2011 150 hp Mercury Optimax/2005 Azure Prop Puzzle

[AZ210]

I am not sure which way to go and wanted to get some suggestions. So I recently repowered with new out of the box leftover 2011 150 hp Mercury Optimax. I love the engine but not so much the prop. My issue is somewhat complicated by the fact that all of the below numbers may be somewhat skewed because they are all with a light load. It is the end of our boating season, and I am just trying to get this figured out before I put the boat away for the winter. The kids are all away at school, so it is just me and sometimes the wife, and there is not much gas left in the tank. Between the weight of additional gas and additional people, figure the usual load will be about 500 to 1000 pounds more, and on those rare occasions when we get 8 people in the boat, there may be 1200 to 1300 pounds more in the boat. So, going through the required info:

1. The problem with the current prop is that I can only trim up a tiny bit before it blows out. And before I make any turns, I have to trim all the way down or else it'll blow out, and even trimmed all the way down, it'll blow out on a sharp turn.
2. The shop rigged it with a 15 X 17 pitch Mercury Black Max aluminum prop.
3. WOT RPM and speed were actually fine, about 5550 RPM by my tach and 40 mph by the speedo (manual says top RPM should be 5250 to 5750).
4. As stated above, the engine is a 2011 150 hp Mercury Optimax XL (25" shaft). Engine is all the way down (top hole). I took it out of the water this past weekend to look at the mounting height, and the cavitation plate is pretty even with the bottom of the boat. So from what I've read, that is about right.
4. The boat is a 2005 Azure AZ210. It is 20'6" long. That company has gone through many permutations, but if you want to get some idea of the hull, the 2014 version of the boat is a Southwind 200 SD Deck boat. The boat is 2500 pounds dry. The engine is about 437 pounds. As stated above, you can probably assume that all of these performance numbers are with an additional, say 350 to 400 pounds, so assume all these perfor numbers are with about 3300 pounds total, and I want to prop the boat for probably 3800 to 4000 pounds.

I went on to the Mercury prop selector site and it gave me a range of alternative choices. So I went with a Mercury Spitfire 4 bladed 14 X 17 pitch aluminum prop. My thinking was that the 4 blade would be much better at holding the boat up when trimmed. I realized that I may lose a little speed on the top end, which was fine. But I also figured that if I was able to trim up with this prop and get the boat in a better position, then maybe I wouldn't lose any speed at the top end.

I was very surprised at the performance with the Spitfire.
1. As I suspected, I was able to trim up very well with the Spitfire. It trimmed up nicely and I was able to take most turns even while trimmed up. Now it was just me in the boat with very little gas, but omg, the hole shot was out of this world. As soon as I hit the throttle I was up on plane and doing 20 mph. But it was all downhill from there.
2. WOT RPM was a totally unacceptable 6200 RPM. Top speed was also a ridiculous 34 mph. I wouldn't mind losing a few mph, but between the WOT RPM and the top speed, obviously I'm not going in the right direction.

I'm thinking maybe I'll keep this 14 X 17 spitfire prop for when I have 8 - 9 people in the boat, a full tank of gas and need to pull up a skiier. But even with another 500 pounds, I don't think this is my everyday prop.

So where do I go from here? Maybe it is as simple as moving up to the 14 X 19 pitch Spitfire. I was also looking at a 15.5 X 17 SS Mercury Mirage Plus prop. With my previous engine, I've always had satisfactory performance with the aluminum props, but if I need to go stainless, I could do that. I'll probably have one more day with the boat this fall, so I may not get this resolved this fall, and may need to continue playing around with it next year. But in the meantime, any suggestions are appreciated. Sorry for the long post.

 

[Chris1956]

Something is not right. A four blade prop of the same pitch as a 3 blade prop would lose WOT RPM, not gain it as you report. Maybe you are ventilating?

 

[steelespike]

I've seen 2 supposedly independent tests of the Spitfire. In both the prop was same pitch as the original 3 blade both improved hole shot And retained rpm and speed of the original. Your original prop has about 15% slip, high normal, usually considered to need improvement.
Your Spitire is at a very high 34%.Speedometers are always suspect. You may want to get some GPS speeds with both props.
​ Also take into consideration any currents or tides that may affect your speed.
It does appear that the Spitfire is wrong for your application.Though you may want to confirm the actual size.
Not unusual to be miss labeled or packaged in correctly.

 

[AZ210]

I agree, as I was typing the numbers, they just weren't making sense to me. My understanding is that Mercury makes the Spitfire with a lesser diameter (14 for Spitfire v. 15 for the Black Max) and that is supposed to make equivalent pitch 3 and 4 blade propellers do roughly the same RPM's. But the rest doesn't make sense. Let me do over again and I'll report back.

I don't think I was ventilating and I was pretty sensitive to that with the previous Black Max propeller. But I'll trim all the way down and take it from there.

And I will definitely look at the model number on the propeller. I didn't even think of that. I remember looking on the box and confirming that it said 17 pitch. And it was a new 4 blade propeller. But I didn't even think to match the model and pitch number on the prop itself.