A question about cruising at "Trawler Speed" MPGs

I guess having started my life with 30 ft tides and a sailboat on an estuary that was very shallow, transiting via Switzerland where only the money counts to the North East Coast of the USA where it is almost all big water and now to the PNW and the BC coast where we have tide rips in 200 Ft of water and lots of rapids between islands I have become fairly used to the fundamental principles of relative motion. (BTW Einstein was at least partly educated in Switzerland so maybe that's where I caught it :lol: )

Being only recently retired, my objective is still to get someplace desirable at a reasonable cost. Don't tell my brother who lives for the "fun" of crossing the Irish sea in a sailboat, filling up with Guiness and saling back again in crappy weather.

My big pleasure today is virtually the same as it was with our CD22, I can go slow if the conditions warrant/permit and I can haul A$$ for 50% less economy if I need to. For me anything over 10 Kts (through the water) is almost the same mpg as 15-20 Kts so I have a wide range of choice in the mid range but currents over long distance are still very important and comfort and safety are above all.

One of my biggest frustrations in life was in having a 40 KT plus deep V and having to do 12 Kts for many hours because of weather, loose dental fillings and a shortage of clean underwear.

You have to admit it's a hell of a lot more fun than driving down the interstate.

M
 
thataway":365juas1 said:
Before GPS, all we had were paddle wheel , or chip logs...
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I had a Walker Excelsior patent log on a trip to Alaska back in the late 70s, but I stopped using it when something ate my "fish". :shock:
 
I have a Floscan fuel meter on my 22 C-dory Cruiser with twin Honda 50's. The other day, I went to the trouble of writing down fuel use at a range of over the ground speeds (GPS). It was lightly loaded, one person onboard, about half full of fuel.

Here's what I found (sorry, I did it in gallons per hour, not miles per hour)
Knots GPH
5 0.80
7 1.00
8 1.80
10 2.00
12 2.40
13 3.00
14 3.20
15 3.60
16 4.00
18 4.40
20 4.80
21 5.00
22 5.20
23 5.60
 
It tells you where the internet has gone: When Mike D wrote the article in 2003 in Boat Design.net, he got 528 hits on google: Boat "hull speed" max. Today the same google search gives 78,200 hits. There are two books which every serious student of boats should own--David Gerr's "The Nature of Boats" and one of the "Voyaging under power" I own 3 editions--I understand that there is a fourth in preparation. The latter show fuel consumption/speed curves for a number of trawlers. Mike D contributed a lot of very good information on the year he was on the design forum.

What we have to understand is that all of the C Dories are planing boats (including the Tom Cats), which begin to plane at a very low speed. They are not semi displacement boats, nor displacement boats. Planing boats, lift, they draw less, and the length waterline becomes less (wetted surface less, which requires less energy) as they come onto a plane. They will be less effecient at the lower speeds, but still can be very effecient.

A full displacement boat--for sailors think of a West Sail 32, or power boats one of the big old tug boats which are double ended at the waterline. They will never go faster than a certain speed. They begin to sink as the speed increases, since the wave steepens, and there is apparently less floatation.

The semi displacement boat --typical of "down east" or Rosborough type of boat--will have a rounded bilge flat run, and run more effeciently in the speeds between displacement (generally below 1.34 x sq rt LWL) and planing speed. The boat begins to lift and often runs at a somewhat bow up position. Some of these boat can plane--with enough HP. But they are less effecient at planing speed, than a planing hull.


From Mike D's post (and Mike is deceased) and a lot of the essence of the post is in these paragraphs.
In the quote at the beginning of these notes is the notion that a boat can climb up and over its bow wave. Agreed that a planing boat may rise and may travel very fast but the wave is still there at the position where the point of contact is. On a displacement boat the idea of climbing up and over is foolish to put it kindly. It’s like asking somebody to run instead of walk so the chin can pass the nose.

The hull speed is only a guide to a speed that should not be exceeded in the interest of fuel economy but then it might as well be 1.3 or maybe 1.4 or 1.2 perhaps. But what is just as important is the power setting of the engine and sufficient power in reserve to overcome bad weather. A gas engine has the characteristic of efficient operation at almost full power whereas a diesel should not run at more than about 70% output. It is unfortunate but it means a gas engine is inefficient at lower, cruising speeds whereas a diesel is more suited.

We use the notion that a planing boat climbs over the bow wave, because the bow wave moves further aft. In some cases to a "delta pad" just in front of the motor, and in other instances, the boat rides mostly on its prop (racing boats), or foils in the world of sail boats (which was actually well known in 2003). In fact what happens on a planing boat is much different than what happens in either a displacement or a semi displacement hull.

The last paragraph, shows that even in 8 years, at least the notion of how a diesel can be run has dramatically changed. In fact the thinking of this paragraph is classical--but not entirely in tune with today's thoughts.

Finally, Mike D did a calculation on a boat not too unlike our C Dory 25, as a planing hull (Actually 8.5 meters and 4500 lbs). His conclusion was that 180 hp would give 30 knots, and gives graffs with range vs speed etc.
http://www.boatdesign.net/forums/option ... -1068.html, click on the first PDF.
 
Richard,

I think your paddle wheel calibration plan makes the most sense. Good plan. Most practiclly useful. Thanks.

Harvey
SleepyC :moon



NewMoon":28mxkdh9 said:
On calibration of paddlewheel speed:

We've noticed that the paddlewheel-reported speed can be very accurate at planing speeds, and very inaccurate at displacement speeds, or vice-versa.

For our purposes, it seems more useful to have the paddlewheel calibrated to show accurate speed over water at displacement speeds. This way, by comparing paddlewheel speed with GPS speed, we can accurately gauge how much current we are running with or opposing, at the slow speeds where current makes the most difference.

When we're going faster, we're more interested in GPS speed over ground.
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I read last night that the approximately 4600 largest cargo vessels are have recently dropped their cruising speed to 11.7 knots, although they are capable of 20 to 24 knots because of the increasing cost of fuel.
 
Rowing this size of boat it makes more sense to stand up--make a frame for the cockpit and use Thole pins. My grandfather rowed his 30 foot sailboat (full displacement) with 18 foot sweeps, standing facing forward, using the legs rather than the back/arms to row. It would also be much easier to fit Thole pins than oar locks, and give more "power" I'll have to scan a photo when I recover from my latest procedure.
 
THE INVISIBLE WORKSHOP
SAILING ADVENTURES IN THE FIRST LIGHT TROW—A HOMEBUILT, 15FT 6”, PLY AND EPOXY, YAWL RIGGED, SAIL AND ROW DAY-BOAT NAMED ONAWIND BLUE.
 
Here are the results of my cruising for the last couple of days.

In crossing the Gulf of Mexico. 1900rpm speed 5=6 MPH 4.5 miles per gal.

At 4000 rpm speed 20 mph. 70 miles used 23.3 gal = 3 miles per gal.

This is loaded heavy 58gal gas, 20gal water plus all the junk to be gone a month.

Today 1400-1500 rpm speed 5 mph traveled 42 miles. = 7 miles per gal.
I actually did a small amount better than this there is still about a cup of gas in the 6 gal tank.

I saw very little difference in speed from 1400 to 1900 rpm but big difference in mpg.

Today I did not run more than 1500 rpm.
If you can stand to go 5 mph it makes a big difference.

There was also a difference in the gas I burned today 100% gas vs E10

Roger
 
After cruising for several days at 1500-1600 rpm. The result I consistently got was 5-6 mph 7.2 miles per gal. This was using an external 6 gal tank with exactly 6 gal each time and distance measured over ground with Garmin nuvi GPS. The results were the consistent so I fell confident of this result.

I never did get a good controlled situation at high speed 20-24 mph. I either didn't have exact measured amount of fuel or had to change speed. But that being said. It averaged out to 3.2 mpg at 4100 to 4300 rpm.

Much below 1500 and I would actually lose mpg.
 
flrockytop":2r8fal6v said:
After cruising for several days at 1500-1600 rpm. The result I consistently got was 5-6 mph 7.2 miles per gal. This was using an external 6 gal tank with exactly 6 gal each time and distance measured over ground with Garmin nuvi GPS. The results were the consistent so I fell confident of this result.

I never did get a good controlled situation at high speed 20-24 mph. I either didn't have exact measured amount of fuel or had to change speed. But that being said. It averaged out to 3.2 mpg at 4100 to 4300 rpm.

Much below 1500 and I would actually lose mpg.
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It's good to find the sweet spots for displacement speed and fast cruise speed RPMs that gives you the best mpg. It's nice to have the option of slowing down and improving range or speeding up (at a fuel cost) and making miles.

Good info.

Best wishes,
Jim B.
 
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