Join Date: Sep 2007
Location: Pasadena Md - Magothy side
Thanked 31 Times in 31 Posts
Rep Power: 12
Re: Overpropped; What to do?
Your 4JHE should have come with a 17x12 two blade.
That's based on a Hull speed should be 8 knts, not 7 (LWL = 35).
3240 rpm (90% of max) / 2.17 reduction x 12 (pitch, inches) with comes out to 1493 feet per minute.
Once you figure the slippage (per David Gerr's chart) at 48% you get 776 feet per minute - just under 8kts (810 feet per minute).
It's important to note that Gerr's slip chart is an ESTIMATE - so you get some variation in numbers.
Okay, that's what you should have had, now you have a 3 blade of unknown pitch
You can get a pretty good estimate on your pitch by reverse engineering the speed/rpm:
Since you are saying you are going at 1800 rpm, that means you have a shaft rotation of 829 RPM and are moving 708 feet per minute (7 kts).
That comes out to a pitch of almost 20 (829 * 20 (pitch) *.52 (slippage) / 12 (convert feet to inch) = 718 feet per minute
That's a bit much in pitch and explains why you can't get 90% of max RPM.
I'm running the same engine on my Irwin 38 - with a 16 inch 3 blade MaxProp, pitch set to 12.
Lessons learned are opportunities earned.