propellerator theory

[tomcat rio]

pretty sure pitch is the number of inches a prop should move the boat through the water in one revolution, assuming no slip.
slip may be the result of many things(cavitation,improper hull clearance, etc.)
..i think current albin 28's use a 19x18 prop with the yanmar 315. not sure what the transmixer ratio is.
...
i have an 18x26 nibral with a 2.00:1 ratio.
..
anyway, if pitch is what i think it is, what effect does diameter have on the equasion?
for one thing, i guess a higher diameter means more mass and more surface area, so maybe less slip.
would like some propulsion guru's input.
if you are not a propulsion guru, your input will also be appreciated.
even some inputs on gear ratio's, too.

[tomcat rio]

. i'm thinking that 3 blades MAY have a slight advantage because the water over each blade may be less disturbed.
when i ran my old 18x19 prior te getting the new 4 blade 18x26, the speeds were about 3 kts higher for a given rpm, but the fuel consumption was also higher by maybe 3 gph. go figger.

[tomcat rio]

..a 19" prop on their 28 TE?
prettty sure 19" would be max.

[Mariner]

Propeller dimensions are expressed by two numbers. First is always the diameter, and second is always the pitch. Diameter should be self explanatory. Pitch is a number which expresses the THEORETICAL distance a propeller will move through the water with each revolution. Keep in mind that ALL propellers slip. It merely represents the angle and shape of the blades. Think of the pitch of being kind of like gears in a car. The higher the pitch, the higher the gear. Just like a car, you have advantages and disadvantages to both higher and lower gearing. A low pitch prop will accellerate the boat quickly, push more weight, but achieve a lower top speed. A higher pitch prop will increase speed (and therefore fuel economy), but if it is too high, it will put excessive load on the engine, and may not allow it to reach it's optimal operating speed. A boat which cannot reach it's maximum RPM's is considered to be "over-propped", as where one that easily exceeds the maximum RPM's of the engine and does not achieve the most efficient speed for the boat, is considered "under-propped". Genearally speaking, to find the perfect prop, you want to find one that with the throttle wide open, you end up right at the rated maximum continuous RPM's for the engine. In the case of a boat like an Albin, I would generally assume that Albin Marine has already found the optimal pitch and diameter prop for the boat, though if you find that your use paterns are different than the designer expected, you may be able to toy with it.

Determining the appropriate diameter can be a little more tricky. A larger propeller, will definitely slip less (meaning it will be closer to it's theoretical pitch in actual performance). But because it is heavy, it can reduce the efficiency of the entire rig. This is where experimentation plays a big role. In outboard and I/O boats, the only limitation for diameter is usually the distance between the propeller shaft and the anti-cavitation plate. For inboard boats with large bronze propellers, it is a bit more complicated.