Project: New Prop
(click on any of the small pictures for a large one!)

Why

I knew that something was up when I first put the boat in the water and ran the engine up to 2200rpm and she was still not even doing 7 knots. Even then, the pilothouse was extremely LOUD and would be less than desirable for a long cruise.

I checked all of the usual stuff, bottom fouling, excess weight, engine loading, etc. It all pointed to the prop.

The engine would wind all the way up to 2800 (max. spec rpm) and keep going beyond to 3000 where the automatic governor would not let it continue. It was obvious that the engine was still not loaded up even at that rpm.

Where

I keep the boat on a boat lift (don't ask), so it made swapping propellers a lot easier. I just had to make sure they dropped into the dinghy and not the drink when they came off!

I only later learned to leave the nuts on the very end of the shaft........


The Job

The big job here was getting old prop off. It was a 21 x 16 three-blade. It even looked small on the boat.

It had been on there for quite some time. I went out and bought an automotive wheel puller (being the cheapskate that I am) and proceeded to bang on it, tighten it, and bang on it some more.

I noticed that the fingers were doing a number on the back of the prop hub, so I Penetrol-ed the entire thing so it could soak and went in. While it was soaking, I made this:

It is a piece of starboard-like plastic cut to fit around the prop shaft. The fingers of the wheel puller can dig into this all they want and not hurt the bronze hub.

More tightening and banging finally loosed the old prop and it slid off.

I contacted Tom at Big Rock propellers in Morehead City and he patiently went over all of the characteristics of my boat with me including the "time trials" I had made with the old prop. The final word was a 4-blade 24 x 20 as a starting point (it is only ever a guess --- re-pitching is done to fine tune later).

The new prop was quite a big difference as you can see in the picture.

The new prop went on without a hitch and I was off to run my "time trial" again.

I am keeping the old prop as a "spare" in case of a major catastophe.

Results

Well being the geeky engineer that I am, I made a spreadsheet of the two props and their performance with my boat. Click on the unreadable thing to the left for a marginally readable larger version.

With the new prop, I could easily make 7 knots at a very quiet 1800 rpm. This was much more tolerable noise-wise in the pilothouse.

However, the new prop causes the engine to top out at 2500rpm. This is 89% of the max rated rpm (2800). According to Gerr (the big-cheese prop guy), if one gets to within 95% of the max rated rpm then the prop is "close enough".

I am not at 95%, but this 89% is "close enough" for me as
a) I will probably never need 2800rpm, let alone even 2600 (I make nearly 9 knots at 2500) and
b) if I reduced the pitch and inch or so that the engine could acheive 2800, I would have to run at a higher cruise rpm to make my goal of 7knots. This would mean more noise.

I think I'll leave it for now.
 
web design by:
All material, text, and graphics copyright 2001-2004 RCHDesigns.
No part of this web page may be copied without the written permission of the author.
email here
at home in: