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Old 01-25-2018, 11:16 AM   #21
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Let’s face it.
All we really need to know is approximate anyway. If you need to know more because your range is getting close to max just slow down. If I run 5.5knots I’m sure I could go from Seattle to Ketchikan on a fillup.
It’s impossible to make accurate burn numbers and the only reason to have them is to post on TF. And of course everybody rounds off and of course on the negative side. Wouldn’t wanna be known as a fuel hog.

TF is full of chat and arguments about stuff that really isn’t that important. Like 15W-40 haha. Like the guy on TV. “Tell me something I don’t know”.
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Old 01-25-2018, 12:45 PM   #22
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Quote:
Originally Posted by jleonard View Post
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.
Lots of blowby on the old Perkins?
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Old 01-25-2018, 12:55 PM   #23
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Lots of blowby on the old Perkins?
No. It was running very well.
I repowered because I could not source Perkins parts in my area. Plus the raw water exhaust manifold was getting to the failure point and I didn't think it was worth a $2500 investment.
I also wanted to extend my range and the extra ponies did that.
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Old 01-25-2018, 01:08 PM   #24
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Gaining 57% in speed with same GPH and fuel type is impressive indeed. Quite a testament to the Cummins.
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Old 01-27-2018, 08:05 AM   #25
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More of a testament to a modern engine.
I was actually able to keep the same prop without changing the pitch because I went from a 2.1:1 gear to a 1.5:1 gear.
Also it turned a great boat into a fantastic boat.
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Old 01-27-2018, 10:30 AM   #26
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I know the BR 44, as a good friend had one until recently. Great Cruising sailboat.
I also had a cruising sailboat for 20 years. I rarely made a wave under power. The usual trip was under power until the wind picked up, then under sail. I never kept track of my fuel consumption, as it was so small I filled only "occasionally".
Since I have had my trawler, I have kept track when I could. I usually cruise at 8 knots. Once i did a whole crossing of Georgia Straight at over 9, but the wave at that speed was at least a foot higher than at 8, so I didn't keep that up, as it was going to be expensive.

Ted has it right, the only thing that reliably tells you how much fuel you are going to be using is the height of the stern wave. Sailboats rarely, if ever, make a wave of any significance, hence their excellent fuel economy.
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Old 01-27-2018, 10:46 AM   #27
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Our boat has a semi displacement hull. While underway at somewhere around 7 knots one of our dogs had an "accident" on the aft deck. I grabbed a bucket, put a foot on the swim step, grabbed the rail tightly with the other hand, and leaned out to fill the bucket from the back of the boat.

I braced myself for the bucket to fill quickly and then be dragged backwards by the movement of water away from the boat, but was shocked at how there was zero pull on the bucket. It took a second to realize the flat transom of the boat was dragging an amazing amount of water behind it...it was like dipping the bucket into a still pond.

That in itself is going to burn some extra fuel.
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Old 01-27-2018, 11:11 AM   #28
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Murray,
Indeed yes,
Looking at stern waves from SD or planing boats one can see that more than moving aft the water seems to be jumping up and down. IMO the turbulence is a bit like a wind rotor on the lee side of a mountain. The water is kind of rolling. Very disturbed and rolling. At some low speeds I’ll bet one could see the water on the surface moving toward the boat transom.

How much this takes place is largely dependant on how much transom is below the WL. 3” then not so much rolling and jumping. But 6 or 9” or so and there’s lots of tumbling/frothing going on. The water is trying to fill the hole the boat made as it passed. And it basically does until one gets up enough speed that the water isn’t fast enough to fill (or back fill) the hole. When the hole appears the boat is to some degree planing. The water appears to be moving aft fast w a rather smooth surface and almost no jumping or frothing. The water just looks like it’s moving aft. Aft of the clean hole the water comes together in a big hump (following wave) and the faster you go the further aft the humped up following wave is. In this state on can see all of the transom. The turbulence is mostly gone and the water is making a clean break from the boat. A nice speed for a faster SD boat.

Re the toplc of the thread it takes lots of power to make all that turbulence at the stern. You don’t put lots of heavy water in motion w/o considerable power expended.
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Old 01-27-2018, 11:56 AM   #29
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Quote:
Originally Posted by alfamike View Post
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 01-27-2018, 04:25 PM   #30
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The ratings I have for Perkins is 113hp for natural and 157hp for the turbo. Hull speed is a big factor and the hull shape. If both engines were run near rated hp, the turbo engine will use 2-3x the fuel (in my experience).
I've operated and owned many marine engines in my life. Starting in the 1950s. I've run the same models of several engines both turbo and natural. The turbo always uses more fuel, even at idle. Part of the reason is a turbo is like a restriction in the exhaust line. It takes power to push out the exhaust with a restriction and turn the turbo blades, it's not free power. It just takes less power than a supercharger. Racing engines have very short exhausts because it takes power to push out the exhaust in long lines.
The goal with adding a turbo is more hp, not fuel economy.
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Old 01-27-2018, 06:23 PM   #31
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Lepke,
It’s economy in a big way. The turbo engine can be much much lighter than the natural aspirated engine. Costs half as much. Burn less fuel .. not as much heat loss from the smaller engine. Smaller fuel tanks. Lighter boat not so deep in the water means less power required to push at a given speed. Less fuel burn again.

Sure if you compare a given boat w a 6-354 Perkins with and w/o the turbo of course it’s going to burn more fuel w the turbo. But you should compare a given boat w X amount of power w a turbo engine and X amount of power w/o a turbo. Different engines of course. The turbo engine would be about half the size and weight of rge NA. Then you’ll find the turbo is very much more efficient.

Tell me if I’m wrong about any of this as I’m not very turbo savy.
But I think it’s true.

But of course this has nothing to do w the M34 being not very efficient. By the way the M34 is probably quite efficient at 20 knots for a boat of that size and weight.
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Old 01-27-2018, 07:32 PM   #32
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I think the word you are looking for concerning turbo-diesels is efficiency.
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