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+--- Thread: RPM's and Prop Issues (/showthread.php?tid=693)
RPM's and Prop Issues - torchlake99 - 04-28-2014
Ok, have a1999 35 Open with cummins 6BTA5.9M3 engines, about 650 hrs on them. Bought the boat late last fall, props on the boat when I picked it up were alter from factory specs of 23x29 3 blade nibral to 23x30. Unknown why they were altered. Running the boat last fall, light load could only get 2700 rpm's, engines plates show rpm's wot should run 3000.
Was advised to send props in and try to get a few more rpm out of them, they now are dialed in to 23x26, this was what was recommended. Over the winter I bought a Cummins manual and read in the manual section on trouble shooting......What I know is we tested the tach on the boat last fall and they were within 1% so not a tach reading problem. This was the 1st thing to look for , number 2 was to check the throttle linkage to make sure it is not misadjusted. Wish I would have had the manual before speeding the time and money on working the props over.
Only time will tell this spring, my concern is with the 23x26 I'm sure the rpm's will run higher and maybe up to 3000 but am I going to be loosing efficiency and performance? In a perfect world I wish a tech would have checked the throttle linkage then if all is well there work on the props.
Any advice? My advice to all that read this is if you do not have an original operation and maintenance manual buy one asap and read it. Most likely will save you a lot of time and money over the years.
RPM's and Prop Issues - Crown Royal - 04-28-2014
Kent, The 6BTA 370's are rated at 3000 rpm but realistically they should be propped to turn 3050 to 3150 loaded as you would use it. This will be way easier on your engines and they will last way longer not running in an over loaded condition. Your economy will not suffer appreciably doing this. My advise is to join and read all the articles on the Cummins 6bta there is a wealth of information on maintaining these engine and will allow you to have 1000's of hours 0f trouble free service from them. The forum administrator Tony Athens is one of the best Cummins mechanics in the country and will answer your questions. These are great boats enjoy.
Chuck
RPM's and Prop Issues - torchlake99 - 04-28-2014
Thanks Chuck
I will check out the site.
Kent
RPM's and Prop Issues - trit21 - 04-29-2014
I have a 2000 Tiara 3500 and I turned my props from 23x29 to 23x28 to gain a little extra RPM as was stated in by Crown Royal. What you will lose by turning them down to 23x26 is a little top end speed, a little bit when docking and a slight jump out of the hole to get on plane. I don't think it will be a hugely noticeable difference.
Tom
RPM's and Prop Issues - sawdustdad - 04-29-2014
On my 3700, I took an inch off my props to get up to 2700 rpm (was barely getting the spec'd 2600). I noticed the engines seemed quieter at cruise. I lost a 1/2 knot at 2200, so I just bumped them up to 2250 to regain the speed (22 knots). Fuel consumption actually is better now, and less soot in the exhaust. You'll be glad you made this change.
RPM's and Prop Issues - torchlake99 - 05-06-2014
Thanks Folks! Once back in the water I am going to run a rpm vs. speed chart and see what happens. Light load on boat, hope to get up to or close to 3000 rpm. Looking out my window this morning the ice pack was pushed back in, could bed a while till boating season starts on the Great Lakes this year.
RPM's and Prop Issues - Chris Burkard - 05-06-2014
Never Tune Props until you have ruled out all other sources of RPM loss.
Your Cummins should turn 3050 RPM under a full heavy load (100%, 100% Water, people and gear onboard). If you are light on fuel, people or gear, you may turn closer to 3100 or perhaps just a bit over.
Reasons for low RPM - 1) Hull Bottom Condition - Is it clean and smooth?; 2) Prop Condition - Are they clean, smooth and free of damage?; 3) Fuel Condition - Is your fuel good, less than 45 days old (use new fuel when testing for proper RPM), free of water and free of debris and algae?; 4) Fuel Filter Condition - Are your fuel filters clean and free of obstructions?; 5) Air Filters - Are your air filters clean and properly lubricated?; 6) Engines - Are your injectors, turbos, fuel pumps all working properly (have tuned if questionable)?
Once you know your vessel is tuned, clean and operating at 100%, only then can you test for proper RPM. With mechanical engines, never trust the tachometers. They are notoriously inaccurate. You must use a photo tach gun in order to get proper RPM readings. Pick a nice day with light winds and light sea conditions to run your test. Make sure to leave the engine synchronizer off during testing. The synchronizer can keep the engines from reaching full throttle and can mask if you have a problem with only one engine. Make two runs in opposite directions recording Actual RPM and GPS Speed at each RPM increment. Once you have reached your RPM target, let the speed stabilize for at least 10 seconds (check your trim and make sure you are level and optimally trimmed) and then record your findings. Do this for each RPM Level. I suggest the following for Cummins 6BTA-M3s @ 370 HP ea.
Record Fuel Level, Water Level, Number of people on board, Gear, Air Temperature, Water Temperature, Wind Direction and Speed.
600 RPM (No Trim) - let speed stabilize for 10 seconds - record speed
1000 RPM (No Trim) - stabilize for 10 seconds - record speed
1500 RPM (Little to no Trim) - Stabilize - record
2000 RPM (Moderate Trim) - Stabilize - record
2200 RPM (Moderate Trim) - Stabilize - Record -
Keep going from here in 200 RPM increments all the way to Wide Open Throttle.
Then repeat traveling in the opposite direction. Then average it out. Take this date to the prop shop and they should be able to tune your props accordingly.
Good Luck!
RPM's and Prop Issues - torchlake99 - 05-09-2014
Hi Chris
Thanks for the information! Will be taking on new fuel as soon as we get the boat into the water. Good advice on using the synchronizer, will do the above as soon as we can.
Cheers
Kent
RPM's and Prop Issues - torchlake99 - 06-07-2014
Wanted to give an update on what I learned about the above adventure..... After spending $1400 changing the pitch on the props there was almost no change in rpm's with the boat. I tried ordering a Cummins manual for the engines late last year when we bought the boat but it was out of print. I finally did get a copy late this spring. In the back of the manual there is a trouble shooting area. Under "engine will not reach full throttle" it says to make sure the throttle linkage is not misadjusted or damaged. I call in the local cummins service rep. and he came to the boat. Turns out this is exactly what my problem was. In one hour we turning 3300 rpm's with no load which is what he said it should spec out at. With full load in the water I am now turning about 3150. Now I have props that are off factory specs but at least we solved that issue. Just wanted to share my adventure. What I learned bottom line is there is no one better than your local Cummins service rep. to take care of these engines. He is going to come back for the 600 hours maintenance in a week or so.