Propeller q

[brak]

I was considering auto/max-prop for my boat to improve reverse handling, reduce prop walk etc.

Today I spoke with someone who is, generally, an excellent specialist and definitely knows engines and boats and what not. His opinion is - max-prop will not help and might make things worse. Instead he suggested that a fixed 4 blade propeller may be preferable for better power and handling.

This sounds interesting but does go against a lot of things I read elsewhere. What does everyone think?

[Petar]

I have an 16" MaxProp on my 31 ft sailboat and it works like magic with excellent handling capabilities and of course produces way less drag when sailing. I'd never ever go to fixed prop even though i am not into racing.
Petar

[brak]

I am partial to the idea of max-prop myself, not so much for drag but for better reverse handling. I also think this guy is *very* conservative when it comes to boat equipment, which is good - but his choices may be driven by "traditional" approach.

What I am curious about is - are there sailboats out there using 4-blade props? I see a lot of single-screw powerboats, in particular with keels and props on a shaft, using those - which is a configuration not unlike a cruising sailboat. I don't think I ever saw one on a sailboat though. Would drag be significantly worse than a 3-blade?

[Valiente]

I have yet to install it, but I have purchased a four-blade feathering Vari-Prop with separately adjustable pitches for both forward and reverse.





Basically, I use reverse to stop (I am never in a hurry to reverse) and so want a different pitch for "torquey" application. In forward, I want to hit a sweet spot for motorsailing, which means a very different pitch to hit a spot on my engine's power curve.

The feathering is just gravy, frankly, although it is a huge bonus if you intend to sail distances in steady winds.

My friend has an Auto-Prop and is able to dock his 40 foot, 36,000 lb. steel ketch like a minivan. It's fascinating to watch. My only advice is to purchase a drive-saver, because the "clunk" of the blades deploying can be tough on the cones in the tranny if you are doing a lot of shifting to get into dock.

[JohnRPollard]

Wow, that is some nice hardware!! Do you sometimes sneak in and pet it when no one's looking? I know I would.

Originally Posted by Valiente

I have yet to install it, but I have purchased a four-blade feathering Vari-Prop with separately adjustable pitches for both forward and reverse.





Basically, I use reverse to stop (I am never in a hurry to reverse) and so want a different pitch for "torquey" application. In forward, I want to hit a sweet spot for motorsailing, which means a very different pitch to hit a spot on my engine's power curve.

The feathering is just gravy, frankly, although it is a huge bonus if you intend to sail distances in steady winds.

My friend has an Auto-Prop and is able to dock his 40 foot, 36,000 lb. steel ketch like a minivan. It's fascinating to watch. My only advice is to purchase a drive-saver, because the "clunk" of the blades deploying can be tough on the cones in the tranny if you are doing a lot of shifting to get into dock.

[brak]

Originally Posted by Valiente

My only advice is to purchase a drive-saver, because the "clunk" of the blades deploying can be tough on the cones in the tranny if you are doing a lot of shifting to get into dock.

That is one cool device Do they have a web site to see what they sell, prices etc?

Also, what is a "drive saver"?

Something to think about - again. I am not sure what my engine's power curve is or where to find the information. Is the match of engine to prop size/pitch something that an engine mechanic would know about? Or who would I talk to about that?

[christyleigh]

Originally Posted by brak

Also, what is a "drive saver"?

A "plastic doughnut" that goes between your tranny and shaft with opposing bolt patterns that are 'supposed to' take up some shock in daily use and self-destruct if you hit an immovable object with your prop at high speed. There is metal inside the plastic to keep your prop shaft and tranny tail shaft connected in that extreme case. I installed one when I installed an AutoProp on my Catalina 320 more for the 1 inch it moved the shaft out away from my strut to allow for the room that the AutoProp needs to litteraly reverse its blades when you shift. It still clunked but maybe the actual impact on the tranny was lessoned.
Nauticats come standard with a similar shock absorbing device so my current tranny is dampened from the AutoProps shock load.

[Valiente]

What Christyleigh said is correct.

Yes, I sometimes go in to the back room and drool on it. I have considered leaving on the coffee table as "modern art", too.

Back to business: You should see the power curve in your documentation and there are several resources that will tell you the proper prop size and pitch. Part of this is related to the engine shaft output, and part to the geometry of the boat, the maximum shaft size, the aperture or space in which the blades arc (rule of thumb is 10% clearance minimum, so a 12 inch prop would need 1.2" of gap between prop edge and boat hull) and so on.

Dave Gerr has at least one book out on the whole subject and the manufacturers will also give suggestions because it is their interest to sell you the appropriate prop for your usage. There are also web sites devoted to arcane formulas on the topic.

The "gold standard" installation for me, with a full keel steel boat intended for passage-making, is a universal coupler for the shaft-engine connection and "soft mounts" for the engine itself.



While this involves the welding in of a thrust bearing, it eliminates a lot of noise, vibration and all alignment issues. It's not cheap, but it replicates what a lot of commercial boats have and makes sense for me.

For a coastal cruiser, a Drive saver-style coupling "cushion" is sufficient.

http://www.defender.com/category.jsp?path=-1|311|53222&id=314197

[brak]

very interesting.

It seems like as long as I have enough clearance behind the propeller and can move the shaft by cushion thickness, it is essentially installed in place?

Edit: you did have the link right there, I just spaced it

[christyleigh]

Originally Posted by brak

It seems like as long as I have enough clearance behind the propeller and can move the shaft by cushion thickness, it is essentially installed in place?

Yes, as long as you have your feeler gauge handy to re-align your shaft.