I own an E-41, Island Bird, and recently pulled the shaft and had it inspected. Turns out I need to replace it. The two shops that inspected it both agree its beyond straightening. I decided to have a new one machined and sent the specs to Marine Hardware out of Redmond WA. It seems that the machinist has a different idea of the length of shaft exposed beyond the aft end of the cutlass bearing to the forward edge of the prop than the specs i gave him. According to him the distance should be the width of the shaft making it only one inch. I have no reason to doubt him. The amount of exposed shaft I have with the shaft I am replacing is close to six inches. The total length of this shaft is 53 inches with a standard tapered prop end and a straight coupler end. The shaft I'm replacing had half of the threads cut off and was using a zinc nut as the prop nut. I sent him the prop from the shaft I'm replacing and when he fit it to the standard tapered end it seated further up than normal leading him to believe the prop may have been over bored. My questions, one; what amount of shaft exposed between the cutlass bearing and the prop is standard? Does engine size and gear ratio determine that? and question two; it looks like im going to replace my prop, two blades or three?
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Maine Sail
Member III
According to him the distance should be the width of the shaft making it only one inch.
He is SPOT on..
I have no reason to doubt him. The amount of exposed shaft I have with the shaft I am replacing is close to six inches.
This is a probable cause for your bent shaft. Anything more than 1X shaft dia can lead to "shaft whip". 6" is a TON of overhang and WILL cause shaft whip. What your shaft guy is quoting you is standard industry practice and right from ABYC P-06 and SAE J756. Some shafts are so sensitive to shaft whip that the thickness of a Drivesaver, roughly 1 inch, can often be enough to induce whip. I have solved a few vibration issues by simply removing a drive saver and getting rid of the shaft whip it induced by sliding the shaft back just 1 more inch. The longer the unsupported distance between the cutlass and coupling the more likely you will be to induce shaft whip. Anything over about 36" is where I often see trouble with excess overhang. I consider "excess" 1.5 - 2X shaft diameter you are at 6"..
My questions, one; what amount of shaft exposed between the cutlass bearing and the prop is standard?
1X shaft diameter.
From ABYC P-06
"6.5.5.4 The distance between the forward end of the
propeller hub and the aft end of the last strut bearing shall
be limited to one shaft diameter. "
Does engine size and gear ratio determine that?
No, engine size and gear ratio have nothing to do with that. It is determined by shaft diameter. 1" shaft gets 1" between forward end of prop hub and strut or cutlass bearing. 7/8" shaft gets 7/8"..
and question two; it looks like im going to replace my prop, two blades or three?
That is entirely dependent on how you use your own boat. I use a three but we frequently battle tidal rivers and a two blade has proven to be a little light for that ind of work. If you don't battle wind and sea or tidal currents then a two may be fine.
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on shafts and props
thanks Maine Sail for the input, much appreciated. I'll resend info on shortening the shaft to specs. As far as props go, I'm in the Puget Sound and deal with alot of tidal current. I'm going with a three bladed prop as most boats in the yard seem to have three blades. The current in the channel I live near rips up to six + knots at times. I figure having this extra thrust will help in maneuvering around while docking.
thanks Maine Sail for the input, much appreciated. I'll resend info on shortening the shaft to specs. As far as props go, I'm in the Puget Sound and deal with alot of tidal current. I'm going with a three bladed prop as most boats in the yard seem to have three blades. The current in the channel I live near rips up to six + knots at times. I figure having this extra thrust will help in maneuvering around while docking.
Maine Sail
Member III
thanks Maine Sail for the input, much appreciated. I'll resend info on shortening the shaft to specs. As far as props go, I'm in the Puget Sound and deal with alot of tidal current. I'm going with a three bladed prop as most boats in the yard seem to have three blades. The current in the channel I live near rips up to six + knots at times. I figure having this extra thrust will help in maneuvering around while docking.
You may want to consider the Campbell Sailor prop. It has very low drag for a fixed three blade and is also very smooth and efficient. Call Norm at North By West..
not always a perfect world
I don't think this applies to your situation, but just for general knowledge in propeller discussion, I thought this worth mentioning. It is important to maintain adequate clearance between the tips of the propeller and the hull or a variety of nasty things resulting from cavitation can occur, like I think you can even burn the tips off the prop. The rule of thumb I'm familiar with is the minimum on a sailboat prop is about 10-12% of the total diameter of the prop. As you change shaft length, the position of the prop to hull should be noted to maintain this. This became and issue for me on Emerald as there is a definite aperture with the modified full keel that the prop has the most clearance, but this doesn't necessarily result in the ideal distance from the strut as Mainsail has properly indicated. Again, I don't think it is an issue in your situation, but file it away as part of your prop knowledge.
I don't think this applies to your situation, but just for general knowledge in propeller discussion, I thought this worth mentioning. It is important to maintain adequate clearance between the tips of the propeller and the hull or a variety of nasty things resulting from cavitation can occur, like I think you can even burn the tips off the prop. The rule of thumb I'm familiar with is the minimum on a sailboat prop is about 10-12% of the total diameter of the prop. As you change shaft length, the position of the prop to hull should be noted to maintain this. This became and issue for me on Emerald as there is a definite aperture with the modified full keel that the prop has the most clearance, but this doesn't necessarily result in the ideal distance from the strut as Mainsail has properly indicated. Again, I don't think it is an issue in your situation, but file it away as part of your prop knowledge.