Half of this 50m pool is pretty shallow, less than meter I guess, when I pass the border of the shallow area to the deep water, I suddenly feel that I slow down by lots. Is it just an illusion or could it be real ?
If you’re going by how it feels, it’s probably in your head. The deep end of the pool is usually faster than the shallow end because deep water absorbs turbulence better than shallow water (which tends to bounce it back toward the swimmer)
It’s why big meet pools tend to be built with a ‘shallow end’ with a 2 meter depth.
Paralax. The closer the bottom is to you, the faster it looks like it’s going by. You’ll be swimming the same speed (unless you’re negative splitting!)
Yeah - what thunderlegs said. When I stare at the sun while running, it looks like I’m standing still. But when I watch the ground - I’m really moving fast. Same concept.
I also always feel faster in shallow water. It is pretty much the only time I feel fast. I just assumed it was because I was closer to the pool bottom and therefore I perceived I was swimming faster.
there *is *actually a difference, it is not imagined. but it should be the opposite, slower in shallow water.
the reason? for a given energy, surface waves are bigger in shallow water, and that should slow you down.
If you’re going by how it feels, it’s probably in your head. The deep end of the pool is usually faster than the shallow end because deep water absorbs turbulence better than shallow water (which tends to bounce it back toward the swimmer)
It’s why big meet pools tend to be built with a ‘shallow end’ with a 2 meter depth.
Actually, they HAVE to be built with a shallow end of 2m depth according to FINA. 
Better yet, anyone ever noticed the redshift/blueshift where the black stripe and the white pool bottom meet? Easier to see at an outdoor pool.
Though the 2m mandate is only for world championships and I think world cup meets. US Swimming still holds the occasional national meet at the Hall of Fame pool in Ft. Lauderdale, and FINA had no problems accepting Natalie Coughlin’s 100M back world record time from there a couple years ago even though the shallow end there is only something like 3’6"
Swimming is definitely faster in a deeper pool.
However, how come when in a boat, you can feel the boat pick up speed as you enter shallower water?
about the boating, is this a perception, or do you actually have a speed measuring device that objectively tells your waterspeed is increasing ?
No, it’s not perception. I have been around boats my whole life and worked on a commercial ferry boat service for ~6 years.
A power boat with a planing hull will definitely pick up speed as it enters shallow water.
The most noticeable, yet worst, time this happened was as the boat I was travelling on rapidly accelerated and then proceeded to run aground. We were in the center of the channel, however, the channel was mismarked.
i am stumped. i can answer questions about why airplanes do strange things (and, like airplanes, boats are a little similar in that they are also propelled machines moving through a fluid), but i am not sure why that would happen …
Hmmm…maybe someone with more knowledge will see this and answer the question.
It never seemed strange to me until I realized that deep pools are actually faster so why should shallow water be faster for a power boat?
Maybe it’s because of the nature of a planing hull? Versus a swimmer who is plowing through the water? I don’t know.
But when the tides were right and there was enough water in shallow sections that we could cut across them our trips were always faster and you could see it on the speed on the GPS.
Better yet, anyone ever noticed the redshift/blueshift where the black stripe and the white pool bottom meet? Easier to see at an outdoor pool.
Man, you must be FAST!
ah, now i am getting it.
the gps is the clue. i don’t think you can’t use gps speed for this puzzle.
the speed i think you should be looking at is the speed relative to the water (which is equivalent to swimming speed). perhaps your waterspeed actually does slow down, but your gps speed increases (due to currents, or tides)?
I don’t think so. I mean there is also a speedometer on the boat.
And believe, it is a noticeable change in hull speed. And just the opposite happens as well, when you hit deep water, you can feel the stern of the boat sink and the boat slows down considerably.
It has to have something to do with the nature of the hull.
how long and deep is the hull?
about how shallow does the water have to be before you can notice or measure an increase in waterspeed as you go from deep to shallow water?
The body of water I worked on was a relatively shallow bay. “Deep” is anything over 8 feet I would say.
On the commercials boats, they were 60-90 feet long, with a draft of ~3 feet. There was a noticeable change in speed if you enter 4-6 feet of water compared to 8-12 feet of water.
My parents boat is 33 feet long with about the same draft and same speed increases.
I had a 17 feet long Boston Whaler that drew about a foot of water and it noticeably sped up as you ended very shallow water, 2-4 feet, compared to anything over 5 feet or so.
in aviation, there is a phenomenon called, ‘ground effect’. basically, an airplane can fly faster (less drag) whenever it approaches the ground at distance somewhat less than the plane’s wingspan (so, that is usually less than 100 feet to just a few feet, depending on the aircraft size).
it would make a lot of sense if nearly the same thing occurs when a boat approaches water shallow enough so that the ground is somewhat close to bottom of the boat’s hull …