[PCW] Bow thrusters, windage, ocean crossing Powercats

[John Winter]

Jim, Jonah, Jeff and listees!

We built an 86 ft Tennant cat and motored her 9000 ocean miles from NZ to
Florida a number of years ago so can contribute a few real time thoughts.
Jonah- see below, not necessarily the way to go, sorry Dennis! 
We had no thrusters at all, 28ft beam, reasonable windage from high topsides
but a slippery superstructure. Quite a few instances we really wished we had
thrusters. 6 boats later we are now fitting stern thrusters as well as bow
in most new builds over 60ft and find the docking procedures much more
relaxing so wives and family like to go boating as its not so stressful.

On our next Powercat (a 56ft ocean going liveaboard we hope to keep as our
personal boat) we are considering the Wilbo system from Holland,
http://www.willdo.nl/index.php?subject=168
the water jet nozzles instead of propeller tunnels for thrusters. They can
operate from hydraulics and when doing bow and stern they say it is less
expensive than propeller tunnel thrusters. Advantages of smaller openings-
jet nozzles, so less drag, no anodes to change and no blades to clean or
mooring lines to suck into the blades. You can see how much power a jet ski
can get from a tiny nozzle so I believe they will work.
Being 4 nozzles they will work well on a cat- 1 in each corner so the
thruster wash isn't being pushed into the other hull. Also a central power
pack or PTO from each motor can operate the windlass and davit/tender
platform, as all 3 don't usually operate simultaneously so battery weight
saving will be a bonus.

Many years ago an experienced ferry driver showed me how he shuffles his
50ft ferry evenly sideways with no thrusters by using the rudders hard over
and forward/reverse gear with the thrust against the rudders. This didn't
work well on our Tennant cat as we had rudders canted over so when turning
at speed the cat leaned into the turn. (Craig Loomes designed them for wave
piercers, ocean race yachts use them also) Not a good idea for manoeuvring,
don't try it, as how often do you need to lean into turns at high speed vs
better rudder power in marinas and docking. We had poor low speed control
from the rudders.
Now Mr Tennant has passed on I will give my opinion of the Tennant hulls at
sea. The narrow waterline canoe body hulls we found were way too slim on
open ocean, good for harbour ferries with controlled payloads but immersion
rates too great from the narrow water plane area for serious sea-going.
Ocean going means large fuel capacity, loads of spares and too much live
aboard gear in all the wrong places so overweight and poor weight
distribution is a factor. When the narrow hulls sunk low, and/or easily got
out of trim they gave poor economy. When they sliced through the wave too
deep the tunnels slammed, not good when you have 1000 miles to go and it's
all on the nose. 
Now we have found some qualified Cat Naval Architects who can get Tennant
level fuel economy with much wider hulls that aren't crippled in ocean going
mode so that's the way we are headed. Also gives engine room and
accommodation spaces usable for senior age comfortably built humans, not
skinny contortionist monkeys which I was attempting to be in service
operations. We've also added better tunnel design for breaking up the waves
and distinctively different planing hull mold change options for the go fast
cruisers as a displacement hull is too often pushed to speeds where it is no
longer efficient. Also they aren't a morphed sail cat, something I have seen
and believe is not successful. In my opinion, Sail cat powercat conversions
have hulls that are too far apart for head seas, letting in too much wave
volume and creating havoc with cross chop on swell. They are designed for
sailing off the wind and counter-acting sails, not head seas. Just go longer
if more room is needed, you'll need to pay for the dock space anyway as most
marinas like to charge beam factors on cats for extra revenue.
Hope I haven't offended too many readers, just stirring up some food for
thought!
Cheers from Down Under!
John Winter
www.adventurebaypowercats.com