Super Secret Settled Science of Sailing?
- Thread starter Will Gilmore
- Start date
Hulls and keels experience lift, for sure. The first time I realized just how all that worked was while training in my kayak. I raced whitewater slalom kayaking in high school. My coach was trying to make the Olympic Kayak team. He'd missed it the previous Olympics by one place. They didn't include Kayaking in the next Olympics
. Anyhow, I had an Equipe Elite slalom boat and was out on the reservoir flatwater training when I noticed it got much harder to paddle when I was close to shore. In deep water, I'd move along well, but in shallow water the boat would bog down and actually sink lower in the water. The wake would be bigger and the stern rode below the water level in a hole created by the wake.I experimented with leaning forward to put the center of balance on the bow wave or leaning back to put the center of balance on the back side of the wake trough. I never got much sense that it made any real difference to how I rode through the water. What was happening? I would have thought the shallow water would have forced the boat to ride higher, not lower. Lift was happening.
Moving through the water causes a bow wake as the water moves to get out of the way of the hull. At first, it plows up, so to speak, but you also are vacating a space behind and that leaves a hole in the water that has to be filled. Sails and wings do this in air also. At the surface of two mediums, you begin to generate a vertical wave of water piling up in front as it builds energy to accelerate aft to fill the vacuum aft. this accelerated water races across the middle of the hull and creates downward lift. In deep water, there is more space for the displaced, racing water to move while equalizing. in shallow water, the bottom pinches against the hull and causes the water to have to move faster to get aft and fill the void left by the boat. This creates a larger wake, more friction on the hull and greater downward lift.
-Will (Dragonfly)
Yeah, that's about right. Anywhere from 1'-3' vs 5' or more. My guess, even 6' would have a measurable effect on my small kayak. For larger boats, you could probably measure the effects in 8 or more feet of water.
Remember, waves break in water that is 3 times as deep as their height. That means they start feeling the effects of the bottom well before they get into that shallow of water.
- Will (Dragonfly)
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i always heard and observed that waves break at twice the depth of their height, but i may be wrong.
on ships, it seemed that we started 'grabbing the bottom' at around double it's draft.
when a ship with a draft of 28' is running in 32' of depth your not moving full speed through the water. waves do not create current to my knowledge. i have never observed this flow from the bow wave back to the trough. the bow creates a wave. behind the bow wave is a trough behind the trough is another wave, some call the stern wave, and when the stern of our boats fall off the stern wave we are at hull speed cause the distance between wave crests is determined by the speed of the wave.
but maybe there is a flow off the wave sorta like how gradient winds are created in the atmosphere. or not
on ships, it seemed that we started 'grabbing the bottom' at around double it's draft.
when a ship with a draft of 28' is running in 32' of depth your not moving full speed through the water. waves do not create current to my knowledge. i have never observed this flow from the bow wave back to the trough. the bow creates a wave. behind the bow wave is a trough behind the trough is another wave, some call the stern wave, and when the stern of our boats fall off the stern wave we are at hull speed cause the distance between wave crests is determined by the speed of the wave.
but maybe there is a flow off the wave sorta like how gradient winds are created in the atmosphere. or not
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when the wind rolls down the a high pressure dome towards a low pressure trough is the air from the surface of the dome or is it from the atmosphere above it?
There is a mixture of ideas being debated here re: hypothesis testing.
The idea of a null hypothesis is borrowed from the world of statistics. For example, if I sample two separated populations of fish, 25 specimens each, and determine the mean number of dorsal fin rays in each sample, the two means will not be identical even if the populations are genetically of the same make up. But is the difference between the means a significant one, or is it one where the difference makes no difference b/c it is caused by inherent sampling variation?
The statistical null hypothesis Ho would be: there is no [significant] difference in the means of the two samples. If the null hypothesis is rejected, then one concludes that the difference is significant. Rejection of the null hypothesis depends on the probability of seeing by chance alone a difference in the means as large or larger than the one observed. If the probability (p) is greater than 1 chance in 20, i.e., p > 0.05, the null hypothesis is not rejected. If the probability is less than 1 in 20 (p < 0.05), the null hypothesis is rejected and the difference is considered "significant."
Less quantitative approaches to hypothesis testing but of similar nature are widely applied under the Popperian idea of falsifyability. A true (in form) hypothesis must be falsifyable. There must be some prediction emerging from the hypothesis that allows it to be tested, and subsequently falsified if it is incorrect, which normally comes via experimentation. Sometimes these hypotheses are formulated as null hypotheses. If the prediction is not seen, the "null" hypothesis is not rejected; if it is seen the "null" hypothesis is rejected. To lessen the confusion of testing with the above semantics, the "null" is sometimes formulated as the one where the prediction fails; an "alternative hypothesis" is formulated as the one where the prediction succeeds, or is observed.
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@Kings Gambit thanks for the clarification. My background is indeed in statistics, however, it has been decades since I actually had to apply them in a experiment. Now I just read and interpret studies.
I have felt that ground effect as I approached the shore in my keeled boat.
But no. While I can perceive a lift force of some quantity in a flattish bottomed boat (in shallow water) compressing the water between the bottom and the boat, the water would need to be very shallow and the boat would need to be traveling with some speed in order to develop a ground effect lift.
My thoughts were looking at the hull design of a typical sailing boat. Stand the hull up on the stern and they look like a ‘Fat Bottomed Girl’. 2 nice and symmetrical curves. In the water as we increase the speed of the hull in the water and heel the boat, that curve takes on the shape of a wing with the same wing lift effect identified in the sail, only the vector forces are reversed.
But no. While I can perceive a lift force of some quantity in a flattish bottomed boat (in shallow water) compressing the water between the bottom and the boat, the water would need to be very shallow and the boat would need to be traveling with some speed in order to develop a ground effect lift.
My thoughts were looking at the hull design of a typical sailing boat. Stand the hull up on the stern and they look like a ‘Fat Bottomed Girl’. 2 nice and symmetrical curves. In the water as we increase the speed of the hull in the water and heel the boat, that curve takes on the shape of a wing with the same wing lift effect identified in the sail, only the vector forces are reversed.
does water compress?
i prefer skinny girls butt i have noticed the trend towards wide bottomed boats
i prefer skinny girls butt i have noticed the trend towards wide bottomed boats
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Isn't it obvious that ground effects can be used either to add lift or down force? At sailing speeds, it will only be the latter. In boats there are two unrelated causes. The first, more likely to be observed by sailors, is the effect of shallow water on wave steepness. The second, observed by freighters with big flat bottoms, is more akin to a lift effect. The kayak may have felt both.
And don't confuse surface wave effects with fluid dynamics without free surface. It hurts my eyes. Hulls and keels have different math.
And don't confuse surface wave effects with fluid dynamics without free surface. It hurts my eyes. Hulls and keels have different math.
It is nearly incompressible. In practical terms, no. That's why pressure vessels, like SCUBA tanks, are tested with water instead of gas (hydrostatic testing), because the liquid can't store enough energy to hurt anyone it the tank fails.
..........'fokker effect' on my yawl. sloop crews are unaware of these things
No, you are right. I mis-wrote.
It is my understanding that the build-up of water at the head of the moving hull needs to go somewhere. It can't stay piled up so it flows toward the hole left behind the moving hull. Gravity and the energy imparted to the water by the moving hull transfer to rearward motion. The water under the boat is also getting displaced by the movement but it can't go down because water doesn't compress well so it flows outwards and up (stern wave). Bernoulli's work shows that the fluid volume entering a pathway in a given amount of time is equal to the volume exciting the pathway in the same amount of time. This would also be true at any point along that path. Since the cross sectional area at midship is more restrictive to that pathway, both by width of the hull and by the depression of the trough, the water, in order to equal the volume entering and leaving the pathway has to move faster towards the stern of the hull, just to catch up.
I also think that the wavelength is determined by the medium's natural frequency. Longer wavelength mean faster traveling waves but the frequency stays near the same to a standing point.
- Will (Dragonfly)
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Waves start to break near shore when the depth of the bottom shoals to 1/2 the wave length. What actually happens, I believe, is that the height of the wave rises relative to the wavelength (or wavelength shortens relative to wave height) when it starts to "feel" the bottom at 1/2 L, eventually to the point where the ratio of H/L becomes > 1/7, where it breaks. This can happen at sea as well, H/L > 1/7, for large (high) waves of comparatively short wavelength, etc.
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In the words of Einstein, "a consensus of 100 scientists can be undone by a single fact." More importantly, in the words of Michael Crichton in a lecture to the faculty at Cal Tech: "There is no such thing as consensus science. If it's consensus it isn't science; if it's science, it isn't consensus." Science is "settled" only provisionally, awaiting discovery of some new fact that ushers in new insight. We must always distinguish the difference between what the press and others sometimes call "consensus science" or "scientific consensus" and what it really is, a consensus of individual scientists (and sometimes others) who may not have the facts or experiments necessary to "make" the science. Also, discovery and establishment of the truth about nature is driven forward by skepticism of the "accepted" explanations, not by agreeing with them to some government (or other) agency with a social agenda.
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