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Old 10-16-2010, 04:42 AM   #1
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My Roberts 45' cut-away full keel cutter has a Perkins 4-236 85hp diesel and I'm trying to determine fuel consumption.

The boat weight is around 30000 lb. Unfortunately my generator runs from the same tanks which makes it difficult

for me to get some actual real life consumption data, even just ball-park values. I suspect the published values are

far removed from the actual ones and would appreciate some real data. Particularly, I'm interested in what rpm's

have been found most economical to get the best mileage and at what level one would expect to double consumption

per mile run. A plotted curve of consumption vs boat speed or rpm would be just dandy.

A second question is how to monitor consumption on an ongoing basis, keeping in mind there is a constant flow of excess

fuel back to the tank. Using a day-tank, accurately calibrated? Some gizmo? Monitoring the tank under way is not that easy

due to access and the fact that some of the tank walls are actually the hull and very much sloped. A dip stick or depth gauge

is not that feasible, though I may need to calibrate one during the next refill.

What have you done to solve this on your boat?

BTW: I have identical dual tanks on opposite sides of the engine, of about 115gals each. The return flow goes to one side only but I can draw off either side. Yes, a legacy PO design but it does allow for some load balancing. It also allows for diesel running out of the overfill escape if you forget to switch tanks at the right time. ( the EPA, Coast Guard and manatees all get their knickers wadded up and twisted into a knot when that happens! )

Thank you for your suggestions. They will be most beneficacious and welcome, no matter how squirrel-brained they may be

Ivo
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Old 10-16-2010, 05:32 AM   #2
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Fuel Consumption being dependent on so many variables is very difficult to provide even a 'guesstimate'.

The LWL? How Clean the hull and Prop? The gear reduction? The RPM? The Sea Conditions? Etc.......

That 85 HP engine - if in good condition, with all the correct appendages, should easily drive a Roberts 45 cruising boat at hull speed.

Maybe the simplest method of ascertaining acceptable fuel consumption, would be to fill the diesel tanks to the brim (don't run the generator) then for several hours in good sea conditions, motor at 2500 RPM, then refill the tanks. Then do the same at 1800 RPM (making sure that hull speed is sustained)

Calculate consumption - my guess it will range between 0.9 GPH and 1.4 GPH
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Old 10-17-2010, 02:29 AM   #3
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OK, that's a good suggestion about the refill and running at two different speeds. The guesstimate still is a WAG but it's true,

much of science has been based on such at one time or another.

Now, has someone with a similar hull configuration actually kept a log of consumption and arrived at some max/min figures

perhaps?
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Old 10-17-2010, 03:44 AM   #4
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Quote:
Originally Posted by linnupesa View Post

OK, that's a good suggestion about the refill and running at two different speeds. The guesstimate still is a WAG but it's true,

much of science has been based on such at one time or another.

Now, has someone with a similar hull configuration actually kept a log of consumption and arrived at some max/min figures

perhaps?
Surely to be relevant, the comparison boat would to have the same engine (in exactly the same condition) the same gearbox, drive train, propeller, the boat would be have to travel the same waters, in the same conditions.Carry the same payload etc......

A similar hull configuration, is only a single small part of the equation.

Even if one could find an exact match, and that boat could achieve

0.75 GPH at best and 3.0 GPH at worst. One would still have to run your boat through a series of fuel trials to obtain reasonably accurate figures.

However, another way of determining the rate of consumption at varies engine speeds, requires the fitting of 2 fuel flow meters, one on the fuel line feeding the high pressure fuel pump which feeds the injectors and the other on the fuel line returning unused fuel to the fuel tank/s. The reading from the second meter is subtracted from the first.

Here is a link to a good meter for diesel fuel flow.

FLOW METER
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Old 10-17-2010, 05:03 AM   #5
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Well, a nice try but still no cigar this time. Correct, for a comparison to be very exact it would require identical conditions, hulls etc. Yet this

accuracy is not what I'm looking for. Some plus or minus 10-20% readings will be fine, like a port to port trip and what it consumed under

an average of speed/wave conditions etc. I'm not looking for rocket science here.

The fuel meter link gave me only equipment that measured from 2" diameter pipe and up, the smallest one with a 315 g PER MINUTE

capacity. A small rotameter ( ball floating up in a conical tube principle ) might work at the very low total flow-rates we'd be talking about but it would likely still be too insensitive to measure an actual delta between gozinta and gozouta fuel amounts.

In my mind an accurately calibrated day tank is perhaps the only ( cheap and nasty but still very accurate way ) to get actual figures

of consumption. It is unlikely weather conditions during a test run would hold steady for more than just a few hours at a time.
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Old 10-17-2010, 05:42 AM   #6
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There are other fuel flow meters that will do the job on the Perkins engine.

As you are in California here is an article on the subject:-

Making Sense

BTW have you ever ascertained the fuel consumption on that engine?
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Old 10-17-2010, 04:35 PM   #7
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Bingo! Thank you for that second link. That article has a lot of very good info but I've not yet had time to digest and study it much. It sure points out all sorts of things that make up the equation. Unfortunately, with legacy equipment it's hard to plug in just the right HP and prop size and pitch without a good outlay of boat-bucks.

As to being "in California" AND "making sense", surely you must be kidding me. Don't you know those two phrases are total oxymorons when juxtaposed like that? To me CA immediately brings to mind the phrase "fruits, flakes and nuts".

I've looked at the published mfr. power/consumption curves but they are somewhat unrealistic as to real life. The basic HP I feel is empirically enough/overkill/to spare to drive that size hull. Now, the prop size I can change but only within limits.

The current prop size is 3-blade 19" x 19". The PO changed it from a 19 x 16 originally to the current 19x19 but his log book notes say it should have been a 22x14. My own inspections tell me the prop cut-out may be too small for a 22" size while still maintaining a minimum 15-20% dia/hull gap ratio.

Also, his further notes say to look for a 20" prop with a 17" pitch as ideal. I plan to do mostly sailing and a larger diameter prop would presumably add substantially more drag. Efficiency while motoring may be better though. Going for a folding type is precluded by the rudder post immediately behind the prop IMHO.

BTW: I'm still looking for real life data from similar 4-236 /boat combinations, even if only anecdotal.

Ivo
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Old 05-05-2012, 06:55 AM   #8
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Hi Ivo,
We have a 4-236 in our swan57, which came with a fuel consumption/rpm/range chart. This has proven to be very accurate over the 20+ years and 5,000 hours we have been tracking fuel. In smooth water (we have a 4 blade Variprop, adjustable pitch) 1400 rpm is max torque/best fuel consumption of 3 ltr/hr at 6.1 knots, and worst case has been 8 ltr/hr in headseas and big winds. If you are interested, I can scan and email it to you. As others have mentioned, boat gearbox ration, etc etc all have affect, but if you can adjust the pitch, the Perkins horsepower/torque/gram per hphr curves give you that 1400 rpm for fuel economy.
Fair winds!
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Old 05-06-2012, 05:18 AM   #9
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Hi Louie

thank you for the offer of the data. A real but welcome blast from the past, as I'd been looking for this data/info for quite a while. As you have quite a bit more waterline your efficiencies may be better than mine, but are nonetheless still very welcome.

Ivo
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Old 10-27-2012, 04:23 PM   #10
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In the past a quick down and dirty way to measure fuel consumption under load conditions is to isolate fuel circuit for said engine . Have an alternate fuel source (I.E. 5 gal fuel can secured to frame away from heat) . Locate vessel under weather conditions to measure. Switch to alternate fuel souce making sure not to loose fuel prime (air in fuel circuit). * Note, I used to use a rubber line and screw a large nut on the OUTSIDE of fuel hose before dropping into 5 gal can. This insures hose stays on bottom of tank. Also, trim hose at a 45 degree on this end so it doesnt suck to bottom of tank under load. Also, have enough slack in line so vibration doesnt cause it to walk up or jump out. Tie the line with cord to secure. Sounds like a lot but actually quite simple. This is what diesel techs would do when flowmeters (quite pricy) were not available. Hope this helps. Be safe. Do not use this method with gas/ petrol engines. The cost is basically rubber fuel line, clamps, nut and appropriate fitting.
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