Georgs,

I am a mechanical engineer, my  specialty is system design - specifically
wind turbine systems.  What I do know is that we spent a lot of a time
developing  computer models for predicting wind turbine performance - and that was
relatively simple compared to hull design because we were operating in  a single
medium, namely air.  Boats operate at the air/water interface  which is
inherently much more difficult to model.  I can't  say I've stayed current in this
field, but the last I knew, there was still  plenty of tank and tow testing
going on in the boat world.  This tells  to me that reliable performance
prediction models are still not readily  available.

But, even with perfect design tools, a  naval architect has still got use a
certain amount of creative (some  might say artistic) talent to create a good
hull design. The computer may serve  as a guide, but it's the designer who
decides how  much importance to give to each variable - often there are direct
conflicts, and compromises must be made. Fuel economy is but one  variable, it
may not get priority number one in the design process.

Although there's been plenty of discussion  on this subject, I'm not sure
there is yet any real agreement on what  actually makes the most economical
powercat.  Of course it  depends very much on speed.  Much like monohulls, cats can
be  optimized for displacement, semi-displacement or planning speeds.

It is my personal opinion that the catamaran  offers little efficiency
advantage over a monohull at displacement speeds.  I've never rigorously documented
this, but my experience tells me that  a well designed trawler can carry the
same weight at the same speed for the same  (or less) fuel consumption, at
least up to the normal "hull  speed" associated with that length boat.  So, if
you're content to  cruise at 7 knots, and your primary concern is fuel economy,
I doubt that a  powercat is what you'll choose.

If however, you'd like to go faster than 7  knots (in a small cruising boat),
the powercat does appear capable  of offering huge fuel savings.  Operating
in that wonderfully nebulous  "semi-displacement" mode somewhere between 15 and
18 knots, the  powercat can beat the pants off any similarly sized monohull.

Although our average fuel consumption on  Sno' Dog has been around 3 gal/hr,
I suspect our actual consumption at a  steady 16 knots is more like 5 gal/hr.
Still, this has got  to be less than one-third the amount consumed by a
similarly-sized  monohull.  Here is where a well-designed powercat shines -  and
this is why we chose the PDQ.

As to why the PDQ outshines some - or most  - of the others (which I believe
was your original question), I  can really only speculate.  I understand
weight is very  important.  PDQ puts a huge amount of effort into keeping their
boats  light.  Virtually all the interior woodwork and panels,  including the
doors (which appear to be solid wood) are actually made from a  foam-core
material.  The weight of a finished dry PDQ is about 11,000  lbs.  Second, the hulls
are slim and well designed. They are not intended  as planing hulls and seem
optimally designed for this mid-range  speed.  Third, PDQ learned from
experience (with their original  32-footer) and added more flat lifting surface aft.
This helps keep the  boat level at speed.  You'll notice that a PDQ doing
15-16 knots does not  appear to be going uphill as some other boats do.  Finally,
the  Yanmar 100-hp engines (which weigh no more than the original  75's) seem
to be an excellent match for the boat.  Because they turn  slower to move the
boat at a given speed, they consume less fuel.  This was  duly noted during
last year's flotilla when, with boats traveling equal  distances at the same
speed, they found the 100-hp models actually consumed  about 15% less fuel.

Perhaps that's more (or less) than you  wanted to know.  In either case, I
apologize.

Henry Clews
aboard Sno' Dog in Newington,  NH

In a message dated 7/24/2006 3:51:55 P.M.  Eastern Daylight Time,
georgs@powercatamaranworld.com writes:

Henry,  you're an engineer, aren't you? Why do you figure the PDQ 34
is so  efficient through the water and thus so miserly with fuel  burn?

--Georgs

Henry,

5gph at 15knots is quite good.  Do you have a fuel flow meter - just curious if the consumption is fairly linear with speed up to that point or if there are big gains  by dropping back to ~11-12?  It seems that most folks either stay at 7-8 or crank it up to 15-16 if they are able.

Could you also comment on the interior noise at cruise.  Often the lightweight designs don't do great with noise isolation.

Thanks,

Mark

HClews@aol.com wrote: Georgs,

I am a mechanical engineer, my  specialty is system design - specifically
wind turbine systems.  What I do know is that we spent a lot of a time
developing  computer models for predicting wind turbine performance - and that was
relatively simple compared to hull design because we were operating in  a single
medium, namely air.  Boats operate at the air/water interface  which is
inherently much more difficult to model.  I can't  say I've stayed current in this
field, but the last I knew, there was still  plenty of tank and tow testing
going on in the boat world.  This tells  to me that reliable performance
prediction models are still not readily  available.

But, even with perfect design tools, a  naval architect has still got use a
certain amount of creative (some  might say artistic) talent to create a good
hull design. The computer may serve  as a guide, but it's the designer who
decides how  much importance to give to each variable - often there are direct
conflicts, and compromises must be made. Fuel economy is but one  variable, it
may not get priority number one in the design process.

Although there's been plenty of discussion  on this subject, I'm not sure
there is yet any real agreement on what  actually makes the most economical
powercat.  Of course it  depends very much on speed.  Much like monohulls, cats can
be  optimized for displacement, semi-displacement or planning speeds.

It is my personal opinion that the catamaran  offers little efficiency
advantage over a monohull at displacement speeds.  I've never rigorously documented
this, but my experience tells me that  a well designed trawler can carry the
same weight at the same speed for the same  (or less) fuel consumption, at
least up to the normal "hull  speed" associated with that length boat.  So, if
you're content to  cruise at 7 knots, and your primary concern is fuel economy,
I doubt that a  powercat is what you'll choose.

If however, you'd like to go faster than 7  knots (in a small cruising boat),
the powercat does appear capable  of offering huge fuel savings.  Operating
in that wonderfully nebulous  "semi-displacement" mode somewhere between 15 and
18 knots, the  powercat can beat the pants off any similarly sized monohull.

Although our average fuel consumption on  Sno' Dog has been around 3 gal/hr,
I suspect our actual consumption at a  steady 16 knots is more like 5 gal/hr.
Still, this has got  to be less than one-third the amount consumed by a
similarly-sized  monohull.  Here is where a well-designed powercat shines -  and
this is why we chose the PDQ.

As to why the PDQ outshines some - or most  - of the others (which I believe
was your original question), I  can really only speculate.  I understand
weight is very  important.  PDQ puts a huge amount of effort into keeping their
boats  light.  Virtually all the interior woodwork and panels,  including the
doors (which appear to be solid wood) are actually made from a  foam-core
material.  The weight of a finished dry PDQ is about 11,000  lbs.  Second, the hulls
are slim and well designed. They are not intended  as planing hulls and seem
optimally designed for this mid-range  speed.  Third, PDQ learned from
experience (with their original  32-footer) and added more flat lifting surface aft.
This helps keep the  boat level at speed.  You'll notice that a PDQ doing
15-16 knots does not  appear to be going uphill as some other boats do.  Finally,
the  Yanmar 100-hp engines (which weigh no more than the original  75's) seem
to be an excellent match for the boat.  Because they turn  slower to move the
boat at a given speed, they consume less fuel.  This was  duly noted during
last year's flotilla when, with boats traveling equal  distances at the same
speed, they found the 100-hp models actually consumed  about 15% less fuel.

Perhaps that's more (or less) than you  wanted to know.  In either case, I
apologize.

Henry Clews
aboard Sno' Dog in Newington,  NH

In a message dated 7/24/2006 3:51:55 P.M.  Eastern Daylight Time,
georgs@powercatamaranworld.com writes:

Henry,  you're an engineer, aren't you? Why do you figure the PDQ 34
is so  efficient through the water and thus so miserly with fuel  burn?

--Georgs

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