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Seeking tips for improving range

Discussion in 'Viking Yacht' started by Greg Roy, May 25, 2022.

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  1. Greg Roy, May 25, 2022
    #1
    Greg Roy

    Greg Roy New Member

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    Hi All, I'm refitting a 1979 Viking Double Cabin boasting the world-famous twin DD 671's. I started another thread titled "Viking Rising" if anyone has an interest in watching the progress. My question is can you all please give me input on ways to improve fuel economy/range. I know the basics of keeping on plane without running WOT, adding fuel tanks, and keeping the hull clean, but specifically, things related to the power plants like downsizing injectors, etc. I'm willing to give up top end for the sake of range. Thanks in advance for your advice.
  2. Pascal, May 25, 2022
    #2
    Pascal

    Pascal Senior Member

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    Slow down to hull speed.

    you can play with props and injectors and everything else you want to spend money on but you can’t change physics. It takes X HP to push a boat at Y knots and you need Z GPH to produce That X HP

    on most boats, slowing down to hull speed will double your range. Hull speed is generally accepted to be 1.33 x square roof of LWL. You didn’t tell us the length of your boat but if 45’, guessing LWL of 40’… hull speed will be 8.2 kts

    what you absolutely don’t want to do is to run just above HS. If your hull speed is 8.2 kts, running at 9.2 will double your fuel burn for only 10% more speed. Either run at HS or on plane

    which planning speed is best depends on the boat and weight distribution. Some boats will be on plane but still bow high and run less efficiently until you go a couple of knots faster. Trim tabs on most boats are critical to get on plane and optimize speed at set RPM
    SeaLion, Capt Ralph, captholli and 1 other person like this.
  3. Greg Roy, May 25, 2022
    #3
    Greg Roy

    Greg Roy New Member

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    That's very helpful, most grateful!
  4. mapism, May 25, 2022
    #4
    mapism

    mapism Senior Member

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    Pascal already said it all, really.
    But don't expect his last suggestion on planning speed optimization to be a life changer.
    It's only hull speed vs. planning that can increase the range dramatically, the rest can only bring marginal results.

    Trouble is, if your boat is capable of cruising at 20+ kts, HS can be VERY boring.
    I spent 17 years of happy boating with a wooden trawler, and we didn't regret one bit cruising for hours if not days at 8kts.
    But for some reason, I always found that annoying in faster boats!
    Fishtigua, Greg Roy and JWY like this.
  5. Capt Ralph, May 25, 2022
    #5
    Capt Ralph

    Capt Ralph Senior Member

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    Keep the engines stock. Just keep your foot out of the engines like Pascal recommend.
    Keep excess weight off also.
    We push slow and avoid the generators when not required.
    However, it is nice to power thru an ugly inlet or pick up some speed to avoid a weather event when needed.
  6. rtrafford, May 26, 2022
    #6
    rtrafford

    rtrafford Senior Member

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    Pascal nailed this. I'll add this...when running at hull speed be patient. Calculate your hull speed and round down a bit. Make sure that you don't push for that extra quarter knot. Your wallet will thank you, both at the pump and with the mechanic. I found my hull speed running at 1475 RPM. I backed off to 1440-1450 and let one engine run at 1350, swapping periodically. In doing so I just cut my fuel consumption on a very long leg by 15-20% from historical trips.
  7. Capt J, May 26, 2022
    #7
    Capt J

    Capt J Senior Member

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    I agree with running at hull speed, but also keep in mind to run at cruise every 4-6 hours for 30 minutes to keep the engine cylinders and air boxes clean on the old detroits. Also a clean bottom and running gear will help too. If you've got 15 years of bottom paint on the bottom, having it soda blasted, new barrier coating and bottom paint will usually pick up 10% speed/efficiency.
    Fishtigua likes this.
  8. Greg Roy, May 26, 2022
    #8
    Greg Roy

    Greg Roy New Member

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    Thanks again for all your input. Capt J, I soda blasted prior to the hull repairs in my post about restoring the boat. The poor guy doing the job had 4 or 5 inches of paint on top of his head at one time! It made a huge difference in hours required to get the hull under control, well worth the $2500 or so to do it in labor savings alone. I'll probably have the hull from waterline down 100% done by end of next week.
  9. olderboater, May 26, 2022
    #9
    olderboater

    olderboater Senior Member

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    Everyone talks hull speed, but the real dramatic savings are one knot or so below hull speed.

    If range trumps enjoyment then easy to accomplish.
    Capt Fred likes this.
  10. Capt Ralph, May 26, 2022
    #10
    Capt Ralph

    Capt Ralph Senior Member

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    Like mother in-laws, adding weight reduces MPG even at hull speed.
    Keep it simple and light.
    Sometimes, even purchasing fuel along the way offers better cost per mile.
    When you start stretching your legs beyond fuel pumps, Bring up bladder tanks at that time but for local cruising, just keep it light and slow.
    You will be amazed how little 671s sip without your foot in them.
    As Skippy J tried to say about opening them up from time to time to blow out the slobber, it may help but the straight 6 does not have the large air box as the V blocks do.
    I advise to my customers, open them up on the way to the fuel pump before every big deployment.
    This ensures your engines are ready to start your trips, blows out the dock grunge, fresh fuel on board from a reliable supply pump, water passages are clear and your refreshments taste better when the hook is down.
    Before you need the power, open them up to ensure you have reliable power. All other times, keep you feet out of them..
    There is no better 2 stroke than the DDC straight 6.
    :):):)
    T.T. and chesapeake46 like this.