Mercruiser greatly overpropped, need advice

[Steelguitarman]

Hello, I have a new to me boat. What I've slowly been finding out is that the previous owner didn't have a clue about maintenance or engines in general. After fixing the wrong timing, hole in fuel pump diaphram, shift cable, Bellows, you name it, I finally think I have it running good. Except I am over prop'ed. Here is the situation

V bottom fiberglass with cuddy 24 ft
Dry weight 4050lbs
50 gal fuel tank
Weight with fuel and gear I feel Is close to 5000-5500lbs
Alpha 1 with 1.47 gear ratio
1988 305 with 230 hp. Wot range 4200-4600
Current prop- mercury 19p 14.5in diameter
Current wot 3700 rpm at 35 mph if calm. Like 32 or 33 if wind or chop.
Using a prop slip calculater, it appears I have about 23% slip
The prop calculator on the mercury website is recommending a 15p 15.5in or a 16p 16in. With a calculated prop of 15.3 at 4600rpm
Does this all sound correct? What would be the difference between the 16 and 15 pitched prop? I feel I would want to go to 16 as it would be 900rpm difference allowing my 3700 to reach 4600, or is that not the case?
Thanks for your help, just want to make sure before spending the money. I know outboards being over or underproped can greatly decrease the life. Not sure about inboard though. With this different prop, what type of performance would I be seeing?

 

[havoc_squad]

Hello, I have a new to me boat. What I've slowly been finding out is that the previous owner didn't have a clue about maintenance or engines in general. After fixing the wrong timing, hole in fuel pump diaphram, shift cable, Bellows, you name it, I finally think I have it running good. Except I am over prop'ed. Here is the situation

V bottom fiberglass with cuddy 24 ft
Dry weight 4050lbs
50 gal fuel tank
Weight with fuel and gear I feel Is close to 5000-5500lbs
Alpha 1 with 1.47 gear ratio
1988 305 with 230 hp. Wot range 4200-4600
Current prop- mercury 19p 14.5in diameter
Current wot 3700 rpm at 35 mph if calm. Like 32 or 33 if wind or chop.
Using a prop slip calculater, it appears I have about 23% slip
The prop calculator on the mercury website is recommending a 15p 15.5in or a 16p 16in. With a calculated prop of 15.3 at 4600rpm
Does this all sound correct? What would be the difference between the 16 and 15 pitched prop? I feel I would want to go to 16 as it would be 900rpm difference allowing my 3700 to reach 4600, or is that not the case?
Thanks for your help, just want to make sure before spending the money. I know outboards being over or underproped can greatly decrease the life. Not sure about inboard though. With this different prop, what type of performance would I be seeing?

Combustion engine fundamentals do not change whether inboard or outboard. If you significantly overload an engine either the transmission and/or engine is going to prematurely fail earlier or accelerate wear and tear to unacceptable levels rather quickly versus the expected lifetime.

Have you seen if there are any service shops nearby you that will provide a loaner prop (with a loaner/rental deposit) in with those specifications in exchange for buying a prop through them?

If not, I would probably recommend going with what they call out for since you have actual WOT test data. 15 pitch looks spot on (-4 pitch at 200 RPM per degree) for 4500 RPM.

As to which prop manufacturer and model is best for your application, I'd see if you can get the top mercruiser forum member's opinions on how they perform with sterndrives, as I don't work on boats for living to get enough time to have an informed opinion on that.

 

[jimmbo]

If the tach is accurate, then you are over propped by quite a bit. However, you engine might not be putting out its Rated Power, for many reasons. So, a Compression test is in order, Is the spark advance working? Is the initial Timing correct? Is the Throttle Cable adjusted so the carb is opening fully? Likely a 4 barrel, are the secondaries opening? I don't mean Fully in their case, as a 5 liter doesn't need much more than 370 CFM at 5000 rpm

 

[alldodge]

I have a new to me boat.

Start with a compression test, motor may be tired.

 

[Steelguitarman]

If the tach is accurate, then you are over propped by quite a bit. However, you engine might not be putting out its Rated Power, for many reasons. So, a Compression test is in order, Is the spark advance working? Is the initial Timing correct? Is the Throttle Cable adjusted so the carb is opening fully? Likely a 4 barrel, are the secondaries opening? I don't mean Fully in their case, as a 5 liter doesn't need much more than 370 CFM at 5000 rpm

Tach is accurate, checked with timing gun. Yes spark advance is good, throttle is opening up completely. And secondary. It's a rochester, timing is at 8 btdc. I have not done a compression test. I can easily do that though, is the compression im looking for around 110- 120 even?

When I first got it, it was only doing 3400, but between the fuel pump diaphram hole and retarded timing, it brought it up to 3700. I wanted to be sure I had everything checked out before even considering a new prop, after seeing that the prop calculater recommended me a 15, I feel confident that's the issue.
Thanks, and the motor has about 650 hrs, although, that's not saying much. The last guy didn't have a clue. Pretty sure he just drained the oil in the bilge, along with other things. Engine does sound smooth and not irregular. Im most certain the old prop either fail or was corroded, as this new one almost has all the paint still

 

[alldodge]

A new motor would see 150, anything below that is reduced power. A motor will run with 100 but not much power left

 

[QBhoy]

Hi. That’s a hell of an unusual ratio for a 5.0 carb and a 24ft boat. Part of me suspects either the outdrive or the engine isn’t original to the boat. A 1.47 ratio would be a rare thing on any 5.0 carb set up on even the smallest of boats. I’ve a 1.47 ratio on an 18ft 5.0 mpi and that’s likely a rare enough thing. But I’d suspect it would be almost unheard of with a 5.0 carb and a boat that size.
Anyway…apart from that, if that’s your set up…you’re going to need considerably less pitch than a 19”. Most 20/21ft boats with a 5.7 and 1.47 ratio would likely struggle with a 19” into the rpm range. I’d start with a 15” and go from there. You might just be lucky and be able to run a small diameter 17” close to the bottom of the rpm range…but still a big ask, I’d think.
What light just be a good all round for you is the 16x16 black max big diameter prop. Can’t think of much else that might be suitable for a guesstimate starting point

 

[Steelguitarman]

Hi. That’s a hell of an unusual ratio for a 5.0 carb and a 24ft boat. Part of me suspects either the outdrive or the engine isn’t original to the boat.

To clarify, I have not inspected the gears myself, but the previous owner had all the paperwork of the work done to the boat since new. The original owner had ran the gearcase with water in it. The paper work says lower unit unit was completely replaced whith the upper unit being replaced with 1.47 gears.

Knowing everything it wouldn't surprised me if the wrong gears were put in.
I do have a 14 inch 17 pitch prop. It has the replaceable hub. Not sure if that would be suitable or if I can get a matching hub. Or if the hub on the mercury prop would work. The 17 is a turning point

Thanks for your thoughts

 

[QBhoy]

To clarify, I have not inspected the gears myself, but the previous owner had all the paperwork of the work done to the boat since new. The original owner had ran the gearcase with water in it. The paper work says lower unit unit was completely replaced whith the upper unit being replaced with 1.47 gears.

Knowing everything it wouldn't surprised me if the wrong gears were put in.
I do have a 14 inch 17 pitch prop. It has the replaceable hub. Not sure if that would be suitable or if I can get a matching hub. Or if the hub on the mercury prop would work. The 17 is a turning point

Thanks for your thoughts

17 is worth a try. Give you a benchmark for reference for sure. Suspect that it might be beefing on a little much for all round use or if you’re heavy loaded perhaps. But will definitely be a worth while exercise for sure. If nothing else. Yeah, I’d think that 1.47 gearing would be considered a little highly geared for that set up perhaps.
Just on your rpm range for that engine. I know there are some variations, but I thought most of those engines around that age and especially the 230hp 4 barrel…would be 4400-4800 rpm. Does yours state less than that you think ? Either way…I think it’s perhaps usually considered better for these things to be propped around or above 4600 rpm in general. Anything less…and I think personally that they aren’t at their best for peak power and at the very least…will lug, strain and be hellish down low for getting going.

 

[Steelguitarman]

QBhoy, yes, it state wot between 4200 to 4600 on Valve covers

 

[Scott Danforth]

Tach is accurate, checked with timing gun. Yes spark advance is good, throttle is opening up completely. And secondary. It's a rochester, timing is at 8 btdc. I have not done a compression test. I can easily do that though, is the compression im looking for around 110- 120 even?

When I first got it, it was only doing 3400, but between the fuel pump diaphram hole and retarded timing, it brought it up to 3700. I wanted to be sure I had everything checked out before even considering a new prop, after seeing that the prop calculater recommended me a 15, I feel confident that's the issue.
Thanks, and the motor has about 650 hrs, although, that's not saying much. The last guy didn't have a clue. Pretty sure he just drained the oil in the bilge, along with other things. Engine does sound smooth and not irregular. Im most certain the old prop either fail or was corroded, as this new one almost has all the paint still

What are your compression numbers. As AD posted 150 for a good motor


A 1988 with only 650 hours means the boat sat for many many years

 

[Scott Danforth]

A 1988 GM motor can be spun to 4800-5000 rpm without issues

 

[QBhoy]

What are your compression numbers. As AD posted 150 for a good motor


A 1988 with only 650 hours means the boat sat for many many years

Fair point Scott. But quite convinced the unusual gearing is a tall ask of that 5.0 carb in a 24ft boat. You’d know better than I perhaps, but suspect that wouldn’t ever have been a match from new ? Madness when you think about it really. Just knowing the circumstances and numbers from my 5.0 with its 1.47 ratio. Despite it being a world of different boat…and certainly hitting the rpm limiter if I had a 19” on it…I suspect even if I was to have the boat running down at his rpm range…say about 4600 rpm (with 19” and 1.47)…the boat would be doing mid 50’s at a guess. To expect a 24ft boat with only a 5.0 carb and loads less power to do the same whilst in its stated rpm range, is just ridiculous. That’s what makes me think that it didn’t have such a tall ratio from new. That’s if the engine is original to the boat. That’s my thinking anyway. There’s just no way that engine could be expected to get anywhere near the rpm range with that ratio and any normal pitch of prop. I don’t even think a 17” is going to do the job. Then you’re down to the very lower end of available pitch of props. 15” and around there. Is there even any props much less than that off the shelf that would suit ? Got to be a **** up somewhere along the line with the drive train ?

 

[Steelguitarman]

What are your compression numbers. As AD posted 150 for a good motor


A 1988 with only 650 hours means the boat sat for many many years

I plan on doing that tonight and will report results.

Am I correct in that all I need to do is disconnect the coil lead, or do I need to disconnect the wires too? I was getting conflicting results when I search it.

 

[alldodge]

Disconnect the purple wire going to the coil on the + side, this removes power so there will be no spark

Open throttle wide open and leave there until all cylinders are tested Have it crank over at least 3 times for each cylinder

 

[Scott Danforth]

I normally keep the key off and use a remote start switch

 

[QBhoy]

Can’t hurt doing a compression test…but still think it’s very obviously related to ratio and prop. That engine can be as healthy as new…but it still won’t turn a 19” through a 1.47 ratio to anything remotely close to its rpm range. Just impossible. A 5.7 260hp wouldn’t either. Not on that boat. Anyway. Won’t do any harm to find out. But even if you are a little short on compression…I wouldn’t put it all down to that. Then there is the whole issue around accuracy of almost any compression gauge that hasn’t been calibrated or of questionable quality…not to mention anything around the actual process of performing the test for a first timer. Good luck in any case. Let us know how you get on.

 

[jimmbo]

A 1988 GM motor can be spun to 4800-5000 rpm without issues

While it can be spun that fast without flying apart too soon, older engines may have used different Cam Grinds and 4800-5000 might be out of their intended Rpm range, as well if the Intake and Heads are restrictive, there is nothing to gain by over revving it past the 4600 mark. In the 70s, OMC and MerCruiser an Rpm range of 3800-4200 for a lot of their engines

 

[Steelguitarman]

Compression test is in. Lowest came in at 145(cylinder 1) highest was 160(cylinder 7) most of them were around 155.


I also double checked my compression tester with my portable air pump, atleast at 90 psi, they are the same.

Engines is healthy. What's next?

 

[QBhoy]

Compression test is in. Lowest came in at 145(cylinder 1) highest was 160(cylinder 7) most of them were around 155.


I also double checked my compression tester with my portable air pump, atleast at 90 psi, they are the same.

Engines is healthy. What's next?

Good news there. Much less pitch is next. Pretty convinced it’s just all as I’ve said above. Makes total sense to me that it is. Best of luck