[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.

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]

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]

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]

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.

 

[brak]

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

I thought this thing was supposed to remove the need for the precise realignment? I.e. it would take up small mismatch in angle (and presumably gross misalignment should not happen this way?). Or does it not?


In all honesty, last time marina disconnected the shaft coupler (to install new dripless seal) they didn't do any preciese alignment other than, once connected, rotate the shaft and look at it I am not sure this is the best procedure, but it is what it is.

 

[christyleigh]

I thought this thing was supposed to remove the need for the precise realignment? I.e. it would take up small mismatch in angle (and presumably gross misalignment should not happen this way?). Or does it not?
In all honesty, last time marina disconnected the shaft coupler (to install new dripless seal) they didn't do any preciese alignment other than, once connected, rotate the shaft and look at it I am not sure this is the best procedure, but it is what it is.

Being plastic it could be said that "precise alignment" is no longer possible any way ...... but don't forget that's part of their advertising. If I didn't know how many thousandths clearance the tranny shaft plate and propshaft plate were supposed to have I would - get out my feeler gauges and feel all around for a common number and also to see if there was "gross mis-alignment to start with (bottom of shaft at x and top of shaft at 10x). The actual number is not as important as having the same number all around the plate as that is the key to the motor/tranny to shaft alignment.
It seems like the yard that did your work made some fairly large assumptions about how perfectly your motor mounts were set to start with, but I suppose if you didn't come in with any complaints about vibration to start with simply re-tightening up the bolts All To The Same Torque would do.
When adding a new piece (drive saver) to the puzzle though I would not do it without a feeler gauge

 

[Valiente]

I thought this thing was supposed to remove the need for the precise realignment? I.e. it would take up small mismatch in angle (and presumably gross misalignment should not happen this way?). Or does it not?


In all honesty, last time marina disconnected the shaft coupler (to install new dripless seal) they didn't do any preciese alignment other than, once connected, rotate the shaft and look at it I am not sure this is the best procedure, but it is what it is.

Here's the idea: You get the universal joint bolted properly (and aligned!) to the shaft coupler and then to the engine coupler. It is the joint that moves slightly to accommodate misalignment in use. So, you get it aligned the first time, and then you don't have to worry so much subsequently because the joint allows for the engine to "lean" on its soft mounts even as it is turning, meaning a lot of the rumble associated with the engine bolted to its stringers via the mounts is lessened, and your shifting action is "dampened" in the rubber parts of the universal joint.

It is merely a refinement of the joint seen on nearly every truck, where the engine shaft to the rear wheels articulates to compensate for bad roads, uneven or frame distortion loads, and so on.

 

[Valiente]

When adding a new piece (drive saver) to the puzzle though I would not do it without a feeler gauge

Same here. You don't know if the drive saver surfaces are perfectly plumb or if the shaft coupler is perfectly flush to the shaft.

Of course, I drilled a hole through the grub screw holding the coupler to the dimple on the shaft and then moused the whole thing together, so maybe I'm a little paranoid.