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Old 12-28-2013, 06:30 PM   #81
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If we are collecting data:

Grand Banks 47'
56k lbs (it's a heavy boat)
Twin Cummins 500s
7.1kts, 2.25 NMPG (7.1 is the closest data point that I have)
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Old 12-28-2013, 08:46 PM   #82
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Ok, some more data:

Ocean Alexander 50 Mk1
60,000 # half load
Twin John Deere 6068, 201 HP each
7 kn, 2.7 NMPG

But it seems I have a lead foot. Looking at daily logs for 2000 nm since my repower I typically am getting 1.3 NMPG at 9.3 kn.

I did a bit more research, and here are a couple of very good pdf's. If you used boat design.net then you might remember Eric Sponberg's series on Design Ratios. On his website he compiled the posts into an updated pdf, and I've attached it. Eric's "primer" is very readable, and whilst it is more sail oriented there are great sections for powerboats.


That file lead me to Dave Gerr's article in Masthead. Also available to download, here it is. The key article is p12-17. In it he discusses things raised earlier in the thread. It is also a very readable discussion for folks like us. He has a great graph for how the 1.34 number varies, discussion of the Quarter Beam Buttock line (and the graphic Eric was looking for) and importantly his updated formula for SLR. It is revised from the one in his Propeller Handbook. It is truly worth printing and keeping pages 12-17.....
Attached Files
File Type: pdf Sponberg - Design Ratios.pdf (865.5 KB, 57 views)
File Type: pdf WestlawnMasthead06_June08.pdf (999.1 KB, 65 views)
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Old 12-28-2013, 10:59 PM   #83
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THANKN YOU Insequent.

I remember Spoonberg and that he's into sailboats. He's put out very good stuff for a long time and I now have it (thanks to you downloaded for good) and I'm off to read. All interested should look at pg 14 of the "Westlawn Masthead" DL and see the good illustration of the QBBL (quarter beam buttock line) and it should be big help understanding the difference between full disp and semi disp hull design.

"lead foot"? You are probably a sucker for a nice big wake fullmof action (like me) and just like to see the seawater foam. Pleasure boating isn't it?
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Old 12-29-2013, 07:28 AM   #84
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Quote:
Originally Posted by Insequent View Post
Ok, some more data:

Ocean Alexander 50 Mk1
60,000 # half load
Twin John Deere 6068, 201 HP each
7 kn, 2.7 NMPG

But it seems I have a lead foot. Looking at daily logs for 2000 nm since my repower I typically am getting 1.3 NMPG at 9.3 kn.

I did a bit more research, and here are a couple of very good pdf's. If you used boat design.net then you might remember Eric Sponberg's series on Design Ratios. On his website he compiled the posts into an updated pdf, and I've attached it. Eric's "primer" is very readable, and whilst it is more sail oriented there are great sections for powerboats.


That file lead me to Dave Gerr's article in Masthead. Also available to download, here it is. The key article is p12-17. In it he discusses things raised earlier in the thread. It is also a very readable discussion for folks like us. He has a great graph for how the 1.34 number varies, discussion of the Quarter Beam Buttock line (and the graphic Eric was looking for) and importantly his updated formula for SLR. It is revised from the one in his Propeller Handbook. It is truly worth printing and keeping pages 12-17.....
Thank's for the links...

I posted parts of the Sponberg article (pretty sure it was that one) in one of the last threads here on hull design/efficiency. It was in defense of my trying to post similar info but was rebuffed when someone was adamant (as always) that 1.34 was a hard and real number for all displacement boats as well as the QBBL as the "determining factor" with little regard to prismatic coefficient or other key design features.... Also they suggest that a quick glance at a bad photo and magically telling you your boat is displacement over semi-displacement is not really practical.

I would hope reading these links will give TF members some insight to the reality of boat design and see that while complicated to a degree...that the complications of design are for fine-tuning small changes in performance...not gross ones....and that trying to "guess" performance from a few tidbits of internet data is ridiculous.
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Old 12-29-2013, 07:47 AM   #85
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... trying to "guess" performance from a few tidbits of internet data is ridiculous.
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?
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Old 12-29-2013, 10:06 AM   #86
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I personally like dollars/nautical mile.....last year it was about $1/NM ...this year should be the same....

Makes trip planning easy....3000 NM trip is gonna cost me $3000 in fuel..
Sorry guys, just got back on line and am feeling feisty having just entered Saddle Cay from the east during a easterly blow with the current coming out. Never saw less than 9 feet, but that's another story.

Bring in the Bahamas, I have finally had the time to analyze fuel and electricity on Dauntless.

Between tt and the other rick, I actually think you are all saying the same thing, though in a bit convoluted way.

The gph is exactly where I expected it, and based on engine rpms. At 1600 rpm, I get 1.6 gal per hour. At 1800 it goes up to 2 and at 1400 about 1.4 gph.

So gph is a good lowest common denominator, to compare engines, but not necessarily boats!

Now, mpg is dependent on do many factors that it can really only be used on a case by case situation.

Once boats are thrown into the mix, then, just changing the prop will change nm per gal or mpg, let alone everything else.

Mpg is a tool I can use on a case by case basis. E.g. I discovered that the last 100 miles of the icw, my mpg went from 4 to less than 3, mainly from all the bridge wait times. Also, current plays a big role.
If I take the time to calculate avg speed, I can then use that to call mpg, but I pretty much don't bother anymore.

So, in sum I have been pleased that my initial expectation of $1/nm has held up well.

Later.
Richard on Dauntless in the Bahamas.
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Old 12-29-2013, 10:41 AM   #87
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Dear Ann Landers

In May we will be driving from Arizona to Seward Alaska. Part of the driving trip will be on a ferry. How do I calculate MPG for the Suburban? Then during June through September we will be cruising on the West Coast in our boat. I'm really befuddled as to how to calculate efficiency, GPH and NMPG for the entire summer's journeys.

BTW, my credit card which I will use to cover the cost of the ferry, truck gas and boat fuel may have been hacked at Target. Do the bank fees to make good on this count towards my dollars per nautical mile?

Awaiting your reply, I remain,

Anxious in Arizona
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Old 12-29-2013, 11:20 AM   #88
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Dear Ann Landers

In May we will be driving from Arizona to Seward Alaska. Part of the driving trip will be on a ferry. How do I calculate MPG for the Suburban? Then during June through September we will be cruising on the West Coast in our boat. I'm really befuddled as to how to calculate efficiency, GPH and NMPG for the entire summer's journeys.

BTW, my credit card which I will use to cover the cost of the ferry, truck gas and boat fuel may have been hacked at Target. Do the bank fees to make good on this count towards my dollars per nautical mile?

Awaiting your reply, I remain,

Anxious in Arizona
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Old 12-29-2013, 11:35 AM   #89
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MPG, would be the only way to compare this subject. GPH means nothing to the fellow on the other boat. I realize there are contributing factors and a decently accurate number is difficult to get. It is still the best comparison for these purposes. My opinion is that a house on a square bottom barge, 40 foot lwl and 14 feet wide weighing 50,000 lbs pushed by a pair of appropriately sized diesel engines will get close to the same fuel usage (in mpg) as a similar sized "trawler".
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Old 12-29-2013, 12:51 PM   #90
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anyone that can't do ALL these different calculations in their head doesn't belong behind the wheel or earn the title captain anyway...my Ouija board is way more accurate than any floscan.....

so what's the question?

ps...real men use metric anyhow....
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Old 12-29-2013, 01:49 PM   #91
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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.

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Old 12-29-2013, 02:27 PM   #92
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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
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Old 12-29-2013, 03:28 PM   #93
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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.
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Old 12-29-2013, 03:37 PM   #94
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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.

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Old 12-29-2013, 03:51 PM   #95
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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.
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Old 12-29-2013, 04:03 PM   #96
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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.
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Old 12-29-2013, 08:18 PM   #97
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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.
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Old 12-30-2013, 03:01 AM   #98
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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
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Old 12-30-2013, 06:47 AM   #99
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Old 12-30-2013, 08:10 AM   #100
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Here are some numbers on gph and gpd vs speed. Yikes!
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