posted 09-07-2008 09:28 PM ET (US)            

Now, I am not a engine manufacturer - or engineer - but I look at the WOT as being a "red-line" - as on a car/pickup/truck engine. Since the efficiency of an engine varies as the engine RPM to some point of maximum efficiency and then decreases - I also suspect that the WOT is not far from the point of maximum efficiency.

posted 09-07-2008 09:38 PM ET (US)            

With a 13x17 prop I can turn up 5600 rpms and go 41 mph and at 3500 I make 23 mph and at 3000 I go 19 mph.

posted 09-08-2008 01:52 PM ET (US)            

Did you dealer tell you 5000 RPM or 5500 RPM? You have stated it both ways both.

posted 09-08-2008 02:19 PM ET (US)            

BRP sent a notice to its dealers with the Optimum RPM range recommendations. I do not know the precise technical reasons for it, but here is a thread where I reproduced it:

http://continuouswave.com/ubb/Forum4/HTML/006093.html

I was interested to see this chart because it fit well with personal experience with the E-TEC 90 which is a motor that does not seem to really like to spin all the way up to its redline. Attempts to improve performance by reducing pitch to gain RPM does not seem to bear much fruit above 5000 RPM.

posted 09-08-2008 06:49 PM ET (US)            

"I also suspect that the WOT is not far from the point of maximum efficiency."

Well, not exactly.

"Efficiency," as specific fuel consumption (fuel flow per horsepower for example) cannot be easily determined, except maybe at WOT. We have no real way of knowing how much power is produced at lower RPM and power settings without a dynomometer at hand.

However, once two-strokes have come up well above idle, and are functioning as gas dymamic engines, efficiency is pretty constant, except, of course, for internal friction, which increases its power demand as the cube of the RPM. Furthermore many engines are calibrated to run rich at near WOT to help them cool themselves.

For those reasons it is exceedingly unlikely that "efficiency" is highest near WOT. Nor, of course is the hull most efficient at high speeds.

posted 09-08-2008 09:28 PM ET (US)            

As I have not read the majority of the posts in this thread - I am not sure of the point you are trying to make. But, be assured that the prop designers are trying to get the most thrust (change in momentum) at a given rpm and as compromised by manufacturing costs. And be assured that a boat makes one hell of a poor dynomometer - given the difference in engines, the differences in hull design, the differences in loading, the differences in the water, the difference in .........

posted 09-08-2008 09:58 PM ET (US)            

Fuel efficiency is a big part of how outboard motors are designed and marketed today. But we are getting off track (as usual) in regard to bloller's original question.

I just grabbed one of my Mercury factory Service Manuals to see what it says about propeller selection. This manual covers the 135-225 V-6s but is probably typical of the advice for bloller's smaller Mercury as well.
[note: the emphasis is Mercury's, not mine. ]

PROPELLER SELECTION

1. Select a propeller that will allow the motor to operate at or near the top of the recommended full throttle RPM range with a normal load. Maximum engine speed (RPM) for propeller selection exists when the boat speed is maximum and trim in minimum for that speed. (High RPM, caused by an excessive trim angle, should not be used in determining the correct propeller.) Normally, there is a 300-500 RPM change between propeller pitches.

2. If the throttle operation is below the recommended range, the propeller MUST BE changed to prevent loss of performance and possible engine damage.

3. For better acceleration, such as is needed for waterskiing, propping up to 500 RPM above the recommended range is advised. Continuous operation above the recommended maximum speed, however, is not permissible.

4. After initial propeller installation, the following common problems may require that the propeller be changed to a lower pitch:

posted 09-10-2008 12:13 PM ET (US)            

Well, literally this could mean the peak could by anywhere above idle, but I don't think this is your intent. I think you mean efficiency is high at high RPM, until something happens and it eventually falls off.

That sounds like you are describing volumetric efficiency, which is closely related to torque. VE climbs with icncreasing RPM, reaches a peak for typical 4 stroke engines around 75% of their max (like 3500 or so) then falls.

Many people are confused with the relation between VE and fuel efficiency. Generally there isn't one. VE is the efficiency of the pumping; you pump both air and fuel together, so it does not make the fuel burn any more efficiently by itself.

Generally, engine friction so seriously deteriorates efficiency that increasing RPM is almost always detrimental to sfc. If you want to experiment, try it on your car with overdrive gearing and instantaneous fuel flow. The results are quite dramatic.

What is your theory on why s/f/c increases with increasing RPM? I have never heard anything like that.

posted 09-20-2008 01:09 PM ET (US)            

Your idea that fuel us used solely based on throttle setting is totally wrong. Both carburated engines and FI engines (using different mechanisms) meter fuel based on (a) manifold pressure, set by the throttle, and (b) RPM. The higher the RPM at any given throttle setting the more fuel is used. On FI engines the throttle regulates air, the FI system meters fuel based on manifold pressure and air density and RPM.

All engines are air pumps that use a fuel air mixture (about 14:1) regardless of RPM or manifold pressure. Increasing RPM pumps more air and requires, obviously, more fuel.

At less than full power, the same power can be produced by increasing RPM and lowering manifold pressure, or by increasing manifold pressure and lowering RPM. Generally, IF detonation is not occurring, and if vibration is not a problem, you gain efficiency by lowering RPM and increasing manifold pressure accordingly. This works for cars, planes, and yes, boats, with piston engines.

"Lugging" an engine is a problem that dates mostly from before the computer controlled engine management era. It describes operating at low RPM and high throttle, with detonation and/or excessive vibration. Lugging is not usually encountered today because the EM software prevents detonation, and because valve timing and other related parameters have allowed low RPM high throttle operation. So don't confuse reducing RPM in midrange with lugging.

In the future you will see multi gear transmissions on small planing boats, to take advantage of the reduction in hull resistance at middle range cruise by allowing a higher (numerically lower) gearing, lowering engine speed, reducing friction and noise, and probably reducing wear.

"The notion that an engine will use less fuel at WOT just because the RPM's are lower is ludicrous. You've still got the throttle wide open. It's still dumping in the same amount of fuel, you're just getting less RPM's out of it, due to higher rotating resistance."

As I attempted to discuss above, there is so much incorrect with this statement I don't know where to begin. Plus, the author seems to think it is "ludicrous" to think otherwise. O well. It remains a mystery to me how people on this forum seem to know, with certainty, so much that is totally wrong, but whatever.

posted 09-21-2008 05:46 PM ET (US)            

In 1968 middle size GM cars had a 2 speed auto trans. Fuel was cheap.

Now, topend cars have 8 speeds.

More ratios mean more performance AND more economy. More ratios allow the engine to run at the optimum speed for the requirements.

Small planing boats do have a "hump" of high resistance followed by a "valley" of smaller resistance, eventually building back up.

Increased economy could be realized by slowing engine speed in the midrange cruise. However, with only one speed to choose from, this results in gearing that makes the engine strain (sometimes) to get on plane, and at the high end makes the engine unable to reach full power RPM.

posted 09-22-2008 09:54 PM ET (US)            

Assume you are going any particular steady state speed under full speed.

How much power does it require? Lets assume a light planing hull with a 200 hp engine goes 45 mph at WOT at 5000 RPM, and maybe 100 hp at 28 mph at 3200 RPM. (values just for illustration).

Now, the engine can produce 100 hp at 3200 RPM, 23" of manifold pressure (you don't measure this in a boat, but it is there), OR 2300 RPM at 32" MP (you cant get there at sea level) OR 2500 RPM at 28" MP, etc.

Now in general all IC piston engines will be more efficient at high MP, low RPM, for the same power output. This is assuming things are not so wierd as to produce detonation, overheating, or unusual vibration.

If you have only one ratio, you must compromise between acceleration, getting to full RPM at WOT, and getting your RPM down and your MP up at cruise.

Cars, airplanes, and boats all have this built in. Cars have gears, airplanes have (some) props that change pitch. Small boats, now, are stuck with 1 ratio.

One day soon it will change, I bet, considering fuel costs.

posted 09-24-2008 01:51 PM ET (US)            

We had a variable pitch propeller on our sailboat back in 1998. I actually still have the $5,000 propeller sitting in my garage. It's a pretty amazing piece of engineering. As the price for this kind of technology comes down, you will start to see it adopted by the recreational boating community. It's already quite common on larger commercial vessels.

An engine is most efficient at the peak of it's torque curve. "Overgearing" it so that your most common cruising speed is below that point will still result in it using more fuel per force exerted. I realize that both carbureted and fuel injected engines do meter their fuel input based on more than just throttle position. But if you've got vehicle that is under extreme load such that it cannot accellerate, and you open the throttle farther, it's going to consume more fuel, even if the RPM's do not increase. That's essentially what you're talking about doing.

posted 09-25-2008 01:37 PM ET (US)            

Not true, if you mean fuel efficiency.

"Volumentric efficiency" does not determine fuel efficiency. Best VE, which occurs at or near peak torque, is simply the point where the most air is pumped.

Why do you think cars have (today) such tall gears, sometimes keeping engine revolutions below 1600 at 60 mph? That is not where peak torque or peak VE is, far from it. It is because engine friction always subtracts from specific fuel consumption, and friction increases exponentially with RPM.

A multi speed trans will offer superior economy at midrange cruise. In the past, this just was not important enough to warrant the complexity. Now it probably is.

"But if you've got vehicle that is under extreme load such that it cannot accellerate, and you open the throttle farther, it's going to consume more fuel, even if the RPM's do not increase. That's essentially what you're talking about doing."

Well, not exactly.

If you increase manifold pressure by opening the throttle more, you increase torque and assuming you are at steady state (not accelerating) you will accelerate.

Air and fuel are metered together. The throttle meters primarily air. Carburators keep a relatively constant fuel mixure, enriching for acceleration and sometimes for WOT. The more air the more fuel. Fuel injectors meter fuel depending (usually) on the air mass, i.e. the density times the volume. Other circuits monitor transient response, such as sudden acceleration, but at steady state the mixture, i.e. the amount of fuel per unit of air, is relatively constant,and the power output per pound of fuel per hour is also pretty constant. Taking from this power is engine friction, which is significant and very significant at higher RPM. Which is why, as long as combustion is occurring correctly (no detonation), you always get your best efficiency at the lowest possible engine speed that will produce adequate power and drivability.

The auto makers fully understand this, as is obvious from any modern vehicle. Why people think that boats engines do not obey the same laws of physics as car and airplane engines, I cannot imagine, but the view is common on this forum. O well.