posted 09-17-2009 11:52 AM ET (US)            

The above comments are obvious to any experienced sailor, and have to do with a boat/motor moving at a sustained speed. What hasn't been discussed yet is the possible effect of current on acceleration. I would think that, as an airplane takes off more easily into the wind than downwind, a boat would accelerate up onto plane more easily and swiftly up-current than down.

posted 09-17-2009 12:48 PM ET (US)            

Nope. Read your first para.

The airplane on the runway is not in the same medium (surrounding environment in which something functions and thrives. Or, an intervening substance through which something else is transmitted or carried on.). The airplane is on the ground, not in the air. But airplanes "cheat", their wheels are on the ground while their wings are in the air.

The boat is already in the water and dead in the water (not moving), so it makes no difference which way it goes, the acceleration will be the same - relative to the water.

Similar to watching an airplane right after it has taken off. If take-off speed is 150 knots and it's taking off into a 75-knot wind, it can take-off while rolling at 75-knots into the wind because the wind is going over the wings at 150-knots. When the airplane gets airborne, it is traveling at 150-knots thru the air, but only at 75-knots over the ground.

posted 09-17-2009 02:13 PM ET (US)            

Running with the favorable current would produce an effective 2-MPH headwind giving increased wind resistance at the bow, slightly increase loading on the engine and possibly a small drop in RPM.

Running against the adverse current the lesser effect of a 2-MPH tailwind at the stern is less likely to change or
decrease loading on the engine or change in RPM.

posted 09-17-2009 03:10 PM ET (US)            

In the water with no current, if the boat moves forward at a speed s , it creates an apparent headwind of speed s .

In the water with favorable current, the boat goes faster, so the apparent wind is of speed s + 2.

In the water with adverse current, the boat goes slower, so the apparent wind is of speed s -2.

If we consider the boat speed to be in the range of 40-MPH, then the apparent wind is in the range of 40 plus or minus 2 MPH.

posted 09-17-2009 03:46 PM ET (US)            

Nope.

To the boat in the water - there is NO current. The boat is sitting there dead in the water. Nothing is moving between the boat and the water.

BUT relative to the land (the bottom) both the water and the boat are moving (but the boat is still not moving in the water).

So when you apply power to the engine, the boat is starting from sitting DIW in a body of water that it had no relative motion against.

Looking at it from the beach is different.
When the boat starts up stream against the current, she appears to get on a plane quickly - relative to the observer on the beach, she appears to cover a short distance of advance.

When the boat starts down stream with the current, she appears to get on a plane slower - relative to the observer on the beach, she appears to cover a longer distance.

But the time to plane and the distance to plane are both the same relative to the water and the boat. Not to the observer on the beach.

It's all relative motion. A vector diagram using a universal plotting sheet would illustrate it nicely.

posted 09-17-2009 03:50 PM ET (US)            

One more way to visualize. Pretend you're in the Gulf Stream, well offshore, out of sight of land. Now run your boat in all directions and see how long it takes to plane. With no other factors, it should take the same in all directions. Even though the Gulf Stream is moving at 1-2 knots.

Relative motion can be a nasty thing.

posted 09-17-2009 04:44 PM ET (US)            

Being the jackass that I am at times, I will notice when my buddy is running the boat against the current and have him note the speed, 24 mph, and the mpg of 2.2. Then when we turn the corner around an island and I know that the current will change, I will take the wheel and show him how much better I am at running the boat because now it is going 27 mph and the fuel economy is 2.5 mpg.

John

posted 09-17-2009 05:55 PM ET (US)            

Attempting one of those Einsteinian thought experiments (like trying to visualize what would happen in a weightless vacuum to a guy moving "downward" in an elevator), let's look at it this way:

I'm tied to a dock facing upstream on a swiftly-moving river. Let's say the current is running against me at 10 knots. I start my motor, throw it into gear, and with a small forward touch on the controls I'm swiftly up on plane (my little 15 ft Whaler planes at 12 or 13 knots). On the other hand, facing down-current, still tied to the same dock on the same river with the same current flowing swiftly past my boat, seems to me that I'd first have to catch up to the current and then "pass" it at 12 or 13 knots to achieve plane. Mebbe not, but still...

posted 09-17-2009 06:09 PM ET (US)            

Tony

posted 09-17-2009 06:29 PM ET (US)            

Facing upstream, your 15-ftr is going thru the water at 10-knots while still tied up (or the water is already going by the boat at 10-knots). Remember relative motion - this example only concerns your boat and the water she sits in. You have to advance the throttle to 10-knots to get a strain off your mooring lines - and you haven't moved, but the water has still moved by your boat at the rate of 10-knots. It's easy to advvance the throttle as the mooring lines are doing the first "10-knots of work".

While facing downstream & still moored, you are "backing" into the current at 10-knots just sitting there. To get the boat going forward at 12-13 -knots thru the water you first have to overcomg going backwards at 10-knots. By the time your going 12-13 knots thru the water you'll be going 23-knots OVER THE BOTTOM.

Maybe ground speed over the bottom (lets say you're using GPS) is confusing you compared to speed thru the water (your boat speedometer hooked to your pitot tube).

Years ago I took a 180-ft Coast Guard buoy tender up the Umpqua River to Reedsport, Oregon against a 6-knot current coming down river due to heavy rains (all of the buoys were pulled underwater by the current and you could see the whirlpools/disturbances on the surface from the buoys just under the surface). When we got to the shipyard in Reedsport we slowed to 6-knots THRU THE WATER and stayed directly off the dock - not moving relative to the shipyard. To the people on the dock we weren't moving, yet the ship was making turns for 6-knots and going thru the water at 6-knots.

posted 09-17-2009 06:57 PM ET (US)            

Years ago I had a CG Captain give me pretty much the same scenario but he wanted to know why the boat wasn't doing 23-knots to start with!

posted 09-17-2009 07:14 PM ET (US)            

According to the Beaufort Wind Scale a 1-3 mph wind will produce ripples and could even affect the rpm deviation from 5,500 in both directions.

Really the question is does increased apparent wind speed of two mph on the bow cause increased engine load and decrease of two mph lessen the load. +/- 4.

Most of us would probably agree it is increased and decreased slightly, but is not discernible.

I would like to hear Jim's explanation of how the results would differ in the same situation but when the boat is backing rather than going forward.

posted 09-17-2009 07:17 PM ET (US)            

This is how we make something easy become something difficult.

Some of y'all have added issues not present in the initial post. Wind? Apparent wind? Acceleration? Where did these factors come from? Should we not add gravity to the issue? If we have current we could be going up or down in altitude in a river or a stream, or not.

Too funny.

posted 09-18-2009 09:17 AM ET (US)            

Wind appears to have a direct affect on engine speed. If the boat moves against the wind, the wind loading on the boat creates a force opposing the movement of the boat. This has the effect of increasing the load on the engine, which will reduce the engine speed. Thus a boat moving against the wind will find its engine speed reduces compared to a no-wind situation. If the boat moves with the wind, the wind assists the boat in moving, and the load on the engine decreases. Thus a boat moving with the wind will find its engine speed increases compared to the no-wind situation.

The force of the wind is proportional to the velocity of the wind and the area of the boat on which the wind acts. It is also influenced by the shape of the area. You can describe the wind force as

F = C x A x ρ x V²

where

F = force
C = a constant related to the shape of the boat
A = frontal area of boat facing the wind
ρ = density of air
V = velocity of the wind

The important factor is the velocity of the wind, whose influence is exponential. This means that as the wind speed increases, it effect grows more significant. At low speeds, such as the 2-MPH apparent wind we considered in the discussion of the current, the influence is much smaller. If the wind increases to 20-MPH, a tenfold increase in speed, the force that results will be one-hundred times greater! If the wind increases to 40-MPH, the force that results will be 400 times greater.

The influence from the wind will be much more noticeable on engine speed when comparing engine speed on runs against the wind and runs with the wind.

It is also important to note that the influence from wind is proportional to the frontal area. An increase in frontal area, such as occurs when a boat has put up its weather canvas, including a windshield, will increase the influence of the wind.

I raised this topic for a particular purpose: to analyze the influence of wind and current on the engine speed at wide open throttle. As many know, I like to collect data about propeller performance. It is not always possible to test propellers in water which has no current or no wind.

In testing propellers, an important parameter is the speed the engine can reach at wide open throttle with a particular propeller. I was curious how wind and current would affect engine speed. Based on this discussion, it seems that engine speed at wide open throttle should not be influence very much--if at all--by effects from current. However, engine speed is likely to be influenced by effects from wind.

Tests made in water with current but calm winds may not need any allowance or correction for the maximum engine speed reached. Knowing this may allow some useful data to be collected. Of course, the best test method is to make runs in opposite directions, and average the results.

posted 09-18-2009 07:20 PM ET (US)            

We operate a boat in conditions where a wind of known strength exists, and with a wind strength which we anticipate will affect the engine speed. For example, we operate the boat with a 15-MPH wind. We make runs both upwind and downwind. Let us assume a boat speed of 35-MPH is obtained. In the upwind case we have an apparent wind of 50-MPH. As we don't have actual data for a test like this, we can assume the variation in engine speed might be 600-RPM.

posted 09-19-2009 10:38 AM ET (US)            

If the pilot chose to allow the set and to maintain a constant compass course the distance traveled would be greater than if a straight course over the ground was maintained???

A few weeks ago I removed some paint from the skeg of my engine by maintaining a steady course directly toward a distant water tower in a wide body of water. Without realizing it the very strong incoming tide was setting me aside very quickly. Thank goodness for a working low water depth alarm.

I am sometimes amazed at the stupid amateur mistakes I can make even after a lot of years of boating. Fatigue may be the most common factor.

posted 09-19-2009 11:17 AM ET (US)            

This article tries to examine the effect of the current on the maximum speed the engine can reach. The effect of the wind on the engine speed was included as it became the means by which the water current influenced the engine speed.