

Roy ive been thinking about your claims that your boards travel over 30mph and im still not convinced that it is possible (on the size of the waves you surf)

Going by all the videos ive seen of you surfing you have never really surfed anything over head high and its always been fairly mellow crumbling waves

So lets say hypothetically that your surfing a 3m (9.8ft) wave. According to experts on average depth a wave will break in 1.3 times the height of the wave. To get the speed of the wave when it breaks would be 3.13 x square route of depth according to airy wave theory the wave speed is only governed by the depth so

3m x 1.3 = 3.9m

So the speed of a 6m wave

3.13 x √ 3.9m
= 6.2 m/s
= 13.8 mph

so the maximum speed the wave travels at when it peaks and breaks would be roughly 13.8 mph, so as the wave hits the sandbar it refracts that allows you to stay on the face of the wave and travel down the line just staying ahead of were the wave is breaking but given that you boards are a lot longer your likely to be able to travel further ahead of the breaking part of the wave generating more speed so lets say you can travel 30% faster than the wave breaking before you drop out of the wave that’s 4.14mph giving you a top speed of 18mph a lot slower than your 30mph claim and that’s on a 10ft wave a lot bigger than anything ive seen you surf on your videos

so there are a lot of factors I haven’t included but I don’t think they are going to effect the speed by a great amount ± 10% maybe? At least not enough for you to hit 30mph, some one on a shorter board pumping it down the line or staying on the peak generating more speed would possibly go faster, but from what I’ve seen in your videos you just drop in and stay in trim with the wave cruising down the line

where in the world did you get that information? The speed a wave forms up and breaks is not a constant. It varries with the angle of the wave across the shelf or reef or beach that is causing it to form up and break. Depending on which way the current and cross currents are travelling, you get slower and faster waves at the same location on different days. If a wave is comming straight in accross the break it closes out and the break is almost instantaneous. If the swell is commin g at a steep angle to the break it will make a wave that peels very slowly. If yo are talking about the speed of the swell itself, that varries with the current. That Tsunami in india last year was only 20 ft high but traveled at nearly 200mph.

[Phil]

by current do you mean wavelengh?

from my understanding (maybe i got it all wrong)

In shallow water the speed of a wave is proportional to the square root of the depth. on a point break the angle of the wave plays a part but on beach with well formed banks which a wave hits head on would slow as it hits shallower water and peel off?

[Baratacus]

by current do you mean wavelengh?

from my understanding (maybe i got it all wrong)

In shallow water the speed of a wave is proportional to the square root of the depth. on a point break the angle of the wave plays a part but on beach with well formed banks which a wave hits head on would slow as it hits shallower water and peel off?

wavelength is proportional to the speed of the wave. The longer the wavelenght, the faster the wave period. Current is water moving sub-surface. In a current, water particles are moving allong the current. In a wave, the wave oscilates while the actual water particles only move a little bit if at all. In shallow water (the waters depth is less than half of the wavelength) and calm wind conditions, the trough of the wave slows while the crest remains the same. Once the trough of the wave deminishes past the crest, the wave peaks and spills over. Depeding on how rappidly the trough is receding and how fast the wave is moving will determine the shape, and how hollow your wave is going to get. The speed of the crest of the wave can be affected by strong winds and pressure. A strong onshore or offshore wind can either blow the wave down or hold the wave up . A beach break is often a slower more gradual break, but depending on the angle that the waves hit the beach, and the angle of the banks, the break can be a close out or a nice peel. The steepness of the beach will also affect the shape of the wave and how fast it breaks.

(sorry the explanation is so breif, I lost my first post and didn't have the motivation to type it all out again.)

[Roy Stewart]

that’s 4.14mph giving you a top speed of 18mph a lot slower than your 30mph claim and that’s on a 10ft wave a lot bigger than anything ive seen you surf on your videos

Hello Phil I did 22.5 mph yesterday on a head high sloppy onshore wave, it was a slow day for sure.

I don't see what basis you have for deciding that my board can travel only 30% faster than the wave, but whatever your reasons are, they are incorrect, as I have just gone a lot more than 30% faster than the wave, and that's on a slow day. Judging by what we achieved yesterday, I am confident that I will be able to crack 30mph plus on a wave around head high, but time will tell.

[Roy Stewart]

Going by all the videos ive seen of you surfing you have never really surfed anything over head high and its always been fairly mellow crumbling waves

Well you can't judge by the videos, I have been surfing for about 38 years and only taking videos for about 2 years, I have ridden a lot of waves in the 3 times and 4 times overhead range, but recently have been riding smaller stuff, partly due t the fact that I have lots of children who like to get out in the water. . . .they suffer disappointment if they have to stay on the beach because the surf is too big.

Please allow me to point out that judging by your videos you have never surfed at all !

Lets not make too many erroneous assumptions

[San Diego Longboarder]

Well, I didn't learn much about epoxy boards on this page, but I'm glad to know that I will always have 1.3 time the height of the wave in depth below me while I'm surfing my favorite reef break.

[Phil]

well if you read what i posted you would notice i was talking about the waves in roys videos *not* a reef break

roy im only making assumptions on what ive seen from your videos why not post a video of you in 4 times over head waves on a 18ft longboard, then ill belive you

also how are you geting these speeds GPS? theres a mass of infomation on the net about the accuarcy of GPS when caculating speed, people have tested various diffrent GPS units all gave a diffrent results

[Roy Stewart]

Hi Phil,

Regarding GPS readings we are following the method used by windsurfers. . . . this method has been tested extensively and gives readings accurate to 0.4 of a knot. If you would like to read about it and download the track analysing software for your own use then have a look at

http://www.gps-speedsurfing.com/gps.asp?mnu=forum

http://www.gpstm.com/

http://gpsactionreplay.free.fr/index.php?menu=1&choice=6

One of the excellent features of this system is that it allows speed tracks to be emailed for independent verification.

Now regarding this "show video of you surfing in 4 times overhead and I will believe you" what are you trying to say? Your initial post was an attempt to prove that i can't surf faster than 18 mph in the waves I surf. . .. I have already proved you wrong there, and in passing merely corrected you regarding your irrelevant comment that "you have never surfed anything over head high" now you are saying that you will only believe it if you see video of it. . .. . well stuff you. . . . . I don't like being called a liar, which is what you are effectively calling me. . . .. . like I said, over the past 38 years of surfing I have surfed 3 and 4 times overhead waves often. . . .. no need to question it, it's not a big deal, anyone can do it, now stop trying to wind me up on a personal level, we were having an interesting physics discussion, so don't turn it into a Shitfest.

And if you are going to be a jerk about it I must repeat that we have seen no evidence that you have surfed at all. ... . am I supposed to doubt that you have ever ridden a wave because you haven't proven it ? No. I take your word for it. . . .




[/i]

[Phil]

yes you proved me wrong and im not trying to be a jerk, just cant get my head around an 18ft longboard holding it in a 4 times over head tube is all, but ill take your word for it

[dougirwin13]

Calculating the travel speed of the wave is only part of the formula.

Angling across the wave makes a difference to distance coverred.

-doug

[Baratacus]

yes you proved me wrong and im not trying to be a jerk, just cant get my head around an 18ft longboard holding it in a 4 times over head tube is all, but ill take your word for it

big gun surfing... I'm not talking about the rediculously labeled 6-7 foot guns you see in the surf shop today... I mean a real GUN. You can't take a short board on a huge wave except to drop in and jump out before you pick up too much speed. They are quick and can turn sharp, but you need a board that can move fast, hold the line and remain stable. A short board doesn't do that. Especially when you hit the chop or catch the wind off the face of the wave... a little light board will blow right over the top. I surf 3x overhead plus and I don't take anything smaller than a 9 on em. (well... I did, but my poor little 8'4 tripple stringer thruster got snapped in half)

[dougirwin13]

Hi Roy,

Thankyou for the fascinating mind exeriment, it reminds me of an Ancient Greek 'proof'. . . . . . and is just as illogical.

Excuse me? That sounds a little insulting. Has the been a communictaion break down here?

I was actually a bit shocked by that, since I thought we were having a mature discussion. If that wasn't your intent perhaps reading your posts before hitting "Submit" might be in order - tone is very easily mistaken in this medium.

I have been trying to understand what you have been saying. But some of your statements seem contradictory (no doubt communication breakdown) and I've had difficulty coming to grips with statements which say "you are wrong" and supply no supporting principle, citation, formula or other reference.

I have held off responding to this over the last few days so that I could put some serious hours of research into nailing down exactly what you are getting at. You see, I am trying to understand and discuss, not insult or fight.

What I have done is take some of your core statements and research them both on the internet (wider searches on Google, but mostly focussing on NASA sites and the nice array of Wikipedia physics articles) and in the library. The statements I focussed on were:
* mass increases with the cube of size
* drag increases with the square of size
* thrust/drag ratio
* force/drag ratio

The last couple didn't yield much. But the first two tend to lead to articles on Terminal Velocity. Is this what you are trying to discuss?

I'm looking forward to bringing this back to discussion directly relevant to surf craft. Especially since many people on here can't quite see how it is relevant.

-doug

[Roy Stewart]

Hi Roy,

Thankyou for the fascinating mind exeriment, it reminds me of an Ancient Greek 'proof'. . . . . . and is just as illogical.

Excuse me? That sounds a little insulting. Has the been a communictaion break down here?

Hi Doug, no insult intended, just commenting on the quote in question. . . .I do find it really fascinating to wrap my head around these things, and the quote did remind me of the Ancient Greek style of thinking. . . .

What I have done is take some of your core statements and research them both on the internet (wider searches on Google, but mostly focussing on NASA sites and the nice array of Wikipedia physics articles) and in the library. The statements I focussed on were:
* mass increases with the cube of size
* drag increases with the square of size
* thrust/drag ratio
* force/drag ratio

The last couple didn't yield much. But the first two tend to lead to articles on Terminal Velocity. Is this what you are trying to discuss?

I am discussing the relationship between the force of gravitational attraction which drives surfboards and the drag forces which hold them back. ... . terminal velocities are part of the discussion certainly.

In relation to surfboards the experiments which assume a vacuum don't tell us much, because there is no drag in a vacuum. I have been working on the force which drives the board and the drag which holds it back. . . . words my two main points are:

1) That the force which drives a board( or rather the force of gravitational attraction which pulls the board ) increases in direct proportion to increases in its mass. (really I should be saying board and rider, because the mass of the rider also increases the force which moves the board)

2) That the drag which occurs does not necessarily increase (all else being equal) in direct proportion to increases in mass, in fact it can increase at a lower rate, which is demonstrated by the fact that mass increases with the cube as an object is scaled up whereas surface area increases only with the square.


In order for the discussion to make sense, it has to be understood that the force which moves board and rider does increase as their mass increases, this is often not understood, in my opinion due to the fact that there is a maximum possible acceleration in free fall. . . . . this tends to make people assume that the force which makes objects fall is of the same magnitude for all falling objects. . . which is obviously false.


Nice to hear from you Doug.

[dougirwin13]

Hi Roy,

Apologies for another delay... Things have been rather hectic this week

Hi Doug, no insult intended, just commenting on the quote in question. . . .I do find it really fascinating to wrap my head around these things, and the quote did remind me of the Ancient Greek style of thinking. . . .

In that case none taken Thanks mate and likewise.

I am discussing the relationship between the force of gravitational attraction which drives surfboards and the drag forces which hold them back. ... . terminal velocities are part of the discussion certainly.

Very much so, I should think. Since it is about drag, gravity and velocity.

In relation to surfboards the experiments which assume a vacuum don't tell us much, because there is no drag in a vacuum. I have been working on the force which drives the board and the drag which holds it back. . . . words my two main points are:

With you now. I wouldn't say "nothing" tho. For one thing they give us the gravitic constant for a given gravity field (like Earth) It also proves that mass has no part in pure acceleration (i.e., dragless environment). More below, anyway.

1) That the force which drives a board( or rather the force of gravitational attraction which pulls the board ) increases in direct proportion to increases in its mass. (really I should be saying board and rider, because the mass of the rider also increases the force which moves the board)

F=MA, yes. And the A (acceleration) comes from the wave energy, as released by the force of gravity. Also, as a craft is moving and the wave catches up it lifts the board a bit more, gravity comes into effect and the board is able to harness that energy as a result.

One nitpick tho - I think it's probably more accurate to say the mass of a board imparts it more momentum (force with direction - i.e. a vector) at a given velocity. That momentum allows it to overcome more drag than an object which is identical in all respects except of less weight/mass. This explains how mass has an indirect impact on acceleration.

2) That the drag which occurs does not necessarily increase (all else being equal) in direct proportion to increases in mass, in fact it can increase at a lower rate, which is demonstrated by the fact that mass increases with the cube as an object is scaled up whereas surface area increases only with the square.

U-huh. Because momentum is a factor that can overcome drag... To some extent.

In order for the discussion to make sense, it has to be understood that the force which moves board and rider does increase as their mass increases, this is often not understood, in my opinion due to the fact that there is a maximum possible acceleration in free fall. . . . . this tends to make people assume that the force which makes objects fall is of the same magnitude for all falling objects. . . which is obviously false.

Again, momentum increases when an object increases in mass, but not (necessarily) velocity. F=ma is a vectorless measurement. Whereas "p=mv" (momentum = mass x velocity) is vectored. Momentum has an important part to play!

Something to consider. If we place a large lump of lead and a feather inside a thick, long, clear cylinder, pump the air out and upend the cylinder both items fall at the exact same rate (well... The difference would only be measurable over several thousand kilometers). Gravity is the same and the mass of the objects hasn't changed. So how does air magically make the rock fall faster? It doesn't, it makes them both fall slower.

Something else. A surfboard is planing along on a glassy surface, approaching a lump in the water. When it hits that lump it will lose some of it's energy and slow down to a certain extent. Now make one change - make the board heavier (but otherwise identical in every respect). The board will now slow down less because it has more momentum.

With falling objects this means the heavier of two objects is able to reach a higher terminal velocity because it's mass is able to "push the air aside" more easily than it's lighter counterpart. Note you would need to drop the two objects from over one hundred meters for this to be noticable (or have a weight difference of at least several hundred times).

Some preliminary calculations using the Vt formula show some interesting results. Of course things are a little more complicated than the pure results suggest, but more on that later, if you are interested.

Nice to hear from you Doug.

Likewise Roy... Although I am almost wishing I didn't show my wife and kids some of those videos... They want us to order a matching set of day-glo pink wetsuits I mean, pink really isn't my thing!

Hope you have a good weekend.

-doug

[Roy Stewart]

Hi Doug,

Thanks for your reply, a short one because I can't find any point of disagreement

Anyway I am very interested in your calculations using the Vt formula when you have time to explain them.

by the way Pink grows on you and it's great for safety. . . except it's a bit dangerous sometimes being chased by all those pink loving women. .. . .orange is another good one, a touch more aggressive perhaps

[dougirwin13]

Hi Roy... And anyone else keeping tabs on this thread

This time a very long, two part, reply

OK, Vt calculations. Terminal Velocity is:
Vt = sqrt ( (2 * m * g) / (Cd * r * A) )
Or
Vt = sqrt ( (2 x mass_of_object x gravitic_acceleration) / (drag_coefficient x fluid_density x area of drag) )

At first I simply played with the formulas in a spreadsheet - very handy way of poking around with physics equations. And the initial results certainly supported the importance of mass that Roy was talking about.

Then I got a bit more serious about things and made sure the appropriate constants were correct (which was pretty easy). Now I did some interesting what-if analysis based on some real board dimensions, basing mass on real world reports of weight for composite, PU and HWS/solid wood boards where possible. Where real weights couldn't be found I calculated a weight based on the differences between boards of those type at smaller sizes (inexact, but probably close enough at this stage). Note that this initial batch of investigation focuses on pure Vt, without anything else being factored in. So that represents a board dropping through the air bottom down. Like an air drop down the face of a two meter wave, for example. I set the drag coefficient to a standard of 0.2, which probably isn't too far off for a surfboard (don't know for sure).

Since this is a longboarding forum I'll report on the "standard" longboards first. Results:
- 5kg 9'0" compsand. Vt = 16.1 m/s (4.7 km/h).
- 9kg 9'6" PU . Vt = 21.1 m/s (5.8 km/h).
- 20.3kg 9'3" wooden board (Roy's). Vt = 32.0 m/s (8.9 km/h).
Now, that speed is the terminal velocity of that object falling through the air bottom down in meters per second. So the difference does seem quite marked.

Now the wooden board has a Vt twice that of the compsand board... But is more than four times heavier.

But there is something else important we need to consider, too. These objects will ACCELERATE (i.e., speed up) by 9.8 meters (less the effect of drag) every second until they reach their Vt. So over a drop of two meters (or five meters) the difference wouldn't be huge. Probably measurable without too much difficulty, but not as different as the numbers above suggest.

Now what if we put an 80kg rider on the board (i.e., add 80kg to the mass)?
- 85kg 9'0" compsand. Vt = 66.7 m/s (18.5 km/h).
- 99kg 9'6" PU . Vt = 66.35 m/s (18.4 km/h).
- 100.3kg 9'3" wooden board (Roy's). Vt = 71.3 m/s (19.8 km/h).
Woah! Look at that gap in Vt drop! And with the ACCELERATION caveat I discussed above the difference will be even less noticeable.

Very interesting.

Also interesting is that the effect of drag on an object "falling through water" due to water's higher density is more pronounced (so the impact of drag area is greater), since water is about 1,000 times denser than air.

The drag equations themselves start out very simple. And become increasingly complex. Drag is fascinating.

This is by no means the end of the story!

With a smaller board carrying the same rider Vt almost reverses!
- 2.2kg 6'2" compsand RIDERLESS. Vt = 14.2 m/s (3.9 km/h).
- 2.7kg 6'2" PU RIDERLESS. Vt = 15.7 m/s (4.3 km/h).
- 5.8kg 6'2" HWS RIDERLESS. Vt = 18.9 m/s (5.2 km/h).
No surprises so far... Now add the rider:
- 82.2kg 6'2" compsand. Vt = 87.0 m/s (24.1 km/h).
- 87.2kg 6'2" PU. Vt = 87.2 m/s (24.4 km/h).
- 85.8kg 6'2" HWS. Vt = 73.0 m/s (20.2 km/h).
What the?! Don't be alarmed. The HWS board is WIDER. So it has a larger "A" value. Making that the same results in 88.9 m/s (24.6 km/h) for that board Told you I used real dimensions where I could.

So that deals with the drop for speed/power generation. Less steep waves will have less of an "air drop" factor, meaning more contact with water. More of the board in contact with water will further negate the effect of mass on acceleration (I can go into this too if people are interested).

Oh yeah. I did these calculations for boards in the 6', 9', 11.5' and 17' ranges. Figures available on request.

Next I moved on to where I think mass really matters... Momentum!

Follow-up post coming shortly.

-doug

[dougirwin13]

OK all, part two now... Sorry about the "essay" for anyone who isn't interested in exactly how and why surfboards work.

I took the weight of "boards with riders" from the previous post and applied the momentum formula, keeping velocity at a neat 66 m/s (18.3 km/h). Momentum (p) is calculated with:
p=mv
Or
p=momentum x velocity

Here even a small increase in weight has a noticeable effect.
- 85kg 9'0" compsand. p = 5,606.70 kg/ms.
- 99kg 9'6" PU. p = 5,874.00 kg/ms.
- 100.3kg 9'3" wooden board (Roy's). p = 6,616.50 kg/ms.

Hhhmmm.. Interesting. Heavier boards most definately have more momentum than lighter boards.

What does this mean in practice? I believe the key is in the fact that a board with more momentum retains energy better than one with lower momentum. Add to this the "topups" of energy we can get from the wave.

This neatly explains why smaller/lighter boards have to expend all of their energy quickly (they have no choice because drag will force them to loose it anyway). They need a big energy charge to get going (they have to drop down the wave face to really get going). The one thing a lighter board has going for it is that the rider can impart some energy directly to the board via that ugly hop or with a powerful cutback or similar. This applies equally to "modern progressive" longboards and shortboards. For real glide you will need a certain amount of mass.

Personally I feel we are really just touching the tip of the iceberg here. Contours and other design features have a powerful interaction with water/drag that becomes very complex. For example, there's the way momentum (paired with other design features) overcomes the drag of Roy's tunnel fin and turns his big, heavy boards in mean machines!

Fascinating, in my opinion.

-doug