Reply
 
Thread Tools Display Modes
 
Old 04-29-2015, 11:40 AM   #1
Veteran Member
 
Just Bob's Avatar
 
City: Oriental, NC
Country: USA
Vessel Name: Liberdade
Vessel Model: Ocean Alexander 54
Join Date: Apr 2011
Posts: 78
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!
__________________
Advertisement

__________________
Just Bob
May you always have at least a hand's width of water beneath your keel!
Just Bob is offline   Reply With Quote
Old 04-29-2015, 12:05 PM   #2
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,884
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.
__________________

Ski in NC is offline   Reply With Quote
Old 04-29-2015, 12:32 PM   #3
Guru
 
caltexflanc's Avatar
 
City: North Carolina for now
Country: USA
Vessel Name: Small Incentive
Vessel Model: Boston Whaler 130 Sport
Join Date: Aug 2011
Posts: 3,789
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.
__________________
George

"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"
caltexflanc is offline   Reply With Quote
Old 04-29-2015, 02:30 PM   #4
Guru
 
Northern Spy's Avatar
 
City: Powell River, BC
Country: Canada
Vessel Name: Northern Spy
Vessel Model: Nordic Tug 26
Join Date: Feb 2012
Posts: 2,666
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 is offline   Reply With Quote
Old 04-29-2015, 02:56 PM   #5
Guru
 
City: Satsuma FL
Country: United States
Vessel Name: No Mo Trawla
Vessel Model: Hurricane SS188
Join Date: Jul 2014
Posts: 1,675
Quote:
Originally Posted by Northern Spy View Post
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.
Could you translate all that into Dick and Jane language?
Donsan is offline   Reply With Quote
Old 04-29-2015, 03:23 PM   #6
Guru
 
Northern Spy's Avatar
 
City: Powell River, BC
Country: Canada
Vessel Name: Northern Spy
Vessel Model: Nordic Tug 26
Join Date: Feb 2012
Posts: 2,666
Maybe after work if still required. That was a lunchtime snippet.

I'm sure others will chime in.
Northern Spy is offline   Reply With Quote
Old 04-29-2015, 03:48 PM   #7
Guru
 
ancora's Avatar
 
Join Date: Dec 2007
Posts: 3,489
As any Predicted Log contestant will tell you, the Spyman is correct in his explanation.
ancora is offline   Reply With Quote
Old 04-29-2015, 05:04 PM   #8
Guru
 
AKDoug's Avatar
 
City: Kenai, Alaska
Country: USA
Vessel Name: Melanie Rose
Vessel Model: 1999 Willard PH
Join Date: Feb 2013
Posts: 570
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.
AKDoug is online now   Reply With Quote
Old 04-29-2015, 05:24 PM   #9
Guru
 
mbevins's Avatar
 
City: Windsor
Country: Canada
Vessel Name: Keeper IV
Vessel Model: 44 Viking ACMY
Join Date: Nov 2013
Posts: 1,305
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.
__________________
"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

mbevins is offline   Reply With Quote
Old 04-29-2015, 05:41 PM   #10
Veteran Member
 
Paul Swanson's Avatar
 
City: Ventura CA
Country: USA
Vessel Name: Proud Mary
Vessel Model: Pacific Trawler
Join Date: Apr 2015
Posts: 77
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)
Paul Swanson is offline   Reply With Quote
Old 04-29-2015, 05:58 PM   #11
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,881
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%27s_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.


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.
psneeld is offline   Reply With Quote
Old 04-29-2015, 06:03 PM   #12
Guru
 
twistedtree's Avatar
 
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 3,187
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.
__________________
www.MVTanglewood.com
twistedtree is offline   Reply With Quote
Old 04-29-2015, 06:03 PM   #13
Guru
 
caltexflanc's Avatar
 
City: North Carolina for now
Country: USA
Vessel Name: Small Incentive
Vessel Model: Boston Whaler 130 Sport
Join Date: Aug 2011
Posts: 3,789
Squat effect certainly does not apply to real planing boats. The rule of thumb in shallower waters for those is to stay on plane.
__________________
George

"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"
caltexflanc is offline   Reply With Quote
Old 04-29-2015, 06:04 PM   #14
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,262
Quote:
Originally Posted by Paul Swanson View Post
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)
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.

__________________
Kar-KEEN-ez Koot
markpierce is offline   Reply With Quote
Old 04-29-2015, 06:16 PM   #15
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,884
Quote:
Originally Posted by Paul Swanson View Post
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)
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.
Ski in NC is offline   Reply With Quote
Old 04-29-2015, 06:26 PM   #16
Veteran Member
 
Paul Swanson's Avatar
 
City: Ventura CA
Country: USA
Vessel Name: Proud Mary
Vessel Model: Pacific Trawler
Join Date: Apr 2015
Posts: 77
I stand corrected. The Bernoulli principle does apply to both compressible and incompressible fluids. It seems to be well known to naval architects.
Paul Swanson is offline   Reply With Quote
Old 04-29-2015, 06:33 PM   #17
Guru
 
ulysses's Avatar
 
City: Gulf Shores, Ala.
Country: USA
Vessel Name: Ulysses
Vessel Model: Romsdal 1963
Join Date: May 2013
Posts: 876
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
ulysses is offline   Reply With Quote
Old 04-29-2015, 07:25 PM   #18
Guru
 
JDCAVE's Avatar
 
City: Lions Bay, BC
Country: Canada
Vessel Name: Phoenix Hunter
Vessel Model: Kadey Krogen 42 (1985)
Join Date: Apr 2011
Posts: 1,605
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 is offline   Reply With Quote
Old 04-29-2015, 07:36 PM   #19
Guru
 
twistedtree's Avatar
 
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 3,187
Quote:
Originally Posted by JDCAVE View Post
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
I thought it was because they used the wrong micron filters and the wrong anchor?
__________________
www.MVTanglewood.com
twistedtree is offline   Reply With Quote
Old 04-29-2015, 07:37 PM   #20
Senior Member
 
City: Leesburg, VA
Country: United States
Join Date: Mar 2015
Posts: 109
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.
__________________

ssobol is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 12:12 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012