I'll take some of those points - It was me, not Neal, that pointed out
that the gearing could well be different in reverse. I also pointed out
that it would be easily noticeable.

Curiously, his first answer, that the prop was mounted backwards, may
have been a better answer. That doesn't change the handedness, but if
the blade is not symmetrical it will affect the efficiency.



N1EE wrote:
5 points to Neal.

Inboard diesel powered boats commonly have different
gearing in reverse.

Incorrectly installing a RH or LH pitched prop when
the opposite was called for could cause a boat to
run at slower than expected speeds, albeit it would
move forward in the reverse position and vice versa.

Neal immediately grasped that the gear ratio might be
wrong and this could be explained by the boat running
in reverse, and at a lower gear ratio.



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


"Capt. Neal®" wrote

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

CN

"Bart Senior" wrote in


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?

Subject: Seamanship Question #15 Props
From: Jeff Morris
Date: 11/26/2004 10:29 AM Pacific Standard Time
Message-id:

I'll take some of those points - It was me, not Neal, that pointed out
that the gearing could well be different in reverse. I also pointed out
that it would be easily noticeable.

Curiously, his first answer, that the prop was mounted backwards, may
have been a better answer. That doesn't change the handedness, but if
the blade is not symmetrical it will affect the efficiency.


Wish I had a prop handy.
My own feeling is that different gearing is a rarity. However, props are
designed to be most efficient when ahead, which means less efficient when
astern, so basically I agree.

Shen

Shen44 wrote:
Subject: Seamanship Question #15 Props
From: Jeff Morris
Date: 11/26/2004 10:29 AM Pacific Standard Time
Message-id:

I'll take some of those points - It was me, not Neal, that pointed out
that the gearing could well be different in reverse. I also pointed out
that it would be easily noticeable.

Curiously, his first answer, that the prop was mounted backwards, may
have been a better answer. That doesn't change the handedness, but if
the blade is not symmetrical it will affect the efficiency.



Wish I had a prop handy.
My own feeling is that different gearing is a rarity. However, props are
designed to be most efficient when ahead, which means less efficient when
astern, so basically I agree.

I can't say about ALL small diesels, but Yanmar is one of the most
common. Here's the spec sheet for the new 30HP, which, with its
siblings, may become the most common engine of its size fitted in the US.
http://www.yanmarmarine.com/products...0_TechData.pdf

While the saildrive gearing is symmetrical, the normal transmission has
three gearing options, none of which are symmetrical.

I remember an article (by Pascoe, I think) about a large power boat that
used reverse gear to have a counter-rotating prop. The shift linkage
was reversed, of course. The owner couldn't figure out why the
transmission needed service every 50 hours.

While the saildrive gearing is symmetrical, the normal transmission has
three gearing options, none of which are symmetrical.

I remember an article (by Pascoe, I think) about a large power boat that
used reverse gear to have a counter-rotating prop. The shift linkage
was reversed, of course. The owner couldn't figure out why the
transmission needed service every 50 hours.

Interestin. G we learn something new everyday.
I do note the reduction difference is not all that great between ahead and
astern and would have to wonder how noticeable the difference would be, versus
putting the prop on backwards. I also have a problem seeing how someone could
put the prop on backwards, other than in a twin screw application.

Shen

True. 2 points to you Jeff.

Would you know if tapering the shaft is the
standard? I haven't seen any otherwise, but my
experiece is limited to puling props on only a few
boats.

I was wondering if some shafts come straight with
just a key way to lock them in position?

Bart Senior

Jeff Morris wrote

I'll take some of those points - It was me, not Neal, that pointed out
that the gearing could well be different in reverse. I also pointed out
that it would be easily noticeable.

Curiously, his first answer, that the prop was mounted backwards, may
have been a better answer. That doesn't change the handedness, but if
the blade is not symmetrical it will affect the efficiency.

(N1EE) wrote in message . com...
True. 2 points to you Jeff.

Would you know if tapering the shaft is the
standard? I haven't seen any otherwise, but my
experiece is limited to puling props on only a few
boats.

I was wondering if some shafts come straight with
just a key way to lock them in position?


Ive pulled and replaced hundreds of props and have never seen a
un-tapered shaft. Why you ask...Because props are made to turn one
way.
Joe




Bart Senior

Jeff Morris wrote

I'll take some of those points - It was me, not Neal, that pointed out
that the gearing could well be different in reverse. I also pointed out
that it would be easily noticeable.

Curiously, his first answer, that the prop was mounted backwards, may
have been a better answer. That doesn't change the handedness, but if
the blade is not symmetrical it will affect the efficiency.