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S&S Swan General - Swan 48 performance under engine
18 November 2010 - 18:58
#1
Join Date: 11 November 2010
Posts: 17

Swan 48 performance under engine
Anyone know what sort of speed under engine I should be getting for a Swan 48. I have a Yanmar 110hp and 20" 3 blade max prop. At the moment we're playing with pitch settings on the prop as only getting max rpm of 3100 which then only gives 7 knots.

18 November 2010 - 21:44
#2
Join Date: 01 February 2007
Posts: 234

Snow wolf

Too many revs for that speed. Look astern and see how far down the hull is pushed into the water or not. Sounds like big pitch problem, speak to the prop manufacturer about the picth angle, he will want a lot of details ref engine, hull etc.

John 

19 November 2010 - 09:21
#3
Join Date: 21 May 2007
Posts: 73

Hi,
I have just replaced the old westerbeke engine with a new mitsubishi solè 62 hp. at 2.500 rpm my speed is 7.5 kn , at 3.000 m( max rpm ) is 8.0 . I have a autoprop propeller ( self pitch) which works very well
Sergio Clarion of Skye 042/48

19 November 2010 - 15:59
#4
Join Date: 05 February 2007
Posts: 102

Dear Snowwolf,
Just for reference: on Swan 47 with a Yanmar 56HP and Maxprop 19" @ 2800rpm (max is 3800rpm)we get 7kn and the pitch is still to be tweaked next time we come out of the water as it is a little high. But after a long and tiresome struggle to find out why (since owning Aorangi) we had terrible performance and consumption, and big vibrations on the shaft, it was a small sacrifice for a season. It turns out the blades were mounted backwards, which is not easily visible but horribly inefficient;-(. good luck and please post progress. thanks!
Jayne
Aorangi 47/047

19 November 2010 - 21:03
#5
Join Date: 02 January 2008
Posts: 1547

Dear Snowwolf

For 7 knots about 24 hp is needed so there are really some big question marks in your case.

If you could give the engine type notation and the reduction ratio this enables  checking of the basic parameters.

Kind regards

Lars

21 November 2010 - 15:50
#6
Join Date: 02 January 2008
Posts: 1547

Jayne

This was interesting information - were the blades installed with the curved edge forward? The blades could then not feather to sailing position, and probably had a pitch angle exceeding 30 degrees, with the mentioned consequences. This is likely to be very unusual

Best regards

Lars

24 November 2010 - 16:00
#7
Join Date: 16 February 2007
Posts: 199

Hello Jayne,

Is it possible at all to mount the blades with edge facing forward? I doubt it. Perhaps lars can illuminate the issue?
pic enclosed

Cheers/Philippe
(47/050 Farouche)

24 November 2010 - 17:02
#8
Join Date: 05 February 2007
Posts: 102

Dear Lars,
I may have translated from Italian in an improper way, as it seems that the "backward" blades is improbable. I can only tell you the symptoms, and the results of the changes made. That is until I get back on board next week, and can post photos hopefully.

First and foremost before the changes the propeller was left handed and in reverse this was extremely evident. we changed stuffing boxes almost yearly, and the vibrations as I mentioned were extreme. was able to do roughly 6knts @2700rpm, and consumption was appx. 5.5lt/hour. the propeller is now righthanded (as per Yanmar specs), and reverse is much less dramatic and slightly to the left instead of immediately to the right. we now have roughly 7knts @2800rpms and consumption is around 4.5lt/hour. being the novice here I will let you draw your conclusions and am very interested in your thoughts on the change.
I truly apologize for my lack of English technical vocabulary, but am still "learning" in Italian, although this forum really helps me to become more familiar with my boat in my mother tongue!

Thank you for your patience.
Jayne Aorangi 47/047

24 November 2010 - 21:49
#9
Join Date: 01 February 2007
Posts: 234

Snow wolf

We meet briefly at Gosport earlier on this year when you were fitting out. My earlier message was done it a bit of a rush, but one of the key things I learnt when working on Avista my yacht over the years was not to assume that work done by previous owner was 100% correct. We can frequently make expediant decisions rather than the right ones. You should seriously check with the prop manufacturer if you have the right prop for the yachts mechanical installation. They will want to know LWL, displacement, engine HP and all importantly gearbox ratio and of course prop details. Its no good having a powerful engine if you can't get that power to the wheels!!

Good luck

25 November 2010 - 21:08
#10
Join Date: 02 January 2008
Posts: 1547

Dear Jayne

Thank you for the additional information.

Referring to the vibration and bad performance you experienced, this possibly was a result of improper assembly of the propeller. It sounds like one blade had a different pitch than the others, and this is known to happen now and then.

The propeller pulls the stern to the side when reversing, and this is related to its handing. When you changed the handing the pull is to the other side.

The simple rule is that this works like the prop was rolling on the sea bed. If you have a right handed prop in forward it will pull the stern to port when reversing.

It is worthwhile to use this to your advantage, and approach the dock so you have it on the correct side, and by reversing move the stern towards the dock and stop the yacht.

With skeg rudders it is difficult to steer the yacht when going in reverse. The recommendation is to not use a lot of throttle, this will just pull the stern to one side. Instead start very gently with the rudder centered, and when the yacht has picked up some speed the rudder begins to bite. This requires that there is space enough for this manoeuver.

The reported 4.5 l/h at 7 knots is about right. Does the engine at full throttle achieve the manufacturer's specified maximum rpm? You may remember that this topic was brought up earlier on this Forum in connection with propellers.

Best regards

Lars

 

26 November 2010 - 15:38
#11
Join Date: 05 February 2007
Posts: 102

Dear Lars,
Thank you for your interpretation of the 5yr. saga, I should certainly have asked sooner, and from now on will be less timid about such issues!!!
But on a lighter note, (may even cause a chuckle) the problem at the time instantly made me learn on my own (not being an expert) how to handle Aorangi, and I spent hours alone in open waters learning how to maneuver in the tightest of conditions(pretended I was surrounded by new Swans...:-). You would be proud that even with 47feet of space in the channel where I'm often moored I receive compliments on how I manage my boat with the bow thruster! (HA, the funny thing is I don't have a bow thruster!!!) So thank you for explaining the whys behind the how to...makes perfect sense to me now.

Kindest regards
Jayne Aorangi 47/047

26 November 2010 - 17:20
#12
Join Date: 02 January 2008
Posts: 1547

Jayne

For a manouvering expert it would be appropriate to know the actual reason behind the influence of the propeller handing in reverse.

The propeller slipstream hits the hull and it depends on the slipstream direction of rotation to which side the hull is pushed. A deeper V-shape will accentuate the effect, a flat bottom reduce it.

The simple rule about the wheel rolling is easier to remember though.

Regards

Lars

 

 

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