Big HP vs Small HP

[O C Diver]

Cummins 6CTA is a mechanically injected engine. 4045 is electronically controlled (though I don't think common rail?). Among things you can change with software programming is the fuel injection timing, which can have a large effect on both power and efficiency. With a mechanical engine, you have to pick a fixed injection timing that is inevitably a compromise at some corner of the operating envelop. That can easily make 10% difference in power or efficiency. With electronic controls, you can set the timing arbitrarily to achieve what you want, at any operating point.

The 4045 has been produced for decades and was mechanically injected. My version (Tier 2) is mechanically injected but the injection pump is electronically controlled. That means the timing of injection doesn't change, only the quantity is controlled by the ECM. I believe the newest versions are electronically injected common rail.

Ted

 

[Arc]

Reading data graphs is only a guideline. My QSB starts the same at idle on the graph and ends the same way. But the middle curve does not match up at any given RPM. cummins load testing is very linear. A boat going through the water is not and my hull will not act like your. Some push easy are a given speed while others may suffer from turbulence

 

[DDW]

According to the current Deere literature, the current 4045FM75 is mechanically injected with a rotary pump, with electronically variable timing. Don't know how far back that history goes.

Common rail with piezo injectors gives much more control, but of course is far more complex.

 

[Don L]

I
There's a lot more to the equation than fuel.

Peter

That is a different topic that the drifters will get to.

 

[markpierce]

with exception of windows needing to be re-bedded, the wide transom Sunseeker was a decent sea boat, at least headed south.

that is the more difficult direction

 

[OldDan1943]

Here is a hint
The N46 had a 120 HP Deere
Definitely a 7.5 to 8 kn fullly loaded boat

 

[Nomad Willy]

Another variable,
Power a small (26’?) V bottomed light plywood boat that was a typical PNW cruising gasoline powered boat for more than several decades w a small diesel (25hp?).

With the small diesel it would probably be more efficient than a typical rec. trawler of the same size.

I’m think’in the displacement of the boat matters more than engine size and all the rest. An efficient trawler gets her fuel efficiency from hull form, slow speed and diesel power. Another boat though could get the same efficiency or more just by making the boat very light. My opinion ..

 

[rslifkin]

Another variable,
Power a small (26’?) V bottomed light plywood boat that was a typical PNW cruising gasoline powered boat for more than several decades w a small diesel (25hp?).

With the small diesel it would probably be more efficient than a typical rec. trawler of the same size.

I’m think’in the displacement of the boat matters more than engine size and all the rest. An efficient trawler gets her fuel efficiency from hull form, slow speed and diesel power. Another boat though could get the same efficiency or more just by making the boat very light. My opinion ..

I agree, weight matters, and that's why the higher powered, less efficiently shaped faster boats often come close to the efficiency of the slow trawlers (although they can't match the most efficient slow hulls). The faster boats tend to be lighter, offsetting a lot of the disadvantage in hull shape. You'll never get sailboat like efficiency out of a fast hull being run slowly though even if it's light.

 

[OldDan1943]

The fastest way to repowering the boat is to determine the original HP and then go hunting for for the exact replacement.

 

[Nomad Willy]

I agree, weight matters, and that's why the higher powered, less efficiently shaped faster boats often come close to the efficiency of the slow trawlers (although they can't match the most efficient slow hulls). The faster boats tend to be lighter, offsetting a lot of the disadvantage in hull shape. You'll never get sailboat like efficiency out of a fast hull being run slowly though even if it's light.

I have looked at a lot of boats that had known power and displacement numbers. I did that trying to determine what engine to buy for my Willard 30 repower.

Willy had 5hp per ton of displacement with the replacement engine I eventually chose. I considered my choice a bit in excess (37hp) as I know I could have done everything I did do except the WOT periodical engine test to insure it’s full ability hadn’t gone south. Of course if I had installed a 32-33hp engine the WOT test may have been at a different rpm. There was a 33hp Mitsubishi engine that may have been a bit closer to a perfect choice.

It was fun to discover how much power there was in many boats that I looked at. The lowest power to weight ratio boat was an 85’ wood boat in So. Cal. w a Gardner engine that gave her 2hp per ton. Most rec. trawlers were 7hp and up. One of the Alaska State Ferries was about 8. From what I learned about it about 3hp per ton seemed perfect for a FD hull.

 

[Barking Sands]

The question focuses on high HP engines running at much lower RPMs than what they were designed for and producing low HP efficiently. The point of the thread was that the 6 cylinder wouldn't be as good as the 4 cylinder at 40 HP, as far as efficiency. There is a point below which an engine no longer produces prop HP efficiently.

I found that at 40 HP with my Cummins 6CTA 450 HP.

Ted

Just thinking out loud here....it seems there are quite a few reasons why this is true. I did read the thread and I didnt see several additional reasons why this seems generally true. Especially when compared to an engine like your Cummins.

1) The Cummins has several accessories sized to cover the 450hp. Fuel pumps, coolant pumps, seawater pumps, alternators, oil pumps, etc. All of these are going to be much larger and have far greater power requirements than smaller versions on smaller HP engines. Obviously these components are engineered and selected to cover the entire rpm range and HP output range for that engine and have a certain upper limit number in mind. The greater the delta between that upper limit design and actual HP produced will result in the greatest losses in efficiency when comparing to smaller components for the same target HP. Just as a mental exercise imagine putting all those components from your 450HP Cummins on to a the 40HP engine in the example...lol. Many times not all of these components are contained in BSFC charts and graphs.

2) Same idea for all frictional losses. Rings, bearings, valvetrain, seals, oil surface area, windage. This increase is mainly due to cubic inches and configuration such as cylinder count.

3) Fuel burn efficiency. When the large engine is run at part throttle at say 1000 rpm well outside of its peak BSFC and is loaded to 10% to achieve the 40 hp VS a small engine that is loaded to 75% at 1700 rpm and is closer to peak BSFC

IMO these are the fundamental and largest considerations when comparing very large engines such as your Cummins to much smaller engines when run at the example of 40 hp . IMO whether the engine is common rail or mechanical injection is secondary when the differential of the examples is so high. Obviously all parameters count.

But...the engines selected (and prop) must cover all areas of the mission. It does no good to compare an engine placed in a vessel with such a broad range of capability that it not only can idle away from the dock but put a 30k-50k lb vessel on plane for extended periods.

IMO its better to know your mission first. I guess selecting the smallest engine that satisfies the mission will be best for lowest fuel consumption. Add in modern tech like common rail, computer controls and turbos to get the same HP (and broader mission capability) from smaller displacement engines

 

[rslifkin]

It was fun to discover how much power there was in many boats that I looked at. The lowest power to weight ratio boat was an 85’ wood boat in So. Cal. w a Gardner engine that gave her 2hp per ton. Most rec. trawlers were 7hp and up. One of the Alaska State Ferries was about 8. From what I learned about it about 3hp per ton seemed perfect for a FD hull.

And then you look at faster boats. I just did the math for my boat and it comes in at just about 50 hp per ton (short tons). Horribly overpowered for our typical slow cruise, but the hull shape can make use of the power, so I don't consider it overpowered in general.

 

[dannc]

Regarding prop curves for engines. The prop curve is just an equation which you can read about here, https://continuouswave.com/whaler/reference/propellerPowerCurve.html

HP = C X RPM^2.7 where C is some constant for a specific hull
and propeller combination

To be a bit more accurate the equation is really HP = C X RPM ^ N. Where N can be between 2.2 and 3.0.

So one has to solve for C which requires one to know the engine HP and the RPM and is easy enough.

The N variable is the real point of this post. Engine prop curves can use a different value of N. For example, JD seems to use 3.0 for N and Beta uses 2.7.

One has to be mindful of this difference when comparing different engines from different companies.

Later,
Dan

 

[Don L]

so is the real reason a big HP engine running low at displacement speeds use a little more fuel at that speed than a smaller HP would is that the prop is sozed and pitched for higher speed.

In other words it is mostly a prop efficiency loss

I of course will have no choice in engines when buying a 1980s trawler. But if it comes down to 2 boats all else equal it is a definite item to consider I feel.

 

[KnotYet]

so is the real reason a big HP engine running low at displacement speeds use a little more fuel at that speed than a smaller HP would is that the prop is sozed and pitched for higher speed.

In other words it is mostly a prop efficiency loss

I of course will have no choice in engines when buying a 1980s trawler. But if it comes down to 2 boats all else equal it is a definite item to consider I feel.

Based on both the anecdotal and actual data there seems to be little enough
difference, big HP vs small HP, that other factors will probably matter more, IMO.

If a boat is a true displacement design, 2.5 to 3 hp/ton is probably all you'll need.
This was my experience in my last boat with ~30 tons and a 80 hp Perkins 4.236.

 

[chanprr]

Thanks FWP, post #69. When running about 7.4 kts, fuel consumption was 3.03 gph for my 380. Fuel consumption was a bit higher than what you posted for the 380, but given hull, running gear and sea conditions I am happy with the fuel burn. For the entire cruise, running in variable weather and occasionally fairly rough sea conditions, for 1789 nm: average speed 6.15 kts; 2.68 gph; and 2.3 nm/g. The Cummins QSB 6.7 380hp is a pretty good engine in my opinion.

 

[Simi 60]

I've mentioned it on here before that our vessel is an ex prawn trawler that was converted side by side another to the same style at the same time.

Both were re- engined during the conversion but with what they had before
One with 10.45 litres and 127hp
One with 14 litres and 325hp (ours)

Both cruise at around 7.5 to 8 knots
But one runs at near full noise to do it
The other runs at a fast idle
Fuel burn is near enough the same.

Which would you prefer?
The hard runner with little if anything left when needed?
Or the hardly working one with plenty of horses to spare?

 

[rslifkin]

I've mentioned it on here before that our vessel is an ex prawn trawler that was converted side by side another at the same time.
Both were re- engined during the conversion but with what they had before
One with 10 litres and 127hp
One with 14 litres and 325hp (ours)

Both cruise at around 7.5 to 8 knots
But one runs at near full noise to do it
The other runs at a fast idle
Fuel burn is near enough the same.

Which would you prefer?
The hard runner with little if anything left when needed?
Or the hardly working one with plenty of horses to spare?

As long as the hull, prop setup, etc. can do something useful with the extra power in tougher conditions, I'd take option 2.

 

[KnotYet]

I've mentioned it on here before that our vessel is an ex prawn trawler that was converted side by side another to the same style at the same time.

Both were re- engined during the conversion but with what they had before
One with 10.45 litres and 127hp
One with 14 litres and 325hp (ours)

Both cruise at around 7.5 to 8 knots
But one runs at near full noise to do it
The other runs at a fast idle
Fuel burn is near enough the same.

Which would you prefer?
The hard runner with little if anything left when needed?
Or the hardly working one with plenty of horses to spare?

At 60 tons, 2.5-3.0 hp/ton is 150 to 180 hp.
So the 'puny' 10.5L doesn't quite cut it and may not having enough oomph for hull speed
in some instances.
With only 12.15 hp per liter, it could probably go forever at full output, though.

 

[Nomad Willy]

KnotYet wrote;
“So the 'puny' 10.5L doesn't quite cut it and may not having enough oomph for hull speed”

Cruising at hull speed w a FD hull is not something one should do especially w an engine that needs or is close to WOT. It takes so much more power and fuel that the benefits are too small to recommend it … and then there’s the fuel. But w a SD hull .. hull speed may be the best cruising speed.
Like Mark always says “run a knot below hull speed”.

 

[Delta Riverat]

I have looked at a lot of boats that had known power and displacement numbers. I did that trying to determine what engine to buy for my Willard 30 repower.

Willy had 5hp per ton of displacement with the replacement engine I eventually chose. I considered my choice a bit in excess (37hp) as I know I could have done everything I did do except the WOT periodical engine test to insure it’s full ability hadn’t gone south. Of course if I had installed a 32-33hp engine the WOT test may have been at a different rpm. There was a 33hp Mitsubishi engine that may have been a bit closer to a perfect choice.

It was fun to discover how much power there was in many boats that I looked at. The lowest power to weight ratio boat was an 85’ wood boat in So. Cal. w a Gardner engine that gave her 2hp per ton. Most rec. trawlers were 7hp and up. One of the Alaska State Ferries was about 8. From what I learned about it about 3hp per ton seemed perfect for a FD hull.

Sounded like a cool ratio to explore, so I did.

Have (2) 235 HP engines and 21 tons weight.

21 HP / ton. I got a Hi-Po!

But the flip side is the motors are advised to run above 1400 rpm. Fourteen hundred rpm will be just about hull speed (8 knots) Flat out (2500 rpm) will produce 13 knots. So, engine minamum produces hull speed and the rest is there for currents and other times when needed.

 

[Nomad Willy]

Delta Riverat,
Like most rec trawlers you’rs is overpowered. Glad to see you’re not overpropped.
I wonder why they advise to run more than 1400 … are yours turbo engines? I’d run 1500-1700 w an inch of pitch taken out. And insure your temps are up where they should be.
Do you have a heat temp gun? I did on my boat but when we sold the boat I took the heat gun home. Use the heat gun for several things but mostly for cooking with the microwave. Even warming up my coffee (140 degrees).

 

[Delta Riverat]

Yeah, a pair of Volvo TAMD-60B, 235 HP, water cooled turbos. Start fast (less than a second) and run smooth. The engine manual advises "do not run below 1400 rpm for extended times", don't know why.

Coolant temp 170 on both. I usually run 1550-1600 rpm as they run smoother there and speed gets to 9-10 knots.

Of course in 5 mph (no wake) zones I'm putting down the river at less than 8-900.

Hey, you run what you got eh? But yeah, the boat - engine - prop thing seems to be pretty well sorted.

 

[rslifkin]

Engine manufacturers typically publish what the max continuous output of a given engine is. It would be nice if they published a minimum continuous output as well and maybe indicated what the limiting concern is (as far as load to keep combustion temps up, RPM for oiling to some component, etc.)

 

[dannc]

Yeah, a pair of Volvo TAMD-60B, 235 HP, water cooled turbos. Start fast (less than a second) and run smooth. The engine manual advises "do not run below 1400 rpm for extended times", don't know why.

My guess is they are concerned about properly loading the engine and/or oil contamination....

I have done used oil analysis on my engine oils for decades. For my tractor, which has a Yanmar engine, and my truck diesel, I was getting a bit of fuel in the engine oil. It was within specification but who wants fuel in the engine oil? In the tractor's case, I increased my engine RPM a bit from 1600 to 1800 and the problem went away.

In the truck, I was driving in a heavy traffic for 10-15 minutes of a 45-60 minute drive. I thought/hoped that the 30-45 minutes of the drive that were at high way speeds would help burn out any issues from the heavy traffic time. It might have but the fuel was in the oil. Eventually, I did not have to drive in that heavy traffic and the problem went away.

Later,
Dan

 

[DDW]

Both mechanical engines? It is a lot less likely with common rail.

 

[OldDan1943]

Has anyone put a ‘kicker’ on the swim platform as a ‘last ditch’ get home?

 

[SteveK]

Has anyone put a ‘kicker’ on the swim platform as a ‘last ditch’ get home?

If I need to it is attached to the dingy, most have one there. pushing works better than towing with dingy

 

[rslifkin]

Has anyone put a ‘kicker’ on the swim platform as a ‘last ditch’ get home?

On anything that carries a reasonably capable dinghy it's probably more practical to hip tow with the dinghy in last ditch situation.

 

[markpierce]

A pair of engines almost always means a semi-displacement hull designed to exceed hull speed. Not for a full-displacement hull or a true recreational "trawler" designed to move below and up to hull speed, which means 80 h.p. or less for most such boats.