Prop size

Joker2 · #1 07-17-2011, 11:59 AM

I have a 75' V20 1995 150 Ocean Pro ( rebuilt completely in 2010 ) The boat has a 15/12 X 15 Prop on it. After breakin it in last season, able to open it up this year Ifound out I was only getting 4500 RPM's, had the Prop reconditioned for more RPM's, not sure why but now when idling along or going below 2000 RPM engine vibrates I even feel it at the wheel when I bring it up I can't notice anything, I might be crazy but could the prop be out of balance or something like that. Also I have my doubts about the mechanic who sent out the prop, it came back painted but they just went right over all the old peeling paint, no sanding or anything, thought that was lame. Makes me wonder what kind of job they did on the prop. The prop guys are a major prop house in this area. Is there any way to find out what I need to know without using a known good prop?

Joker2 · #2 07-17-2011, 12:10 PM

From what I read on this site is 15-1/2 X 15 the wrong size should I have a 17 ?

bradford · #3 07-17-2011, 01:46 PM

Most folks run a 15 x 17 or 14 1/2 x 19 prop. MJ used to run a 15 x 15 to get added torque for carrying heavy loads.

You should be getting more RPM's with the 15 x 15. Are you sure she's firing on all cylinders? Makes me think a cylinder or two are not with the low rpms and the low speed vibration.

Joker2 · #4 07-17-2011, 01:59 PM

I can check that out, she's not stalling though.

tsubaki · #5 07-17-2011, 03:41 PM

Good call bradford on the low rpm's and vibration, could likely be a bad plug or two.
And I agree, a 17" stainless or a 15" aluminum should be about right.
Joker, did they rehub the prop (if applicable) too and restamp the numbers?

Joker2 · #6 07-17-2011, 06:34 PM

not sure about the rehub, but I did not see any stamping. I almost think the #*$%hole mechanic just polished up the blades and charged me for it, because on the main housing the paint was peeling from when he originally gave me the boat last year ( he resprayed a used prop ) Then when I brought him the boat recently to replace my fried Ignition system do to a direct short caused by the metal clip that holds the big red plug together, I asked him to send out my prop to be reconditioned for more RPM's. When I looked at it all the blades were nice but the main housing had all the old chipping paint still there underneath new paint. I never had it done before but for 82.00 you think they would wheel it out and respray. maybe it's me.

Joker2 · #7 07-17-2011, 07:04 PM

How does the diameter work, can I go from a 15 to 14, not sure what changing the diameter does. I never had to mess with props before, trying to learn. Thanks

tsubaki · #8 07-17-2011, 07:45 PM

Normally when they repitch a prop, they heat and bend the blades causing the need to rehub. Actually, if I'm not mistaken they have to remove the hub to fit their presses.
Lets say you repitch from a 15 to a 17, you will lose diameter. If you repitch from say a 17 to a 15 you may need grinding to fit beneath the cavitation plate or miss the steering torque.
When buying new, there are numerous changes that can be made toward performance with subtle changes of the prop, this is where the pro's come into place.
Depending on the circumstance, keeping a certain pitch but changing the diameter or even the blade design cup or lack there of can make a huge difference in performance. The same can be said for keeping a certain diameter and changing the pitch, having a cupped or regular blade.
The diameter basically dictates how much water the prop grabs BUT there is almost always a relationship with the pitch.
What I do is find a normal manufacturer of a prop (Michigan Wheel preferred) and work within the perimeters of their props. Easier to replace with the same if damaged.
Lets say I've got an engine not turning up enough rpms and have verified the engine is running correctly and at the correct height. I'd decrease the pitch a couple of numbers, test and go from there. You will see the average manufacturer does not carry the same diameter while also changing pitches.
Now, problematically, different prop materials will change your prop designations. Stainless, aluminum or composite materials perform differently even if you have the same exact pitch, diameter and blade design between the three.
Sorry this is not a good definition but about the best I can muster.

Joker2 · #9 07-17-2011, 08:08 PM

It's helpfull, thanx

Joker2 · #10 07-18-2011, 06:42 PM

I checked all the cylinders pulling the wires one at a time didn't really notice the Tach move, but I did hear a drop in the engine with each one. Also on the lower Starboard plug there was a little build up of black. This happened once before when I was running double oil on break in, just thought it was the extra oil fouling the plug, should I be conceerned?