Displacement speeds/economy

K

Karl

New member
Assume someone is willing to forgo planing speeds in a CD22 or similar hull, and is willing to run slow displacement speeds, just for the sake of being able to get out on the great lake and scoot around economically for many hours, perhaps sitting in the pilot house using a laptop...a slow alternative to anchoring, I guess.

What kind of gallons-per-hour could be expected?

Would this be better done on a kicker (like a Honda 9.9), or would a typical contemporary 4-stroke (like a Honda 90HP) on idle be just as suitable and economical?

The fuel crunch really has me examining alternatives, without giving-up the retirement dream on a limited budget...

Thanks-
 
Hello Karl - For what it is worth - with some experience running a CD 22 with a Honda 90 at 1,000 rpm for many hours monitored by a flo scan - you can expect between 10-12 miles per gallon depending on the seas, wind, current, etc. But between 1,000 rpm and say 3,000 rpm expect less effciency, with the rpm just before going on plane the least efficient of all. Some say running at 1,000 rpm is hard on the engine, causes heating, etc., but that was not my experience. The new EFI Honda 90 may be different than the 4 carb system. Don't know. Would be interested in hearing the experience of others. Of course the total weight being pushed is a very important variable, one passenger versus four, full fuel and water, etc., etc. Yellowstone
 
I have been out cruising quite a bit lately at displacement speeds on my 22. I use my 8hp yamaha 4 stroke kicker for 2 reasons. The first is I am not sure it burns any fuel. Just kidding. I haven't figured it out but the fuel burn seems almost non existent. Secondly , the cost of my 90 yamaha 4 stroke is substantially higher than the 8hp and I wouldn't unneccesarily accumulate hours on it. I figure the value and longevity of the more expensive motor is better preserved if I use the kicker when fishing or just putting around.


regards

ryder
 
I have never run one of the C Dories on a kicker for a long time, but I have run comperable sized sailboats for over a thousand hours on 4 to 9 HP, and they are very effecient. If you only want 5 knots, --it will be 10 to 12 miles a gallon--if you want 8 knots--then it will be much less. As long as you keep the speed less than 1.2 x sq root LWL (assume that the LWL of the CD 22 is 18 feet, sq root is 4.24 feet and 1.2 x that is 5)
 
Yeah - what Bob said :) It takes me three or four fishing trips to drain a 6 gallon fuel can with my 6 horse two stroke evinrude and that is trolling for hours and hours on end each time. I would say 10 mpg could be a very realistic number.
 
thataway":241ick2o said:
As long as you keep the speed less than 1.2 x sq root LWL (assume that the LWL of the CD 22 is 18 feet, sq root is 4.24 feet and 1.2 x that is 5)
Click to expand...

How do you calculate the LWL and hull speed of the TC 255 since it has two hulls?

Thanks,
Warren
 
Karl,

The previous posts are consistent with my experiences with displacement cruising in a C-Dory. On plane (4200-4400rpm) Naknek gets around 3.5-3.9 smpg. If we slow down to displacement speed (about 2200rpm gives 6.3mph) the mileage increases to 6-6.5smpg. Although I haven't done it, even slower cruising gives even better mileage (10-12 really is possible) as indicated on the fuel scan gizmo.

As mentioned in other posts, we have found that having an autopilot really helps relieve some of the tedium of slow cruising, and make it more enjoyable.

Unlike the usual "dayboat" a C-Dory offers the ability to comfortably cruise and overnight, so the cruise to a destination is a major part of the fun. Having a boat that will "plane" is fun as well, but costly; having a boat that will displacement cruise is slow but much more economical; with a C-Dory we have both!

As boating becomes more expensive the C-Dory and it's utility as well as it's economy will Shine. We can trailer it virtually anywhere yet be comfortable for extended periods afloat - and those qualities really help stretch the recreational boating dollar!

...just some thoughts.

Casey
C-Dory Naknek
The Villages, FL
Lake Montezuma, AZ
 
Fuel-flow monitoring that shows instantaneous mpg sounds like an attractive option for cheap bastids like me. Does the flow sensor just get installed in a common fuel line and rely on an external speed sensor, so no switching is required to show mpg of either the primary motor or the kicker? Or, am I missing something?

BTW, I appreciate all the replys...I'm new here and already sense
that the forum is a supportive group.
 
Karl,

You need to look at the specs for the flow sensors. The ones I've seen don't work at the very low fuel flow of a kicker at low power.

For a gas engine, the fuel flow (gal per hour) at max horsepower is about 10% of the hp rating. A 9 hp kicker should burn about a gallon per hour at high throttle. At about half throttle you're burning around 1/2 gal per hour and that's near or below the lower limit for some or most flow sensors I think. I expect you'll get poor data or no data.

Does anyone have good experience with a sensor for low flows? Maybe there's one I haven't seen. I'm sure one could be had for LOTS of money if necessary.

You might want to get a marked transparent fuel container and hook up an outboard fuel supply hose to it and run it for a while and time how long it takes to burn off a certain amount of fuel. Or run it much longer and refill it back to a known starting level and calculate from that.

Jeff
 
The general formula for gas engines is 14 horsepower per gallon an hour. So if you are using 7 hp you would be using 1/2 gallon an hour.

The Navman and Lowrance use very similar turbine flow sensors--and I agree are not super accurate at very low fuel flows. I think your best way to measure the low flows is to use an external fuel tank, and record fuel used vs hours run.

Warren. The Tom Cat's water line is relitatively similar to the standard C Dory--but at least our boat-- is not a true cat at displacement speeds. The tunnel is not open, until the boat is up on a plane, and there is immersion of the aft part of the bridge deck. The design of the engine bracket does immerse it and there is some bouyancy--but not a true hull extension. I would call the water line of the Tom Cat at about 21 to22 feet--but it is an estimate.--you are still best off at 5 to 6 knots--not 8 knots for best fuel economy.
 
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