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Old 01-22-2018, 11:37 AM   #1
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Why is the 34 so fuel thirsty

I had a 44' Bruce Roberts center cockpit steel ketch that was powered by a Perkins 6.354 NA Diesel. It weighed 32,000 pounds witha displacement hull and deep keel. At 1800 RPM, 7 knots, it used less than one gallon an hour.

Yes, it was NA, but that shouldnt make a difference since the horsepower needed is the same either way. The Turbo just means the top end is higher.

The Mainship 34 is half the weight and has a semi-displacement hull. Why does it take more fuel with the same engine?? Should take far less, shouldnt it?
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Old 01-22-2018, 11:54 AM   #2
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At what speed?

Sure, an SD hull won't be as efficient as your slippery FD sailboat hull.

My Nordic Tug 37 (SD, Cummins 330hp, 28000 lb loaded) does 7 knots at 4 nmpg.
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Old 01-22-2018, 11:56 AM   #3
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I suspect hull speed from the longer waterline length of the sailboat is a factor as well
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Old 01-22-2018, 12:16 PM   #4
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I had a 44' Bruce Roberts center cockpit steel ketch that was powered by a Perkins 6.354 NA Diesel. It weighed 32,000 pounds witha displacement hull and deep keel. At 1800 RPM, 7 knots, it used less than one gallon an hour.

Yes, it was NA, but that shouldnt make a difference since the horsepower needed is the same either way. The Turbo just means the top end is higher.

The Mainship 34 is half the weight and has a semi-displacement hull. Why does it take more fuel with the same engine?? Should take far less, shouldnt it?
An unscientific way to answer your question is look at your wake. The 44' probably had little or no wake. The 34' at 7 knots probably has a significant wake. Simply, it takes energy (fuel) to make water stand up. If you slow the 34' down till the wake goes away, it will require a lot less energy (fuel).

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Old 01-22-2018, 01:21 PM   #5
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Six knots for our 14-ton, 35-foot, FD-hull, 80-HP motor boat consumes about 1.5 gallons an hour for 4 miles a gallon.
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Old 01-22-2018, 01:31 PM   #6
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It weighed 32,000 pounds ..., 7 knots, it used less than one gallon an hour.
That's impressive no matter how you cut it.
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Old 01-22-2018, 01:42 PM   #7
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Most people power their boats cruising relative to how much noise they make.

You were out in the open air w the sailboat and now w the Mainship you’re in a box w a diesel engine running in it. You may think you were running them at the same load but not. Because of the FD hull on the SB just a bit more speed and rpm could double the fuel consumption. The fuel consumption to a greater degree follows the engine rpm w the Mainship.

But unless you were overdriving your SB it’s as simple as the difference in hulls. Ships mostly all have FD hulls because the efficiency would be unthinkable if they didn’t.

Some SD hulls would be more efficient than the Mainship. The Main is really a planing hull. Another trawler is similar .. the Californian. The only thing displacement about them is the keel and extra weight. They are a hull that will go 20+ knots. SD hulls don’t do that. If you had a CHB, a Nordic Tug or a GB your fuel consumption pushing those boats w the same weight would be less less at low trawler speeds until your speed got to 11 knots or so. Then the straight run aft would give the Main the advantage. The 11 knots is just a guess. Could be higher or lower. But at 7 knots the Main will definitely suffer and at 15 the main will fly right by the true SD boat like a CHB.

As a previous sailboat owner I would think you’d get a trawler closer to a SD or even a FD.
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Old 01-22-2018, 02:21 PM   #8
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Mainship 34 hulls are fat and flat.

A BR 44 has a fine entry, it's a foot narrower, has a rounded bilge, a 10 foot longer waterline and much less windage.

It's like comparing a box and a fish in the water.
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Old 01-22-2018, 03:46 PM   #9
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OK, I thought a FD hull would have more wetted surface, therefore harder to push through the water. Guess I was wrong. Should have kept the BR and just cut the masts off her.
But no, the MS 34 has more living space in it that the SB. It was 10 feet longer but almost the same beam and 1/5 of the hull was taken up with the engine room and workshop. The masters quarters were larger but the Main Salon was smaller.
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Old 01-22-2018, 03:56 PM   #10
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I kinda doubt a 6.354N at 1800 pushing a 44' boat at 7kts would burn under one gph. I do fuel calcs as part of my business and that is well below most under similar conditions.

The 354 would burn about a half a gph at 1800 in neutral.

And I will say that the MS 34 is not a particularly slick hull shape. Not as easy to push except at well under hull speed.
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Old 01-22-2018, 04:07 PM   #11
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well, your doubts would be wrong. I used that boat for 10 years so am fully aware of its economy.....and I dont think you should question my veracity. Apparently there is an error in your calculations.
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Old 01-22-2018, 04:26 PM   #12
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I am with Ski. 1 gph at 1,800 might produce 10 hp at the flywheel, due to the parasitic load as Ski mentions. I can’t believe that 10 hp will push that boat to 7 kts.

Don’t be insulted. Convince us that you are correct.

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Old 01-22-2018, 04:32 PM   #13
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I cant convince you of it because I no longer have the boat. The logs went with the boat. And personally I dont care if you are convinced. Apparently the BR 44 was well designed.
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Old 01-22-2018, 05:48 PM   #14
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Quote:
. Should have kept the BR and just cut the masts off her.
.
Ever been on a yacht that's dropped its rig?
Pretty unpleasant snappy roll.
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Old 01-22-2018, 06:35 PM   #15
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Fuel usage curve for true full displacement Hull KK42 example

At displacement speeds FD is more efficient than the main ship box shape. Hull shape, taper, fairness & size and load among other things determines the fuel consumption based on how many HP needed to push it through the water.
In my discussion here I am looking at Fuel usage at displacement speeds only at this time so that is what we are talking about here.

Efficiency of the propeller, how many blades, pitch, etc. also makes a difference as do a lots of other things like prismatic coefficient & beam, angle of the prop & shaft, & even a big flat stern has a huge effect on Fuel usage, how much of a wake the boat makes too, etc.

So their are a lot of variables. To many to list here. BTW - Water line length makes a huge difference as it changes the S/L ratio for a given hull speed as measured in knots. Beam in the water makes a big difference as you have to push the water farther to get it out of the way for the boat to move forward. We all generalize, but their are truly many variables.

NOTE: Every boat is different.

Here are some approximate example numbers for my Kadey-Krogen 42.

being an approximate 42 foot boat overall hull length with approximately a 39 foot 6 inch water line length. Lets look at the approximate numbers.

Starting point is to get find out what the Square Root of water line length is, for this example it is approximately 6.25 & at a S/L factor of: S/L ratio of 1.0 it works out to be about 6.25 knots.

in this example remember S/L ratio of 1.0 gives better fuel consumption that S/L ratios of 1.1, or 1.2 or 1.3 , 1.34 etc... Higher the S/L ratio the more fuel consumed for that particular individual boat design & shaping at that speed through the water on that individual boat design.

so KK 42 Displacement Speed length Ratio's are approximately as follows

S/L Ratio - 1.0 = 6.25 knots

S/L Ratio - 1.1 = 6.875 knots

S/L Ratio - 1.2 = 7.5 knots

S/L Ratio - 1.3 = 8.125 knots

S/L Ratio - 1.34 = 8.375 knots

Full displacement boat is not going much above these hull speeds by any significant amount with out pegging your fuel usage meter.

Advantage of a SD boat is it can move up into the planning speeds & FD can't efficiently do that.

I have seen with my own eyes that a similar dimension FD boat & same water line length SD boat with flat sides & no taper to the rear will burn maybe for example 4 GPH at 7.8 knots & FD burns maybe 2 GPH at 7.8 knots. All due primarily to different hull shaping, but other stuff too.


Like I said - every boat design is different here & there & it adds up.

Kadey-Krogen 42 has a true full displacement hull with a wine glass stern & leaves very little wake. Here is an example Fuel consumption curve with the Ford Lehman 135 - for my KK 42 the fuel usage curve is as follows:

Fuel Usage Curve – 700 Gal
Kadey Krogen 42 - 148 w/ Ford Lehman Sp135 & 4 Blade Prop
----------------------------------------
RPM--- Gal/HR--- Spd Kts--- Range
1800 ------- 2.00 ----- 7.8 ----- 2730
1700 ---- 1.75--- 7.2--- 2880
1600 ---- 1.55--- 6.6 --- 2981
1500 ---- 1.45--- 6.2--- 2993
1400 -----1.15--- 5.7--- 3470
1300 -----1.1-----5.5 --- 3500
1200 ----- 0.80--- 4.8--- 4200
1100 --- 0.70 --- 4.6--- 4600
900 ----- 0.50 --- 3.8 --- 5320
So there is an example where at displacement speeds the SD still used more that the true FD.
But the FD is stuck at maybe under 9.0 knots no matter what they do HP wise.
SD can get up & plane at higher speeds like 12, 14, 16 etc or more knots, but uses more fuel & power to do that.


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Old 01-22-2018, 07:05 PM   #16
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well, your doubts would be wrong. I used that boat for 10 years so am fully aware of its economy.....and I dont think you should question my veracity. Apparently there is an error in your calculations.
His doubts are indeed correct.
But as David said don’t be insulted. If you calculated that consumption you made a mistake. Most skippers make the mistake of using their hour meter and the number of gallons put in the tank. Sounds like it should be correct. But in only rare circumstances is it?
The reason it’s not right is that lots of fuel is consumed at less than cruise power. Almost never do we go above cruise power. And every time we go slower (or stop) the hour meter just keeps running at the same speed as it does at cruise.

Then there’s all the “slow no wake” signs and the time entering port and getting into your slip. There’s lots and lots of time you go slower. But we tend to forget about it. But the hour meter dosn’t. And most everybody says “it’s just fly stuff”.

I decided some time ago most of the fuel burn numbers on this forum are considerably on the shy side. But most people tend to think they and all the others are correct. Most are not ... IMO.
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Old 01-22-2018, 07:06 PM   #17
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Ever been on a yacht that's dropped its rig?
Pretty unpleasant snappy roll.
No, but I have heard that. Thats why I didnt do it. But my wife became too frail to work the foredeck so I had to sell her. But I hated what happened to "Millennium Falcon" after I sold her.

The new owner put her in a boatyard and after about 5 years of not paying the bill (over $18,000) she was given away just to move her. The next owner put her on a flat bed trailer and hauled her to MO where she was set with keel in the dirt to be used as a lake side cabin. But better that than cut up for scrap, I guess.
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Old 01-22-2018, 07:10 PM   #18
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Great post amike,
What is your WOT rpm. I know the natural FL should be 2500 but is the 135 the same?

50% load on an FL 120 is 3gph. WOT is 6. Somewhere at the top of your numbers should be 3gph. Looks like you are going almost 8 knots and your still way under 50% load. That would compute if you were considerably underpropped. Like getting 27-2800rpm at WOT.
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Old 01-24-2018, 12:55 AM   #19
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Nomad Willy,

I just don't run the engine over 1800 rpm. It is a sweet spot that the engine seems so smooth & comfortable & happy running at - & @ 7.8 knots it is fast enough for a pretty good fuel burn of 2.0 GPH. Really starts sucking the fuel above that speed.

I don't remember what the WOT RPM was as I never usually ever run the FL above 8.0 to 8.3 knots max which is maybe 2000 or 2300 rpm respectively. Just Guessing I would say maybe in the 2500 or 2600 rpm area for WOT or so maybe ? -- No data, just a guess.--

I do not know exactly above the 7.8 knot figure, but the fuel usage curve does start to get real steep from 7.5 knots & up with more & more fuel for smaller & smaller increments of increased speed.

Prop pitch is I think a tall ratio, but don't remember what that is, it is stamped on the prop, but I am not going swimming to go read it.... LOL !

I would concur with your fuel usage numbers as it uses about a touch over maybe 8 GPH at very close to full throttle for the 135 HP engine, but the speed is like maybe on a lucky day 8.8 knots ! A good rule of thumb number is usually .06 of a gallon per hour for each HP used to move the boat hull at what ever speed. More HP means more GPH & more speed, but at a ever decreasing amount of speed increase as you approach hull speed.

Just a guess, but Probably 3 GPH at 8.1 knots & 4 GPH at 8.35 knots might be a good guess. Definitely diminishing returns for the fuel burn. However, honestly I don't have the data to back up my guess once over 7.8 knots & 1800 rpm's, as I just don't run there. so the 0.2 or 0.3 of a knot faster is just not worth another 1 or 2 GPH - IMHO.

So I can only publish what I put in the other post where I have the data & the data actually supports those figures. - Above that is just a guess.

Sorry I don't have more exact info for the higher speeds, but I just don't run there or even try to run much faster then what I posted at 7.8 knots.

So I hope the limited info helps some of our friends here in TF understand the diminishing returns of speed & fuel burn relationship as we approach & try to go close to & even try to go faster than hull speed in a FD boat. Your just throwing fuel away at the faster speeds.

thanks.

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Old 01-25-2018, 07:19 AM   #20
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I owned a 34 Mainship model I for 14 years. Original engine was a Perkins T6.354 at 160 hp.
I never measured gallons per hour but I did measure Nautical miles per gallon.
At about 7 knots I got 2.3 NmPG.
It was very repeatable.

As an additional note, I repowered it with a Cummins 6BTA at 270 hp.
When I cruised it at 7 knots efficiency increased to 3.2 NmPG.
When I cruised at 11 knots it would be back to about 2.3.
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