[C38] performance

[Les]

You have already received several replies, but none mentioned what
causes it except the term propwalk.  So, here is the physics course...
	Water gets denser as it gets deeper.  Our props are about 16-18" inches
in diameter(I just don't remember the right number)...  Thus the water
at the top is about 
1#/sq inch less dense than at the bottom, and the prop covers about 3"
width, so there is about 18 square inches driving at each of the top and
bottom (only thinking of the widest part at the outer half of the
prop)... This translates to about 18# of force driving the boat away
from the rotation at the bottom.  The prop turns counter clockwise in
reverse, and so it drives the boat to the left (port).  The only thing
counteracting that is the lift across the rudder and keel.  In our boats
1Kn generates about 20 lbs of lift on the rudder, so once the boat is
moving a bit over 1kn, the rudder begins to take control, and by 1.3-1.5
knots the rudder overcomes the propwalk, and you go where you steer.
That is why the practice all of us use (reverse, throttle a bit,
throttle down, neutral wait a bit and repeat) works. The boat is massive
(8tons), so the propwalk takes time to accelerate the boat sideways, a
short burst will produce some back motion, and then neutral removes the
propwalk, and the rudder has control. three times usually works for me.

	There is a neat book called Dockmanship which details some other
techniques using lines to control the boat both coming in and going out.

	A neat exercise on a quiet day is to take the boat to calm waters, and
use the propwalk to turn the boat in its own length. 

	the procedure:
	stop the boat in a calm and out of the way area.  let it just float a
bit and feel the absence of motion.  
	Turn the wheel to port (left).  
	Put the transmission into reverse, and goose it a bit on the throttle.
but
		don't let the boat move backwards more than a foot.
	The boat will start to swing to port.  Put the throttle in neutral, 
	quickly turn the wheel to starboard, change to forward and goose it.
When going forward, the prop wash (water pushed by the prop) hits the
rudder to help turn the boat.  Goose the throttle a bit and then before
the boat moves forward, throttle to idle and shift to reverse, swing the
wheel over to port and goose it again.  

	Now the boat will be swinging pretty well.  Take it easy.  It is easy
to get the boat spinning wildly if you do this too many times, and what
you want is to be able to get about a one minute swing for a full 360.
Practice this and you will get a good feel for the propwalk and a new
skill, that of being able to turn your boat around in its own length.  
	
	Practice also getting the boat to swing 90 degrees and 180 degrees so
you learn how to stop the swing (put the boat into forward with the
rudder to port).

Some really good folks can do this in the opposite direction only using
forward, with just enough reverse to keep the boat from going forward.
I haven't mastered that one yet.

	It is worth while to note that the fin keel makes this possible.  A
full keel boat may be able to do the turn in the direction of propwalk,
but not the other way due to the physical resistance of the full keel.

	This ability will help in getting into and out of narrow slipways or
changing your approach in a narrow channel as well.  

	Finally, slow crashes do less damage.  When docking or rafting try to
go at a very slow walk or waltz speed.  Also NEVER let anyone try to
stop this big a boat by pushing with their legs against something.  They
can be badly injured.  Remember that 8tons is pushing and even slowly it
is still 8 tons.

Regards,
Les H
On Sat, 2007-10-27 at 16:01 -0700, Ken wrote:
> I recently bought a '83 Catalina and I am in the process of getting to
> know the boat. It has the standard 21 hp Universal and two blade prop.
> My big problem is when I am trying to back out of my slip the stern
> moves hard to the port no mater which way I turn the wheel. My slip is
> pretty tight length and width wise.
> Is there some way I can get more power and control without getting a
> bigger engine? What's the best way to turn the helm when I back up in
> a tight area to keep the stern swinging to the left?
>  Ken 
> Catalina 38 hull #251