Prop Resizing Help

[Beeliner]

I’m hoping some prop experts can help me out here to see if I’m on the right track. After my first year of cruising with my Crosby 26 tug, I think I need to resize my prop. The boat has the following specs:

10,000 pound displacement hull
25’ waterline length
10’ beam
Yanmar 4jh3e 55 hp, 3800 max rpm
Velvet drive trans, 2.57:1 reduction
Hull speed: 6.7 knots (this my preferred cruise speed which I reach at 2500-2700 with current prop)

The current prop is 20 X 13 and appears to be over-propped. I only reach 3200 WOT in gear and start getting vibration and cavitation (sounds like churning water) above 2000 rpm. (engine reaches 3800 rpm + in neutral). Considering the boat was a harbor tour boat in its previous life, I'm guessing they never ran above 2000 rpm (about 5 knots).

I believe the 2 factors affecting this vibration are 1) the shaft overhang is 2.5” which is 1.85x the shaft diameter, above the 1.5x recommended maximum and 2) the clearance from tip of prop blade to hull is 2.5”, less than the recommended 15% of prop diameter.

Based on this, I am thinking to 1) shorten the shaft length by an inch or so, bringing it within the 1-1.5x overhang and 2) modify the prop to an 18.5 X 13, which should give me a little more rpm and increase the the prop to hull clearance to 15%. The down side may be a little higher rpm to achieve hull speed, but hopefully with a smoother ride.

(Note that engine mounts are new and shaft alignment is good so I am ruling out these factors for now.)

I’d greatly appreciate any comments or alternative suggestions before I proceed.

( I left several messages with Crosby Yacht Yard with no call back yet so not sure they can help)

 

[PierreR]

Sounds like you are geeky enough to handle calculating the correct prop. Get Dave Gerr's Propeller Handbook and go through the calculations. You are going to get a lot of rabbit hole suggestions for a fix here.
Your 18.5x13 does not sound way out of line on my back of the envelope calculations based on what you have given me. The vibration is most likely tip clearance unless the prop is two blade instead of three

 

[Beeliner]

Sounds like you are geeky enough to handle calculating the correct prop. Get Dave Gerr's Propeller Handbook and go through the calculations. You are going to get a lot of rabbit hole suggestions for a fix here.
Your 18.5x13 does not sound way out of line on my back of the envelope calculations based on what you have given me. The vibration is most likely tip clearance unless the prop is two blade instead of three

It's a 3 blade prop. Will definitely check out the handbook. Thanks.

 

[SteveK]

Did you use VICPROP or similar calculator?
Shortening the shaft, that is interesting to consider. Beyond me to consider.

 

[ranger58sb]

Sounds like you are geeky enough to handle calculating the correct prop. Get Dave Gerr's Propeller Handbook and go through the calculations.

Or just talk with your local prop shop. Could just be a matter of cleaning up, slightly re-pitching, maybe adding or subtracting cup... and they may be able to tell you all that with info from a scan.

With our guys, it's been a "simple" as telling them what WOT RPM we've got, what WOT RPM we want... getting the props to then... and then magic happens.

-Chris

 

[C lectric]

Unless you have already done the checks then I agree that those should be done before changing anything. THe prop and /or shaft may be fine or maybe they are not.
Find out before considering all kinds of alterations.

Then take a good look at the shaft for good support and straightness, no tweaks.

 

[Comodave]

I think you are on the right track. You don’t want too much overhang on the prop shaft and the tip clearance is likely what is giving you the vibration.

 

[Fish53]

I had a similar issue where I couldn’t reach peak RPM, about 90% but also experienced a significant vibration at 75% of peak. My 1.25 inch shaft was overhung from the stern bearing by 2 inches. I pulled the shaft and had it cut back 1 inch and the vibration ceased. My RPM still only went to about 95% but I deemed that acceptable for my use. Just my experience, other results may vary.

 

[SteveK]

I had a similar issue where I couldn’t reach peak RPM, about 90% but also experienced a significant vibration at 75% of peak. My 1.25 inch shaft was overhung from the stern bearing by 2 inches. I pulled the shaft and had it cut back 1 inch and the vibration ceased. My RPM still only went to about 95% but I deemed that acceptable for my use. Just my experience, other results may vary.

The vibration is gone, great. But what caused it, and did you have the prop checked?

 

[Comodave]

The overhang on the prop shaft caused the vibrations. He said he cut the shaft down and the vibrations went away.

 

[SteveK]

Dave, I can read. However vibration is caused by other than shaft length. That allowed the wobble to display as vibration. An out of balance prop, slightly off blade May still be in play, just now wearing on bearing. JMO

 

[Comodave]

There can be multiple things that can cause vibrations. Does not mean that the shaft length being cut to the proper length can’t fix the vibrations. It could be the prop but the OP said the shaft length was the problem.

 

[Fish53]

The vibration is gone, great. But what caused it, and did you have the prop checked?

I always balance and check for true on propellers and lap them to the shaft before fastening them and the stern bearing was new. I knew it was overhung but I was anxious to launch the the boat so I took a shot. Alignment was good so I suspect my engine mounts are a tad too soft, but of course it’s fine now.