Surfing vs. Plaining

Moore Play

Member
64
20
Hawaii
Thank you all for the clear and concise responses. They allow me to hone in and clarify my inquiry. Obviously dinghies & multi-hulls have different parameters for "surfing vs. planing", and to muddy the water, (sorry for the pun), there is now so much development in lifting foils, T-foils, hull shape and stability that our very basis for what is "hull speed" may not be so clear anymore. Good possibility I am wrong but I believe in the era of the age of sail up to and including the clipper ships, hull speed was determined to be the square of the weighted waterline length, as the twentieth century progressed it became 1.34 the square of the WWL. Love them or hate them, the America's Cup T-foilers WWL has absolutely nothing to do with hull speed, and they are in flat water. Obviously they are not surfing but are they really planing? Are they just an anomaly? Do we just sort them into a range of their own, (not dingy, not multi-hull, developmental?). The wakeboarding video's bring out a part of this. Notice the difference in the length of the different boards. In the 1st video the board on the outside is noticeably longer and the guy is just cruising, the others are shorter and obviously more responsive but kinda need to be "worked" to stay on the wake. Then you have the foiler who can go faster than the boat creating the wake, until he runs out of wake. When I'm at the helm of my boat I'm more like the guy with the long board, my younger crew are much more aggressive on the helm, knowledgeable pumping of the sails seems to help us all out equally. 

 

El Borracho

Sam’s friend
6,234
2,326
Pacific Rim
Well...hull speed is definitely a real thing, @fastyacht.. It is defined as the speed at which the wavelength of a boat's bow wave equals the waterline length of the vessel. The value  does not vary except with waterline length except for a tiny amount due to water salinity and. The incremental amount of energy to increase the vessel speed beyond the hull speed varies with the characteristics of the boat. However in engineering, it is now (for 200 years?) replaced by the more scientific Froude Number.

Planing is defined as the mode where hydrodynamic lift overtakes buoyancy (hydrostatic lift). Surfing is being propelled by a steep or breaking wave without regard to planing. Surfboards are always planing while surfing. Waves in the open ocean are unable to accelerate a floating object unless somehow steepened. A floating ball will not roll down the face of even a huge ocean swell, for example. Nor do boats change speed in such seas. Steepening can be from entering shallower water or from being driven by wind. Heavy slow boats entering a channel with following seas often surf for a moment, without planing, just before embarrassing themselves. In heavy following seas heavy displacement boats will surge somewhat beyond hull speed. Such boats tend to just create more wave drag until they either slow down or sink bcuz hydrodynamic lift never overcomes buoyancy. So the thread title Surfing vs. Planing is improper. Surfing and Planing would be better.

I don't think one can plane at less than hull speed. The water moves out of the way, around the hull, rather than producing any upwards force. 

As a boat begins planing the wave-making drag is reduced. For some boats the wave-making drag is already small so there is not much difference at the transition to planing. 

 
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ROADKILL666

Super Anarchist
2,340
520
FREAKVILLE, FL
With my Olson 29, 30 knots wind and 10-15' waves, boat speed peaking above 16 knots we were exceeding the speed of the wave and driving into the back of the next wave(if you've ever been on an O29 in these conditions, we are talking burying the bow in the next wave). Was I surfing or planing?
You were having fun.

 

El Borracho

Sam’s friend
6,234
2,326
Pacific Rim
With my Olson 29, 30 knots wind and 10-15' waves, boat speed peaking above 16 knots we were exceeding the speed of the wave and driving into the back of the next wave(if you've ever been on an O29 in these conditions, we are talking burying the bow in the next wave). Was I surfing or planing?
Surfing, planing, and submarining. Big fun. Can get crowded in the back of the boat. 

 

Bruce Hudson

Super Anarchist
3,251
846
New Zealand
This discussion promotes a false dichotomy - it is not either surfing or planing. 

Boats can surf without planing, or plane without surfing. Boats can plane while surfing. There is an overlap between surfing and planing.

On a planing hull (or board), once you exceed your hull speed (so long as you aren't foiling), you are planing.

 

Chris in Santa Cruz CA

Super Anarchist
5,303
911
earths surface
This discussion promotes a false dichotomy - it is not either surfing or planing. 

Boats can surf without planing, or plane without surfing. Boats can plane while surfing. There is an overlap between surfing and planing.

On a planing hull (or board), once you exceed your hull speed (so long as you aren't foiling), you are planing.
bravo, sir, for ending our mildly entertaining circle jerk

hope you're happy with yourself :)

 

El Borracho

Sam’s friend
6,234
2,326
Pacific Rim
Already told my sons we would be crowded in either the port or starboard stern quarter when flying the chute on the hotfoot 20. 

550 lbs on a boat like that is statistically meaningful :)
Have them watch for kelp patches while back there. I recall a collision with kelp ahead of a very rare black squall was the demise of the Hotfoot down in Monterey. Still on the bottom I assume. Many years ago. 

 

shaggybaxter

Super Anarchist
4,338
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Australia
As a boat begins planing the wave-making drag is reduced. For some boats the wave-making drag is already small so there is not much difference at the transition to planing
Thanks Borrachio, that was all very informative and succinct, nicely put.  The 12.50 had a lurch onto the plane, smooth but it was there. Makes sense with a flat transom. 

We forgot one thing, noise. Surfing is noisy, planing is not ...well, at least till you get up into the teens and the harmonising starts. But prior to that, it amazes me how quiet everything becomes. Sounds more like a muted radio hiss than water.   

 

 
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JimC

Not actually an anarchist.
8,099
961
South East England
In the dinghy world defining planing is an old argument. The powerboat boys don't reckon they are planing properly until they are at about 3 x root L, which precious few sailboats are capable of. Truth is, it is, to borrow the trendy phrase of the time, a spectrum. At 0.1 root L knots boats are pretty much 100% displacement, at 3x root L predominantly dynamically supported, in between its pretty much impossible to draw a rational) line. Sufficiently light boats can have significant dynamic support well below "Hull speed", and sufficiently narrow boats very little dynamic support well above that point. In any case "Hull speed" is an approximation anyway, not a boundary. The "Hull speed" of a low prismatic and high prismatic boat of the same nominal length will be subtly different. 

 
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fastyacht

Super Anarchist
12,831
2,537
Well...hull speed is definitely a real thing, @fastyacht.. It is defined as the speed at which the wavelength of a boat's bow wave equals the waterline length of the vessel. The value  does not vary except with waterline length except for a tiny amount due to water salinity and. The incremental amount of energy to increase the vessel speed beyond the hull speed varies with the characteristics of the boat. However in engineering, it is now (for 200 years?) replaced by the more scientific Froude Number.

Planing is defined as the mode where hydrodynamic lift overtakes buoyancy (hydrostatic lift). Surfing is being propelled by a steep or breaking wave without regard to planing. Surfboards are always planing while surfing. Waves in the open ocean are unable to accelerate a floating object unless somehow steepened. A floating ball will not roll down the face of even a huge ocean swell, for example. Nor do boats change speed in such seas. Steepening can be from entering shallower water or from being driven by wind. Heavy slow boats entering a channel with following seas often surf for a moment, without planing, just before embarrassing themselves. In heavy following seas heavy displacement boats will surge somewhat beyond hull speed. Such boats tend to just create more wave drag until they either slow down or sink bcuz hydrodynamic lift never overcomes buoyancy. So the thread title Surfing vs. Planing is improper. Surfing and Planing would be better.

I don't think one can plane at less than hull speed. The water moves out of the way, around the hull, rather than producing any upwards force. 

As a boat begins planing the wave-making drag is reduced. For some boats the wave-making drag is already small so there is not much difference at the transition to planing. 
Hull speed is a fixation in peoples brains. There is no such thing except as a convenient abstraction.

But glad you enjoyed writing all that. I concur substantially on the rest.

Froude number has nothing to do with hull soeed. It is merely the nondimentional ratio of vessel speed to a reference dimension. The reference can be length, depth etc. Many intetesting curves have been drawn using it. The principle importance being similitude.

 

fastyacht

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Jim brought up the effects of dispkacement vs kength. This is really, along with prismatics and buttock shspe, why I make fun of "hill speed"

:^D

 

fastyacht

Super Anarchist
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Jim brought up the effects of dispkacement vs kength. This is really, along with prismatics and buttock shspe, why I make fun of "hill speed"

( There is domrtiing misleading in there about dynamic lift though). I have never seen any way to acheive dynamic lift at Fn less than0.4 except eith foils. In fact vrssels even loght develoo suction and bow down trim at Fn lrss than 0.4

:^D

 

Steam Flyer

Super Anarchist
40,035
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Eastern NC
Jim brought up the effects of dispkacement vs kength. This is really, along with prismatics and buttock shspe, why I make fun of "hill speed"

( There is domrtiing misleading in there about dynamic lift though). I have never seen any way to acheive dynamic lift at Fn less than0.4 except eith foils. In fact vrssels even loght develoo suction and bow down trim at Fn lrss than 0.4

:^D
Is "foiling" a subset of planing?

"Dynamic lift" is kind of a clumsy and inexact phrase. Water being incompressible, i assume we can't really postulate a high-pressure region of water pushing a planing hull upward. We -can- postulate that a hull may be moving so fast that the inertia of the water it has to push downward to go by, is greater than the weight of the hull & it's contents....

FB- Doug

 

RATM

Anarchist
852
45
Surfing can take place underwater just ask all the dolphins and seals out there. There does not have to be a gravity component. If you have ever bodysurfed and done an underwater takeoff, you know what I'm saying.

 

fastyacht

Super Anarchist
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Is "foiling" a subset of planing?

"Dynamic lift" is kind of a clumsy and inexact phrase. Water being incompressible, i assume we can't really postulate a high-pressure region of water pushing a planing hull upward. We -can- postulate that a hull may be moving so fast that the inertia of the water it has to push downward to go by, is greater than the weight of the hull & it's contents....

FB- Doug
Coprdssibility is not the issue.

The classical  thing is measuring cg rise. At below Fn 0.4 (wave speed of hull wl) the cg actually falls due to suck. As you get on the back of the bow wave you are still not lifted therefore classically not planing.

From a thought experiment standpoint iti s semantics. "Chines dry" is another planing idealisation. For thought experiment can you have that without maximum cg rise? (The answer varies because of tricks!)

The whole subject is slippery lol

 

Steam Flyer

Super Anarchist
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Eastern NC
Coprdssibility is not the issue.

The classical  thing is measuring cg rise. At below Fn 0.4 (wave speed of hull wl) the cg actually falls due to suck. As you get on the back of the bow wave you are still not lifted therefore classically not planing.

From a thought experiment standpoint iti s semantics. "Chines dry" is another planing idealisation. For thought experiment can you have that without maximum cg rise? (The answer varies because of tricks!)

The whole subject is slippery lol
OK, this is intriguing but I don't really understand. CG rise and fall? Chinese dry?

And yes, slippery subject, fraught with old wives tales and misinfo, such as the pernicious lie that Flying Scots and J105s can plane......................

FB- Doug

 
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