The "kick" Of The Propellor

[tjg1677]

4 hours ago, Wyndham said:

So, if I have read this right it would explain a situation I was in earlier this year. I was on a wild mooring and it was very shallow. When leaving the mooring I turned the wheel hard to starboard ( the bank) , gave a burst of forward expecting the stern to kick out from the bank and nothing happened, tried again, nothing happened. The boat hook was my friend!

Have I understood correctly? Is that the effect you're talking about?

Spot on but other factors come into play too in very shallow water. In some instances it is possible to get a sort of suction effect between the boat bottom and the riverbed. When that happens the boat don't go nowhere. I once had a small tanker run aground on the humber on a Mudbank, no problem thought I, the next incoming tide will refloat it- wrong!!.  

The ship was stuck fast in the mud and the suction would not let go, we watched the tide rise and cover the draft marks then decided it was time to panic as the engine had no effect.  In the end we had to get a tug to haul her off and when the suction broke she shot up by 6 inches like a cork. I guess the moral of this is that things that float have a total mind of their own in shallow water and normal handling simply doesn't happen.

Cheers

Trev

[tjg1677]

On 7/16/2017 at 2:16 PM, Vaughan said:

Yes. What is more there are gearboxes such as PRM which are "twin disc", so that they give the same power in either direction. So in a twin engined boat, you just have to change over the cables leading from the morse controls, in order to "hand" the propellors.

Can't remember which "hand" goes on which side. Probably Jonathan can tell us that!

Usually in a twin screw set up the props rotate outboard I.e. port goes anti clockwise and stbd goes clockwise. Doing it that way makes max efficiency of the props .Also in board rotation tends to increase cavitation. 

[ChrisB]

Going off subject a bit, when we took delivery of our V20 in 2008 it was fitted with a standard Honda 20 and was frankly a nightmare to control. 

In those days the local Honda dealer was Fineway at Hoveton and I mentioned the problem to Steve and that I was thinking of fitting a rudder device but was a bit worried because of the large amount of lilly at Broadsedge.

He confirmed that he saw a lot of damage where rudders came into contact with the prop due to weed or touching the bank when turning. However his solution was to fit the PowerThrust Honda prop which is huge compared to the standard.

It transformed the boat both ahead and astern. When crossing Barton in the calm of very early morning with no other boats underway I can set the trottle to run at a logged 4.8mph and leave the helm to put the kettle on or have a quick heads visit.

The engine now runs at much lower revs and will not achieve anywhere near it's potential of 6000 so the few hundred spent on buying a 20 over a 15 was wasted as the extra 5hp are in the last 500 of the rev range.

An extra 80 kilo of ballast in the bow completed the job.

[Vaughan]

I think it was in The Art Of Coarse Sailing, that they defined sailing as mucking about in boats; and sailing on the Broads as boating about in muck! I think this thread shows this to be true!

Whenever I have had to go and tow a boat off the mud, you always have to break the suction of the keel first. 

I also had the "wheeling effect" of the prop one day when coming in to an alongside mooring between two boats at Sutton Staithe. I got the boat parallel to the bank about 4 ft away, but it would not go any closer. I had to get lines ashore and pull it in.

Displacement.

 

Here is a displacement hulled barge on the Canal du Midi, near Montauban. It weighs 400 tons, but does not add any weight to the aqueduct as it passes over. It is displacing its own weight in water.

 

A bit nearer home, here is "Little B.A." pushing the water out in front, when at speed.

 

Squat effect.

 

This is a classic example of squat effect. You can see the barge pushing up a mound of water ahead of it and on a still day, you can see the hump reflected in the water, around 400 yards ahead of the barge. If you are following behind in a cruiser, it is impossible to overtake, through the rushing current of water, that is going back down the sides of the barge. The coasters on the Yare used to do this.

 

Here is an attempt to describe it! The boat is pushing the water ahead of it (displacing it) and the level drops sharply alongside the boat as it passes. But water has to find its own level, so the extra water in front rushes back down the sides of the boat, to find the normal level again, once the boat has passed. This will usually create quite a large, breaking stern wave.

If you are in a narrow channel, such as Rockland or the Chet, have a look down the side of the boat. If the water is going past faster than the boat is, and the level is dropping sharply away from the bank as you pass, you are going too fast! This, by the way, has nothing to do with whatever the speed limit might be.

I have proved many times, on the long straight canals in France, that if you slow down and reduce the squat effect to a minimum, you will actually go much faster over the distance. And use a lot less fuel! You can't tell the hirers this, I am afraid, as they feel the boat going slower, so they naturally want to accelerate. But they are just making it worse!

The boat will also be sensitive to change in depth of the water and will "shy away" from shallow sides which create a back pressure against the hull. On a tidal river, the water is always shallow on the inside of the bend, and this is what they call canal effect. If you are handling a long boat, like a barge, you have to slow well down before the bend, and get the bow right up into the inside of it. You then accelerate, and hope that the prop and the rudder will bring you round.

This change in bottom level is also why you might find it difficult to steer in a straight line going down the Ant but when you come out onto Barton, you go straight as an arrow. Open water, so no squat effect!

 

 

[LondonRascal]

Fascinating stuff this - and on a practical point of view, you can really experience how different depths and general areas of water can change handling.

In the Wet Shed at Stalham, we are fortunate in so far as even on occasions where the water levels generally are low B.A is still floating and that is just the way we like her to be! But some berthers will find their are aground or have some trouble getting out, even around the basin and leading off down the Ant from this location there is only a couple of feel under the keel.

As you maneuver you feel the boat does not respond in the same fashion as in deeper water and you just have to adjust and take this into account. Likewise on shallow, narrow rivers like the Bure above Coltishall Common and the Chet the sheer displacement of the boat and the 'squat' that can form has a big difference on performance. You often have to go a couple of MPH slower than the speed limit to avoid excessive wash being created as the river level ahead of you reduces - at the side of you 'flows out' and then 're-fills' behind you. Things like steerage, speed, and generally how the boat reacts to inputs all changes.

 

 

[tjg1677]

Once again bang on the nail vaughan. What you have described i have seen many times on canals. It is akin to hydraulic theory the boat is the piston, the canal the cylinder and the water the fluid.  You have almost the same effect too in shallow water. The movement of the boat creates a low pressure zone under the hull and actually drags the boat downwards slightly, the more revs you put on, the slowesr you go.!! 

As a slight aside when a displacement hull is in deep water it requires approx 4 times the power to double the speed. E.g.. if you are doing 2mph and using 10 horsepower, to do 4 mph you will need 40 up. I am not saying this is what power you need, it is just an illustration. So just think how much fuel you save when you only do 5mph in that 6 mph zone, answer - loads!!

Dead interesting stuff this and I reckon Vaughan should get the member of the month award for bringing it all up.!!!  :-) 

Cheers

Trev

[Vaughan]

I forgot to mention that this is what the pilots do at Potter Bridge. Going through the bridge at speed, the squat effect in the narrow arch drops the boat down by sometimes several inches, and so it goes under.

Not my kind of boat handling I am afraid. Too "hit and miss"!

[Islander]

20 minutes ago, tjg1677 said:

Dead interesting stuff this and I reckon Vaughan should get the member of the month award for bringing it all up.!!!  :-) 

Quite agree. Learnt a few things and confirmed others. Love to see more like this. Thanks Vaughan, now I know why 'The Green Lady' is such a #*%^ in astern but 'Lady Linda' is much easier and steers reasonable well, even though in our narrow channel at St Olaves I always have the bow thruster on and ready to use.

Colin

[Malanka]

I have been the victim of the squat effect from a barge on the Canal du Midi a few years ago. The resultant tug and flow from the heavily displaced water pulled up the rond anchors and dragged me almost into the water holding onto the line so the boat didn't get dragged into the barge. Silly person did not slow down one bit. Literally I had my feet set holding the rope and I was dragged like a comic strip character both feet set across the gravel. Was funny later but serious at the time.

 

M

[ChrisB]

I have used this type of snubber of my springs and warps. Fishing boats would pass our mooring at speed and these did take the worst of the snatch out

[Malanka]

To be honest if we had tied it to a tree it would have pulled the cleet out. It was going way too fast weighed thousands of tons and was fully loaded. We were very lucky because nobody was onbaord at the time so the boat would have been adrift and off down the canal. With or without life jacket you can't swim in the canal du Midi either to go after it unless you want to catch something horrendous.

[Wyndham]

Resurrecting this old thread...

This is my propeller..

 

So am I right in thinking that's a left handed prop and if so when in reverse the stern should "walk" to starboard?

Only it doesn't, it "walks" to port....or have I got it all wrong.

[DaveRolaves]

If you clean the prop up you should see the size and LH or RH marked on the centre boss. My prop is right handed and in reverse the boat walks to port.

[quo vadis]

31 minutes ago, Wyndham said:

Resurrecting this old thread...

This is my propeller..

 

So am I right in thinking that's a left handed prop and if so when in reverse the stern should "walk" to starboard?

Only it doesn't, it "walks" to port....or have I got it all wrong.

That is a left hand prop for sure can’t help explain anything else 

[Guest]

My prop is left handed and the walk when going astern is quite substantial, ok now I understand, in fact it really helps getting in and out of my home mooring, but it confused the watsits out of me when we first got the boat.

[TeamElla]

Like many others, we've put our starboard "prop walk" effect to good use over the years. On Ella it is hugely amplified by starting a turn to port (while still slowly moving forwards) before going astern.

So strong is this effect that when coming in at an angle to moor stbd side on, (in a small slot) after going astern to both stop the forward motion and swing the stern in, a short burst of forward thrust with full stbd helm applied can be the only thing that will stop the stern from hitting the bank.

Foregoing anything to do with tide and wind, It makes for a snappy single handed arrival snug into the bank, but it needs a bit more clearance in front of the boat to accommodate the extra foot of forward motion that occurs.

Re changing props:-

Before looking to try other diameter/pitch props, it's worth taking a good look at the old prop. Ours was so old and battered that just replacing like for like made such a difference it was like having a new boat. After all, the manufacturers knew what they were doing when they fitted the original prop and 30 years of who knows what had taken it's toll.  We even discovered that the original prop had a hidden crack in it!

 

 

 

[DaveRolaves]

info from the web which may help.....Put your engine in reverse at one-quarter throttle while tied to a dock and look over the stern quarters. If you see prop wash on the starboard side, then your propeller is right-handed and your stern will walk to port. If the wash is on the port side, your prop is left-handed and will kick the stern to starboard.

Right handed prop walks to port in reverse

Left handed prop walks to Starboard in reverse.

[Guest]

1 hour ago, DaveRolaves said:

info from the web which may help.....Put your engine in reverse at one-quarter throttle while tied to a dock and look over the stern quarters. If you see prop wash on the starboard side, then your propeller is right-handed and your stern will walk to port. If the wash is on the port side, your prop is left-handed and will kick the stern to starboard.

Right handed prop walks to port in reverse

Left handed prop walks to Starboard in reverse.

A quarter throttle could be a bit extreme if you have a powerful engine and the tests could be can I:

Hear the popping of fenders

Or

See a section of the mooring is attached to the boat as we break away from the riverbank. 

[annv]

You could try one of these they work very well in reverse but still kick the stern with a quick burst on throttle

[annv]

Hi Wyndham i would suggest you replace your split pin it is incorrectly fitted the ends should be bent around the nut left for one tail and right hand for other your pin is sticking out in the  oncoming flow and could pick up weed and bend it straight or break it of. easy to do while its out. John

[Vaughan]

Simon (Broads 01) has just started a thread called "prop walk" about stern on mooring and refers to a previous one where we had discussed it, so I thought I'd save repetition by referring back!

You can see that "prop walk" is described as a different thing from "prop kick".

Prop walk happens in very shallow water where the prop is almost paddling the stern of the boat sideways through the mud.  Prop kick is the natural tendency of a propellor to thrust a bit sideways, which is felt more when going astern and is usually to starboard as most river boats have left hand propellors.

Anyway, have a read of the discussion and see what you think . . . .

Meantime, I suggest that good stern on mooring has a great deal to do with judging where the wind is coming from and getting your boat into the right position before coming astern.

[CambridgeCabby]

Just read your original post on this thread , have to say , what a fantastic and informative post , Thank you Vaughan 

[Broads01]

Thanks Vaughan for resurrecting this thread. I'd forgotten just how informative it is.

[Cheesey69]

What a great discussion and a great reference work for the rest of us!

Everyone in this thread stand and take a bow. 

[CeePee1952]

Excellent information!  I've always been aware of the issue but never really understood the practicalities of it until now  I'll try to remember it when mooring stern on in a couple of weeks time 

Chris