Water depth its effect on boat speed

When I first started paddling a few months ago the blokes I was paddling with would complain about the difficulty/resistance of paddling in the shallow water.And how it was much easier and faster in the deep water.At the time I couldn't tell as I could fall out just as easy in the shallow as the deep.But now im staying in the boat I do notice a definite difference.Can someone tell me why this is?And is it the same in fresh water?

I think its called the Bernoulli effect. Not sure how it all works though.

I took a marathon canoeing course many years ago and the instructor told us that shallow water causes the boat to slow down. He called this "suck water", and thats what i've called it since then.

But he also said that at certain depths of shallow water, paddlers can do a short interval of high intensity paddling, causing the boat to "pop" up, or plane across the surface. Once the boat "pops" up and begins to plane, the paddlers can maintain their speed with little effort. Not sure how much truth there is to this.

The effect is caused by the wave from the bow of the ski bouncing off the sea floor and impacting the ski half way down the waterline length.

You normally only see waves on the surface spreading out from the bow of the ski as you move through the water. But there is really a wave reflecting out at all angles under the water.

The wave that is pushed down into the water normally just disappears into the ocean depths. But in shallow water it bounces off the sea floor and back onto the ski - but half way down the length of the ski since the ski has moved forward through the water since it was created at the bow. This can have a really odd effect on the way the ski travels through the water making you feel like you are paddling into a wave (or trying to paddle over the top of a wave).

The 'sprint' float effect noted above is probably from getting the wave just right so you actually start to surf the wave created by your own ski. Once you are planning you don't need quite the same effor to maintain that speed.

I believe the effect is caused by the bow wave reflecting off the bottom and back up to the boat, slowing it down by causing additional drag. Don't quote me on that, though - I need to verify it. At any rate, it is definitely true that you can "pop" the boat in shallows and pick the speed back up. Some boats are easier than others to pop, and Mohicans are noted for being nearly immune to the effects of shallows. As a midwestern USA river racer, I've had more than one race where I had to pop the boat most of the race due to the shallow conditions.

With a kayak or surf ski, I change from the typical stroke form to a rapid, low-angle stroke to pop the boat and keep it popped. Some heavier sea kayaks and such may need to get a running start on the shallows to be popped easily, but boats such as the Thunderbolt and most surf skis can easily be popped once you find you're in shallow water.

What sort of depth are we talking about?

I barely notice shallow water on my ski, but in my kayak, any hint of shallow and I'm sure a passing whale grabs the back of my boat.

I really notice it when the tide is out and im paddling in 3 to 4ft of water and if I drop into the Chanel of over 12ft of water the boat becomes much easier to paddle along.A mate of mine told me that this is why all competition kayak events have to be in a certain depth of water to be sanctioned,and maybe this is why competition Olympic pools don't have a shallow end.

I start to notice an effect in about 3 feet of water and in anything less than 2 feet the drag really kicks in.

The wave riding effect become really noticeable when you have been paddling at a good speed then you move over a shallow area for say 20-30 feet,... the boat hasnt had a chance to slow down from the drag but the underwater "wave" hits and gives that unusual sensation. I hadnt really thought much about it until now but now that you guys have explained it it's like a lightbulb going on in my head

Yep, around 3-5 feet depth will slow you down.

Paddling in from the deep over a shallow sand bank will really reveal the drag - watch your GPS!

Different boats will have different effects - sea kayaks will have a bigger wave and bigger impact whereas racing skis with little wash will have less impact.

Here is a nice explanation of the effect of shallow water on boat speed, with an example from a large british naval vessel. The graph says it all. I think in a surf ski you cannot take advantage of this principle unless you have a kick up rudder, as you need to be in less than 2 ft or so of water.

www.navweaps.com/index_tech/tech-091.htm

You can pop a surf ski, although the skis (particularly the HP skis) often seem less affected than some other boats. I had an Evo with an understern rudder that I tested in the shallows on a lake. Sure enough, it slowed noticably, but once popped I could run it over 7 mph through the shallow area. Obviously if it's so shallow your rudder drags or there are obstructions to whack your rudder, you have other problems to worry about.

I've always known it to be called the 4 foot factor.
Anything deeper than that and your fine but 4 foot and shallower and it feels like your towing a skier.

Tidal flow makes a big difference though. Paddling against a tide/current will make the 4 foot factor less noticable because small tidal waves will be formed and will help slightly.

Paddling with a tide, stay deep and stay fast..

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