Wheel spins to Port while motoring - Why?

[G. Richard Stidger]

I never noticed this before, but when motoring, especially at WOT, if I were to let go of the wheel, it would rotate to port until hard over.This happens at all throttle settings, but is most rapid at higher rpm's.I have a RH rotation 3-blade prop vs the original 2-blade prop. It occurred to me that perhaps this is just prop walk to the starboard while going forward?Does this make any sense?

 

[Pete Vente]

Same here ...

I have a 3-blade prop on my 37c, and the same thing happens. I just contribute it to prop walk, nothing more.

 

[Mickey McHugh]

Prop walking

My 40.5 walks to starboard as well. It is the standard 2 blade. It is a little scary at high speed since letting go of the wheel would be disaster. The 40.5 turns on a dime and everyone would get tossed around. So, I try not to get in a hurry and keep my speed around 6 knots. Using the prop walk during docking is like having a stern thruster. It can be used to move you against or away from the pier. Practice on a T-head pier.

 

[G. Richard Stidger]

I feel better now

Thanks for your responses. I can't believe that I never noticed this before my third season.Rich

 

[Rod Leonard]

Wheel spin

Boy was I glad to read this. My 45 makes a hard turn to port when motoring. I just bought it this spring and put on a Flex O Fold Folding Prop. Thought it might be that. My 31 never did this.

 

[Ron Hughes]

autosteer

My understanding of this common motoring trait is that as the prop turns and sends its thrust aft, there is more wash directed to the starboard side of the rudder blade as a result of the right-hand rotation of the prop and shaft. This makes the water on the port side have to try to keep up as it too passes the blade. As in the case of an airplane wing as the air passes over the top of the wing it has to go farther due to the shape of the wing, creating lift. This compares nicely to the increased thrust on the starboard side of the rudder. The faster moving water on the starboard side creates a relative vacuum and therefore lift on the port aft portion of the rudder. Since the balanced spade design on our boats reduces steering effort, it also reduces resistance to side forces on the rudder surface. The passing wash creates lift on the aft port section of the rudder causing it to want to turn its leading edge to port and thus a starboard turn of increased force in relation to speed of the prop. Two engines with counter rotating props fix this problem nicely. Facinating eh? Best fix for our boats in open water tho' is to let the autopilot do the driving for the long haul as it has no arms to tire as we do! Now if it could just see to stand watch from 0000 to 0400! Ron

 

[Wade Spurrier]

corkscrew effect

Ron, You are correct about the right side (descending) prop producing more thrust. I think another big factor (similiar to a single-engine prop airlplane) is what, in aviation, is refered to as "spiralling slipstream". That "corkscrew" of water is spinning behind the prop in a clockwise motion (as seen from behind) and striking the port side of the rudder. It hits the front of the rudder first naturally, and forces it to starboard (wheel to the left).-wade