Propeller Pitch vs Cup Question

[JimP]

My 6-71 is turning about 140 RPM less than rated max load RPM. Unloaded max RPM goes to full rated 2650. I currently have 24x26 three blade cupped props.

Spoke to 2 prop reconditioners a got 2 different recommendations. First one said take 1-2" of pitch out. Second suggested leaving the pitch alone but take the cup out.

I'm not sure which way to go on this one. I've always though cupping provided many benefits, but maybe that only applies to smaller go-fast boats.

Suggestions?

 

[captcoop]

Prop pitch

If you change one or the other I would change the pitch. The cup helps hold water against the blade. If your engines turned the props before I would suggest a tuneup. Just because the engine will turn the no load RPM does not mean there isn't a problem, it only means that the governor is set properly. Good Luck

 

[JimP]

Prop

Captcoop, I bought the boat last year and it never hit max load rpm. Even after tune up. Had the bottom redone this winter but that did not change the rpm issue.

Spoke to the previous owner and he says he never went over 2100 rpm so max load rpm was not important.

After reading recent posts about needing to allow an engine to "breath" easier I think I want to get the loaded RPM back to where Detroit designed it.

 

[Genesis]

Remove the pitch

I'd remove 1" of pitch, and leave the cup alone.

 

[Genesis]

Oops...

Make that 2" - you say you're 140RPM short.

Take off 2" of pitch if you can.

Be aware that if the props have been repitched before this is not necessarily "safe" in that props can't be bent back and forth indefinitely before metal fatigue gets you.....

 

[captcoop]

Prop pitch

I think that you can also cut an inch off the diameter and get the same results and you may be able to keep some of your speed. I agree that 2 inches is tough to bend especially if the props are nibral.

 

[Genesis]

Eeeeek....

Be extremely careful cutting off the diameter.

Props on boats like this are highly loaded. The blade area must be sufficient for the load applied, or you will get cavitation which will rob you of speed and also cause erosion damage to the prop surface.

If you're going to cut diameter you need to talk to someone who can run the calculations to see what kind of blade loading you have now, and whether doing that is safe or not.

 

[jim rosenthal]

I agree with G...

I would not take diameter off the wheels...I would adjust the pitch. Also, check the rest of the drivetrain (cutlass bearings, struts, etc, is everything in line and is there minimum friction at rest?). You may be losing a little to frictional losses in the bearings or misalignment.
You need the blade area to absorb the engines' power and move water to push the boat. Removing blade area may increase the loading on the remaining area. Probably not a good idea, plus if you ever need to sell the props or trade them you will have a difficult time getting someone to take them.

 

[longshanks]

What model are we talking about...

I have a 42C w/ four blade 24X24. Seems also I have the same power plant GM/J&T 6-71 (310hp/side) with 2650 rpm no load. Under load with the four blade (cupped) wheels I turn up to the recommended 2350rpm on both sides... provides about 17.5knots... hope this is useful

 

[longshanks]

Correction...

I get 17.5 knots at cruise rpm of 2100; @ 2350 and flat calm it maxes out at around 19.4.

 

[longshanks]

One more correction...

:\ I made a typo, sorry! My 42C has 24x26 Four(4) blade wheels.

 

[JimP]

Performance

At WOT I'm turning 2365 RPM vs nameplate loaded rpm of 2500. My 6-71's are pumped to 410 hp.

Even though my engine rpm is low, the boats speed is good. I get 23 knts at 2050 and 27 knts at 2365.

 

[Genesis]

DO NOT allow low RPMs on diesels!

You WILL blow things up that way.

You're short enough that you need to find and correct this problem immediately. The result of not doing so will be dramatically shortened engine life.

You're in the "high performance" category with those engines - nearly 1.0hp/cid of displacement. If you can't pull 2500 RPM find out why.

 

[jkp1]

Re: Propeller Pitch

Ditto on the above.
I was at Hale Propeller here in CT and ran your situation by them. They plugged it into their computer and came up with a target (cupped) design pitch of 24.12". Genesis was correct with his recommendation of 2" pitch removal. They also recommend against reducing diameter, removing cupping or EVER painting a bronze prop. I was shown a painted prop where electrolysis made a mess of it.

Only other thing I can add is to make sure your tach readings are accurate before you spend the $ on prop modifications.

 

[JimP]

Propellers

Ok, I'm sold on dropping pitch 1-2".

If I shed 2" of pitch and it's a bit more than required, my loaded WOT may come in at 2550-2600 RPM. To me this seems worst than just shedding 1" but only getting the WOT up to 2450. Are there any risks to under propping and over reving? Also, any ideas what I may lose in speed by dropping pitch?

 

[Genesis]

Better overrev than under

There is no harm in overrev - the fool with the throttle has complete control over that.

You will likely lose a knot or so of cruise speed. You want to be 50 rpm over spec ideally when heavily loaded, but with a clean bottom - this gives you a bit of slop for when the bottom is less than clean, or there is some "stuff" on the running gear.

Being 50rpm short isn't fatal - unless you run at WOT, in which case it will be very damaging...

 

[jim rosenthal]

proper sizing

None of us run at WOT routinely...or if we do, we replace the engines frequently. Even the trawler guys don't run at WOT, and their engines are derated singnificantly from what we have.
Basically as I understand it, this is all about how much work you are asking the engine to do, how much load it is handling. If the boat is propped so that the engine can't reach its' rated rpm, the engine is having to struggle with that load all the time, even at lower revs, and that's what leads to short engine life. That's why it's better to prop the boat a little "loose"- it leaves you some room for load increases which inevitably arise from growth on the bottom, which increases drag and power loss, and additional weight on the boat, from things like belongings and filled tanks.
Periodically I go through my boat and ask myself if all the stuff on there is really necessary. Usually I find 100 lbs of stuff that isn't. Boats, like people, benefit from periodic weight reduction regimens.

 

[JimP]

It's Fixed!

Thanks for all the advice, you guys are great!

Thought I'd fill you in on how things turned out. Picked up my props this morning and was a bit shocked when I was told the shop took out 2.5" of pitch. Yes, I now have 24x23.5 cupped props. Had a diver put them on and immediately went out for a test run.

Port WOT is now 2500, Starboard is about 2475. Speed at WOT dropped 1.25 knot. Engines seem to run much smoother and sound better. A side benefit I was not expecting is it's now easier to dock because there's less tendency to "dig" when shifting gear.

A final question for the group - what's the optimum cruise RPM for a 6-71 TI? I'm sure it's somewhere between 2000-2300, I'd just to to poll the group to see where others are running.

 

[Genesis]

2100 RPM

6-71s should be run around 2100 RPM, assuming you can reach the full 2500.

6V92s should be run around 2000-2050, assuming again you can reach full 2300.

What you do not want to do is run them too slow; you WANT boost. If you're off the boost curve the engines will load up and the EGTs will be way too high.

If you have EGT meters you can see where the "sweet spot" is....

 

[jkp1]

Exhaust Gas Temperature meter?

Genesis,

How do you use an EGT meter to determine the "sweet spot"? I was thinking of installing a Flowscan for the same purpose but decided they're too expensive for what they do.

EGT meters don't cost that much and if they can confirm the location of that ever elusive sweet spot, it may be money well spent.