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quote:
Originally posted by maladat:
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).


From my experience it is a brick wall when it comes to a trawler or yacht. A friend of mine repowered 6 of these 42' Grand Banks that went to Norway and Sweden about 7 years ago (along with 48's, a few Europa's etc. etc.). I ran all of them to the ship 50 miles away. They needed to have motors newer than 2001 to meet the EU standard to import a yacht, the replacement new Lehmans are 160HP instead of 120 HP. The hull ONLY does 7.2 knots no matter what you do, at 2/3 throttle it did 7.2 knots now, giving it full throttle created a huge stern wake (like 5'), lifted the bow a bit and still 7.2 knots. Surprisingly I've run several displacement hulled boats that were seriously overpowered, whereas hull speed was like 2/3-3/4 the rpm of rated cruise RPM, they wouldn't go any faster if you gave them more throttle. One was a 60' defeaver with 6v53 TI's, hull speed was at 1600rpms, rated cruise 2400rpms and WOT 2800 rpms, on survey/seatrial we did WOT, it didn't go any faster either. Also if you tried to exceed hull speed and in less than 10' of water, it would do a 180 in 2 boat lengths.

Please keep in mind I run 100 different yachts per year on average, for 20 years, and do 10,000-15,000 NM's in deliveries every single year. While long narrow boats like a sailboat can exceed displacement speed given enough power, a fat portly trawler isn't going to. Most power trawlers will hit hull speed at 65-80% power or less, so you're cruising at hull speed (not WOT). The larger diesels don't burn anymore fuel running them at less of a load to hit hull speed......load determines HP needed, determines fuel needed to make that HP.

As for the OP, you'll end up averaging 2.5 nmpg in a 42' Grand Banks trawler most likely, 60 NM a day, 25 gallons of fuel per day. The loop is around 6200NM with all of the marina pull ins and outs......at a 5 knot average (which is what you'll average when you figure waiting on all the locks, current, etc. ). 1248 hours/ 8 hours a day= 155 days underway.....doesn't leave much time to stop and smell the roses.
 
Posts: 21335 | Registered: June 12, 2005Reply With QuoteReport This Post
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Picture of Blume9mm
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quote:
Originally posted by jimmy123x:
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.


It amazes and pleases me that every once and a while on this forum you find someone who is a real expert on the item being discussed. Wow! I no longer sail but I sure learned a lot from the above post. Thanks Jimmy123x


My Native American Name:
"Runs with Scissors"
 
Posts: 4441 | Location: Greenville, SC | Registered: January 30, 2017Reply With QuoteReport This Post
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Thanks for all the terrific replies, especially you Jimmy. I was hoping you'd comment...that was sure enough some great info.

It was hard for me to wrap my head around the much larger and powerful engines not using more fuel.

Of course I was trying to gauge fuel costs, but that is only one of many. I'm also still trying to see how many days we could comfortable go between marina stops. At $5~/ft/night that'll add up in a hurry too.

The wifey was looking through the boattrader with me. I can promise you none of those 25 year old boats made the cut! Wink She did like one of those newer Europa's. Gulp!
 
Posts: 1828 | Location: Fayetteville, Georgia | Registered: December 08, 2005Reply With QuoteReport This Post
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Originally posted by mjlennon:
Thanks for all the terrific replies, especially you Jimmy. I was hoping you'd comment...that was sure enough some great info.

It was hard for me to wrap my head around the much larger and powerful engines not using more fuel.

Of course I was trying to gauge fuel costs, but that is only one of many. I'm also still trying to see how many days we could comfortable go between marina stops. At $5~/ft/night that'll add up in a hurry too.

The wifey was looking through the boattrader with me. I can promise you none of those 25 year old boats made the cut! Wink She did like one of those newer Europa's. Gulp!


Don't overlook the power catamarans. Check out Leapords and Lagoons....in the 40-45' range. IF you planning on doing the great loop, workable side decks are a must. As you have to have someone on the bow and stern tending to the lines in the locks. Diesels barely sip fuel at lower rpms (2300 rpm diesel will sip fuel at 1000 rpms and less for example). Dockage is pretty cheap for 2/3 of the loop. It's the East Coast mostly where it's pricey.

The 75' Hatteras MY if I kept it at hull speed 1000rpms/ 10.2 knots we used 13 gph with the generator. At cruise 1900 rpms/17.5 knots we burned 90 gph.
There are a lot of loopers that tie up on the lock walls and such for free or almost. Some had only 30amps electric. But if you need 50 amps, you could have a 50' 30 amp cord and a splitter to make 50 amps 240v out of 2 outlets. You can also anchor, but need a cruising guide and charts to find areas out of the way you can anchor. I did the loop in a 75' Hatteras MY and was able to find a place to anchor every night from Chicago South, except we stayed in a marina every 3rd night.

Anchoring is a whole nother ball of wax. IF you do the loop on the right schedule, you can get away without A/C mostly. We ran the generator all of the time (used around 1 gph or so on this yacht), needed to in that yacht, but when you add up generator maintenance and fuel it's just as cheap to stay at a marina ar $3' or less. Some people stay on anchor without running a generator and have an inverter bank and use minimal electricity.
 
Posts: 21335 | Registered: June 12, 2005Reply With QuoteReport This Post
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