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Friday, May 17, 2013

Single or Twin Engines?

Single of Twin Engines?
A clean, single engine installation in a Back Cove 37

Which is best?

I'm sorry, but I have to start with: it depends.

But wait!  Spoiler alert!  I'll tell you which is best right here and right now.  A single screw is better than twins.

But then again, it depends on a few things...

First, for some light background reading, check out these various resources on single vs. twin installations:

Bill Parlatore addresses the subject in PassageMaker.
An unknown author referencing Trawler FAQs.
iBoats forum discussion.
BoatDesign.net forum discussion.

Then, know that Nordhavn - that's right, the ruggedest (word?), meanest, baddest boat in town, designed to go through hell and high water - uses single engine installations.

BUT, they do it intelligently.  This is just one instance of how Nordhavn has solved some of the potential mishaps of a single-engine and consequent lack of redundancy:

In lieu of a conventional wet exhaust, P.A.E. has invested years in developing a well-engineered, dry exhaust system. Well worth the effort, the dry system eliminates the need for a saltwater supply thru-hull, an engine mounted raw water pump, heat exchanger, anti-siphon valves and sea water injected exhaust hoses. Instead, the engine is cooled by a simple closed system that circulates the coolant through a captive series of tubes recessed in the vessel's hull. This keel cooler has no working parts and produces negligible hull drag. In addition to the advantages of not introducing the corrosive effects of saltwater into the boat's system, the dry exhaust system eliminates annoying exhaust gas and noise as the exhaust is vented far above the aft deck.

Well, duh.  I love it when there are plain and simple alternatives to overly-complex, unreliable systems.  Less stuff to break means less risk and better economies from a single engine.  For the longest time, when confronted with this debate, I would have said 'TWO ARE BETTER THAN ONE!'  But, given all of these facts, I think a change of mind is in order.  Principally, these are the points that have changed my mind:
  1. To quote Bill Parlatore, 'nearly one hundred percent of the commercial fishing and workboat vessels (of all sizes) are single screw/one engine boats.'  Of course, some of those boats probably have spares and a marine mechanic on-board.  But then again, we recreational boaters can carry spares and learn to install them too.  
  2. If there's a problem with the FUEL system (which frequently IS the cause of engine failure), then both engines will shut down, and you'll have two engines to repair.  Hmmm....
  3. One engine is cheaper than two.  One engine is cheaper than two.  One engine is cheaper than two.  Get it?
  4. A single, center-line propeller is protected by the keel and rudder.  I've seen this numerous times in boat designs, and yet never gave it much thought in comparison with two, exposed props.  
  5. If you learn how to drive a boat, you can manage without fiddling with twin-engine sticks.  Two words: bow thruster.
  6. Most engine manufacturers, who make money selling ever-more engines, recommend a single-engine installation.  If THAT doesn't get you thinking, I don't know what will!
  7. Most boaters are never out of sight of land to begin with, and therefore SeaTow is only a call away.  Before you lambaste me for that one, consider the cost (economically and emotionally) of SeaTow the .05% of the time you need them, versus the cost of operating the boat with two engines 100% of the time.  Then remember that Nordhavns cross oceans.
All in all, this debate really centers on your comfort level with a single vs. twin installation.  If a second engine makes you feel safer, by all means do it.  If you've learned how to change water pumps, impellers, hoses and fan belts, and carry spares (all of these things should apply if you are any kind of boater), then you may just find that a single screw boat is cost-effective, reliable, and simple.  

In any case, maintain your engine(s) diligently, and they'll treat you to a safe arrival at your next port o' call!
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  1. Yes,
    It never ceases to amaze me that if one person can't see the need for two engines, then nobody else needs two engines either.
    Some of the reasons given here for single engine installations are sophistry at best and untrue at worst.

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