What's wrong with a bit of prop walk?
- Thread starter john_morris_uk
- Start date
Searush
Well-Known Member
I wonder if you'll modify your view.....
Why?
What is the relevance fo a sub to manouvering Yachts in confined spaces? Is it perhaps to do with there being little room inside a sub?john_morris_uk
Well-Known Member
I was going to ask the same question. Furthermore I have been on on the bridge (as in on the fin) of a sub when it was being brought alongside extremely skilfully using prop walk and helm to put the several thousand tonnes alongside the jetty without any dramas or incident.Why?
Perhaps it should be called propulsor walk with a sub as subs have propulsors and not propellers. Prop walk will do for both...
DaveS
Well-Known Member
OK, I'll be the daft laddie. What's the difference between a propeller and a propulsor?
john_morris_uk
Well-Known Member
Its a technical term for the fancy circular devise (propellor look alike thing) that propels a submarine.
And (joking apart) the design of them is very very sensitive. People would get very twitched if you started to take photographs - not that you would get a chance...
Clyde_Wanderer
Well-Known Member
Propwalk should have very little effect on a properly controlled boat going astern.
Its a balance of the right amount of throttle and the correct rudder angle that will guide a boat astern.
Rudder angle and water flow over it.
Folk tend to point the rudder too far to the opposite side of the propwalk and give too much throttle, as opposed to a slighter angle and gradually increase the flow.
I use propwalk to my advantage when comming alongside, nearly always on portside.
C_W
Its a balance of the right amount of throttle and the correct rudder angle that will guide a boat astern.
Rudder angle and water flow over it.
Folk tend to point the rudder too far to the opposite side of the propwalk and give too much throttle, as opposed to a slighter angle and gradually increase the flow.
I use propwalk to my advantage when comming alongside, nearly always on portside.
C_W
BlueSkyNick
Well-Known Member
Another option
you could always get yourself a Southerly; they have an optional stern thruster, ie a bow thruster fitted into the skeg.
The make an awful noise when both bow and stern are in use !!
you could always get yourself a Southerly; they have an optional stern thruster, ie a bow thruster fitted into the skeg.
The make an awful noise when both bow and stern are in use !!
capnsensible
Well-Known Member
Propulsor.
http://en.wikipedia.org/wiki/Propulsor
Its no secret that a submarine has an electrically driven secondary motor in a trainable pod for berthing, similar to the drive motors on a cruise ship. Its not prop walk.
Common term, eggbeater.
capnsensible
Well-Known Member
Oh yeah, on topic, prop walk is brill. Use it! ( I am lucky to be able to do this nearly every day 
)
)
Dockhead
Well-Known Member
In my opinion, this is not really on topic. Once there is enough water flowing over the rudder to give it authority, I think most sailors are able to manage. Prop walk stops being a major issue.
The problem is during the phase between full stop and necessary speed astern to gain rudder authority. A boat with a lot of prop walk will go off in a circle. Which is perfectly ok if you have space in that direction, and can be actually useful if you don't have space behind. But it can be the devil otherwise.
Another issue not discussed here so far is relative position of the rudder and the prop. When these are close together, then the stream of water produced by the prop can act on the rudder, and create rudder authority at a lower speed. Boats with sail drives often lack this useful feature, so going astern can be more difficult with them.
My previous boat had a bad combination of long keel and small rudder -- would not gain rudder authority until two or three knots, and meanwhile could just about make a 180 degree turn to starboard. This behavior was rarely useful; if I had to leave a berth in astern I usually just had to warp off -- forget using the motor.
Sometimes you can improve the situation by modulating the throttle. So a firm burst of power to get the boat moving, swing the stern out (if you are lucky enough to be starboard-to), then shut down the throttle and hope that momentum gets you up to rudder authority speed. Sometimes it works.
My present boat has a bulb keel and large rudder, with shaft drive and prop quite close to the rudder. The rudder starts to work at a knot or less, and there is not much prop walk before that. Believe me, it is much easier to maneuver.
Searush
Well-Known Member
From reading this thread, am I right in thinking that people have problems when using the engine to stop & then go astern?
Cos my solution to that is to approach so slowly that we stop where we need to without any engine use. This usually requires using the engine at tick-over & putting it in neutral some time before we arrive. OK, it will need a slight boost or brake action, but at slow speed that's not an issue.
Hard reverse thrust for braking will always push my stern to port, but that's a benefit if slotting into a tight berth port side to. If berthing starboard side to it has to be a slow approach & maybe a short kick ahead onto a hard to Port rudder to get the stern in.
Cos my solution to that is to approach so slowly that we stop where we need to without any engine use. This usually requires using the engine at tick-over & putting it in neutral some time before we arrive. OK, it will need a slight boost or brake action, but at slow speed that's not an issue.
Hard reverse thrust for braking will always push my stern to port, but that's a benefit if slotting into a tight berth port side to. If berthing starboard side to it has to be a slow approach & maybe a short kick ahead onto a hard to Port rudder to get the stern in.
BlueSkyNick
Well-Known Member
I hate agreeing with you SR but on this point, I do.
Father's first boat on which I learned was a Leisure17 with a Seagull 40 hanging off the back. This had no clutch so we had judge when to kill it, either by turning off the fuel or hand over the air intake, to glide on to our mooring.
Now on bigger boats I try to use the boat's own way as much as possible and just a little nudge forward or astern as required.
capnsensible
Well-Known Member
Except, of course, if its windy, or tidey, or both. Some people actually go out on boats when its more than a force 2
And sidways wind. Er and lots of current. Faint heart never (expression not allowed) 
Cantata
Well-Known Member
Couldn't agree more. I have long been puzzled by boat testers' comments about this. Sure it's a nuisance sometimes, but I reckon it's more often a benefit.
Modulation
Well-Known Member
Biggest advantage of pw is that I can turn the boat 180 degs in VERY tight spaces - only one way mind.
Searush
Well-Known Member
Except, of course, if its windy, or tidey, or both. Some people actually go out on boats when its more than a force 2
And what great difference does that make? Simply use the minimum force needed to overcome the resistance. Current is usually a great help as one can ferry glide sideways into a berth & even hold station with the engine as you step ashore with the warps.
Sideways wind either blows you in or off, off is the only issue & the best way to deal with that is to go in bow first & use the engine to hold you there as you step ashore with both warps, make the bow fast & warp in the stern.
But I do these manouvers solo & need to set stuff up before it is needed.
And this is not about "winning fair ladies" is it?
DaveS
Well-Known Member
I've read this several times and still don't get it. Surely when going astern the stream of water produced by the prop goes forwards under the boat and not past the rudder?
I have a sail drive about 1m ahead of the rudder, negligible prop walk, and reversing is no problem at all.
wolf
Active Member
Prop wash cause of
For any that don't know why their boats have prop wash, it is the angle of the prop to the water column, sail drives tend to have less as the more down angle of the engine the worse it will be.
For any that don't know why their boats have prop wash, it is the angle of the prop to the water column, sail drives tend to have less as the more down angle of the engine the worse it will be.
Dockhead
Well-Known Member
I've read this several times and still don't get it. Surely when going astern the stream of water produced by the prop goes forwards under the boat and not past the rudder?
I have a sail drive about 1m ahead of the rudder, negligible prop walk, and reversing is no problem at all.
The prop creates a stream of water which begins in front of it and ends behind it. It acts on the rudder in both directions, if the prop is close enough.
nigel1
Well-Known Member
Not heard that theory, how does that explain the transverse thrust on say a ship, where the prop shaft is pretty close to being parallel to the keel plate.
Here's a simple explanation
http://www.free-marine.com/i3prop.htm
Never used a sail drive,but my initial thought is that if transverse effect is less than for a normal prop, it may be due to the fact that the prop on a sail drive is closer to the center of rotation of the vessel
