V-20 Outrage: Effect of propeller diameter

[FullThrottle]

I have a 1980 Outrage V20 with a 2000 Yamaha 150-HP Ox66. The propeller that came with the boat was a beat-up aluminum propeller, 14-inch by 17-pitch. [The OUTRAGE V20 boat speed reached] 44 to 45-MPH at WOT, with engine speed barely 5,000-RPM.

I just tried out a 15-inch by 19-pitch stainless steel propeller. [The boat speed was] 42-MPH at engine speed 4,700-RPM at full throttle.

I'm [going to try] a 14.25-inch by 19-pitch propeller. Will a change in diameter [of 0.75-inch to 14.25-inch from 15-inch] at the same pitch [allow the engine to accelerate an addition 800-RPM and reach} the 5,500-RPM ballpark?

Thanks

[jimh]

A propeller moves the boat by accelerating water and pushing it astern, with the resultant effect the boat goes forward. The greater the volume of water pushed astern and the higher the speed of the water pushed astern, the more thrust imparted to the boat. The total thrush imparted is thus a combination of the volume of water and the speed the water is moved astern.

If two propellers are alike in every way except one has blades of less diameter, then the propeller with less blade diameter will be easier for the engine to turn, because when that propeller turns it moves less water.

If the smaller diameter propeller is easier to turn, the engine will tend to accelerate to a higher rotation speed under the lighter load. This means the water the smaller propeller pushes astern will be moving faster.

The other propeller, with larger diameter, is harder to turn, so perhaps the engine cannot accelerate to the same speed as it can with the smaller diameter propeller. The larger diameter propeller turns slower, but moves more water.

Which approach to creating thrust is better? Moving the water faster? Or moving a larger volume of water?

Recall from basic Physics: FORCE = MASS × ACCELERATION

Will the total force be higher with more mass or with more acceleration?

These are difficult questions. People who design propellers might be able to answer this. A great deal of analysis is necessary to find the answer. In general, there is a tendency for some certain ratio between pitch and diameter to be more efficient at producing thrust that others. And for some particular blade shape to be more efficient at producing thrust than others.

With this in mind, we now face the question: between two propellers with the same rated pitch but of unknown blade shape and design, with a difference in diameter of a certain amount, will an engine of unknown horsepower and torque curve accelerate to a particular higher speed with one compared to the other?

This is a difficult outcome to predict. About all that can be said is that the propeller with smaller blade diameter would be expected to be easier to turn (for the reason mentioned earlier), and this should let the engine accelerate to a higher speed before the power produced by the engine and the load imposed by the propeller are equal and the engine stops accelerating.

As far as predicting the performance of the boat, this is also very hard to know. The smaller diameter propeller will likely end up rotating faster, thus accelerating the water it moves to a higher speed, but it probably will be accelerating a smaller volume of water.

If you want to find out what will happen when you change to a particular propeller, the only way to discover that outcome will be to test it.

[jimh]

A further element in the problem that must also be considered is the engine that will try to turn the propeller. In general an engine produces more power as its rotational speed increases, but the curve of power and RPM is not a straight line. The shape of the power curve of an engine varies due to variations in the engine's design. Perhaps as an engine rotates faster there are frictional drags that increase and drain off useful power output. The engine manufacturer may provide a detailed specification about precisely what rotational speed range for the engine produces the peak power output. When that information is available, then in a particular application if the desire is to get the maximum power from the engine, the load should be chosen so that the engine can accelerate to that optimum speed.

If the particular engine has a very narrow range of RPM where its maximum power is delivered, then it becomes very important to adjust the load so the engine runs right at that peak power--any other combination of load and engine speed will result in the engine delivering less power output.

So back to the original question: will a certain change in load (due to propeller diameter change) cause a certain change in engine speed? Predicting this is very hard to calculate, without knowing the new load and the engine power curve. Again, about all that can be said is the engine will likely be able to accelerate to a higher speed under a lighter load. The faster the engine turns the propeller, the greater the load. The engine keeps accelerating until the power to turn the load and the power the engine can provide are equal, and the engine stops accelerating.

[jimh]

The goal of all this is probably to propel the boat faster. Now we are back to the basics of the propeller itself. What will make more thrust: more water being pushed astern (mass) or pushing that water astern faster (acceleration)? The outcome depends on the EFFICIENCY of the propeller at the particular speed it winds up turning. The propeller itself will likely also have a particular rotation speed region where it produces the thrust most efficiently.

If the goal is to get maximum thrust applied to the boat, then the engine power curve and propeller efficiency curve have to be aligned so that the engine runs at a speed where it delivers maximum power and turns the propeller at a speed where the propeller works most efficiently. When those factors are aligned, the thrust will be greatest, and the boat speed should be the fastest.

The GEAR RATIO will have to be considered here. The gear ratio reduces engine speed to a range more suitable for the propeller. What generally occurs in outboard engine design is the outboard engine maker selects a gear ratio that he feels will provide a propeller shaft speed that will produce good results with the typical propellers available.

If a crude survey of outboard engines were made, it would likely find the gear ratio on engines where the maximum horsepower produced occurs at very high engine speeds, say 6,000-RPM, will likely be a higher reduction ratio than used on engines which produced maximum power at lower engine speeds, say 5,500-RPM. The effect is that the propeller shaft speed for these outboard engines will be the same when the engine speed reaches its maximum power range.

[jimh]

On the most crude analysis:

The diameter change is 0.75-inch in 15-inches. This is a reduction of 0.75/15 = 0.05 or 5-percent.

The targeted engine speed increase is 800-RPM in 4800-RPM. This is an increase of 800/4800 = 0.16 or 16-percent.

The question becomes: will decreasing propeller diameter by 5-percent cause engine speed to increase 16-percent?

If the relationship between these values were strictly a linear inverse relationship, then the chances are not good for success. The desired change in engine speed is more than three times greater than the change in propeller diameter.

As mentioned, there is more going on than strictly linear relationships. To get a better notion of how engines can turn propellers, read my article:

Propeller Power Curve
http://continuouswave.com/whaler/reference/propellerPowerCurve.html

[jimh]

A bit further into the general discussion, it should be noted that in most cases a propeller doesn't know much about the engine that is going to be turning it. The propeller doesn't know what speed the engine likes to run, what the gear ratio will be, what the engine power band is like. The propeller just imposes a load on the propeller shaft. That load is translated by the gear ratio and imposed on the engine.

It seems reasonable that a propeller designer will have some notion of what propeller shaft speed is likely to be used with a particular propeller, and from that has an estimate of the speed of advance of the propeller through the water. Knowing these factors will affect the design of the propeller in areas like the total blade surface area, the shape of the blades, the number of blades, the rake angle, the pitch, use of progressive pitch, use of cupping, variations in blade tip designs, and so on. All of those elements of propeller design will affect performance. If the propeller designer anticipates the propeller will be used at speeds of advance of 60-MPH, the propeller design may be different than a propeller designed with anticipation of only having a speed of advance of 25-MPH.

It is interesting to note that there is at least one propeller, the Mercury ENERTIA propeller, that was, ostensibly according to all public accounts, designed specifically to be used with a specific engine, the Mercury VERADO L6 engine. In the VERADO engine it appears that the more RPM the engine can produce the more power it provides, and the ENERTIA propeller is widely noted as allowing the VERADO engine to run several hundred RPM faster than other propellers of similar pitch and diameter. The propeller appears to exploit some aspect of the VERADO in which the faster the engine runs the more power output. This is possibly related to blade flexing, as the material used in the ENERTIA has been cited for the property of being able to flex. Whatever the actual mechanism, the ENERTIA was designed for a specific engine, the VERADO, and it seems to give good outcomes with that engine. When used with other engines, the outcomes do not seem to be as favorable. This tends to support the concept that a propeller designed for a specific engine and that engine's power curve might produce better performance by optimizing its blade efficiency for a certain rotational speed and speed of advance.

It seems a bit unlikely that all of this could be reduced to a simple relationship that could predict with great precision how propeller diameter will affect the speed of any engine trying to turn it . The only certain method to discover how a change in propeller diameter will affect the speed of a particular engine on a particular boat with that particular propeller is to actually try it. Don't be put off by advice to actually try something to find out if the device produces the desired outcome. This is the best method to find the answer.

A further and most significant problem in developing any sort of relationship with propeller diameter as an isolated variable: there are just about NO propellers made where the propeller pitch, blade shape, rake, and so on remain identical and only the blade diameter changes. If we could find a series of propellers of the same pitch, blade shape, rake, etc., but available in an assortment of diameters, we could test them with various engines and see what sort of effect blade diameter produced on engine speed. But without those propellers with all things constant but blade diameter, I don't see a method to develop any data that might lead to a rule of thumb estimation method to predict engine speed change with blade diameter. As I mentioned much earlier, the expectation is engine speed will increase with smaller blade diameter, but to quantify it is going to be hard.

[Phil T]

Two things:

Without saying what brand and model of propeller you are talking about, any results are meaningless. Propeller blade design makes each model unique and sizes are NOT universal, even within a brand.

If someone gave you a recommendation of X manufacturer's model Y in a size of 14 1/4 x 19, you need to try that exact manufacturer, model and size. If you drop down to a 14 1/4 x 17 or go up to a 14 /14 x 21 in the same manufacturer and model, then you can discuss the differences due to change in pitch.

There are several good recommendations in this thread from the archives for your engine.
http://continuouswave.com/ubb/Forum4/HTML/004260.html