GL: Roll Causes Yaw? Was "Appropriate trawler for Caribbean cruising"

[Bill Donovan]

Jonathan, et al:

The yaw premise, included below, fascinated me.  So, I did some 
paper-napkin calculations.  Lets assume a typical 40 ft twin screw 
"trawler" with a 14 ft beam and, say, 7 feet between shafts.  If the 
boat rolls some 45 degrees, the two propellers will differ in depth 
about five feet.  With a hydrostatic pressure gradient (fresh water) 
of .433 psi/ft, the pressure difference on the two screws would be 
about 2.165 psi, or equivalent to about 15% of one 
atmosphere.  Doesn't seem like much.  Not compared to a 15-20 ton boat.

Now I thought about a practical case.  Of course the premise is 
correct that the deeper propeller is pushing denser water and 
therefor will push a little harder (due to less cavitation 
probably).  However, with the brief roll period of the 40 ft boat, 
the time the slight additional thrust has to act is very small, 
unlikely to be sensed by a passenger on the boat.  And, I dare say, 
the motion of the waves causing that deep roll will have enormously 
greater effect on the boat.

I also suspect that anyone rolling 45 degrees in a seaway isn't going 
to care very much about a little yawing! ;-)

I also thought about the tighter turn of a twin screw versus 
single.  If the twin has both shafts at the same speed, the tightness 
of the turns of either vessel will depend on the rudder size, not the 
single vs. twin screw.

And as Bob says, a long, deep keel will tend to help a boat track 
straighter regardless of what outside forces are at work.

Bill


At 01:44 PM 2/20/2009, jonathan olenick wrote:
>I have heard ( but not confirmed) that when a twin screw vessel rolls, the
>deeper prop has a bite on denser water , and hense more thrust, thus causing
>yawing in conjunction with the roll. Also single screw with long deep keel and
>large rudder tends not to yaw, flip side, it does not make sharp turns. Jon on
>Valentine, Nordic tug 37, full keel, single screw, large rudder.
...snip...