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Old 17-05-2014, 12:09 PM   #21
Captain slow
 
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Country: england
Location: surrey
Boat name: terminal velocity no.120
Boat make: phantom20
Engines: 2.5efi smillin jack
Cruising area: solent

Join Date: Mar 2006
Location: surrey
Posts: 478
No Bob, engines still in bits.
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Old 31-10-2021, 11:33 AM   #22
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Posts: 48
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Quote:
Originally Posted by Paul E View Post
This is a topic that really interests me, and I've thought about it an awful lot.

The reason a boat would have a natural tendency to roll to port is because of the propeller's rotation. Every action has an equal and opposite reaction, so the propeller spinning clockwise is going to want to spin the boat anti-clockwise. Of course with the boat having a much greater mass than the propeller the propeller wins, and the overall effect is pretty small, but still enough to cause a problem sometimes.

By mounting the motor off centre, all you are effectively doing is shifting weight. The weight of an outboard is quite a large percentage of your overall weight, so moving it slightly can change the balance of the boat quite a lot. If you now imagine a boat with the motor shifted quite a bit to the starboard side, that uses a propeller that has a tendency to provide lift, you're lifting the starboard side of the boat more than the port side, causing more roll to port, counteracting what you're trying to achieve by shifting the motor in the first place!

Another thing to consider is that the tendency to roll to port is exaggerated when trimmed in. Once you're trimmed level, or out, the roll reduces a lot. On flighty boats that get a bit wild when trimmed out, yet roll to port when trimmed in, adding some weight in the bow would help. You can then use more trim, without getting flighty, and with more control which should mean more speed overall...

The above is just my interpretation, based on a lot of thinking and some experimentation with my own boats.

Pretty much every high performance boat built and rigged today, be it a drag racer, offshore boat, ski racer boat, are mounted central. It's far easier to add a few kg of ballast than it is to guess how far over to mount an engine.
You are mostly right Paul but it’s not just the weight of the engine it’s the paddle wheel effect of the prop on a RH box trying to walk up out the water as well as going forward that’s what makes the hull tilt to the left in fact most offset the engine to the right it works better to the left as the prop is trying to tilt more weight
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