|Topic Review (Newest First)|
|02-03-2008 04:18 AM|
Bow and stern thrusters...
|02-03-2008 03:03 AM|
|trantor12020||How I wish Propwalk could be switch-on to aid manoeuvre and switch-off when backing straight or to stop a forward movement. Any such luck ?|
|02-01-2008 03:09 PM|
Hmm. You're right. My full keeler's shaft comes out of "the deadwood" (actually just the steel hull) nearly "flat", whereas my fin keeler is a good 15 degrees below 90. I don't get propwalk with the full keeler, but I didn't notice to this point because I have been preoccupied with its less than effective ability to back down in reverse!
I use the "coast into the slip" method even more now, because the dynamics of a full keeler are very different than a fin keeler in both subtle and gross ways.
|02-01-2008 04:43 AM|
Originally Posted by Plumper View Post
|02-01-2008 01:35 AM|
Originally Posted by Plumper View Post
|01-31-2008 03:24 PM|
My shaft line comes straight out and I still get prop walk. All the single screw ships I have driven also get propwalk, to some degree, regardless of the angle the shaft left the hull.
It is always difficult, especially when there is wind and current. That's why they invented tugs. When I was Captain of a 100 ton classic yacht I always had a dinghy in the water to check the swing from prop walk, wind or whatever because it was often unmanageable and always different. The only thing you know for sure is that it isn't going to go straight
There is no pat solution. Each day is different even at the exact same dock. Each boat is different. Just keep practicing but as some one here says in their signature block. Only go as fast as you want to hit ....
|01-31-2008 12:31 PM|
Originally Posted by AndrewScullion View Post
|01-31-2008 09:51 AM|
Getting your boat to behave exactly how you want it too is mostly just down to practice, but understanding exactly what prop walk is, can help.
Most people do not know why the boat moves sideways in reverse other than it is something to do with the prop.
The truth is, it is nothing to do with the prop itself. It is caused by the downward angle at which the prop shaft leaves the boat and the hull shape around it.
Yachts with a sail drive experience almost no prop walk as the prop shaft is aligned horizontally.
Lets take a yacht with a prop shaft sloping downwards as it exits the boat.
When the prop spins, a rapid flow of water is forced away from the blades. The flow of water is not contained so the spinning blades results in the flow twisting and rotating out at the edges.
When in forward, this heads off on a slight downward angle towards the rudder and has no effect on the boat.
When in reverse, this flow is sent up towards the hull. The flow of the water on one side of the prop shaft is twisting off away from the hull towards the deep and on the other side is twisting up into the hull. This results in a greater force and flow of water over one side of the hull causing the boat to move sideways.
It is important when controlling a boat under engine to only use bursts of intense throttle and then back to neutral. Do not leave the engine ticking over in drive, forward or reverse. You should be drifting or adjusting.
This means you limit the time the rush of water is pushing the hull sideways in reverse and gives you a kick across the rudder in forward to spin the boat without it moving forward.
|01-24-2008 11:55 PM|
Larry, you really need medal!!!
I have the same problems with full keel and single 3 blades prop (I guess LH or RH works the same way) on 46' boat. Here is how I dock stern-in if I must to(!):
Since some 30,000 pounds is very dependent on inertia I need to neutralize it as soon as I start backing. Once on reverse gear I will try to back without turning steering wheel and holding it dead center (damn hard not to move the wheel – our brains always think of it as another Cadillac). When the boat starts moving and conditions (wind/current) help to go straight I just keep going until I can throw at least one line to deckhand (not allowing him to pull until I say so) and if successful, I am at the bar! If the boat starts to swing out from the intended course I usually switch to forward and kick hard ahead until I fill that the boat stall, then switch back to reverse (no wheel movement). What will happen is, that the bow will loose momentum to bear off and if lucky boat will start moving backwards - usually under different angle to the dock from the one I originally wanted. Once this happen and boat have low speed, I slowly start turning the wheel to the desire course as needed (no more then 5-10 degrees) and I try to steer into the slip. It will take about 30-60 feet for these maneuvers. Not always works and not always successful, but there is no other way that I know to succeed it. In deed it is widely dependent on the current and wind, which must be accounted into the whole process.
Much more easily is to turn on the “dime”. Many times I was able to turn around 40-50’ boat in 60’-70’ wide channel. I stay within about 2/3 of the channel turn the wheel hard to port/starboard and hold it. At the same time for few seconds I will kick forward to about 3/4 RPMs and when the boat is turning fast I will switch to reverse gear with high RPMs (still holding the wheel hard to). This technique has to be repeated several times and no turning the wheel at all, until the boat is facing the other way of the channel or desire course.
If you want to practice it, try it and see if it works for you. It did work for me although I hate to back my boat at all – turning on a “dime” is actually fun and I kind of like to do this.
|01-24-2008 11:30 PM|
Good link, thanks for posting it.
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