Trawler vs Convertible fuel burn comparison

Ok, I had a question posed to me yesterday that I'm pondering today.

Fellow said to me, OK this 48 ft twin 200hp each diesel trawler cruises at 12.5 knots and burns 15 GPH isn't much different from a 48 ft Convertible with Twin 800hp Diesels that cruise 25 knots and burn 35-40 GPH. Yes Trawler burns less but you get distances half as slow vs the Convertible. He states the trawler will burn close to same fuel similar distance, I don't agree with that.

I know the range of the trawler is much greater holding 1000 gallons of fuel, the convertible holds 700 gallons. Is it just a time difference from port to port ?
Thanks

Slow the trawler to about 8-9 knots and the fuel economy may almost double.


Speed kills....mostly fuel economy.


Plus the Trawler may be much heavier which is a penalty when trying to move faster.

If you run boat boats at the same speed, fuel usage will be similar. Hp required and therefore fuel usage is based on length, weight and hull type for a given speed. Less important are number and size of the engines.

But fuel usage per mile goes up exponentially with speed, so you will not use the same fuel by running one boat slow and the other boat fast for the same miles.

David

You can't have a hard-and-fast(or slow) rule that covers all types of boating and fuel burn. There are just too many variables. You can waste fuel in a trawler and save fuel in a motoryacht. It is all a matter of your expectations. We went from a 8mph/2gph trawler where it took us 6-ish hours to get from New Bern, NC to Morehead City, NC, (a trip of about 40 miles) to a dual 320hp/+12gph at ~20mph motoryacht that reduced travel time to 2 hours. One boat burned 12 gallons and one 24, so yes, our old trawler did save us on fuel. Do we care? Not one bit. We needed the speed and time saved to more enjoy the destination and the $35 or so that we saved in the trawler more than made up for it based on the time spent in Morehead (or wherever). HOWEVER, it doesn't scale up for long-term cruising. We can slow down and close the gap between the two boats and will do that on longer trips, but in our case. What we did and how we spend money on fuel works for us.Your friend isn't completely wrong, nor is he completely right. One thing is certain... Fuel is not the biggest concern or the greatest expense in owning a boat. Someone that shops for a boat based on fuel savings needs to understand this ahead of time.

Trawler Expert said:

Fellow said to me, OK this 48 ft twin 200hp each diesel trawler cruises at 12.5 knots and burns 15 GPH isn't much different from a 48 ft Convertible with Twin 800hp Diesels that cruise 25 knots and burn 35-40 GPH.

Click to expand...

12.5 kts is probably approaching the worst speed possible for fuel economy.

Between 7-9 kts should be better.

IOW, his comparison is flawed.

We could reach almost 2 NMPG (less than 4 GPH total, IIRC) with twin 450s on our previous 42' Convertible... at around 7 kts, depending on wind/current/tide/etc.

-Chris

Trawler Expert said:

Ok, I had a question posed to me yesterday that I'm pondering today.

Fellow said to me, OK this 48 ft twin 200hp each diesel trawler cruises at 12.5 knots and burns 15 GPH isn't much different from a 48 ft Convertible with Twin 800hp Diesels that cruise 25 knots and burn 35-40 GPH. Yes Trawler burns less but you get distances half as slow vs the Convertible. He states the trawler will burn close to same fuel similar distance, I don't agree with that.

I know the range of the trawler is much greater holding 1000 gallons of fuel, the convertible holds 700 gallons. Is it just a time difference from port to port ?
Thanks

Click to expand...

"48 ft Convertible with Twin 800hp Diesels that cruise 25 knots and burn 35-40 GPH" Sounds pretty optimistic to me.

If you're in the ocean, many times you may not run 25 knots due to ocean conditions. And 800hp are optimized for higher rpm and may not give good economy at slow speeds. My experience with several boats with 900hp twin diesels was 60-80gph depending on how close to max hp.
And the bigger diesels cost a lot more to maintain and rebuild. Bigger oil capacity, more injectors, filters, etc.

Trawler Expert said:

Ok, I had a question posed to me yesterday that I'm pondering today.

Fellow said to me, OK this 48 ft twin 200hp each diesel trawler cruises at 12.5 knots and burns 15 GPH isn't much different from a 48 ft Convertible with Twin 800hp Diesels that cruise 25 knots and burn 35-40 GPH. Yes Trawler burns less but you get distances half as slow vs the Convertible. He states the trawler will burn close to same fuel similar distance, I don't agree with that.

I know the range of the trawler is much greater holding 1000 gallons of fuel, the convertible holds 700 gallons. Is it just a time difference from port to port ?
Thanks

Click to expand...

In the example you gave:
the Trawler cruising 12.5 kts is burning 1.5 Gallons/mile, or .66 nautical miles/gallon

The 48' convertible is also burning about 1.5 gallons/mile (which I think is probably not very reasonable at 25 kts, but we'll let that pass for now.

Third example, our boat, at 57' overall cruises at 7.5 kts, and uses 2.4 gallons/hr, or a little over 3.1 nautical miles/gallon. Oh, and for our boat, that fuel burn is also running the 12kw generator, on a boat weighing on average around 85,000 lbs.

Is one better than the others? No, just different. For us, the journey IS the destination, or expressed another way, the getting there is half the fun! So we don't mind getting there slower and in a more sedate manner, but it's not for everyone.

For us, the big plus of fuel "economy" means being able to stay out longer, travel longer, and not have to be tied to fuel stops, or at the mercy of the "only fuel stop for the next 200 miles of coastline". It's a big plus for extended cruising in remote areas. But if you cruise from marina to marina, want to get there fast, and don't mind the fuel bill, the convertible is probably a better bet!

What’s not mentioned is two key factors.

Displacement. A 48’ trawler may be quite significantly more efficient in moving the same weight when operating within its design parameters.

And speaking of water pushed aside. Prismatic coefficient, lwl, wave piercing or bulbous bow, parasitic drag and so many other factors effect fuel burn. So the geometry of the hull effects efficiency as well.

If you’re going to compare boat efficiencies LOA may not be the best the fixed parameter. A narrow, light long hull of for X displacement operating below hull speed is very much more efficient than a heavy beamy boat which would have much less lwl at the same displacement.
Rather would think displacement is a more meaningful one or total contained volume. Hence the half ton or one ton ratings for some old school sailboat races. In powerboats engines run constantly well below their HP ratings may be less efficient and even with common rail enhancements have less service life.
Wrote this as slowgoesit posted. He’s right. Get the boat that makes you smile.