Prop to hull clearance

[28islander]

I am currently repowering my 2I8 with a Beta 20. This of course requires a new prop also. The old MD7A had a 12" prop installed. The clearance between the prop tip and the hull is about .5". I read that prop tip to hull clearance should be 10% to 15% of prop diameter. So, my question, what have others who repowered done in this area? Did you just stay with the 12" prop or downsize to an 11" prop? Has anyone noticed any problems with less than optimal tip to hull clearance?
Thanks for any input,
Paul

[jb1528]

Paul,

I can't speak directly to the Islander clearance, but I believe that the percent of diameter is a good. My friend who did his cutlas bearing had to shorten his propshaft for some reason because of something unrelated. He took out a spacer or some darn th ing. Anyway, the result was a close tolerance between the hull and prop. He experienced a vibration he didn't have before and perhaps slightly less efficiency because of it too I believe.

Clearance is good.

Jim

[DelmarRey]

Dave Gerr gives the following values in the propeller hand book.
Minimum clearance of the propell diameter:
RPM 200-500 S/L less than 1.2 = 8%
RPM 300-1800 S/L 1.2-2.5 = 10%
RPM more than 1000 S/L over 2.4 = 15%
Planing crafts S/L over 3 = 20%


More sources:

propeller clearence

[28islander]

Yes, I have read the references on proper tip to hull clearance. Hence the questions I posed. I was hoping to get a response from other 2I8 owners who have repowered. I suspect they have the same issue and was wondering how they dealt with it and/or if they noticed any issues with the 12" prop and the stock shaft/strut setup. I would think all 2I8's have the same setup. The original setup on my boat only had .5" clearance and was like that for 33 years. Maybe it is a non-issue.
Paul

[joelozzy]

Paul,

You've probably already figured this out, but I just got back on the Sailnet forum and thought I'd chime in since I replaced the prop & shaft on Opti during haulout back in Fall of '09.

On this project I replaced the original fixed blade prop with a Martec folding prop, hoping for better sailing performance and reduced prop-walk in reverse. Clearance for the original prop to hull was 1/2". We decided to move the prop towards the leading edge of the rudder to increase the hull clearance, all calculated prior to cutting & threading the new shaft. I have a photograph, but can't figure out how to post. (???) Any help for a 'newbie' to the site would be appreciated.

BTW - close-winded sailing performance in heavy weather was greatly improved with a folding prop. Disturbed waterflow coming from the fixed prop was separating as it streamed along the rudder, resulting in loss of tiller control at high degrees of heel/speed. With the folding prop, flow is more laminar across the rudder, providing a dramatic increase in helm response at the tiller.

Forward motor thrust hasn't been noticeable, due to the already very underpowered MD6B pushing her along. Reverse is slower to engage, thrust is much lower, but once reverse movement starts I have helm control. Which has been very helpful in docking. ~Joel

S/V Optiminium
1976 2I8