Believe I'm over-propped.

[Stamey]

When I bought the boat it had engine issues so I could not figure out anything regarding the prop last season. Now that I have fixed the leaky intake valve and changed the exhaust from one-piece manifolds to brand new two-piece, I'm ready to examine the prop.I also put in new spark plugs and set the timing to factory spec. The former owner told me it would do 43mph when the engine was working right.
This is a 1996 Rinker Flotilla, 24' deck boat. I believe it weighs 3,450, empty, and has a 43G fuel tank. It has a 4.3LX engine, that is 180hp, from what I understand. Engine SN: 0F821477

My current prop is a Quicksilver 48 16316 A5 19P, stainless. (14.5" x 19P) I put this prop on because the original that came on the boat had a few nicks in it where this one had none, and it was a leftover from my last boat. The original prop that was on it when I bought it was a Michigan Match, PN: 033004, 14" x 19P.

My load yesterday was about 450 lbs plus a full tank of gas. Best I could get out of it, with the trim just below blow out was 32.3 mph, per GPS, at 3920RPM. I'm looking for top speed and fuel mileage, as I hope to take some long river trips (70-100 miles one way). I may pull a float here and there, but doubt I will ever be trying to get a skier out of the water quickly.

I'm thinking I need to be in the range of 4400-4800, for RPM at WOT, which suggests that I may need a 17P or 16P. The Mercruiser prop selector was suggesting 17P 3 blade or 18P 4 blade, when I put in my boat info, including weight. I do not absolutely know what the red line for this engine is, I am only guessing from other posts I have read here.

Looking for ideas as to what prop to try next. I am open to 3 or 4 blade but want to stay with stainless if possible.

Thanks,
Chris

 

[SkiDad]

Re: Believe I'm over-propped.

If you think the engine is running right I would say 17 pitch 3 blade. That is a lot of weight or that engine. Also are you trimming the engine up after take off ? I'm not sure if you can hit 43 with that boat

 

[jestor68]

Re: Believe I'm over-propped.

Your 1996 4.3LX is 190 HP with a 4400-4800 rpm range. It should have a 1.81 gear ratio.

A 17 pitch will only get you to the bottom of the rpm range(about 4400 rpm).

I suggest a Solas Amita 15-4/5 X 15 prop to get you to 4500-4600 rpm (where the motor develops it's full HP) and provide the blade area to more efficiently push that boat. That should reduce your slip factor to a more reasonable 11-12 percent(now it's showing about 18%).

 

[Stamey]

Re: Believe I'm over-propped.

The engine runs nice, but I cannot say for sure there is no performance problem. I will verify the timing. I set it in November when I put the heads back on it and believe I used the correct procedure, but I will go through it again to make sure before I buy a prop.

I had it trimmed to the point of any higher and it would have blown out. I was doing fine tuning of the trim as I watched the RPM and GPS. I then took pics of the dash with all gauges and GPS in frame, so I would have a snapshot to look at, depending on what you guys asked about.
You are right, the former owner could have lied about the top speed. It may not be able to do 43mph with a 4.3L.

Thanks,
Chris

 

[Stamey]

Re: Believe I'm over-propped.

Ignition timing verified. It was a couple of degrees short. Made no difference in boat performance with the 19P prop.

I managed to get a hold of a Ballistic 14.5" x 17P prop and it brought my RPM up to just under 4100, with speeds as high as 35.4 mph, per GPS, trimmed all the way up. Had same weight in boat as previous tests. Appears I am going in the right direction, but if I look strictly at the numbers, as compared to the 19P, it appears that to get myself into the correct RPM range, I will need to drop to a 13P prop, which should get me to about 4500 rpm, with GF, cooler and full tank of gas. I wonder if I should go further, to get to 4600 or 4700, with a 12P or 11P?
Does this sound right to you guys?

Thanks,
Chris

 

[jestor68]

Re: Believe I'm over-propped.

The reason you didn't realize the anticipated rpm change is most likely because you changed from the plain jane Mercury (Vengeance?) SS prop to the more aggressive Ballistic prop.
Any time you change material or prop designs, the results can be problematic.

 

[Stamey]

Re: Believe I'm over-propped.

I am limited by what's available to borrow to try out. I hate to spend hundreds of dollars on props just to find out they aren't the best for my boat. This 17P came to me by way of a trade for my old 19P so I couldn't beat the deal. I did find a 15P to try out, but I quickly found out that the prop, while the same shaft diameter and splines, had too tall a hub to fit a Mercruiser, so I'm still on the hunt..
Thanks for the info.

Thanks,
Chris

 

[Stamey]

Re: Believe I'm over-propped.

OK, so I found a Michigan, PN 033002, SS 14.5x15P prop. Put that on and got surprised. Since the 17P Ballistic I had on there only got me to just under 4100 RPM, I figured the 15P would get me to around 4300 RPM. I could easily hit 5100 RPM with this 15P prop. I was surprised by the jump in RPM. I don't think I ran it full throttle because I was scared of over-revving and damaging something. I only got 33 mph out of it with the 15P. I am thinking of the next step being a 16P 4 blade aluminum, as it's cheaper, to allow me to test that config out, before I get a stainless version.

Thanks,
Chris