Prop Walk Forwards

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Gypsyjoss

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I've been musing whether on long motoring trips, say 20 miles, there is any significant sideways thrust from the prop which needs to be factored in to course setting calculations?

On my Sadler 32, the prop is not on the centreline of the boat. It is offset (angled) to starboard, presumably to allow the propshaft to be withdrawn past the skeg.

On other boats, the prop is spot on the centreline. Discuss?
 
I don't see where it has to come into any calculations about course to steer. Think of a sailing boat on a reach. The centre of effort driving the boat forward is well out to one side.
 
davidej said:
If you have a rudder position indicator, you will probably find it is slightly offset to maintain a straight course.
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It's a Sadler 32, tiller steered. My 34 has the same arrangement of offset prop, which avoids the really lousy job of removing the skeg. There is always slight 'weather' helm when motoring, as with sailing but other than that the boat goes where it is pointed. Don't really see where any calculation would be necessary but it's not an activity I participate in very much anyway. I have done it for long channel crossings, Lizard to L'Aberwrach and once for Holyhead to Isle of Man but the figure obtained was only a guideline, certainly not accurate enough to be influenced by rudder angle.
 
Gypsyjoss said:
I've been musing whether on long motoring trips, say 20 miles, there is any significant sideways thrust from the prop which needs to be factored in to course setting calculations?

On my Sadler 32, the prop is not on the centreline of the boat. It is offset (angled) to starboard, presumably to allow the propshaft to be withdrawn past the skeg.

On other boats, the prop is spot on the centreline. Discuss?
Click to expand...

I skippered a Thames Barge for a season & that prop was well to starboard of the centreline.
Made it interesting when parking & needing some astern to prevent squashing the plastic motorboat along the pontoon (normally crew jumped off with bow spring, but sometimes missed cleat), since stern swung quickly to port, whereupon rested expensive yacht Leopard, with attached bowsprit, not something you'd want to hit with a heavy wooden barge.
 
The offset prop will generate some sideways thrust which has to be resisted by the keel and therefore will cause some amount of "leeway" (or, slight crabbing) just as happens when sailing. This is a bit different to sailing on a reach, mentioned by a previous reply-er, where there is some sideways thrust but mainly offset thrust, which causes a turning moment and weather helm to counteract it.
My guess would be that the "leeway" due to an offset prop is barely noticeable, and most likely dwarfed by other influences on the boat's course ie tide and wind.
 
Any offset whether parrallel to the centreline or an angular offset will impart a turning moment under power, but this is simply couteracted by a small rudder angle and the boat then tracks in a straight line - so no "leeway" needs to be allowed. Manouvreing can be a different issue, though! At less than cruising speeds the boat is effectively under acceleration so even a centreline prop will give some prop walk, but the same applies, the rudder couteracts this but when manouvreing these effects should be used to best effect.

I have always noticed on the last two boats I have sailed, which both have an angular offset of the prop, when motoring I tend to stand astride the tiller and it always pushes against my right thigh. This is equivalent to the effect when using the autohelm and you will notice that under calm conditions the tiller will settle off centre.

Rob.

P.S. A wing engine, where the shaft is significantly away from the centreline, is usually set at an angle to couteract the turning moment but that will be calculated for the speed (power input) most likely when using that engine. Otherwise, with lower or greater power the effect of the offset will be come noticeable. A similar situation is experienced when towing alongside. As this is commonly done with a smaller vessel as the tug, the prop should ideally be aft af the towed vessel's transom and the thrust angled toward the bow. I've done this with an unmanned dinghy alongside a long-keeler and adjusting the angle allowed the yacht's rudder to centralise giving much improved authority, as well as reducing drag.
 
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I have a friend who once had a Sadler 32 with an offset prop. I understood that part of the reason for the fitting was to offset the prop walk that would result from a central prop, thus leaving a neutral helm. I didn't handle the boat often enough to see if this was the case.

Incidentally, one result of having a shaft that is easy to remove is that it is also easy to lose if it comes off inadvertantly. This happened to my friend when the boat was newly acquired, since there was nothing on the shaft to prevent it from disappearing when it decoupled from the gearbox during a burst of astern. Fortunately, the prop was horizontal when it reached the skeg and it stopped there. A jubilee clip on the shaft was quicky installed.
 
NormanS said:
I don't see where it has to come into any calculations about course to steer. Think of a sailing boat on a reach. The centre of effort driving the boat forward is well out to one side.
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No matter what point of sail the boat is on, apart from running, the sail, if trimmed correctly, will have exactly the same orientation to the wind, so the COE theoretically stays pretty much in the one place.
Propwalk is a phenomenon which is produced when the prop starts to rotate, and disappears when steerage way is established.
 
My boat has an outboard well slightly offset to port and forward of the transom hung rudder.

When motorng in a calm with the autohelm on there is a tiller off-angle of maybe 8 degrees; I can almost adjust this out by vectoring the angle of the engine in the well ( about all the swivel adjustment I have, can't use it as a lateral thruster when I cock things up ! ) - so obviously there is a slight sideways vector and loss of 100% forward propulsion there so it seems the boat keeps a straight course to to target, at a slight loss of speed.

Rightly or wrongly, as an ex dinghy sailor I feel having a constant rudder off angle working as a brake too much to watch.

If it was a fixed offset prop like the Sadlers described, I'd think the angle tracked - CMG - in a pure calm worth keeping an eye on for long cross Channel trips for example, but it's always a bit of a juggle with tides, surface drift from recent gales etc.

I try to think of it as if I was rowing a boat like John Ridgway & Chay Blyth & co, if I had to do it across the Atlantic I'd be pretty keen to sort out the least draggy rudder angle !
 
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Spirit of Glenans said:
No matter what point of sail the boat is on, apart from running, the sail, if trimmed correctly, will have exactly the same orientation to the wind, so the COE theoretically stays pretty much in the one place.
Propwalk is a phenomenon which is produced when the prop starts to rotate, and disappears when steerage way is established.
Click to expand...

I don't think so... firstly whilst the sail may be in a relatively similar position relative to the wind, but the position of the sail relative to the boat will be very different and so as was explained earlier, the centre of effort can be nearly over the centre line when hard on the wind do being well outboard when broad reaching.

As for prop walk, that exists throughout the engine rev range, but is just concealed as the foils take effect. The correction from the helm will have to be applied otherwise boat will steer gradually around in a circle...
 
Not too sure about the last bit; prop walk can be used in short bursts of power to move the stern sideways to one's advantage - with very little hull speed on so the foils are little do do with it apart from rudder maybe steering thrust and keel giving a centre of lateral resistance, on fin or relatively high aspect keel boats a pivot point

I agree for long motoring trips some sort of offset is required, if the prop is off centreline then handing of the prop will be important; if good in forward, even worse in astern but boats are a compromise and it's all ' fun ' especially if a spectator watching & commenting from the bar.
 
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Spirit of Glenans said:
Propwalk is a phenomenon which is produced when the prop starts to rotate, and disappears when steerage way is established.
Click to expand...

That is definitely not the case. For example -If my boat is allowed to gather weed on the hull the pull on the tiller at full revs can be really tiring to counter due to extreme propwash as the boat pushes against a lot of resistance through the water. With no weed there is still a marked pull on the tiller
 
Spirit of Glenans said:
No matter what point of sail the boat is on, apart from running, the sail, if trimmed correctly, will have exactly the same orientation to the wind, so the COE theoretically stays pretty much in the one place.
Propwalk is a phenomenon which is produced when the prop starts to rotate, and disappears when steerage way is established.
Click to expand...

Think perhaps a better term for it, is 'paddlewheel effect'.
 
Every single engined boat under power will try to turn in the direction of the paddle wheel effect but it is automatically accounted for by either the helmsman of autopilot. Therefore nothing to do with plotted courses.
 
As soon as a lateral component is introduced to the propulsion and some sort of correction applied, it makes sense to try to get tabs on exactly what is happening.

A helmsman or autopilot will hopefully constantly correct if following a GPS display so a plotted electronic track may not be any guide, unless one judges the tiller offset required, sets an autopilot on manual and sees what happens over a decent distance run.

We all know it is quicker to cross the Channel doing the classic ' sine wave ' with tidal offset first one way, then the other - rather than constantly steering to stay on the bee line as a cross track display would suggest and an autopilot slaved to a GPS would follow.

This is all academic for interest in efficiency, but I suppose if fuel is critical or say one is trying to get an injured crew to help ASAP it may suddenly become more important.
 
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