Quote:
Originally Posted by Spruill242
. . . If I decided to go the cheap aluminum route to find the right prop, will a Stainless prop react the same when I find a pitch and diameter that works for me?
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That's a good approach and is what I did when I first started testing props, except that I screwed up by not testing at maximum load! I started out with a Michigan Vortex 4B 14x20P prop; the corresponding stainless prop was the Michigan Apollo, but they added 1/8" to the diameter to compensate for the thinner SS blades! (This comparison is only valid for the same style prop . . . same manufacturer, blade shape, rake, etc.!) The 14.125x20P SS Apollo initially looked like a killer prop, giving me about 5.0 mpg at cruise and almost 50 mph @WOT with a real light load at ~5200 rpm. However I then found out that the
optimum WOT RPM for my motor was 5300-5500, and one day when I had 6 divers on the boat I discovered it would only turn about 4600 rpm and took forever to get on plane, so I was seriously over-propped! I ended up running a 15x15 4B PowerTech prop that will turn 5400-5450 pretty much regardless of load, and the boat jumps on plane easily with a heavy load at less than half throttle! It also has great stern lift and will hang on plane easily at about 12 mph. 3 blade props are typically faster and more efficient, but I need the extra stern lift of a 4B because my boat is so stern heavy with the bracket and relatively heavy motor (although it's about 50 lbs lighter than equivalent 4-strokes.
The blades on an aluminum prop have to be thicker to minimize deflection because aluminum's elastic modulus (stiffness) is only about 1/3 as stiff as SS. In other words an aluminum part will deflect 3X as much as a dimensionally identical steel part with exactly the same load on it! A SS prop with a given pitch and diameter will typically spin a little faster than it's aluminum twin because of the difference in blade thickness.
An aluminum prop is easy to modify if it needs to be tweaked to really dial it in. A good shop can repitch it to a lower pitch if it doesn't turn enough rpm or add some cup or more pitch if it over revs. That way you can get it dialed in just right before spending big $ for a SS version. (Make sure you test with at least as much load as you normally intend to carry. I always test with full gas tank, full live well and galley seat water jug, plus 65 and 70 qt coolers filled with sea water for ballast, because I run pretty heavy when diving or cruising.) I would also pay attention to min planing speed, because there are times when running offshore, that it's nice to be able to slow down but hang on plane to keep from beating up your passengers. (This is much more important on a much lighter 20, which tends to start going airborne at about 20 kts when seas get over 3'!)