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Old 02-08-2013, 06:45 PM   #41
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It is not a toy boat surveyor's job to approve or disapprove anything on your boat. He should keep his opinions to himself unless you ask, otherwise his role is to document condition so far as can be determined by visual examination. Period.

Unless the surveyor is representing a class society and is conducting a survey related to the vessel's classification or statutory status he has no mandate to bless or condemn anything.
What is the proper procedure for doing a mechanical survey? I am most concered with the engine and transmissions condition than other systems.
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Old 02-08-2013, 06:48 PM   #42
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Old 02-08-2013, 08:57 PM   #43
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GB slightly overpropped all their boats back when the typical powerplants were one or two FL120s or FL135s. As they began putting larger and larger engines in their boats starting in the mid-80s i don't know if they continued the overpropping practice or not.
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Old 02-08-2013, 09:08 PM   #44
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Fred:

We have had this debate before on the merits of overproping. Let's do it again but with facts rather than opinions. Take a look at the Cummins spec sheet for the 6BT 210 hp engine at: http://www.sbmar.com/Engines/PDF/6BT...20Nov%2000.pdf

This is an engine used in some of our trawlers and is a real workhorse. The spec sheet is one of the few that has fuel consumption curves for both wot loads and prop loads which makes it easy to compare the two.

So look at the prop curve and you will see that 100 hp is produced at a bit more than 2,000 rpm and it takes about 6 gph or 16.7 hp per gph. That is the place many trawler owners would operate at fast semi displacement speed, say a GB 36 at 8+ kts.

Now overprop the same boat and engine severely so that the 100 hp is produced at about 1,200 rpm which is on the wot output curve. The fuel consumption is about 7 gph.

Now these are extremes and no one should think about overpropping that much, as you will quickly ruin the engine. And I have no doubt that overpropping a little will actually result in fuel consumption below 6 gph. But not by much- maybe to 18 hp per gph or about 5.5 gph.

So I think that a 10% fuel consumption improvement is all you are ever going to see by overpropping. In my mind that just isn't worth the risk of overloading your engine if you ever push the throttles forward.

David
David, is it correct that the only downside of slight over-propping (my case) would be experienced at WOT (never happens)? Or are there other issues at lower rpms I'm missing? Thanks.....
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Old 02-08-2013, 09:12 PM   #45
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Spy,
I'm going to assume you actually have an appropriate idle rpm. I think that propeller speed v/s boat speed affects the idle boat speed. Lets say you have a direct drive gear and another identical boat has 10-1 reduction. I think the 10-1 reduction boat's idle speed will be considerably slower than the direct drive. This is admittedly a very extreme example but sometimes extremes will make it obvious what small differences fail to show.

Could it be that you have a 1.5-1 gear? My W30 has a 2.57-1 ratio and has an acceptably slow idle boat speed. And I idle at 900. I have the idle set a tad high so I can get underway soon w/o any danger of my engine quitting while shifting gears. The engine idles nicely at 700 but I could back out of my slip and possibly into another boat if I set it at 700.

Eric:

I don't get what you are driving at. I think boat speed at idle has nothing whatsoever to do with gearing.

I have always related idle speed to full speed. For Example, if I have a boat with a full speed of 9 knots at WOT of 3600 rpm, and you have a boat with a full speed of 25 knots at WOT of 3600 rpm, I will expect your idle speed at 750rpm to exceed my idle speed at 750 rpm by the same ratio, 25:9. So if my idle speed is 3.5 knots, I will expect yours to be 9.7 knots.
This explains the guys in the fast sport fishermen who have to go in and out of gear to get below the no wake speed when coming through the marina.
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Old 02-08-2013, 09:24 PM   #46
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What is your idle rpm and speed?
850, ~3.8 knots
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Old 02-08-2013, 09:27 PM   #47
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Some of you might be interested in this thread that is running on an ag forum that I follow. It shows some videos taken inside the cylinder as the piston approaches TDC and at the moment of ignition. Its pretty cool that someone could even take the pictures, let alone what they show.
Diesel Combustion High Speed Imagery - The Combine Forum
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Old 02-08-2013, 09:32 PM   #48
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Spy,
I'm going to assume you actually have an appropriate idle rpm.

...

Could it be that you have a 1.5-1 gear?
I do right on spec at 850. I choose a 2.27:1. In hindsight, I should have chosen the 2.74:1. I was trying to hit a sweet spot at the 1800 rpm mark.
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Old 02-08-2013, 09:36 PM   #49
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Every marine diesel that I have seen specifies the maximum hp and a single rpm that goes with it. Which specific Volvo model do you have? We can look it up and advise.

If you look at the max hp curve (not the prop curve) of a common rail engine, they often are flat for the last several hundred rpm. But there is always a max rpm and a max hp.

David
D2-75. Every technical document says the same, 2700-3000. It is not common rail, but as a turbo engine it has a great flat torque curve in the usable rpms.
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Old 02-08-2013, 09:37 PM   #50
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Some of you might be interested in this thread that is running on an ag forum that I follow. It shows some videos taken inside the cylinder as the piston approaches TDC and at the moment of ignition. Its pretty cool that someone could even take the pictures, let alone what they show.
Diesel Combustion High Speed Imagery - The Combine Forum
That is way cool...
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Old 02-08-2013, 09:53 PM   #51
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David,

Do you suppose Volvo may be saying WOT rpm is blessed by them over a range of 300rpm? Meaning that 2700 to 3000rpm full load is OK?
Bingo. I think that when the Swedish VP engineers marinized the Shibura N844LT-C, their panties weren't so tight that they demanded some theoretical finite rpm.
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Old 02-08-2013, 09:54 PM   #52
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Eric:

I don't get what you are driving at. I think boat speed at idle has nothing whatsoever to do with gearing.

I have always related idle speed to full speed. For Example, if I have a boat with a full speed of 9 knots at WOT of 3600 rpm, and you have a boat with a full speed of 25 knots at WOT of 3600 rpm, I will expect your idle speed at 750rpm to exceed my idle speed at 750 rpm by the same ratio, 25:9. So if my idle speed is 3.5 knots, I will expect yours to be 9.7 knots.
This explains the guys in the fast sport fishermen who have to go in and out of gear to get below the no wake speed when coming through the marina.
I think your forgeting to take into consideration hull design, the design size and pitch 0f the prop. Also the fact that the fast boat is a planing hull and the slow one a displacement hull. Your engine wot is pushing aqgainst a brick wall so much power is wasted trying to over come hull speed. The fast boat isnt limited to that. When she is on step the boat is mostly out of the water reducing drag and making her go fast
another factor is prop slipage, kinda like stripping out a screw. For each revolution of the prop in a perfect world she will travel if a 19" pitch ninteen inches but add a load and she now travels maybe 16 inches. Lower pitched prop will rev up faster and may even give the same or close to the same top speed when compared to a higher pitch prop but at a higher rpm. You can use a smaller diameter prop to give the same effect as a lower pitch.

There are many many factors that govern idle speed over ground.
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Old 02-08-2013, 10:02 PM   #53
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Spy,
OK good ... your idle speed and/or gear ratio is not an issue here that would cause a speed too high at idle in gear. And I know you have a semi-disp hull so the ultra low resistance of a FD hull is not a factor either.

But 3.8 knots does seem fast. I can't remember what Willy does. Oh .. how much total hp do you have. Perhaps you're just over powered.
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Old 02-08-2013, 10:12 PM   #54
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Spy,
OK good ... your idle speed and/or gear ratio is not an issue here that would cause a speed too high at idle in gear. And I know you have a semi-disp hull so the ultra low resistance of a FD hull is not a factor either.

But 3.8 knots does seem fast. I can't remember what Willy does. Oh .. how much total hp do you have. Perhaps you're just over powered.
Dosen't the 3.8kn at 850rpm simply reflect the fact that drag factors are at a minimum? Trawlers don't usually rev very high do they? Don't most people stay under 2200rpm. If Spy has a large diameter high pitched prop at this low speed slipage will be minimal as well as drag factors resulting in high kntph.
This is all new to me as most of my experiance has been with go fast vessels that rev up to 4-6000rpm
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Old 02-08-2013, 10:13 PM   #55
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850, ~3.8 knots
I'm 700 rpm and 3.8 knots. I don't have a problem with the 3.8 as I just kick it in & out of gear when in the marina. 850 does seem a tad high, though.
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Old 02-08-2013, 10:16 PM   #56
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Can't troll for salmon at 3.9 knots. That's the big problem.
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Old 02-08-2013, 10:28 PM   #57
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Keith wrote;

"I don't get what you are driving at. I think boat speed at idle has nothing whatsoever to do with gearing."

I haven't read of this by any expert but it's a theory of mine based in seeing numerous propeller thrust curves. I think as you back off on the throttle the thrust for a bigger slower prop drops at a faster rate that smaller faster props.

Imagine an example of two very heavy boats the same except that they are geared quite differently. Say one has 1.6 -1 reduction and the other has a 2.9 - 1 ratio. Both run at 2500rpm at WOT and at that rpm the lower geared boat that has more thrust and is faster (for that reason) than the smaller propped counterpart. But if you measured the thrust at a reduced rpm like 1500 to 1700rpm I think the larger prop/lower geared boat would loose more (as a percentage) of thrust than the smaller/faster prop boat. The low geared big prop unit at half speed should have less thrust.

The performance of big prop boats is better at full speed but looses efficiency at lower speeds. The performance of higher speed prop units (like a smaller outboard) is less totally but the performance over-all is more uniform. The smaller prop OB needs flexibility very badly as it's not even known what boat (load) it will be asked to perform on.

Any of that compute to you Keith?


Floyd,
If Spy had a large dia high pitched prop he'd be over propped and yes then his boat speed at idle would be too fast.

Spy,
What's your WOT rpm? And David is right you should have numbers that say your engine develops X amount of power (hp) at X rpm expressed in hundreds. The 2700 to 3000 should be labeled as a specific range for some specified purpose. But it's max power will be expressed as 2700, 2850, 2950 or 3000 ..... or some other specific engine speed. A specific rpm ... not a range.
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Old 02-08-2013, 10:35 PM   #58
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Can't troll for salmon at 3.9 knots. That's the big problem.
I would have bet good money on that being the reason you want to go slower. Have you looked into getting a trolling valve? Have you tried lowering your idle to 650-700 rpm? That just might put you at 2.5 knots which is a speed I've caught a lot of salmon at.
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Old 02-08-2013, 11:13 PM   #59
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Spy,
What's your WOT rpm? And David is right you should have numbers that say your engine develops X amount of power (hp) at X rpm expressed in hundreds. The 2700 to 3000 should be labeled as a specific range for some specified purpose. But it's max power will be expressed as 2700, 2850, 2950 or 3000 ..... or some other specific engine speed. A specific rpm ... not a range.
2950ish. The same engine is marketed with various WOT max rpms by different vendors. My engine is rated by VP at 75 crank horsepower at 3000rpm for leisure use. The hp curve is flat from 2700 to 3000.
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Old 02-08-2013, 11:13 PM   #60
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Walt I didn't need the arrow.

I think there was a fellow w the same problem that had a 32 Nordic Tug and was from northern BC. I think his username started w T and could probably be found on the members list by usernames.

Spy I see your post #59. I give up. Just catch fast fish. When you go fishing pull up the engine compartment hatch and adjust your idle down some. Should have a very noticeable effect.
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