Re: Going from Aluminum to SS
I don't believe the OEM prop is cupped, it doesn't look like it.
Leaning toward this:
Solas New Saturn Stainless Steel Boat Propeller #3331-111-13 for Yamaha 40 HP Outboard 1984 & Newer Includes 40C - iboats
3x11-1/8 13 pitch
Thoughts? Almost an inch more diameter, I don't believe it is cupped either. Should I expect it to turn about the same RPMs?
On cupped or not, maybe the smaller diameter Solas aren't cupped. Most larger hp SS props are cupped and the reason is holding power in high trim positions to reduce ventilation in rough water conditions and tight turns at high engine rpms. Why don't you ask Solas.
For the record. I was fooling around one day and decided to buy one of the Turning Point Hustler alum props because they were advertised to be everything a SS was with the exception of the thin blades....which does matter. I wanted a 23P prop but could not get better than a 21 in my 4 1/4" lower unit size (90 hp), but one was available for the larger 4 3/4" LU diameter which is used with the big V6 engines.
I checked clearances and ordered the prop. I had to add a shim to get the prop to match up to my LU as the prop outer tube (thru prop exhaust) and the LU were the same diameter. Regardless, I rigged it and ran it and the data are posted below:
Original prop: SS Ballistic 13 1/2 x 24 XL
New prop: Hustler Alum 14 1/2 x 23
Both props cupped, high rake (bow lifting), and the Hustler was ported, Ballistic wasn't at the time.
I was alone in the boat and trying to drive, read my instruments and record data all at the same time so the following data are the best that I could do:
---------------------Ballistic------------------------Hustler
------------Calculated---RPM---MPH------CS----RPM--MPH
------------Slip %
On Plane-----------------3k-----18--------------2.8k---20
20 mph--------35--------3.1k--------------------2.8k -
25--------------26--------3.4k--------------11---3k -
30--------------21--------3.8k--------------09---3.5k -
35--------------17--------4.3k--------------08---4k -
40--------------12--------4.7k--------------07---4.5k -
45--------------11--------5.1k--------------06---------Hustler topped out at 5k rpm and 44 mph..engine ran out of whoopie with it.
48--------------------------------------------------------Ballistic topped out at 48.5 (Explorist 200 gps, 50 speedometer) at 5.6k rpms.
The larger diameter prop with less pitch but thicker blades gave me a very smooth hole shot (could not detect any assistance with ports in this prop), very smooth 2k to 3k rpm operating range like the rpm range where you would cruise around on a Sunday afternoon, and I really liked it. It just lugged the engine as 5k is the lower limit on my operating rpm range and I will not run my engines down there. Other thing was the alum held tight in tight turns and the SS ventilated somewhat, but these were high speed very tight turns. The added diameter probably did that.
So all things considered, I dropped 1" in pitch but lost some with the alum blade thickness, doubt the tube diameter had any impact on anything as it was the same diameter as my LU and was not a drag, but I increased a full 1" in diameter.
Numbers are 7.5% dia incr. 12% rpm decr. 10% mph decr.
If I were to choose an alum prop for this engine it would be the Hustler but I would drop down to the tube size for my engine which would reduce the OD to 13+ and I would drop the pitch to 21. This should put me up around 5400 at WOT which is fine with my 5500 suggested upper limit and should provide for a nice ride. But that's not going to happen because I have 4 props for this engine and that's enough.
HTH,
Mark
