|Topic Review (Newest First)|
|06-13-2011 02:45 PM|
I read L. Francis Herreshoff state that the offset prop is better under sail and power. I think it was in the book: The Common Sense of Yacht Design.
As the 2 posters below mentioned, it performs better under power because the prop gets a clean flow and it gets better performance under sail because you don't have a propeller arpature (spelling?) in the keel/rudder. From what I got from reading the book: A little bit of weather helm helps windward performance because there is a little bit of lift when the rudder is pointed to leeward. The arpature diminishes this lift and it also creates a lot of drag as a ball of turbulance.
<< I read once that it is to clean up the water flow over both the rudder and the prop... in other words its so they both have their own "clean" water to bite into. In full keel boats it also prevents you from having a big hole in the rudder so the water flows better and the rudder is more effective. >>
|06-13-2011 12:05 PM|
I don't know the exact reason why the offset prop is used; but here are some observations regarding them. My boat; a 41' Newport (C&C) has an offset prop. The advantages are that it moves the engine to beneath the galley counter/dinette seat area, giving excellent engine access compared to beneath the companionway and cockpit. In addition the area beneath the cockpit becomes a massive storage area for large (lightweight) items if needed. You would not want things that can damage your thru-hulls and plumbing down there; but items like extra fenders, your inflatable dinghy, etc store perfectly in there. Also; shaft log access is excellent for servicing the packing gland, and if you need to pull the prop shaft it can be done in the water with an appropriate dowel insert to take the place of the shaft.
The big disadvantage is lack of prop-walk and torque induced steering at low speeds. It's better to have prop walk than to have none, and if you need to back & fill to turn the boat around it can be very difficult with an offset prop. In addition the torque induced by the offset prop wants to overpower the rudder when in reverse; so you end up going straight back with the rudder hard to port until enough velocity builds. With the thrust of the prop to the side, the rudder wont get prop induced flow across it; and the boat is also sluggish to turn to port from a stop for both reduced wash on the rudder, and torque steer for the offset prop.
All of that being said, I still like the easy access to the engine and open area beneath the cockpit. Maneuvering near the dock can be dealt with given practice and since you only really need the engine to get in/out of the marina or motoring in no wind it is not much of a problem unless you can't get the hang of the idiosyncrasies.
|06-13-2011 06:27 AM|
|IslandRaider||2 cents worth, I belive the ofset is for both, shaft pulling and clean water running, a lot of older full keel boat ran with ofset props/ shafts for these reasons. Privateer is a full keel with a 16/16 prop an the shaft straight out the back, prop cavitates @ anything over 1800 rpm an I have been told this is a result of the turbulance caused by the "deadwood" at the back of the keel, and the only way to remove the prop or change the cuttlass bearing is to pull the engine forward into the boat as removing the rudder shoe an steering quadrent ect, is even more of a pain in the...|
|06-13-2011 06:00 AM|
My Sabre has an offset prop. It's not a problem imo..
You can remove the shaft without pulling the rudder. The designers also thought it gave better flow. My boat steers fine..
|06-13-2011 03:50 AM|
|sidewinder585||I read once that it is to clean up the water flow over both the rudder and the prop... in other words its so they both have their own "clean" water to bite into. In full keel boats it also prevents you from having a big hole in the rudder so the water flows better and the rudder is more effective.|
|06-12-2011 08:15 PM|
|Quickstep192||I've seen several boats like that and always assumed that it was to avoid drilling through what is in many boats a very massive keel structure. The other reasons make sense too.|
|06-12-2011 04:17 PM|
|OtterGreen||im going to lean toward keeping it in a straight line. when i was in flight school i learned that certain aircraft are designed to overcome flight characteristics by offsetting the rotorblades and vertical stabilizers, i.e. a blackhawks tailrotor is angled to produce lift and counteract translating tendancy which is where the aircraft drifts in the direction of thrust. maybe its something along those lines..|
|06-12-2011 09:40 AM|
|PBzeer||My Ontario 32 (a C&C design) also has an offset shaft, though to starboard rather than port. It does seem to mitigate the propwalk when in reverse. Only negative I can really think of, is that it might give you a bit less thrust when motoring into a strong current or tide. That's just a guess though.|
|06-12-2011 08:53 AM|
|bobperry||I have offset props for the reason JRD mentions, so you can pull the shaft without removing the rudder. In the old days an offset prop was very common. I think in many cases it was so they didnt have to bugger up the rudder with a big hole for the prop.|
|06-12-2011 01:35 AM|
I've got a C&C with an offset prop.
It's positively so you can pull the shaft without dropping the rudder. I thought everybody knew this.
Has no effect on steering!
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