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

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

Why?

Daddyo wrote:
Why?

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*Why what?

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

About 3kts... I have driven big sportfishes that idle around 8-9kts....pretty freaky.

We're over 4 knots 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?"

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.

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?"

Click to expand...

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

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.

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.

*

I think most propeller people would'nt recomend a cuped propeller on a trawler unless it runs faster than most.

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.*

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.

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.

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.

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*That's what I do except I try not to move the lever too quickly (maybe that's what you meant?)

***

*** Almost 6..... I am use to bumping in and out of gear

Good analysis, Eric.

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.

Sharkey wrote:
***

*** Almost 6..... I am use to bumping in and out of gear

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*Sharkey:

I'm sure it is posted somewhere here, but I'm too lazy to search. What make/length/model is your boat? Love your avatar/helm picture.

What make/length/model is your boat?

*

65' Fleming.....

I'm not the owner, I am the full time captain.

*


-- Edited by Sharkey on Sunday 16th of October 2011 07:15:47 PM

ktischler wrote:
... I usually come into the marina with only one engine in gear to keep it down to 2-3 knots.*

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*Ah, yes...another good reason for twins!* I'm about 3kts with both in gear, 2 kts with one in neutral.

3.2ish kts at idle

*

In close quarters I cannot run with just one in gear without getting on the thruster .... And with both in gear I start to*build a pretty sizeable wake...... I need to be moving really slow anyway, hard to stop 50+ tons.........................

*

*

*

A fellow in our marina with a 55' Mikelson cannot idle slow so is on his thrusters all the way in from the no wake sign. Trolling valves are a*popular option for many big sport fishers*- I'm surprised a 65' Fleming doesn't have them or a good prop /idle speed balance, given Tony's dedication to perfection at spare no cost. On 55's I'm familiar with, there seems a less than 5 knot harbor speed.


-- Edited by sunchaser on Monday 17th of October 2011 08:03:04 AM

If I came into our marina on both engines I would get booted.... I don't use the thrusters much, just the props and no touching the wheel.

*

*

Sharkey, what kind of horn(s) you got to warn others you are coming?

I come in very slow. I have never needed to sound off at anyone..... Duel airs scary the heck out of everyone ,,,

I do a prolonged*signal when entering and exiting*my marina in case someone is hidden behind the other side of*breakwater.