To Modify Skeg — Or Not ... That is the Question ... installing Autoprop

[a64pilot]

Originally Posted by Uncle Bob

Curious, do you put the transmission in gear after stopping the engine?

No, I used to put it in reverse, then one day forgot, then no longer bother.
The blades on an Autoprop are not geared to each other nor do they have any kind of pitch stop etc.
You can take a blade and spin it around in circles forward or reverse until you get bored.

It took awhile for me to understand the operating principle.
It’s really very simple, centripetal force forces the blades flat, but they do have an aerodynamic shape, so zero pitch still will provide lift, in this case, thrust.
When you start out with zero boat speed and Autoprop seems to have very little “Bite” engine revs increase without a whole lot of thrust, it takes a little getting used to, having to goose the engine more than normal for very low speed maneuvering.
However as the boat begins to move, the induced water flow drives the blades to higher pitch, which in a perfect world would mean that engine loads stays the same regardless of boat speed, but at least on mine or sort of means the faster she goes, the faster she goes if that makes sense.

You really see it motorsailing, I’m light wind we may be making 3 or 4 kts. Crank the engine and only push it to 1000 RPM and we are at 7.5 kts, roughly double what we were making and at the lowest speed the engine seems to run smoothly and a good speed for battery charging.

Normal fixed prop we would only get a kt or two.

Reason of course is a fixed or regular feathering prop is a fixed gear if you will, where an Autoprop will increase pitch to maintain the engine load, even when going downhill so to speak which is sort of what motorsailing is compared to an automobile for instance.

[Uncle Bob]

Originally Posted by a64pilot

No, I used to put it in reverse, then one day forgot, then no longer bother.
The blades on an Autoprop are not geared to each other nor do they have any kind of pitch stop etc.
You can take a blade and spin it around in circles forward or reverse until you get bored.

It took awhile for me to understand the operating principle.
It’s really very simple, centripetal force forces the blades flat, but they do have an aerodynamic shape, so zero pitch still will provide lift, in this case, thrust.
When you start out with zero boat speed and Autoprop seems to have very little “Bite” engine revs increase without a whole lot of thrust, it takes a little getting used to, having to goose the engine more than normal for very low speed maneuvering.
However as the boat begins to move, the induced water flow drives the blades to higher pitch, which in a perfect world would mean that engine loads stays the same regardless of boat speed, but at least on mine or sort of means the faster she goes, the faster she goes if that makes sense.

You really see it motorsailing, I’m light wind we may be making 3 or 4 kts. Crank the engine and only push it to 1000 RPM and we are at 7.5 kts, roughly double what we were making and at the lowest speed the engine seems to run smoothly and a good speed for battery charging.

Normal fixed prop we would only get a kt or two.

Reason of course is a fixed or regular feathering prop is a fixed gear if you will, where an Autoprop will increase pitch to maintain the engine load, even when going downhill so to speak which is sort of what motorsailing is compared to an automobile for instance.

Thanks, the reason I asked is that a few years ago I seriously considered this prop but on closer examination and asking the question of the seller here in Aus it was indicated that a brake was a requirement in most installs, of course available from Brunton's which pushed the price beyond me.
Reports on it's performance have been extremely good.

[a64pilot]

It’s a complex thing which requires maintenance, plus it has a special anode that is quite small and doesn’t last very long. It is also a rather expensive thing, prop that is.
Islers diving used to make an adapter so that you could fit an ordinary prop B anode. I have that adapter but don’t know if it’s still made.
The prop is also heavy, and mine vibrates above 2200 RPM.
As it will only vibrate above 6.5 kts or so, I strongly suspect the source of the vibration is the rather large amount of deadwood that I have, and as the prop is pitched from a balance of centripetal force and induced flow, the loss of induced flow right behind my rather thick keel causes pitch changes, pitch changes cause drag changes which result in vibration.
At least that is my theory.
You can tie her to the dock and run it at any RPM it’s as smooth as glass, but underway it’s not.

So if possible a try before you buy may be in order.
I keep telling myself I’m going to get a Campbell Sailor fixed prop, but instead keep my engine at or below 2200 RPM

[a64pilot]

This may help visualize how it works.
https://youtu.be/_pkrxZwI2_Y
A key is that a regular prop when paced in reverse the leading edge and trailing edge swap, meaning the trailing edge is now the leading edge.
Of course this greatly reduces thrust.
An Autoprop the leading edge is always the leading edge, so if your transmission ratio was the same, you would have identical levels of thrust in ahead and astern.

I can’t find it in writing, but I have been told that the design is quite old, that it came from landing craft in WWII as a way to get one off of the beach, of course a landing craft needs a strong reverse like nobody else.

[randygva]

Really Appreciate all the comments and suggestions. Am weighing the next step, and will post whichever decision we make. It will come soon as I plan to splash by the end of March. Thanks.

[a64pilot]

I’d cut off the skeg.
My thinking is if you don’t what is remaining is so weak so that instead of protecting the prop, it may pretty easily be bent right into the prop.

You already have more overhang than ideal, the Autoprop is heavy and I believe more likely to cause problems with too much shaft sticking out, the max suggested is I think 1.5 times the thickness of the shaft?
So if you shorten the shaft length, you’ll have to cut even deeper into your strut, plus as the engine mounts determine prop positioning, under way the prop moves forward just a little too as the mounts flex.

I had to grind two scallops into my keel for prop clearance, so it’s not uncommon to have to modify for an Autoprop.

[Suijin]

Another option is a Maxprop which does not have this issue. Mechanically more complicated but with routine maintenance will last the life of your boat.

[a64pilot]

Maxprops or any other prop with a flat blade with no aerodynamic shape or blade twist are relatively inefficient props, plus they can’t adjust pitch under varying conditions.
The best advantage of an Autoprop is on my opinion motorsailing.
You can literally add several kts when sailing by just bringing the motor up to 1000 RPM, on my boat I will double my speed.
Reason is the Autoprop will increase pitch rather dramatically until it loads the engine, a fixed prop can’t do that.

Any prop that the blades are flat plates are pretty inefficient, people want to argue that of course, but there is a reason why all regular props both have an airfoil shape and blade twist

[randygva]

After much consideration, we've decided to make the mods and go with the autoprop installation. Here is a picture of the modified skeg. The material is very study -- even at the bottom -- so I decided to leave it in place to provide an initial amount of protection.

[randygva]

Originally Posted by randygva

After much consideration, we've decided to make the mods and go with the autoprop installation. Here is a picture of the modified skeg. The material is very study -- even at the bottom -- so I decided to leave it in place to provide an initial amount of protection.

And a pic with the installed Autoprop. Launches 18 April so will report the results.

[a64pilot]

You will like it.
But it looks like you primed the zincs on the shaft? If so you want to remove that, but leave the zincs on the shaft, the autoprop anode is a joke, and not even a good one at that.
Expensive and too small, so keep your other zincs.

[Tricolor]

Me think it might be a good idea to cut away that thin piece left of the skeg. If you hit something with it it might bent into the propeller damaging the prop.

[randygva]

Originally Posted by Tricolor

Me think it might be a good idea to cut away that thin piece left of the skeg. If you hit something with it it might bent into the propeller damaging the prop.

Taking a calculated risk on the skeg. I whacked on it ( not super hard with a hard rubber mallet) and it didn’t deflect at all in the newly cut area.

I’m going for the smaller branches and floating wood over hitting something huge.