Boat Speed Affected by Water Depth

We lose between 1 and 1 1/2 knots in water depth less than about 20 feet. I don't know the first thing about physics, but I know that water depth has a substantial effect with our hull.

As an aside, when we did our sea trial, I almost passed on the boat because the speed was so much less than the specs stated at given power settings. My surveyor assured me it was because of the shallow water (less than 10 feet). I thiught he was nuts until we got into deep water (over 40 feet). Sure enough, the speed picked up to spec.

So, we bought the boat.

The “reverse squat effect” noticed by Ulysses ( which should really be called the inverse squat effect ) is caused by the well-known principle that two rigid objects cannot occupy the same space at the same time. In detail, it has to do with conservation of energy. Since you are no longer going forward some of that kinetic energy is converted to potential energy in lifting the boat vertically. This energy is eventually recovered when the tide comes in and the boat returns to its normal waterline.

Northern Spy said:

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.

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And humble too!
And, no thanks needed. I was just stating the facts. Keep up the great posts!

ssobol said:

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.

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I bet the first Captain that did this had a pucker factor of 150%

Brisyboy said:

I bet the first Captain that did this had a pucker factor of 150%

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Sounds weird to me...love to see the risk management model explaining to the corp types and insurance company if anything went wrong.

Paul Swanson: I stand corrected "inverse squat effect" it is. Thank you for the science behind it. I find it very common while doing the old green-right-returning thing.

dan

Never experienced it on my Willard but quite noticeable on my downeast hull.

A number of years ago, the QE2 tore a hole in its' bottom in Vineyard Sound near my home. The Capt tried to blame it on poor American charts but he hadn't heard of squatting in shallow water either. The investigation showed that, at the speed he was making (much to fast for conditions) he was squatting 6'.

Years ago I noticed a speed reduction in my kayak when paddling over shallow water. I would use a handheld GPS, man that thing ate batteries, to show distance and speed I had traveled. The slowing of the kayak was around points with shallow water and I could FEEL the boat slow down and my paddling effort would increase. I can't remember the exact number, but I think I would loose 1/2-1 mph which was a significant loose of speed since my max sustain speed was 5.2/5.3 mph.

I also noticed it happened where there were large numbers of weeds as well. There is a nearby lake that grows a huge amount of water weeds that don't quite make it to the surface. If I padded over a patch of weeds it was the same affect as going over shallow water.

Eventually, I read about the affect in a book, I think it might have been Van Dorn's "Oceanography and Seamanship."

Later,
Dan

Greetings,
The queen squats? Well, We are not amused...

When the Krogen Manatee first appeared on the market in the mid 90's, it was tested by a National magazine where the writer registered a WOT speed of the 90 HP Volvo powered Manatee at 7.5 knots in shallow Biscayne Bay. James Krogen asked the Writer to come back and retest the boat in deeper water to demonstrate what Krogen claimed as a more realistic speed of 8.5 knots. The magazine said "no thanks" and published the report at the originally determined 7.5 knots. I never saw a 90 HP Manatee with a reasonably clean bottom that couldn't do the 8.5 knots that Krogen claimed.

RT, you should spell "we" with a capital "W" when a royal refers to him/herself.

Greetings,
Mr. mp. Thanks. Good catch. I'd better be careful or I'll be off of Liz's xmas list....

Northern Spy said:

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.

Click to expand...

That speaks to me also. I'm an ENTJ, as is the Dauntless' First Mate.