Volvo Penta Shaft Seal and flexible coupling

Discussion in 'Boatbuilding' started by yodani, Oct 29, 2011.

  1. yodani
    Joined: Nov 2010
    Posts: 190
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    Location: Danube Delta

    yodani Senior Member

    Hi there everyone,

    I just purchased a new boat that has a Volvo Penta D2-55 fitted with a M25 reverse gear box. The engine is on flexible mounts and is connected to the straight shaft with a flexible coupling type bullflex. In the attached picture you can see the set up.

    The rubber shaft seal is Volvo Penta water lubricated seal. I was looking for some information about the seal maintenance and discovered at the end of the manual the following note:

    Note! The rubber stuffing box is not recommended for
    installations where flexible couplings are being used.

    I know the engine and shaft line where fitted by a Volvo Penta dealer in Germany so I have no idea what to make of it?

    The shaft is long ... about 2.2m and it has a support bearing by the stuff box. At slow speed the shaft spins without vibration but when accelerated to about 2200 rpm the shaft starts to vibrate when currents are changing at the propeller or when rudder is slightly turned.

    At 2500 rpm you can hear from time to time a sound in the shaft line just like the shaft would touch the tube.

    Any idea why this is happening and what I should do about the seal?

    The propeller is large 21x14 (53cm in diameter) with 4 blades... seems too large but the reduction ration on the reverse gear box is 2.7 and the engine is reving up nicely with no sweat.

    Any advice would be appreciated.



    Attached Files:

  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    I think that if the manufacturer says it is not the correct installation, there may be an issue with durability. Probably it will work for a while but wear out or break too soon. Contacting Volvo Penta directly is a good option.
  3. Frosty

    Frosty Previous Member

    A few things wrong here. First of all a simple fix 'could' be to re-alighn the engine to shaft.

    But if you have an engine with flexible mounts you should not have a bearing on the shaft at the stuffing box as it not able to move so as the engine moves its trying to move the shaft. It will work for a while but its not right.

    Your shaft is very long. you could try removing and trueing or even go up a size.

    Your prop is pretty close the the rudder and I see you have to remove it to get the prop off. If it was a balanced rudder you might have been able to cheat a bit and cut a bit off but you cant do that without moving the rudder back. You might be able to move the prop forward a bit by using a shorter stern tube.
  4. yodani
    Joined: Nov 2010
    Posts: 190
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    Location: Danube Delta

    yodani Senior Member

    Thanks Frosty and Gonzo for the reply.

    I know it is not the correct installation as it doesn't feel right. The engine is running fine but when mounted on flexible mounts it has a big movement on start up. The bullflex coupling does a great job attenuating the vibration but still I am not happy with it.

    Another thing I have noticed is the sound the gear box does when shifting forward and reverse. The propeller is pulling and pushing the engine and the gear box sounds a bit loud. I have to say this is my first boat with flexibly mounted engine so I don't know how it should sound.

    I think I will need a thrust bearing and maybe to avoid all alignment issues a CV joint. I have build such system for my other boats and it worked great. Aquadrive sounds good but the price of that thing is way too high for me.

    I don't think I can modify the boat at the prop as it would be a major structural change. I have the boat for two weeks and have used it for 24 hours just to test and see. That huge propeller is putting a great strain on the engine mounts when changing forward - reverse.
  5. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Don't worry about the rubber seal at all! We use them in general just the way yours is installed, and they work fine. Just as Frosty said, make shure there is no radial bearing at the inner end of the shaft tube; the rubber seal has its own, which takes radial loads away from the sealing lips.

    Remember to let any air out of the unit when she goes back into the water (squeeze the sealing end so it opens slightly until pure water comes out on top of shaft, then add some grease through the opening (use Volvo special, the blue stuff supplied with the sealing).

    What you should worry about, however, is the shaft centering in the tube! While dry, loosen the shaft seal and slide it forward on the shaft. Adjust engine pads until shaft is centered and reconnect. Note that the Bullflex can take some angular displacement. Concur with Frosty regarding your propeller "environment". The prop is too close to the rudder, and the deadwood is far too wide and square, giving a wide wake that causes torque variations in the shaft. Also check that all four rubber pads are intact; if one has lost vulcanizing contact with its carrier, the engine will twist when thrust is applied.

    The MS gear is a mechanical unit with cone couplings; it is a bit rough in manoeuvering, but check that it has the correct oil and quantity, since they can be a bit sensitive.
    1 person likes this.

  6. yodani
    Joined: Nov 2010
    Posts: 190
    Likes: 2, Points: 18, Legacy Rep: 16
    Location: Danube Delta

    yodani Senior Member

    Thanks baeckmo,

    That is good to know. I took the boat again for a spin yesterday and re-aerated the seal. After doing that the metallic sound in the tube was gone and had less vibration. It is possible that air or gas made its way in the tube somehow (we drive in shallow water and when the prop disturbs the bottom there is a lot of methane and hydrogen sulphite released... a lot...) well that or it just wasn't aerated well.

    Now I can go to 2600 rpm no problem. The vibration caused by the turbulence at the prop is still there so I don't think I will be able to solve it this year. The engine and shaft are new with only 40 h on them (that was the reason I bought this boat) so I think I will have to fiddle a bit with the alignment. The rubber pads are ok but I have found one screw a bit loose.

    I have found this:
    Or look at the video:
    Alignment Eliminator a sort of Aquadrive but with a new type of CV coupling. I have seen this Thompson coupling discussed also here on the forum.

    Any thoughts about this?

    Thanks again for the comprehensive answers.

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