Range of C-Dory at Hull Speed

J

Jim Gibson

Member
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?
 
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.
 
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.
 
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.
 
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
 
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.
 
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"
 
Day-Dreamer":3ko8d5u5 said:
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.
Click to expand...

All very true, but, unfortunately, the table for the 75 hp Honda starts at 2500 rpm and 8.5 mph, which is beyond the most efficient displacement hull speed.

I think that the boat easily speeds up until it "hits the wall" at about 7-8 mph where extra power just mostly equals inefficiency, and the boat just drags an increasingly larger wake while it's still below planing speeds. My personal experience is that the boat is easily driven up to about 5- 6 mph at about 1600 rpm with either a 75 or 90 hp engine.

The factory's table gives 5.67 miles per gallon as the fuel consumption at 2500 rpm at the aforementioned 8.5 mph. A slower speed should, within limits, be more efficient, but it would be tough to extimate what it would be at the maximum point. Maybe 8 mpg? 50 gallons at 8 mpg would = 400 miles! Even at the 6.7 mpg, 50 gallons would produce a range of 283.5 miles.

Of course, the instantaaneous figures quoted above do not count start and warm up time, idling, maneuvering, loss due to currents, etc.

Who's got a 22 with a good fuel management system from which to get some real numbers!

Joe.
 
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
 
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.
 
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
 
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.
 
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.
 
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
 
Jim,
The Honda prop tables for a 50 on a 22 cruiser show 13.3 MPG at 4 MPH at 1500 RPM, and 7.5 MPG at 6 MPH at 2000 RPM. Yikes, quite a drop! Phil Bolger wrote somewhere that 6 is the magical psychological number to make you think you're actually getting somwhere so the C-Dory seems likely a poor choice for a displacement boat. But that's with a 50. I'd love to know what would happen with a 15 or 25 which would be more in line with the engine sizes on the Nimbles.

If you had to get to Mexico, slower is better!

Mike
 
Not that a C Dory is a poor choice for a displacement boat, but that the length water line of a 22' C Dory is short, and the more effecient speed is the one which is close to sq root of LWL--or even 1.2 x sq rt LWL.

Could you make a more effecient hull? Certainly--for example I have a friend who made a wave piercing cat, which is 32 feet long, and weighs 1200 lbs, all up, including a 15 hp outboard. The 15 hp engine will push that boat at 12 knots--at a gallon an hour. A very effecient boat and hull--but a very light boat, and very limited accomidations--enough room for a sleeping bag on the floor of the cabin, a camp stove in the cockpit for a stove and a couple of gallons of water. If you increase the weight, the effeciency will decrease.

Or you could make a very narrow C Dory, but it would be much less stable, have less accomidation and probably would not do that much better MPG. Back to all boats are compromises.

The engine smaller than the 50 hp, may do 10 to 15% better, but that is all you can expect. For at boat this size, it is as about effecient you can get with the accomidations which it has. There are some sailboat type hulls which might be slightly more effecient, but again only in the 10 to 15% better effeciency. Could you take a C Dory 22 down the Pacific coast of Baja?--yes, you could--there are enough places to pick up fuel if you had a range of 300 miles. But what about comming back up the coast? You would be most likely pushing into wind and waves which would be most uncomfortable--and would decrease the range, perhaps to the point of marginal range.

We averaged over 150 miles a day when sailing--or about 6 knots. If you are averaging 4 knots, then 100 miles a day (24 hour day). Some small sailboats only do this speed. On our West bound Atlantic crossing we made about 170 miles a day--At the same time a 24 foot sailboat made under a 100 miles a day--he took 30 days, and we took about 17 days. Not sure how that relates to "going anywhere"--both boats crossed the Atlantic safely. I would not consult Phil Bolinger on passage making--he has designed multiple small boats, often of unique design and written both books and magazine articles on his concepts of simplistic design. But how many passages has he made? The point is, that speed is very relative. We have met one man who was very happy pushing his 28 foot garvey type house boat to Sitka AK up the inland passage at about 3 knots average. with an 8 hp outboard. Others want to go 25 knots all of the time....
 
These #'s make the idea of trolling slowly back home very attractive. A big spread can cover some territory trolling, doubt if the increased drag would make a huge difference. 10mpg on the way back would be impressive.
 
Incidently when cruising we had a minum speed under sail, we started the engine--and that was averaging less than 3 knots over an hour's peroid of time. When racing, of course there was no minimum speed. We have anchored to keep from going backward in currents, and drifted for a day in the middle of the Pacific on the way to Hawaii...
 
On my great loop trip I averaged 3.75 mpg over 7000 miles, but one day I traveled on one engine at a time, with the other engine off and out of the water (I have twin Honda 40s on my CD22), at about 6 knots. I got between 6 and 7 mpg over a distance of about 50 miles on that day. With twin 20-gallon tanks that gives me a range of about 240 miles.
 
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