New prop and shaft.

[questionmark]

Our surveyor noted some small corrosion spots on the shaft, and wanted to pull it for a dye penetrant test. Since the boat is a single, and I plan to be outside of tow range (insurance wise anyway) I thought I'd err on the side of caution and replace it during my haul out. Good thing.

Here's what it looks like where we couldn't see it...

That big pit on the photo is well over an eighth of an inch deep.

Of course, I have to get a new prop as well, since the shaft was a crazy custom taper. I've been researching the three vs. four blades idea. The boat previously topped out at about 8 knots, but I felt it was under propped with the existing 4 blade. Most of the Google and search hits are for boats much faster than mine. Any trawler owners have experience in changing props on slower boats?

 

[DavidM]

Here we go, another over propping thread!!!

But seriously, an eighth of an inch is bad, so you are right to replace it. Take accurate phototached rpm measurements at wot and work with a good prop shop. As Ski said recently, prop selection is part science and part art with a bit of luck thrown into the mix.

David

 

[FF]

The best hope of changing/improving a prop is done by using a good tach and a GPS to create a graph of RPM / boat speed.

Every 200 RPM should make a smooth graph.

IF you do change the shaft and prop, be sure to specify a std taper so you can change props in the future, and carry a spare..

 

[O C Diver]

The boat previously topped out at about 8 knots, but I felt it was under propped with the existing 4 blade. Most of the Google and search hits are for boats much faster than mine. Any trawler owners have experience in changing props on slower boats?

Are you referring to other boats of the same make and model with the same engine? Hull shape and engine HP have a great deal to do with speed. As an example: my boat cruises at 7 knots. To reach 9 knots would require more than 4 times the HP at 7 knots.

Ted

 

[jleonard]

The boat previously topped out at about 8 knots, but I felt it was under propped with the existing 4 blade. Most of the Google and search hits are for boats much faster than mine. Any trawler owners have experience in changing props on slower boats?

I always thought mine was "underpropped" but I get full rpm at WOT with a calibrated tachometer, and when I compare to other boats of the same make and model I am about the same speed and fuel consumption as them.
Even though I only top out at about 8.3 knots.