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S&S Swan Maintenance - Autoprop on Swan 47
28 November 2016 - 09:32
#1
Join Date: 29 January 2007
Posts: 1018

Autoprop on Swan 47

Dear Lars,

I have been offered a 19" three blades Brutons Autoprop (H6-534).

On Vanessa I have a Yanmar 4JH3 HTE, Kanzaki gearbox KM4A gear ratio 2,63, shaft 40 mm.

Previous Vanessa's Owner installed a four blade MaxProp Easy, not a bad prop but quite noisy (maybe because the blades are very flat?), and four blades are (in my opinion) a bit too much for a sailing boat.

Autoprop promises miracles with their props, not only in terms of speed/consumptions, but even with noise/vibrations reductions, which they say are very low.

I would like to know your opinion on this!

Thank you very much, and fair winds!

matteo (47/069 Vanessa)

28 November 2016 - 13:46
#2
Join Date: 02 January 2008
Posts: 1547

Dear Matteo

This is an interesting subject, and it is suggested that you try this propeller but make sure that it can be returned if not fulfilling the promises.
 
To begin with I would suggest 19" is not a good choice with this engine power, a larger diameter would be preferable, assuming that you aim at keeping up a decent speed in a strong headwind. It is also necessary to check that the blades do not hit the strut or hull when they swing forward, I believe this is considered on the datasheet you will fill in.
 
In order to have a reference against which the Autoprop can be compared, the present performance under power needs to be established. Accurate measurement of a number of rpm/speed combinations, say six of them about equally spaced over the normally used speed range, plus full throttle and finally the achieved rpm and pull with the yacht held stationary would all be helpful. 
 
It would be preferable to use a portable digital rpm-meter in order to get more accurate readings, and the sea has to be calm and at least 15 m deep. Will you use both GPS and speedometer? 
 
Do you know the present exact propeller diameter and pitch setting? If not, can be determined when the yacht is lifted for the propeller replacement.
Kind regards
Lars

28 November 2016 - 14:08
#3
Join Date: 29 January 2007
Posts: 1018

Dear Lars,

thanks! Unfortunatelly Brutons is not going to get the prop back if it does not work as they promise, which is annoying as this is quite an expensive propeller.

Will check if a diameter larger than 19" suits the boat when the boat will be ashore (January) and if they can freely swing.

I will do the 6 measurment before hauling Vanessa with a portable RPM/meter, thanks, and will take the data with both a GPS and speed on water.

I don't know the present pitch, and will tell you exact diameter of the four blades MaxProp.

Thank you very much!

Fair winds!

matteo (47/069 Vanessa)

28 November 2016 - 15:56
#4
Join Date: 02 January 2008
Posts: 1547

Dear Matteo

The 47 originally had 21" propeller diameter with about 40 mm clearance when the blades were in normal position.

If there is a Max-Prop manual on board the pitch may have been marked in it, check the table with pitch angles.

Would you know the maximum full throttle rpm? This gives an indication of the pitch setting.

Kind regards

Lars

 

 

28 November 2016 - 16:22
#5
Join Date: 29 January 2007
Posts: 1018

Dear Lars,

unfortunately they did not mark the pitch, anyway at full throttle and clean hull/prop I gan get 3,300 RPM, I usually cruise at 1,800 (7,2-7,4 kn), which is fine with me, as  speed is great and fuel consumption drops drammatically to about 3,5 L per H and gets me a long range

Fair winds!

matteo (47/069 Vanessa)

28 November 2016 - 20:49
#6
Join Date: 02 January 2008
Posts: 1547

Dear Matteo

Thank you for the information.
The present pitch setting is a suitable compromise providing good cruising speeds, but I have an additional comment about the engine loading.
As a general rule it is advisable to use at least 25% of the engine maximum power when cruising, Further it is a turbo engine, and this may also require some certain rpm. It is suggested you check if the engine manufacturer gives some advice about this in the engine manual. 
In order to avoid the engine collecting soot, 25% output is required, and in your case this means 2100 rpm/8 knots if running continuously. 
Provided you rev up the engine to a higher output, say 3000 rpm now and then, this blows out the soot - should be visible - and allows 1800 rpm to be used in between. 
Four blades do not provide an advantage at cruising speeds.
Kind regards
Lars

28 November 2016 - 20:54
#7
Join Date: 29 January 2007
Posts: 1018

Dear Lars,

thanks, yes, this is exactly what I usually do (running the engine at 1,800 and every few hours pushing it at top revs in order to blow out the soot.

I agree with you, four blades are not an advantage when cruising but a disadvantage when sailing (and, as you know, not at all considered in rating rules!)

Fair winds!

matteo (47/069 Vanessa)

28 November 2016 - 21:13
#8
Join Date: 29 January 2007
Posts: 1018

Dear Lars,

sorry, did a confusion!

I am usually cruising at 2,400 RPM and get 7,3-7,5 kn (not 1,800), and 1,800 just when I have to get just a few NM.

Fair winds!

matteo (47/069 Vanessa)

01 December 2016 - 19:41
#9
Join Date: 02 January 2008
Posts: 1547

Dear Matteo

Here some additional information.

Found a document presenting the measured propeller coefficients for Autoprop and Max-Prop, both with 3 blades,  These coefficients enable calculations to be made for evaluating the theoretical performance for each one.
 
You have 4 blades, but a comparison using 3 blades still gives some indications. The calculations suggest an Autoprop with the same diameter should provide slightly more thrust than a Max-Prop over the cruising range, but the performance in a strong headwind (bollard pull) is a questionmark. 
 
The strong point of the Autoprop is motorsailing, i.e. increasing the speed under sail with assistance from the engine. This propeller enables the engine rpm to be fairly low in such conditions. You would need a fuel consumption meter for knowing how much power the engine then delivers.
 
I have the impression that making the Autoprop so it self-pitches correctly is a delicate balance, sometimes it works, sometimes not, and this should be kept in mind.
Kind regards
Lars

01 December 2016 - 19:57
#10
Join Date: 29 January 2007
Posts: 1018

Dear Lars,

thanks!

Speaking about noise (MaxProp, maybe because of the shape of its blades, is quite noisy), do you think there will be a real improuvment with the Autoprop, as they say?

Thanks, and Fair Winds!

matteo (47/069 Vanessa)

01 December 2016 - 20:36
#11
Join Date: 23 October 2011
Posts: 150

Dear all,

I remember I read an interesting article with a test comparing various folding propellers.

I found it and here it is:

http://www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807

 

These tests were made on a smaller boat, and it seems that the Bruntons Autoprop has a lightly higher drag coefficient when sailing but is more efficient, having self regulating pitch. It reaches higher speed at the same rpm comparing it against the three blade MaxProp. Therefore there is less fuel consumption with Bruntons

Anyway I agree totally, these tests are only a hint for the right decision to be taken.

In your case, Matteo, I understand that that your problem is the noise coming from this propeller. I do not understand if you are meaning vibrations, because I am not able to perceive the noise of my propeller  as distinct from the engine when it is on. Clearly, when you are sailing engine off you keep the gear on the reverse. 

Regarding my three blade Max Prop, about fifteen years ago I was feeling there were too many vibrations in the boat and, apart from changing the boccola, methinks cutlass bearing in English, I left the propeller at the maxprop workshop in Milano, they cleaned the propeller, and they equilibrated perfectly the ambaradan, which looked balanced, shining and new.. Perfect final result in the water..

You live in Milano and you might check with them. In case try to swap your four blade propeller with a three blade from them... Four blades are more expensive and may be good only if there is no free space underhull, which is not our case.

I have to check, but I think I have a 21 inch propeller..

 

fair winds,

Matteo45 GRAMPUS 47/016

 

01 December 2016 - 21:13
#12
Join Date: 02 January 2008
Posts: 1547

Dear Matteo (47/069)

The Max-Prop blades have fairly thin edges, and if they are not properly rounded off but have sharp corners this will cause noise and possibly vibration. Check this when you lift the yacht.
The propeller shop should be able to improve this if you do not want to do it yourself.
Kind regards
Lars

 

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