Sorry but I already mailed the new prop back. I do have a picture of my old prop on the motor. I attached that picture. The new prop would be hitting right about where the arrow is pointed. The new prop would just barely touch the skeg when I rotated it by hand. I thought about filing down the skeg but fortunately did not have a file handy at the lake. I understand aluminum props flex when they run so I would have had to do a lot of filing. I also thought that a bigger thrust washer behind the prop would have brought it out a bit. But then I am not sure how much room I would have for the cotter pin. So after thinking about all this for 10 minutes I decided to put the old prop back on. Then when I go home I went to the Michigan Wheel website and saw their caution about this prop and my application.
I got up to 4800 rpms now with the old 10x13 prop this weekend, probably because the temperature was in the 50s and 60's. I think I will search for an OMC prop on eBay in the 11 pitch area. A 2 inch drop in pitch should get me in the low 5000 rpms. Do you think I am on the right road?
Will you give me the exact RPM you are turning with how many people are on the boat at that time at what speed and at what elevation those numbers are at. I have 4,600 and 4,800 RPM, but I don't know how many people or the elevation you are running at.
Sure thing. This weekend I was turning 4800 rpm at WOT and got a speed of 22.5 mph measured by GPS. The elevation of the lake is 3,058 feet. Air temperature was 65 degrees F when I got these measurements. It was just me in the boat (150 pounds). I had about 3 gallons of gas and two batteries all in the stern near the transom. The boat weighs about 275 pounds and the motor is about 115 pounds. I do not have electric tilt and I was running in the third pin hole. And I am running an OMC 10x13 inch aluminum prop. The boat is a deep V style runabout (old style closed bow and windshield).
The gear ratio is listed as 13:28 (0.465) which I think is equivalent to 2.15:1. The rating for my motor is 4500-5500 rpm. My OMC repair manual says "20hp at 5000 rpm" and says I should select a prop to get near the mid range with an average load in the boat. The powerhead on this motor is the same as the OMC 30hp and for that motor 30hp is achieved at 5500rpm. So I think this motor can go at a higher rpm. To get 30hp there is a different carburetor and enlarged hole for the intake manifold according to what I have read on iBoats about this.
Ok, I ran the numbers with the data you just posted, and this is what you can expect with a change to an 11" pitch in the OMC prop. Your RPM will rise about 800 or so RPM, and your hole shot and load carrying ability will change and be much better. You prop slip at this time is 18% and hopefully will be better with the change in pitch. Here are my numbers.
Wow! Dropping from a 13 to an 11 inch pitch results in a big jump of 800 rpms and puts me over the max rpms for the motor. Would you think I should go to 12 inch pitch?
No, in my opinion you will be fine and I believe you will be closer to 5,600 RPM. How often do you run the motor wide open, most people don't do that very often, you cruise.
With the numbers hwsiii came up with I think you should give the Rascal in the 11"pitch a close look here at iboats.Historically these props usually give an overall improvement in performance same pitch to same pitch.Unfortunately hwsiii doesn't yet have the specs for the Turning point props so can't run the numbers.
Update on this old post. I bought a Turning Point Rascal prop 10.5X11 to replace my OMC 10x13 prop. I will be going to the lake this weekend and give it a try. I was turning 4800 rpm with the old prop and will see what we get with this new prop.
Not much luck with the new prop. I put on a 10.5x11 Turning Point Rascal prop. I only got 4500 rpm and a very sluggish hole shot. I put my old prop back on which is a 10x13 OMC and I got 4600 rpms with a better hole shot. This was with two people in the boat.
Very confusing to me since I thought for sure I would go up some rpms with the lower pitch prop. I did notice that the Rascal prop has much bigger blade area and looked really big compared to the OMC prop.
P, the Turning Point Rasal and Hustler are about the most aggressive aluminum props made. I very seldom reommend them only beause I do not know the blade geometry, but I do believe they are higher rake props with up on the trailing edge.
If you will buy the OM in the 11" pith you will be very happy.
Question on this old thread. I have an opportunity to buy an OEM stainless steel for a very good price. The previous advice recommended I get an OEM aluminum 10.5x11. Would it be okay to get an OEM stainless steel 10.5x11? Would I get more or less rpm's with a stainless steel prop of the same dimension and make?
It is an OMC SST. On iBoats it is described as this: Evinrude / Johnson / OMC SST 390728 Stainless Steel.
I understand that SST props are cupped. The Turning Point 10.5x11 prop I previously bought was also cupped. It did not do well for my application.
Boating season is over for me now and have until next spring to get a new prop. So I am not in a big rush and can continue looking for an aluminum 10.5x11 if that is the best choice.
My mistake. The prop is actually omc part number 387525. That is supposedly interchangeable with OMC SST 390728 from what I have found in cross reference charts.
P. I think I see the one you are talking about and I would offer him around $ 75 as it is used,and yes that prop will do fine on your boat and raise the RPM quite a bit up above the Hustler you have now.
Update on this very old post. I finally bought an OMC Stainless steel prop. It is 10.5x11. I currently have an aluminum OMC 10X13 and was running about 4800 rpms WOT. The 1989 20hp Johnson motor is rated for 4500-5500 rpms. I hope to give the new prop a run in the middle of November weather permitting.
I do have general question though. This stainless steel prop is really heavy. I weighed it and it was over 4 pounds. I would suspect my aluminum prop would be in the 1 pound range. So my question, is what effect does weight have on a prop? I know they make composite props which must weigh next to nothing. On the one hand, I am thinking this heavy SST prop is really going to slow things down and also I am really going to feel a bigger clunk when I shift. But on the other hand, prop makers must make some type of compensation for the weight of a prop.
I have never seen a post on this issue of weight so I was just curious.
The weight has no affect on performance;just be sure you idle down good before you shift as you should do with any type prop.
The stainless prop grips the water better that could also cause a little more "clunk" as you shift.I'm sorry and a little surprised the Rascal dsidn't work. If there is a good prop shop around they may be able to remove a little cup or pitch and improve performance.Looking forward to results from the stainless.
I wanted to give a final update on this very old post. Yesterday I ran an 10.5x11 OMC Stainless steel prop. I previously had an aluminum OMC 10X13 and was running about 4800 rpms WOT. The 1989 20hp Johnson motor is rated for 4500-5500 rpms. The boat is rated for a 40 hp so I am under powered a bit.
With the new prop I got up to 5300 rpms. Much better hole shot and much better cruising at lower rpms. My top end speed remained the same at 22.5 mph for both props. Overall, I am real happy with what I am running now.
My next test (next year) will be to see how it does with more that 2 people in the boat. Before I could not get up on plane with 3 people.
