Re: [PCW] Catamaran Drive system Comments
Hi Gary,
The varible transmissions, the controllable pitch props, etc, etc are beyond
the scope of this drive unit I speak of. Those are all worthy considerations,
but they fall outside the realm and scope of this project.
Our desire is simply to address the subject of getting the power transmitted
from inboard the vessel to the prop outside. In that context we will keep it
to a 1 to 1 drive ratio with the stipulation that the max RPM of the input
and
output shafts be 1000 RPM or closely thereabouts.
So one can chose the prop size and type accordingly, and the engine and
transmission gear ratios accordingly that they wish to put on either end of
the drive unit. We simply want to transmit that power to the prop without
having to 'line-up' with the prop shaft. We want to be able to place the
engine over the prop, if that is deemed advisable, or an advantage with it
comes to access to the engine for maintance, and/or, keeping the engine(s)
out
of the living spaces.
We hope also that this drive unit can be incorporated into an aftermarket
unit
configuration that can be added to existing 'sailing vessels' to provide for
a
'motorsailer vessel' with greater capabilities in their power nmode. (take
older sailing catamarans and make them better performers in the power mode)
Brian
------ Original Message ------
Received: Sun, 04 Apr 2010 10:07:38 AM EDT
From: Gary Hagstrom ghagstrom@yahoo.com
To: power-catamaran@lists.samurai.com
Subject: [PCW] Catamaran Drive system Comments
Brian,
If you guys are working on a new drive system as you described
consider including something like the Nissan continuously variable speed
automatic transmission in the design. You are aware of the typical
significant difference between the rpm and power producing/absorbing
characteristics of a diesel engine and the propeller. The prop is
designed to
absorb the total hp the engine can produce at rated RPM. At RPMs below
this
level the prop does not absorb as much power as the engine could
produce.
Properly loading the engine at lower RPMs would also solve the problem
of
a
large engine not having sufficient load at low rpms to fully warm up.
Most
engines have an optimum specific fuel consumption considerably below
their max
rated RPMs. The new Vovlo D3 has its most economic specific fuel
consumption in the 2500 rpm range. The Nissan transmission is used in
300
hp
vehicles and gets better mileage than a manual transmission version. It
has
been in service a number of years now and seems both reliable and
efficient.
I looked at controllable pitch propellers but they lose efficiency due
to
the
hub size and are quite a compromise in the smaller diameters. I also
thought
of a two speed gear that used high gear for lower rpms and low speed for
the
highest rpms but the standard available items did not have appropriate
ratios.
Envision a single 'speed' lever that integrated prop, engine and
transmission characteristics and as you pushed it forward it determined
what
ratio to set the transmission to and what rpm to set the engine at. With
today's electronic controls this seems like a relatively trouble free
way
to
achieve the maximum economy and performance. From the 'back of the
envelope'
calculations I have done it looks to me there is a 5-7% economy
improvement
possible and a significant decrease in engine noise and wear with the
same propeller thrust (equates roughly to boat speed) available at
500-700
rpms less than a traditional fixed pitch single speed prop.
Give it some
thought
Gary Hagstrom
building CRACKERJACK, a Maine Cat P-47
Iron River,
Wisconsin
ghagstrom@yahoo.com
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