2.5 Mercury 175 on Champion 184 Prop

[DieselDood]

Hey guys

I have a 1985 184.

Over the winter I rebuilt a 2.5 Mercury, 175hp. (30 over Weisco pistons, CCMS Reeds and stuffers)

It had a 150 OMC on it, so I have a 24 Raker that is on the new Merc motor now.

Right now I get excessive prop slip (happened with the OMC too) no matter what Prop I run. The OMC had the lowest slip numbers with a 23P 4 blade (ballistic), but I don't have a suitable 4 blade to try anymore. I thought I did, I have a 25 4 blade Renegade bass, but the hub barrel size is to small, so I get CRAZY slip on the Merc (45%) due to the exhaust going over the hub. (gets out of the hole nice, then I see like 45mph max and if I jump on the throttle it cavitates)


Current Setup: Prop to pad... I'm not sure, I had the motor too high, so I was loosing water pressure at WOT, so I dropped it 1/2". 8" manual Slidemaster Jack Plate. I mostly run 2 people, full fuel, full livewell. I fish tourneys, so I'm running 5 batteries (36v Trolling, 2 system due to 4 graphs and livescope), I removed the front fuel tank and built a battery tray for the additional batteries. So it's a little give and take for weight. If it had both tanks, vs the additional 3 batteries, I think it's a pretty even trade off.

With the 24P 3 Blade Raker I see 6200 RPM at 53MPH fully loaded. I calculate that to be about 28% slip. (OUCH!) I also see the same Speed and RPM with the livewell Empty and one person with half a tank of fuel.
I ran a 23P once and I had to back out of it because the RPM was headed WAY into the red. (Motor is Balanced and Blueprinted, but I don't want to spend much time over 6500, it makes me nervous...yes I know the racers run 9k... still not for me), but I saw 55mph at 6500 RPM, which is also 27%.


So my question is this... Should I try and find a larger diameter 25P 3 blade to borrow and try? Or is a 4 blade going to be better?
What are your thoughts on a Solas Scorpion on these rigs? Has anyone run one? I didn't see one in the database, but there isn't a way to search it either.

Thanks!

[pmgoffjr]

25M Yamaha pro series is the exact right prop.

[DieselDood]

Thanks Pat!

Any idea why I get so much slip?

[pmgoffjr]

Usual reasons.
Motor too high for that particular prop.
Something like a transducer is blocking water flow to prop.

[DieselDood]

Well... 25p Trophy and it's the same as a 25p Shooter

55mph at 6000rpm. 27% slip

The trophy does get out of the hole better.

Loosing almost 30% is brutal.

On pad neither of the Ducers I have are in the water.

I'm going to try lowering the motor another 1" and see what it does.... but so far, I'm not finding what my causes are.

[DieselDood]

Well I reset the motor on the jack plate and dropped it down farther. I have to remeasure, but I don't think I can get much lower. The bottom of the lower cowl is in the water when I'm trimmed down and not on plane. I started at 3" Prop to Pad, and have gone down 1/2" and ran it, over and over. The best I have gotten was 57mph at 6100rpm.

I checked the ducers, they are well to the side and above the bottom of the hull.

Next chance I get I'm going to put the ole boat on the scale just to see what it weighs.

Any guesses with an EZ-loader trailer with folding tongue weighs? 1000lbs?

[DieselDood]

Oh... and I forgot to mention... for fun I tried a friends 27P Tempest. I didn't think I'd be able to get on plane, but it did get up... 55mph again but at 5500rpm. Which makes sense, go up 2 prop sizes..250rpm per step up, everything adds up.

But that takes the prop and prop hub size out of the picture.

[Almag6]

verify your tach is reading correctly

[pmgoffjr]

27 tempest turning 5,500 should be all over 70.
You need someone with a punched clue ticket to look at what you’ve got going on.

[DieselDood]

27 tempest turning 5,500 should be all over 70.
You need someone with a punched clue ticket to look at what you’ve got going on.

That's what I'm trying to do here.... So far other than prop to pad and possible obstructions I haven't gotten any other answers as to what could be causing this.

I've changed prop to pad from loosing water pressure to 5" below Prop to Pad... going by 1/2" increments with very little change.
I've tried props with the appropriate hub diameter, and props with the smaller hub. Both have the high slip. I'm currently running the 25P Trophy Plus (57mph @ 6100rpm) which has the smaller hub size. The 27 Tempest (55mph @5500rpm) I tried, had the proper hub size to match the lower unit, and it obviously still had that slip.

[pmgoffjr]

Yeah, something is amiss.
Drastic changes should have drastic results. And you're not getting that.
Internet diagnostics is limited because we can't see it, don't know what's the cause of your dilemma, so at this point, anything suggested is at best a wild guess.
Maybe one of our guys local can come put an eyeball on your rig and see what's up?

[DieselDood]

That would be great. The local marine shops just say "I don't know, never heard of that before"

I did try calling Andy at Team Marine in Wisconsin, as I'll be down that way next month and it could be worth the trip down there if he has any ideas, but they are so busy right now he hasn't returned my call.

[castforcash]

Have you verified your tach is accurate? Are you sure the motor is reaching full throttle? I would post this on the mercury boards. Lots of help over there. Make sure you post your serial number when posting there. Good luck!

[Almag6]

verify your tach is reading correctly

try this. Buy a tiny tach for $10 on ebay and verify

I chased my tail for 18 months. Speed was okay but slip % was 18-20%. Frustrated to no end. In the end, I checked my tach and it read 9% high in all ranges. So, it would read 6000 rpm and I thought I was good.....but in reality I was only turning 5400.

Before I verified the tach, I put on 2 jackplates, 6 props, and new lower unit. Lots of frustration only to find out I was running 8% slip afterall.

[DieselDood]

I've verified with a Photo-Tach that it is set properly. It's a new tach and I wasn't sure, so I set the idle to 1500rpm with the photo tach, then set the new tach in the boat.

I'll double check it, but it should be correct.

The photo Tach is like the link below. You put a piece of reflective tape on the flywheel and it counts the times it passes. Pretty simple. Used quite a bit in the diesel world.

https://www.amazon.com/Nicety-Handhe.../dp/B009T4REQG

[rboren]

GPS speed ? If so you need to find a local guy that will look at it. As Pat said! Best prop I ran on a 184 with a 2.4 Merc was a 25 M Yamaha. Some 4 blades will probably work good too. You don’t have a transducer hanging off the transom that could interfere with water flow do you???

[DieselDood]

I've talked to all the local guys about it. They say "Never heard of that problem before" or "Ya, that's odd, I'm not sure what to tell you" or my favorite after I explain all the stuff I've done "Wow, you've done your research on this and tried everything I'd suggest. I don't know what to suggest"

The ducers are up high. As soon as I'm on plane they stop reading, and are a couple of inches above the bottom of the hull. They are also both a few inches outside the width of the jack plate.

Yes my speed is via GPS.

[pmgoffjr]

Here’s what I’ve got.
Put a known good prop on. The Yamaha M is proven the best on your exact rig. Set it at 3 1/2” under pad.
Go run the snot out of it. Wide open throttle and full trim should be 5,600 ish rpms at 65ish mph.
If you’re not getting that then you know it’s not your prop or setup and there is something else causing your issue.

Until then you’re chasing rabbits down holes.

[DieselDood]

Well I don't have a Yamaha M, but I set it to 3.5" under pad, 25p Trophy 4 blade.

Wide open (hot day 89degrees, warm water 84 degrees) I got 54mph at 6000rpm.

I'll keep trying to track down a Yamaha 25M to try.

I wish I didn't have to pull the fuel tank off to pull the jack plate so I could try that. I doubt that 8" plate is making that much difference...but at this point I'm not sure what else it could be.

[pmgoffjr]

That's a TON of prop slip.