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Old 04-29-2015, 08:50 PM   #21
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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)
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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Old 04-29-2015, 08:59 PM   #22
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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!
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Old 04-29-2015, 09:04 PM   #23
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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.")


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Old 04-29-2015, 09:12 PM   #24
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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.
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Old 04-29-2015, 09:34 PM   #25
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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.
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Old 04-29-2015, 09:46 PM   #26
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Originally Posted by Nsail View Post
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.
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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Old 04-29-2015, 09:53 PM   #27
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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.
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Old 04-29-2015, 10:32 PM   #28
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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.
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Old 04-29-2015, 11:15 PM   #29
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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.
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Old 04-29-2015, 11:31 PM   #30
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Yeah, right.
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Old 04-30-2015, 12:02 AM   #31
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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.
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Old 04-30-2015, 11:24 AM   #32
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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.
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Old 04-30-2015, 04:19 PM   #33
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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.
And humble too!
And, no thanks needed. I was just stating the facts. Keep up the great posts!
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Old 04-30-2015, 05:50 PM   #34
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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.

I bet the first Captain that did this had a pucker factor of 150%
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Old 04-30-2015, 06:11 PM   #35
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I bet the first Captain that did this had a pucker factor of 150%
Sounds weird to me...love to see the risk management model explaining to the corp types and insurance company if anything went wrong.
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Old 04-30-2015, 07:29 PM   #36
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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
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Old 04-30-2015, 08:11 PM   #37
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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'.
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Old 04-30-2015, 08:27 PM   #38
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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,
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Old 04-30-2015, 08:31 PM   #39
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Greetings,
The queen squats? Well, We are not amused...

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Old 04-30-2015, 08:33 PM   #40
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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.
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