Twin-screw manoeuvering

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prv

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Idle curiosity from a sailyboat person here :)

In another thread, someone mentioned that while bowthrusters are very useful, stern thrusters are unnecessary because you can generate the same effect using just the props.

I'm reasonably competent at manoeuvering boats like mine - keel in the middle to pivot around, big rudder that works at low speed, single propeller sticking out of the bottom halfway between the two - but I've never driven anything with twin engines.

I'm sure whoever it was in the other thread is correct, but I'm curious how it works. How do you shove the stern of a boat sideways using just its props (and possibly rudders, though I suspect not)? Are there other unexpected tricks you can so?

Cheers,

Pete
 
Theoretically but each boat differs:
Going to starboard example without bow thruster. (Apparently works better on shafts I've never managed it on legs but havent tried too much)
Turn steering 45 degrees to port.
port engine forward / starboard engine reverse. Manage throttles to maintain no forward or aft movement. The boat should slowly move to starboard.

With bow thruster.

turn rudder hard to port. Use port engine and bow thruster in synchrony.

You can see how the theory works using this simulator. http://theboatdocker.com/

Having said all that my close quarter handling skills are legendary. Harbour masters sell tickets when they hear I'm coming. Punters always have a hoot and laugh as I yell at the crew and slowly wilt in embarrassment :p
 
Murv said:
I've posted a few questions re twin screw boat handling. Some interesting replies here :) http://www.ybw.com/forums/showthread.php?456232-Twin-screw-boat-handling-tips&highlight=
Click to expand...

I am finding this thread and the one you linked enormously helpful. I am just about to switch from a single screw long keel sailing boat - with all it's perverse behaviour in astern - to a twin shafts motor boat, I have been pondering on just these handling problems myself so am grateful for all the helpful advice in these threads.
 
I have a joystick with sterndrives + no bowthruster. It will go in any direction and has low and high power modes if it's windy. Piece of cake! Anyone worried about parking needs one of these ... don't listen to old stick-in-the-mud 'you should be able to do it without' types ... I can and this is easier! Now available with shaft drive.
 
Twin shafts I never use the rudder when manoeuvring, just use either for/aft port/stbd as required, one forward the other reverse spins you on the spot, just the starboard forward turns the boat port but steps the back out to starboard, reverse the opposite but backward obviously. I have a bow thruster but only ever use it if the crew is a tardy in getting the bow rope when docking and the wind / tide is on a blow off. If you go a little touch of port forward and the a touch of port backwards you stay in the same spot but crab to starboard, the opposite using port forward. / backwards in small dabs.

Amazing what you can do with just a bit of experimenting
 
prv said:
I'm sure whoever it was in the other thread is correct, but I'm curious how it works. How do you shove the stern of a boat sideways using just its props (and possibly rudders, though I suspect not)? Are there other unexpected tricks you can so?
Click to expand...
Put one engine into forward gear and the other into astern gear with the rudders centered and the stern will move towards the side that is in forward gear. You can use the rudders to speed up or slow down this movement but it isn't essential. However there are subtle differences between using the engines to move the stern sideways and using the sternthruster. First the thrust from using the props is much greater than using the thruster so if you want to make small adjustments, the thruster is more user friendly. Second when you use the engines, the boat pivots about a point 2/3rds forward of the transom so whilst you may be moving the stern sideways you are also moving the bow in the opposite direction which might be a disadvantage if, say, you're trying to nudge the stern onto a quay. On the other hand, using the sternthruster pivots the boat about the bow so all you're doing when you use the sternthruster is to move the stern. A sternthruster is not essential to manouvering a twin screw mobo but its one of those nice to have things that makes parking that bit less stressful
 
Note: Everyone here is talking about spinning the boat on it's axis to bring the stern in where as post #3 should produce a parallel sideways movement without using thrusters.
 
BruceK said:
post #3 should produce a parallel sideways movement without using thrusters.
Click to expand...
Huh? As I understood, you suggested that with port engine fwd and stbd engine in reverse, by steering 45° to port you can make the boat move sideways to stbd without pivoting, right?
Well, if that's the theory behind the simulator you linked, I hope nobody will try to put that in practice while mooring anywhere near my boat! :ambivalence:

That said, in reply to the OP - which btw I guess is referring to the "docking difficulties" thread, where I also said that s/thruster is unnecessary:
There is no way to replicate EXACTLY the stern thruster functionality with a twin screw mobo, because as Deleted User said, with props you always make the boat pivot to some extent (depending on each vessel), while the s/t by definition "pulls" only the stern sideways.
But if you've got (as is almost always the case) a decent b/t, you can counteract the pivoting motion, hence actually making the boat move sideways, rather than pivoting.
I mean, using the above example, with port engine fwd and stbd engine in reverse, the stern moves to port and the bow moves to stbd (pivoting motion).
But if you use the b/t to pull the bow to port, eventually you achieve the result of moving sideways to port (with the appropriate combination of throttles and b/t, of course).
Rudder position is so close to irrelevant at maneuvering speed to be not even worth mentioning, as whitelighter said.

Actually, there is another subtle trick that can be effective, depending on the specific boat - and typically works better with heavy D or SD boats (no prize for guessing how I know... :)).
It's based on the fact that all mobos (with the only exception of a few extremely fast boats) are rigged with left hand port prop and r/h stbd prop.
This implies that, when only one engine is engaged in fwd, on top of moving the boat fwd, there are two effects which tend to move the stern towards the side of the running engine: the asymmetric thrust and the prop walk. And the latter is particularly interesting, because it's effective also at zero speed.
Now, heavy D/SD boats don't jump forward as you engage the engine, as opposed to most P boats.
Therefore, by engaging say port engine in fwd, and adding a small throttle burst for a second before disengaging, you can actually make the stern move slightly to port even before the whole boat begins moving forward - or with an almost irrelevant forward motion, anyway.
Takes a bit of practice though, and as I said it's very much boat-specific, but worth trying (in open waters first...! :rolleyes:)
Otoh, what I said in the previous paragraph is more generally applicable.
 
Hi MapisM that exactly what Im saying with the provisio that it's afaic theoretical and certainly I havent achieved it. However many claim to have mastered or used the technique.
[video]http://www.youtube.com/shared?ci=cSCz95Ndxlg[/video]

http://www.thehulltruth.com/boating-forum/238833-can-someone-explain-walking-your-boat-dock.html

Try a google search on walking boat sideways without thruster will find a lot more.

Of course you cant believe everything on the net. Myths abound.
 
Neither have I.....but as a general overall picture....unless we want to talk about thrust and rudder angles....you get my drift?....or no drift depending on wind.....
So touchy some people!!!!
 
I used to skipper an 87' steel M/Y with twin old Gardners, a single rudder and no thrusters.

That, chaps, sorted the men from the boys while maneuvering in a tight marina. Patience, planning and practise were called for, hammering the throttles fore and aft did very little good. Getting the bow where you needed it first was the priority, the arse-end would always get there in the end. :)
 
Regardless of where the props will put her, ALL boats sterns will seek the wind when running astern, so use the effect to advantage, and never ever fight it.
 
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