FUEL ECONOMY

P

Pat Anderson

New member
Live and learn - that is what we all hope for anyway! A year older, maybe a bit wiser. Last year, we blasted Daydream around at WOT whenever the water conditions would allow. We also ran with the engines in full down position all the time, because we didn't want the bow any higher than it already was. This year we learned a couple of things.

First, slow(er) cruising. WOT for us is just a little north of 5500 rpms (standard factory issue three blade aluminum props). This year a conscious effort to run at 4400 - 4600 rpms most of the time. Instead of 23 - 25 mph, we are now running anywhere from 14 - 20, set the rpms and let the speed float with wind, current, load, etc. This is where the Honda 40s are most efficient, I just take that on faith from the published info.

Second, trim engines up. We get the same rpms at lower throttle, or higher rpms (and speed) at same throttle setting, compared to full down position. Patty just has to sit on two cushions to see over the wheel now! Bow up is the natural position for a C-Dory they say!

The result - a significant increase in fuel economy, old way was always south of 4 mpg. A lot of the time we were in the 3.5 - 3.7 mpg range. New way - we are creeping up on 5, have not got there yet, Blakely trip (cruise over, whale watch cruise, cruise back = 97 miles on 22.9 gallons, or 4.6 mpg). I think with a light load, calm water and favorable winds and currents, we could turn in a bona fide 5 mpg...The other benefit is invisible, we are HOPING that this program will extend engine life as well, again taking this on faith.

 
Pat,

Could you let us know if your speed, distance, and mpg are for knots(nautical miles per hour) and nautical miles or statute miles per hour and statute miles?

Nautical miles are 15% longer, and knots are 15% faster than the land based equivalent.

Many speed/log instruments can be set for either.

Thanks,
 
Pat,
I appreciate your comments on fuel economy. On our NW Lake Superior trip back in June we had a rare opportunity to run a long distance and then fuel up at a metered pump. The result was 3.6 mpg. I was somewhat disappointed as the published statistics for twin forties are in excess of 5 mpg. Although, we probably did have in the neighborhood of 1000 lbs. load vs. the 800 in the stats. Then too, the seas were not always calm (!!!). Anyway, your points are good ones.
Al on the MOOSE
 
Sure, I'm a taker!

There are numerous things that effect fuel MPG, and I'll just name a few. Bottom condition - just a little crud on the bottom makes a lot of difference. Weight - this is significant, because I can even notice the difference between going solo and adding a passenger. If you are full of fuel and water plus lots of gear you will notice a huge difference. Load distribultion - if you are not loaded properly and must use engine tilt or trim tabs to get the most efficient attitude you will definitely lose MPG. Speed - it's best to run the numbers on you own combination of hull/engines, but if your speed varies much from optimum you will notice a big difference in MPG.

Many other things are significant - just a little wind on the bow or wind generated chop will zap you. How you drive the boat makes a difference. Every time the engines turn from center drag is induced -- loss of MPG. If you let the wind/current move you off your measured course line -- more distance, less apparent MPG.

Trim tabs do help a lot. Easy to trim the boat for its most efficient attitude, and then adjust the motor(s) for their max efficiency (usually thrust line parallel to the water surface).

With the 22, full of fuel and water, two people, and estimated 200 pounds of personal gear, I can average about 4.0 - 4.1 nautical miles per gal. I also find it very easy to drop to 3.5 if I'm sloppy... and that's often. Good fuel flow indicators ready are worth the price -- and they are spendy...

HTH

Dusty
 
Yes, this is statute miles / mph / mpg. I have the GPS set for statute miles, only switch over to nautical when I am in a channel with the speed limit in in knots...


Nancy H":1gh1k6ku said:
Pat,

Could you let us know if your speed, distance, and mpg are for knots(nautical miles per hour) and nautical miles or statute miles per hour and statute miles?

Nautical miles are 15% longer, and knots are 15% faster than the land based equivalent.

Many speed/log instruments can be set for either.

Thanks,
Click to expand...
 
Dusty is absolutely correct. There are endless factors involved in trying to calculate (...and improve) fuel mileage. Fuel mileage in a vehicle is difficult enough to get in-hand, but when you move into the boating World even more variables come into play.

We just returned (last night) from two months and 1350sm on the Inside Passage (AK). "Naknek" is a 2003 CD22 with a Suzuki 90. We were quite loaded (approx 4000#) and the best mileage I got was 3.3smpg. The best cruise seemed to be 4200rpm which resulted in 15knots (about 17.3mph). The boat is equipped with trim tabs and a Navman fuel flow meter ( http://www.navman.com/marine/products/fuel/index.html ). the meter really lets you see your fuel useage (maybe not a good thing...). At one point I thought the meter might need calibration, so went through that procedure for a total of 97 gallons. The end result was that over the 97 gallon calibration period, I used 8/10 of a gallon less than the meter said I would use. Pretty darned accurate. (On one occasion a gas dock attendant asked how much fuel I would probably need. I told him "...about 24 gallons..." we took 24.4 - he was impressed.)

There are endless variables, but given our weight, I fuel-planned at 3 statute miles per gallon and never had a problem. The next items I plan to explore are a different prop (we used the 14x17P that came with the motor, and a 13x18P (four blade) graphite ProPulse prop during the trip - results seemed almost identical). I also will explore the use of a DoelFin type gizmo on the lower unit. I've used one on our Whaler with negligible results, but given the stern heavy condition of the C-Dory, the fin might help some on Naknek.

One thing I did notice was that "displacement speed" (in our case 2200rpm = 6.5smph) brings the mileage up to an honest six statute miles per gallon. It's just slower than hell. But we sometimes found it was fun to just turn on the autopilot and motor (slooowly....) along a route for a hour or two; rather like a sailboat.

a future project will be to try and figure-out how to graph the many sets of performance data I collected, in an effort to come up with a fuel performance chart. Somewhere out there is the "best" set of factors for our boat and use, and I would like to find it.

Overall, I'm not overly impressed with our fuel mileage on this trip, but I'm not disappointed either. I just want to maximize the mileage to the extent possible given our style of boating.

A great trip; expensive but fun. Highest fuel on the trip was in Tenakee Springs at $2.99 gallon. (I didn't cost-adjust the fuel costs in Canada - but they were expensive.)

Now we're back to trip planning, but first there's a boat to clean (...we should rename Naknek "Dirty Dory") and some travel kitty funds to replenish!

Casey*
(* the one who left his heart in Elfin Cove(AK)....)
 
Welcome back, Casey!

Color me green with envy. Absolutely love that trip and would like to do it again and again. Doubt I can do another, but dream a lot.

Your numbers vs weight fit on the line of my graph... and I'm with you on the hull speed cruise. If I'm not rushing to get to the old fishing hole, I'm doing about 6.0 knots and watching the sailboats drift by. Good way to live, double the pleasure and cut fuel costs in half.

Our numbers are absolutely great when looking at other breeds. I was lucky to get 2.0 nmpg on both the Stamas hogs, and always less than 1.5 on the 24 Whaler... looks like the Orca will do between 2.0 and 2.5 or 2.6, but still breaking in.

Again, congrats on the great trip and welcome back.

Dusty
 
I just got back from a 505 SM trip on the Chesapeak Bay with my CD25/BF150. The boat was heavily loaded for cruising and conditions were great for half the week, and lousy to horrible for the other half. Saturday, we left Annapolis heading north in 2-3 foot seas, and it quickly deteriorated to 5-6 footers. This might not seem big to you left coasters with your long rollers, but the Chesapeak Bay is all very short, very steep chop. The typical bay watermans workboat is very long for its size, designed to span three waves. Anyway, the total for the trip was 195 gallons, or 2.6MPG. I consider this to be a great figure, especially since my other friends with their twin big blocks were burning three to four times that doing the same speeds. And they didn't seem to be much more comfortable than I had been in the same conditions.
 
In the spring of 2003 we took our 25CD from Baltimore, MD to the St Lawrence River. We totalled about 750 miles, and averaged 2.9 mpg. This included a lot of locks, and the overall mpg includes all the waiting both in and outside the locks with the engine running. I thought this was pretty good!
 
Those are GREAT numbers on the 25 in the slop -- and boy do I know that area and its water. Boated out of Norfolk and Little Creek for many years -- taught landing craft beaching tactics and small-boat handling for quite a spell at Little Creek and commanded two big ships based there!

The 135/150 are sweet engines... and absolute skinflints when in the cruise range. Sure wish I had the same technology in the 225, but I'm happy with it. May go for another Tomcat if they change the interior -- then a pair of 135 or 150... dream on!!!!

Sure is fun when the big guys are getting less than 1 nmpg and having a rougher time!

Dusty
 
I recently returned from a four day trip to Prince William Sound, motored one morning from the Valdez area to SW PWS, about 65 nautical miles as figured by the chart plotter. Cruising speed varied from 18 to 24 knots, the twin Honda 40's would not turn more than 5200 rpm. Took me 12 gallons to refuel, averaging just over 5 nautical mpg. I watch the speed vary with the tilt of the motors (constant rpm), and leave them in the position that results in the highest speed for the water condition and rpm setting. Met a fellow C-Dory owner (the only other pleasure craft I had seen that morning) who had a 90 hp Suzuki, claimed he could get up to 6 mpg (nautical or land I don't know, assumed nautical) and up to 32 knots on his circa 1984 C-Dory.
 
We have a Suzuki 90 on "Naknek," and the best I've ever gotten was about 3.3mpg (statute!). I fuel-plan at 3mpg, and cruise at 15knots (4200rpm). Slowing to displacement speed (6mph @ 2000rpm) increases the mileage to about 6mpg.

On the recent trip on the Inside Passage (1350 statute miles), we were quite heavy (I would guess about 4000#). I used both a 14x17P Suzuki prop, and a ProPulse 14" graphite prop adjusted to 18P. Both seems to give very similar performance as I recollect.

I'm slowly collecting peformance data in an effort to home-in on just the right prop for Naknek. At the moment I'm leaning toward a 4-blade, but they're pretty pricey! ...you don't just out and buy one on a hunch!

I may try a doel-fin type planning device to see if it helps, but want to isolate the various "improvements" so I can determine what does/doesn't work!

Casey
 
Last trip logged 139 nm, lightly loaded with two on board. Honda 90- 4 stroke. Cruised often at 4500rpm giving a speed of 18-19 kts in a 1-2 foot wind chop. Trim tabs used to reduce pounding upwind and set for bow up attitude downwind. The last leg was 30nm downwind with 15-20 kt breeze. Fuel used was 33 gallons netting 4.2 nmpg, 4.8 statute. Decent numbers!
Levity's Michael
 
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