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Old 01 September 2012, 23:10   #1
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Prop Torque / Leaning to port (free advise?)

Hello guys,
I have a 5.5 meter RIB that has a considerable lean to the left side when going flat out. Does anybody have any ideas how to counter this. It feels quite dangerous when coming off a big swell as the port side tube will dig into the water. Its very level and balanced when on its mooring and weight is evenly distributed on board. It has a Mercury 90 with 21 pitch prop.
Any ideas guys ?

Pluto 2
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Old 02 September 2012, 03:01   #2
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RIBase
Move the plate that sits above the prop to the left slightly (it may be the right) and this will adjust for you - it's teh lug that spins in it's holder left and right.

I'm sure some one will be along with the correct term for it shortly......
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Old 02 September 2012, 03:39   #3
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Trim tab
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Old 02 September 2012, 03:46   #4
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Also consider where you have the engine trimmed at speed.
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Old 02 September 2012, 04:02   #5
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Not sure that the trim tab will be the issue. It is worth having a look at first, but the normal effect of the trim tab is to neutralise the steering, that is to make the effort required to turn to port and starboard a similar effort.
It would help to know more about your boat. Engine offset is one possible issue, although with a 90 the effect should not be too pronounced, or it could be down to weight distribution in your boat.
I had a very similar problem with my boat. This was down to poor set by the the guy who installed it (me ). I tried to get the static balance right by putting the two batteries on the port side to balance out the driver on the starboard side, as I would mostly be out by myself.
I completely missed the effects of prop torque. I moved the batteries to the starboard side and the boat now handles as it should at speed.
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Old 02 September 2012, 05:37   #6
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you would expect prop torque to make the boat lean to starboard which is exagerated if slightly overpropped or trimmed in.the engine is usually set slightly off centre to counteract this yours is possibly set too far over or not at all.alternatively (but more unlikely) is the engine from a handed pair & counter rotating as its reacting the way you would expect if fitted with left hand prop
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Old 02 September 2012, 05:47   #7
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Probably an offset engine issue. It is surprising how much a boat will drop on one side due to prop torque.
On my 6.3m Ocean Pro I offset the engine mounting by 1.5ins and even offset the console and seats by 1in to counteract the effect, and it turned out great.
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Old 02 September 2012, 06:20   #8
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Quote:
Originally Posted by beamishken View Post
you would expect prop torque to make the boat lean to starboard which is exagerated if slightly overpropped or trimmed in.the engine is usually set slightly off centre to counteract this yours is possibly set too far over or not at all.alternatively (but more unlikely) is the engine from a handed pair & counter rotating as its reacting the way you would expect if fitted with left hand prop
Prop torque effect on a boat is the opposite direction to that of the prop you are using.

RH prop = Lean to Port

LH prop = Lean to Starboard
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Old 02 September 2012, 06:34   #9
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Two really basic checks are:

1..What's the offset of the engine on the transom and is it to the right side?

2..Weight distribution on board. For even running you might have to consider moving ballast around the RIB for your journeys. The smaller and lighter the RIB the more you'll have to consider this one. This is easy to check though. On your next trip take a 20kg bag of builders sand with you. You can experiment with this by moving it around the deck until your happy with the handling. Once done you'll have an idea and can consider where to move batteries and kit on board to make the changes permanent.

One other thing to look at is the size of prop you are using. Smaller pitches mean a greater prop torque (geared lower). Perhaps a less aggressive prop or a bigger pitch might be worth considering?
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Old 02 September 2012, 15:17   #10
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Leaning to Port

Quote:
Originally Posted by Ian M View Post
Not sure that the trim tab will be the issue. It is worth having a look at first, but the normal effect of the trim tab is to neutralise the steering, that is to make the effort required to turn to port and starboard a similar effort.
It would help to know more about your boat. Engine offset is one possible issue, although with a 90 the effect should not be too pronounced, or it could be down to weight distribution in your boat.
I had a very similar problem with my boat. This was down to poor set by the the guy who installed it (me ). I tried to get the static balance right by putting the two batteries on the port side to balance out the driver on the starboard side, as I would mostly be out by myself.
I completely missed the effects of prop torque. I moved the batteries to the starboard side and the boat now handles as it should at speed.
The only thing that is off centre is the 30 litre fuel tank , which is at the port side at the transom. The engine is Merc 90 four stroke spinning a 13x21 prop. When powered up to 30 knots and over, it gets a bit unnerving coming off a wave and down on to your port tube. The hull is in good shape, cant see anything odd about it, just wondering why this is happening, anyone else experience it I wonder ?
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