Repoweing... How do you compare a diesel to a gas?

Discussion in 'Propulsion' started by cmb1998, Aug 24, 2009.

  1. cmb1998
    Joined: Aug 2009
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Portland Oregon

    cmb1998 New Member

    I have a 24ft aluminum pilot house. Boat weighs about 5400LBS wet.

    The boat is presently powered by a VP 5.7L 260HP motor and a douprop outdrive (1993 vintage). The original motor has about 2000hrs on it and is getting tired. I am planning on repower this fall / winter.

    With the present setup the boat cruises at about 24kph 3600RPM and WOT is around 32kph at 4300RPM. My goal is to increase this a little and decrease fuel consumption with a modern EFI engine.

    I am either going to repower with a new 5.7GXI VP and douprop or a diesel mated to a VP Douprop. I am looking at the Yanmar 6LP-300 or the Yanmar 6LP-250 if I go with the diesel. Weight of the gas / diesel engines is about the same.

    Here is my question… How does the power of a diesel translate to the power of a gas engine?

    Obviously the diesel makes much more TQ than the gas. Would the 6LP250 (210 continuous HP / 250 max) by fairly comparable to the 260HP Volvo I have now?

    Anyone have any feedback on the 6LP250?

    Thank you.

    Joined: Oct 2002
    Posts: 4,519
    Likes: 111, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    WE always compare engines on fuel use , not rated HP.

    IF you figure that you get 10 (12 max) hp from each Gallon an hour of gas consumed . You can easily guesstimate the HP you actually are using.

    A modern high speed diesel will generate 18 hp (maybe 20 ).

    The diesels (if of industrial stock) will live longest with 70-90% of their rated loads.

    A diesel car transplant will probably require the same 50% power reduction from gas engine ratings , at cruise.

    After you figure the HP you are using , see where that HP is on the power graph of the diesel folks you are contemplating.

  3. Jarek
    Joined: Apr 2003
    Posts: 61
    Likes: 1, Points: 0, Legacy Rep: 14
    Location: Canada

    Jarek Junior Member

    Outboard on a bracket.

    I know this is slightly off topic, but since you are planning renovations and contemplating a switch to gasoline, how about enlarging your cockpit by repowering with a modern outboard on a positive floatation engine bracket, behind the transom?
  4. mark775

    mark775 Guest

    How does "repowering with a modern outboard on a positive floatation engine bracket, behind the transom" "enlarge the cockpit" (are you assuming that the boat was designed with an above deck engine box?) and why would one want an enlarged cockpit? The reason I asked, and I'll give you a break here, is that most people think they want an enlarged cockpit for fishing until they can no longer effectively fish because there is a bunch of stuff hanging back there in the way. I have seen and heard of more fish being lost to outboards than one can imagine (I sport-fish for a living and hear about fish lost every day!). Indeed, if improved fishing is the goal, any set-up is better than an outboard. If just cruisin' around, I concur with your points.

    Imagine a salmon cutting hard down and over or a halibut thrashing in the current behind this mess. Warm water - how about a tuna's death spiral or a dorado's crazy antics?
  5. Jarek
    Joined: Apr 2003
    Posts: 61
    Likes: 1, Points: 0, Legacy Rep: 14
    Location: Canada

    Jarek Junior Member

    Yes, I assume that on a 24 ft. boat that cmb1998 has, an inboard engine sits in a "doghouse" in the middle of the cockpit - I may be wrong.
    Also, I do not see any mention of fishing in his post; maybe he does not fish?

    There may be lots of reasons against using outboards, but if one is open to exploring options, I invite you to visit my website to inspect my engine bracket design.

  6. gonzo
    Joined: Aug 2002
    Posts: 16,394
    Likes: 1,430, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The power of a diesel and a gas engine are the same. For that matter the power of a horse is the same as an internal combustion engine. The difference will be that the diesel engine will have more torque so will produce the power at a lower RPM. That also means you will have to change the gear ratio. You need to check the weight and dimension differences more than anything.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.