prop rpm

Gentlemen - this forum seems to have the most informed/techincal readers - so I'll pose my query here. I ran a mini-seatrial on a 42 trawler with twin 160 hp Turbo Lehmans today and could only get 1800 rpm vs the rated 2500. They ran fine otherwise but smoked at WOT. Were even worse at first. Upper/lower helm tachs are close, so I think this is close to accurate. Made only 8 kts vs expected 11-12.

The bottom was covered in barnacles and slime - will it rob the rpm that much? I certainly understand the speed loss, but I would not have expected that much impact on props. They are 24x19 three bladed through 2:1 velvet drives if that is pertinent.

Comments? We plan to re-run after a haul and cleaning on Monday. I fear it will not help that much and something else is wrong. Also, I checked the turbos and the spin freely.

Also, this boat apparently has two different gearboxes with 2:1 ratio/24x19 prop on one side and 1.9:1/24x20 prop on other. Is this just totally crazy? I am guessing the engine was swapped at some point and they created this bastardized system. The mounts are different elevation and the shafts different lengths to accomodate the gearbox dirreneces. I just realized this whie investigating the paperwork in searching for history in the files/records. The present owner doesn't even realize what he bought - I bet he'll be upset.

Thanks to all - JeemboNC

 

Props with big diameter relative to power will grip well - not much slip. It is after all a "trawler" so is designed to drag a heavy load with some degree of efficiency. Not quite a tug but you want good grip in a trawler. If you load the boat up in any way you will certainly bog the motors.

If the hull is fouled as bad as you indicate it will make a big difference to drag on the hull. At 8kts you are below hull speed so not a lot of power going into the wave making. Not sure if you will see 12kts but should easily get over 8kts with the installed power.

You say that it did improve throughout the trial. My bet is some of the slimy growth was worn off.

I have never heard of odd twin drives but I do know identical drives "beat" or drone if the speed is close. Maybe the shaft vibration was an issue???

I used to own a deep keel yacht and there was no point racing on Sunday if you did not spend an hour or so on Saturday cleaning the boat's bum when living in the tropics. Week-to-week growth would make enough difference to be noticed in a race. 6 months growth and it would be encrusted and take two days to clean off. The anti-foulings I used back then liked to be operated to keep clean so sitting on a mooring was not good.

Please give a report after you have cleaned it down. I would expect at least 10kts.

Rick W.

 

Thanks for the input. I think there was some engine change somewhere in the history, but who knows when? The loss of speed is more easily understood - I just had no idea the engines would bog down that much?! I guess those big-a props just bite like a rubber tire on dry pavement. Will report tomorrow. JeemboNC

 

Jeembo
That is a good analogy. Maybe more like damp pavement.

Some rough numbers indicate that the 24X19 prop could absorb a maximum of 200HP at 1250rpm doing 6kts. It would absorb 180HP at 8kts doing 1250rpm. So the 160HP motor will not have the torque to get it to full rpm if the drag is higher than design.

The prop will absorb 115HP at 8kts doing 900rpm. The same torque gives 160HP at 1250rpm so the motor seems to be developiong the full torque just too much hull resistance for the pitch - gearing too high or drag more than it should be. You would think the latter and we will soon know.

If the 24X19 prop is a good match then it should absorb 120kW at about 10.3kts. If it gets over this speed then the power is less than 120kW so it could actually carry a bigger pitch untill the next time the hull gets fouled.

Rick W.

 

To have counter rotation, it is quite common to have one gearbox simply running in reverse. Unfortunately this also means a difference in ratio and in your case looks to have been accomodated by a different propeller.

I think you will be pleasantly surprised after a trial with a clean hull and props.

 

Rick - this was VERY helpful - thank you so much! Can you share with me the reference guide so I can conduct further calculations? I have some other projects that would benefit from this capability. I know engines much better than props. Again - thanks!!!! JeemboNC

BTW - been to Australia - chartered a bloboat in the Whitsundays - loved it!!

 

Frosty - so this is not uncommmon for two different tranny configurations/ratios/prop pitches/shaft lengths/engine mounts? Seems kinda kluged up - like I would do on a scavenged home project!! JeemboNC

 

I should tell all the I had another similar experience on a 20' deep vee outboard with a 175 Merc. The boat would only do 38 mph when I originally seatrialed her - vs expected 47+. RPM was 4600. Engine and prop were healthy so I bought her.

The bottom was covered in dried slime over gelcoat (no bottom paint). Pressure washed, acid scrubbed, etc and ran again. 50 mph at 4700 rpm. I figured equvalent to 70 hp. Was amazed at speed improvement for just slime, but did not see that much change in rpm. I guess a high speed 15x21P prop slips a LOT more! JeemboNC

 

Your first problem, of course, is the dirty hull bottom. The propellers are probably dirty as well and covered with barnacles. So clean all that off and that will be the new starting point to analyze the problem. And the real problem is, why are the engines not reaching their design RPM?

First, the props could be too big diameter or two much pitch for those engines. The don't sound too big, but without doing a prop and power analysis, this should be checked.

Second, the governors on the engine could be improperly set. Make sure they are set to allow the engines to go up to full RPM.

Third, the fuel injectors or the injection pumps may be faulty and the engine is not getting enough fuel. Make sure the pumps and injectors are all in good working order. Make sure that the fuel lines from the tanks to the engines are clear. Make sure the fuel/water separators/filters are clean with new filters.

Fourth, the engines may not be getting enough air! If they are in a closed and well sealed engine room, maybe not enough air is getting to them. You can tell if this is the case by trying to open the engine room door or hatch with the engines at full RPM. When opening the door, do the engines rev higher? If the door or hatch is hard to open because of suction, then that is part of the problem. Also, try running the engines with the door or hatch open so that enough air is getting to them. Do the engines rev higher?

Fifth, check the exhaust lines and mufflers. Are these free and clear? Is there marine growth in the exhaust outlets? Are the mufflers plugged? Is there are water trap in the exhaust lines that the exhaust air cannot get through? Make sure the exhausts are clear.

These are the most common faults for engines not achieving revs. Do these checks and let us know what you find out.

Good luck.

Eric

 

Eric - great advice. I checked the exhaust outlet - seems to be clear. I did the open hatch test - no change. The Racors have differential gages - seem okay. Inlet filters to turbo are clean. That leaves (other than the crud on the props/bottom):
1. governor
2. injectors
3. fuel lines
4. prop sizing (I don't think this is a problem)
5. old fuel

We'll run the tests and go from there. Thanks for the input! I'm in Jacksonville - wanna go for a ride tomorrow/Tuesday? JeemboNC

 

Jeembo
I have my own prop analysis software but for your analysis I used JavaProp. Here is a link:
http://www.mh-aerotools.de/airfoils/javaprop.htm
You run the Java Applet by clicking on "The Applet". It takes a little time to learn it but it is more flexible and easier to use than playing around with data sets. This is an analytical tool that relies on the physics involved not data sets measured on a series of props. It works for water and air props providing the parameters are set to suit.

If you want to try it you need to set the Options page to water parameters and make a few other changes. I can guide you through it if you want.

My estimates on your prop were rough as I assumed the number of blades and guessed a blade profile but these should not result in gross error. If you have the boat out of the water it will pay you to take some measurements on both props. Things like number of blades, blade maximum blade chord, hub diameter and angle of shaft. Also take a few photos of the hull and I can do the drag analysis for a clean hull if it does not get to the 10kts.

The main reason I did the analysis was because I felt your expected performance of 11 - 12kts might be a bit high so I wanted to get an idea of what the props were designed to do. Might get to 11kts in calm water when all is clean. If it gets to 12kts at 1750rpm then, like I say, you are not using full engine power.

Rick W.

 

Jeembo
Attached is a screen dump of JavaProp with the data I guesstimated for the 24X19 prop you are working with. I tried 3 and 4 bladed prop but it does not make much difference. The 4 blade would do slightly better speed.

If you are doing any test or design work with props then JavaProp will be useful. You can load actual blade data to do the analysis if you have the measurements.

Rick W.

 

Example ZF 630 2.5:1 forward.--2.54:1 reverse. As one gearbox is run in reverse to get counter rotation you have a different gear ratio. This results in one engine being slightly worked more. To get around this and to stop the booming of different RPM to balance the thrust another propellor is fitted.

My boat is exactly this set up and I was offered a different prop but I refused prefering to leave it as it was.

I did not say different 'trannys", shaft lengths or engine mounts.

 

I had a chance to speak to the owner today. He claims (he has been pretty candid and trustworthy thus far) the first owner changed the setup way back (I assume ~1986) due to a common problem with the Borg Warner trannies set up on these engines. The one (port side) creating the counter-rotating prop (I assume LH rotation) failed and the different tranny/realignment/changes were common. Apparently he spoke to the Ford guru Bob Smith who said that many Ford-powered trawlers have been similarly modified.

The boat used to do 11 kts clean, but that may be just a GPS guess and he never ran it that hard except when he seatrialed during purchase 3 years ago. He usually ran 1600 rpm yielding 8 kts. Also, though the book seems to say full power is made at 2500 rpm, he only saw 2350. Therefore, she may be a little overpropped. Is that bad as I will usually run slow for economy and that will help load then engine? As long as it doesn't smoke?

While were on the subject - I will be ferrying her to NC up the ICW starting upcoming weekend (8 day trip). It is 600 miles, so I will put 100 hours on her. Should I try running on just one engine? Freewheel or lock the other prop in gear? Will it save fuel? I do not have FloScans so I won't really know.

Rick - I tried the site - will spend more time on it this week. I'll take the pics for you - thanks!

Again, thanks to all the respondents - very helpful! JeemboNC

 

And Frosty - I think your relatively straightforward run in reverse with everything else the same makes lots more sense. JeemboNC