posted 01-17-2011 06:29 PM ET (US)            

The warmer the air, the less dense it is and the less power a given motor will generate.

posted 01-18-2011 12:07 AM ET (US)            

http://www.csgnetwork.com/h2odenscalc.html

We know that there can be significant different in water temperature between the Florida Keys and New Jersey, but I have not seen any good data about salinity differences. One source says salinity in the Atlantic varies from 33 to 37 parts per thousand, or a variation of about 4 parts per thousand or 0.4 percent.

When I enter a variation in salinity in the calculator above, say from the default value of 39000 PPM to a value of 39156 PPM, I see only a slight change in density, from 1027.014 kg/ml^3 to 1027.133. On the other hand, if I enter a change in temperature from 50-degrees to 80-degrees, I see a change in density from 1030.106 to 1025.871.

Comparing the change due to salinity (0.119) with the change due to temperature (4.235), we see a much greater change due to temperature. The change in density due to temperature is 35-times greater than the change due to salinity.

We have yet to establish in any meaningful data that the water in the Keys has greater salinity than in New Jersey, but, even allowing for the maximum variation in salinity cited above, we see that salinity is only a minor influence on water density compared to water temperature influence on water density.

posted 01-18-2011 09:11 AM ET (US)            

Hee is a chart of worldwide ocean salinity if anyone is interested.

posted 01-20-2011 09:39 AM ET (US)            

Suggestions that the warmer (and less dense) air affected performance are contradicted by the engine turning the same speed. Also, less dense air should reduce air resistance, which should have a (very small) positive effect of boat speed.

Water which is colder is more dense. If the propeller turns at the same speed in both locations, then the weight of water being thrown from the propeller will be greater in the colder water. A greater weight of water being thrown will produce more thrust. More thrust should make the boat go faster. The general theory that water temperature, and thus water density, would influence boat speed, however, is shattered by the later report that the water temperature in South Florida was very cold and about the same as in New Jersey water.

posted 01-20-2011 11:47 AM ET (US)            

I find that the comparison is completely analogous to my initial representation. As air density decreases, performance of aircraft propellers decreases. In fact, we see that aircraft driven by propellers all tend to have a maximum altitude or service ceiling, above which they cannot operate, because the density of the air is too low to permit flight. This analogy is very good to boats operating on hydroplane. Denser water produces more thrust from the propeller, and denser water provides more lift for the hull.

posted 01-20-2011 12:10 PM ET (US)            

Less dense air should offer less resistance and allow greater boat speed? Common, surely you jest. We all know that as air density declines, so does the power output of an internal combustion engine. Warmer air is less dense.

Gearcase drag in denser water might increase but insignificantly so. Denser water makes a boat float higher and thus there is less drag from the hull's surface area in contact with the water and greater boat speed.

Furthermore, I suppose a propeller will have greater grip in denser water and offer a greater apparent pitch, but again I am sure it is also completely insignificant in the context if this discussion.

We don't need to argue theories about why a boat might be slower or faster in fresh vs. saltwater, the difference is demonstrable.

My boat is 1-2 MPH faster in saltwater than in freshwater. Seattle is bordered on the West by the saltwater estuarial waters of Puget Sound and on the East by freshwater Lake Washington. Most boaters here understand boats are faster in saltwater.

There is a very significant difference in water density between saltwater and freshwater elsewhere too and it is equally demonstrable. Last month I was on the West coast of Mexico. In the ocean I could just float on my back without swimming, but hiking upriver to swim in the river pool at the base of a local waterfall in freshwater, I could not float at all; I sink like a rock (I have very low body fat).

However, we are talking here, not about fresh vs. saltwater but the saltwater of New jersey vs. the saltwater of Florida. The difference in salinity is very small but it is Florida that has the saltier water yet it is Florida where the boat apparently is slower which is the opposite result of what we would expect so I reject the notion that it is the salinity of Florida's water that is slowing the boat.

posted 01-20-2011 12:37 PM ET (US)            

http://www.csgnetwork.com/relhumhpcalc.html

Used in conjunction with Jim's "Crouch's Speed Calculator" we can actually estimate the difference in speed of vin1722or's Outrage 17 between New Jersey and Florida.

[urlhttp://continuouswave.com/cgi-bin/crouchcalc.pl[/url]

posted 01-20-2011 05:55 PM ET (US)            

There is insignificant change in density in water between 50 and 70 F. There should be a change in density due to the salinity - if it exists.

Air temperature will change the performance of an internal combustion engine - as Tom and others have stated.

But - there are other factors in play here as well. That is, the warmer air increases the density - which increases the drag.

We don't know the difference in the water conditions, the wind conditions (velocity and direction), the load on the boat - or the fuel consumption.

The mere fact that the rpm was about the same - only indicates that the engine rpm was about the same. But, realize that the engine/prop performance (at any given instant) is at the conditions where energy input is equal to the energy output - by definition. Of course, the energy input is from the fuel consumption. The energy output is based on the thrust from the prop and the air/water resistance - which are also balanced.

posted 01-20-2011 10:28 PM ET (US)            

quote:

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"Less dense air should offer less resistance and allow greater boat speed? Common, surely you jest."

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The influence of air density on the air resistance of the hull has an influence on boat speed. I made no observation about influence on engine performance.

In the case under discussion, the engine speed was the same in Florida as in New Jersey. The engine appears to not have been affected by any air temperature variation.

It seems quite difficult to say that air temperature affected the engine when the engine ran at the same speed in both places. I will need an explanation of how the air temperature affected the engine, but somehow the engine ran at the same speed.

posted 01-20-2011 10:32 PM ET (US)            

quote:

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"I strongly suspect a measurement error is actually the cause for this odd mystery."

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Yes. I agree. The data presented is contradictory, and there is no good fit for the data into the principle of propulsion and Physics.

posted 01-21-2011 08:39 AM ET (US)            

If GPS, you will get an accurate speed over ground reading but it neglects the effect current has on the boat, either a slight boost or a drag.

To eliminate the you will need to take two runs in opposite directions at the exact same RPMs and exact same trim settings. Then repeat it in the other locations exactly, with the same configuration (top up or down) and vessel loading (fuel, passengers, gear).

Too many uncontrolled/unknown variables to draw an accurate conclusion.

posted 01-22-2011 09:46 AM ET (US)            

A boat is driven by a propeller. The propeller is turned at the same speed in two locations. We assume there are no other variables in these two instances except for the location, that is, the boat condition, engine condition, weight, or other factors related to the boat are all the same.

Even though the propeller turns at the same speed, the boat moves at a substantially different speed in the two locations. In one location (Florida) the boat moves at 39-MPH, and in the other location (New Jersey) the boat moves at 44-MPH.

We can assume the atmospheric conditions are similar in both locations. We now ask what factor could account for the 5-MPH difference in speed.

The most likely influence on boat speed that remains as a variable is water density. Water density is affected by temperature and salinity. We are told, however, that water temperature in both locations was nearly identical. This leaves salinity as the only possible influence.

We know that water density varies with salinity, but the difference in salinity between the two locations is small, and the difference in density of water due to variation in salinity of a small amount is correspondingly small.

The ratio of the boat speed in the two locations is

44/39 = 1.1282

We know from the work of George Crouch with moderate planing hulls that to produce a change in speed of a factor of 1.1282 we would need a change in horsepower of a factor of (1.1282)^2 or 1.2728.

In order for the engine to produce 1.27-times more power output at the same engine speed, the engine would have to produce more torque at the same engine speed. More torque would imply the load on the propeller was greater. The load on the propeller must be due to having to turn in water that is more dense. Since the propeller turns at the same speed in both locations, we have to assume that the increase in boat speed is due to the propeller turning in water that is more dense and therefore creating more thrust. The propeller can create 1.27-time more thrust in only a few ways:

--it can turn faster so it accelerates the water to a higher speed

--if can accelerate more water

--if can accelerate heavier water

We are told that the propeller turns at the same speed, so the first option for increased thrust is not possible.

It may be possible that when operating in higher density water a propeller becomes more efficient, however it is difficult to image that the efficiency could improve by a ratio of 1.27:1 due to a relatively minor change in water density. Therefore we anticipate that a propeller turning at the same speed will accelerate the same amount of water, more or less, without a great influence from the density of the water.

This leaves the third mechanism, an increase in the weight of the water, as the most likely mechanism to produce more thrust while turning at the same speed. Since the temperature of the water is the same in both locations, we are left with salinity as the only mechanism to explain to difference.

Water density increases with salinity. Unfortunately, the water has greater salinity in the location where the boat goes slower. This outcome is the opposite of the anticipated effect of salinity on water density.

posted 01-22-2011 11:32 AM ET (US)            

Why is this? Did I miss something? I saw where Vinny noted the water temps were similar but was air temp and humidity noted as being similar?

posted 01-22-2011 05:00 PM ET (US)            

quote:

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I...could not understand how a 4-5 mph difference is possible.

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vin1722or -- It's not possible. No, let me be more clear. Ceteris paribus, it is not possible for a boat/motor with the same propeller to travel two radically difference speeds at WOT yet turn the same RPM.

What is possible:

- A boat can be faster in saltwater vs. freshwater.

- A boat can be faster in colder air.

We can correctly argue that colder air is more dense, that colder water is more dense, that saltier water is more dense and that density and drag are related.

We can correctly argue that the pull of the Moon will influence boat speed.

We might even to be able to argue that water depth influences water speed.

But all these things are completely insignificant on the context of your situation. Discussion of them ignores the elephant in the living room: the difference in reported boat speed is 3-4 MPH.

You reported that it is not a measuring error and I accept that. Boat speed and RPM have been measured accurately. You are not making this up. But as with most mysteries, there is probably a simple explantation.

Nothing discussed above can accommodate such a great difference in boat speed...except this:

quote:

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the boat ran at 34 knts when I bought it in the spring and repropped it,raised the engine 1 hole up and achieved 38.5 knts which is where I and many CW people thought it should be. I get to florida and 39 mph is all I get is 39 mph.

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I note that 34 knots is about 39 MPH.

You inadvertently put the old prop on the boat before going to Florida.

posted 01-22-2011 07:25 PM ET (US)            

The only two items that can make the boat go a different speed is the air temp/humidity, and maybe the salt water. The moons pull would be to small to calculate. And I do not think water depth changes anything. But the rpms being the same does not add up...

posted 01-23-2011 10:13 AM ET (US)            

Ambient pressure has direct influence on the density of the air containing typically 20,95 % volume Oxygen.

If the ambient pressure goes up the air becomes denser and
the relative volume % Oxygen increases proportional to the
ambient air.