Fuel Economy

[Pat Anderson]

OK, somebody who knows, please come back here...

I always thought that at a given RPM an engine would have a steady fuel burn rate - say at 3500 RPM, you would be burning 4 gallons per hour, and the the effect of current and wind would be that you might be going at a lesser rate of speed, i.e., your miles per hour would drop, but your gallons per hour would remain constant at that RPM. If you were going faster, your miles per gallong would be better. Of course, if you were going slower, your miles per gallon would be less too.

But it doesn't seem to be working that way, and I am puzzled. Although propped up, and I am now going 13 - 14 mph at 3500 RPM instead of the 9 - 10 mph I was doing before, I don't seem to be getting better fuel economy. This is calculated with the standard GPS trip odometer and the gallons used on filling up...

So what is going on here????

[dogon dory]

Sorry, Folks - Post Deleted By Author

[TyBoo]

The fuel you burn is the result of the work you are doing. Period. It's like driving your car up a hill - you have to push the gas pedal down farther to go the same speed, and so there is more gas going through. Your GPH doesn't remain constant because to turn the same RPM with the motor working harder, you have to give it more throttle. It's not as easy to sense that with the controller handle as it is with a gas pedal, I guess. Now that you are going 13 MPH instead of 9 MPH, your motor is doing more work, so your GPH is affected accordingly. You still get the same MPG, but you are going faster, and turning the prop that propels you faster requires more work to be done by the motor. One hour later, your GPH has gone down, but your MPG has stayed about the same because you have gone farther in that hour. Nothing is free, nor will it be until DoD has his PMM in his garage perfected.

Or look at it this way if you don't think you are giving it more throttle to maintain 3500 RPM. At WOT, you max RPM is a few hundred less than with the other prop, no? Before you could turn your new max RPM with the throttle handle, say, 3/4" less than wide open. Now, to get 3500, the throttle handle is 3/4" further open than before. (Actually, it is more like 23/32", but you'll have to get SW Joe to do the math.)

Now I just read back through that, and it doesn't make a lick of sense. Just read the first sentence, and there's your answer. It has nothing to do with the size of molecules you can shove through your stern hole.

[TyBoo]

Dammit. I wanted to beat DoD to the post on this one but my wife kept yakking. Once he sees that his explanation was a lazy one, and that mine, flawed as it may be, is the result of deep consideration, he is really going to pound me. Worse yet, he has to throw that HP stuff into the equation, so now here comes Chuck with the torque talk. And I don't understand all that stuff so well.

[dogon dory]

Sorry, Folks - Post Deleted By Author

[TyBoo]

Well I don't know. But the Cummins in the Dodge has a rev limiter, so it won't blow up even at its preset max RPM. And at 3500, it is going a damn sight faster than 78 MPH - whether it is towing the TyBoo or not.

And speaking of stern holes and ionic polarity.....

[CatyMae n Steve]

Does that tiny print mean Mike's mumbling?

[dogon dory]

Sorry, Folks - Post Deleted By Author

[Chivita]

TyBoo,

I would not worry too much about DoD pounding you over your posting, he can't even get his boat's length correct. I think he's been in Alaska too long. Just like our neighbor's claims about the 12 foot tall Grizzly bear that visits the backyard of our new house every fall, I bet he's only 6 feet tall at the most!

Course, if he is 12 foot, I'm going to use every last horse tucked away in our Dodge getting our butts outta there, rev limiter or no rev limiter!

[Byrdman]

Hey Pat! Glad to hear you are gaining on things with your prop. I guess you got rid of the ventilation issue with this new prop since the Blakely whale trip we were on?? Hope so. Your speed increase seems to match which ever other CD25 that was running with us in the islands...? After a very long up river run on my boat, a TomCat not a mono hull.... for kicks and grins, I lowered more of the boat into the water, as to more accurately... have less of a nose-high-but lower and dragging effect... (I know there are nautical terms for this...but what the heck...) ..change of the boats "attitude" I think.... and I'll be. I had been running my boat with the noise too high by about 1/2 of those little marks on the engine trim gauges. .... Now, to be real accurate, I'd need to fill up at the same dock, take a run both up and down river about 25 miles each way, change only the tilt, and measure the difference in the fuel it takes to top off the boat IN THE WATER....with the same exact everything.... SO, I figured.... I got a good enough idea.... and close enough fuel mileage to the highest fuel mileage of 22s... (actually Bill and El on Halcyon...who I know have spent many hours testing...), and I got real close fuel mileage...with the size/weight difference of the CD22 and the TC24.... In fact.... figuring my boat was large enough to have two couples on board for the trip...and split the fuel cost 1/2...that doubles my mpg...because I only bought 1/2 of the gallons used... making my boat get even better mileage than Bills. Now that made him swallow twice at the restruant.... grin.

[Forty Two]

Patrick in particular: I have Honda 90s on my TC - came with 3 blade, aluminum 13.25 diameter, 15 pitch props. WOT was 6000 rpm at about 28 knots - which seemed too high (rpm, not speed).

I changed over to everything identical except 17 pitch props, and now WOT is 5200 rpm at about 28 knots. This is too low.

Rats.

What next? Grind bottom a little to reduce the prop diameter?

What props do you use??

[Byrdman]

I looked just the other day sitting on my noodle floating about in the cove...they are the Honda Stainless steel props that came on my boat...when I got it from Les and Dusty... I could not see the size/pitch markings on them.. I would have to email Kathy at EQ and find out. She sent this info to me once, but I had a major crash and lost a lot of stuff on my old CPU.... I can only get up to about the 5500 rpm range and about 31.5 to 32 mph is about all she'll go.... I spend a lot of my time at about 6mph....with one motor running, just cruising....with the auto pilot going...with the remote in my hand either sitting up front on the deck, or out back in the cockpit, or stretched out on the dinnette bed let down into a nice take a nap day bed... real high impact stuff. I love to just cook, eat, cruise and relax... Real cruising is between 3800 rmp and 4200 rpm.. and the closer to the 4200 range the boat seems to like it. ..not the answer of just which props I have....but the best I can do tonight.

[rogerbum]

Byrdman - glad to hear you'r e OK and weathered the bit of the storm that came your way. Got an update of your day watching the boats? (probably better on another thread)...

[Sea Wolf]

fishbob-

You obviously need an in-between prop of the two you tested.

If you can't find a 16 inch pitched one, take a look at cupping. Cupping adds about an inch to the straight pitch of a prop.

If your 15's wernt't cupped, have it added to them.

If the 17's are cupped, have it removed.

Best deal: take them to a very good prop shop and discuss it with them.

My $0.02. Joe

[Forty Two]