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Discussion Starter #1
Last year I installed a mid-1980s 4b (NA) Cummins motor in my 32' salmon gillnet boat. I've been running the motor at a steady 2000 rpm and have been burning about 0.8 gallons per hour of #2 diesel. So far I've got about 60 hours on the motor in this boat and I'm comfortable that the motor is settled in and broken in nicely.
Question: What have users of this forum found to be good running speeds for their motors?
While 2000 rpm is smooth and it sounds great, I know this motor is capable of more...but how much more?
 

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Last year I installed a mid-1980s 4b (NA) Cummins motor in my 32' salmon gillnet boat. I've been running the motor at a steady 2000 rpm and have been burning about 0.8 gallons per hour of #2 diesel. So far I've got about 60 hours on the motor in this boat and I'm comfortable that the motor is settled in and broken in nicely.
Question: What have users of this forum found to be good running speeds for their motors?
While 2000 rpm is smooth and it sounds great, I know this motor is capable of more...but how much more?
With a stock engine your most efficient engine speed is UNDER 2000 RPM's. Peak torque is generally around 1800 RPM's. In your case you could end up having 2 things working against you. They are engine RPM speed vs prop pitch. If your prop is designed for peak efficiency at a higher RPM your fuel economy will fall as you exceed 2000 RPM's. To me it sound like you have the right combination for economy.
 

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I offer this up to all who come here with marine motors-JOIN boatdiesel.com. It is a pay site-to participate-but lurking is free. Since you have a commercial boat that can have a widely varied load I'd suggest propping/gearing so that you can at the very least hit max RPM at WOT when "empty". Otherwise you have a higher chance of lugging the engine trying to maintain similar performance when loaded. BD has several neat calculators as well as data sheets on most marine engines.
 

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Discussion Starter #4
Thanks BobS, this is exactly the information that I am looking for. These boats started out in the early 50s with straight 6 Chrysler Crowns (85HP), when I got it there was a Chevy V8 gas motor installed with a 2.52:1 gearbox. I am currently using the 2.52 gearbox with my Cummins. I'm thinking I'll see if I can't find a lower reduction gear and work the problem from that angle. This boat will not be hauling big loads. Thanks again.
 

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Talk to a prop maker that uses "Propscan". They will plug everything in the computer & tell you what is best. Amazing results with that system.

Ed
 

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I think a good way to check your running gear combination is to find out what the rpm is at wide open throttle in neutral then find out what rpm is at wide open throttle in gear. If you are within 150 - 200 rpm you are probably in good shape. If not then it's time to start making changes to the gear and or prop.
 

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Discussion Starter #7
I think a good way to check your running gear combination is to find out what the rpm is at wide open throttle in neutral then find out what rpm is at wide open throttle in gear. If you are within 150 - 200 rpm you are probably in good shape. If not then it's time to start making changes to the gear and or prop.
Wow- that is an excellent method to assess load on an engine. I'll definitely give this a try and report back. Thanks!
 

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Last year I installed a mid-1980s 4b (NA) Cummins motor in my 32' salmon gillnet boat. I've been running the motor at a steady 2000 rpm and have been burning about 0.8 gallons per hour of #2 diesel. So far I've got about 60 hours on the motor in this boat and I'm comfortable that the motor is settled in and broken in nicely.
Question: What have users of this forum found to be good running speeds for their motors?
While 2000 rpm is smooth and it sounds great, I know this motor is capable of more...but how much more?
I´m not arguing your fuel consumption numbers, but if a truck with a 4BT is doing 60MPH and getting 20MPG, it is burning 3 gallons of fuel per hour. Even with wind resistance, I can´t imagine the drag of a boat displacing a lot of water being lower than the rolling resistance of said truck. I would expect the boat to burn a lot more fuel than that.

Can someone explain it to me?? :confused:

Thanks and congratulations on your results.
 

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The aerodynamic drag on the truck at 60 mph is greater than the hydrodynamic drag on the boat at probably 8-10 mph. He didn't say what speed he gets at 2000 rpm but I bet it's in that range. It's probably a displacement hull as opposed to a planing hull. If it was a planing hull the fuel consumption would be much higher. Displacement hulls can be very efficient until you push them past their "hull speed" at which point the hydrodynamic drag goes exponential and fuel consumption skyrockets. Say he is doing 10 mph at the .8 gal/hr fuel burn, that's only 12.5 mi/gal.
 

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that's only 12.5 mi/gal.
And in powerboats that is phenomenal! Mine gets 3-4 MPG with a single diesel at 6-8+ knots. But in boats (pleasure anyway) you don't say I've got a 40 mile trip ahead of me. You'd say I'm taking a 5-6 hour cruise! Its all in your perspective.


To the OP-Are you burning 0.8/hr per engine? I'd be astounded if you have 8L of diesel engine burning less than 1 GPH at 2000 RPM. Even if its 1.6 total GPH at 2K RPM its great!

Typical Ford Lehman 120s will consume about 2 GPH in a twin boat The 80HP 4 cyls do about 1.5 GPH-mind you this is in 36-40'semi planing hulls ran at "trawler" speeds of about 7 kts
50 HP Yanmar singles I've seen burn about .5-.75 GPH in 30' boats ran the same way

I've known of Westerbeke and Yanmars in the 30 HP range in sailboats (which are the most efficient displacement pleasure hulls out there) that sipped .25 GPH!! Granted they are usu making about 5 kts max at those numbers.
 

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Yeah, 12.5 mpg IS amazing, I would like to know more details of the boat from the OP. That has to be in the 6 knot range. What is hull design, material, beam, draft,etc?
 

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Discussion Starter #12
Thanks for the replies. The 0.8 gals per hour is legitimate: it's based on a Hobbsmeter and fuel pumped in the tank. I've been holding the rpms at 2000 rpm to get baseline fuel consumption numbers, and to get the whole system settled in. Yep, the boat is a displacement hull, at 2,000rpm I do about 7.5 knots, so if I'm doing the math right, that's just over nine and one-third miles per gallon.

I was very surprised also when I first started running the boat. I was anticipating 2 or 3 gallons per hour fuel consumption. I'd put 10 or 15 hours on it before I saw any air in the sight glass...at 1/2 tank, I thought that maybe the sight glass was clogged, so I unstrapped the tank and physically lifted one end to confirm that there really was that much fuel in the tank. I think that mattm's test to compare engine rpm at an approximate known fuel flow rate in gear and then in neutral will show that the engine is hardly pulling a load. My current reduction gear is about a 2.5:1 ratio, (2.5 motor turns for one prop turn). I may see if I can get or borrow something around a 2:1 ratio and see what that does. There are 2 factors that make me reluctant to do this. These are, 1) fuel consumption (ie energy expended) goes way up as soon as you begin to go faster than hull speed...10mph I suspect would take 2-3 gallons per hour or more to do, and 2) I like my dead slow speed while I'm parking when I've got the motor pulled back to 700rpm, if I change gearing I'll be moving faster and will be more likely to have an "oopsie" while getting in my slip.

Thanks again for the insight and suggestions.
 

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On a pure displacement hull a rough rule of thumb is 2 HP/Ton for diesel. Like my boat, yours is over powered- but being a commercial vessel design, yours had to contend with a length to beam ratio that would change as the boat was loaded-thus increasing the HP required to maintain speed. Mine is a planing pleasure boat that is commonly used at trawler speed but the "extra" power allows it to climb the bow wave and plane out. Coming from a commercial fishing raising I can tell you from experience that available engine cost usu dictates what repower is used on small vessels. Our commercial boats were overpowered due to pushing/pulling nets while trawling/skimming. The 2HP/Ton is used in pleasure boats with a static "good" hull aspect. I would have my props tuned before swapping transmissions. Re-pitching or adding cup may be all thats needed to allow you to get back down around 1600 RPM. Do this and your boating experience will drastically improve as it'll greatly reduce NVH. You shouldn't notice a fuel burn change unless its to the good as you still won't be putting any load on the engines but you should bring them back down the fuel:power curve to the engine's sweet spot. You could very likely cruise on one engine at a time (use both for docking and close quarter maneuvering). There are devices to lock thr shafts in position so they don't freewheel the trans:nuke: This is commonly done with over-powered boats to reduce fuel burn and to try to load a diesel. There are high speed and low speed diesels. The 4B falls into the low speed (not EMD low though)

Also if you have a displacement hull 7.5 kts is in the range you your hull speed. If you try to go faster your stern will fall into the trough and your bow will try to climb the bow wave but you won't go much faster- just burn more fuel. My boat is a planing hull so when I exceed my "hull speed" the water falls back from my stern as the sharp edge keeps the water from being able to travel up-effectively adding its weight to the stern.
 

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Discussion Starter #14
Report on load on my 0.8 gph 3.9 4b marine unit

I think a good way to check your running gear combination is to find out what the rpm is at wide open throttle in neutral then find out what rpm is at wide open throttle in gear. If you are within 150 - 200 rpm you are probably in good shape. If not then it's time to start making changes to the gear and or prop.
Okay- I had her out today. Running at 1800 rpm, doing about 5.6 knots, I dropped it into neutral without touching the throttle. Engine rpms popped up to 1900...some load on the motor, but not much. The current reduction gear is one I had with the V8 that came with the boat: a 2.57 Velvet Drive. I'm working on a lead on a 1.5 ratio Velvet Drive. The motor is still running great.
 

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Your numbers are GREAT. Just have the prop done by a Propscan shop (Full computer analysis & tuning). It may gain you some, but you are already on the miserly side of fuel consumption.

Ed
 

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I know those trollers are very fuel efficient. I have a Mainship 34 with a 160hp Perkins and get about 2gph at 7.5 knots. With a sleeker hull I am sure that could be improved upon. If you have the cash you should look into a CVP.
 

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Discussion Starter #17
Thanks for the responses. A second one of these boats came along for me a year or so ago. I really like the 4b motor. Anyone have any ideas where I might come up with another one of these motors? The one that I've got in my main boat was a Craigslist score...it was factory marinized, rigged with a Sendure cooler, and a Sherwood raw water pump. TIA for the information and responses.
 

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With a stock engine your most efficient engine speed is UNDER 2000 RPM's. Peak torque is generally around 1800 RPM's. In your case you could end up having 2 things working against you. They are engine RPM speed vs prop pitch. If your prop is designed for peak efficiency at a higher RPM your fuel economy will fall as you exceed 2000 RPM's. To me it sound like you have the right combination for economy.
I'm going to use a 4bt turbo 130 HP in 24 ft Wellcraft. I installed tunnel to accommodate 16" dia. wheel using a 2:1 AND not going less than 19° pitch. It draws 19". What is your prop?
 

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1" dia equals 2° pitch on engine load. Every revolution of prop should cover distance in inches equal to degrees in pitch on planing hull. Otherwise your wasting power!
 
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