Rebuilt 12-71Tis Sea Trial results & cost- finally

[Tawney1]

He guys,

I would like your comments on the sea trial results and suggestions of Covington DD here in Wilmington -- after months of delays waiting for parts, etc., the rebuild of our engines is complete, and a sea trial (actually three) was recently completed. The total cost for the rebuilds of both engines was six figures. That included new camshaft on port engine and new starters and alternators for both engines. If you would like more specific cost and/or breakdown info, send me a PM and I will be happy to provide them for you. I will say that labor on the stbd engine was 377.51 hours, but was substantially reduced by Covington.

During the first two sea trials there were numerous small problems, but largest was problems with the injectors. During the third sea trial, the injectors seemed to be fine.

Here are results:

As I understand it the engines should turn up to 2300 rpms at WOT (is that correct?). I thought factory was 2500 Max no load, and max full load is 2300).

The stbd engine at WOT turned up only 2144 rpms and port 2182.... Covington indicates that the reason for the difference is that I need to get some pitch taken off the props to free the engines up so they'll turn 2300 rpms at WOT. Covinton indicates that would allow the engines to live longer and we can get 2300 rpms at WOT. Now, as is without prop changes, they are suggesting that the running rpms should stay around 1800 to 1850, and should not exceed 1850. If I am reading their notes correctly, cruise speed achieved 14 knts, full speed WOT 21 knots. But I will have to confirm that.

What say you guys? Comments and observations will be appreciated! Does this sound reasonable? Should I have the props' pitch changed? Is that even possible using my same props?

Many thanks,

Paul

 

[Pascal]

congrats

you shoudl be able to get 100/120 rom back by taking pitch off your props but what kind of rpm were you turning at WOT before the rebuilt ? were you turning rated RPM ?

14kts cruise is just about the worst possible speed, that cant' be accurate. if you get 21kts WOT then i would imagine your cruise would be fully on plane around 17/18kts. at 14kts, you're off plane, pushing a big wave and overloading those engines.

 

[captddis]

I take care of a 1984 61MY. We [ myself and another] rebuilt the engines in 3/99. The boat turned up 2180 after overhaul. The PO replaced the props and they were too big. Had Blackdog tune and repitch the props for 2300+ fully loaded. Cruise @ 2050 is 16 kt. WOT with a new bottom is 19.9kt @ 2320
All speeds per GPS. 9 years later and no leaks or problems, just oil + filters and impellers everyother year. Also had 2 coolant changes. Installed 24 new injectors at overhaul and no failures.
They overhaul was done in the boat and had everything from new cranks to new intercoolers. Hit six figures in 99 and that was with my lower labor rate.
Dave.

 

[REBrueckner]

Your should read the thread here "Larger props for efficiency and longer engine life" started by Brian. It will addresss your question. The bottom line is that if you want to cruise at moderate RPM and speed for fuel efficiency,say 1300 or 1500 RPM and are willing to give up some top end speed, keep the props a bit big like they are now. What Covington means is that if you want to run at higher RPM and cruise speed, say 1800 RPM, then you'd help the engines at this range by reducing pitch an inch or two to increase RPM.

 

[SKYCHENEY]

Paul,
I had the same issue re: props. My no load WOT was fine, but I could not make it there under load. I had 2 1/2" of pitch taken out of my props and now all is well. As long as your props are in good shape and have not been worked on too much in the past, they should be able to change them at a good prop shop. I usually cruise at 10kts, but I wanted to keep the ability to get up and go without lugging the engines. That 2 1/2" of pitch seemed to make no real difference in top end speed or fuel economy either.

 

[Genesis]

Do not run the boat on step without fixing the wheels.

You are SEVERELY overloaded and this will have a major impact on engine life if not addressed. This sort of overloading is one of the primary causes of premature engine failure.

 

[Tawney1]

Thanks very much guys. If I read Covington's notes correctly, 14 knts is correct. I am meeting with them on Wednesday, however, and will confirm that. It seems that the consensus is to change the prop pitch....

 

[Boatsb]

I would still look at the performance figures before the rebuild or better over the years leading up to them. If all was OK with the engines and props before they needed the rebuild why do the props need to be changed now?

 

[Airpilot]

I have 12-71s and they produce 2350 rpm at wot. At 2100 rpm 19 kts. My props are 31/30.7. All rpms were read by a mechanic with electric tach. Hope this helps.

 

[Brian Degulis]

Your engines are most likely putting out rated power. The next question would be then why won't they turn full RPM unless you de pitch the props. The reason is that everyting is maxed out when the boat was built Hat wanted to be able to get the most speed they could so they set it up with props that would absorb every bit of power the engines could produce. That was with a brand new boat everything was clean and perfect. Now it's 20 + years old weights been added and re distributed props and shafts are not perfect and on and on so it's not un usuall at all to have to de pitch a little to turn full rated RPM.

If you want to run up on plane at or near WOT then you must de pitch. If you plan to run slower and want to know at what point you will overload check the exhaust temp before the turbo that will tell the story. It's all about finding the RPM you can run up to before you overload your mechanic may have already done that and come up with 1800 RPM.

I think unless your running at or near displacment speed I would de pitch. The whole prop loading curve is so mis understood you would probably be wise to de pitch for full rated RPM to protect your warrantee.

Brian

 

[Tawney1]

Paul,
I had the same issue re: props. My no load WOT was fine, but I could not make it there under load. I had 2 1/2" of pitch taken out of my props and now all is well. As long as your props are in good shape and have not been worked on too much in the past, they should be able to change them at a good prop shop. I usually cruise at 10kts, but I wanted to keep the ability to get up and go without lugging the engines. That 2 1/2" of pitch seemed to make no real difference in top end speed or fuel economy either.

Thanks all....

Sky,

What size props do you have? I usually cruise at 12 knts. When I was told about the props by the mechanic my concerns were, if I did that, what about fuel economy, speed and getting on plane--to the extent the hull ever gets there anyway even when playing with the trim tabs. I understand that you noticed no difference in speed or fuel economy. That's good news, but to me it seems if those Detroits are turning 200 more rpms--and with those turbos humming, they're gonna gulp more fuel.... I am certainly no expert here, so what am I missing?

After taking off that 2 1/2 " of prop, how much did that affect your ability to "get up and go?"

We have a heavy boat. With only about 1/2 tank(s) of fuel and no water, our 61' recently weighed-in at over 80,000 lbs in the sling when we dropped her back in the water.

I have a spare set of props, and they are turned, pitch wise, identically to the ones on the boat. I assume the PO had no issues with the pitch. But if I should or need to take off 2 to 2 1/2", my spares are in great shape and should hold up well for the process.

Thanks,
Paul

 

[REBrueckner]

"... but to me it seems if those Detroits are turning 200 more rpms--and with those turbos humming, they're gonna gulp more fuel.... I am certainly no expert here, so what am I missing?.."

You only get 200 RPM more at the top end...WOT..and you use more fuel because you are developing more HP...going faster. If you don't want that, don't bother reducing pitch.

If you used to cruise at, say, 1200 RPM and 8 knots, with smaller pitch you'll cruise a bit slower at 1200 RPM and your fuel consumption will be a bit lower as well. (less HP, less speed, less fuel.) If you increase RPM to achieve your prior cruise speed, say 1300 RPM, fuel consumption should be very close to what it was...you need more RPM but not so much fuel rack advance as this small pitch prop sucks up a bit less power. In this example, 1300 RPM produces about the same prop HP as the old prop did at 1200 RPM, so you get about the same speed.

As noted in a recent thread, a larger prop develops more HP at a given RPM and this helps boost operating/exhaust temps which may be desireable if you cruise at modest RPM for long periods. Also, a larger prop is slightly more efficient than a smaller, but I'm not sure if that's for just diameter or includes pitch size as well.

"...After taking off that 2 1/2 " of prop, how much did that affect your ability to "get up and go?".."

If the bottom is clean and props true, in a planing hull you should get a bit more top end speed.

 

[SKYCHENEY]

Paul,
My props were originally 28x32 from the factory. When I bought the boat, she had 28x31's on her. I repitched the spares to 28x30. This helped but I could still not get to rated WOT and one prop was "singing". So, I then repitched the original set of props to 28x28.5

The 28x28.5 are working well. My boat weighs in at about 60,000lbs and she has 6v92's that are rated at 465hp. I did not notice any change in getting up on plane or fast cruise, still about 15kts. I did notice that my fuel consumption seemed to go up at 1400rpms. I have no flowscans so this is just a perception (real or not I cannot say). I now run at 1300 most of the time and get about 10ghp at 9.5kts. Fast cruise is about 40gph, 1950rpm, 15kts.

Other factors here that are not known are how different the 2 sets of props are that I have used and just how close they were to being what they said they were to begin with. I also have about 3800hrs on my engines. They start well and run clean, but I have no idea if they are putting out at, below, or above the original design HP.

These boats are heavy and they take alot of fuel to get up and go. When I first decided to tackle this problem, I asked a lot of questions, but received many conflicting responses. It is costly to play around with different props and I didn't want to chase something that I couldn't achieve. Unfortunately, I think that in most cases you just have to play around with it until you get it right. A good starting point is probably to just do what the DD guys are saying. I wish I could be of more help.

 

[Brian Degulis]

In general if you reduce pitch or diameter to achieve full rated RPM or for any reason. Your RPM to cruise at slower near displacment speed must go up because the props are now producing less thrust per RPM than they were before. If you turn higher RPM to achieve the same near displacment speed you will use more fuel and combustion temp will be lower. So you will be running in a less favorable condition on the low end of the RPM range but will have the ability to use all the HP and get up on plane. Add pitch or diameter and the reverse is true better near displacment cruising with lower fuel consumption but you can't use all the HP without overloading and proubably won't be able to plane.

I have the same boat as the poster with the same engines I decided I didn't want to plane so I proped larger for better near displacment cruising. That works very well for me 70 GPH to go 18-19 kts 15 GPH to go 10.5kts 19GPH to go 10.5 and still be able to go 18-19 kts. Since I was never going to run at 18 kts the choice was easy. I now run quieter smoother combustion temp is much higher and I save $14 an hour in fuel cost.

Brian