Max Prop or Not?

[FPNC]

Most of your feathering propellers, (Max Prop, J-Prop, Vari Prop, Auto Stream) will have a flat blade section. Versus the twisted blade section of a fixed propeller. For flat blade section propellers, it is always better to turn the prop shaft as slow as possible, and try to get a pitch that is at least 2/3rds the diameter. Less pitch can adversely effect performance and efficiency. Of course you have to balance pitch selection with maintaining as much diameter as practical.

[Dockhead]

FWIW, the Autoprop requires a lot of maintenance. 3000 hours on mine, and I've done two complete overhauls with replacement of all of the expensive bearings and seals, and I'm probably about due for another one. It needs greasing every time you lift out, and the seals are not too durable.

The Autoprop is a wonderful piece of gear, but you sure pay for it, both up front and as you go along. Professionally done, the overhaul would cost more than $1000.

Originally Posted by Oceansailor

The other one that have been maintenance free is the Gori

[FAST FRED]

If the prop aperture is large enough , you might find a 2 blade prop with the broader blades used on power boats to have the surface area required for your engines HP.


The advantage (besides no maint) is the prop can be lined up with the water flow while underway with sail . With 2 wrenches you can usually find a spot where the shaft will not attempt to rotate, mark the shaft and install a prop brake.

This is way easier to live with and install than any of the folding units

[kenbo]

Fast Fred, in his last 2 posts has given you very solid advice for your situation. Flat bladed feathering props suffer from cavitation when their tip speed exceeds a certain speed. Steve Dashew had a good article on this years ago, unfortunately I can't find the reference. Your high reving, low torque Yanmar really isn't ideal for a flat bladed featherer.

I've had years of ownership with an Autoprop and the reported "bearing failures" is a misnomer. For a number of design and cost reasons, Bruntons went with water lubricated, open ball bearings. Not a bad decision when you consider that the blades aren't spinning around, they're simply rotating at most 360 degrees or less. The problems with this design was the attachment method. When the bearings were adjusted, many felt the bearings should be slightly loose rather than slightly tight. This resulted in slight vibration as the components wore/settled in and the early cap screws or later nuts could loosen over time. Think knucklehead Harley's. Proper bearing adjustment and vibration proofing thread lockers (VibraTite) solved the issue.
King Propulsion in Virginia Beach is a distributor/dealer for Autoprop.
If you want a feathering prop that better deals with your high reving Yanmar call Luke in Maine. Their featherer has true, twisted blades. But I believe a 2 bladed fixed locked behind your deadwood is your best value play.

[kenbo]

As I think more about your boat an interesting option for you could be a 2 blade Campbell. As you know, 2 bladed props see cleaner fluid and the ogival shape of the Campbell prop provide a lot of lift for their reduced surface area. Less drag, especially hidden behind your deadwood.

But nothing beats an Autoprop for motorsailing.

[a64pilot]

Any prop will begin to cavitate if the tip speed is high enough, for an airplane it’s when the tip speed approaches the speed of sound, it’s the transonic flow that separates from the prop making it inefficient and very noisy.

I don’t know what the speed is for boats, but would imagine that it’s a whole lot higher than any normal sailboats prop turns.
I don’t believe cavitation is an issue on sailboats, not really. I know most think it is, but to look at it logically, a 350 HP outboard has a smaller prop than my 40 HP sailboat, and it turns at much, much higher RPM, and yet it doesn’t cavitate, it can and will ventilate which many think is cavitation, but it’s not.

Sailboats props are relatively large and slow turners, they are this way for efficient operation on a displacement hull, and a large slow turning prop just isn’t the type of prop that cavitates.

[ctl411]

My 2 blade 19" maxprop would get cavitation at high loads.
Has to do with not enough blade surface area for the horsepower according to a design book I have. The 3 blade 18" maxprop is better but still not perfect. Bought both used...

[noelex 77]

The Autoprop requires some clearance forward of the hub to allow the blades to rotate. I am not sure an IP38 has enough room for this.

[a64pilot]

Surface area has a lot to do with it. As does blade design. The prop on that 350 Outboard I spoke of is I can assure a very highly refined design. Simple flat plates have their advantages, and disadvantages

[a64pilot]

Originally Posted by noelex 77

The Autoprop requires some clearance forward of the hub to allow the blades to rotate. I am not sure an IP38 has enough room for this.

It’s been on for I guess three years now. I did after a year or so when chasing a vibration problem that I still have, scalloped out the keel some to allow the shaft extension to be about the same as it’s diameter.
I’m nearly certain my vibe issue is due to design of the boat and not a prop issue.

[ctl411]

Originally Posted by a64pilot

Surface area has a lot to do with it. As does blade design. The prop on that 350 Outboard I spoke of is I can assure a very highly refined design. Simple flat plates have their advantages, and disadvantages

Yes, flat blades one more compromise in the boat equation. One of these days I will go up a couple inches in diameter so I can back off in pitch. I think this will help with efficiency and know through trials will help lessen prop walk.

[Pauls]

Cavitation on a sailboat prop is caused by excessive blade loading, meaning too much power per square inch of blade area. For anyone looking to do a good job optimizing their boats prop I highly recommend Dave Gerr's Propeller Handbook. You will find in it a formula for calculating cavitation for your particular boat. Cavitation is to a degree driven by the specific boat as one of the factors involved is the depth of submersion of the prop.