Twin VS single

Discussion in 'Boat Design' started by claydog, May 10, 2011.

  1. claydog
    Joined: Dec 2010
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    Location: michigan

    claydog Junior Member

    I've searched the forum and come with a lot of conflicting information, and not having anywhere near the math skills and overall boat knowledge needed to figure it out for myself, I’m hoping for some help in understanding the pros and cons of a twin engine set up beyond cost and having a backup in case of failure. My question is, is there any performance gain on a twin engine set up over a single, assuming all other variables are the same on a planning hull boat?

    Thanks Andy
     
  2. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Claydog,

    It depends on how you define performance, but there are some advantages each way.

    A single engine set up has
    1) less stuff in the water and therefore reduces drag from the second engine.
    2) Less installed weight for the same horsepower (Older boats may not be able to handle the weight of twin modern engines)
    3) Half the maintenance (since half the number of engines)
    4) Less dollars/hp (generally, even with a kicker)


    A twin set up
    1) allows differential stearing (unequal thrust) greatly enhancing low speed manuverability
    2) More options in installing location allowing for better balance
    3) Less weight and thrust at a specific point
    4) Better hole shot (low end torque)
    5) No Torque stearing
    6) Less prop ventelation (Not on centerline)

    In short there is no one size fits all aproach to selection. But for me, the one advantage you excluded (as a back up engine) is the most important. I fish off shore as far as 45 miles in a 26' open cat. Knowing if I loose one engine (I amso have two seperate fuel tanks, and isolate the engines from one another electrically) I can still get up on a plane with one engine and make it back in something like a reasonable amount of time is very important to making these trips safely,.
     
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    In addition to Stumble's observations:
    For the same speed the total engine power of a twin-engine boat will be higher than the engine power of a similar (same EHP) but single-engine hull. Various authors claim various percentile increase, ranging from 15% to 30%, depending on hull type and speed.
    The difference lays in the fact that a twin-engine installation will have smaller propellers (which are less efficient) which, in addition, won't benefit the influence of the hull's wake. It will also have a higher drag due to double propeller struts, shafting and (eventually) double rudders. Furthermore, two smaller engines will be somewhat less thermodynamically efficient than a single big engine.
    Cheers!
     
  4. whitepointer23

    whitepointer23 Previous Member

    i run twins, this is the first twin engine boat i have owned and the last. the handling is good but not good enough to out weigh the negatives. the 2 biggest for me are having to buy 2 of everything and very inefficent, 450 hp when the same model boat is faster with a 260 hp sterndrive.
     
  5. keysdisease
    Joined: Mar 2006
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    keysdisease Senior Member

    The fastest boat in the world has a single engine. Steve


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

    daiquiri Engineering and Design

    2x225 HP engines + stendrives weigh around 800 kg + fuel
    1x260 HP engine + sterndrive weigh around 450 kg + fuel
    Considering that the weight is a single most important factor in planing boat power requirements, it's no wonder that you have lost performance.

    A more correct powering for your boat, imho, if you wanted to change to twins, would have been 2x(160-170) HP, which would weigh around 550-600 kgs. Still much more than a single engine + sterndrive, yet much less than your actual configuration.
     
  7. whitepointer23

    whitepointer23 Previous Member

    the twin 225 chryslers are standard in my old 70's boat, they are shaft drive which also contributes to the equation. the ones that have been refitted with 260 sterndrives perform a lot better. my boat is over 4 metric tons. i just think twins are to much to look after, i would rather look after one engine .
     
  8. u4ea32
    Joined: Nov 2005
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    u4ea32 Senior Member

    I agree that twin engines can be a bad idea, for all the reasons mentioned. My current boat is also my first and last twin engine boat.

    A big reason twin engines are a big fail in most boats is the lack of access for inspection, maintenance, and repair. Therefore, the maintenance costs are much more than double due to the great increase in hassle to do anything. Therefore, the likelihood of failure is much greater, because for a given amount of money you can only afford so much maintenance, so some maintenance will not be done, and some failures will occur.

    Very few commercial boats have more than one engine. Including the Sea Tow boats. The key to their ability to cross oceans, fish far offshore, and to rescue recreational boaters, is the ability to maintain their vessels and powerplants.

    Most recreational boats are not designed to be maintained, even single engine boats. Most recreational boats are designed to be bought and sold. This directly leads to that old saying about the best days in the life of a boat owner is the day the boat is bought, and the day the boat is sold. Only on those two days is the boat actually doing what it was designed to do.
     
  9. Easy Rider
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    Easy Rider Senior Member

    daiquiri,
    Unless it's a flat bottom the twin can run larger props w the same draft.
    There is no prop walk that the rudder must constantly correct for = less drag.
    Two smaller shafts should be about the same drag as one big shaft.
    What does this mean? "won't benefit the influence of the hull's wake."
    The twin is more expensive and has more vulnerable props but other than that I see twins as superior from a performance standpoint. But I know my opinion is not shared by most. I see the prop walk issue a huge advantage to the twin.
     
  10. claydog
    Joined: Dec 2010
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    claydog Junior Member

    Thanks to all, this tread has helped me a lot in understanding the pros/cons of twin engines.

    stumble, the only reason I exclude the second engine as back up is that it is the most obvious benifit. It's also my prime reason for considering a twin, I don't get as far out in the great lakes as you do in the gulf, but I do have to run into any weather that pops up to get back to port.
     
  11. Submarine Tom

    Submarine Tom Previous Member

    No, only losses.

    -Tom
     
  12. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Claydog,

    As you can see the jury is split on the issue. Personally I have owned, and would own in the future both set ups. For off shore work the marginal efficiency losses from a twin are more than made up for with the added safety. And if eeking out the last possible MPG is critical then slow down 10% and in any configuration you will save money.

    That being said for inshore work on smaller boats I prefer the simplicity of a single engine.

    I don't think one is necessarily better than the other, Ijust think they have different purposes.
     
  13. whitepointer23

    whitepointer23 Previous Member

    the most common setup here in aus is an auxillary outboard on the transom for back up. stumble , i can see your point but i am convinced that 1 well maintained engine won't let you down .
     
  14. Pierre R
    Joined: May 2007
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    Pierre R Senior Member

    The first twelve post on this thread are some of the best that I have seen on this topic yet.
     

  15. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    In a performance boat its the drag that kills you
    Just look at rigs that have 3 outboards instead of 2
    What you can gain in a heavy boat adding an engine is getting the same cruise speed but with lower engine rpm and that could make a big difference in fuel consumption especially with automotive engines.
     
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