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

[OldManMirage]

Yeah, Dashew is probably nothing like the average cruiser, certainly nothing like me. I'm a guy who is just trying to get there, and by "there" I mean let me go to the Bahamas. I'm doing it on a SS budget and it won't be pretty, but all the pretty I need is the water in the Bahamas.

[Simi 60]

Originally Posted by RedneckRedcoat

Not even the engine manufactures go off gallons per mile. It’s gallons per hour at engine revs. You could be doing 3500 revs and go 1 mile in 4hrs or do 1500 rev and travel 5 miles in 1 hour, one would use far less diesel than the other.

We are not replacing sails or lines every year but on average they last about 10 years. Even at $20k that’s only $2000 over 10 years. We have traveled 1000 miles since December. If I sail 1000 miles in a year that’s $2 a mile almost half the cost and will be even cheaper if I clock up more miles were as in a the example above the Trawler the cost per mile is going to go up by $3.75 for every mile traveled

Sailing is way cheaper and economically

It really depends on the boat

The last cat I was building for ourselves if done in sail would have had north of $100,000 in winches, pulleys, ropes, rig and sails on it.

Ringing wire and sail replacement every 10 years or less ( prior cat we built blew up its mainsail after 5 years) adding another $30,000 (guess) to the mix.

It likely would have motored on one litre/nm doing 8 knots.

You do the math

[mvweebles]

Originally Posted by RedneckRedcoat

Sailing is way cheaper and economically

Do you have direct experience? I know of three people who circumnavigated on sail, then switched to power for long range coastal passagemaking. All three say their long term costs between power and sail were roughly equal.

Now, if goal is to cross oceans vs Alaska to Maine (Coastal Passagemaking), a well equpped powerboat is more expensive than a suitably equpped sailboat. But that's only for cross-ocean cruising. And of course if you want to live on a shoe-string and are okay with taking a performance hit with baggy sails, you can really go ultra-cheap with a sailboat.

[HankOnthewater]

I wonder if Anton (OP) would be any wiser or better informed after nearly 100 replies to his original question. Or just more confused? He has not come back to this thread again to post.
I gather he is a first time boat buyer, and considering his options.
Checking his other posts, he lives in Europe and looking for a liveaboard boat between 30 and 45 ft, and a budget of 50k (i assume that is euros).
Maybe someone clever (and with time) can summarise all the good info of these posts, as obviously lots of people are interested; this thread has sofar 4570 views.

[boatpoker]

38' Benford Fantail with 50hp Beta
7.4nmph at 1800rpm using 0,75gph
Is that as cheap as a sailboat ?

[Icarus]

Originally Posted by Anton Tokar

Hey all!

I just for a minute considered getting a motorboat instead of a sailboat as my first boat (for liveaboard).

and I asked to chat with GPT to give me a fuel consumption comparison.

And I cannot believe my eyes,

how could a boat eat 30-100 liters an hour???

just impossible.

what would be my budget for motoring 24 somewhere???

A diesel engine uses approx. 200gr of fuel per hp.
If the motor boat uses 100hp, that's about 20 litres/hr

[Adelie]

Originally Posted by Icarus

A diesel engine uses approx. 200gr of fuel per hp.
If the motor boat uses 100hp, that's about 20 litres/hr

It’s more than 20l/hr, diesel weights about 850g/l.
So about 23.5l/hr.

Also fuel efficiency depends on the size of the engine and the percentage of max power being delivered.

Larger engines have better efficiency. There is a bunch of data on farm tractor engines. Last I checked there was only 1 engine that had exceeded 20hp-hr/gal and that was a 600hp engine. When you get down around 50hp peak efficiency is around 14-16hp-hr/gal.

Also for vessels with fixed pitch props, including feathering and folding props, efficiency drops precipitously as speed drops, if you look at a fuel map for an engine the prop curve goes thru some pretty inefficient areas when not at or near full power. The reason mpg improves as speed drops is the total demand falls faster than efficiency declines so mpg improves.

With a Controllable Pitch Prop pitch can be adjusted so demand on the engine is kept at or near the most efficient rpm for a given speed.

From my research I noted that even though smaller engines are less efficient than larger engine when both are operated at their max power, the smaller engine is more efficient than the larger engine when they are both operating at the same HP.

To rephrase a 70hp engine is more efficient at 70hp than a 40hp engine operating at 40hp but when they are both operating at 30hp the 40hp engine will be more efficient.

[becrux]

Originally Posted by Sailing Hanse37

Well, i just want to add our real life experience when we motored from the Canary Islands to the Capverdian islands due to no wind at all. We had, with our beneteau 43 and a 55 hp yanmar, a fuel concumption between 1,7 and 1,8 litres per hour. The motor was running at 1500 to 1600 rpm and average speed was 5,5 knots. So that makes about 0,22 gallon/hour for 5,5 miles. Thus consumption per nautical mile is 0,22 / 5,5 = 0,04 gallon/nm. In metric units that makes 0,33 liter/nm.There is no motor boat capable of even approaching that consumption… a sailboat is built to be sleek in the water, offering the least water resistance possible… a motor boat, and certainly a trawler, never is.

Your figures are off. 1.7 or 1.8 lph is 0.45 to 0.476 gph- approximately double your claimed 0.22 gph. Still your consumption would figure in at about 0.09 l/nm, which is excellent. I suspect current may have been your friend, but there is no doubt that you were operating at a sweet spot for your equipment. Well done.

[Adelie]

Originally Posted by Anton Tokar

Hey all!

I just for a minute considered getting a motorboat instead of a sailboat as my first boat (for liveaboard).

and I asked to chat with GPT to give me a fuel consumption comparison.

And I cannot believe my eyes,

how could a boat eat 30-100 liters an hour???

just impossible.

what would be my budget for motoring 24 somewhere???

Fuel consumption is going to depend on boat length, hull design, displacement and speed.

Let’s start with whether you are content to motor at displacement speeds (4-7kt) or if you want to plane (14-18kt). At displacement speeds you will be using 0.5-1.5gal/hr depending on boat length and displacement. Longer boats use generally need leas power per knot of speed but typically weigh more which requires more power per know.

If you want to plane then fuel use goes up 3x, 5x, 10x, … depending on speed, length and displacement. Increasing any of those increase fuel consumption.

Hull shape also plays into this. If you try to push a displacement shape to go faster at some point more power just burns more fuel and makes bigger waves.
If you throttle way back so a planing hull is at displacement speeds it will get about 1/3 the mpg of a displacement hull.

If you decide you never need to go faster than displacement speed the hull can be designed to be much more efficient than it you also want to plane.

Depending on whether the boat is a motor vessel or a sailboat also affects this. For motor vessels there is generally more propulsive power available so displacement vessels are designed for more living space at the cost of efficiency. These hull shapes are still much lower drag than planing hulls.

Because sail power is generally significantly less than engine power their hulls are designed for the least drag at the expense of living space. WHO’s means that sailboats under motor typically get significantly better mph than most displacement motor vessels.

[Simi 60]

Originally Posted by Icarus

A diesel engine uses approx. 200gr of fuel per hp.
If the motor boat uses 100hp, that's about 20 litres/hr

We have a 350hp engine pushing 60ft and 65 tonne
At 1150rpm it's using a bit over 100hp
We consistently do 7.5 knots burning 15 lph

[Simi 60]

Originally Posted by Anton Tokar

,
how could a boat eat 30-100 liters an hour???
just impossible

Easy

Near 100ft fast catamaran ferry that goes out to a nearby island, approx 50nm uses 2000 litres each way and does the trip in a bit over 2 hours
We do the same trip and burn a bit over 100 litres but take close to 7 hours.

[Jeff F]

I've traveled over 11,000 NM in my current power boat, averaging 4.5 nmpg to date. 49' wll, 36,000 lbs displacement, 135 HP. Average speed around 7 knots with occasional higher cruise.

I've done the AICW in three boats - a Jeanneau Sun Fizz (sail) at 3.8 nmpg, a Mainship 34 (power) at 4.2 nmpg, and my current boat at close to 5 nmpg. All at about the same speed.

I'm a sailor, but for my budget and cruising desires the power boat offers a lot more. If you choose the boat and operate it with a goal of minimizing fuel consumption it's not hard to get fuel economy that rivals a sailboat offering comparable amenities.

Of course I'm motoring 100% of the time vs 80% that seems typical for cruising sailboats that travel my routes. That's the downside.

[Lee Jerry]

Originally Posted by Adelie

Fuel consumption is going to depend on boat length, hull design, displacement and speed.

Let’s start with whether you are content to motor at displacement speeds (4-7kt) or if you want to plane (14-18kt). At displacement speeds you will be using 0.5-1.5gal/hr depending on boat length and displacement. Longer boats use generally need leas power per knot of speed but typically weigh more which requires more power per know.

If you want to plane then fuel use goes up 3x, 5x, 10x, … depending on speed, length and displacement. Increasing any of those increase fuel consumption.

Hull shape also plays into this. If you try to push a displacement shape to go faster at some point more power just burns more fuel and makes bigger waves.
If you throttle way back so a planing hull is at displacement speeds it will get about 1/3 the mpg of a displacement hull.

If you decide you never need to go faster than displacement speed the hull can be designed to be much more efficient than it you also want to plane.

Depending on whether the boat is a motor vessel or a sailboat also affects this. For motor vessels there is generally more propulsive power available so displacement vessels are designed for more living space at the cost of efficiency. These hull shapes are still much lower drag than planing hulls.

Because sail power is generally significantly less than engine power their hulls are designed for the least drag at the expense of living space. WHO’s means that sailboats under motor typically get significantly better mph than most displacement motor vessels.

"Fuel consumption is going to depend on boat length, hull design, displacement and speed."
This is correct, but even more simply, just displacement and speed (design or cruising) can tell you the fuel consumption within +/- 20% (at least on the low end, there's almost no limit to ways to make things less efficient). The displacement could be broken down a little more to payload and range if desired. Everything past that is just details.

However, I'm going to have to challenge / disagree with you on several other of these statements.

"...so displacement vessels are designed for more living space at the cost of efficiency."
No, this goes to the displacement / payload above. I suppose this depends on how you define efficiency, but IMHO deciding to carry more stuff isn't less efficient. (Maybe wasteful, but not less efficient.) For example, a school bus may be less "efficient" than a sedan in terms of fuel economy, but if you have to get 30 kids to the same place at the same time it's "more efficient" than 30 sedans. Efficiency is better used when comparing two things doing the same thing.

"These hull shapes are still much lower drag than planing hulls."
That depends, as you've indicated previously, entirely on the speed - displacement has lower drag at "low" speeds (~<hull speed) while planing has lower drag at "high" speeds (~>hull speed).

"Because sail power is generally significantly less than engine power their hulls are designed..."
Every sailboat I have owned, from 25ft sportboat (similar to Melges 24) to 38ft racer/cruiser to 52 ft heavy cruiser, and probably every boat I have ever sailed on (many, many more within that same range), has achieved its max speed under sail. This is without considering surfing. Drag is a function of speed, so that means more power (thrust) is generated by the sail plan than the engine.

"WHO’s means that sailboats under motor typically get significantly better mph than most displacement motor vessels."
Even though it's probably something simple and obvious, I can't figure out what you mean by "WHO." Also, did you mean "mpg" or "mph?" Regardless, I would say that a heavy cruising sailboat (Oyster, HR, Passport, etc.) have both very similar speed (mph) and fuel economy (mpg) to a trawler - i.e. two boats designed for a similar purpose.

[Tedd]

Thirty litres per hour is about 110 hp and 100 litres per hour is about 375 hp. Those are realistic numbers for many powerboats.

[Adelie]

Originally Posted by Sailing Hanse37

Well, i just want to add our real life experience when we motored from the Canary Islands to the Capverdian islands due to no wind at all. We had, with our beneteau 43 and a 55 hp yanmar, a fuel concumption between 1,7 and 1,8 litres per hour. The motor was running at 1500 to 1600 rpm and average speed was 5,5 knots. So that makes about 0,22 gallon/hour for 5,5 miles. Thus consumption per nautical mile is 0,22 / 5,5 = 0,04 gallon/nm. In metric units that makes 0,33 liter/nm.There is no motor boat capable of even approaching that consumption… a sailboat is built to be sleek in the water, offering the least water resistance possible… a motor boat, and certainly a trawler, never is.

1.75l/hr is 0.46gal/hr. That means at 5.5kt you were getting about 11.8nm/gal.