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Old 01-06-2010, 10:08 PM   #21
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RE: HP vs. RPM vs. Economy

Bernoulli is a factor in the generation of lift. It is not THE generator of lift. You can have lift without the Bernoulli effect. But I'm no expert--- I'm simpy quoting what NASA and our aerodynamicists say.

As to the Bernoulli effect with regards to hulls, logic would suggest that since a boat hull is normally curved identically on both sides the Bernoulli effect would not come into play here since the pressure on both sides of the hull would be equal, regardless of how it was reduced by the Bernoulli effect of the water flowing over the curved surfaces. If there is no pressure differential, there wil be no "force" acting on the hull. Whether the Bernoulli effect on each side of the hull reduces the resistance of the hull to the water I have no idea. You'd need to find a hydrodynamicist for that.

But whether it does or doesn't, at 8 knots and 27,000 pounds it's not going to make a g*ddamned bit of difference to my boat or my fuel bill.
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Old 01-07-2010, 05:40 AM   #22
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HP vs. RPM vs. Economy

Quote:
Marin wrote:Whether the Bernoulli effect on each side of the hull reduces the resistance of the hull to the water I have no idea.
In the pressure domain surrounding a hull there are high pressure areas forward and aft and a large low pressure area midships. This is what causes "squat" and "bank effect."*

Squat is the phenomenon of the draft increasing with speed in shallow water. Bank effect is the tendency of the ship to yaw rapidly when passing close to another ship or the bank of a river or canal. It is what really screws people up when trying to rapidly motor out of the Ballard Locks and they appear to be stuck to the sidewall.

The effect doesn't suck the boat forward and reduce the power requirement, just the opposite, it takes more power since the draft increases. In a large ship, you can tell from the depths of the engine room when the ship has entered shallow water as the vibration increases a great deal and the power output increases to maintain the same shaft revolutions.


"But whether it does or doesn't, at 8 knots and 27,000 pounds it's not going to make a g*ddamned bit of difference to my boat or my fuel bill."


It would if you ran around in shallow water all the time. You might not care and it might not show up as a big increase in fuel consumption but it would make a difference.

*


-- Edited by RickB on Thursday 7th of January 2010 06:44:21 AM
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Old 01-07-2010, 10:45 AM   #23
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HP vs. RPM vs. Economy

From operating narrowboats in the UK for many years I'm well acquainted with what a hull does in very shallow water since the clearance between the bottom of the 60', 20-ton boat we use and the canal bed is a couple of feet at best and often less. In fact the technique for not running aground when forced out of the middle of the dish-shaped channel is to slow down to reduce the "squat." However I was told-- perhaps erroneously-- that the stern squat of these boats when more power is applied is due to the propeller pulling water out from under the hull. Reducing the power reduces the amount of water being moved back by the prop and the stern will come up a few inches. In fact when powering the boat around in a tight turn with almost no forward speed, adding a burst of power to put a lot of thrust against the rudder also pulls the stern down enough to see and feel.

When these boats meet and pass, the width and contour of the canal usually requires them to pass very close--- perhaps about two or three feet apart. I have never noticed a tendency for the boats to be pulled together until they get about halfway past each other when the bow of each boat is pulled in by whatever the force is just beyond the stern of the other boat. You have to anticipate this and be ready to counter this inward yaw with the tiller.

Anyway, interesting forces at play here--- I know how to deal with them on a narrowboat but I don't really understand the dynamics that create them.


-- Edited by Marin on Thursday 7th of January 2010 11:45:50 AM
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