Quote:
Originally Posted by 2Hulls
End of good advice, IMHO. And this contradicts the good points made above. If you've consulted with other owners and prop makers, you'll arrive at the correct diameter and pitch and won't have any need for variable pitch props, which are fancier and more expensive.
Dave
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The choice of a prop isn't quite as simple as it seems at first glance:
Firstly, no two engines, even ones of the same; make, model, year, & production run (even with only 1 digit of difference, seriel # wise) are the same. They can, & do, at times have more than enough difference in terms of HP & Torque output at various RPM's so as to make getting them perfectly matched with a
propeller, a thing requiring a bit of fine tuning.
For example, you can pull two Chevy's off of the lot or production line, which meet the criteria state above. And they can have several percent of HP difference (sometimes in the double digit range, percentage wise). And that's in engines which have built in computer control modules. As well as several other features to optimize their engine's output.
As opposed to, say, a
Marine Diesel, having mechanical
fuel injection. And also with no/to minimal built in systems for optimizing the various aspects of timing, for example. Or to "re-tune" the engine (many times per second) for things based on; the sensed octane of the
fuel, the fuel's temp., ambient air temp., measured air density, engine temp., engine load, etc., etc.
Not all diesels are so low tech as to not have
some of these features, but... there are plenty which have few to none of them. At least in (relatively) low priced yacht engines.
The reasons that one (Chevy) Mustang may be 3-5mph faster than another, or be 0.5 sec quicker in the quarter mile. Both of which constitute a Lot of HP & Torque, BTW. Are due to many of the above stated factors (& more).
For example, one could have more aerodynamic drag, or in a boat's case, hydrodynamic, than the other. A common thing. For which I could list 2 dozen+ factors.
And too, said vessel could have more significantly more or less aerodynamic drag than it's average sisterships; on which the prop selection model is based.
Bottom line. These types of things all add up to make 2 different vessels of the same type, differ when it comes to what kind of loads they put on their engines while powering through the water at different speeds. So then, being able to fine tune one's prop is a grand perk. Adding to efficiency, plus, in the long run, engine life as well.
Then, for instance, a common external influencing factor is this. In some locales the seas will vary greatly in; frequency, size, & shape, etc. as compared to other locations. Which again, would add up to wanting a differntly pitched prop than that provided by a generic recommendation. And such factors might be seasonal, or based on the environmentals of a specific voyage.
Yes, one can have a fixed bladed prop re-pitched, but, there's a reason that whole
classes & designs of vessels have controllable, variable pitch propellers.
Meaning that, such vessels even able to alter their propeller's pitch while underway. In order to accomodate some of the above mentioned factors, & thus optimize their engine's transfer of power into the water. IE: Get the most possible speed, for the least amount of fuel burned.
Such as, for instance, when making a long, primarily uphill voyage, most of it under power, or
motor sailed. Like the infamous
Baja Bash, or when going from
Hawaii back to the lower mainland.
Me, I'd love to have such a
propeller, especially for such trips... if my Visa card gave it the nod.
Thus my comments about it being nice to have the option to tune one's prop to the boat.
My apoligies for the dissertation. I'm simply attempting to clarify the why behind my statement(s).
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