Seamanship Question #15 Props
 

5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?

Remove it, turn it around, put it back on.

CN

"Bart Senior" wrote in message . net...
5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?

Close Neal. Because of the taper fit, that won't work.

Care to elaborate on your answer?

"Capt. Neal®" wrote

Remove it, turn it around, put it back on.

CN

"Bart Senior" wrote in message
. net...
5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?

If the shaft has a taper there is no way to put it on
backwards so, perhaps, they shipped a LH prop instead
of a RH prop or vice versa? Diameter and pitch could
be identical but it would be so slow if turning
direction were wrong.

CN


"Bart Senior" wrote in message . net...

Close Neal. Because of the taper fit, that won't work.

Care to elaborate on your answer?

"Capt. Neal®" wrote

Remove it, turn it around, put it back on.

CN

"Bart Senior" wrote in message
. net...
5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?

You replaced it with one that was identical. Try
replacing it with one that isn't old and brittle.
S.

"Bart Senior" wrote in
message
. net...
: 5 points (Impress me with your genius)
:
: You just bought a new boat and notice the prop
is old and brittle.
:
: You replace it with an identical prop. After
motoring around for a
: while you find the top speed of the boat is much
lower, estimated to
: be 2.5-3 knots and the prop seems to be
slipping.
:
: What is the most likely cause, explaination, and
cure?
:
:

Putting a LH prop on a RH setup engine would require running in reverse.
This will work on some engines/trannies, others will be destroyed by
constant running in reverse, and many will have a different gear ratio.

In any case, I think it would be noticeable on any modern setup.


Capt. Neal® wrote:
If the shaft has a taper there is no way to put it on
backwards so, perhaps, they shipped a LH prop instead
of a RH prop or vice versa? Diameter and pitch could
be identical but it would be so slow if turning
direction were wrong.

CN


"Bart Senior" wrote in message . net...

Close Neal. Because of the taper fit, that won't work.

Care to elaborate on your answer?

"Capt. Neal®" wrote

Remove it, turn it around, put it back on.

CN

"Bart Senior" wrote in message
. cv.net...

5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?

The brittleness is an indication of surface hardening and roughening, either
by mechanical or electrochemical means. Since the neutral axis of the chord
of the blade lies in the concave region of the cusp, the entire blade acts
intension and the surface hardening has caused the cusp to increase (shorter
surface length on the inside of the cusp) and the water displaced with each
turn of the propeller is more.

Now to your anticipated answer, which is wrong. The new prop does not slip
because it has less surface friction. The surface friction only drags the
water in the direction of the propeller rotation, it cannot create a water
trajectory normal to the propeller surface. In fact, propellers with less
surface friction are faster.

Amen!

Bob Crantz


"Bart Senior" wrote in message
. net...
5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?

On Sat, 20 Nov 2004 04:29:47 GMT, "Bart Senior"
wrote this crap:

5 points (Impress me with your genius)

You just bought a new boat and notice the prop is old and brittle.

You replace it with an identical prop. After motoring around for a
while you find the top speed of the boat is much lower, estimated to
be 2.5-3 knots and the prop seems to be slipping.

What is the most likely cause, explaination, and cure?


Outboard or inboard, dumbass? I've never heard of an inboard with a
prop that slips. But on my Evinrude 9.9, it didn't have a sheer pin,
and the rubber ring caused it to slip. I had to buy a new prop.


BTW, how can a prop be old and brittle? What's it made of? plastic?




Pathetic Earthlings! No one can save you now!

Bart,

The First thing that came to mind was that the old prop had been
"re-pitched" and wasn't truly what the stamping indicated. In the old
days (Depression) this was a cheaper solution than a replacement.

????
Ole Thom
P/S remove the new prop and go with the Old.

On Sat, 20 Nov 2004 14:45:22 GMT, "Bob Crantz"
wrote this crap:

The brittleness is an indication of surface hardening and roughening, either
by mechanical or electrochemical means.

That's the stupidest thing I ever heard.

Since the neutral axis of the chord
of the blade lies in the concave region of the cusp, the entire blade acts
intension and the surface hardening has caused the cusp to increase (shorter
surface length on the inside of the cusp) and the water displaced with each
turn of the propeller is more.


That's not likely. I think you're making up this bull****.


Now to your anticipated answer, which is wrong. The new prop does not slip
because it has less surface friction. The surface friction only drags the
water in the direction of the propeller rotation, it cannot create a water
trajectory normal to the propeller surface. In fact, propellers with less
surface friction are faster.

This is bull****.





Pathetic Earthlings! No one can save you now!