Mileage vs Consumption

[JDCAVE]

An interesting discussion, for sure! I did a few theoretical calculations for the KK42 at different loads: the oft quoted 39,500 displacement and with a full fuel load (760 ,gallons diesel at 7.1 lbs and 350 gallons of water at 8.3 lbs for 47,800 lbs displacement). FWIW I found this article by Charles Doane, "A Better Way to Estimate Hull Speed"...
CRUNCHING NUMBERS: A Better Way to Estimate Hull Speed

I'm not saying Charles Doane's model is correct or not, only that this is what I used to calculate the "Maximum Speed to Length Ratio" (MSL) where the value 1.34 is often quoted.

First, I was able to replicate Charles Doane's model for the 28 foot vessel. Note that he didn't provide the detailed calculation for the displacement to length ratio (DLR). DLR is calculated by dividing a boat's displacement in long tons (2,240 pounds) by the cube of one one-hundredth of the waterline length (in feet).

The example in Charles Doane's article is for a 28' vessel of 12,000 lbs. The displacement to length ratio, DLR is 244. Max S/L ratio = 8.26 ÷ D/L ratio raised to the power of 0.311 which is 1.49.

Assuming the KK42 LWL is 40' (a guesstimate) at the oft quoted 39,500 lb displacement the MSL would be 1.439 and the nominal hull speed would be 9.1 knots. at the 47,800 lb displacement, the MSL would be 1.356 and the nominal hull speed would be 8.6 knots. I had seen somewhere that the 39,500 lb displacement is for a half load, in which case the full load would only be 43,700 and the MSL would be 1.395 and the nominal hull speed would be 8.8 knots.

While the above is interesting, and perhaps relevant if you are designing a vessel, the findings of Larry and Richard are more important to me, and are similar to the findings for my KK42: I'm getting about 1.8 gallons per hour at 1,800 rpm and 7.5 knots without the paravanes in the water. Richard's findings suggest I'd be saving money by dropping half a knot and running at 1,600 rpm. I don't thinkl many of us on the forum are running around at the calculated nominal hull speed. I think I was able to get up there with the throttle at full stop during the sea trial when I had an offer on the boat last spring.

Jim

 

[Wxx3]

An interesting discussion, for sure! I did a few theoretical calculations for the KK42 at different loads: the oft quoted 39,500 displacement and with a full fuel load (760 ,gallons diesel at 7.1 lbs and 350 gallons of water at 8.3 lbs for 47,800 lbs displacement). FWIW I found this article by Charles Doane, "A Better Way to Estimate Hull Speed"...
CRUNCHING NUMBERS: A Better Way to Estimate Hull Speed

I'm not saying Charles Doane's model is correct or not, only that this is what I used to calculate the "Maximum Speed to Length Ratio" (MSL) where the value 1.34 is often quoted.

First, I was able to replicate Charles Doane's model for the 28 foot vessel. Note that he didn't provide the detailed calculation for the displacement to length ratio (DLR). DLR is calculated by dividing a boat's displacement in long tons (2,240 pounds) by the cube of one one-hundredth of the waterline length (in feet).

The example in Charles Doane's article is for a 28' vessel of 12,000 lbs. The displacement to length ratio, DLR is 244. Max S/L ratio = 8.26 ÷ D/L ratio raised to the power of 0.311 which is 1.49.

Assuming the KK42 LWL is 40' (a guesstimate) at the oft quoted 39,500 lb displacement the MSL would be 1.439 and the nominal hull speed would be 9.1 knots. at the 47,800 lb displacement, the MSL would be 1.356 and the nominal hull speed would be 8.6 knots. I had seen somewhere that the 39,500 lb displacement is for a half load, in which case the full load would only be 43,700 and the MSL would be 1.395 and the nominal hull speed would be 8.8 knots.

While the above is interesting, and perhaps relevant if you are designing a vessel, the findings of Larry and Richard are more important to me, and are similar to the findings for my KK42: I'm getting about 1.8 gallons per hour at 1,800 rpm and 7.5 knots without the paravanes in the water. Richard's findings suggest I'd be saving money by dropping half a knot and running at 1,600 rpm. I don't thinkl many of us on the forum are running around at the calculated nominal hull speed. I think I was able to get up there with the throttle at full stop during the sea trial when I had an offer on the boat last spring.

Jim

Quoting from "Voyaging Under Power" the kk42 has a:
D/l of 293
Lwl of 39'2"

I figure my current displacement is about 43k #

Lastly, the PO cruised at 1800 rpm, but in talking with Larry, he suggested 1600-1700 and the engine does like that. So does my pocket book.

When the circumstances warrant, I'll go faster or slower, but will only cruise any length of time, between 15-1800

Richard

 

[markpierce]

MPG, would be the only way to compare this subject. GPH means nothing to the fellow on the other boat. ...

Might not it be all about time navigating on the water when considering fuel use. So GPH is relevant in that equation.

 

[JDCAVE]

Richard: Thanks for the revision on the LWL of 39'2". I guessed at 40'. I hadn't posted the DLR in the above post, but if I substitute the LWL of 39'2" and use 39,500 for displacement, I get your reported 293 number.

These so-called Max S/L ratios have imbedded in them "factors" that include a whole host of variables, relating to bow shape, friction coefficients and so-on. Important if you are a marine engineer designing hulls. What is most relevant (to me anyways) is what speeds and fuel efficiencies I am actually getting.

Jim

 

[Wxx3]

Voyaging Under Power was the first book I read over 6 years ago.
I re read it this summer and its amazing how much more I got from it.

It's also what pointed me towards kk in the first place, as I knew I could not afford to do what we wanted, unless we had the most efficient boat we could find in the size we could afford.

 

[Tom.B]

For me... I usually don't care too much. I use GPH since I have two speeds... moving and stopped. We do very little idling as a tiny fraction of time spent at 1800-2150 rpm forward movement. I certainly didn't pick Skinny Dippin' based on consumption. I mean, maybe a little, but I knew she'd be pretty good sipping fuel and that was good enough for me. I just watch the sight tubes and get fuel when I need it. Yes, I do the calculations, but I am set on a GPH number and it is the "standard" that works for me. As long as I know I get 1.5-2.5 GPH, I can estimate, closely enough, how far I can travel before the next fill up. Simple.

 

[kulas44]

I will see if I can talk to some of my pushboat captain friends and ask them what it takes fuel wise to move my proposed barge at 7 knots. I'm guessing it will go at close to 2.5 mpg. I'm just saying that at 7 knots our boats are all pretty close in fuel consumption, diesel that is. I will concede that a true trawler has other things in its favor, but much greater economy is not one of them.

 

[Northern Spy]

That's not likely to stop anyone.

This thread brings to mind the Weather Channel gimmick of naming storms that meet some self-defined parameter. Why shouldn't a boat designer contrive a ratio of some sort and sell it?

I just developed a new one myself ... it is the ODS ratio and is a measure of how many angels get their feet stepped on while dancing on the head of a pin per angel per unit of pinhead area; it is the Overlap per Dance Step ratio.

Give me some time to fine tune the formula and I will include dance efficiency in there somehow. Speaking of which ... what happened to the "efficiency" idea that was tossed around so freely at the start of this thread?

42

 

[RickB]

42

 

[Larry M]

Here are some numbers on gph and gpd vs speed. Yikes!

 

[psneeld]

Here are some numbers on gph and gpd vs speed. Yikes!

Is that why cruising experts say leave your guns at home?

Dragging around firepower at speed costs big time....

 

[kulas44]

Wow, at 24 knots my sportfisher gets at least twice as good as that.