Darcy - You couldn't be more correct. Its those 'one wave in a million' that we snag that keeps the fire burning.
Back in 1976, I broke my ankle skating a pool just 3 weeks before a month long Baja trip. I went anyway and ended up dragging my right ankle behind me, unable to kick. Bad news since I was a kick-only, no arms style KBer.
I'd been having a hard time picking up waves on the way down the coast: Big Rock; Ocean Beach; San Miguel. At Quatros Casas, I wasn't doing much better. If I caught a wave (the rights at the main peak), I really couldn't put any pressure on my right ankle to turn. So I just planed straight down the line. The left was weak and didn't go very far.
Then, one day, a left popped up way out at the point where there hadn't benn any rideable ways before. A combination of swell and tide created a huge, rocket ship, horseshow bowling LEFT! But only one wave every once in a while was rideable.
So, I paddled out and waited. Finally, one set wave came to me and I backed doored this massively horsehoeing monster. From my take off spot I could look straight across at the other end of the horseshoe...about 40 feet way.....the wave was curved way way back. So I went, backdoored this thing on total autopilot and made it.
And then the wave shut down: fini; kaput. I didn't see it break again.
But I had my one wave and was able to spend the rest of the trip, happy as a clam, covered in dust and still with a busted ankle. But that one wave is still with my today and keeps me pumped.
Fastest board quad or tri?
Moderator: Moderator
Speed, smeed
Sound like a bunch of petrol heads
What - you scared of getting a bit of lip on the head? Your board only needs to go fast enough to make the wave you are riding. If you not making them then you are taking off too deep for your ability/gear.
The key to decent hotdog surfing is to retain speed through turns (or accelerate, if you're Farrer). To do this you need minimal stall before, during and after the turn. Having to sink the tail in order to get the board to turn is extremely negative to this end. Find a board (1,2,3,4,5,6 fins) that will turn without being stalled and you'll go faster ALL the time.
It's not what you got - it's how you use it!
Sound like a bunch of petrol heads
What - you scared of getting a bit of lip on the head? Your board only needs to go fast enough to make the wave you are riding. If you not making them then you are taking off too deep for your ability/gear.
The key to decent hotdog surfing is to retain speed through turns (or accelerate, if you're Farrer). To do this you need minimal stall before, during and after the turn. Having to sink the tail in order to get the board to turn is extremely negative to this end. Find a board (1,2,3,4,5,6 fins) that will turn without being stalled and you'll go faster ALL the time.
It's not what you got - it's how you use it!
-
Bryan Jackson
- Ripper (more than 100 posts)
- Posts: 158
- Joined: Fri Mar 28, 2003 12:14 pm
No, MTBarrels, I was correct in referring to “cavitation” and you are quite mistaken in this case 
, as both surfboards fins and boat propellors do indeed undergo cavitation.
From, www.keelynet.com/keely/cavity1;
"CAVITATION There are four basic types of cavitation. Fundamentally cavitation results from a drop in pressure on a liquid creates pockets or bubbles in the liquid - an increase in pressure causes these bubbles to collapse resulting in a tremendous "local" force which can cause damage to metals, emulsification, de- gasification, sonoluminescence and many other strange and wonderous phenomena.
CAVITATION: Cavitation is the formation and activity of bubbles (or cavities) in a liquid. Here the word 'formation' refers, in a general way, both to the creation of a new cavity or to the expansion of a pre-existing one to a size where macroscopic effects can be observed. These bubbles may be suspended in the liquid or may be trapped in tiny cracks either in the liquid's boundary surface or in solid particles suspended in the liquid. The expansion of the minute bubbles may effected by reducing the ambient pressure by static or dynamic means. The bubbles then become large enough to be visible to the unaided eye. The bubbles may contain gas or vapour or a mixture of both gas and vapour. If the bubbles contain gas, then the expansion may be by diffusion of dissolved gases from the liquid into the bubble, or by pressure reduction, or by temperature rise. If, however, the bubbles contain mainly vapour, reducing the ambient pressure sufficiently at essentially constant temperature causes an 'explosive' vaporization into the cavities which is the phenomenon that is called cavitation, whereas raising the temperature sufficiently causes the mainly vapour bubbles to grow continuously producing the effect known as boiling. This means that 'explosive' vaporization or boiling do not occur until a threshold is reached.
There are thus four ways of inducing bubble growth: 1. For a gas-filled bubble, by pressure reduction or increase in temperature. This is called gaseous cavitation. 2. For a vapour-filled bubble, by pressure reduction. This is called vapourous cavitation…
…we may distinguish between four different kinds of cavitation according to how it is produced: 1. Hydrodynamic cavitation is produced by pressure variations in a flowing liquid due to the geometry of the system. 2. Acoustic cavitation is produced by sound waves in a liquid due to pressure variations…
HYDRODYNAMIC CAVITATION: In a flowing system, the liquid velocity varies locally and at the points of highest velocity, low pressures and cavities occur.
The conditions which mark the boundary or threshold between no cavitation and detectable cavitation are not always identical. For example, the pressure of disappearance of cavitation has been generally found to be greater, and less variable, than the pressure of appearance. Three cases of flow cavitation arise: 1. Travelling cavitation occurs when cavities or bubbles form in the liquid, and travel with the liquid as they expand and subsequently collapse. 2. Fixed cavitation occurs when a cavity or pocket attached to the rigid boundary of an immersed body or a flow passage forms, and remains fixed in position in an unsteady state. 3. Vortex cavitation occurs in the cores of vortices which form in regions of high shear, and often occurs on the blade tips of ship's propellers - hence the name "tip" cavitation…"
And in regards to boat propellers/ships screws and surfboards fins, yes indeed they are quite similar in that both must provide lift (the act of turning a surfboard involves capturing the energy of the surfboards forward motion and redirecting it, fin design plays a large role in this process) while minimizing resistance
.
So there is a definite trade-off between the two desired goals. For example, the most minimal fin resistance would actually be no fin at all! Of course, at the same time turning the board would be extremely difficult.
So if at some point you do want to turn the board, then the lift provided by fins will do the job quite nicely (of course, some will do it better than others ). The only problem is that, at the same time, fins also create drag (i.e., resistance) 
.
When first invented, George Greenough’s high aspect ratio single fin made him the “fastest man alive in the surfing world”
because it so efficiently accomplished both tasks at the same time. For example, such a fin does not suffer from tip cavitation as much has do other fin designs. It is no accident that high aspect fins look like a tuna fish fin 
. Greenough was inspired by one of the fastest of fish in the ocean and incorporated tuna fish fin design into his surfboard fin designs 
! But guess what, the tuna fins he copied were not its pectoral fins (the fins on each side of the animal’s body) which serve merely as guide fins, providing no propulsive power of their own, but rather their tail fins, which do provide propulsive power (i.e., lift) 
.
, as both surfboards fins and boat propellors do indeed undergo cavitation.From, www.keelynet.com/keely/cavity1;
"CAVITATION There are four basic types of cavitation. Fundamentally cavitation results from a drop in pressure on a liquid creates pockets or bubbles in the liquid - an increase in pressure causes these bubbles to collapse resulting in a tremendous "local" force which can cause damage to metals, emulsification, de- gasification, sonoluminescence and many other strange and wonderous phenomena.
CAVITATION: Cavitation is the formation and activity of bubbles (or cavities) in a liquid. Here the word 'formation' refers, in a general way, both to the creation of a new cavity or to the expansion of a pre-existing one to a size where macroscopic effects can be observed. These bubbles may be suspended in the liquid or may be trapped in tiny cracks either in the liquid's boundary surface or in solid particles suspended in the liquid. The expansion of the minute bubbles may effected by reducing the ambient pressure by static or dynamic means. The bubbles then become large enough to be visible to the unaided eye. The bubbles may contain gas or vapour or a mixture of both gas and vapour. If the bubbles contain gas, then the expansion may be by diffusion of dissolved gases from the liquid into the bubble, or by pressure reduction, or by temperature rise. If, however, the bubbles contain mainly vapour, reducing the ambient pressure sufficiently at essentially constant temperature causes an 'explosive' vaporization into the cavities which is the phenomenon that is called cavitation, whereas raising the temperature sufficiently causes the mainly vapour bubbles to grow continuously producing the effect known as boiling. This means that 'explosive' vaporization or boiling do not occur until a threshold is reached.
There are thus four ways of inducing bubble growth: 1. For a gas-filled bubble, by pressure reduction or increase in temperature. This is called gaseous cavitation. 2. For a vapour-filled bubble, by pressure reduction. This is called vapourous cavitation…
…we may distinguish between four different kinds of cavitation according to how it is produced: 1. Hydrodynamic cavitation is produced by pressure variations in a flowing liquid due to the geometry of the system. 2. Acoustic cavitation is produced by sound waves in a liquid due to pressure variations…
HYDRODYNAMIC CAVITATION: In a flowing system, the liquid velocity varies locally and at the points of highest velocity, low pressures and cavities occur.
The conditions which mark the boundary or threshold between no cavitation and detectable cavitation are not always identical. For example, the pressure of disappearance of cavitation has been generally found to be greater, and less variable, than the pressure of appearance. Three cases of flow cavitation arise: 1. Travelling cavitation occurs when cavities or bubbles form in the liquid, and travel with the liquid as they expand and subsequently collapse. 2. Fixed cavitation occurs when a cavity or pocket attached to the rigid boundary of an immersed body or a flow passage forms, and remains fixed in position in an unsteady state. 3. Vortex cavitation occurs in the cores of vortices which form in regions of high shear, and often occurs on the blade tips of ship's propellers - hence the name "tip" cavitation…"
And in regards to boat propellers/ships screws and surfboards fins, yes indeed they are quite similar in that both must provide lift (the act of turning a surfboard involves capturing the energy of the surfboards forward motion and redirecting it, fin design plays a large role in this process) while minimizing resistance
.So there is a definite trade-off between the two desired goals. For example, the most minimal fin resistance would actually be no fin at all! Of course, at the same time turning the board would be extremely difficult.
So if at some point you do want to turn the board, then the lift provided by fins will do the job quite nicely (of course, some will do it better than others
). The only problem is that, at the same time, fins also create drag (i.e., resistance) . When first invented, George Greenough’s high aspect ratio single fin made him the “fastest man alive in the surfing world”
because it so efficiently accomplished both tasks at the same time. For example, such a fin does not suffer from tip cavitation as much has do other fin designs. It is no accident that high aspect fins look like a tuna fish fin . Greenough was inspired by one of the fastest of fish in the ocean and incorporated tuna fish fin design into his surfboard fin designs ! But guess what, the tuna fins he copied were not its pectoral fins (the fins on each side of the animal’s body) which serve merely as guide fins, providing no propulsive power of their own, but rather their tail fins, which do provide propulsive power (i.e., lift) .
Sorry, but you're wrong. Your lengthy reproduction of the definition of "cavitation" essentially says what I said, albeit in more detail. It does not prove that you can get cavitation with a surfboard fin operated at the typical speeds at which a surfboard operates. However, because of the proximity of the fin to the sea surface, it's quite easy to get ventilation. How come you apparently refuse to even discuss the possibility of ventilation as an alternative?Bryan Jackson wrote:No, MTBarrels, I was correct in referring to “cavitation” and you are quite mistaken in this case
But since you apparently don't believe anything that I say, go to the US Patent office site and check out Patent #s 5,471,942 and 5,309,859 to see the problems that a windsurfer/physicist (Richard Miller) has had with ventilation--not cavitation--with his windsurfer. And that's at speeds substantially greater than those of a surfboard on a typical ride.
Alternatively, go to the International Hydrofoil Society web site (www.ihs.org), go to their forum, and post a question (Foil, strut, design etc. section) about the relatively likelihood of experiencing cavitation vs ventilation on the fins of surfboard when riding on a wave. I know that there are some guys there who are very familiar with cavitation and hopefully you will get a reply from one of them.
Or, if you've got the XFOIL program, calculate the peak negative pressure just before stalling (it will drop down immediately after stalling, inhibiting the onset of cavitation) as a function of speed and see what speed you have to be going to get the negative pressure down to the vapor pressure of water (at the normal temperatures at which one is surfing).
MT
Experience gained is in proportion to equipment ruined.
-
hart
- Legend (Contribution King!)
- Posts: 928
- Joined: Sun Sep 14, 2003 4:46 pm
- Location: Dee Why, Sydney.
Drag is just, a drag
Yes, but that depends on just how badly, you are dressed.
( icon here, intentionally omitted)
Hey Wax..at least you have some research on the friction co-efficient of aerated water.. could be good. Do it and tell me if you snag something.
Anonymous.
( icon here, intentionally omitted)
Hey Wax..at least you have some research on the friction co-efficient of aerated water.. could be good. Do it and tell me if you snag something.
Anonymous.
Speed is cutting back in close-outs - and still making the wave!
Anything else is a bit like p!ssing over your head - nice to do occassionally, but not necessarily the best way to empty your bladder.
PS Beware of the yellow dew-drops in the hair and keep your mouth closed when practising.
PPS People with longer boards may find this easier to do
Anything else is a bit like p!ssing over your head - nice to do occassionally, but not necessarily the best way to empty your bladder.
PS Beware of the yellow dew-drops in the hair and keep your mouth closed when practising.
PPS People with longer boards may find this easier to do
-
Beeline2.0
- Legend (Contribution King!)
- Posts: 1873
- Joined: Mon Apr 07, 2003 9:02 pm