Best propeller pitch for Frankenstein build.

[Serf27]

Hello everyone.

I have an odd build here and am looking for help trying to identify the proper propeller pitch.

It is a 3.8l V6 engine OMC sterndrive.
Someone previously swapped the drive with a 400/4cyl drive.
Unaware of this, I ordered a 800 lower drive without knowing the upper was a 400.
I now have a v6 bolted to a 400 upper bolted to a 800 lower.

Everything is bolted up and spins fine.

The upper gear case ratio on the 400 is 21:18
The lower gear case ratio on the 800 is
14:26.

I’ve ordered a 17 pitch propeller. I was told by a local shop that it would get me out of the hole quick but my RPMs would be high while driving.
Can anyone help me in identifying the proper pitch for this?
Thanks.

 

[jimmbo]

I think your final gear ratio is 2:1 or 2.16:1, but I could be wrong

So what exactly is this Cobbled together Frankenstein unit going to be installed in?

 

[Serf27]

Its on a 1984 19ft “Sun-ray”.
The engine is the original and now so is the lower drive. I’m keeping an eye out for the correct upper drive, but since my current 400 upper works, I’d like to take the boat out for now.

Here is the manual showing the different gear ratios.
I am still not all too sure how ratios work but the upper gear case ratios look relatively the same.
400 drive 21:18 while the rest are 21:16-21:17 and 21:20.

Thanks.

 

[jimmbo]

What prop were you running before you changed the lower unit? and what rpm was it doing?

From what I can see, the configuration you currently have is like that of a High Altitude 5 liter motor, which would spin the prop 11% more per Engine RPM, but will load the Engine up more and drop the RPM. Dropping the Pitch 11% should return the load on the engine close to where it was prior to the Lower unit replacement. Actually the reduction in Pitch might need to be greater, to account for the larger Diameter of the 800 series Props

 

[Serf27]

It had a 15 pitch prop before I swapped the lower drive. So it was a complete 400 upper/lower.
I don’t know how to determine the speed, but at 4k rpm I was I’d say “cruising”..?
At 2K rpm, I was moving but not that fast.
I was told the ideal rpm for “cruising was somewhere between 2800-3k RPM.

So 11% drop would roughly be a 15pitch propeller?
I cannot use my old 15pitch prop because the splines are different but I can order one. I didn’t know which to order for the new 800 drive so I just took a stab in the dark and got a 17pitch.

 

[jimmbo]

What was the Max RPM you were getting at WOT? Also, what is the recommended WOT range of your engine? You do not chose a Prop, for a speed at a certain rpm. A prop is selected by what RPM it allows engine to attain at WOT

 

[Serf27]

I did not get to try WOT.
I believe I had a carb issue so at 4k, the engine would die after about 30 seconds.
So I was just driving at 2k rpm until the lower drive went bad.

The throttle lever was not all the way down at 4k RPM though.

How do I determine recommend WOT?
I will look in the manual to see if it mentions it.

 

[Serf27]

I’ve just read on this forum that the safe operating rpm is 4600.

1985 OMC 3.8l Max safe RPM operating range

Just installed a 3.8l that was purchased from a running boat. It appears the engine was rebuilt recently but i am not sure if the engine was rebuilt from a GM 229 automotive based engine. I am not 100% sure that the cam was changed to a marine cam. What would the operational difference be...

forums.iboats.com

 

[jimmbo]

If that is the case, the recommended WOT Range would be 4200 - 4600 rpm.
One would choose a Prop that lets the engine run in the upper half of that range. Too much Pitch, and the Motor is overloaded, and can die a horrible twitching death, not enough Pitch leads to over revving, which too, can harm the engine

It is important to know what it is, because
In the 1970s, some OMCs had a WOT range of 3800 - 4200. Since the late 90s, most V8 I/Os have a WOT Range of 4400 - 4800. Some were even a bit higher. V6s may or may not have followed the 8s

 

[Serf27]

What pitch would you recommend with my current gear set up at 4600 rpm range?
I don’t need to launch from the hole or hit max speed. Something that can drive the boat around with a little bit of fun.

Thanks.

 

[jimmbo]

Sorry, without knowing what the RPMs are at WOT, one cannot be certain if the Prop is correct
What was the WOT rpm attained with the old Gearcase? And what was the Pitch on that prop?
Once you get the Upper Gear Case replace, here is a Chart from the 1981OMC Accessory Catalog
A 17 might be the correct choice certainly a good place to start
As for the current configuration, see what it does, and if need be drop to a 15.

 

[Serf27]

Thank you for the chart.
I did not get to WOT last week with the old set up. All I got was 4k rpm on a 15 pitch prop, but the throttle lever still had room to move forward.

If I’m reading these other threads correctly, the max rpm for the complete 400drive/4cylinder is also 4600 rpm.

The 17pitch prop should get here tomorrow and I am going to take it out on the water.

I will see if the stalling issue was just old fuel.
If allowed by the carb, should I hit WOT for a few seconds once I’m out there and see what the RPMs are at?

 

[jimmbo]

If it exceeds 4600 don't remain there too long, but if it still climbing at 5000, then the prop is really too short.

 

[Serf27]

Perfect. I will try it out and post results.
Thank you.

 

[jimmbo]

Old Fuel, can leave its Mark on a Fuel System for a long time, in the form of Gum and Varnish, plugging/restricting Passages and Air Bleeds

 

[Serf27]

I drained all the old fuel out of the tank last week. First there was some water/fuel mixture and orange fuel followed it. About 7 gallons.
I replaced the main fuel line from tank to the water separator and replaced the separator too.
All cyl compression is at 150.
The carb is the last thing to rebuild/clean.
I managed to lift up half of the carb yesterday to inspect the float needle and it looked fine.
Maybe the float adjustment or accelerator pump…?

At 2K rpm I was able to drive and drive, but slowly. At 4k rpm it would last for about 30 seconds and die.
My theory is that the float bowl couldn’t fill up fast enough at 4k…?

Thanks.

 

[jimmbo]

Could be a Restriction somewhere in the Fuel System, including Tank Venting, Fuel Pickup in the tank, even a weak fuel pump. Have you tested the Pump for Flow and Pressure? Is there a Filter at the Carb? Is it a 2bbl or a 4bbl?

 

[Serf27]

i have not looked at the fuel vent. That did also cross my mind but I never checked it out.
I was able to pump fuel out of the tank relatively easily though from the main line that goes to the pump. I felt no restrictions there when pumping old fuel out. It flowed in a steady stream.
There is a filter on the carb, it is new.

I have not checked the pump/pressure either.

When the pump line was disconnected from the carb and I cranked the engine, fuel would not shoot out in a steady stream, more like pulses.
I figured this was due to because the engine was only cranking or it was a normal amount, enough to fill the bowl.
I am not sure now.
It is a 2 bbl.

 

[Serf27]

Somewhat successful day today.
Took it out with the new prop and the new 800 lower case. The new prop, only 2 pitches larger than the 15 pitch was much larger in overall dimensions.

I was moving a lot faster at 2K rpm than with the old set up.
I took it up to 3500-3800 and I was really moving. Faster than 4k rpm with the old set up. I didn’t get to hit WOT because it was windy and wife was telling me to slow down, but the throttle lever still had room to go.

I drove around for about 35 minutes at varying rpms and not a studder from the boat.
Great, all is well—-nope.
As I approached a beach area to land, my gears were incredibly stiff and it was very hard to maneuver the boat.

Headed back to the dock after the boat sat for about 1.5 hours and within 10 minutes it died on me the same way it did before.
I sucked fuel from the main tank line to be sure there was no blockage, removed the fuel water separator to verify there was fuel in it, still no start.

I then open the fuel fill cap to see if that was an issue and the boat started. This time I kept it at 1500 rpm and just coasted back. Gave it a bit more gas as I got near and it died but started back up after about 3 crank cycles.

Made it to the dock but it’s one issue after another.



Main topic point, I feel comfortable with the current prop and new boat speed/rpm.
I still want to hit wot to see what my rpms are, but the engine didn’t sound stressed today and rpms looked good for the speed.
No clue how fast I should be going at any said rpm, but it felt ok.

 

[Serf27]

I hit wot today and maxxed out at 3900 rpm.
Do I need a bigger prop than my current 17 pitch?