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S&S Swan General - Swan 48 performance under engine |
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Join Date: 11 November 2010
Posts: 18 |
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Swan 48 performance under engine |
Join Date: 01 February 2007
Posts: 234 |
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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 |
Join Date: 21 May 2007
Posts: 73 |
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Hi,
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Join Date: 05 February 2007
Posts: 102 |
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Dear Snowwolf, |
Join Date: 02 January 2008
Posts: 1547 |
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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 |
Join Date: 02 January 2008
Posts: 1547 |
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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 |
Join Date: 16 February 2007
Posts: 199 |
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Hello Jayne,
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Join Date: 05 February 2007
Posts: 102 |
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Dear Lars, |
Join Date: 01 February 2007
Posts: 234 |
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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 |
Join Date: 02 January 2008
Posts: 1547 |
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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
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Join Date: 05 February 2007
Posts: 102 |
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Dear Lars, |
Join Date: 02 January 2008
Posts: 1547 |
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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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