Disadvantages to an auto pitching propellor

[Amp1ng]

Are there any disadvantages??

I'm looking to increase motoring performance at engine speeds less than full chat. I've read many a thread advertising the virtues of an auto pitching prop (increased boat speed throughout rev range, increased sailing speed, good stopping power, less prop walk). There must be some shortcomings to one of these units.... mustn't there?

 

[ChrisE]

Are there any disadvantages??

I'm looking to increase motoring performance at engine speeds less than full chat. I've read many a thread advertising the virtues of an auto pitching prop (increased boat speed throughout rev range, increased sailing speed, good stopping power, less prop walk). There must be some shortcomings to one of these units.... mustn't there?

Yes, they are more expensive than a fixed prop but IMO worth the money and they, or well the Bruntons on our boat, does what it says on the tin.

 

[Coaster]

As ChrisE says, the only substantial disadvantage is cost.

One further disadvantage is the increased speed when crawling around a dock or marina. That's dealt with by using the gearbox as necessary, i.e. taking it out of 'forward' when needing to go slower.

We're very pleased with our Bruntons Autoprop.

 

[vyv_cox]

The only negatives compared with a fixed prop are that they need an anode, which can be quite pricy, and they need greasing each season, which takes about half an hour. The need for anodes is reduced by also having a shaft anode. A very short list compared with your positives, which are all true of an Autoprop.

 

[Tranona]

An Autoprop may well be an improvement on your present set up as the variable pitch is able to make better use of the power whereas a fixed pitch is only at its most efficient at maximum revs.

However, your current set up is possibly the least effective. The combination of a marginal powered engine and the least efficient prop is very limiting. It is a sailing boat with an auxilliary that can just about get hull speed. A fixed 3 blade prop will improve speed a bit at lower revs and be better at stopping and in reverse. There is a limit though as you need the revs to produce the power. So the simple way to improve speed is to increase revs (and therefore power), or install a larger engine.

 

[Ian_Edwards]

I've had an Autoprop fixed to my last 2 boats and find that they work well. In addition to the "down sides" noted above, I've found that they work a bit like a constant torque load on the diesel engine, the prop "pitches up" when motor sailing and the boat gradually accelerates when running on tick-over, as the prop "pitches up" with increasing boat speed, so in calm water we can be doing 3.5 to 4 knots at about 800rpm ..... too fast for a marina.
The constant torque works the other way when heading into a strong wind, I find I have to wind-up the revs to keep the boat moving, because the prop has "pitched back" in response to the lower speed through the water and the boat can be slow to accelerate when she's stopped by three big waves in a row. The drag when sailing is also higher than a good quality folding prop, but still much less than a fixed prop.
As I've said before, in previous posts, we get about 7 knots at 1800rpm in flat water with a clean hull and burn about 2L per hour, which I think is quite good for a 40ft lump of plastic.

 

[Albert Ross]

Our Brunton's Autoprop is all good apart from the anode.
Despite having a shaft anode, our prop anode lasts little time.
Last year we had to be lifted out early to deal with leaking stern gland. This was just 3 weeks after we'd put on a new Autoprop anode ... our Autoprop anode was half gone in that time (and we'd been on a swinging mooring for most of that time .. so no galvanic action from marina metalwork).
Our autoprop was supplied with plastic bolts to hold the anode on with (presumably so that the bolts cant corrode), I don't think these are very secure.

Has anyone lifted their boat after more than a few weeks and found they still have an anode on their autoprop?

 

[Albert Ross]

Oh .. and they cost £22 plus VAT each .. for a small piece of zinc and 3 plastic bolts.

 

[charles_reed]

Price.

x7 that of a fixed prop.

Regarding anodes - no problem is you fit a shaft anode as well. Mine lasts 3 seasons in the Med - providing you antifoul the fixing holes to prevent them eating away and the nearly whole anode falling off.

As to greasing - mine has water-lube bearings (since replaced by greased ones on more modern Autoprops) - need adjusting about every 700 hours and rebuilding every 3000 hrs running.

 

[Hoolie]

We have an Autoprop and it works fine when it's clean. But because it relies on a subtle balance between centrifugal force and water pressure on the blades it hardly works at all when covered in barnacles.
First time out last season it seemed OK at first then wouldn't allow any revs on the engine and we thought we'd picked up line or netting. But no, it was just thrown completely out of balance by barnacles
Regular prop scraping is now on the agenda!

 

[Robin]

Our Brunton's Autoprop is all good apart from the anode.
Despite having a shaft anode, our prop anode lasts little time.
Last year we had to be lifted out early to deal with leaking stern gland. This was just 3 weeks after we'd put on a new Autoprop anode ... our Autoprop anode was half gone in that time (and we'd been on a swinging mooring for most of that time .. so no galvanic action from marina metalwork).
Our autoprop was supplied with plastic bolts to hold the anode on with (presumably so that the bolts cant corrode), I don't think these are very secure.

Has anyone lifted their boat after more than a few weeks and found they still have an anode on their autoprop?

If it really has half gone in 3 weeks you have a problem somewhere with stray current I would guess.

The cone anodes on an Autoprop last fine if you paint inside the mounting holes, I used Primocon but Charles Read has said antifoul works too. This simply stops the anode corroding around the bolt holes first and getting loose enough to fall off. I changed ours every year but could probably have got two years out of them, but I figure I'd rather throw £20's worth of zinc away than a £2,000 prop. We also had a separate pear shaped hull anode wired to the shaft and prop. Tip, if you buy several anodes at a time they usually offer a better price.