Boat Speed Affected by Water Depth

I'm curious to hear how many of your boats are effected by water depth and by how much. This is assuming hull speed for displacement boats. At what depth do you notice the difference and how much speed do you loose or gain?

This is our first displacement speed boat and this winter was our first of full time cruising, having kissed that 40 hour/week nonsense adiós last year. We've run in every conceivable condition over the winter from open ocean to turning up mud.

Yes I've factored currents into the equation. With no current we cruise comfortably at about 8 knots in the ocean and if we get into anything less than about 20' of we loose about 1 knot at the same rpm. We draw 5'

Is this typical?

Thank you!

My planing hull boat is slower in shallow at hull speed, but faster in shallow at planing speed. Ground effect. Changes the way the water squeezes around the boat.

Surprised you see an effect at 20'. I usually only notice it when depth is less than about twice my draw.

My experience is the same as Ski's. 20 seems awfully deep to have that kind of effect. On our SD Hatteras (or should I call it SP?) we definitely noticed a reduction at hull speed , below say 10 feet and it continued to decrease as the water got shallower. Of course, as we got down to a foot or two below the keel, (5 ft draft) we began to ease the throttles down close to idle.

Water velocity increases due to the narrowed gap between the hull and the bottom. Increased velocity, decreases pressure, decreased pressure, decreases buoyancy, with decreased buoyancy the hull squats.

Furthermore, long waves can't travel at their desired propagation speed in shallow water, so there is a increase in wave making resistance.

Northern Spy said:

Water velocity increases due to the narrowed gap between the hull and the bottom. Increased velocity, decreases pressure, with decreased buoyancy the hull squats.

Furthermore, long waves can't travel at their desired propagation speed in shallow water, so there is a increase in wave making resistance.

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Could you translate all that into Dick and Jane language?

Maybe after work if still required. That was a lunchtime snippet.

I'm sure others will chime in.

As any Predicted Log contestant will tell you, the Spyman is correct in his explanation.

I come to a grinding halt in less than 3'8" I honestly have never experienced that phenomenon! Being a retired fireman, I know about hydrodynamics but have never experienced the effect in my boats. I will be watching for that if I ever get into water shallow enough for any significant length of passage.

Interesting...You've answered a puzzlement I've experienced for years.
When on plan running in a dredged shipping lane (32') I'm doing 20knts, when I get near my club I turn out of the channel and proceed in about 12' of water. The speed drops to 18.5knts every time.

I can't agree with "Gilligan" . The physics he is describing, Bernoulli's principle, is for a compressible gas. Water is incompressible. The water does not move relative to the sea floor and buoyancy is strictly dependent on water density which does not change with any kind of water motion, therefore the boat does not sit lower in the water. The wave action is a distinct possibility since waves get shorter and steeper as the water gets shallower due to the drag on the bottom. This would certainly slow a displacement hull.

Paul (Physics PhD)

Squat effect - Wikipedia, the free encyclopedia

The squat effect is the hydrodynamic phenomenon by which a vessel moving quickly through shallow water creates an area of lowered pressure that causes the ship to be closer to the seabed than would otherwise be expected. This phenomenon is caused when water that should normally flow under the hull encounters resistance due to the close proximity of the hull to the seabed. This causes the water to move faster, creating a low-pressure area with lowered water level surface (See Bernoulli's principle). This squat effect results from a combination of (vertical) sinkage and a change of trim that may cause the vessel to dip towards the stern or towards the bow.[1]
Squat effect is approximately proportional to the square of the speed of the ship. Thus, by reducing speed by half, the squat effect is reduced by a factor of four.[2] Squat effect is usually felt more when the depth/draft ratio is less than four[2] or when sailing close to a bank. It can lead to unexpected groundings and handling difficulties.


http://en.wikipedia.org/wiki/Bernoulli's_principle




This article is about Bernoulli's principle and Bernoulli's equation in fluid dynamics. For Bernoulli's theorem in probability, see law of large numbers. For an unrelated topic in ordinary differential equations, see Bernoulli differential equation.
http://en.wikipedia.org/wiki/File:VenturiFlow.png

In fluid dynamics, Bernoulli's principle states that for an inviscid flow of a nonconducting fluid, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.[1][2] The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738.[3]

The only thing I really have a hard time believing is the 4X the draft for water depth....it may start taking effect but I have really never noticed much till the water was about 2x-3x the draft of the vessel.

I noticed this big-time in our Grand Banks when we went down the ICW. The boat draws just under 4', and it would start to slow at about 10' depth and get progressively slower as the depth drops from there. It was quite pronounced. I found the best thing to do was throttle back and not fight it. A very narrow waterway makes it worse yet. Places like the Dismal Swap, but that's a whole other horror story.

Squat effect certainly does not apply to real planing boats. The rule of thumb in shallower waters for those is to stay on plane.

Paul Swanson said:

I can't agree with "Gilligan" . The physics he is describing, Bernoulli's principle, is for a compressible gas. Water is incompressible. The water does not move relative to the sea floor and buoyancy is strictly dependent on water density which does not change with any kind of water motion, therefore the boat does not sit lower in the water. The wave action is a distinct possibility since waves get shorter and steeper as the water gets shallower due to the drag on the bottom. This would certainly slow a displacement hull.

Paul (Physics PhD)

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Notice the far end of the Coot's wake where it has entered the shallower waters of Napa River. The far end appears to have speeded up.

Paul Swanson said:

I can't agree with "Gilligan" . The physics he is describing, Bernoulli's principle, is for a compressible gas. Water is incompressible. The water does not move relative to the sea floor and buoyancy is strictly dependent on water density which does not change with any kind of water motion, therefore the boat does not sit lower in the water. The wave action is a distinct possibility since waves get shorter and steeper as the water gets shallower due to the drag on the bottom. This would certainly slow a displacement hull.

Paul (Physics PhD)

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The Bernoulli principle absolutely applies to water. Makes no matter whether compressible or not. Any fluid flow has an inverse relation between velocity and pressure.

I do think more of the effect is due to wave dynamics vs Bernoulli, but that's a guess. Surface wave stuff is complex.

I stand corrected. The Bernoulli principle does apply to both compressible and incompressible fluids. It seems to be well known to naval architects.

At normal cruising speed of 8 knots and 15' or water then when I get to 10' of water same rpm my speed will reduce to about 7 knots with a 7' drafted vessel the squat effect takes place at about 10' of depth and is noticeable. The squat will be overcome and the boat actually raise up (reverse squat effect) and speed will come down to 0 knots when the depth of the water gets to 5'-6'.

dan

The squat effect was considered to be partly responsible for the sinking of the Costa Concordia.
http://gcaptain.com/wrong-costa-concordia-analysis/


Jim
Sent from my iPad using Trawler Forum

JDCAVE said:

The squat effect was considered to be partly responsible for the sinking of the Costa Concordia.
So What Went Wrong on the Costa Concordia? [ANALYSIS] - gCaptain Maritime & Offshore News

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I thought it was because they used the wrong micron filters and the wrong anchor?

The squat effect is also used by some large ships to get under bridges. They head for the bridge at high speed and take advantage of the squat effect to lower the air draft.

For instance, the Oasis of the Seas class of ships rely on this effect plus lowering the smoke stacks to get under a bridge that crosses their route from the shipyard where they were built.

Paul Swanson said:

I can't agree with "Gilligan" . The physics he is describing, Bernoulli's principle, is for a compressible gas. Water is incompressible. The water does not move relative to the sea floor and buoyancy is strictly dependent on water density which does not change with any kind of water motion, therefore the boat does not sit lower in the water. The wave action is a distinct possibility since waves get shorter and steeper as the water gets shallower due to the drag on the bottom. This would certainly slow a displacement hull.

Paul (Physics PhD)

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You're new around here and it's generally wise to take many, if not most posts with a grain of salt as they're only personal opinions and not facts. However, the one person that I've never seen wrong on a post is Northern Spy. In fact, I've never even seen anyone question him until your post. Take what he says to the bank, and rest assured that he's certainly no Gilligan.

Wow, this was quite a physics lesson. Maybe it is shallower than the 20' I listed. We're underway again this weekend and will be in a shallow part of the ICW for a few days, I'll pay more attention to the exact depth where I notice it.

Thank you all!

I've seen no difference. We must live in a different universe. ... Only noticed a difference when the Mahalo Moi slowed down, and then us, as our keels slid through the slough's soft bottom. (Hint: only transit beyond the Hwy 101 bridge near high tide on the Petaluma "River.")

With my trawler I was able to notice about 0.5 kt decrease when the depth dropped to less than 1 ft under the keel. Its about 6 miles of shallow canal with no current on the run to my storage yard. I could not feel it but the GPS clearly showed the drop in speed.
On the other hand, on my 17 ft center console, I can feel the boat slow and behave different when the depth gets less than a foot under the hull. The boat smooths out just as the depth increases. (No, I'm not churning mud!) The GPS speed on that boat is too inconsistent at any speed to see the effect. Its a 30 minute ride each way to the ICW though many shallow spots for plenty of opportunities to prove this out. Absolutely no doubt in my mind the squat effect is real.

I have never noticed a big change of speed, though with less than a foot under the keel (with a mud bottom of the ICW), maybe 0.5 kts loss.

However, I finally realised that the Krogen wants to stay in the channel. IN the "S" curves of the ICW, even through the hydralic steering, I could feel that my course, according to the markers, was not what the boat wanted to do.

Finally I let go of the wheel and discovered that the boat would turn quite a bit on her own to stay in the deepest part of the channel.

As the channel widened, this effect would stop and I actually had to steer again.

The lesson learned, was that a light hand in such conditions kept the boat in deeper water than just using the buoys and charts.

Nsail said:

You're new around here and it's generally wise to take many, if not most posts with a grain of salt as they're only personal opinions and not facts. However, the one person that I've never seen wrong on a post is Northern Spy. In fact, I've never even seen anyone question him until your post. Take what he says to the bank, and rest assured that he's certainly no Gilligan.

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Ummm. I don't even know how to respond to this. Thanks?

I am the epitome of an incomplete education. Mainly because I have a fantastically short attention span. I would classify myself as a guy who is lucky enough to know a little about a lot, or at least recall that I learned it a one time in my past. Particularly if I found it remotely interesting. I am no expert on any subject, but am fortunate enough to have worked with many of them in various fields.

Essentially, I glean nuggets of wisdom from smart educated people. That and I am an ENTP.

I run across a few hard sand bars with less then a foot clearance at 7 knots. I feel the boat slow as I cross them and speed up after returning to deeper water. Maybe close to a knot loss overall. I know I'm not touch bottom because I have had that surprise also.

We slow down for shallow and speed up for deep too. However, in our situation that's a direct result of reducing and increasing throttle.

We notice a speed drop when we get under about 5' of water. We also notice the autopilot stops tracking correctly and the boat seems to drift towards deeper water.

Yeah, right.