In the sailing "slower in shallow water thread", an interesting disagreement has emerged.
AU64 says,

Select to expand quote

I am planing on a SUP at 10.5-11.6 kph, --- I can tell the difference in speed between planing (30-40 knots) in glassy flat shallow water and glassy flat water beside a wall or sand spit.

This goes against my understanding of what planning is.
I thought that planing is when the board exceeds the bow wave speed, and is in front of the bow wave.
I also thought that this produces a positive outcome from ground effect in shallow water. not a lot with our narrow boards, but we shouldn't be going noticeably slower.
Can this be where the extra lift occurs? if it's behind the center of gravity it will push the nose down, increasing whetted area and slowing the board.

Also, can a board plane at a slower speed than the bow wave?
I guess if it has enough area, and a long water line, that could be so.

Thoughts, experience, facts, anybody?????

No I think you're absolutely right Decrepit.
On a skinny displacement hull, you'll absolutely feel ground effect, because the hull is compressing/shifting water sideways away from the hull, this leads to the lack of water under the bow to support its mass.


A planing hull on the other hand, still technically "displaces" a small amount of water, but the flat bottom causes "compression" under the hull, which helps to overcome ground effect more and more the faster you go.

as to what is planing and what is not planing? It's the exceeding the bow wave part that's the critical difference. All proper planing hulls have a "hump" speed where the pressure increases and acceleration actually slows (briefly) before it slides over its bow wave and planing is in progress, acceleration then can start increasing exponentially.
important to keep in mind that what we're sailing around on are flat bottomed boards that spread the load of everything on top over a larger surface area. As soon as we have some movement our boards do a much greater job of using the waters surface tension to stay on top and displace less water than say a kayak.


Is Lake George fresh or salt water? I've often wondered how much of a difference sailing on fresh as opposed to salt water might make to overall speeds/trim?

Select to expand quote

decrepit said..


Also, can a board plane at a slower speed than the bow wave?
I guess if it has enough area, and a long water line, that could be so.

Short answer, no


i used to race cherubs a couple of decades ago, at some points sailing up wind, it would feel like the boat was planing. There were a few markers of planing present, like clean release of water from the transom/bow slightly raised, but it was quite clear on further observation that the boat was still sailing inside its own bow wave. They were capable of planing upwind given the right conditions/trim, and sailing a slightly lower angle, but there was always a noticeable mode change.

a longer waterline increases the length of the bow wave, which allows a higher displacement speed, but no planing. During WW2 some warship designers found that the addition of a bulbous bow allowed some more speed to be leant to the hull by altering the bow wave formation (perhaps made the wave smaller/shifting its formation further back in relation to the bow? I dunno, just guessing).

I've just read this.
smalltridesign.com/Trimaran-Articles/design/planing.html.

It's got no mention of relative bow wave speed and hull speed.

It defines fully planning as when the hydro dynamic lift is much more than buoyancy lift.
partial panning is when there is an element of both.
So it's back to semantics really, it depends what you mean by planning.

Select to expand quote

Subsonic said.. >Is Lake George fresh or salt water? I've often wondered how much of a difference sailing on fresh as opposed to salt water might make to overall speeds/trim?

LG is salt, it's opened to the ocean every no and again. I've sailed fresh and hyper saline, haven't noticed any difference in sailing, but there is a difference in water starting.
I think you'd have to run from one to another to notice a difference.
Cold wind seems to have more push than hot wind, so it seems logical that denser water would have more lift and drag.
But the greatest difference at LG is the oil on the water from the weed

I regularly change from cold fresh water lakes to hyper saline ones. I don't feel any difference once on a full plane, but the difference in shlogging is absolutely noticable. Even more so as I mainly sail freshwater lakes in the Bavarian winter with all the weight and cumbersomeness of the hooded gimp suit, boots and gloves. Christmas cookies might play a role too, so early January my boards seem to have lost at least 10 litres of volume, only to miraculously regain them in spring. Going off to Southern France (Etang de Leucate) tomorrow, so I'm looking forward to experiencing the prodigial board transformation soon.
I'll pay attention but have never noticed different drag between deep and shallow.
I can only agree with Decrepit: cold winds feel a lot stronger while in summer on the Greek Islands wind readings are sometimes misleading on hot days. It blows like hell but nobody is planing. You even feel the wind and see the ripples on the water with everey sense telling you that you ought to be planing but no way. Must be lack of density in the air, it feels as if the wind speed is right for a 5m but the wind only hits half of the sail. Is it just me?

sounds like you sail in a big range of wind temps, so you are well qualified to judge the difference in power.
I'm not surprised you don't notice a difference in lift/drag between salt and fresh. Because they often go together, and cancell each other's effect out.

I would be very surprised if there were not slightly less drag at planing speed in shallow water. As in 14-16cm fins almost dragging on the bottom, which I have some experience with. It always felt very fast to me on that depth, but that could just be the heightened fear factor in my head!!

Think of a skimboard. Definitely less drag in very shallow water where the ground effect is stronger.

It's probably the reason that the WSSRC also specifies a minimum depth of water for records (according to hull width)

With displacement hulls, the bottom drag is really noticeable in shallow water. I used to paddle Racing Skis and Kayaks, and it was really obvious.

Select to expand quote

sailquik said..
I would be very surprised if there were not slightly less drag at planing speed in shallow water. As in 14-16cm fins almost dragging on the bottom, which I have some experience with. It always felt very fast to me on that depth, but that could just be the heightened fear factor in my head!!

Think of a skimboard. Definitely less drag in very shallow water where the ground effect is stronger.

It's probably the reason that the WSSRC also specifies a minimum depth of water for records (according to hull width)

With displacement hulls, the bottom drag is really noticeable in shallow water. I used to paddle Racing Skis and Kayaks, and it was really obvious.

I'm glad you have the opposite experience to AU64, confirms my idea of what is going on.

Select to expand quote

decrepit said..

Cold wind seems to have more push than hot wind, so it seems logical that denser water would have more lift and drag.

Cold dry air under high pressure has the greatest density, hence more lift is created. For a given sail size it will produce more lift when it is cold and dry rather than in warm moist conditions. Denser water has more buoyancy than fresh water, so for a given board you will plane earlier. The drag on the board will be a function of surface friction which is higher in salt water but the difference to fresh water is almost negligible.

Apparently, there are faster and slower Olympic swimming pools due to the depth. So, there would have to be a difference in super shallow water windsurfing with the fin just about hitting the bottom.

OT in the context of "ground effect" planing, but one thing to keep in mind is water viscosity - nearly doubles going from just above freezing to 25?C, which will increase skin drag considerably.


On topic - there are papers on this sort of thing.

apps.dtic.mil/sti/tr/pdf/ADA016682.pdf

Select to expand quote

The sensitivity of the resistance of a planing boat to shallow water through the critical speed range (Frnh =0.5-3.5) is analyzed and compared with the deep-water results. Near the critical speed(subplaning), horsepower increases dramatically due to shallowness, but at planing speeds the power is reduced.