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Old 12-17-2015, 03:16 PM   #21
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Tach inacuracy

I searched around and found an old survey that was done on my boat in 2005. The surveyor used a digital tach and compared it to the installed. At 2200 rpm the installed tach was low by 176 rpm ( digital tach was 2376) and at full load the installed tach read 2625 while the digital read 2713.
So this tells me two things, one the installed tach is very inaccurate as many of you have been saying and two with the original prop the boat had achieved the max rpm rating of the engine.
The prop on the boat now has one degree more pitch than the original, also according to Michigan prop it has a #3 cup. So I'm wondering how much effect the cup value has on engine max rpms.
From some research I have done .1 degree of cup equals the same as adding 1 degree of pitch. So does a #3 cup add 3 degrees of pitch?
Seems like going back to the original prop may solve the issue.
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Old 12-17-2015, 04:58 PM   #22
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Mike:


That is a question for a prop guy, but I can speculate ;-).


So the prop that gave 2,713 true rpm at wot has 1 degree or 1" less pitch and more or less cup, we don't know. I know that 1" more pitch causes about 100 rpm drop in wot rpm. I also remember from boatdiesel that cup only affects rpm about 50.


Cup is a slight inward turning of the trailing edge that limits cavitation. The # values are degree of inward turning. My prop has a #1 cup and is barely visible.


So if you are now seeing 2,200 rpm at wot which is (we think) 2,376 true rpm and if you adjust for 1" more pitch then with the old prop you might be at 2,476 true rpm. The cup differences might add or subtract maybe 50 rpm. You seem like you are a lot closer to where you need to be.


This is what I would do, in addition to the 2-3 simple tests I have outlined previously:


Get your spare (the old prop) scanned and trued so you really know what it is. A prop shop will charge $300-400 for this work (well maybe more in Ct. Ct yard work is outrageous). Then do a quick haul or get a diver to swap it with the one on it now.


Then once you have done the other tests and calibrated your tach, see what the scanned prop will do at wot. If it is really now under 2,600 (I think that this is your rated rpm), then for another $300-400 you can get the prop you took off repitched to pick up the presumed (from the above analysis) couple of hundred rpm shortfall.


Your entire problems may be differences in prop, tach calibration, a slightly fouled bottom and weight added over time, and could have nothing to do with the engine.


David
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Old 12-18-2015, 07:43 AM   #23
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"Seems like going back to the original prop may solve the issue."

Just what "issue" are you trying to solve?
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Old 12-18-2015, 08:40 AM   #24
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Low RPM at WOT

The problem I am trying to resolve is getting the engine to full RPMs at WOT. Currently indicating 2200 (installed tach) engine rated at 230 HP @ 2600 RPM.
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Old 12-18-2015, 11:09 AM   #25
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Quote:
Originally Posted by djmarchand View Post
Mike:

This is what I would do, in addition to the 2-3 simple tests I have outlined previously:

Get your spare (the old prop) scanned and trued so you really know what it is. A prop shop will charge $300-400 for this work (well maybe more in Ct. Ct yard work is outrageous). Then do a quick haul or get a diver to swap it with the one on it now.

David
Mike
If you are looking for a prop shop to do the work David suggested - I have had great luck w/ Gabe at Presision Prop in Newburgh NY
He checked and cleaned one for me and I think it ran $100 a few yrs ago.
I've seen some of his rebuild work and he truly works miracles.
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Old 12-18-2015, 01:53 PM   #26
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"The problem I am trying to resolve is getting the engine to full RPMs at WOT."

Is that because you wish to operate at WOT most of the time?
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Old 12-18-2015, 03:22 PM   #27
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Quote:
Originally Posted by FF View Post
"The problem I am trying to resolve is getting the engine to full RPMs at WOT."

Is that because you wish to operate at WOT most of the time?
FF, I don't think going down the "being over propped is ok" road is a good discussion at this point. These boats generally are operated on plane. They are extremely tender off plane. And running a turbocharged engine over propped like this under significant load(not necessarily WOT") WILL harm the engine. I think your arguments as they relate to slow turning lightly loaded diesels are valid as long as you understand what you are doing. I don't think they are valid here.
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Old 12-20-2015, 07:25 AM   #28
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Any highly loaded engine will be far safer with an EGT gauge ,

at only $100 or so (plus install) finding out how hard the engine is actually working might prove helpful., before and after a prop swap.

But I would start with an accurate tach first.
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Old 12-20-2015, 09:07 AM   #29
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Capt Mike - I own a 2000 Mainship 30 soft top with my 210hp Cummins 6BT. Below is the Seatrial data for your information.

SPEED TRIAL DATA:?

Fuel ¾ full

Engine hours 447.62

Water tank: ¾ full

rpm Oil (psi) TEMP (F) *S.O.G. (Kts)

600 (Idle) 30 140 3.8

1000 45 145 6.2

1500 50 150 8.1

2000 55 160 11.8

2200 55 180 13.4

2500 55 180 16.2

2650 (WOT) 55 180 17.7

2650 (WOT OPP) 55 180 18.1

*Speeds were obtained without the influence of trim tabs. Noted: Utilizing trim tabs at approximately 2250 RPM’s +.5 knots was gained in trimming the vessel.

The engine turns a 1 1/2 inch stainless steel shaft through a TWIN DISC 2.39:1 ratio reduction gear terminating in a HYTORG “22RH27”, “H-CUP” four (4) bladed propeller.

My vessel in on the hard a few towns over from you at Shennecossett Yacht Club, if you ever want to compare notes. Over the few years I have owned the boat I have rebuilt the exhaust, installed an EGT gauge, serviced injectors etc.
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