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Old 10-13-2011, 10:37 PM   #1
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Your speed at idle

At idle (minimum RPMs), the Coot moves at about 4 knots, slightly over half maximum speed of 7.8 knots.* What's with your boats?


-- Edited by markpierce on Thursday 13th of October 2011 10:39:38 PM
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Old 10-13-2011, 10:47 PM   #2
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Your speed at idle

We had our props pitched down an inch a few years ago so now our idle speed through the water (as measured by our knotmeter, however accurate that is), is about 2.8 knots.* However I suspect it's a wee bit faster than that, probably more like 3 knots.


-- Edited by Marin on Thursday 13th of October 2011 10:48:00 PM
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Old 10-13-2011, 11:52 PM   #3
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RE: Your speed at idle

Why?
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Old 10-13-2011, 11:59 PM   #4
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RE: Your speed at idle

Quote:
Daddyo wrote:
Why?
*Why what?
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Old 10-14-2011, 05:01 AM   #5
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Your speed at idle

My Albin runs at about 2 knots at idle. Idle rpm is around 550

My "ex" Mainship ran at 4.2 after I repowered and that was too fast many times in close quarters.


-- Edited by jleonard on Friday 14th of October 2011 07:36:26 AM
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Old 10-14-2011, 06:57 AM   #6
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RE: Your speed at idle

About 3kts... I have driven big sportfishes that idle around 8-9kts....pretty freaky.
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Old 10-14-2011, 07:27 AM   #7
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RE: Your speed at idle

We're over 4 knots at idle.
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Old 10-14-2011, 07:30 AM   #8
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RE: Your speed at idle

Mark

Assuming your idle* is about 650 RPM, either your prop is "too big" or the gearbox ratio is incorrect. I'm in same idle speed range as Marin and Jay - with oversized twins! *

If this subject were on boatdiesel.com the question would be "Can your vessel*attain full rated RPM without overheating?"
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Old 10-14-2011, 07:31 AM   #9
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RE: Your speed at idle

It's the "cruise prop" thing. Being over propped is like being in high gear. Also most engines idle at about the same rpm but top out from 2500 to 3800. If you have a boat that idled at 800 and topped out at 3800 AND was propped for WOT at 3800 you'd be going very slow at idle. With a 2500rpm engine that is over propped and w an idle speed of 1000rpm you'd probably make 5 knots at idle. A higher gear ratio also would tend to make your idle speed of your boat high. But probably the greastest element causing high idle boat speed is the fact that the power required to push our boats is VERY nonlinear. More so w full disp hulls like the Coot. To push a 40' FD hull 2 knots may only require 2hp and to push a TT w it's big square and deeply submerged transom may require 8hp.....4 times as much.
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Old 10-14-2011, 12:15 PM   #10
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Your speed at idle

Quote:
sunchaser wrote:
Mark

Assuming your idle* is about 650 RPM, either your prop is "too big" or the gearbox ratio is incorrect. I'm in same idle speed range as Marin and Jay - with oversized twins! *

If this subject were on boatdiesel.com the question would be "Can your vessel*attain full rated RPM without overheating?"
The JD 4045D engine's*idle is 750 RPM and max is 2400 RPM.* (Governor*keeps the engine from reaching its rated maximum of 2500.)*The 1650 RPM difference is low compared to some other diesels having max RPMs in the 3000-range.* At max RPM, the boat barely exceeds hull speed.* The Coot displaces a lot of water for its size (35 feet LOA)*as it weighs 14 tons.* If prop was repitched for slower speeds at 750 RPM, I wonder/doubt if hull speed could still be reached.* The higher boat speed-at-idle "issue"*has only come into play when I'm in the marina heading for the berth.**For much of the time*I'm out of gear to slow down, but am*mostly in gear when making turns.* Engine cooling water temperature doesn't climb above the*normal 180-degree temperature at max throttle.


-- Edited by markpierce on Friday 14th of October 2011 12:35:03 PM
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Old 10-14-2011, 12:52 PM   #11
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RE: Your speed at idle

Mark--- Your engine's rpm range is about the same as the old Ford Lehman 120, only the FL120's max governed rpm is 2500. And our idle rpm is more like 600 or so. If your JD engine can achieve its 2400 rpm at full throttle with the prop you have now, conventional wisdom says you have the right prop pitch and diameter. If it can't then you have the over-propped condition that Eric described. Over-propping is not necessarily bad--- Grand Banks did it for decades. Over-propping can give you a bit more cruise speed at a given rpm and as long as you don't run hard and lug the engine--- Eric's high gear in a car scenario--- it's a valid way to get more efficiency out of the boat.

While our boat no longer had its original three-bladed props when we bought it, the four-bladed props it had were set at the original props' pitch settings. Since the props were flour blades, this overpropped the boat even more. At full throttle the engines turned at about 2200 and 2300 rpm respectively. So we had the props completely reworked and pitched down an inch from the original prop specs.

If your engine cannot achieve 2400 rpm with the prop pitched as it is now, you might want to talk to a good propshop to determine if you wold be better off with the prop pitched down to the point where the engine will achieve 2400 rpm. If you can get 2400 rpm with the prop the way it is now, then I guess you're about where you should be unless you want to pitch down for a slower idle speed. This means you'll need to run at a somewhat higher rpm at cruise. But with the prop pitched down, the engine won't be working as hard at that higher cruise rpm as it does now at that rpm.

Everything's a trade-off.
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Old 10-14-2011, 12:58 PM   #12
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RE: Your speed at idle

Another approach is to remove some pitch to keep your idle and "harbor" speed down, then add some cup to reduce your WOT rpm.

This has been discussed in boatdiesel in the past and is successful in certain situations. That was actually what I had done with the repowered Mainship I had.

*
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Old 10-14-2011, 09:35 PM   #13
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RE: Your speed at idle

I think most propeller people would'nt recomend a cuped propeller on a trawler unless it runs faster than most.
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Old 10-15-2011, 11:50 AM   #14
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RE: Your speed at idle

About 5 knots, I wish it were less. I usually come into the marina with only one engine in gear to keep it down to 2-3 knots.*
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Old 10-15-2011, 01:26 PM   #15
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RE: Your speed at idle

When I want to go really slow I just keep bumping it in and out of gear as necessary. I don't find that to be any objectionable amount of work or activity and if it dos'nt go on too long I actually feel more in control as I'm not trying to stay in gear. And as Long as one moves the lever quickly I'm sure the clutches are not suffering at all.
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Old 10-16-2011, 01:57 PM   #16
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RE: Your speed at idle

With a clean bottom, we idle at 5.5kn (650rpm). It is a bit too fast and I've already taken an inch of pitch out.*I've learned to live with it and just bump in and out of gear when manoevering.
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Old 10-16-2011, 02:19 PM   #17
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RE: Your speed at idle

Quote:
nomadwilly wrote:
When I want to go really slow I just keep bumping it in and out of gear as necessary. I don't find that to be any objectionable amount of work or activity and if it dos'nt go on too long I actually feel more in control as I'm not trying to stay in gear. And as Long as one moves the lever quickly I'm sure the clutches are not suffering at all.
*That's what I do except I try not to move the lever too quickly (maybe that's what you meant?)
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Old 10-16-2011, 02:20 PM   #18
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RE: Your speed at idle

***

*** Almost 6..... I am use to bumping in and out of gear
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Old 10-16-2011, 03:07 PM   #19
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RE: Your speed at idle

Good analysis, Eric.
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Old 10-16-2011, 03:43 PM   #20
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RE: Your speed at idle

Instead of having the idle rpm*set*at 800, I have it set around 700 which produces around 3.4 kts, a nice maneuvering speed.* The engine still works good at this setting.

Doing this, of course, is very boat/engine/propulsion system specific, but it works well for this application.
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