I just replaced my two blade fixed prop with a three blade max-prop,
and had the rudder rebuilt, which was de-laminating. I took it out
this weekend for the first time, and under full throttle it tracked
consistently to the right, and required about five degrees of
correction by applying constant pressure on the tiller to compensate.
Can anyone offer any clues as to the possible cause, or suggestions
for a remedy?
Thanks,
Jon Lang, Jolie Brise
My BCC, “Devon” ex-Bucephalus, also has a Max-prop. I’ve only owned
the boat for six months and have not powered that much (and, of
course, never without the Max-prop). So I can’t compare it with a
two-bladed prop. But I notice exactly the phenomenon. The boat
tends to steer to starboard naturally, particularly as you increase
the RPM. You therefore have to compensate by pulling the tiller to
starboard. I just thought that it was natural. I don’t think that
any sailboat steers straight. Right? I don’t think that it’s a
problem. That’s my two cents. Good luck!
–Peter
It’s the P Factor:
The same thing happens in airplanes while climbing. The relative wind
of the downward turning blade has a bigger ‘Bite’ and pulls the airplane
to the left. On take-off you have to use a little right rudder to counter this
tendancy. Its called P factor. (prop factor)
Fair winds
Bob
Thanks Peter and Bob. Interesting. I might go back to my fixed two-
blade, or ask Max-Prop to trade my three-blade for a folding two-
blade. The boat tracked true with the two-blade, and I don’t like the
constant pressure I need to apply to the tiller. I think, from your
comments, that the bigger bite from the three-blade is what is
causing the problem…
Again, thanks for steering me, and my boat, in the right direction…
Yes, its the bigger bite from the 3 blade. The shaft that turns your propeller
is at a slight downward angle; so as you move thru the water (water going aft
past the prop) the ‘downward’ traveling blade takes a bigger bite (the angle
between the blade and the horizontal_______ water) than the ‘upward’ moving blade.
Therefore…the larger bite blade tends to move the boat in a slight turn.
You can see this easier if you draw a cross section of two blades. (draw a large letter
“X” ) then rotate the drawing counter clockwise. Notice one blade has a much greater angle (compared to a horizontal line_________) than the other? More thrust on the
downward blade because it has a much greater bite.
There you go…
Bob
Granted that the three-blade Max-Prop with it’s greater thrust may pull to
stbd (and to port in reverse) somewhat more than a two blade folding prop,
but my experience has been that it is hard to beat a properly set-up
Max-Prop for thrust and maneuverability in tight quarters. As for the
2-blade MaxProp I know nothing, but I expect the three-blade would have more
oomph powering into a head sea. I do recall that a friend’s Tarten 10 with
folding Martec provided some exciting docking moments, particularly when
going into reverse.
My experience with Itchen has been similar to that with a Victoria 34 w/
3-blade MaxProp that I used to drive a lot. One can back & fill into a very
tight marina slip with much more control than with any other prop I’m
familiar with. True, on a long passage under power she’ll make big clockwise
circles unless one actively steers – or has a tiller pilot (my preference).
I guess the choice depends on how she’ll be used most of the time. Just my
$0.02!
Scott
S/V Itchen