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Does Shallow Water Slow You Down?
Posted: Tue Sep 02, 2008 8:20 pm
by Mishigas
Here is a wierd question but I know someone here will know the answer (I sure don't). This season (perhaps due to running slower than normal) I have noticed that when I cruise into shallower waters about 9' or less my speed slows down as much as a couple knots. Without any throttle input my speed increases back when I get into deeper waters. I assume this is due to some physical reasoning but I need some "splaining' as they say. Has anyone with our PDQ's noticed this? What gives? Thanks, Sandy Kramer MV MISHIGAS HULL 68
Re: Does Shallow Water Slow You Down?
Posted: Tue Sep 02, 2008 8:36 pm
by duetto
hi sandy,
i'm not sure what the hydrodynamics are but it does happen. here is the good news. when the water gets below about 7' you will pick up speed, and in fact the shallower the faster......"so do you feel lucky punk?"
Re: Does Shallow Water Slow You Down?
Posted: Tue Sep 02, 2008 9:28 pm
by eepstein
Hi;
A lot depends on current. Water can not be compressed, so moving water in shallow areas runs faster than deeper areas. If you have a 1kt tide (i.e. tide speed according to tide predictions by NOAA are for the normal navigation channels), in shallower water, the current from the tide will run faster than the predicted tide. So going with the tide (or rain runoff effect, or general current) in shallow water will speed you up, against it will slow you down. So its a good plan to go down current in shallow water and against the current in deeper water.
But here is a fun one, if you are crossing from deep to shallow water 'with' the current you will not do as well, as the effect of the current, as the depth decreases is to pile more 'non-compressible' water towards the shallows, which will raise the sea level, making you motor slightly uphill.
Those of us with little engines on PDQ 36s feel the effects more, not to mention kayaking. Hope that didn't muddy the waters too much.
Eric
Re: Does Shallow Water Slow You Down?
Posted: Wed Sep 03, 2008 7:56 am
by ThomKat
It also depends on whether you are in planing mode or displacement mode. You will speed up when you are in planing mode and slow down in displacement mode. It's all because of the compressibility effects mentioned above.
Tom
Re: Does Shallow Water Slow You Down?
Posted: Thu Sep 04, 2008 11:53 am
by Sno' Dog
Yes, Sandy, we have definitely noticed this effect. Passing through the Dismal Swamp Canal, it was very noticeable, and cruising on the canals in Holland where the water was less than 6 feet deep we noticed the same thing. In fact, the effect works quite well (if you're paying attention) to warn you of shallow water. Several times, I've seen our speed drop and then realized we had wandered into shallow water.
I think the others are right though, this effect happens only at displacement speed. At planing speed, it might actually work in reverse - but I've never tried it.
Henry
http://www.snodoglog.com
Re: Does Shallow Water Slow You Down?
Posted: Thu Sep 04, 2008 12:46 pm
by duetto
my earlier post was talking specifically about planing speeds. it can add a full knot at about 3200 rpm.
Re: Does Shallow Water Slow You Down?
Posted: Fri Sep 05, 2008 9:35 am
by Mishigas
Thanks for all the feedback. Nice to know that others have observed this behavior. These boat do creat a rather large "Stern Wave" at certain slower speeds. That is my "Barometer" that I have slowed down a considerable amount. Regards, Sandy Kramer MV MISHIGAS PC34/68
Re: Does Shallow Water Slow You Down?
Posted: Sun Sep 07, 2008 6:57 pm
by Milky Way
Hi Sandy, At 18 to 20 mph. all the boats I have ever had would speed up as much as 2 mph. in water 10 ft. or less. My explanation is that the bottom under your boat is impacted in a similar way as in flying where we call it ground effect, therefore the boat is lifted somewhat reducing the wetted surface and hence increasing the speed. At hull speed the story appears to be different. Hope this helps. Armin Quickert. PDQ 41/09.