420 Sundancer diesel question

Discussion in 'Sport Cruisers' started by MJS-Sundancer, Feb 17, 2020.

  1. MJS-Sundancer

    MJS-Sundancer New Member

    Feb 17, 2020
    '08 350 Sundancer
    8.1 Mercruiser Horizons, V-drive
    Hello Everyone! First thing, I am new to this site even though I have frequently searched for info here. I boat on the rivers around Pittsburgh, Pa. I have a 2008 350 Sundancer w/ 8.1 Horizons.. I love the boat. Boating on the river primarily consists of putting around (usually 5-8 MPH) and using the boat as a floating camp and rafting up with other boats. Anyway, I have an opportunity to look at a 420 Sundancer with 6CTA diesels. I tried to research the gas vs diesel as much as possible but I have yet to come across the answer I am looking for, so I am going to ask here. My question is.... Are those engines going to hold up over time by being primarily run around 1000 Rpm going up and down the river with only getting the boat up on plane once or twice a season for an hour or so at a time? I would hate to invest in a bigger boat with diesels and end up ruining them by not letting them open up and run. Any thoughts would be appreciated and thank you!
    Express 390 likes this.
  2. Express 390

    Express 390 Active Member GOLD Sponsor

    Sep 14, 2018
    South Shore of Long Island NY
    1986 SeaRay 390 EC
    1993 SeaRay 290 DA single 7.4 /Bravo 2
    1992 SeaRay 230 DA 4.3LX Alfa drive
    Twin 454 Crusaders inboards 4 blades
    First Welcome to CSR ! It's a great site with all kinds of information. With the type of boating you do your not going hurt them , And when you need to get up and run you can run 2400 all day long The 420 happens to be my favorite Good luck with what ever you decide.

  3. fwebster

    fwebster Well-Known Member TECHNICAL Contributor PLATINUM Sponsor

    Oct 6, 2006
    Middle Tennessee ; Panama City Beach, FL
    1996 450DA
    3116 Caterpillars
    As a general rule, diesel engines just like being run.....the more the better. 6CTA's are probably going to be quite a bit more fuel efficient than your 8.1's. The only problem you may see with low rpm running for sustained periods is excessive soot collecting in the lube oil. That comes from idling a lot but can be avoided by running the engines fast enough that they are under a load. That is easily determined by the turbos on the engines. If the turbos are generating some boost, then the engines are loading up enough to burn off the excess soot.

    I have Caterpillar engines that produce more torque than the 6CTA Cummins but this will be a close parallel: We boat in a high tourist area in NW Florida. During tourist season, June to October, we have a 3 mile stretch of minimum-wake channels to get to the Gulf of Mexico. If I run 800-1000 rpm, then my oil samples come Bach with high soot in the oil. If I run up to 1400-1500 rpm, the bow is beginning to lift, you can hear the turbo whine and the soot in the oil comes back normal. Perhaps someone with 6CTA Cummins has a better feel for low speed running and its effects on Cummins 6C's than I do.
  4. MJS-Sundancer

    MJS-Sundancer New Member

    Feb 17, 2020
    '08 350 Sundancer
    8.1 Mercruiser Horizons, V-drive
    Thanks for info! So far it does not sound like I would miss treating them. I will have opportunities to run and plane the boat out... just wont be every weekend. If I pursue this 420 I'll put it in the thread.
  5. Jaybeaux

    Jaybeaux Well-Known Member SILVER Sponsor

    Jan 3, 2016
    Upper Potomac River
    420 Sundancer 2004
    Naught On Call
    Cummins 6CTA-8.3's with V-Drives
    9KW Onan Genset
    I have a 2-3 mile no wake zone, and last year, we spent a lot of time just cocktail cruising...about 1000 rpm. I checked my oil analyses, but didn't see a line item for soot. Could it be "insoluables"? After any length of time at cocktail speed, I would always like to get a good 5-10 minutes at 2200 rpm. Every now and again (that's a highly accurate unit of time measure) I push the throttles to 2700 rpm for a couple of minutes just to let those puppies run.

    You will love the 2004 420. The engine room access is amazing. Easy to work on. In addition to oil changes and belt changes, I have always performed complete winterization, oil changes (don't forget samples for analyses), fuel/water separators, spin on fuel filters. A buddy and I did a complete raw water train maintenance. Removed Aftercoolers, gear oil coolers (replaced with new), heat exchangers, and cleaned all. I have replaced all of my batteries. I do all raw water pump impeller changeouts myself. The point is, if you are inclined, it is very easy to work on these boats. And I am not a wrencher by any stretch of the imagination. There is a great thread devoted to the 420. http://www.clubsearay.com/index.php?threads/420-da-thread.1273/

    And one on the 6CTAs. http://www.clubsearay.com/index.php?threads/official-cummins-6cta-450c-thread.77886/

    Good luck.

  6. MJS-Sundancer

    MJS-Sundancer New Member

    Feb 17, 2020
    '08 350 Sundancer
    8.1 Mercruiser Horizons, V-drive
    That about suns up boating on the river for us... cocktail cruising and the occasional let her run. Thanks for the input.
  7. my3sons

    my3sons Well-Known Member SILVER Sponsor

    Feb 24, 2009
    NY (Lake Erie)
    2004 400DB, Onan 9 kw Gen, Highfield RIB,
    Cummins 6CTA
    Insolubles is anything that won't dissolve or mix into the oil. It can be solid or liquid, like dirt from a bad oil filter or bypass valve that's stuck, to water, to ??
    The soot test is sometimes an extra depending on the oil lab.
    I use Blackstone and soot is (or was) $10 more and you have to ask for it.
    I used it twice a few years ago when I thought that it might not be a bad idea to check it after an Erie Canal trip that had us at no wake speed for 8 days and 350 miles. I pulled the samples immediately upon returning to home port, not days or weeks later when fast running could have changed the chemistry. Turned out it's apparently no big deal as it was not a problem. We do a lot of slow cruising and I no longer check for soot on a regular basis. I try to keep the rpm's around 1100 when I can.
    Jaybeaux likes this.

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