Novice question on fuel economy

You don't want to run a twin engine vessel on 1 engine. It is in-efficient because the thrust is not pushing the vessel straight. But the main reason is because both engines are propped to achieve rated WOT rpm with both engines running and making power. Essentially, the each engine is way over-propped throughout the rpm range if you are only running 1 engine in a twin engine installation.

That being said, I have done MANY trips running hull speed and with diesels in a planing yacht, it is very economical. I'd recommend running hull speed 4-6 hours and then cruise for 30 minutes and back down to hull speed. The newer diesels (electronic) are better at metering fuel and keeping operating temps up, but you can still do it with mechanical engines.....I did the entire Great Loop running a generator 24/7, and the engines like I described 5300NM's and average 1.5 GPM in a 1988 75' hatteras MY. But like I said, the generator ran 24 hours a day 80% of the time.

I'm guessing you're looking at yachts in the 40-50' range. Running hull speed and cruise for 30 minutes every 4-6 hours. With diesels you should see an average of 1.5-2 MPG, close to what many trawlers get.

Find it interesting that you were able to access this website; but not GOOGLE.


Some interesting reading:

https://www.google.com/#q=lehman+diesel+fuel+consumption+rate


P.S.- Welcome aboard and hope this helps.

The info is a little wrong IMO. Hull speed on a 42' Grand Banks is 7.2 knots, it doesn't matter how much you push it with how much HP. I would think a Lehmen at cruise RPM's is going to burn about 2 gph each. I would think in *MOST* cases with most single engine trawlers that 3-3.5 MPG would be acheivable with a single engine. Generally you're looking at around 50' to get to a hull speed of 8 knots in a trawler.

Some boats I'd consider looking at when it comes to non-trawler interiors etc. A 53' Hatteras MY or 58' YF with Naturals. Some of the Hatteras LRC. I can't think of a trawler without a trawler interior from memory......

Internet seems to think less:

Fuel consumption on Lehman 120s

Currently doing the Loop, let me give a few observations.
1) I don't find this talk of MPG very relevant, because the are so many factors that affect it. Also, because if you're counting pennies you have no business doing such a trip. Get a relatively fuel efficient boat and run it as your mood and circumstances dictate.

2) the idea of running 4-6 hours at hull speed, then 30 minutes at cruise doesn't happen. In places like the Erie Canal, most of the Trent-Severn, the Welland canal,many parts of the ICW you'll basically be running at idle speed for possibly days with no opportunity to put the throttles down. When you get to many places you'll have such along distance between ports (unless you're a gunkholer) that you'll want as much speed as your boat will give you, especially if a storm is chasing you.

3) We're currently doing the Loop on a 56 Hatteras. It's a very comfortable boat to cruise, and gives the right speed (we average 10-12 kts). Yesterday we passed another Looper in the middle of Lk. Ontario. With the wind out of the west he was down to about 3 knots. It'll take him forever to get anywhere, and he'll probably be doing some 24 hour shifts at the helm. That said, I consider this boat considerably larger than you'd want for convenience sake. She's got all the comforts of home (your wife would love it), but it's a hassle finding dockage, adequate electric, having to lower the arch, etc. At the Spring AGLCA rendezvous there were as many different style boats as there were families doing the Loop, from ours down to a 25' that's doing the Loop by trailer and being dropped in at different locations. My recommendation would be something in the 35'-38' range, able to get up to at least 12 kts. Single screw is certainly much cheaper, but twins will make docking so much easier and you will bring it into some tight basins.

Capt. J is correct to forget the idea of getting twins, but running on one. You'd be fighting the boat and the boat would be fighting itself every step of the way.

In choosing your Looper boat the first thought has to be about what you can afford, and the second what can you and your honey be comfortable on. Washer/drier is convenient, but you can certainly find them at marina's and towns along the way. So that's very low priority.

Yes, it says Lehmans burn 2 gph at cruise. In the size boat he's looking at, hull speed will and is much less than 8 knots. Let's take a popular boat. 42' Grand Banks, twin Lehmens at cruise buring 4gph......hull speed 7.2 knots....you're looking at 1 3/4 MPG. Prove me wrong in a trawler in the size the OP is looking at 40-50'. Tell me which trawler gets a whopping 4 mpg at 8 knots?

To the OP, take a look at a 43/44' Lagoon power cat. With the big engines it will cruise at 16.5 knots a 1.5 GPM, 12 knots at 2 mpg, and 10 knots at 3mpg....even more efficient at slower speeds.

To NYCAP: you run at cruise when you can to keep the engines clean....obviously there are some places/days you just can't........There are places in the Erie canal you can run at cruise as well......once you get west of Troy.....before Troy on the Hudson river.....etc.....I was able to put that 75' Hatt MY up to cruise at least once or twice a day almost everyday......

Know the route well. One east run the entire Erie from Tonawanda and one west run to Oswego. About 10 + up the Hudson (north/south) to Albany & Troy. Just saying that circumstances dictate more often than desires on much of the Loop.
If you put a 75 Hat up on plane almost anywhere on the Erie Canal except Lake Oneida you risked killing people in small boats tucked in and people on the shoreline, not to mention the damage done to the shoreline.

There's a few places actually. One place before three rivers where the canal runs right along the highway. Lake Oneida is another.....From three rivers North there is nowhere. But I agree, you run when you can, but many times you just can't find a place to run...... Sometimes I would just bring one up to 1400 rpms and drop the other down to 800rpms just to spin the turbo's for 5 minutes and put a good load on the engine and bake off what I could, then swap over to the other one.

However on a trawler that only cruises at 7.2 knots, you can pretty much always run at cruise.....LOLOL......

Once you start cruising you will find that fuel costs are small piece of the "cost" pie. Boats are tools so get the right tool for the job. Real trawlers (Nordhavn like) are 7-8 knot boats. A person could starve to death at that speed ;-)
Stick with T/300 to 480 hp diesels in a 45 +/- boat and you will get a good combination of speed when you need it & reasonable economy. Your fuel burn rate will have wild swings based on prop load like a pick-up truck towing or not. Divine the one engine hp output by 16 on a twin engine boat and that will give you the max cruise burn rate at normal load.
Running a twin engine on one motor is not healthy for extended periods.

You just made a very good point I hadn't thought of, and it may have gotten glossed over. A turbo boat is the wrong boat for doing the loop for exactly what you pointed out.

P.S. I know the spot you're talking about. We brought it up there also. Thankfully I was on the bow watching out. There's a spot there with rocks coming down from the road to the water's edge. There was a guy sitting at the bottom and he looked like just another rock. Had I not spotted him we'd have washed him into the canal.

if you lock the non running shaft or run a supplemental cooling setup and do not overload the running one you can run them singly as long as you like.

What would you say about efficiency & speed on "40 - 48' aft cabin" ?

By running them singly, you are overloading them at any RPM. The boat is propped based upon both engine propelling the vessel. If only one engine was propelling the vessel it would have a propellor with a lot less pitch as to achieve rated top RPM's and a proper propellor curve. So by running one you are overloading the engine at any RPM along the chart.

On the smaller boats it's less efficient to run 1 out of 2 anyways, if you run 2 and keep them off of the turbo's, they're pretty efficient at 1000 rpms and really really efficient at 800 rpms or less. Not to mention the boat is crabbing on one engne because the method of propulsion is off center.