Surf Simply's Question Time: Saltwater vs Fresh water buoyancy & Volume vs Length.
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- Опубликовано: 23 июн 2020
- In the third episode of the Question Time Series, Harry addresses the density and thus buoyancy differences between a fresh water wave pool/ river wave, and salt water ocean wave and the real world effect, plus what happens when you maintain the same volume but change the surfboard length.
Here are the original questions:
I'd like to offer my own opinion on an interaction you mention for I'm not convinced that a low pressure region at the tail of the board is "sucking" water in an upward direction. You say that during trimming, with more rocker we should imagine that there is more weight at the back of the board. This is accurate and is a useful mental tool for surfing, although I think the cause is not a result of the board and water exchanging momentum at the tail, but a difference in the centre of lift. With more rocker, the centre of lift is further forward. To then counter this force so that the net torque of the surfer and board is zero, the surfer must shift their weight further forward than usual, as if "there is a weight on the back of the board". I'd love to hear what you think about this different perspective. Thanks for the great content and food for thought.
From Phill Halloran
Should I be concerned about less buoyancy in river surfing that normal saltwater surfing?
From Private User
What might be the difference in paddling or riding between a 5’8 and 6’8 both with 40 litres and otherwise identical designs.
From Javier Bustinza
Notes from Harry:
3:10 Harry mentioned playing with some shaping software. You can download AKU Shaper (akushaper.com) or Shape 3D (shape3d.com/) for free.
10:47 Harry mentioned Surfing Explained Episode 7, he actually meant Episode 8: • Surfing Explained: Ep8...
12:52 Harry spoke about CFD animations on surfboards • The Sci-Fi enters CFD:... 
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Hi Harry - could you comment on what is happening during noseriding?
Although I haven't surfed the same board in the fresh and salt water I can confirm that from the surfing perspective itself there's no much difference (if waves were the same). But you will definitely feel the difference when you're paddling through the incoming waves or when you're falling into the water - fresh water is much softer and thus it's easier to paddle through and more pleasant to fall into :) And just to mention that besides rivers and pools there are also lakes and some gulfs which could be surfed perfectly
Awesome description thank you
thanks for the video. there is an error in the calculation:
the difference between freshwater and saltwater is not 2.2kg, it's not a percentage of your weight, since your weight is the same regardless. what is different is the weight of the displaced water, which for 30L would be only about 800 grams. this in turn is only one percent of the body weight and has even less impact then 2.2kg would have :)
Hey Harry, pronunciation was fine. Thank you once again for taking your time to respond. I enjoyed your longboarding example and I can't say I wasn't compelled to find answers.
Longboard noseriding is interesting and not something I've yet succeeded at, but from countless video examples I've watched, large amounts of water both flow over and break onto the rear of the board. I think this more than accounts for the stable position of the board with the rider at the nose.
But from there I found it hard to conclude much about the flow of water under the board. Solving fluid flow problems is a complex task and goes way over my head.
Thanks for the response, looking forward to further content. This channel is one of the best when it comes explaining surfing.
Glued to watching this the whole time! Love it Harry!
Glad you enjoyed it Rob!
Thank You Harry! I had in mind a 42 liters Lost smooth operator (6'9 20.88 2.75) and a 42 liters Firewire Glazer (6'0 22.00 2.875)
Ah ok, so pretty different boards. Sorry I didn't understand your original question.
I'd be very interested to hear your opinion on that!
Very interesting!
I was really thinking about this since I went on a surf trip in Costa Rica lately and I brought a new surfboard that is roughly 32 liters (6'0'' x 19.63'' x 2.375'') and I weigh 85kg.
The board felt great and I was feeling it floating in the water.
When I got back home and start paddling in the river's fresh and cold water with my 5-4 wetsuit I instantly felt sinking.
So I made some math based on board to weight ratio
Weight in Costa Rica without a wetsuit = 32liters/85kg= 38%
Weight in Canada with 5-4 wetsuit, boots and gloves = 32/88.6kg = 36%
So then I had a 2% (suit weight) + 2,6% (salty and fresh water difference) 4.5ish% (32 liters *1.045 = 33.44) difference should feel like around 1.5 liters difference if I made it right.
Maybe I'm just surfing too small of a board in the end, but it's interesting to understand the difference in this situation where multi factors apply making the difference slightly notable.
Nice videos!! Congratulations and many thanks!
About the tail rocker question, Coanda effect is the reason, very clearly seen when you put a spoon under the water tap. Very interesting to feel how the water flow sucks the spoon. Realising that was a gamechanger for me. I can't help about the calculation hehe
Thank you for the explanation!
Hello to all
Regarding the first question
A few of the new board designs, like Rob machudo's "seaside", is a much higher volume on approximately the same length/shape
I was wondering how a shorter board is like (5'3" , 30 liters) when compared to the the older designs at (6'0" , 29 liters)
You answer with such precision and clarity, I'm definitely a fan!
May I ask, could you explore the approach to duckdiving from a beginner to intermediate perspective?
Everyone says that even longboards can be duckdived, it just takes practice and developing the right technique. But that goes for everything: every surfboard can be ridden too! Beginners, nonetheless, are advised to use big softboards bc they're easier to learn on.
Isn't there any tips to start duckdiving? For example, is there a suggested ratio between board volume and surfer weight, roughly speaking, considering average levels on every other factor that comes in play (like fitness, age, timing skills,...).
Do few liters make a difference when starting to duckdive?
p.s.: people will say "there's no use in getting a smaller board you can duckdive when you can't catch waves with it" but things aren't so extreme sometimes.
I live in the South of Italy. Most times, we surf in active bad weather, and those are the times I struggle to get out the back, because waves are big, wave period sucks, there's strong winds, strong currents and no channel. The harder part to get past is the one with the white water, where it's much harder to duckdive. With those conditions, I prefer having a smaller board I can (try to) duckdive to get out the back (and I don't even struggle that much with catching waves since they're more powerful and push more), instead of grabbing my fat board and get smashed around the entire session. I'd rather push my limits with little volume (harder to surf but duckdivable) than with a duckdive I still haven't on lock (more volume for easier surfing but can't get to the line up).
So, is there a range of volumes I can look at when I learn to duckdive as a beginner-to-intermediate surfer (around level 3-.5 on your tree of knowledge)?
For reference, I'm 5'2 (160 cm) x 116 lbs (53 kg) and I usually ride either a fat board for mushier waves [5'10x21 1/4x2 3/4 (38L)] or a lighter board for better waves [5'11x20x2 1/2 (31L)]. Well, I've tried a 5'8x20x2 3/8 (28L) and I felt like duckdiving was incredibly easier. Was it a lucky one time thing or can 3 liters REALLY make that much of a difference??
I am so sorry for the length of the comment. I understand if it doesn't get a response =D
Thank you so much anyway!
What's the bowling section of a wave? I'm reading Barbarian Days by William Finnegan and he often refers to bowls.
Hi, Harry. How much difference is there between two identical boards, one made from PU and other from EPS/Epoxy? Would this difference be perceived by an Average Joe?
Great informative videos and great channel that I've been long following !
Regarding your math "challenge" I have an educated guess (most probably very imprecise and simplistic).
Let me say upfront I'm no math or physics student. However in my course I had some physics class and I like the subject.
I basically used the change in momentum(p), because the change in momentum equals Force ( mathematically expressed by F = dp/dt). In words, momentum only changes if an external Force is applied. My calculations:
So, you have 16Kg of water coming in horizontally at 4m/sec and still leaving at 4m/sec but at an angle of 10º.
So your water will now have an horizontal component with speed 4m/sec * cos(10º), and a vertical component with speed 4m/s * sin(10º).
Both changes in the horizontal and vertical component of the water velocity correspond to a change in momentum since p = m*v. The water horizontal momentum decreases which probably reflects as drag in the board, and the change in vertical momentum reflects as a downward force in the board by Newton's 3rd law.
Focusing in the change in vertical momentum, from the formula p=m*v we have that the expelled water has a vertical momentum of p = 16Kg * 4m/sec * sin(10º).
Since the water that came in had no vertical velocity, i.e no vertical momentum, and that changed by "p" in 1 second the change in momentum is dp/dt = (16Kg * 4m/sec * sin(10º) - 0 ) / 1sec = 16Kg * 4m/sec² * sin(10º) = 11 Newtons or about 1.1Kg of downward force in the board.
Since your numbers are conservative the actual force is probably 3x or 4x bigger... (?)
Still, it sounds small, but that is how far my math/physics goes :)
The length of the board we are looking at is only 1 meter, so the dt should be 0.25 seconds for water moving at 4m/s, so the final result would already be 4 times larger.
So I hear epoxy is needed for this reason. Would you say this isnt true in reality. I prefer PUs but now live in texas and need a new board for bsr.
Hi Michael, yes exactly. When combining the overall total weight of the rider and equipment, the difference makes up a very tiny amount. An eps blank might weigh only 1-2lbs different to a pu blank, so a total weight of say 170lb compared to 172lb is negligible to 99% of riders in almost any wave.
Surf Simply sweet man! love the vids and thanks for the help!
A back of a napkin calculation would give 16 * sin(10) = 2.78 kg/s directed upwards. Now kg/s is not a force or anything, someone else in the comments has already worked out the details.
But to be honest, these numbers don't mean anything anyway, unless you put them in perspective! No surfboard has 0° rocker (except maybe some unrefined mini simmons). You mention 10° is average, so maybe it can be compared to 8° and 12°. And then calculate how much your weight would have to be shifted back and forth to compensate for this. I might write a scientific paper on this later
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