Gas or diesel?

Discussion in 'Boats, Motors, Trailers and Towing Rigs Forum' started by Ruff, Nov 16, 2017.

  1. noluck

    noluck Active Member

    i have my cummins 4bt governed at 3100, and cruise 24knt at 2800. 900 degree's on the pyro, 9lbs of boost, and oil temps 225 degree. i think general rule of thumb is cruise 80% of governed on a diesel
    bigdogeh likes this.
  2. bigdogeh

    bigdogeh Well-Known Member

    My D4.2L's were diesels made by VM Motori... Yes, they were made in italy. I think the VW TDI diesels you are thinking of are different. If I was going to run a diesel at the top of its rpm rating for an extended period of time I would definitely want it set up with a pyrometer and alarmed at a certain temp. As well as all the other alarms (oil pressure, water temp, etc.) I recently hooked my Cummins up with low oil pressure switches that kill the engine in case of low oil pressure. And have sensors on the water temp at the outlet of the exhaust mixing elbow that will kill the engine as well if it senses it isn't getting water cooling at that point also. (snap disc sensor) I tend to try to be cautious when it comes to oil pressure and cooling temps... I have a pyro-guage that has an audio alarm and visual alarm as well. Fuel pressure and boost pressure guages as well that are alarmed visually and with audio..
    Last edited: Nov 25, 2017
    noluck likes this.
  3. captmike

    captmike Member

    2 diesel mechanics also suggested an economical speed is just planing + 2 kn
  4. ericl

    ericl Well-Known Member

    Sounds very good as long as RPM/load is kept within reason. I have heard that at around 25kn the drag of all the underwater running gear of straight shafts becomes any issue compared to stern/pod drives.
  5. Sentinel

    Sentinel Active Member

    My opinion is if it is a diesel with an outdrive behind it you will lose all fuel economy savings to increased maintenance costs. I went from a 30’ boat with a single 370 hp volvo diesel and dp outdrive to a 28’ with twin honda OB power. Even though the 30’ averaged 2.0 mpg and current boat about 1.7 mpg the overall cost of ownership is far less with the 28’. Like 1/2 as much or more . Now if we are talking diesels with straight shafts I could get behind that especially if you plan to own it for a long time.
  6. ericl

    ericl Well-Known Member

    Sentinel; great post. I converted my I/O SeaRay to O/B mostly because of engine issues; only I/O issue was a hole in the boot caused a water leak which took out he gimble bearing which caused the engine to flood. By that time I had had enough.

    Considering a re=power on a 33ft Chris craft; twin shaft diesels (time for them to go).

    Right now the inboards provide running hot water & robust battery charging which provide all the comforts of home for evernote use; plus the idea of having my props 4-5 feet aft of the stern makes me depressed when the issue of keeping downrigger/fishing gear separate from the props comes to mind.

    It also seems the diesels beat the O/B's severely when fuel use at hull speed is an issue. I am retired, so I have time to do most of my running at low speed.
  7. Eden Island

    Eden Island New Member

    I switched a 1984 Hourston 23 from the original 260 hp Volvo gas engine and single prop to Volvo D3 200 hp aluminum block with duoprop leg. I get twice the range out of the same tank.

    How? Diesel is 30% more efficient than gas. Duoprop is ~30% more efficient than single prop. 1.3x1.3 = 1.69. Where's the other 30%? The D3 and modern leg were 500 lbs lighter - out of the back 3 feet of the boat - the boat jumps onto plane.

    I cruise at 22 to 30 mph, without exceeding 3500 rpm, burning 5 to 6.5 gph [Imp]. I'll have to live to ripe old age to pay for the switch, but its worth it.

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