What hp??

[Nomad Willy]

Is there a way of telling when your semi-displacement hull is "on the money? I run at 8.5 knots at 2460 RPMs (measured mile) with two 136 horsepower Volvos, and it feels just right.

ancora,
What on earth is "right on the money"? Hull speed? That's too fast for best economy ... MPG. Not knowing what you mean "on the money" may be about .75 of a knot below hull speed. However you've got a long boat and w that much length work'in for you more speed would be fully appropriate depending on how you are re buying fuel.

 

[FF]

For folks that think a diesel is a diesel might wish to view;

https://www.morganscloud.com/2015/06/02/understanding-an-engine-fuel-map/

When you get to the fuel map part it will explain why there is no fixed HP/Gal output at any loading but in the bullseye .

Interesting when using google and "diesel BMEP chart" one can read the entire thing, going to directly , far less info.

????

 

[Trawler Dave]

True, you can't separate torque from horsepower, but the formula above does not mention what torque actually is. Torque = force x lever arm length. The reason diesels make such high torque is that they are long stroke, high compression engines. The long stroke gives a long lever arm on the crank journal. This is why we are able to cruise and turn a high pitch prop at such low RPM. Gasoline engines, on the other hand are much shorter stroke engines. They have a shorter lever arm and much less torque (especially in the low ranges). You can have both a diesel and a gas motor rated at the same BHP, but the diesel will develop the same torque as the gas motor @ 1600 RPM while the gas motor needs to be screaming @ 5000 RPM or more to make the same torque. Which engine would you want in your boat?

 

[BruceK]

.. You can have both a diesel and a gas motor rated at the same BHP, but the diesel will develop the same torque as the gas motor @ 1600 RPM while the gas motor needs to be screaming @ 5000 RPM or more to make the same torque. Which engine would you want in your boat?

Which explains nicely why my turbo 150KW/400NM diesel car so easily conquers hills. First diesel car owned, but I rented several in Europe; I`m sold, performance and economy together. Applying that to a boat as Dave does, diesel is an easy decision.

 

[man n black]

Trawler Dave hit it right on the money.

I would only add that TQ initially will be the primary moving force in any vehicle but it's HP that keeps your speed up/constant once your are moving. Coincidentally TQ can and does break many things....especially if you are running a high gear load...things like axles, propshafts, transmissions and gear tooths come immediately to mind....or in my bonehead case sometimes bolt heads too..

@bruce..now go try yourself a full size diesel truck and tune it....My Cummins truck tows our 8k lb trailer across 14k foot Rocky Mountain passes like it's not even there....600hp/1200ft lbs TQ...crazy..

 

[bayview]

Exactly my point, a trawler needs a lot of force but low RPM. I need a lot of power but don't want to work to hard for it, that is why a diesel is the right power plant, it puts out lots of torque at low RPM, so you can put a large high pitch prop on it and still be able to turn it at low RPM using less fuel and less wear and tear on the engine.

RPM does not affect fuel use. Only load does.

 

[bayview]

I will go back and look at that one again, thanks. I know with my particular engine, 1400 rpm is about 10% of max fuel consumption. Seems to be a big difference.

If that is measured OK.

If it is from the engine curves true only if you use prop curve and reach rated WOT.

 

[bayview]

To understand how unrelated RPM is to fuel use consider that out of gear your engine can rev all the way to max but will still use very little fuel.

Only when in gear will load require fuel and that fuel required will be based on loading not maximum engine rated vs rpm curve.

 

[Fletcher500]

Good discussion, especially on horsepower and torque. I like the three cylinder analogy and the weightlifter. We have a planning hull, so the engines are running at about 80% of WOT. This same engine is Used by a lot of fishermen in my area who also run on the high-end with many hours and no problems . I probably won't get trawler life out of the engines, but they should still outlive me before a major overhaul is needed.

 

[Nomad Willy]

Trawler Dave,
Sorry to burst your concept of torque but long stroke has almost nothing to do w torque. Look at the Buick straight eight and compare it to the V8's that came after. Extreme lomg stroke straight eight had far far less torque than the V8's. I've observed that in many other engines as well.
The force (torque) is re to the force on the piston top and the lever arm created by the stroke via the crankshaft. You either have big force on the piston crown and small leverage via the short stroke or small bore small force on the piston crown and big leverage on the long stroke crankshaft for X amount of torque. Amount of oxygen, fuel and compression also gets into it but the force on the piston top and the mechanical advantage of the stroke is primary to torque.

The above is just opinion but if you care to look you'll find lots of short stroke engines with above average torque output.


Fletcher500'
Perhaps you mean 80% of rpm. Almost nobody runs at 80% of power output. That would be only 100-150rpm down from max. Maybe even less.

 

[Warwgn]

Eric is correct, torque is force x distance, and the main factor for a diesel is the force from a high compression combustion chamber. Stroke is important but the bore is where most of the power is made, the larger the bore the more room for fuel and air. Of course I know CID is stroke x bore as the main parts of the formula but the larger the bore the higher the torque is a good rule to follow as a norm.

 

[Ski in NC]

Lots of reasons for more torque from diesels: Higher comp ratio, longer stroke, turbocharging, no worries of detonation or pre-ignition, injection continuing well past tdc.

And yep, bore is a bigger deal. Displacement varies with the square of the bore, yet linearly with stroke. If you add a half inch to either, bigger bore gets bigger effect.

And someone mentioned BMEP/BSFC charts. That is the gold standard for nailing down engine performance and efficiency. The charts the eng mfrs give us are poor substitutes for the fuel maps they certainly have, yet do not release. Annoying.

Cummins and Cat give enough data that you can reasonably calc efficiency in boat service. But they could just publish the fuel maps.

For trawler service, a very rough way to check loading is to take your cruising hp and compare it to rated power. So using a Lehman 120 as example, if you cruise at 60hp that is 3-4gph. And rpm should be somewhere around where torque peak at WOT. Probably around 1500 to 1800rpm.

So if you are burning 3gph at 1700, the engine is in a sweet spot.

 

[Nomad Willy]

"The larger the bore the more room for fuel and air .. yes but it's a issue of stroke too .. Displacement. And big time the compression but isn't the energy in diesel fuel less than gasoline? They aren't the same but I can't remember which is which. Also diesel don't burn at the same rate as gasoline. I think diesel is faster burning and that may have an effect on torque. Could be a plus for gas??

 

[Fletcher500]

No, I meant 80 percent of WOT. The boat is on plane at around 2700 and I often cruise her at 2800. WOT is 3550. This is SOP for these engines in this configuration.

With that said, I spend most of my time at 1300 rpm enjoying the view and the low Gph.

Fletcher500'
Perhaps you mean 80% of rpm. Almost nobody runs at 80% of power output. That would be only 100-150rpm down from max. Maybe even less.

 

[Baker]

The above is just opinion but if you care to look you'll find lots of short stroke engines with above average torque output.


Fletcher500'
Perhaps you mean 80% of rpm. Almost nobody runs at 80% of power output. That would be only 100-150rpm down from max. Maybe even less.

I think I'm pretty close. Although I'm fairly underpropped so my 2400rpm cruise is likely less than what the fuel and power curves show.

 

[bayview]

"The larger the bore the more room for fuel and air .. yes but it's a issue of stroke too .. Displacement. And big time the compression but isn't the energy in diesel fuel less than gasoline? They aren't the same but I can't remember which is which. Also diesel don't burn at the same rate as gasoline. I think diesel is faster burning and that may have an effect on torque. Could be a plus for gas??

Backwards Willy. Diesel has more energy than gasoline. Burns slower too.

 

[Nomad Willy]

Thanks bayview I couldn't remember.

I did think it burned slower because I thought it should but remembered it was backwards ... and it wasn't.

But I wonder if it burns sooner or later because it would seem that having the greatest force on the piston should optimize power if the downward force on the pistion occured a bit before halfway down from top dead center. And some engine cylinder bores are offset to maximize the advantage of having the downward force at the optimum time.

 

[man n black]

Oftentimes the spark is actually set to just after TDC...this includes diesels with their compression ignition concept...

The theory I think is....it's much easier to fire just after TDC because it keeps the engine going forward/around...ie: an object in motion stays in motion..

Igniting the fuel air mixture too far down the stroke or before TDC has the engine fighting its natural motion and trying to tie itself up in knots

Also...someone else said it more scientifically but basically..there's no replacement for displacement. .'Merica has proven that time and time again in the racing world...

Ch

 

[FF]

Injection operating like spark timing is always Before TDC.

The injector sprays fine fuel dropplets into the cylinder where it needs both heat and time to vaporise and ignite before it can burn efficiently

 

[Trawler Dave]

Gasoline engines are set to between 4 and 10 degrees before TDC to allow for flame to propagate and pressure to build. Sparks timing is variable and advances as RPM increases. My Lehman injection is timed for 20 degrees before TDC.

 

[psneeld]

No, I meant 80 percent of WOT. The boat is on plane at around 2700 and I often cruise her at 2800. WOT is 3550. This is SOP for these engines in this configuration.

With that said, I spend most of my time at 1300 rpm enjoying the view and the low Gph.

Many diesel vessels are run 200 rpm off the pins.

That is the rule of thumb I was brought up with.

Everything from fast planing boats to displacement tugs.

Obviously adjustments were made for different situations, but that was considered "typical" loading rpm unless specifically rated for 24/7 on the pins, or came with specific loading instructions.

 

[Ski in NC]

Diesel is different from gas in that the fuel is injected over a pretty long duration. Injection process might begin at 20deg btdc, but actual injection will start later, and combustion start even later than that. It is all timed so pressure starts rising from combustion after tdc. But the big difference is that injection continues at a measured rate so as piston descends and volume increases, the pressure is kept somewhat constant until injection stops. That way peak cyl pressure can be kept below structural limits. This also helps efficiency somewhat by limiting heat transfer by having heat added later in the process.

In a gasoline engine combustion starts with the spark and the rate of pressure rise is not actively controlled, only limited by the rate of the flame front advancing. Too much pressure or heat and flame initiates spontaneously with bad results. Thus raising compression ratio, which directly increases efficiency, has its limits. Also closing down the throttle to reduce output effectively reduces compression ratio, with significant drop in efficiency.

Fun stuff..

 

[bayview]

IMO 200 off the pins is bad advice in general. For 30 HP per liter engines it was fine but get to 60 HP per liter and you are pushing things hard. Makers do not warranty that level of running on recreational engines for any high duty cycle.

 

[Ski in NC]

"200 off the top" as a cruise point is a very crude tool, but it does apply pretty well to lots of engines.

A 2300 rpm Detroit or Cat, 2100 is considered ok, provided it can make it to 2300.

A 3800 rpm Yanmar, I would not like to run it at 3600 continuously.

If an engine can't make its rated rpm, then you need to give it even more margin.

 

[Bruce B]

The concept of ignition timing tends to confuse people. It is easier to understand when you think of combustion as a chemical reaction that happens over time. In other words it takes time for the process to happen. As an engine increases rpm the time component of each stroke shortens. To visualize this consider an engine at 1 rpm. It would take 30 seconds for the compression stroke at 1 rpm. Now let's double the engine speed to a whopping 2 rpm's. The compression stroke now happens in 15 seconds. Get it?
Ignition timing advances so that combustion has enough time to happen before the exhaust valve opens thus maximizing the power generation of that cycle...
Bruce

 

[Bruce B]

Here is a video by one of my favorite YouTube channels. The subject matter is always automotive in nature but this guy is very capable and he does a good job of explaining the subject matter. I can watch this stuff for hours and there is lots of material. This video is on Horsepower vs Torque...
Bruce

 

[Baker]

IMO 200 off the pins is bad advice in general. For 30 HP per liter engines it was fine but get to 60 HP per liter and you are pushing things hard. Makers do not warranty that level of running on recreational engines for any high duty cycle.

That is in the actual Cummins manual for max continuous power. Does it mean you should run it there??? No it does not. But they do feel the need to define it. And since it is defined, I would assume it is warrantied. My 4LHA Yanmar was rated at 240hp@3300rpm. Again the manual said 3100(190hp) continuous(Gives you an idea how non linear the power curve is with that last 200rpm representing 50hp). I ran my Yanmar at 2800rpm and that yielded 15kts in that Pilot. I run my Cummins around 2300-2400 yielding 16-17 knots. Cummins rated at 315@2800rpm. Both boats were/are fairly underpropped so they seem quite happy.

 

[bayview]

That is in the actual Cummins manual for max continuous power. Does it mean you should run it there??? No it does not. But they do feel the need to define it. And since it is defined, I would assume it is warrantied. My 4LHA Yanmar was rated at 240hp@3300rpm. Again the manual said 3100(190hp) continuous(Gives you an idea how non linear the power curve is with that last 200rpm representing 50hp). I ran my Yanmar at 2800rpm and that yielded 15kts in that Pilot. I run my Cummins around 2300-2400 yielding 16-17 knots. Cummins rated at 315@2800rpm. Both boats were/are fairly underpropped so they seem quite happy.

That may be so for low power models but the cummins specs i remember said not to exceed 1 hour out of eight.

it is also worth mentioning that the max rated load is not a requirement just allowed. Some folks have the idea that their engines should always be run at very high load.

 

[Baker]

I think the one hour out of 8 is for WOT or anything above the max continuous range....hence the term "max continuous". And I fully agree with the rest of your sentiment. Just because it says you can does not mean you should. Basic common sense tells you the harder you run it, the more wear it is causing.

 

[bayview]

The common cummins 5.9 liter engine run at 200 hp ( 34 hp per liter) will last a very long time while the same run at 400 HP (68 HP per) not so much.

Interestingly overhaul expected time sometimes is discussed in terms of fuel used which directly relates to work done. On that basis both rated engines would be the same. One just uses fuel 2X faster and races to overhaul time faster.