Prop Clearance

[Vic Suben]

Surprise has an M25XP engine with a two-bladed fixed (i.e. not folding) prop.  Although there is no noticeable vibration when the engine is running, there is a noticeable grumble when the engine operates in the 2000  - 2500 rpm range. I suspect that the grumble may be caused by insufficient clearance between the tip of the prop & the bottom of the boat.

Is there a rule of thumb about the clearance required between the boat & the prop that I can check when Surprise is on the hard?

Vic & Judy
Surprise
Hull No. 453

[Tom Soko]

Vic,
I have been told that you should have a clearance of at least 10-15% (20% is better) of the prop diameter, which would be about 2".  Hope this helps.

[Ron Hill]

Vic : Jerry Douglas of Catalina (the designer of the C34) has stated many times NOT to go larger in diameter than 15 inches for a C34 prop.  Why don't you give him a call?    

[Vic Suben]

Thanks for the information.  When Surprise is hauled, I will check out the prop diameter & the clearance from the hull.

Vic & Judy Suben
Surprise