My wife and I are approaching retirement age. Considering purchasing a small trawler to make the great loop. Something on the order of 42 Grand Banks.

Most of those produced in the 80's were equipped with couple 6 cylinder Ford diesels that would burn 1.5 gal/hr each at cruise. These magnificently efficient machines produced a grand total of 120-135 hp ea.

In the 90's and later models, these same trawlers are equipped with 300 to 400 hp twin Cat turbo-diesels.

These displacement hulls cruise best at 8 to 9 kts. Obviously you can push faster with more power. The question is, if I operated it at 9 kts, can I still expect to burn the 3 gal/hr the Fords burn?

If the price of fuel is a big concern then boating might not be for you. Diesel in my area was at $6.30/gal and my boat never left the slip this season. Most trawlers are somewhat economical depending on many variables. Finding a single screw with a bow thruster could be the ticket. Five years ago I purchased a Hatteras 53MY with the plan to retire and cruise the east coast Bahamas etc. my bank account is almost empty and I think I’m going to put it up for sale this coming season. Boating is a very expensive hobby but it’s a great lifestyle.

I think you already know the issue with a displacement hull ... there is a point depending on the water line length when you are creating a hole in the water behind the boat and even though moving forward the boat is trying to fall back in that hole and the excess energy is used to keep it out. That is the best way to explain it. For others... there is a mathematical formula for this but you've got to find that right speed for the boat. I could dig around in a book and find it but it should be on the internet for anyone interested. But it will probably be safer to figure this out by rpms rather than actual speed through the water.

My question wasn't so much the cost, it's fuel consumption per amount of work. The amount of work is pushing the boat 9 knots. Is it relevant whether you're doing it with 16 turbocharged cylinders or 12 NA? Relevant meaning would the fuel consumption be more or less the same?

I don't have the numbers you're looking for...but I did really enjoy this family's journey on their Nordhavn 40

https://youtu.be/CgUTYqZHzwQ

They ended up with a motor sailing Nordhavn before disappearing off the tubes.

I think that's the direction I'd tend to go also, except more along the cat hull lines.

I think his playlist includes fuel consumption numbers in real-world cruising.

quote:

Originally posted by Blume9mm:
I think you already know the issue with a displacement hull ... there is a point depending on the water line length when you are creating a hole in the water behind the boat and even though moving forward the boat is trying to fall back in that hole and the excess energy is used to keep it out. That is the best way to explain it. For others... there is a mathematical formula for this but you've got to find that right speed for the boat. I could dig around in a book and find it but it should be on the internet for anyone interested. But it will probably be safer to figure this out by rpms rather than actual speed through the water.

The estimate for hull speed in knots for a displacement hull is 1.34 times the square root of the waterline length in feet.

For a 42’ waterline boat, this is 8.7 knots.

The formula is just an estimate - a particularly chunky hull form might be a little lower, and a particularly narrow one might be somewhat higher, but it’s a good starting point for “normal” displacement hull boats.

You don’t want to get hung up on the exact 8.7 number. In practice, what it really means is that damn near anything will give you 7 or 8 knots, big engines burning a lot of fuel will get you 10 or 11 knots, and you can put 5000 horses in the boat and you still aren’t getting to 15 knots.

I’m not an expert on inboard boat engines, but assuming they’re throttled to the same power output, a more modern turbodiesel should be more fuel efficient than an older non-turbo engine and a higher horsepower engine should be less fuel efficient than a lower horsepower one.

If I had to guess, I’d expect the bigger, newer turbo diesels to be a little more fuel efficient but I wouldn’t be terribly surprised if I was wrong.

Regardless, the difference should be small enough to not really matter.

This summer we did a 13 hour boat day, 104-miles combined bay, river & ocean. Twin 350 mercs nothing crazy on the throttles burned 70 gallons.
Boats LOVED Trump Fuel, they hate Biden fuel.

quote:

Originally posted by mjlennon:
These displacement hulls cruise best at 8 to 9 kts. Obviously you can push faster with more power. The question is, if I operated it at 9 kts, can I still expect to burn the 3 gal/hr the Fords burn?

With a displacement hull the faster you push it the higher your fuel consumption rate per nautical mile.

Some examples in this article: Feel The Burn: The Fuel Consumption Equation

The newer Grand Banks have a different hull shape than the traditional boats. I suspect they are a semi-displacement hull form. Most likely the fuel burn at 8 knots would be very similar, probably within 10%.

I would go to a trawler forum, or a GB forum for more precise numbers.

The 41’ aluminum hulled utility boat I had at my first station had 2 Cummins VT903 engines. It carried ~470 gal of DFM (diesel fuel marine) and we got about 300 nm range, and it would run around 20-25 knots in a flat sea.

No way I’d want to pay the fuel bill for 470 gallons and only have a 300 nm range.

The 903 is a truck engine and the CG had turbos added…they had to govern the engine as the boats were aluminum and anything over 2500rpm would tear the mounts…

If I were to retire and have a boat, it would be party barge..I’d stay on a lake and drink beer and fish.

I have to aplaud your choice of a trawler, Grand Banks if a premier in the trawler world. Plus the 42 is a great choice, most are equipped with a washer/dryer, dual A/C units and a host of other features that true cruisers like.

We cruised for 5 years, albeit, on a 40' sail, we saw a large number of GB's of all sizes and partied on board quite a few. True, diesel is expensive and more so at marinas but it's a totally new life style and worth the costs. Hanging on the 'hook' with nothing but the sound of the water lapping against the hull is simply mesmerizing.

I hope it works out for you !!

quote:

Originally posted by MikeinNC:
No way I’d want to pay the fuel bill for 470 gallons and only have a 300 nm range.

It all depends on what you want to do on the boat.

E.g. there are a lot of US coastal areas where an 8-10 knot cruise makes day trips to fish offshore somewhere in the spectrum from impractical to impossible.

I used to know a fellow sho had a 40 foot Trawler. It was single screw with a IIRC a 40 HP 4 cylinder diesel. Cruising speed was 8 1/2 knots it was pretty thrifty on fuel. My vague memory is that his burn was 1 1/4 gallon per hour but don't take that as gospel. What I do remember clearly was his plan to do the Eastern loop of the US by going down the Mississippi, thru the Gulf, around Florida, and up to the St. Lawrence to get back to Michigan.He projected it would take 2 to 3 years. Last I heard he made it to Florida and then his wife got sick and the boat got sold.

Tow lessons to be learned here. First is that you need to consider your wife's and your won health status before taking something like this on. This means full physicals for both and colon and breast checks for Cancer.

Second, there is a real WALL with a displacement hull and trying to get over that WALL will burn fuel like you would not believe. Cruise at 8 - 8 1/2 knots and you'll only burn something like a 40 HP worth of fuel, try to increase that by just 2 mph and you may see 250 HP worth of fuel burn. Frankly I really don't see the need for twin V8's for a Trawler hull. While it would make sense for a planing hull for a Trawler it's a waste of space, weight, and money.

Finally I wholly endorse the suggestions that you visit a Trawler Forum. These are a special class of bots that operate in a performance envelope that most boaters never experience. The fact is that they are in a lot of ways closer to a sailboat than a powerboat.

I’m sure you know there are groups just for doing the loop. Check that out. The last couple I personally knew that started the loop didn’t finish due to COVID restrictions not allowing them into Canada - nothing to do with fuel issues- love your boat choice, btw.
Good luck.

Most boat manufacturers have forums like ours. For real world results, I highly recommend researching specific models on those forums. You likely can find very specific information based on the exact boat.

I’d guess most run 1.5 to 2 mpg with some rare exceptions being super efficient.

I can't comment on fuel usage but I can tell you the 42 GB is a terrific boat. We bareboated one in the Virgin Islands in the mid '90s. I think we were the second people to take it and we had a great 10 days. Two years earlier we had bareboated a 49 GB in the same waters. That was like and iceberg with propellers, the 42 was nimble had plenty of room for 6 adults.

We had elusions of getting a 40+ foot trawler but came to our senses, we just didn't/don't have the income.

I'm A Captain that does a lot of deliveries, 10,000 NM's per year for the past 20 years. With displacement hulls you cannot exceed displacement speed no matter how much power you give them. Hull speed on a 42' Grand Banks is 7.2 knots, OVER the water. So if you get into a 4 knot current, you're down to 3-4 knots speed over ground. If a storm is approaching you're stuck in it and can't outrun it. I had a squall follow me the entire night and next day in the Gulf making 4-6' seas in a 50' Ocean Alexander trawler and it was miserable. I'd never own a displacement hulled yacht for this reason.

Also I've done the Great loop a few times. If you're planning on doing it at 7 knots, you are literally spending every navigable day underway, with no time to smell the roses and see the towns. Locks slow you down a lot. Some places you will want to run at 16+ knots cause if you hip hop your way across the great lakes, you'll add a ton of mileage to your journey or run overnight and you're not experienced enough for that. Aside from fuel, you better be very mechanical as repairs are expensive and the Grand Banks you're talking about are all 25+ years old. The Europa with the 3208 cats is a semi-displacement, these will do up to 20 knots. However, you can still run them hull speed 7 knots +/- and get virtually the same efficiency as the displacement hulls, however you'll want to run them up to cruise for 30 minutes every 6 hours to keep them clean. On top of maintenance, dockage is pricey on the East coast.

The best compromise as far as price, fuel efficiency, and speed is a 42-43'lagoon trawler with the 300 hp diesels. They have a ton of room inside and outside. A lot less exterior maintenance than the GB's without any teak and varnish. They'll get 3 nmpg at 10 knots, 2nmpg at 12 knots, and 1.5 gallons per nautical mile at 16.5 knots and even better at slow speeds.........I delivered 2 of these, Corpus christi to Ft. Laud, and Ft. Laud to Cancun. Great seaboats.

Honestly, fuel is going to be the least of your concerns compared to maintenance, insurance, and dockage.

quote:

Originally posted by mjlennon:
My wife and I are approaching retirement age. Considering purchasing a small trawler to make the great loop. Something on the order of 42 Grand Banks.

Most of those produced in the 80's were equipped with couple 6 cylinder Ford diesels that would burn 1.5 gal/hr each at cruise. These magnificently efficient machines produced a grand total of 120-135 hp ea.

In the 90's and later models, these same trawlers are equipped with 300 to 400 hp twin Cat turbo-diesels.

These displacement hulls cruise best at 8 to 9 kts. Obviously you can push faster with more power. The question is, if I operated it at 9 kts, can I still expect to burn the 3 gal/hr the Fords burn?

I've owned and been around boats my whole life. The only data on fuel consumption I trust is from people using the boat in real world conditions. A quick Google search for that information on the 42 Grand Banks turned up the following: https://www.trawlerforum.com/f...nsumption-30348.html

Looks like reports range from 3.5-6 GPH depending on engines and speed. Definitely don't overlook the additional berthing and other costs you will incur if you are limited to 50-60 miles per day.

quote:

Originally posted by jimmy123x:

Honestly, fuel is going to be the least of your concerns compared to maintenance, insurance, and dockage.

This is so true.

quote:

Originally posted by jimmy123x:
With displacement hulls you cannot exceed displacement speed no matter how much power you give them.

It is true that starting around the hull speed of a vessel, the drag on a displacement hull starts to increase very quickly, but that doesn’t mean it’s impossible to go faster than the hull speed.

Most displacement hull boats are designed around efficient cruising, and it makes no sense to put in huge engines that can burn ten times as much gas to make an extra knot or two of top speed.

But a displacement hull with enough power and/or a particularly low-drag hull form can absolutely go somewhat faster than hull speed.

Hull speed isn’t a brick wall, it’s an elbow in the drag curve (and consequently, the power vs. speed curve).