Improve fuel efficiency of vessel

[cnfx31]

Hi, I'm looking to purchase a fishing trawler that has been converted into a long distance cruiser. It ticks many of my boxes with the sturdiness of the vessel, proven cruising history, great layout, but at 2 litres/nm at cruise (8 knots, 1200rpm) is a little thirsty for my plans and wallet. I'd prefer to be in the 1 to 1.5 litres/nm range. Anyone have a suggestions for solutions that would make this vessel suitable for me?

 

[bajansailor]

Welcome to the Forum CNF.

You will need to provide some more info about this trawler in order for anybody to make suggestions.
Such as overall dimensions and approx displacement - and ideally a photo or sketch to help show what it looks like.
2 litres / nm is approx 2 miles per gallon - if it is a 50' or 60' trawler doing 8 knots at 2 miles per gallon then that is pretty good really. Throttle back to 7 knots and your mpg will improve dramatically.

Edit - if the cruise rpm is 1,200 rpm at 8 knots, what is the maximum rpm of the engine?

 

[comfisherman]

Keep the bottom clean, shave off as much weight as possible and knock the speed down to hull speed whatever it may be. After that not much left to do, physics being what it is and all.

 

[cnfx31]

Welcome to the Forum CNF.

You will need to provide some more info about this trawler in order for anybody to make suggestions.
Such as overall dimensions and approx displacement - and ideally a photo or sketch to help show what it looks like.
2 litres / nm is approx 2 miles per gallon - if it is a 50' or 60' trawler doing 8 knots at 2 miles per gallon then that is pretty good really. Throttle back to 7 knots and your mpg will improve dramatically.

Edit - if the cruise rpm is 1,200 rpm at 8 knots, what is the maximum rpm of the engine?

bajansailor thanks for the reply. The vessel is 52' x 14.4' with 5.9' draft and displacement of 25000 Kilograms. I don't have information for maximum rpm but the max speed is 11-12 knots and the engine is 450hp Mercedes 8 cylinder diesel

Is it possible to quantify the mpg at 7 knots. I'm in no hurry to get where I'm going!

 

[bajansailor]

I think that this article might help to answer some of your questions -
Feel The Burn: The Fuel Consumption Equation https://www.passagemaker.com/technical/feel-the-burn

 

[Barry]

Slow down,
If you search for trawler boat tests you will find quite a few tables that show fuel consumption at various speeds. The Krogen 50 below put these numbers forward and I believe the engine was in the higher 200 hp range
5.7 knots gives you 5.2 nmpg
7.5 knots gives you 2.5 nmpg
8.9 knots gives you 1.06 mpg

So going from 5.7 knots to 7.5 knots doubles your fuel consumption.

 

[missinginaction]

Yep, the laws of physics don't change. CNFX, I've a much smaller boat than yours @ 26 feet, 10.5 foot beam. Horsepower is about 200 max. I run at about 7.5 - 8 knots typically and travel about three statute miles per gallon. RPM is only about 1800 and slowing down to almost idle speed doesn't help much. Besides, running a small V-8 with almost no load on it isn't really good for the engine anyway.

Back a few years ago I installed a 9.9 horsepower "high thrust" Yamaha outboard. These are typically trolling motors. One day I had a little time and thought "I wonder how far I can go with just the little outboard with it's three gallon tank?". It took a while, but I traveled down river 12 miles and back on that little three gallon tank. Still had about a quart in there when I tied her up. Now I was only making about 4.5 knots so it was a leisurely ride but it goes to show how little fuel you can consume if you're willing to accept a slower pace. I could never had done that with the main engine running though, even at idle. I needed the smaller engine.


A couple of observations. I installed an autopilot as trying to manually keep a boat on track at almost walking speed is really tedious. I haven't used the little Yamaha for traveling much over the past few years as fuel has been relatively inexpensive. That's likely to change this year and I'm glad I have the option.

 

[cnfx31]

Thanks all for your responses, I'm learning a lot!

missinginaction, your experience feeds into a thought I had, is there a solution to replace the engine? On the face of it I assume there is good reason that the existing engine was put into the vessel in the first place, but equally I see many boats of similar dimensions with much smaller rated engines, for example the Gardner 6LXB has appeared a few times, and seems to be very economical engine.

I'd be grateful for all opinions on this. As I say, i'm learning a lot

 

[bajansailor]

If money was no object, then yes, a Gardner 6LXB would probably be ideal for this boat (it is the holy grail that trawler owners dream about) - but you would also need to buy a new propeller as well, and probably a larger diameter shaft, as the reduction ratio of the gearbox would probably be higher.

But realistically, if you really like this boat, then just run the engine a bit slower so that you are doing 7 knots instead of 8 - the improvement in mpg will be on par with the figures mentioned in Barry's post above.

 

[comfisherman]

You can buy a tremendous amount of fuel for the cost of any form of repower. Between brackets, transmission adapters, coolant adapters, exhaust re routing, alignment and all the additional accutremonts it can add up fast even if the gear and everything else can be re used. I've played around on a lot of boats around 50 feet from 50-70k lbds so right around the same weight and size. Usually they cruise from. 7.4 to 7.6 knots at enough rpm difference below 8 knots to make a difference.

 

[kapnD]

A converted fishing trawler is almost certainly running much lighter than it was designed to.
If original gearing and propeller are unchanged, you may be able to improve your mileage by altering those items.
Your best bet is to work with a reputable propeller shop. they’ll crunch your numbers for you, and they’ll have a wealth of practical experience to add.
There’s some really helpful tools available at boatdiesel.com and elsewhere.

 

[comfisherman]

For free you can use the victoria prop calculator tool. For my non volume fisheries we used to swap out a prop that had 2x more inches of pitch to compensate for weight differences. If the boat was going to always be dry probably could have done 3-4 inches compared to my operational normal weight.

 

[rnlock]

When keeping the bottom clean, if you don't have time to do it all, do the forward part first, because the boundary layer is thinner there and roughness will make a bigger difference. The prop and rudder ought to be nice and clean, too. If there are lumps and bumps, gadgetry, etc. in the water, you might be able to fair them in and save some fuel, though I don't know how much. If the boat has a sealed cooling system, using what I think are called bilge coolers on the outside of the hull, you might see if they can be faired in a bit better without interfering with their function. I'll admit I don't know how significant these drag sources are. They might only be noticeable when you've slowed down, significantly below hull speed.

I'm sure different props make a difference, too. Generally, a large, slow prop can be more efficient than a small, fast one. Up to a point, anyway.

 

[sdowney717]

If you want to vastly improve a power boat's fuel mileage, put a giant balloon on it and float it across the water in the air. You also get to go with the prevailing winds for free.
No starwars character would have a boat move through water, they would cruise in the air at high speeds using some fantastic technology that will only ever be found in science fiction movies and book fantasy land.

But of course real life for us today there is the kite
POWER YOUR BOAT WITH KITES: Wind Power by KiteShip (inhabitat.com)

SkySails fly 100 to 400 meters above a ship, to which they are attached by rope. An automated control system adjusts the kite’s flight path, and a monitor on the SkySails’ workstation displays information related to a ship’s operation. The kites can cut a ship’s fuel consumption by 35 percent.