Bow thrusters, windage, ocean crossing Powercats
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
Hi All,
John raises some interesting points here, a boat can never have too much
control, and there certainly aren't many 85fts these days without thrusters!
It allows owners to get larger boats into tighter spaces without having to
be an expert skipper who spends every day on the water. On smaller vessels
where budget is a concern the cats beam certainly does gives the skipper
more control and the option to do without the added expense of thrusters. I
know the cost of a thruster is not that much compared to a whole boat, but
you say that 50 times...........
The jet thruster idea does sounds interesting the decreased efficiency is
well out balanced by the installation options, especially when considering
the infrequency of use. On a similar note I'm sure plenty of you will have
seen the below, looks like a lot of fun!
The other aspect here is hull immersion rates and the effect this has on
both handling and load carrying. For centuries the arguments for short fat
boats or long thin boats has been discussed back and forth, as the virtues
of interior volume, fuel efficiency, roll stability, pitch stability and
load carrying have been played off against each other. There was a Russian
Royal Yacht, circa 1881, http://www.cityofart.net/bship/ru_novgorod.html
which in search of ultimate stability was built with almost a circular plan
form. As well as being very stable she also became the first known
instances where Naval Architects became aware of the damage that can be
caused by dynamic slamming pressures in a seaway, when upon being caught in
a storm this boats immense initially stability made here the unmoveable
object that came up against an unstoppable force of a North Atlantic Storm!
Powercats, unlike mono hulls are more specialised in their design, and the
hull shape should match its intended use. A design that is intended as a
full time passage making powercat will be specialised. John's 86fter
(Pacific Harmony) was designed for duel purposes as a charter boat for
harbour and coastal operation, but to be capable of being delivered anywhere
in the world. She was also based around using and existing hull mould that
was lengthened to suit the new boat. For a pure offshore boat this might
be too much of a compromise to make.
A wide hull can be made fuel efficient if the speed is right and the transom
immersion is kept to a minimum in all load cases. Inevitably the length to
beam ratio of a hull will effect its fuel economy regardless, as the tighter
the radius the water must bend around the hull, the higher the pressure
gradient, the larger wave and the greater the resistance. But this has to be
balanced against many other things. For many owners on an offshore boat
engine access and ventilation are primary concerns, and need to be
considered with the number of hours the boat will operate and the required
efficiency, and range to set the waterline beam.
The second thing that specialises a powercat is the wingdeck height, and
John makes some good points on adaptation of sailing designs and why this is
often unsuccessful as the wingdeck shape, and reserve buoyancy distribution
of a well designed sailing cat will not necessarily have been done for
taking the seas head on, as this is not possible while sailing, where as on
the powercat this is a common occurrence. As well as function the wingdeck
height plays a significant part in the aesthetics of a power cat and is a
constant balance between the profile, wingdeck clearance and the centre of
gravity for pitch stability of a design. On a charter boat like Pacific
Harmony the overall profile of a charter boat is a significant consideration
for the 'desirability' of the boat, so again compared to a purpose built
offshore vessel you may choose to go for a lower wingdeck than you otherwise
would target.
Kind Regards
Tony Stanton
(PS I worked as a designer and engineer for Malcolm, including work on
John, Dennis and Mikes boats)
-----Original Message-----
From: power-catamaran-bounces@lists.samurai.com
[mailto:power-catamaran-bounces@lists.samurai.com] On Behalf Of John Winter
Sent: Thursday, 23 April 2009 5:37 p.m.
To: power-catamaran@lists.samurai.com
Subject: [PCW] Bow thrusters, windage, ocean crossing Powercats
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
Power-Catamaran Mailing List