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Old 06-21-2012, 02:36 PM   #1
Athene of Lymington
 
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Default Autoprop vs. Maxprop

For anyone that's interested in the difference between these two props, I've recently ordered a Brunton Autoprop for our Oyster 435 Athene of Lymington to replace a 25-year old Maxprop. Why? Because Maxprop quoted me over £4,000 for the new generation model (pitch adjustable underwater) and a 4-5 week delivery. Brunton quoted me a little over half that amount and a 7-10 day delivery. Once the Brunton is on, I'll be in a good position to compare the performance of the two.

We had a Brunton on our previous boat, a Moody 346, and the lack of prop walk and self-pitching characteristics when motorsailing were the principal advantages. Whether we get the same benefits on our Oyster remains to be seen. Athene has always kicked strongly to port in reverse with the Maxprop and, since we do a lot of stern-to mooring in the Med using our anchor, that's a real problem that I hope the Brunton will solve. I'll post a report once we've launched on the difference between the two props.
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Old 06-21-2012, 07:20 PM   #2
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I like to think of prop walk as a sorta bow thruster. But for the stern. And it only works one direction. And you can't turn it off.
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Old 06-22-2012, 06:08 AM   #3
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Angry Stern thruster?

Or just a source of entertainment followed by panic for other moored boats. Should I get a fender out? Has he just bought that boat? Should they make boat handling lessons compulsory for the over 60s? Wonder if he's got third party insurance or not? GET THAT TEN LANGUAGE YACHTSMAN'S DICTIONARY OUT NOW!!!
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Old 06-23-2012, 03:52 PM   #4
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Default Autoprop fitting

Well - so far, so good with the fitting of our new Autoprop, which arrived just 10 days after ordering. We had to fit a 10mm spacer on our propshaft coupling, since the clearance between the tip of the blades and our P bracket when in reversing mode was a potential issue. At the same time, Brunton recommend that the clearance between the P bracket and the boss of the propeller should not be more than the shaft diameter (which in our case was 38mm), so the spacer was a careful compromise. That done, fitting the new Autoprop took little over 10 minutes. First, you slide it on without the shaft key and mark where the back comes to on the shaft. That way, you can tell if it's binding anywhere when you then fit it with the shaft key in place. Fit and tighten the retaining nut with a socket spanner, lock the nut with a retaining screw that screws down on to the flat of the nut, attach the anode with the three nylon screws supplied and job done. We take to the water on Monday, so then the serious evaluation starts of how the Autoprop compares with our previous Maxprop.
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Old 08-01-2012, 10:42 AM   #5
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Default Autoprop vs maxprop?

We’ve now had a month to evaluate the performance of our new Brunton Autprop compared with the previous Maxprop and, so far, the results are mostly favourable. We haven’t yet had the opportunity to test it in strong wind conditions with heavy seas or sailing at hull speed, but for what it’s worth here are the observations so far:

Motoring

At any given boat speed, engine speed shows a reduction of 400-500 RPM compared with the Maxprop. To put it another way, any given RPM seems to deliver an average increase in boat speed of between 0.5 and 0.75 knots. For example, at 1500 RPM our speed in flat water is 5.5 knots and at 2000 RPM around 6.5 knots; equivalents with our previous Maxprop were around 4.5 - 5.0 knots and 5.5 - 5.75 knots. Even at 1200 RPM - not much above idling speed - we still travel at around 4.5 knots. So the claims of greater efficiency and therefore likely fuel savings appear to be borne out.

Motorsailing

This is where the Autoprop really scores over the Maxprop. Motorsailing down to Levkas from Preveza in just over 10 knots of wind, we achieved 6.5 knots easily with sails hard in and engine throttled back to little over 1200 RPM. Getting the engine RPM just right for any given conditions does require a little trial and error at first, but once the boat is ‘in the groove’ it’s possible to pull the RPM right back without any sacrifice in speed. The Maxprop, by comparison, tended to cavitate once the sails started drawing and we could rarely get the combination of RPM and sail trim exactly right.

Slow speed manoeuvring

Berthing the boat - or, indeed, any kind of slow speed manoeuvring - does require a different approach to the Maxprop. There’s very little initial ‘bite’, since the Autoprop doesn’t assume its full pitch immediately, as the Maxprop does. This means that you need to apply much more initial throttle when starting off or changing direction and you need a certain amount of anticipation and patience until the Autoprop starts to bite. On the positive side, however, the considerable prop walk we experienced with the Maxprop is substantially reduced. As our boat (Oyster 435) has a semi-long keel, that’s a very welcome bonus.

Sailing

This is where we expected the Autoprop to suffer by comparison with the Maxprop, since (in theory, at least) a self-pitching prop should offer slightly greater drag when sailing than a feathering one. Surprisingly, that hasn’t been our experience at all. We reckon to go to windward at half the apparent windspeed (at least, up to 15-18 knots!) and downwind at a third of the true windspeed. We’re still achieving the same speeds under sail and I would say the difference, if there is one, is negligible.

Fuel economy

It’s still early days, but, as an indicator, our total engine hours since launch after fitting the new prop are 26.6, during which we used 72.66 litres of diesel. In other words, a fuel economy of 2.73 litres an hour. This compares with an average with the Maxprop of 3.3 litres an hour. All our motoring has been in flat water, so maybe this isn’t a fair test yet of the new Autoprop. Having said that, whenever you’re motoring in the Med it usually means there’s no wind and therefore the water is generally flat.

Maintenance

Of course, we’ve had no maintenance yet, but this is the one area where we don’t expect the Autoprop to measure up to the Maxprop. The latter is much simpler mechanically and only needed re- greasing once a year and dismantling, cleaning and re-greasing every other year (at least, that’s what I did). With the Autoprop, annual maintenance, like the Maxprop, is confined to cleaning off the fouling and re-greasing, BUT the bearings need replacement around every 1,000 hours (probably 5-6 years at our rate of engine usage) and that requires the initial purchase of a proprietary set of tools from Brunton at a cost of £120 plus a bearings kit costing £325. Leaving aside the initial tools cost, the replacement bearings cost amortises at around £60 a year (which doesn’t seem too high a price to pay for the benefits).

I hope this information is useful to anyone contemplating purchasing one or other of these two propellers and I would be happy to answer any questions forum members may have.
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Old 10-02-2012, 01:41 PM   #6
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Default End of season report

We're now laying up Athene of Lymington and total engine hours with our new Autoprop are 67 for the season (effectively just three months this year). Fuel usage is 148 litres, which gives an average for the season of 2.2 litres per hour.

That compares, as I said in my earlier posting, with 3.3 litres per hour with our former Maxprop - effectively a 33 per cent reduction in fuel consumption. I've also noticed our stern is clean, whereas it used to get sooty after several hours motoring with the Maxprop (set at 21 degrees pitch). It seems that, for our boat at least, the Autoprop is a more efficient combination, especially when motorsailing.

Otherwise, nothing has happened to change my earlier observations on the motoring or sailing qualities.
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Old 10-03-2012, 01:52 AM   #7
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I don't have a folding prop, but have often thought of buying one despite the fact that I use the auxilliary rarely in comparisson to modern trends. However, I sincerely appreciate your input and the data you have provided. I would take careful heed of your postings in the event that I were to seriously look at buying a folding prop.
Thanks.
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Old 05-20-2013, 03:22 PM   #8
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atheneoflymington: thank you for your posts they were quite informative. I am about to buy an Autoprop for my 44 Irwin center cockpit. I was curious about your engine hp and transmission gear ratio. Autoprop told me with my 75hp and 2.3:1 gear ratio I would need an 18" 19" or 20" Autoprop. There is a considerable difference in price which is one of my concerns. Also which size prop did you choose.
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Old 05-22-2013, 04:56 PM   #9
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Quote:
Originally Posted by tropical fun View Post
atheneoflymington: thank you for your posts they were quite informative. I am about to buy an Autoprop for my 44 Irwin center cockpit. I was curious about your engine hp and transmission gear ratio. Autoprop told me with my 75hp and 2.3:1 gear ratio I would need an 18" 19" or 20" Autoprop. There is a considerable difference in price which is one of my concerns. Also which size prop did you choose.
My engine is a Volvo Penta MD30A, 62HP and gearbox ratio is 3:1. I fitted the 534mm prop as I was afraid the 504mm would leave me under propped. Maybe I needn't have worried, since we do 5 knots now with only 1200 - 1500 engine revs.

Hope this helps.

GORDON
Meganisi, Greece
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