props

[arwelch475]

Greetings fellow boaters, I have a 1999 4100 scb with 3.7 Cummins diesels. I caught a buoy cable right at end of season, tore the starboard prop, and have to replace the props. Can anyone tell me if there is a big difference between 23 pitch and 24? It has 24 inch diameter with 23 pitch on her now, but I run into 24 on 24 offerings and wonder how much it would effect my performance. It's a motor yacht after all, not a ski boat. thank you for any input you can give me. AW

[mrt214]

I've heard the best rule of thumb is plus or minus 2degrees,but you can go on mercruiser website and follow the steps to find the best one for your boat.

https://www.mercurymarine.com/en/us/

[arwelch475]

Thank you for your response.

[shrew]

What was the pitch on the original props? If only one was damaged, why replace both? Obviously both props need to be mirrors with the same diameter and pitch.

The rule of thumb is each inch of pitch will modify RPM by about 200. Increasing Pitch will lower RPM, while decreasing pitch will increase RPM.

You would need to determine the MAX RPM Range for the engine. Then load the boat as it would normally be loaded. Run the boat up to Max RPM (calm conditions (wind, waves, current, tide). Determine if you are close the Max. RPM Range.

Increasing pitch will lower the Max. RPM. Decreasing Pitch will increase Max. RPM. Ideally you would want to be as close to Max RPM as possible.

The idea that if you increase pitch you can maintain speed while lowering max. rpm in order to reduce fuel consumption is a false economy. You will end up overloading the engine over time doing this.

[arwelch475]

The existing props were 24inch diameter, 23 pitch. The starboard is a gonner, the port knicked up pretty good too. I figure to get a set, then see if the poet o e can be repaired, and I'll look for one first starboard to have an extra set in case something happens in season. Thank you for your response.

[Danny4100]

I was wondering how you did with the props?i have a 4100 scr cummins as well... Just did my cutlass bearings n props ,I changed the pitch the boat was way over propped Danny

[arwelch475]

Hello, I was able to find a company in Long Island whose main business is fixing damaged props. I thought my starboard was a goner, but they are repairing both of them, so the original 24 inch X 23 pitch props are going back on in the spring. then I will take the boat out with some people on it and run it up to max RPM and see what she does. Then I'll give all the info to someone and see what the computations come back as. They are heavy boats, with fuel and people almost 31,000 lbs, that may be why there is so much prop on them. I'll post findings in May sometime when our season gets going. Thanks to everyone who responded, this forum is great resource.

[Danny4100]

I to am from Long Island ,I used bossier and sweezy in bell port ,I gave him my info and he told me exactly what I needed...before I was getting 2750 rpm max on bolth engines and those diesels need to see 2950 rpm ..the shortest life of a diesel is being over propped.. I've read and read and read on our boat and I'm hoping this will be a good cure

[arwelch475]

That's who's fixing my props!Small world. What did they say to put on your boat and what did they cost if you don't mind me asking.

[Danny4100]

I don't know the price yet,but boy they look pretty.almost want to hang them on the wall instead lol,just realized u have a scb,I have a scr ...but anyway just be sure not to over prop these engines.. It will take a diesel from 5000 hr expectancy to just about 900 hrs .but they did take an inch of pitch off ,I haven't tried myself but I can't wait to tru

[arwelch475]

Sounds good. Please let me know what they put on yours and what it cost. If it's better for the boat I would consider it next year. They seem very professional, made my day when they said they could fix my props. Good luck, look forward to hearing from you.

[Danny4100]

Sure you bet,I have the boat yard doing the props thru bosler and swezy ,can you let me know how much they are charging you?

[arwelch475]

I certainly will when I pick them up. Repair to starboard one was significant. Poet was less so. My yard called them goners, I found bossler on my own.

[Danny4100]

What did you hit?any engine damage?

[arwelch475]

We think it was a buoy cable. No damage to shaft or Trans or engine.

[Danny4100]

I was out in July just got on plane it was glass out that day,anyway I hit a piece of dock that you could not see so no damage to shafts but props diffrent story.. So I did no cutlass bearings n props etc lol

[Danny4100]

In it for 3 grand so far

[Stray Cat]

Having owned the Cummins B series (5.9 BTD) 370 hp engines, I thought I would chime in.
First thing is, what is the maximum RPM you can currently achieve?
For a 6b 370hp, you should be able to acheive 3000 RPM + 50 = 3050
For a 6b 330 HP you should see 2700 RPM + 50 = 2750
It's important to get to maximum rated RPM on these engines.
If you have 24x24 four blade nibral props (likely) and you aren't getting within 100 RPMs of max rated RPM you will have to replace those with a 24x22 and probable have them repitched to 21.
Yep, they are expensive. About $1500 each, but that is way cheaper than new engines.
Running EGT and boost gauges will tell you exactly what each engine is doing in regards to sync, which is better than a tach sync. Yes RPMs will be the same, however load on each engine may not be. You may have to adjust the pitch on one side. Remember one transmission has a reverse rotation which is less efficient, that needs to be accounted for in propeller pitch as well.

[Danny4100]

Mine were repitched to 21 .but 1500 a piece don't know where your shopping but that's not what I paid

[Danny4100]

I can get a brand new set for less than that,already researched it