Choosing Diameter

[tboltmike]

What is the relation between pitch and Diameter? What is the criteria for selecting diameter? Seems like for a given engine there are ranges of diameters and pitches. Does going up in diameter for a given pitch have the same affect as increasing pitch? Does speed and economy enter the picture for a given rpm?

 

[tpenfield]

Re: Choosing Diameter

You will get an array of responses . . . but . . . in a propeller, pitch and diameter are related by the volume of water that (theoretically) gets pushed per revolution of the propeller. (makes sense?)

There are 2 ways of changing the amount of water that gets pushed by the propeller per revolution . . . change the pitch . . or . . . change the diameter.

You can compare props from a theoretical standpoint by calculating and comparing how much water they want to push per revolution. Other factors about different props designs have an effect as well and those are bettered measured through trail rather than calculation.

my 2 cents

 

[Frank Acampora]

Re: Choosing Diameter

First understand how a propeller works in order to compare diameter and pitch: A boat propeller in cross section is shaped similar to an airplane wing. It operates by "screwing" itself through the water. Negative pressure on the back of the blade and positive pressure on the blade face "pull" it through the water. The amount of water thrust behind the boat is actually the "slip" of a prop. If a given prop were 100% efficient (an impossibility) there would be no slipstream behind the boat. If you tie your boat to the dock and gun the engine in gear, you will get 100% slip and zero efficiency with a large slipstream and no boat movement.

Now: The combination of blade area and pitch will cause a propeller to absorb a specific amount of horsepower. Thus, for a given horsepower range, USUALLY as blade area goes up due to increasing diameter, pitch is reduced. Blade area is increased (again usually) by increasing the diameter of the propeller. Larger or heavier hulls require a greater diameter to increase the amount of pulling power the prop can deliver and stay witin a reasonable slip ratio. (you wouldn't try to move an ocean liner with a 13 inch diameter prop) BUT since this will also increase the amount of horsepower needed to move the boat, the pitch must be reduced to compensate. Thus, for a given engine, the larger or heavier hull it is expected to move, the larger diameter and lower pitch prop it will require.

 

[steelespike]

Re: Choosing Diameter

If the prop is intended for your motor the diameter will follow.
Extremes in diameter toward larger sizes would usually be associated with heavier or more difficult to push boats ie: Pontoons.
If you check diameters you will note that generally as pitch goes up diameter goes down
also adding a blade will usually reduce diameter.
If if your dealing with a boat of a size style and weight of any other typical family boat prop selection is relatively simple if your dealing with a specific problem or activity then selection becomes more difficult.
If you read "read this first" at the top of the page and get us as much detail as possible we can help you tell if your setup is working and if you need a change.

 

[Bondo]

Re: Choosing Diameter

What is the relation between pitch and Diameter? What is the criteria for selecting diameter? Seems like for a given engine there are ranges of diameters and pitches. Does going up in diameter for a given pitch have the same affect as increasing pitch? Does speed and economy enter the picture for a given rpm?

Ayuh,... Ted, 'n Frank have given the explanation of form, 'n function,...

For the practical application of that info is,...
Ya buy a prop for a specific motor, by Pitch...
The Engineer that designed the prop has already covered the diameter for ya...
The blade design, 'n it's diameter are built into the pitch factor...
Generally speakin', as pitch goes Up, diameter goes down...

 

[Silvertip]

Re: Choosing Diameter

If you look at prop calculators there is no entry for "diameter". Calculators consider engine rpm, pitch, speed, slip and gear ratio. You need four of the five values to come up with the fifth. That is why the "Read This First" sticky at the top of the forum asks for this information. When making a prop swap you know what engine you have and any props designed for that engine already have diameter built into them. You cannot install a 16 inch prop on an engine that was not designed to handle that large a diameter due to interference problems. You know what the engine manufacturers recommended wide open throttle rpm is and you know what the gear ratio is. Plug those numbers into the calcualtor and it will give you a realistic (probably not an exact) prop pitch but at least it is a good starting point. You can determine slip (propeller efficiency) or any other factor if you know the other four factors. Use 10 - 12% for the slip number until you've made some test runs and can actually calculate this factor.

http://www.rbbi.com/folders/prop/propcalc.htm

If you look at the iBoats propeller application listings for a given outboard, you will not see a wide range of propeller diameters. You will likely see about a 1-1/2 inch diameter range for that engine. The largest diameter will be the lowest pitch listed and the smallest diameter will have the largest pitch. So you cannot/should not search for props by "diameter". You shop by engine make, year, and HP (that sets the diameter range automatically). You then select the pitch you need and that sets the diameter automatically. To confuse the issue even more, two 15P props from two different manufacturers will very likely have slightly differt diameters. It is due to design. Likewise, two 15P props from the same manufacturer but from two different series will likely have slightly different diameters. And to throw one more variable into the mix, two 15P props will likely perform differently regardless who manufactured them. This is why prop selection is called a "crap shoot." Trial and error after establishing a baseline with an existing prop is the only way to be sure a prop will optimize performance for your boat.

 

[Texasmark]

Re: Choosing Diameter

I too have noticed what the guys said on here about diameter reducing as pitch increases. Looking at the prop blades, especially on high rake blades, it seems like there is pretty much the same blade area within a certain range of pitches and the blade seems to be just laid over hence reducing the diameter.

Again, as the guys said, once you get in the general area of diameter for a given gearbox dimension, like on mid size and larger Mercs they have a small dia gearbox (4 1/4") and a larger (4 1/2") more or less, forgot the exact dimensions, but anyway, the smaller box runs 13+" dia props and the larger 14+" dia props, again the exact dia is determined by the designer for the pitch and prop type.

Now you have my "expert" opinion. Ha.

Mark

 

[tboltmike]

Re: Choosing Diameter

Wow, lots of good information, Thanks Gentlemen

I don't think this is covered in the selection guide.

For a given boat, planing hull, and a given. If economy was the goal, would a larger finer pitch prop give the desired results vs a smaller more course pitch? My thought is the larger dia would have less tip loss.

 

[Bondo]

Re: Choosing Diameter

Wow, lots of good information, Thanks Gentlemen

I don't think this is covered in the selection guide.

For a given boat, planing hull, and a given. If economy was the goal, would a larger finer pitch prop give the desired results vs a smaller more course pitch? My thought is the larger dia would have less tip loss.

Ayuh,... You'll have to be Much more specific than that for a quality answer...

Btw,... Motors are propped to run in their rated rpm window...
Over propping causes a lugging condition, that'll burn holes in yer pistons,...
rather than increasing yer fuel economy....

 

[fishrdan]

Re: Choosing Diameter

To get the best ecomony, select a prop pitch that allows the engine to turn at it's WOT RPM with a normal load in the boat. You might turn more RPM at cruise than a higher pitched prop, but the engine will be more efficient.

I agree with the others that say large diameter props are for heavy loads, pontoon, large cruiser,,, or as a high thrust prop for kicker motors . I found some low priced large diameter props for my Mercruiser I/O and they never worked well since I have a relatively light hull, 2000#. I was using a 15.5x19P and a 15.75x17P and they were slow to plane and limited upper RPM's. About the only thing they did well was hold plane at lower RPM's. I'm running a 14.5x18P 4 blade now and it outperforms both of those large diameter props by a huge margin.

 

[steelespike]

Re: Choosing Diameter

As far as economy keep this in mind.A marine motor needs to rev freely within its rpm range.
A marine engine is working like its going up hill all the time so to high a pitch is like putting it in over drive and straining it whether at cruising speed or wot.
Marine motors fuel use is rated in gallons per hour(gph). A motor will use about 10% of its hp in fuel at wide open throttle(wot) ie: a 100 hp will use about 10 gph at wot.
The boat needs to be setup for the best speed at a given throttle setting while keeping within wot rpm rating.It is a fine balance between speed and rpm. Rpm too low will produce
premature motor problems and poor performance. The savings between a good setup and a perfect setup would require a flow meter to tell the difference.
Higher pitch might produce more speed at lower rpm but at an advanced throttle setting.
also would produce slower hole shot and delayed top end using more fuel.Again unless in an extreme a flow meter would likely be necessary to tell the difference.
The rpm rating for your motor refers to wot.Its fine for you the run a correctly propped motor at whatever rpm you prefer.
If you tell us about your setup we may be able to suggest a prop.