Fuel consumption: sail/motor. are you serious?!

[Johan Leopard51]

Whether it is ‘which 4x4 is best’ or sail vs power, there are always ten angles to consider.

For us, that tend to be 8 or sometimes 10 people, mostly seeking comfortable seas in tropics, the ideal would be a 50-ish foot sail cat that has a proper flybridge and proper motors.

The flybridge is crucial for us in our weather and number onboard. I’d give up some sail area for a flybridge and make up for that with a lighter yacht.

We don’t use our 740Hp to race around, maybe once in a while do 14 knots for 2-3 hours to make a weather window or safe anchorage.

Most sail cats imho have motors that are too small, so their fuel consumption at around 7 knots without sail is horrific compared to a same weight power cat with two 300’s doing same crossings. We’ve done trips where sail cat same trip is 50% more on a 6-7 knot speed 400nm round trip!

But I do love sailing as it keeps you engaged and they’re beautiful.

And yes the best 4x4 is an old LandCruiser …

[Dockhead]

Originally Posted by HankOnthewater

. . .My 16 mt 22 ton boat uses 4.3 lt per hour, doing 5.5 knots. That is in fairly flat seas, clean bottom. That is for me the most economical.

Very similar here. My boat is 16.4m, catalogue displacement 20mt, actual weighed displacement full of cruising gear and spare parts 25mt.


I budget 1 liter per mile with a reasonably clean bottom and reasonably good conditions. In dead calms motoring very slowly 5.5-6k it's 0.4 to 0.5l/mile. Relaxed cruising at 7 knots will be about 0.8l/mile. Any kind of sea state or headwind and it goes up from there.


I can motor at 9 knots using about 3000 RPM in calm weather, but then fuel consumption is completely different.

[StephenF]

Just for info,

I rented a bareboat in Croatia for a week in July. The boat is a Cobra Future 40ft with Fly Bridge. During the week (actually 6 days) I cruised at around 7 knots (not a fast boat) and used Euro 720 of diesel over a distance of 239 NM (Zadar - Frapa - Hvar - Vis - Split - Sukosan).

Boat specs:
Make: Cobra
Model: Futura 40 with fly bridge
Type: Motor yacht
Motor: Yanmar 150hp single diesel engine
Cruising speed: 7knots

Looking at the internet, I budgeted for my consumption to be around 10 to 12 litres per hour, which was spot on. My stats were:
Fuel Spend: 720 Euro
Fuel Consumed: 450 Liters
Distance: 239 NM
Duration: 42 h

Therefore:
Litres/h: 10.7
Average trip speed: 5.7 Knots
Litres/NM: 1.9
Cost/NM: Euro 3

[Johan Leopard51]

Interesting that your numbers (mono hull I deduct?) are not that dissimilar from 51 cat

Chilled cruise good water we do about 650 liters on outer islands say 350nm return trip ten days with the 120nm crossings sticking to around 7 knots so as to depart early light and arrive before bad light. (Do not rely on blue charts below 10m in Seychelles - must do visual)

[anjou]

This is my biggest worry for my retirement . A government could tax diesel to death and my boat becomes a floating static caravan

[Na Mara]

Lots of good info here but there seems to lots of motoring going on at less than the optimal rpm for the engine. Yes slowing down saves fuel, but it’s also increases wear/maintenance on the engine.

Case in point, the recommended rpm for my engine is 2800. At that point, because I have a large engine for the boat (90hp Yanmar 4jh2 dtbe on a 43ft mono) I’m digging holes in the ocean while making just under 8 knots. If I take the revs down to 2000 I’m making a respectable 7 while burning a lot less fuel, however doing that isn’t great for the engine long term. Do others just ignore this or have they crunched the numbers and found that the greater wear/maintenance is less than the cost saving?

[rslifkin]

Originally Posted by Na Mara

Lots of good info here but there seems to lots of motoring going on at less than the optimal rpm for the engine. Yes slowing down saves fuel, but it’s also increases wear/maintenance on the engine.

Case in point, the recommended rpm for my engine is 2800. At that point, because I have a large engine for the boat (90hp Yanmar 4jh2 dtbe on a 43ft mono) I’m digging holes in the ocean while making just under 8 knots. If I take the revs down to 2000 I’m making a respectable 7 while burning a lot less fuel, however doing that isn’t great for the engine long term. Do others just ignore this or have they crunched the numbers and found that the greater wear/maintenance is less than the cost saving?

It really depends on the engine. Some suffer more than others from being lightly loaded. But at 2000 RPM if yours is getting up to temperature and not sooting from the exhaust after a long run then I doubt you're hurting anything. Especially if you run it harder for a few minutes every few hours of runtime to get things good and hot and burn off any carbon buildup. Realistically the engine will probably last longer with the lighter load as long as it's not light enough to carbon up the rings, turbo, etc.

Engine manufacturers typically recommend a maximum continuous cruising RPM. They rarely give as much information about minimum continuous, at least without asking them directly. There's usually a pretty good range where an engine is happy to run, so as long as you're not above the published max continuous or running close to idle, I wouldn't worry much about it.

[newhaul]

Originally Posted by rslifkin

It really depends on the engine. Some suffer more than others from being lightly loaded. But at 2000 RPM if yours is getting up to temperature and not sooting from the exhaust after a long run then I doubt you're hurting anything. Especially if you run it harder for a few minutes every few hours of runtime to get things good and hot and burn off any carbon buildup. Realistically the engine will probably last longer with the lighter load as long as it's not light enough to carbon up the rings, turbo, etc.

Engine manufacturers typically recommend a maximum continuous cruising RPM. They rarely give as much information about minimum continuous, at least without asking them directly. There's usually a pretty good range where an engine is happy to run, so as long as you're not above the published max continuous or running close to idle, I wouldn't worry much about it.

My opinion is run the engine hard to get up to hull speed then slowly back off untill you lose a quarter knot speed 5hen you are in the sweet spot for the engine . Load vs fuel consumption.

[Island Time O25]

Similar to newhaul's strategy. On longish motoring runs mostly run the engine up to 145-160F temps then once in awhile up to 180-190F for few minutes to burn the carbon off. My rpm gauge has been out of commission and I procrastinate fixing it. But that gave me chance to learn rpms by sound and temp of the engine. ))

One consequence I noticed after extended high temp runs is the white smoke which I take that the cooling water epaorates too quickly in its exhaust run. But because I don"t motor that much a flush of CLR mixed with water at the end of the seasln is usually enough to keep the deposits out of the exhaust run. Or so I hope. Next on my "need to buy" list is one of those cheap endoscopes a see touted on my FB feeds. ))

[sailingharry]

This idea of lightly loading diesels being bad is largely a boater philosophy. It probably started with someone being concerned about what happens when you use a big engine to run a 60A alternator for hours on end (or a couple hours every day in the Bahamas without ever moving). Then it grew to being concerned about less than ideal heavy load -- ie, anything less than 80 or 90%. I doubt you'll find any research on the negative consequences of running an engine at 50% for 10,000 hours.


But, like the old mantra "changing your car oil every 3000 miles is the cheapest insurance you can buy" (how many cars die from anything related to insufficient oil changes -- none of mine ever have), who has had an engine "die" of light loading? Most engines die from bearings, or blown seals, or water lead in exhaust manifold, or (probably most common) owners deciding that 5,000 miles is time for replacement. Very few die from "carboned up from light loading."

[jordanbettis]

I went on a deep dive on the whole cylinder glazing argument against oversized engines once because I found it strange that sailboat people talk about it like it's a critical factor but you never hear about it from anyone else who uses diesel engines. This is particularly surprising when you figure that road engines are typically producing less than 50 percent of rated power for many hours on the highway and this is considered an ideal duty cycle!


From that I've seen cylinder glazing and wet stacking tend to occur in two different use cases:


1) AC gensets operating with little load. The genset has to operate at a specific RPM regardless of the load to produce a 50/60 hz wave so the genset is going to rotate at 1800 or whatever it's designed to do even with almost no load on the crankshaft. This ratio of RPM to load is what caused the wet stacking, glazing and other problems.


2) excessive idling, especially in police cruisers or other vehicles that idle for many hours a day to run accessories and heaters. This duty cycle is similar to cruisers charging batteries by idling for several hours per day and may be where cruisers developed the fear of "too big" engines.


The only other thing I was able to find is that typically a new engine needs to be run in using a procedure that slowly brings it up to about 80% of rated power and that run in procedure may be truncated at lower power levels for oversized engines.


But in general, I think the fear of an oversized marine engine glazing is mostly misplaced unless an engine is severely underproped, idled excessively or improperly run in when new.


A final observation is that tug boats have engines that are severely oversized whenever they're not actively pushing a load which should be 50% of the time considering that they have to deadhead to or from the tow.

[rslifkin]

Jordan's assessment above is spot on in my mind. And on top of that, more modern diesels tend to have more precise fueling control, better injector spray patterns, etc. so they burn cleaner at low RPM and light loads, meaning less issues when running lightly loaded.

[skipperpete]

The O.P vanished after…. his OP in june 2023 and the thread lingers on aimlessly with discussions about Harley Davidsons, glazed cylinders, generator sets and theories about why engines fail.
In response to the ancient original post, YES! Plenty of marine engines in motor cruisers easily use 30-100 litres an hour, more if the gensets are included in the calculation. One example of how rubbery the fuel consumption versus distance over the ground is from a vessel I was the engineer aboard, 7,000hp at cruise rpm in bad weather ….12 hours steaming for zero nautical miles over the ground, and 20 hrs for only a few nautical miles in total.

[mvweebles]

We run a 36-foot Wm Garden displacement trawler. We're 2200 nms south of California at Mexico border with Guatemala. Our diesel costs work out to under $1/nm (USD) after around 7-8 months active cruising. We've spent more on beer and tacos. I was talking with a sailor this afternoon on a 43-foot boat. He's about to spend $5k on a replacement mainsail (bargain price -North Sail would have been $9k).

.Depends on how broke you are. I sat in the pool this afternoon having a $40-peso Negra Modelo beer ($2.25 USD). When the bar closed, a frugal sailor came with his own beers (cheapest possible - 75 pesos for a 6-pack), around $0.70 each. Not exactly cold but he saved $3 USD by waiting so he could bring his own beer.