Range of C-Dory at Hull Speed

[Jim Gibson]

Has anyone figured out the cruising range of a 22 foot C-Dory, with 50 gallons of fuel, running it at hull speed, not unlike a diesel trawler or a motor sailor. In other words, if the C-Dory was taken on an entended cruise up to Alaska or down to Mexico, and it was only run at maybe 8 knots or so, would the economy of the C-Dory be similar to a single engine diesel pushing a small pocket trawler at hull speed?

Any thoughts or calculations on this?

[Adeline]

Interesting question. I installed a fuel-flow computer and my cruising results were what I expected. I.E., about 2.5 mpg. However, as I recall, motoring away from the launch my indicated economy was 8 or 9 mpg @ around 5 or 6 mph(no wake speed). I really haven't nailed down these numbers and I'm only going from memory. Next time out I'll pay closer attention and get some firm figures.

[lloyds]

I would guess a C Dory at trawler speeds would get less than half the mileage that a displacement vessel powered by a diesel engine would get.

[sportner]

If all we are doing is guessing then my guess would be that the C-Dory would be equal or better than the full displacement trawler, probably better. I have no statistics but I think most trawlers at displacement speeds will get 2-3 nautical miles per gallon.

[ffheap]

Hi Folks,

If you can operate the C-Dory at about 15 kts, enough to get it planning, I think under ideal conditions you will get about 4 to 5 Nautical Miles per gallon. If you push it faster, efficiency goes down. If you operate it below planning speed (under 8 kts) you will have poorer milage.

A 50 gallon tank could get you about 200 miles at the best.

These are guesstimates.

Fred

[Day-Dreamer]

The factory web site has an rpm/fuel used/mileage table for the C-Dory 22 under the "performance" heading. If you have two 40's on the transom, the low speed mileage is poor, less than at planing speed. If you have one 75 the hull speed mileage is higher than the planing speeds. At really low speeds 4-5 mph (or kph) you usually get very high mileage, but will need lots of time to make the trip. This is consistent with the tables Powerboat Reports (now Boating Magazine) have published over the years for most boats.

[Levity]

The performance log shown on the C-Dory web site does not conain data for speeds below 8.5 MPH. Theoretical hull speed for the 22 is close to 6.1 mph.
It seems logical to my untrained, non engineer mind that the fuel burn at the low RPMS needed to attain hull speed must provide for greater economy. The actual results will be interesting to see.
Mike "Levity"

[Sea Wolf]

[Casey]

I've played with this quite a lot on Naknek (CD22 w/Suzuki 90).

Using GPS statute mileages and s/mph, and observing the stabilized fuel consumption figures (using a Navman fuel indicator) I have found Naknek gets about 3.5sm/gal at 4200rpm cruise (about 14-15mph). I generally fuelplan for 3sm/gal to allow some reserve (in addition to whatever extra fuel I may carry).

If we slow to hull speed (slightly below 7mph [2200rpm with this prop] in a CD22) we get a tad over 6sm/gal. It seems clear that if one was making a long passage, proceeding at hullspeed will GREATLY increase your range - but it'll take Forever to get to your destination.

You can do the math yourself. The formula for a displacement vessel hull speed: Knots = 1.34 x (square root of water;ine length). I make this to be about 6.5knots (7.07mph) for a 22 boat. The key is to keep the speed near hullspeed but well BELOW the speed necessary to begin climbing the bowwave. (Granted, a planing hull may vary somewhat, but this was close enough for my purposes.)

During some long passages we've found that cruising half way at hull speed, then kicking her up on plane to destination can be a reasonable combination for extending range yet not being much older when you finally arrive.

I've confirmed these figures on various occasions, and they seem pretty stable. When we're cruising Naknek is generally pretty heavy, so other folks may get better figures.

If anyone is interested, I have three or four spreadsheets with speed/rpm/fuel consumption data that I'd be happy to share with any C-Brat's. I'm still searching for the perfect prop that will give me a reliable 4sm/gal at cruise; but don't know if I'll ever find it!

Casey
C-Dory Naknek
Lake Montezuma, AZ
(...still home; but getting itchy)
lorencasebeer@aol.com

[Jim Gibson]

Thanks everyone. That is good input. I was thinking by slowing down to hull speed that my range could be increased to 300 to 400 nautical miles. This is about double my normal crusing range. This is good to know in situations where one is more concerned about fuel economy and the distance to the next fuel stop, versus the time needed to get there.

[Casey]

I certainly don't want to end or highjack this thread, but there are some related aspects that might be interesting to explore as well.

With that in mind, I have started a thread that deals with the general topic of Economical Cruising on C-Dory's. Hopefully folks will stop by an share their ideas.

Casey

[thataway]

Yes, if you put on a small engine, and slow the boat down, you will get excellent range. It takes very little hp to move a C Dory at hull speed.

Incidently-I am not sure what the LWL of a C Dory22 is, but I think it is about 19 feet--not 22 feet. The 1.34 x sq rt LWL is the theoretical max displacement speed--not the most effecient speed. (For a LWL of 19 feet this would be 5.8 knots).

The most effecient speed is at the sq root LWL or less. If you assume that the Length of the water line of a C Dory is 19 feet, then the sq rt LWL is 4.3 knots--much less than most folks are noting.

Any of the turbine flow meters become in-accurate at low flow rates, so a meter which is accurate at 5 gallons an hour, will not be as accurate at .5 gallon an hour.

A 5 hp engine can push a C Dory 22 at 4.3 knots. A gas 5 hp engine will use about 1/3 of a gallon an hour--or 15 miles a gallon. This may be a little high, but from my experience from sailboats at low speeds this is probably fairly close.

Don't start trying for that trip to Hawaii (about 2200miles), becuase the "ideal" mileage is is calm water with no wind or adverse currents. (beside, at this theoretical 15 miles a gallon, you would only get 750 miles out of 50 gallons--if you could use all of it).

I have pushed a 48 foot LWL, 60,000 lb boat at 2.5 knots with a 4.5 hp outboard in calm conditions--it takes very little to move a boat at very low speeds.

The other issue, is that you are pushing a boat with a 75 to 100 hp engine, and the effeciency of this engine will be less at the very low speeds than a small engine will be. Today I checked to see what the idle speed of my Tom Cat 255 (two 150's at idle--about 700 RPM, in calm conditions)--and it was 3.5 knots. Although my fuel flow meters register a value, I do not trust them at this speed. To find the consuption you should use a small container and a timed run. So if you really want to get the max range, use the kicker at a low speed.

[flagold]

Boat Test did some extensive reports on all the CD's:

http://www.boattest.com/boats/1455/test_results.aspx

You'll have to join (free).

[thataway]

Actually the boat tests are often mis leading--they are with light boats, and usually minimal crew--partial filled tanks. The one of the C Dory 22 confirms my statement about flow meters at low flows--at 2 knots and 3.3 knots the fuel flow was recorded as being the same--of course it was not--it was below the threshold of the fuel flow transducers. If you were to belive this test, the range at 2 knots would be the same as 12 knots.

[bongo]

For what it's worth, here is a table I got from somewhere in the Internet, I can't recall right now, but is shows the CC going 17.8 MPG, and so on...

Comparizon - E-Tec 115 and 150
115/150 115/150 115/150
R.P.M MPH GPH MPG
1000 4.2 / 5.0 / 0.28 / 17.8
1500 5.9 / 7.2 0.8 / 0.83 7.38 / 8.6
2000 7.4 / 8.6 1.4 / 1.32 5.29 / 6.5
2500 8.6 / 10.1 2 / 2.10 4.3 / 4.8
3000 11.3 / 14.5 2.8 / 4.29 4.04 / 3.3
3500 16.2 / 18.8 3.3 / 5.46 4.91 / 3.4
4000 21 / 26.8 4.5 / 6.24 4.67 / 4.3
4500 26.5 / 34.2 5.3 / 7.33 5 / 4.6
5000 30.02 / 37.0 6.9 / 8.58 4.38 / 4.3
5500 35.0 / 40.0 8.7 / 13.0 4.02 / 3.1
5700 W OT 36.8 /

John B