Propeller Cupping – 38 Bayliner

[Nick]

Hello…my name is Nick…

I have a 1988 Bayliner 3870 repowered with Cummins 6BTA’s 5.9M’s, build 8206, Tranny gear at 2:1, WOT 2600…propellers 21x26x4 Nibral.

My issue…I have some damage in the corner of the tunnel where the trailing edge of the prop has pitted a hole about 3/8″ deep apparently caused by water turbulence/cavitation.

Right now I cruise at 2350RPM/16.5kts, WOT 2600RPM/22kts. I don’t want to lose that performance. Based on internet formulas it seems I have somewhere around 24% prop slippage at WOT and 34% at cruising. Not sure if that makes sense. It would be nice to improve that.

My prop shop has suggested cupping the props to move the water better in the tunnel which will help minimize cavitation. I’m afraid this will also need a pitch change and am concerned that there would be some guesswork involved with more and more changes later.

My question…is there anybody out there that has cupped their 4-blade props and what was the associated pitch change and what effect did it have on cruising and WOT speeds and other boat performance…?

Thanks in advance…Nick (I hope I put this in the right Forum….)

Topic

[Nick]

Steve wrote:

Sir,

Are those the original propellers and were they adjusted when the boat was repowered?

I had my prop serviced by Wilmington Propeller Service in Southern California. Talking with numerous others (including a Long Beach Harbor Pilot who has a fishing boat a few slips down), Wilmington Prop Service has a stellar reputation in the shipping industry.

The owner there told me that generally all props in tunnels should be cupped. He explained that given the propeller’s close proximity to the hull, cupping helps reduce propwash against the hull and generally makes it run more efficiently. I doubt you’d actually even notice any reverse response.

If you take create a table of your performance numbers at varying rpms, your prop guy should be able to optimize your prop and calculate what adjustment is necessary, if any.

Whether the hull pitting was caused by the prop or not, it wouldn’t hurt to see what your prop service says about your performance data and the tunnel/cup question. (I’m assuming that your guy will be as competent and trustworthy as Wilmington Propeller Service).

Good Luck Sir

Thanks, Steve…they are the original props after they tried 5 prop sizes after the repower. The props were stamped 22×26 but then made to 21×26…

I was told the same thing…tunnels should have cupped props to move water more efficiently down the tunnel. Based on online calculators I have about 34% slippage so I’m guessing the cupping will help that.

It is possible I may not do anything this season except fix the tunnel wear…gives me a chance to set a good baseline and data. It depends on what my prop guy says after running his software.

Thanks again…Regards, Nick

[Nick]

Thank you Steve…appreciate the details in your response. That is exactly what I was hoping to hear…somebody else that has been there…

In about a week I’ll hear from the prop guy after he slaps ’em on the computer and inputs my info. If it’s a small cup and drop an inch of pitch, I’ll be comfortable with that…even though my current configuration works, I just can’t imagine what’s going on in the tunnels with a 26 pitch…

Thanks again…

[Steve]

My question…is there anybody out there that has cupped their 4-blade props and what was the associated pitch change and what effect did it have on cruising and WOT speeds and other boat performance…?

And to answer this…Yes. We have a 4 blade cupped prop in a tunnel.

After an inability to reach w/in 100 rpm of our WOT (3000 rpm for us), we had it serviced by Wilmington Prop. They reduced the pitch by 1″ (now at 23×25) and fine tuned the cup. We maintained our cruise speed (17K at 2400rpm) and can now hit 2900-2950 WOT.

On a side note, we put on a spare uncupped 4 blade prop while the work was being done and noticed a significant increase in the hull vibration above the prop.

Who knows…maybe cupping your prop will increase performance, reduce noise and eliminate the pitting!

[Steve]

Sir,

Are those the original propellers and were they adjusted when the boat was repowered?

I had my prop serviced by Wilmington Propeller Service in Southern California. Talking with numerous others (including a Long Beach Harbor Pilot who has a fishing boat a few slips down), Wilmington Prop Service has a stellar reputation in the shipping industry.

The owner there told me that generally all props in tunnels should be cupped. He explained that given the propeller’s close proximity to the hull, cupping helps reduce propwash against the hull and generally makes it run more efficiently. I doubt you’d actually even notice any reverse response.

If you take create a table of your performance numbers at varying rpms, your prop guy should be able to optimize your prop and calculate what adjustment is necessary, if any.

Whether the hull pitting was caused by the prop or not, it wouldn’t hurt to see what your prop service says about your performance data and the tunnel/cup question. (I’m assuming that your guy will be as competent and trustworthy as Wilmington Propeller Service).

Good Luck Sir

[Nick]

Thanks, Tony

Correct…they are recon’s and right again, twins, and boat is around 20,000…

As you can see by the performance in my opening post, it’s running like a top. Running angle about 2deg at plane…

Fill to fill on fuel shows 4.5 – 5.5 gph depending on where I get fuel. My typical outing is 15 mins at 1100 til engines show good warmup, then 2350 for the rest of the trip. She’s very responsive even at idle. Distance between my pulpit and next row of boats is 55ft…come out smooth, opposite gears, turns on a dime, out to the channel…same coming back in…no rudder used.

One of my concerns with the cupping is some loss of response at idle in reverse…would hate to lose the smooth exit and docking in the slip. Plan to discuss with the prop guy before doing any work.

My current thinking is “do nothing” except fix the tunnel wear. I plan to set a good baseline with boost and EGT gauges this season and phototach actual RPM’s to make sure my tachs are good.

Thanks for your reply…my taxes kept me busy this week also…

Regards, Nick…

[Tony Athens]

38 ft Bayliner–Propping Questions

It was TAX week–sorry for the delay.

#1——–Per you serial number (60261469) you DO NOT have a 6BTA’s–You have a RECON 6BT 5.9 210 CPL8206, rated 210 BHP at 2600 RPM ( minimum).Prop as below for best efficiency…….

You have twins, right?

#2——-FR rating is the official fuel rating of your engine / how the engine was “calibrated” as to power output and emission certifications, if applicable.

As to cupping–Up to your prop guy—Check your tach fro accuracy.. Check your prop for QUALITY and design.Prop design/quality can make a noticeable difference…

Prop to an accurate 2650-2700 RPM…………..Running gear and bottom–clean to the max (baby butt smooth), especially the prop and running gear sanded and shinny………………..You now have all this engine has to give on your boat.. Actually, your current numbers are decent considering the low HP engines you have..I would think your boat must weight 18000-20,000 lbs ( or more) ??

Tony

[Nick]

Tony…thought I might have heard from you by now. I did post one of my serial numbers and also wanted to know what “FR Rating” is…pardon the ignorance.

You hit the nail on the head having the same concern about overpropping if cupped.

Anxiously awaiting…thanks, Nick

[Nick]

Pardon the ignorance…what is “FR Rating” that you mentioned…?

[Nick]

Engine Serial Number

60261469…thanks…

[Tony Athens]

Nick,

Post an engine serial number–I want to look up the FR rating..

Then I’ll comment of the cupping thing.. But in many cases, cupping can do what was suggested, but lets be sure that is does not add other issues ( like over propping)..

Tony

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