prop question

[mtadamek]

I have a 1965 41 conv. with 8v53's recently overhauled. Boat has 24x24 nibral props. I am only turning 23-2400 rpm's on the pins. After many hours troubleshooting, we believe it is the props. What can I do with the props to get the rated 2800 rpm's out of the engines. What are the pro's and con's of doing this.

 

[Boatsb]

I have a 1966 41c and turn 23x23 3 blades at full RPM if the bottom is clean and the boat is realistically loaded.

 

[jim rosenthal]

you should take at least one inch of pitch out of the props, if you are certain that everything else is tuned up as well as it can be. That would get you closer to where you need to be.

 

[mtadamek]

We have spent a lot of time working with the engines. They are newly rebuilt and have about 150 hours on them. They are still being broken in. We continue to see decreasing oil burn and less crap in the cans each time we take her out. We have seen some minor increases in rpm as well. We spent some time fine tuning the govenors and racks.

Set the no load at 2950 and always scrape the bottom before we go out. We will be hauling her out this next winter for a bottom job and this rpm problem has me thinking about getting the props repitched.

The wheels are 4 blade 24x24. She makes about 1.2 gal / mile running about 15kts. These engines also have limiting speed gov. on them instead of the variable. I have no idea when or why they were changed. Kind of a mystery. Can't imagine why someone would do this, but nonetheless it was done. Does not seem to make a huge difference except when she is in neutral you can't hold an rpm. Under load it is not a problem. Variable rpms on a following sea, but nothing intolerable.

 

[mtadamek]

boatsb:

what kind of fuel numbers are you getting with the 23x23 3 blades? Wonder what will happen to my burn numbers when I repitch.

 

[Boatsb]

I do not have flo scans so they are guestimates but I probably cruise at 16 Kts at about 23 GPH.

Going slower screws up the calculations because we have so many manatee zones I probably get 7 kts at less than 5 GPH.

Scraping will only do so much. I lost 200 RPMs with slime and scraped off running gear. Try it with a clean hull before you change the props.

FYI I am running with full water, 3/4 or more fuel (250 gals average), a 500 lb generator and lots of tolls and spares. I can lighten the boat by about 1000 pounds if I get rid of the wife but ............

 

[REBrueckner]

The pro of reducing pitch a bit is that you'll be able to turn up manufacturer rated RPM and avoid potential of the overloading the engines. Over propping could potentially result in overheating at high RPM. You'll gain a few knots of speed above 2400 RPM or so as your engines will be able to develop a bit more HP and RPM.


Whether the change reduces your gpm at lower rpm is something you'd have to check carefully....not easy to do in practice. In theory, if you want to run at moderate RPM, say 1500 to 1800 as an example, and not run at higher RPM's and speed, you'll get a bit more prop load at modest RPM and MAYBE a bit better fuel efficiency. The tendency is to kep your engines a bit more loaded and hence better maintain full operating temperature. But if your hull is not in a good spot at this RPM and speed all bets may be off.

In my own case with a 1972 48 YF with 8V71TI's, I was able to turn up only 2140 RPM's instead of the recommended 2300 all during my 11 year ownership...likely due to do boat weight. When new, Hatteras clocked her at about 22.3 knots WOT, presumably 2300 RPM. After engine refurburshment, I clocked 20.2 knots at 2140 RPM....and ten years later at sea trial for my sale last May, she still turned about the same 2140 RPM...we did not check speed over ground. I ran 95% of the time at about 1500 to 1700 RPM so a little extra speed at top RPM was of no interest to me; my turbo exhaust tempt at 1500 RPM was at the low end of appropriate operating temperature...around 500 degrees...so I did not want to do anything to lower operating temperature.....

My mechanic told me thermostats of 160, 170 and 180 are all ok. I used 170 degree thermostats to stay in the middle of that range.....and if you wanted to boost operating temp slightly you could always use 170's instead of typical 160's that might be in your engines. ....
I do not recall ever reading here if higher temp thermostats increase or decrease (in theory) fuel efficiency....in practice, it's likely barely detectable if at all...

If you decide to remove picth, get advice from the prop shop....likely they'll tell you to remove one or two inches pitch. That's just a guess on my part.

 

[REBrueckner]

Here are some prior discussions:
http://www.samsmarine.com/forums/showthread.php?t=13803&highlight=engine+efficiency

and my favorite:
http://www.samsmarine.com/forums/showthread.php?t=8373&highlight=engine+efficiency

Search ENGINE EFFICIENCY for more.

 

[mtadamek]

Thanks for the help. The responses definately helped understand the problem much better.

 

[mtadamek]

I did some funny math on that fuel burn. I said 1.2nmpg. It is more like .8nmpg at 15.5 or 16 kts. My navman fuel computers have never worked right, but on a 240 mile trip at running 200 miles at 15.5 - 16 kts and trolling the other 40 we burn about 250 gallons. That is with the genset running at about 1 gph for 15 hours. So over a fishing trip out to about 90 miles offshore we are getting roughly 1nmpg. Still damn good in my book for a big old heavy 41 hatteras.

I have never seen a power curve on these engines, but it makes perfect sense to take a little pitch out of the wheels once we are good and clean. Get a little more hp out of them all while running a little cooler. We have noticed the temps creeping up if we don't clean the raw water side once a year or even more. Running the boat the way it is, there is not much room for dirty heat exchangers or dirty strainers or through hulls. We always wondered why she was so finiky about that. Always figured there should be a little more room for error.

 

[jim rosenthal]

After a few years of being on this forum, it seems to me that much of the discussion centers around figuring out correct prop sizing, which is quite a black art to begin with, and also cooling system problems. I see why commercial vessels of any bigger size use grid cooling systems and dry stacks. If you think about the amount of maintenance that seawater/exchanger cooling systems require as compared to our cars or trucks, it is really rather a lot. Part of the problem with recreational vessels is that many of them only get occasional use, which doesn't exactly benefit the cooling systems. If you look at the trawlers sold for bluewater passages, they generally have simpler cooling systems; I wish those were compatible with planing hulls of the kind we have.

I've hijacked this thread a bit and apologize, but I think as a general rule cooling systems for pleasure boat engines have not kept pace with the increases in power and speed that we all enjoy using.