[PCW] Outboards on a cat

[Robert Deering]

Having owned a 28' outboard-powered cat for nearly ten years I have some
thoughts on the matter.

The advantages have been pretty well laid out previously and I agree with
them.  There are some disadvantages too:

Efficiency - A modern diesel still beats a modern 4-stroke by about 35%.
The price of diesel has climbed relative to the price of gas lately, so the
operational cost of the outboard is probably only 20% or so higher.  But the
effective range of a gas boat is still substantially smaller, which is a big
deal for those of us in remote areas.

Noise - the new common-rail diesels are amazingly quiet.  So are 4-stroke
outboards when they are idling.  But an outboard running at high RPMs is
still pretty noisy - I had to keep my cabin door closed when running or it
was hard to talk.  There are no effective ways to dampen noise on an
outboard, it's not like you can add sound shielding.

Weight distribution - hanging outboards off the aft transom(s) forces some
design compromises.  Boat design doctrine says that the majority of the
boat's weight should be as centered as possible.

Prop size - outboards are primarily designed for planing hulls.  Running at
slower displacement speeds suggests larger diameter props which can't fit
under the cavitation plate.  And the outboard's gear ratio is fixed, you
can't change the transmission.  Not to say that an outboard will never work,
just that displacement boats, even faster cats, is not the target market.

Vulnerability - All of your expensive propulsion system is hanging out there
ready to be rammed.  I lost an expensive stainless prop to an errant trawler
recently.  Also suffered a cracked cowling due to an unidentified encounter.

Maintenance - outboards should be easier, right?  And generally they
are...if the boat is out of the water.  But a Coastal Cat 34 is not the type
of boat you'll be hauling out very much.  Now you're doing maintenance with
the outboard suspended out over water.  Buy lots of cheap tools - you'll be
offering them to Neptune regularly.

Longevity - I concur with Pat.  I've heard of outboards making it over 2500
hours with 'ritualized maintenance', but no normal boater (including me)
will want to go through the processes these guys did.  1500 hours is a more
realistic number, which is still probably long enough for most.    

Bob Deering
Juneau, Alaska