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Old 01-25-2013, 08:15 PM   #21
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Jim--- For a given engine/shaft rpm, the finer the prop pitch the less water it moves and the slower you go.

Our marina has a "5 Knot or No Wake" speed limit. Or it might be 4 knots--- I never look at the sign anymore.

Our engines idle about about 650 rpm (1973 SW tachs are not the most accurate instruments on the planet).

Before the props were re-worked, our speed at idle was just under 4 knots. With the props correctly set and pitched down an inch our idle speed is 3.2 knots or thereabouts by the paddlewheel sensor. Anyway, whatever the numbers, it's noticeably slower.

Now we never had to bump in and out of gear before, but the slower idle speed we have now is nice. We carry that power right up until we're turning into the slip.

As to the engine loading, putting a finer pitch in the prop blades is a bit like dropping to a lower gear in a car when you go up a hill. You don't go as fast but the engine is working less at 2000 rpm in third than it is at 2000 rpm in 4th.
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Old 01-25-2013, 08:17 PM   #22
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Overpropping is for the educated cruiser...if you know what you are doing then that's that...for newbies...bad idea.

For those that say "no" in all instances have to remind themselves of their own limitations before they impart those limitations on others.
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Old 01-25-2013, 08:29 PM   #23
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Jim--- For a given engine/shaft rpm, the finer the prop pitch the less water it moves and the slower you go. Our marina has a "5 Knot or No Wake" speed limit. Or it might be 4 knots--- I never look at the sign anymore.

As to the engine loading, putting a finer pitch in the prop blades is a bit like dropping to a lower gear in a car when you go up a hill. You don't go as fast but the engine is working less at 2000 rpm in third than it is at 2000 rpm in 4th.

Marin,

Then I generally had the right idea on your point. Somehow though in my mind, at cruising speed and plenty of extra horsepower, the greater rpm of the diesel (with reduced pitch) would have more effect on the fuel burn than the loading factor.
In my particular case, I do not do many 100's of miles per year, so the difference of a few extra gallons of diesel is as much a benefit as a negative in that I can keep fresher fuel.
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Old 01-25-2013, 09:15 PM   #24
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My guess is that the fuel savings from the reduced load on an engine due to some pitch being taken out of the prop(s) is negligible, as you say. Depending on the kind of engine the reduced load might be beneficial to its service life. Or not.

The biggest benefit in your case sounds like it would be the reduction in idle speed.

Grand Banks boats were all over-pitched from the factory, at least in the first few decades. This made all kinds of sense given the power of the engines and the relatively low cruising speeds people went with them. Like driving in a bit too high of a gear, over-pitching can give you more speed for a given rpm, so as long as an engine is not overworked in this process, you get a more economical boat at cruise.

On the recommendation of the prop shop we had our four bladed props (not original to the boat) pitched down an inch not to lower our idle speed but because the shop said that as a general rule, for the same diameter prop on a given boat, a four-bladed prop should have one inch less pitch than a three-bladed prop to get the same performance and economy. The four-bladed props on our boat were set--- badly--- to the same two pitches the original three-bladed props had been.
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Old 02-02-2013, 11:11 AM   #25
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I also have turbo volvos tmd41a. The only issue I see with the 2000 rpm cruse is that you are probably not creating any boost. I produce some smoke until I get over 2200, I typically cruse at 2500. In one of Tony Athens articles he references an approximate 10% increase in efficiency on an engine with boost. Turbo engines should cruse in a rpm range where they produce boost or the turbos carbon up I believe.
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Old 02-02-2013, 12:53 PM   #26
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Delta JimS,
You said your Volvo "tops out at 3600rpm". Something very important you haven't mentioned is what your engine is rated at. RPM wise. At what rpm does your engine develop it rated power. That is the rpm that it should attain at WOT. And one should check to make sure the throttle and cables are advancing the fuel injector to max. One should change pitch to achieve rated rpm.

That was good information from Third-Reef but his cruise rpm may be a way different loading on your engine at the same rpm unless you both get the same rpm at WOT.

Quite a few mechanics support over propping but I've never heard an engineer say anything positive about it.

If your boat is way over powered and over propped you will save some fuel if you stay under loaded and over propped but it's hard to tell when you are and when you aren't. You may burn LESS fuel by rated propping w higher rpm and less load. In most all cases you'll experience more noise. And I think the engine may last longer making more noise. MANY here disagree w me here so be amused by this conversation and seek an ENGINEER'S opinion. I think over 95% of them will say prop to rated rpm. Talk to the engine manufacturer or an engineer at the distributor. Very few commercial applications (perhaps none) will ever resort to over propping.
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Old 02-02-2013, 01:45 PM   #27
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Thanks Eric,

Appreciate your input. Based on my own understanding and input from Marin and Third-Reef, I am definitely in favor of trimming back my pitch. Hope to get lower idle speed as well.

I have included a scan here of the engine datasheet for the TAMD40B.(if it is hard to read, I can send you a better copy if you provide an email addr.)
As you can see, it states "Propeller Shaft Output" at 3600 rpm. I have never taken her over 3500 rpm frankly because that rpm still makes me nervous enough at this point. I still have some throttle arm left to go, so I have no doubt she will do that. Like I mentioned, I have plenty of power out of that diesel and am plowing water big-time at that point at over 9 kts.

The other reason for not running above 3200 much is because of that bad vibration that I attribute to the prop balance - she is shaking pretty hard at that point.

TR makes another good point to support my upping the rpm by reducing the pitch which will allow running more in the Boost mode while cruising. I certainly agree with that approach. For the limited amount of yearly cruising that I do at this point, fuel burn is a lower consideration. It would actually be a plus to be able to run fresher fuel.
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Old 02-02-2013, 02:46 PM   #28
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If it were mine I'd definitely take her to WOT just long enough to stabilize the rpm so you know what it is. I'm not there so don't know how bad the vibration is so be very cautious. If the vibes are really bad you should fix that (at least to a tolerable level) before going to max rpm.

No I can't read it. You should also go to WOT in neutral gear w no load at all. If you've never heard of checking the rev-limitter (governor) it may sound crazy but like the full throttle full load test this should be done carefully and only top out enough (up to about 300 maybe 400 rpm). If you reach 400rpm over the rated rpm your governor may not be working right. Ideally it should top out about 200 above rated rpm.

Then go to a good prop man and tell him "I'm getting ___ rpm and this is my prop. My engine is rated at ___ rpm. What can we do to make the WOT ____rpm?"
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Old 02-02-2013, 07:13 PM   #29
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Then go to a good prop man and tell him "I'm getting ___ rpm and this is my prop. My engine is rated at ___ rpm. What can we do to make the WOT ____rpm?"
This is some of the info the prop shop we use asked for. So we took the boat out and got the numbers they wanted. Most Grand Banks were over propped-- deliberately-- by the manufacturer. We still are over propped a bit, I think. We had an inch taken out of the props because they are four blade instead of three blade but had been pitched (badly) to the boat's original three blade specs.
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Old 02-02-2013, 08:26 PM   #30
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Delta Jim, How clean is your running gear? Is the pic of prop and rudder before or after cleaning? There looks to be remains of barnacles on the prop, and barnacles on the rudder.
I recently hauled out, not for antifouling, the hull a/f was near perfect but the shaft/prop/skegs were heavily fouled with shell, a yard had used the same soft a/f as on the hull. I cleaned up and applied primer and hard a/f which cured vibration and loss of speed that had gradually developed. That could be your main problem, but a prop tune up can`t hurt.
I may apply Prop Speed to the running gear next time.
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Old 02-02-2013, 11:59 PM   #31
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Bruce, I had Propspeed put on the running gear for the fist time last time I hauled and antifouled, and the Micron Extra still seems very smooth when rubbed, and the running gear is not showing vibration at all, and more importantly the speed for given rpm is as good as it was when fresh give or take a rev or two, yet she has not been re-done since winter 2010. I'd swear by it now, remembering what used to happen with just hard a/f on the gear.
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Old 02-03-2013, 12:25 AM   #32
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Bruce, I had Propspeed put on the running gear for the fist time last time I hauled and antifouled, and the Micron Extra still seems very smooth when rubbed, and the running gear is not showing vibration at all... I'd swear by it now, remembering what used to happen with just hard a/f on the gear.
Thanks Peter. I noticed the running gear when the boat came up to fix damage in king tides against the shipwrights fixed wharf (their cost). With planning or more time up, I could have gone to propspeed, it`s good knowing first hand it really works. My 2 coats of hard a/f should last a while.
I am very pleased with Jotun Seaguardian on the hull. No reason to touch it after 17 months, my best a/f experience ever.
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Old 02-18-2013, 08:11 PM   #33
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Al, Thanks for the response, do you have a record or memory of what speeds you get at what RPMs?
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Old 02-26-2013, 07:46 PM   #34
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Al, Thanks for the response, do you have a record or memory of what speeds you get at what RPMs?
We've been out of the water the last 2 years so this is from memory...

1750 rpm = 8kts (approx) this is what I normally cruise

2000 rpm = 9kts(approx)

2200 rpm when I can get it = 9.5 to 10kts and NOBODY is happy
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