Big HP vs Small HP

[Don L]

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.

I haven't seen any trawlers for sale in my price range with such little power

 

[Delta Riverat]

Yeah, mine is a SD for sure. A couple of 120's would have been enough and I think were the standard offering. But you run with what you have eh?

And this will be my first and last boat of this size. A retirement toy to have fun with -

 

[twistedtree]

Back to the original question about big engine run light vs small engine run harder, I have been looking at data sheets for a Cat 3208 which is an old-school mechanically injected, 4 stroke. This notion that higher performance rated diesels have worse BSFC than a lower rated engine operating at the same HP just doesn't wash. At worst they are the same, and most of the time the higher rated diesel have a lower BSFC.


It would be great to see any actual data supporting this notion that a smaller engine is more efficient that a bigger one, because so far it seems that exactly the opposite is true.

 

[Nomad Willy]

Anytime a boat is lighter it’s more efficient. Smaller engine means smaller shaft, prop, fuel tanks (not much) but any weight reduction = more efficient.

And most engines are most efficient running fairly close to max continuous cruise rpm and load. Including gas engines. Most here are sensitive to noise. Bigger engines also make more noise. A little engine running full throttle at high rpm can make more tho.

I’ve had two trawlers. 25’ Albin and 30’ Willard.
The Albin had a 35hp Yanmar and the Willard had a 37hp Klassen (Mitsubishi). The Yanmar made it’s power at 3400rpm and the Mitsu-Klassen made power at 3000. Both engines were sound insulated fairly well.

Never thought either one was too noisy. Ran the Mitsu at 2300 and the Yanmar at 2700. So I was about 700 rpm down from max on both engines. The Yanmar had some or more aluminum parts and I think the aluminum parts (including trans.) transmitted more noise.

The engine I really wanted for the Willard when I re-powered was a 38hp @ 2500rpm all cast iron … I think it was Kabota. Couldn’t get one tho as they quit making it. But the smaller Mitsu was perfect in every way .. I think.

 

[MacPhid]

Back to the original question about big engine run light vs small engine run harder, I have been looking at data sheets for a Cat 3208 which is an old-school mechanically injected, 4 stroke. This notion that higher performance rated diesels have worse BSFC than a lower rated engine operating at the same HP just doesn't wash. At worst they are the same, and most of the time the higher rated diesel have a lower BSFC.


It would be great to see any actual data supporting this notion that a smaller engine is more efficient that a bigger one, because so far it seems that exactly the opposite is true.

I think that using the 3208 as an example would indeed have better BSFC at higher HP by virtue of the efficiency gains with turbo charging as opposed to the NA version of that engine. But at lower rpm out of the turbo they would be very similar.

James

 

[Fletcher500]

Sometimes more, is more.

Interesting thread.

 

[OldDan1943]

The speedometer on my car’s max is 130mph. That does mean I must run it 130mph. I get better gpm when the speedometer or tach readings at 12 o’clock. It is the same for a marine type engine. That is what I was taught in the Navy.

Everything at 12 o’clock. To make it easier, adjust the face so ‘normal’ at 12 o’clock. You can tell at a glance is all ‘normal’. OR put a piece tape at the ‘normal’ level.
Another reason for analog gages.

 

[Nomad Willy]

Interesting Dan.

When I flew UL aircraft i preferred my instruments to be analog for some and digital for others mostly considering how I used them.

Air speed … I did’t need tenths of a knot accuracy but I liked the readout to be rather large so I could (w a very quick glance know about how fast I was going). But w cylinder head temperature I wanted high accuracy and more importantly at what rate it was increasing or decreasing. 400 degrees was fine at continuous climbout if stable temp. 400 degrees and climbing rapidly was NOT good.

So I used large analog for airspeed and small digital for CHT.

 

[Mako]

Agree, Dan, I have followed that same technique.

A needle tells you at a glance if systems are nominal. Digital data needs to be interpreted to become useful information. Either that or just rely on buzzers and alarms.

 

[rslifkin]

Agree, Dan, I have followed that same technique.

A needle tells you at a glance if systems are nominal. Digital data needs to be interpreted to become useful information. Either that or just rely on buzzers and alarms.

Also agreed. Digital information is nice for some things, but for a lot I just want to know if it's approximately normal and if the reading is trending up or down. I don't care if the oil pressure is 39 or 41, I just want to know it's about 40 and steady. An analog gauge is quick to determine that at a glance. Digital less so.

 

[mvweebles]

As mentioned in another thread, I accompanied a friend with his ultra-lux Horizon PowerCat 52 on a 90-nm round trip yesterday. A 2014 model with twin Cummins 435 hp. On the way out we did 11.5 kts, 1.6 S/L ratio and a positively awful speed from a fuel economy perspective ("Hull Speed," the speed at which a hull attempts to escape its wave-trape, is often described as 1.34 S/L, or 1.34 x square-root of waterline length. 1.2 S/L would be slower; 1.5 S/L would be faster). On the way home, we did 8.5 kts, 1.2 S/L, a good speed for economy, and it showed.

45-nm trip; about 7-nms in no-wake zone.

• Out: 4.5 hours, burned about 80-gallons (assume $4.25/ga = $340).
• Back: 5.5 hours, burned around 35-gallons ($127)

Some conclusions

First, by saving an hour we blew through an extra $200 in diesel. Not a great use of resources. Won't do that again - rather spend it on dinner.

Second, either go slow or go fast, not inbetween. Seriously, we could have gone 18-kts and likely burned about the same amount of fuel as we did at 11.5 kts but arrived an hour earlier.

I wish I had taken pictures of the stern wakes at 1.6 S/L and 1.2 S/L. The amount of water mounding when going fast is enormous - really makes sense that it takes a lot of energy to move that much water.

I think the Achilles Heel of semi-displacement boats seeking fuel economy is they probably scooch-up to above 1.2 S/L, at least from the looks of the wakes they're showing. If you think you're saving by throttling-up even a little, there's a decent chance it's false economy. It's one thing to crank-up for a mile to catch a bridge opening, another thing to do so for several hours or longer. If you're going to get-moving, bring her up to full cruise per the engines, not somewhere in between.

Peter

 

[rslifkin]

I think the Achilles Heel of semi-displacement boats seeking fuel economy is they probably scooch-up to above 1.2 S/L, at least from the looks of the wakes they're showing. If you think you're saving by throttling-up even a little, there's a decent chance it's false economy. It's one thing to crank-up for a mile to catch a bridge opening, another thing to do so for several hours or longer. If you're going to get-moving, bring her up to full cruise per the engines, not somewhere in between.

Agreed. I'm always surprised by how many SD trawler types in the 40 foot range run regularly at 8 kts. With my boat, 8 kts is plowing a heck of a lot of water. Somewhere between 6.5 - 7 kts (depending on fuel load) the drag rise and increase in wake for a little extra speed starts to become very noticeable. So we generally slow cruise just below that drag rise and basically consider the speed range between 7 and 15 kts to be un-usable for any sustained period of time. Not that running at 17 kts is particularly efficient, but at least that has the boat on a nice, clean plane without plowing a huge wake.



This old thread is relevant here: https://www.trawlerforum.com/forums/s3/wake-pics-55586.html

 

[Rufus]

Agreed. I'm always surprised by how many SD trawler types in the 40 foot range run regularly at 8 kts. With my boat, 8 kts is plowing a heck of a lot of water. Somewhere between 6.5 - 7 kts (depending on fuel load) the drag rise and increase in wake for a little extra speed starts to become very noticeable. So we generally slow cruise just below that drag rise and basically consider the speed range between 7 and 15 kts to be un-usable for any sustained period of time. Not that running at 17 kts is particularly efficient, but at least that has the boat on a nice, clean plane without plowing a huge wake.



This old thread is relevant here: https://www.trawlerforum.com/forums/s3/wake-pics-55586.html

....might have something to do with the fact that most of the alleged/assumed semi-displacement hulls...are not.

 

[Fletcher500]

....might have something to do with the fact that most of the alleged/assumed semi-displacement hulls...are not.

Yes, and many "trawler" type hulls fall somewhere on a spectrum in between FD and SD.

 

[rslifkin]

Yes, and many "trawler" type hulls fall somewhere on a spectrum in between FD and SD.

And some other "SD" hulls are really just planing hulls without enough power to actually get fully on plane.

 

[OldDan1943]

I have yet to hear of any AT planning, just a bigger bow wave.

 

[rslifkin]

From what I know, the AT34 is supposed to top out somewhere around 18 kts.

 

[OldDan1943]

From what I know, the AT34 is supposed to top out somewhere around 18 kts.

18knts, that maybe true but, I think I would have to take the fuel deck fuel fills caps off to prevent the fuel tanks from collapsing due to suction. LOL

In theory, according to my sea trial data sheet, 3000 rpm, 18 knots, 19.5 gph. LOL

Doncha love theories?

 

[Art]

12' beam / 2'10" keel draft / 12'6" air draft - 34' Tollycraft Tri Cabin, twin screw 350 cid 255 hp Mercruisers / 17K lb. dry / 20 to 21K loaded, hard chine planing hull

3 +/- gph at 7 knots [7.58 calced hull speed] = 2.25+/- nmpg

16 to 17 gph at 16 to17 knots = 1 +/- nmpg

WOT at 22 to 23 knts = OMG gph!

 

[DDW]

Actually the fuel consumption of my AT34 is pretty flat between 10 and 18 knots, right at about 1 nm/g ;-).

 

[long-cours.62]

What I could say concerning the "best" ratio nm per liter ( at acceptable low speed because , of course at 6 kts it will be even better)

on our two last boat it was around 66% of the hull speed it means 0.88 of S/L
with our LC62 around 5nmpg

with our Trawler 72 around 2.45 nmpg

 

[rslifkin]

What I could say concerning the "best" ratio nm per liter ( at acceptable low speed because , of course at 6 kts it will be even better)

on our two last boat it was around 66% of the hull speed it means 0.88 of S/L
with our LC62 around 5nmpg

with our Trawler 72 around 2.45 nmpg

Slowing down even more will almost always burn less fuel, as you've noticed. But at some point you find the "best" slow cruise, where you're only burning a little more fuel per mile, but still going a reasonable speed. Longer boats have a little more flexibility there, as hull speed is higher, so you can get further below it before becoming unbearably slow.

 

[Hippocampus]

I’m a newbie on this. Although we’ve put a few thousand miles on our new to us boat so far either run
WOT briefly when we screw up and need to get out of Dodge. Fortunately that happens very rarely.
At 10kts when we want to make tracks. Will do this for hours and hours at time.
7-8kts when just cruising along.most common speed.

It’s a NT42 with theoretical hull speed of 8.37kts. Has a Cummins QSC 540hp turbo and aftercooled non mechanical controls.
Seems very happy at 10kts. Stable in chop with no SeaKeeper on. Lots of wake at 9kts but not much at 10.
Very happy at 7-8kts as well with all gauges nominal for a 8-10 h run but will need the SeaKeeper in stuff at or over 3-4’ depending upon direction. Genset is a Onan 11kw. and goes on with the SeaKeeper.

What speed should we be running?
Best efficiency? Slow cruise?
Best fast cruise?

 

[rslifkin]

I’m a newbie on this. Although we’ve put a few thousand miles on our new to us boat so far either run
WOT dry briefly when we screw up to get out of Dodge. Fortunately that happens very rarely.
At 10kts when we want to make tracks. Will do this for hours and hours at time.
7-8kts when just cruising along
It’s a NT42 with theoretical hull speed of 8.37kts. Has a Cummins QSC 540hp turbo and aftercooled non mechanical controls.
Seems very happy at 10kts. Stable in chop with no SeaKeeper on. Lots of wake at 9kts but not much at 10.

What speed should we be running?
Best efficiency?
Best fast cruise?

Every hull behaves differently. Wake as well as how easily the boat gains speed with a small increase in throttle are good indicators of the more and less efficient speed ranges. Getting better data than that requires looking at speed and fuel flow. Being that your QSC is electronically controlled, I expect you can get fuel flow data from it.

You'll likely see fuel economy decline slowly as speed increases above idle, and somewhere in the 7 - 8 kt range, it'll start declining faster. You'll probably see an increase somewhere in the higher speed ranges, then it'll start slowly dropping again. That second peak will be your most efficient fast cruise, assuming the boat is otherwise happy to run at that speed (such as not requiring the engine to be run above max continuous to get that speed).

How the boat behaves above hull speed is what varies the most between boats. Some have a really big hump, others don't have much of one and for the most part just go faster as you add power. From your description, the NT42 may be more in that second category.

 

[mvweebles]

Got me thinking - anyone know of a general efficiency vs S/L ratio curve? We always use comparative terms - go slower/less fuel. I realize it would vary by hull type, but for the most part, our hulls (trawler-class) are more similar than not.

I did a quick plot of what I would expect to find - peak would be in the 0.9-1.0 S/L range and drop off pretty quickly after 1.2. Other note is my guess would be that for Semi-Displacement boats with enough power to plane, efficiency flattens out pretty early, meaning might as well throttle-up and get on plane vs push a bow-wake. Overall fuel consumption won't change.

Thoughts? Is there better information vs a semi-educated guess (emphasis on guess)?

Peter

 

[rslifkin]

Got me thinking - anyone know of a general efficiency vs S/L ratio curve? We always use comparative terms - go slower/less fuel. I realize it would vary by hull type, but for the most part, our hulls (trawler-class) are more similar than not.

I did a quick plot of what I would expect to find - peak would be in the 0.9-1.0 S/L range and drop off pretty quickly after 1.2. Other note is my guess would be that for Semi-Displacement boats with enough power to plane, efficiency flattens out pretty early, meaning might as well throttle-up and get on plane vs push a bow-wake. Overall fuel consumption won't change.

Thoughts? Is there better information vs a semi-educated guess (emphasis on guess)?

Peter
View attachment 137457

That's about right up to hull speed I think. Above hull speed, you'll get a hump, then efficiency comes back up and levels off a bit. But efficiency on plane will still be lower than below hull speed.

 

[twistedtree]

Got me thinking - anyone know of a general efficiency vs S/L ratio curve? We always use comparative terms - go slower/less fuel. I realize it would vary by hull type, but for the most part, our hulls (trawler-class) are more similar than not.

I did a quick plot of what I would expect to find - peak would be in the 0.9-1.0 S/L range and drop off pretty quickly after 1.2. Other note is my guess would be that for Semi-Displacement boats with enough power to plane, efficiency flattens out pretty early, meaning might as well throttle-up and get on plane vs push a bow-wake. Overall fuel consumption won't change.

Thoughts? Is there better information vs a semi-educated guess (emphasis on guess)?

Peter
View attachment 137457

What I found on our Grand Banks, which was a planing boat with max speed of about 23 kts, is that there were two distinct "zones" of fuel efficiency. The first was across the displacement speed range, and the second was from displacement speed through planing up to full speed. I think it was you who said earlier that you should either go fast or go slow, and that's pretty much what I concluded.



Across the displacement speed range, the fuel burn grew exponentially as speed increased, just like with a displacement boat. NMPG (fuel efficiency) always dropped as speed increased.


But once you maxed out displacement speed and started to climb onto a plane, things changed. As you would go faster, fuel burn of course increased, but so did speed. The resulting NMPG still always dropped as speed increased, but not nearly as much. In fact, it was almost a flat curve. Not quite, but close. So you would get almost the same NMPG at 10 kts as at 20 kts. Attached is a plot of that boat's actual performance.


My ultimate conclusion was that if I wasn't going to run at displacement speed, I might as well let 'er rip and get where I was going.

 

[mvweebles]

I’m a newbie on this. Although we’ve put a few thousand miles on our new to us boat so far either run
WOT briefly when we screw up and need to get out of Dodge. Fortunately that happens very rarely.
At 10kts when we want to make tracks. Will do this for hours and hours at time.
7-8kts when just cruising along.most common speed.

It’s a NT42 with theoretical hull speed of 8.37kts. Has a Cummins QSC 540hp turbo and aftercooled non mechanical controls.
Seems very happy at 10kts. Stable in chop with no SeaKeeper on. Lots of wake at 9kts but not much at 10.
Very happy at 7-8kts as well with all gauges nominal for a 8-10 h run but will need the SeaKeeper in stuff at or over 3-4’ depending upon direction. Genset is a Onan 11kw. and goes on with the SeaKeeper.

What speed should we be running?
Best efficiency? Slow cruise?
Best fast cruise?

If my theory (chart above) is close, running between 1.2 and 1.7 is tricky because you're not reducing time by much but you are reducing fuel economy quickly. 7.5 kts is good 'normal' cruise number. But for fast cruise, you might want to review your engine control numbers for 12-kts vs 10-kts. I have a hunch the mpg won't change dramatically, but your elapsed time reduces by 20%, which is sort of the goal for 'fast cruise.'

Peter

 

[mvweebles]

So you would get almost the same NMPG at 10 kts as at 20 kts. My ultimate conclusion was that if I wasn't going to run at displacement speed, I might as well let 'er rip and get where I was going.

Yep - I think that's right. Us Trawler owners have a "Prius Mindset" were it's all about reducing consumption. But that ignores the time-side of the equation. And let's face it, vast majority of us have boats capable of near-planing speeds.

If I get a bit bored I may convert the raw data on TTs GB plot to single data plot similar to what I posted above (speeds converted to S/L vs knots).

I'll go a couple steps more granular:

First, I'll challenge the notion of S/L 1.34 as the "right cruise speed." I suspect it's quite a ways past optimum efficiency. Throttling back to 1.1-1.2 delivers a decent fuel benefit without much time penalty.

Second, inline with TT's conclusion, if you're going to go past 1.34, get up and go. I suspect there's a dead-zone between S/Ls of approx 1.3 and 1.8 where small changes in speed (and time) have outsized impact on efficiency. If time is your limiting factor (i.e. wanna get there sooner), crank 'er up. Fewer engine/generator hours and not much of a fuel penalty.

Peter

 

[Senojev]

Suspect there is a difference between best mileage and average mileage over a season. I'm amazed at planing or semi displacement boats that are getting 3.5 nmpg!

Our Bayliner 3888 has twin Hino 6.4l naturally aspirated diesels. When we did the loop we averaged 6.6 knots over 792 engine hours burning 2960 us gallons. We travelled 5237 nautical miles for a total average of 1.769 nautical miles per gallon. Thats over the full year with currents, against currents, idling, occasionally over 8.5 knots etc.

James

We also have a 2010 37 NT with a Cummins QSB 380. Over a season, we average 3.8 nmpg and that includes some minimal genset use. Average speed is around 7.25 kts in relatively calm water and taking some advantage of currents when we can.