Author Topic: More glide = less acceleration?  (Read 5802 times)

Billekrub

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More glide = less acceleration?
« on: May 23, 2015, 01:45:47 PM »
According to the Bert Berger video on board rocker and other topics at:
http://www.standupzone.com/forum/index.php/topic,27240.0.html

For a short time, a banana can outpace a rocket?

The less rocker, the less the acceleration of a board (even though it has more glide), whereas a more rockered board is in a lower/first gear,
AND the faster the less rockered board once up to speed, as if it were in 3rd gear.
All else somehow being equal.

Why is this?  Board weight would be one reason, board size another, but just rocker?

It would imply, when comparing how different boards best catch waves, the heavily rockered boards for advanced surfers should be better for last minute accelerations as the wave approaches, and less rockered boards require a less radical turn prior to wave arrival in order to maintain speed.

Board weight differences would favor the banana over the rocket, when weights differ.

The rockered catch would benefit from higher "revs." in a lower gear and no reason to paddle as much in advance of wave arrival.



SlatchJim

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Re: More glide = less acceleration?
« Reply #1 on: May 28, 2015, 11:06:31 AM »
OK, I'm lost.

Zooport

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Re: More glide = less acceleration?
« Reply #2 on: May 28, 2015, 11:19:27 AM »
I duno either.  I just paddle and ride waves.  Does seem odd though. 

« Last Edit: May 28, 2015, 11:25:07 AM by Zooport »
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spookini

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Re: More glide = less acceleration?
« Reply #3 on: May 28, 2015, 11:28:04 AM »
My preliminary calculations suggest false...

I'd guess (until Pono chimes in) that a rockered board decelerates after every stroke, thus the need to accelerate w/ each new stroke.  A yugo stuck in first gear.
But that doesn't mean the Yugo accelerates faster than a Ferrari NOT stuck in 1st gear.
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Re: More glide = less acceleration?
« Reply #4 on: May 28, 2015, 11:45:30 AM »
That actually makes perfect sense. 


Glide is how long it takes a board to stop moving.  If it takes longer to stop moving, its should also take longer to start moving.

In a wave scenario, a very small board (provided the rider is not too heavy…Bert goes over this too), will get on plane very quickly because it takes less energy to get a smaller planing surface up out of the water.  So small board = quick acceleration.  A big (i.e. long) board will need more energy to get on plane, but will carry that energy more efficiently once it gets moving. 

Bert does a really kick ass job explaining a lot of stuff, but its easy to forget stuff you already know when listening to him :P 


PonoBill

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Re: More glide = less acceleration?
« Reply #5 on: May 28, 2015, 07:52:04 PM »
First, Bert is talking about surfing, which is a very dynamic environment. Rocker in a steep, rear critical wave does a lot of things. One is that it conforms to the curve of a wave and therefore transfers energy effectively. You aren't paddling to get up to wave speed, you're paddling to get to a fraction of wave speed that enables the energy transferred to your board to accelerate you the rest of the way. A flat board won't do that effectively because either the nose and tail are pressed down, or the contact area for energy transfer is small. Conforming to the shape of the wave gives the most effective drag area, which is what transfers force to accelerate the board. A dragless board would transfer no force from the wave and would only accelerate with paddle strokes. The water in a wave doesn't move much, it just goes up and down. The energy wave moving through the water is what you need to catch, and you need a balance between skin friction moving across fundamentally immobile water, and form drag which is picking up some energy from the wave. then there's simple old gravity, which is accelerating you downhill, independently of the shape, except that a nose or tail stuffed under the surface will make an effective brake, as we have all noticed when a pearl tosses us into a faceplant or stepping back too soon pulls us out of the wave.

That same high drag area sucks the life out of the board in flatwater. Downwinders are in between. You need a lower drag to reach and maintain good speed with reasonable effort, and enough drag to effectively transfer energy from the swells. That's why downwind boards don't win flatwater races--too much drag, even when they're skinny. It's also why I hate raceboards for downwinding. Unless you're a 20 year old 140 pound alien the swells just roll under them. You need to reach nearly full wavespeed to catch anything.

Boards don't have gears.
« Last Edit: May 28, 2015, 07:59:42 PM by PonoBill »
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Re: More glide = less acceleration?
« Reply #6 on: May 28, 2015, 08:07:28 PM »
I'm thinking faster with more rocker means the board can lift creating less wetted surface= more top speed.. A board with less rocker will glide better at a lower speeds but get resistance when trying to lift when at higher speeds..

PonoBill

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Re: More glide = less acceleration?
« Reply #7 on: May 28, 2015, 08:23:15 PM »
Lift is negligible at paddling speeds, but it plays a part at planing speed.

Here's a good article on rocker:

http://hosted.verticalresponse.com/235086/6c4116f4ac/441000351/8f0221997c/

Realize though that none of these guys are talking about flatwater race board or downwind boards. The shape of these boards and their rocker has a lot more to do with kayak/canoe/surfski/scull experience and technology than surfing.
« Last Edit: May 28, 2015, 08:39:02 PM by PonoBill »
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Re: More glide = less acceleration?
« Reply #8 on: May 28, 2015, 09:05:21 PM »
That actually makes perfect sense. 


Glide is how long it takes a board to stop moving.  If it takes longer to stop moving, its should also take longer to start moving.

In a wave scenario, a very small board (provided the rider is not too heavy…Bert goes over this too), will get on plane very quickly because it takes less energy to get a smaller planing surface up out of the water.  So small board = quick acceleration.  A big (i.e. long) board will need more energy to get on plane, but will carry that energy more efficiently once it gets moving. 

Bert does a really kick ass job explaining a lot of stuff, but its easy to forget stuff you already know when listening to him :P

This might be true when surfing steep waves where the smaller wetted area on a smaller board on a plane results in higher speed.   For downwind boards, however, we have found that a larger surface area creates more lift and therefore planes earlier.  In some downwind conditions a wider board can actually be faster than a narrower board because of the additional lift created.  So a narrower board is not always faster, even if balancing on the board is not an issue. 

For this reason, Formula windsurf boards are very wide to create a lot of lift which allows them to plane in very light wind.  Even though the additional width is slower in sub planing conditions, once on a plane, these boards are much faster than a narrow board that does not create enough lift to get planing.
« Last Edit: May 28, 2015, 09:10:17 PM by blueplanetsurf »
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Eagle

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Re: More glide = less acceleration?
« Reply #9 on: May 28, 2015, 09:24:02 PM »
I think Bert talked about weight area and speed as important parameters.

You do need to reach close to full wave speed to get on the swell - and being lightweight is key.  Once you catch the swell you do use that wave's energy to plane to the wave ahead.  In small waves it is much easier using all our lower rocker boards and hardest with the M-14 exactly for the reason noted in the Greenlight Surfboard Design Guide Vol.2 - Rocker -

"Therefore, boards that are heavily rockered (very curvy), create a relatively large amount of both lift and drag, and therefore tend to take off later than boards that have what is referred to as relaxed rocker (less curve). Boards with relaxed rocker create little lift, but also little drag, and therefore take off earlier than more curvy boards."

Once your board planes there is even less drag.  It is very fun to catch small waves but does requires high output similar to HIIE.

"As the board’s speed continues to increase until it is finally up and planing, the entry rocker section of the board comes almost completely out of the water, and only the gentle curves of the middle third continue to provide lift. At this point, little lift is required, and the small amount of lift that is generated is ideal, as it also produces only a small amount of drag."

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Blue crab

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Re: More glide = less acceleration?
« Reply #10 on: May 28, 2015, 10:03:47 PM »
The first thing I noticed on my F16 was that it was without question the worst flat water and upwind board imaginable. The drag is extreme & it feels almost like it is sinking between paddles.   In a downwind situation, things change for the better. The board accelerates extremely easily once there is even a relatively pitch on the wave, probably due to conformation with wave shape that Bill describes in conjunction with the lateral force of the energy wave. Once the board starts to plane, there is a positive feedback loop of awesomeness.   Lift -> less wetted surface -> vroom -> more lift -> less wetted surface - > faster speed -> repeat...... It is a 16 foot board that has less contact with the water during a big glide (6-8 feet) than any 12 or 14 foot board I have tried. The lift has got to be a critical part of this.

I am not convinced that narrow boards plane any faster than narrow boards. The advantage is probably a little less effort to paddle the board up to sufficient speed to get in.

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Re: More glide = less acceleration?
« Reply #11 on: May 29, 2015, 08:17:12 AM »
This happens with the F16 and M-14 - "To sum it up, the more rocker a board has, the faster the board must go to get up and planing on the water’s surface. But once it is up on plane, the greater curve of a heavily rockered board lifts more of the board out of the water, resulting in an overall decrease in wetted surface area."

Here are some images from Greenlight -
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Billekrub

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Re: More glide = less acceleration?
« Reply #12 on: June 01, 2015, 12:21:09 PM »
First, Bert is talking about surfing, which is a very dynamic environment. Rocker in a steep, rear critical wave does a lot of things. One is that it conforms to the curve of a wave and therefore transfers energy effectively. You aren't paddling to get up to wave speed, you're paddling to get to a fraction of wave speed that enables the energy transferred to your board to accelerate you the rest of the way. A flat board won't do that effectively because either the nose and tail are pressed down, or the contact area for energy transfer is small. Conforming to the shape of the wave gives the most effective drag area, which is what transfers force to accelerate the board. A dragless board would transfer no force from the wave and would only accelerate with paddle strokes. The water in a wave doesn't move much, it just goes up and down. The energy wave moving through the water is what you need to catch, and you need a balance between skin friction moving across fundamentally immobile water, and form drag which is picking up some energy from the wave. then there's simple old gravity, which is accelerating you downhill, independently of the shape, except that a nose or tail stuffed under the surface will make an effective brake, as we have all noticed when a pearl tosses us into a faceplant or stepping back too soon pulls us out of the wave.

That same high drag area sucks the life out of the board in flatwater. Downwinders are in between. You need a lower drag to reach and maintain good speed with reasonable effort, and enough drag to effectively transfer energy from the swells. That's why downwind boards don't win flatwater races--too much drag, even when they're skinny. It's also why I hate raceboards for downwinding. Unless you're a 20 year old 140 pound alien the swells just roll under them. You need to reach nearly full wavespeed to catch anything.

Boards don't have gears.

Great insight, thanks.

 


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