Help me make the decision ... twin or single diesel?

Discussion in 'Diesel Engines' started by piperca, Apr 1, 2009.

  1. piperca
    Joined: Mar 2008
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    Location: Southern California

    piperca Junior Member

    I am getting close to getting back into my boat project, since my home remodel is getting close to completion ... :)

    I have a 28' Skipjack Flybridge that I have removed the twin gas engines from and am now considering repowering with diesel(s). My dillema is twins or a single. I am considering either a pair of rebuilt Volvo Penta AQ31P-As (one is actually new) or a single rebuilt Yanmar 6LP. The Volvos are 150 hp each and the Yanmar is 315 hp. Price to install either is not an issue, since it works out close to the same ... couple of thousand more for the single.

    Yes, it would be a lot easier to drop twins into the current setup and use the existing outdrives, but I'm looking at maintenance, also.

    There's a bunch of transom patching and stringer reconfiguration to take place if I do the single. I'm a carpenter by trade, so that doesn't really scare me too much, I'm just a little burned out with remodeling projects, I guess.

    Anyway, given the options, those of you that have experience with this type of work; which way would you go? Why/why not?

    I have a terrible time making up my mind, so anything you guys can throw at me would help immensly!
     
  2. El Sea
    Joined: Aug 2006
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    Location: St Petersburg, Florida

    El Sea Junior Member

    FWIW:
    Since I favor the single engine concept, I would go the Yanmar, this would be a good time to get those stringers and transom in shape.

    If you were determined to go twin, I would go a pair of smaller Yanmars, I really don't get a true, warm fuzzy feeling when it comes to Volvo's.

    Luther Carrier
    Absolute Tank Cleaning
    St Petersburg, Fl
     
  3. piperca
    Joined: Mar 2008
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    Location: Southern California

    piperca Junior Member

    Thanks for the response!

    Do you have experience repowering from twins to a single? I am wondering how this would affect the boat, since it was designed to be run with twins? The guy who is going to do the install thinks that a single is a bad idea; he thinks it will affect the boats center of gravity in a negative manner, since it is an 8' beam and is tender on the drift with twins ... comments?
     
  4. El Sea
    Joined: Aug 2006
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    Location: St Petersburg, Florida

    El Sea Junior Member

    I would hope that when the twins are removed along with the shafts, struts, etc a keel would be layed to give stability and protect the prop/shaft.

    Keep in mind with a 8' beam and twins, how are you going to maintain them?
     
  5. piperca
    Joined: Mar 2008
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    piperca Junior Member

    Sorry, I should have clarified; the boat had twin I/Os (sterndrives), so there's two lovely big holes in the transom that will need filled! Here's a pic of what I was left with:

    [​IMG]
     
  6. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    The boat was design for twin stern drives gas engines. The new diesel engines will probably be a lot heavier (first Problem). and if you don't use stern drives what other type of drive are you going to use. You don't have room for much.

    I would go with single diesel, surface drive.... Save a alot of work with rudders etc... Check out Levi drives and you dont have to open holes in bottom.
    I had a 8 foot beam with twin engines, what a maintenance nightmare.
     
  7. piperca
    Joined: Mar 2008
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    Location: Southern California

    piperca Junior Member

    Yes, weight was an issue, but I decided on Volvo Penta 31s, which are very close to the weight of the small block Chevys. Now, if I were to go with a single, I'd be dropping close to 1000lbs ... nice thought!

    Surface drive? I am not familiar with this type of drive; heard of it, but know nothing about it. Can someone guide me to where I might learn a bit about them (installation, maintenance, etc.)?

    And, yes, I have learned that the twins are a maintenance nightmare and would like to leave that behind!

    BTW: I was just looking over some information on these surface drives. How do these drives perform in a fishing environment? Do they become a bit of an obstacle when fighting a fish? They appear to hang pretty far off the stern ... am I wrong about this?

    Update: I don't think the surface drive will work for me. According to what I've read, they are not preferred for slow speed, like the speeds during trolling, etc.
     
  8. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The difference in weight between a small diesel and a gas engine is no issue, certainly not when you contemplate changing from stern drives to surface props.

    Admittedly, maintenance on 2 engines takes more money & time than one, but having two engines makes a lot of difference in handling your boat in narrow spaces, crosswinds etc. And in case of a defect or prop damage, it can make the difference between getting home by yourself or waiting for a tow.

    I changed from stern drives to jets, then after 2 years of jet misery to tunnel drives. Took lots of time and resources, but it was very rewarding.
     
  9. Frosty

    Frosty Previous Member

    That looks pretty tight for two Yanmar 6LP's.

    There is a big intercooler on the port side and a big turbo on the Stbd.

    Twins are better but you need a tape measure.

    Yanmar 6Lp are good engine --high reving for a surface drive but then again your engine room is tight in so much as length with a gearbox.

    If you wait a couple of days I can give you all dimensions of the Yanmars ,--I have a pair.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    Dilema

    Piperca,

    I don't envy your dicision position.

    Pull out a piece of paper and list the pro's and con's on each side and then

    give them an importance value of 1-10. This may help give you a

    numerical summary of your desires.

    Have you considered outboards? Like you need another option...

    How important is weight to you?

    What would you do with the extra bilge space?

    Some other considerations may be:

    resale value, ride comfort and noise level, stability (sea worthiness),

    length overall, cost (install and maintenance), time out of service,

    what the wife wants, and probably several other things I can't think of right

    now.

    Good luck! Let us know what you decide.

    Tom
     
  11. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Just one shallow question - why do you want to switch to diesels? Diesel engines are an excellent choice if you spend a considerable amount of time at sea. In that case the overall fuel cost savings can compensate for the bigger cost of diesel engines in, say, 4-5 yrs of use. So the first thing to do is an estimate of number of engine operating hours each year.
    In case you already did this math and decided that diesels are the way to go, than you have to take care of several things over there:
    - weight of new engines vs. the old ones. You have a planing boat over there and the weight distribution is an important point.
    - dimension of new engines vs. available space.
    - don't forget to leave enough space for a mechanic to visualy control and insert his hands between the engines and do whatever he needs to do. You shouldn't leave less than 30 cm (12") of spacing.
    P.S. - no, surface drives are not what you need if you are looking for good low-speed and trolling capabilities.
     
  12. piperca
    Joined: Mar 2008
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    Location: Southern California

    piperca Junior Member

    I weighed the gas/diesel dilemma and this is what I found:

    -Gas is about 7k a side (Marine Power w/closed cooling).

    -A couple of low hour diesels would cost me about the same and I could use my existing drives (280s).

    -A single Yanmar is around 12K; outdrive, another 5K.

    -I installed new fuel tanks, which have never seen fuel.

    -No GAS under my aft berth to blow up my wife and kids.

    I've got one BRAND NEW Volvo Penta 31P-A (4 cylinder, 150hp), but haven't been able to find a match ... this would be the easiest option. I'd snug these up to my existing Volvo Penta 280s and call it done!

    The Yanmar would reduce weight, reduce overall maintenance (single vs twins), reduce fuel burn, but I'd sacrifice maneuverability. I would probably see better performance from the Yanmar than the twin Volvos, due to the drop in weight and higher horsepower rating of the Yanmar. I'd need to invest in a newer outdrive (Bravo 3x or equivalent), which will set me back a few more dollars.

    So, I think I've settled the gas versus diesel dilemma regarding the difference in cost to repower; therefore, I can shelf that.

    Now, back to the original question; twins or single?

    The Volvos are no bigger than a gas 350, so I know I'll have plenty of room after installation. In fact, I've seen a lot of Skipjacks with twin Volvo 41s installed (6 cylinder), which is the choice setup for this boat, so I'm not concerned.

    I guess maintenance and maneuverability appear to be the main issues, which gives a knock to each category, single and twins ... which can I live with???

    Thoughts? Am I missing anything?
     
  13. Submarine Tom

    Submarine Tom Previous Member

    Diesel

    "Now, back to the original question, twins or diesel."

    Diesel.
     
  14. piperca
    Joined: Mar 2008
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    Location: Southern California

    piperca Junior Member

    Got me! :p
     

  15. Jango
    Joined: Aug 2005
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    Location: Mid Atlantic

    Jango Senior Enthusiast

    Been following the discusion and based on what I've heard, I would go for Twin 150HP diesels.
     
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