E27 prop and shaft

[SchoolBoyHeart]

I just had my new boat surveyed and one thing that came back was that the prop shaft is too long and that the prop may be too big. I think the surveyor decided the prop was too big because there was some diesel coming out the exhaust. Does anyone have any idea what the normal size prop/shaft length is on an E27?

i have a ysm 8 in the boat and the prop is a 2 blade 12 x 12. I would say the distance from the strut to prop is about 10" or so.

This is my first inboard so everything is new to me

Rob

[toddster]

Ten inches of overhang does seem very long, although off-hand a 12" prop does not sound overly large. If your engine was upgraded from an Atomic 4, maybe someone just put the drive train back together with parts on-hand.

I had to replace my shaft and prop last year, which stuck out 4" behind the strut. Everyone over at the Moyer atomic 4 forum was horrified. 1" was the agreed normal distance. But since then, there have been several other threads here showing about 4" behind the strut on Ericson boats.

I would suggest that the real test of whether the shaft is a problem should be whether you have abnormal wear on the cutlass bearing and/or the shaft. (Mine did.) And for prop size, it should be whether your engine is able to achieve the normal RPM range at cruising speed. (I don't know enough about the YSM8 to know about diesel in the exhaust thing.) Kind of the "if it ain't broke, don't fix it" mentality. Worry about it when the time comes to replace these components anyway.

[SchoolBoyHeart]

You may be right and I may be eager to dive into things on the new boat! Here's a pic. The cutlass bearing are great. So maybe I should just let her be. What about diesel in the exhaust water? Could that be something else?

[tenders]

I'm not a diesel expert by any means but I'd want to rule out a lot of simple things more likely to put diesel in the water before I decided to swap the prop for that reason!

[SchoolBoyHeart]

What sort of easy things might those be?

[dt222]

Hi Rob,

Here is a picture of mine. I don't know the pitch, but it is a 12" prop. There is an 1 1/8" clearance from the aft bearing to the prop, and 1 1/4 inch clearance from the tip of the prop to the bottom of the hull.

Don

[SchoolBoyHeart]

That looks more normal to me. Well I guess I'll have to add it to the list!

[Emerald]

Couple quick thoughts. The goal on the propeller clearance to the hull is usually about 15% of the diameter of the prop, with 10% considered the minimum clearance. Tighter clearances cause issues with the water flow around the blade tips (I believe cavitation and ventilation issues). The amount of overhang past the cutless bearing is usually targeted to be the same as the shaft diameter, but this is often not possible due to maintaining the proper prop tip clearance to the hull - getting the prop in the correct aperture in the hull, hence the longer than ideal shaft lengths you often see. A diesel engine that is overproped will labor and run rich, which means dumping fuel into the exhaust which will show in the water, so, until you resolve your prop pitch/diameter etc. being correct, you'll possibly have the other issues. And another quick sign of your diesel running rich will also be black smoke in the exhaust. Any decent prop shop will have formulas to calculate what you should have, and you can usually repitch your existing prop. However, the best thing is to find someone who has already dialed it in e.g. your same engine output, blade type and boat model. In the case of the Independence 31, the "right" prop for the boat with the popular upgrade of a Yanmar 3GM series engine is actually different from what the formula says....

[Loren Beach]

Building on David's excellent reply, I would suggest that there is indeed considerable experience and intuition involved in correct prop selection.

When we were hunting down a drivetrain vibration and reached our almost-new three blade prop (detailed elsewhere on this site) we sent our prop out to this guy,
http://www.props.com.au/propellerworks/default.htm
and found that the pitch was out by about a half inch on one blade (!). Having the pitch equalized for all blades is important for balance.

Also important (to me) would be to never buy another Michegan Prop without having it checked and corrected at their expence.

So, do pay attention to the formulas, but not slavishly so. It's mostly "science" but not completely......

Oh yeah, and initially, cut down or replace that too-long shaft.


LB

[SchoolBoyHeart]

Thanks for all the help. I'm dropping her off with Nigel on Monday and he's going to get her dialed in with the prop shop. Hopefully that cures all!