Boat Performance

[jespchee]

I am in the process of moving down (in speed that is) to a 40 foot Marine Trader Labelle. At the sea trials the engines did not meet the recommended full max. rpm of 2500. The motors are 212 hp Sabre Marine 6 cylinder, turbo(ed). The PO had replaced the original (at least when he bought the boat) 3-blade props with 4-blade ones (D 24 P20 ), which I have. His thoughts were he was buying the boat for the Great Loop and he would get better efficiency at 8 knots with this arrangement. I have the 3 blade props. I plan on doing much more extensive cruising where efficiency at a higher speed would be welcomed. My experience with other machinery is that they are more efficient if run at their "sweet spot". However I have no clue as where it is at this time.
So after all this my question is do I change the props. Here is the performance numbers at the trials.

1100-6.8 Kts
1400-8 kts
1800-9.2 kts
2100-13.4 kts (however I saw 14.5 on my smartphone app)

Any advice is welcomed as I am new to big boat performance.

 

[Comodave]

I have a 41’ President with 225Lehmans. At 1600 RPM i get about 8.6 knots. I have 4 blade 24X24 props. On the seatrial we did come very close to 2500 RPMs. So I think mine are pretty good as to size.

 

[DavidM]

It is a fairly common, but misguided practice IMO, to over prop a boat several hundred rpms in the expectation of lower fuel consumption at lower rpms. That is what seems to have happened with your boat.


Since you plan to run it at higher rpms than 1,400 for 8 kts, I would go back to the original 3 blade props and make sure the engines hit 2,500 at wot.


David

 

[Shrew]

Overpropping: Saving hundreds of dollars in fuel and losing thousands of dollars in engine lifespan and total duty cycles as a result of overloading.

 

[Shrew]

Moved from "How To Use The Forum, Site News & Account Concerns" to "Power Systems"

 

[RT Firefly]

Greetings,
Mr. j. Welcome aboard. The "sweet spot" for any Lehman we've owned has been 1750 RPM. 380 cu in. 120 HP. The seem to like 1250 RPM as well...

 

[SoWhat]

Start by calibrating your tach with a handheld device, then do the trials at slack tide in calm water with no wind. If you have to run inlets against the tide then I would want max rpm's and speed. I suspect the 3 blade will give you higher speed at top end.

If strictly cruising then I would probably pick up some digital flow meters on EBay and figure out what most efficient settings are for various conditions. I'm guessing the 4 blades would give you better efficiency at the cost of losing some top end speed. You could always repitch the 4 blades by reducing pitch by an inch or two to get them to max rpm but then you sacrifice some efficiency.

Prop shop calculations usually aren't that accurate. Trial and error works.

 

[Nomad Willy]

SoWhat,
I’m guessing the 3 blade will give better efficiency because the 4 blade will not have enough pitch and too much blade area. Top speed may depend on the same variables.
But I see you’ve got a 24x20. That would be ok (probably) but when you depitch it to get propper rpm you’ll likely wind up w 24x18. Then the three blade may be better.
Check w Michigan Propeller or find a good local prop guy and feed them w all the correct variables.

 

[Lepke]

Most people moving from a faster boat to a larger slower boat are unhappy with the speed realities of a big boat. They look for ways to wring out an extra knot or two. If your plan is to run at 2500 rpm over long periods, with a turbo diesel, you will shorten the life of the engine.
Heat (and dirty oil) is the enemy of engine lifespan. At full hp, the internal temps created will be slowly wearing away the rings, pistons, cylinder walls and valves. Most experienced operators of small turbo diesels run them at 80% of hp except for short periods of WOT. The difference is usually double the lifespan. The goal of most makers of small yacht diesels is to make it thru the warranty period. After that to sell parts or another engine. Few yacht diesels, if any, have all the expensive metal alloys that are used in large commercial diesels. A small turbo diesel run hard may not have the lifespan of a good gas engine.

 

[FF]

These engines are not industrial so increasing HP and RPM may seriously shorten the engine life.

Is an extra K or two with 3-4 times the fuel burn and perhaps 1/2 engine life?

Perhaps a different hull style of boat might make a better cruiser for you?

 

[North Baltic sea]

Overpropping: Saving hundreds of dollars in fuel and losing thousands of dollars in engine lifespan and total duty cycles as a result of overloading.

100% same

And diesel fuel on the US is really cheap

NBs

 

[Nomad Willy]

Shrew and NBS,
Probably not saving that much fuel except over a long period of time.

Lepke wrote;
“Most experienced operators of small turbo diesels run them at 80% of hp except for short periods of WOT. The difference is usually double the lifespan. The goal of most makers of small yacht diesels is to make it thru the warranty period. After that to sell parts or another engine. Few yacht diesels, if any, have all the expensive metal alloys that are used in large commercial diesels. A small turbo diesel run hard may not have the lifespan of a good gas engine.“

I agree but think you’ve over stated it. 80% to WOT isn’t that much when you go from one to the other if you’re not overpropped. So many are though you could be right. I don’t think it’s rpm that wears an engine out. It’s load. Just above the middle of the stroke it’s how hard the piston is pushing sideways against the cylinder wall and how much lube oil is there to protect it .. or keep them apart.

But if overpropped you could be very correct. And I agree 80% should be fine for an industrial engine.

 

[jespchee]

Sorry it took so long to get back to this subject, working on getting the papers signed etc. My concern is not so much as speed as it is economy. When we were doing survey at the 2100/2200 rpm the engine temp went up quire rapidly which led me to believe there was an issue with the props. I recognize there is a big difference in go fast boats and what I am buying however as my son-in-law said "why are you in a hurry?". At my age I decided I am not. Looking forward to having time to look at the scenery. Thanks to all. Tuesday the 4th is the big day!!!

 

[O C Diver]

It is a mistake to assume that over proping an engine will shorten the engines life. That assumption doesn't take into account how the engine is used. My boat is over propped simply because a 4:1 gear doesn't exist for my engine with a down angle. The engine diagnostics show at my normal cruising RPM, the engine is producing less than half of the allowable HP for that RPM. The dealer's comment was," it will never wear out at that load percentage ". Now consider the alternative, if the engine was correctly propped and I ran it wide open all the time, how long do you think it would last? Life expectancy is more about how hard you run an engine. Finally, consider a slow speed generator. They are set up to optimize maximum load at a much lower RPM than the motor was designed for. As long as you don't exceed the load for a given RPM, you can certainly optimize the load for that RPM.

While there may be some small savings in fuel from over propping, it's pretty much insignificant. There can be a significant difference in noise levels though.

Ted

 

[Nomad Willy]

Jespchee,
You’re saying there’s something wrong w the prop because the coolant temp went up w the engine working hard?

All the prop can do is increase the load. If an engine is in good shape and especially the cooling system the coolant temp should match the thermostat rating under any load.

But I’ve been saying for many years engines should at least be able to achive their rated rpm at WOT.

 

[FF]

"But I’ve been saying for many years engines should at least be able to achive their rated rpm at WOT."

Perfect if you operate a lot at WOT.

Most trawler have vastly over sized engines 2-3 GPH is usually 30 to 50 hp, at best.

Overproping is NOT overloading an engine , it is simply matching the engine power curve to the prop required HP at cruise.

The best (but most difficult) way to set up a boat to cruise with lowest noise , longest engine life and least fuel burn is with a Fuel Map or BMEP graph for that engine.

Sadly its usually easier to get the plans for a nuke ICBM than the fuel map from an engine assembler.

 

[djones44]

Most people moving from a faster boat to a larger slower boat are unhappy with the speed realities of a big boat. They look for ways to wring out an extra knot or two.

An autopilot helps overcome the tedium of steering boats at slower speeds than you may be used to. When the kids ask "Are we there yet?" you can look up from the NYTimes on your iPad Poort and announce "OMG, we are!"

 

[DDW]

I'm also in the camp that says over propping only overloads the engine if you run it overloaded. At substantially less than full rpm, there is a large gap between max engine torque and prop absorption. That gap represents a light load on the engine. In normal circumstances there may not be much economy difference, but there will surely be a noise difference. If you overprop enough to close that gap at normal cruising rpm, then yes, the engine will not last as long - but that is a very extreme case of over propping for most trawlers.

 

[ksanders]

Some engines are more susceptible to damage from overproping/overloading, the Cummins B series being a prime example.

If you overrprop to make the engine produce closer to it’s rated output at a low RPM, you will be significantly overloaded at higher cruise RPM’s, and this wil reduce life, if you actually run at those higher rpm’s

In a full displacement hull boat you might never run at the higher rpm settings, but in a Semi Displacement boat you probably will.

Frankly I see very little to gain by intentionally overpropping, and quite a bit to loose.

 

[O C Diver]

Some engines are more susceptible to damage from overproping/overloading, the Cummins B series being a prime example.

If you overrprop to make the engine produce closer to it’s rated output at a low RPM, you will be significantly overloaded at higher cruise RPM’s, and this wil reduce life, if you actually run at those higher rpm’s

In a full displacement hull boat you might never run at the higher rpm settings, but in a Semi Displacement boat you probably will.

Frankly I see very little to gain by intentionally overpropping, and quite a bit to loose.

While you would not want to overprop an engine and then try to run it at near maximum RPM, overpropping by definition should mean giving up an RPM range that you will never uses. In most overpropping examples on this forum, the boat can't plane either because of hull style or lack of HP. In essence, the owner knows or should know what his reasonable maximum attainable speed is, and is now optimizing the RPM range to match the realistic speed range. Clearly the owner and or operator need to understand the concepts involved and new lower RPM limit.

Ted

 

[psneeld]

As most discussions here on TF......absolutes and 100% and always and every other expression alluding to the same has to be applied to a specific set of parameters.

Which, for the most part is never done.

So whether overpropping can be or can't be done safely and for efficiency is more of when and where....more so than never or always......

 

[sunchaser]

Over-propping will show up during a proper sea trial. It can kill a deal. The current owner of the vessel in question should be requested to prop the vessel correctly to ascertain if full rated RPMs can be safely obtained under full tank scenarios.

If the owner refuses, the potential buyer then must shoulder the risk or walk. A good broker knows this buying dance.

 

[Nomad Willy]

FF and the rest are right ....... to a point.

The problem is no one knows where (what rpm) the overloading starts.
If you run your boat through it’s rpm range while tied to the dock the upper end of what you can achieve (rpm wise) is overloaded. Close to the top is very overloaded. The problem is you don’t know where the overloading starts. Lets say you can achieve 1900rpm WOT. Where does the overload start? At 1800? Probably quit before that. May be at 1600, 1700 or 1500.

It is true at a certian rpm (lets guess 1400 in the above example) the engine runs as if it were at a 2500 rated rpm. That is at the same load. And then you run at 1200 thinking you’re saving fuel. At some point you are. But at a very narrow range .. but what rpm is that? Probably arange of 100rpm approx. Very likely less than that

The following is mostly subjective but my boat has been overpropped 125rpm for quite awhile and now (a month or two ago) I put the old prop back on and I really like the way it runs. Runs right up to rated rpm. She just easily sings along seemingly w less effort ... and that part is true as I’ve been cruising at the same rpm. I like it. Actually I’d rather be 100rpm underpropped.

 

[O C Diver]

FF and the rest are right ....... to a point.

The problem is no one knows where (what rpm) the overloading starts.
If you run your boat through it’s rpm range while tied to the dock the upper end of what you can achieve (rpm wise) is overloaded. Close to the top is very overloaded. The problem is you don’t know where the overloading starts. Lets say you can achieve 1900rpm WOT. Where does the overload start? At 1800? Probably quit before that. May be at 1600, 1700 or 1500.

It is true at a certian rpm (lets guess 1400 in the above example) the engine runs as if it were at a 2500 rated rpm. That is at the same load. And then you run at 1200 thinking you’re saving fuel. At some point you are. But at a very narrow range .. but what rpm is that? Probably arange of 100rpm approx. Very likely less than that

The following is mostly subjective but my boat has been overpropped 125rpm for quite awhile and now (a month or two ago) I put the old prop back on and I really like the way it runs. Runs right up to rated rpm. She just easily sings along seemingly w less effort ... and that part is true as I’ve been cruising at the same rpm. I like it. Actually I’d rather be 100rpm underpropped.

It depends on your engine.

I have a tier 2 engine which essentially means mechanically injected electronically controlled pump. Because a computer is controlling the injection rate (fuel consumption) and knows the rpm, it can determine load or more importantly % of load for any RPM. There is a gauge on my engine display that shows % of load. So, all I need to do is keep below 80%. From 80% to 100% the computer tracks time value and after so many minutes, it electronically slows the engine down to below 80%. Usually I'm around 50% at cruise RPM, so the load isn't an issue. If I increase speed from 7 to 8 knots, I'm running at 70% of allowable for that RPM. I'm still ok to cruise continuously at that setting. If I go over 80%, I think I can do that for 30 minutes. 100% load I believe is limited to 15 minutes. I never reach these percentages as the speed increase is less than a knot, so I'm not absolutely sure on the time values.

Ted

 

[motion30]

Over propping will allow the engine to have more load at lower RPM if one wants to run slow that will help get to the optimum operating temperature at at lower RPM. But I agree it probably will not want to run at full RPM

 

[rolomart3]

I am in the process of moving down (in speed that is) to a 40 foot Marine Trader Labelle. At the sea trials the engines did not meet the recommended full max. rpm of 2500. The motors are 212 hp Sabre Marine 6 cylinder, turbo(ed). The PO had replaced the original (at least when he bought the boat) 3-blade props with 4-blade ones (D 24 P20 ), which I have. His thoughts were he was buying the boat for the Great Loop and he would get better efficiency at 8 knots with this arrangement. I have the 3 blade props. I plan on doing much more extensive cruising where efficiency at a higher speed would be welcomed. My experience with other machinery is that they are more efficient if run at their "sweet spot". However I have no clue as where it is at this time.
So after all this my question is do I change the props. Here is the performance numbers at the trials.

1100-6.8 Kts
1400-8 kts
1800-9.2 kts
2100-13.4 kts (however I saw 14.5 on my smartphone app)

Any advice is welcomed as I am new to big boat performance.

IMHO, there is the question about accuracy of the tachometers. It could be out of adjustment. Also, I would rev the engine in neutral to see that the governor in the injection pump is not limiting it below the 2500 RPMs.


Lastly, based on my experience with my 120 HP Lehman, going from a 3 to a 4 blade propeller resulted in a reduction of 240 RPMS at FOT. The fuel efficiency performance was not changed but the sweet spot I found it to shift down from 1800 to 1650-1700 RPMs. Of course, YMMV (Your methods may vary).


Rodolfo

 

[Nomad Willy]

OK Ted,
How many of us have engines like that?
My comments were directed at the older mechanical engines like my 2003 Mitsu.
What elements of engine operation are considered to arrive at “% of load”.

 

[O C Diver]

What elements of engine operation are considered to arrive at “% of load”.

Most engines built in the last 20 years have manufacturer's performance graphs. One is usually a horse power curve. Often there is a fuel consumption curve. It would be my assumption that a manufacturer develops that information through testing. If you assume the line represents 100% of the load for a given RPM, then it should be pretty easy to develop a computer program that determines the percentage of load for that RPM based on the ammount of fuel consumed. As an example my engine might be able to develop 80 HP (100% of load) at 1,500 RPM with a 4 GPH fuel consumption. In my boat, when cruising at 1,500 RPM, I'm only burning 2 GPH. Theoretically, if I'm burning half the fuel, I should be running at 50% of load.

Ted

 

[markpierce]

Installing a propulsion system that can result in overheating the engine at normal/capable engine speeds is a harmful waste of time. If one wants optimal fuel-consumption/speed efficiency, move at a knot below hull speed.

 

[DDW]

It's really not that hard. The engine has a torque/rpm curve. The prop has an rpm/torque curve. You overlay them, the gap between them is the excess torque available. The prop torque over the engine torque at any point is the percentage load. Where the curves cross is the max rpm achievable. You just need to decide what constitutes overload. The 80% assumption is just a guess, and there is no one right answer.

Determining the fuel economy is much more complicated, requires the fuel map, and can probably really only be determined with tests.