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Old 02-03-2017, 11:33 AM   #1
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Will engine meet rated RPM if boat in gear secured to the dock?

Or will it only reach it's max rated RPM moving through the water?

Will engine reach max rated RPM if the hull is severely fouled?
I know it will go slower.
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Old 02-03-2017, 11:38 AM   #2
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No and no.

Tied to the dock will lug the engine down. No way will it make rated rpms. And a good chance you will tear the cleats off the dock, depending on how big the engine is. Static pulls like that involve a LOT of force, much like putting your car against the wall of your garage and flooring it in gear.

Fouled prop and bottom will also drop full power rpm.
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Old 02-03-2017, 11:45 AM   #3
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Thanks, what % drop of engine RPM would you expect say if prop was clean, hull fouled?
If my engine max is 4000, would it only be able to do 3000, 2500? etc...
I mean how bad would it be.
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Old 02-03-2017, 11:57 AM   #4
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Thanks, what % drop of engine RPM would you expect say if prop was clean, hull fouled?
If my engine max is 4000, would it only be able to do 3000, 2500? etc...
I mean how bad would it be.
I performed some Bollard Tests on my displacement trawler Boomarang several years ago when evaluating different propellers. This is what I found:

High Idle RPM: 4250
Engine Rated RPM: 3800
Bollard Pull RPM (Prop #1): 3300
Bollard Pull RPM (Prop #2): 3050

Prop #1 was a 3-blade
Prop #2 was a 4-blade - same diameter, 1" less pitch but more blade area

Both props delivered rated engine RPM+ at WOT.

This is not meant to be scientific in any way, just my observations.
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Old 02-03-2017, 11:59 AM   #5
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As an old engineer friend of mine use to say "its all relative" So it all depend on the level of fouling, in the end the only way to know is haul the boat and get it fully de-fouled!
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Old 02-03-2017, 12:03 PM   #6
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Larry's results match my own experience.

Also prop fouling has much more effect than hull fouling underway. On a sailboat, a fouled prop would drop wot rpm by 500-800 rpm. When the hull was fouled (but never badly) it would only drop a few hundred rpm.

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Old 02-03-2017, 12:30 PM   #7
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................ And a good chance you will tear the cleats off the dock, depending on how big the engine is. ..........
A friend of mine "accidentally" hit full throttle while tied to the (floating) dock. He didn't tear off any cleats but the force of the water pushed the plastic float out from under the dock and everybody watched it float across the marina to the other dock.
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Old 02-03-2017, 12:51 PM   #8
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No, it won't reach full rpm with fouled hull and prop.

As to the dock routine and full rpm at the dock I only have to ask "why?" If it does, it's a great way to find yourself with a huge insurance claim and/or dock repair bill.
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Old 02-03-2017, 01:05 PM   #9
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Actually never done full rpm tied to the dock. I have gone to 1500 RPM for a short time.
The 3/4 inch nylon lines were very tight.
I partly did it to clear the slip of muck and run the engines in gear to look at the prop shaft spin at the trans coupler after I adjusted it.

My question relates to hull fouling and how that affects engine RPM's and if tied to a dock.

If this would allow an engine to reach max rpm tied to a dock, it would be a way of testing engine power in combination with a given prop.

How much pulling force do you think is involved?
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Old 02-03-2017, 02:21 PM   #10
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100% slip

THis is a question that typically only tugboats and drag racers are worried with.

I've done prop slip measurements at all kinds of engine RPM's, but only with a "free" hull.
If you are determined to get an idea of rpm at rest, simply go full throttle at a dead stop. That initial rpm will be close.
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Old 02-03-2017, 09:35 PM   #11
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[QUOTE=sdowney717;5 to run the engines in gear to look at the prop shaft spin at the trans coupler after I adjusted it.[/QUOTE]

I would say that this test is probably not an accurate gauge of actual conditions when underway.
Your boat is engineered to move forward in response to the propeller's input, and bollard pull has little to do with this.
Depending on how the vessel is tied off, hull stress patterns will be very different than when actually underway.
Measuring the "squish" of the motor mounts when underway will shed some light on what your coupling is being asked to do.
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Old 02-04-2017, 07:55 AM   #12
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When selecting the prop the cruise speed and desired engine cruse RPM are considered.

If the boat is expected to cruise at 6 -9 K the prop will be selected to accelerate water at that speed.

0 speed will overload most diesel cruisers , and even the sport fish killers with huge engines will belch a black smoke (overload) on a heavy throttle start.
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Old 02-04-2017, 09:50 AM   #13
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Yes, doing these type of tests does put more than usual stress on the system. High loads at low rpm induces lots of torque. There are instances of shaft fouling with line causing an engine to spin off its mounts. I would hope that a free shaft won't do that much damage, but full throttle at zero speed tends to make lots of engine mount stress.
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Old 02-04-2017, 08:21 PM   #14
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I believe that is what the US Navy calls "Fast Cruising" as in tied fast to the dock. Its what you do a lot of if its a new boat or fail your operational readiness tests.
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Old 02-04-2017, 11:44 PM   #15
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No, it won't reach full rpm with fouled hull and prop.

As to the dock routine and full rpm at the dock I only have to ask "why?" If it does, it's a great way to find yourself with a huge insurance claim and/or dock repair bill.
I've done it. Did it a few days ago actually. Way down on rpm and w only 37hp no lawyers were called.
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Old 02-05-2017, 12:16 AM   #16
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I've done it. Did it a few days ago actually. Way down on rpm and w only 37hp no lawyers were called.
When I did it with Boomarang's 56hp, no lawyers were needed either. Now, with the new boat and over 600hp, no way. I'll look for other ways to measure like time to plane, rpm, speed etc.

Using the formula BHP x 0,9 x 1,10 / 100 = (tons of static pull), for fixed pitch propellers, a bollard pull with 56hp should produce about 1,265 lb pull. 600 hp would produce over 13,500 lb of pull which might be harder on cleats, docks etc.
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Old 02-05-2017, 12:20 AM   #17
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If you're wondering about your boat's capability, untie it from the dock and take it out from the marina.



Regardless, clean the hull and prop!
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Old 02-05-2017, 12:37 AM   #18
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An annual haul-out and bottom treatment has kept my boat free from poor boat performance due to vegetable and animal growth.

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Old 02-05-2017, 01:06 AM   #19
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Depends on the wheel, especially pitch. As noted above about tugboats. My dad used to tow logs all over Puget Sound and lower BC from just after WWII until the early 60's with a lot of different tugs, and I can well remember him saying about one boat or another, "she gets all her turns tied to the dock". Indicating likely being a good puller on logs which obviously are slow and hard to tow. The difference is between what they called a good "log wheel" or a good "scow wheel". He and other tug boaters also liked to engage in the occasional pulling contest and the same thing applied there.
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Old 02-05-2017, 02:45 AM   #20
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I've done it. Did it a few days ago actually. Way down on rpm and w only 37hp no lawyers were called.
Then you didn't do it at full rpm so you can't say you did it. You did it at reduced RPM, way down on RPM. You also didn't have any form of accident.
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