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The "kick" Of The Propellor


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When you come in to moor, and put the engine in astern, the stern of the boat will swing over, usually to starboard. If you can understand why this happens, it is a big help when handling the boat.

I have looked for pictures on the internet to describe this, but can't find find any, so you will have to put up with my scribbles!

596907df610af_Prop1.thumb.jpeg.5ab47b01cb432263a904289329963d05.jpeg

Fig (1) shows a standard 3 blade Broads type propellor. As it turns, the main thrust (a) is straight back but as the blade is angled, there is also a small sideways moment (b). The mean of these two is a moment (c) which comes out at an angle to the shaft line. This is why the wake from a propellor goes back in a corkscrew pattern (fig 2) which you can see clearly in underwater films.

59690826282e3_Prop2.thumb.jpeg.460bd92fb7742b1e5e7cdb00bd000348.jpeg

 

Next thing to consider is water pressure. We all know that if you go snorkelling or diving, as soon as you put your head under, the pressure increases rapidly and you have to clear your ears. The same thing happens to a propellor, even a small one. So when the prop turns, the pressure is greater on the lower blades and they are therefore making more "effort", some of which is sideways (fig.3).  This means the propellor is always pushing the stern slightly sideways.

When you are going ahead this is happening all the time but you don't notice, as you are steering to compensate for it. When you go astern, however, the rudder now has no effect, as it is in front of the propellor, not behind. So the "kick" of the propellor will move the stern sideways.

Almost all Broads boats will have a left hand propellor, which kicks the stern to starboard when going astern. This is because you are keeping to the right of the river, so when you want to turn round, you will do so to the left. In that case, the propellor will help you round, when you go astern.

Leaving out wind and tide, it will therefore be easier to approach a stern on mooring so that you have to turn out to the left, before going astern, so that the astern gear will help to turn you in. You don't normally do this in a bath tub, as you can see easily down the port side, but you can't see anything to starboard! In which case, leave the wheel, step up onto the bench in the forward well, and you can see over the cabin top.

While we are at it, you may have sometimes wondered about the size of propellors. Most Broads cruisers will have something like a 19 X 17 prop, where 19ins is the diameter of the blade circle, and if you can imagine the prop turning in mud or soft sand, where there is no slippage, then 17 ins is the distance that the prop would move forward in one revolution of the shaft. (see fig 4).

A fine pitch prop will give you high speed at high revs, but a coarse pitch will give you thrust at low revs, when manoeuvring . This is what we want for Broads cruisers.

Why don't they tell you this on the trial run? 

How much time have you got to spare?

 

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I've always wondered why that happens.

When I come into a stern on mooring on my port side I swing the stern round, go astern and the stern starts to then drift back where it came from (if you see what I mean)

Looking forward to trying next stern mooring. Roll on October.

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Great explanation Vaughan!  I agree, it's something that could be mentioned on a trial run if it is progressing well.  It could also be written in the on-board handbook for those who are interested.

Two comments:

1.  The propellor,blades are not flat.  There is a "twist" from hub to circumference which is designed to compensate for the fact that the tip is travelling faster than the hub.  there is also a bit of "dishing", curving the surfaces to provide more thrust, in the same way as an aircraft wing or a "full" sail.  Both of these shape optimisations are for forward motion and make the propellor less efficient when it is going backwards, so it does a lot more "churning" and the thrust force ("A" in the first diagram above) is reduces but the outward ("B") force is probably more; so the resolved or combined force ("C") is at a greater angle off-axis from the centre-line of the boat.  Also, because the reverse thrust is less efficient, more engine rev's may be used!

2.  Fine and coarse pitch:  A fine pitch is analogous to a low gear in a car.  It provides more "pull" at low speeds but is not good for going fast.  A coarse pitch (like a high gear) allows lower engine rpm for a higher speed through the water but it works less well (is less efficient) at low speeds.

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1 hour ago, Vaughan said:

Leaving out wind and tide, it will therefore be easier to approach a stern on mooring so that you have to turn out to the left, before going astern, so that the astern gear will help to turn you in.

 

Bit like this but a tad slower !! 

 

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Thanks Vaughan. I had no idea about this, I only knew that when going astern the boat rarely stays in a straight line but never understood the technical reasons. It explains why sometimes I can complete a stern mooring with no bother at all and other times needs a lot of messing about to put the boat in the right place. 

How does speed act on the effect? Is it a case of the more throttle, the more the sideways thrust? If so, why does a super slow reverse somewhere like Neatishead Dyke still require multiple corrections? 

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I have a boat with a right handed prop and when in reverse the stern will swing to port. Boats with steep shaft angles will have a more noticeable swing than craft with shallow shaft angles. On a boat with a shaft angle parallel to the water then the swing is vastly reduced.

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Johnathan, thank you very much your interesting post and I am sure you and I could chat about this for hours. Maybe we will one day, on the Broads?

I am sure you are correct in what you say about pitch. I don't pretend to know anything about the science of propellor design (a very dark art) and what I have posted above is actually based on a film about ship handling which was shown to us when I was at naval college. I have also tried to keep my post to the basics, in order to try and explain it as easily as possible.

I am also talking about displacement hulls, so "high speed" is a relative term! The design of high speed propellors for speedboats is a very different matter, and they are a very different shape. Something I forgot to mention is that, owing to the high and low water pressure that I have described, a propellor will always seek the surface, as there is more power coming from the bottom than the top. This will assist offshore powerboats to get up on the plane and it also explains the "rooster tail" that you see coming from the back of racing speedboats on Oulton Broad.

This is also why twin engined cruisers have "handed" or contra-rotating, props. So the kick from one side cancels out the kick from the other.

I have driven big harbour work boats when I was at the military port at Marchwood. These have variable pitch propellors and with them, the use was quite clear - fine pitch for a fast passage (and less fuel) and coarse pitch for towing and manoeuvring.

Choosing a propellor for a boat is really a matter of trial and error, and if you think your boat is not handling well it is worth talking to a boatyard and trying a different one. You may be amazed at the difference it makes.

At Crown Cruisers, the original 42ft Classiques had a 20X20 prop on a Nanni 4220 engine. I find them lovely to handle but the hirers seemed to find them too slow to respond, so on later boats we fitted a Nanni 5 cylinder, swinging a 22X18 and the difference in response, and astern power, was remarkable.

When we built the smaller version, the Crusader, it had the same 20X20 prop but that gave it too much power and the engine was overheating at revs over 2000. We then fitted the 19X17 and this did the trick perfectly. As we have hour meters on the boats, I was able to prove a reduced fuel consumption of just over 1.5 litres an hour.

It is a fascinating subject but for the benefit of easier Broads boat handling, I hope I have helped for a better understanding of what is going on under the aft deck!

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A great and informative post Vaughan. I suspect that there are many trial run drivers, myself included, past and present, who would benefit from a greater understanding of this matter and might, just might, encourage them to pay more attention to this important and basic skill of boat handling. In doing so they would help to dispel the anxiety which so many holiday makers experience.

Andrew

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On 7/14/2017 at 11:48 PM, Broads01 said:

How does speed act on the effect? Is it a case of the more throttle, the more the sideways thrust? If so, why does a super slow reverse somewhere like Neatishead Dyke still require multiple corrections? 

Morning Simon, I got a bit busy yesterday so I haven't answered your question!

The trick, as in most manoeuvres, is to get yourself in the right position before you start. In this case, you are lined up stern towards the dyke, but with the stern a bit too far over to port. Put the rudder hard over to port and start to come astern. Slowly and steadily, no acceleration. If you have got it right, the kick of your left hand prop will bring the stern over to starboard and as the boat picks up "steerage way" the rudder will start to counteract the kick of the prop and you will be lined up with the dyke. As you pick up a little more speed you will be able to bring the rudder back towards midships and you then have steering, even though it is very slow to react.

Don't go too fast as all you want is steerage way. The rudder will not work unless it has a flow of water going past it.

The other big factor in this is hull design. A Broads type displacement hull has a nice long deep keel which will keep it straight in the water. You will also find these much easier when going ahead. Other types, with an underwater shape more like a soap dish, will start to yaw off to the side after a short distance and you cannot control this unless you have a bow thruster (which is cheating).

The fascination of boat handling is that it takes loads and loads of practice, so you never actually stop learning how to do it. I never have!

So to sum up. there are some boats that you can reasonably steer in astern and there are others, that you just bloody well can't!

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Thanks Vaughan. I had in mind reversing out of Neatishead from the bottom between two lines of moored boats which I've done several times over the years and always enjoyed. In that situation because you have so little width it's easy to see when you're starting to veer off straight and need a forward correction. I always wondered why I needed several forward corrections and I think you've explained that now. 

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Great post, thank you Vaughn. I had always known about Prop Kick but it's nice to know why it happens. Am I right in thinking that twin screws normally have props which contra rotate to solve this, meaning that they have dedicated port and starboard props?

 

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1 hour ago, Paul said:

Great post, thank you Vaughn. I had always known about Prop Kick but it's nice to know why it happens. Am I right in thinking that twin screws normally have props which contra rotate to solve this, meaning that they have dedicated port and starboard props?

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!

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This 'Prop Kick' here is what Iv'e always known as 'Prop Walk'.

Of the many boats Iv'e helmed over the years, it's never been

a problem........till now!

Our Sheerline was built for a customer on the Thames and was

fitted with the Nanni 5.280 (5cyl).

Goes like the stink in forward but is a real pig going astern!

It has a right hand prop and pulls quite significantly to port and

it has taken me the best part of 30 months to find a way to get

it to behave! Iv'e been told to do what Vaughan mentioned above +

many other ways even by professional pilots,and that doesn't

work on ours. What does work is short medium bursts

of revs, around 1500/1800 rpm, and then put back in neutral.

The rudder needs to be about 45deg to starboard.

It gives a slow, gentle glide without any change of direction and a few

short bursts of throttle from time-to-time, about 3 seconds, keeps it on

track and the Bowthruster is almost essential in even slightly windy

conditions. Coming in to moor on the Port side is easy though :default_norty: 

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I have always known it as propeller bias. A propeller also works like a paddle wheel giving a sideways thrust. In forward gear the helmsman automatically compensates for this by just pointing the boat where he wants it to go. I reverse, until the boat picks up a bit of speed and has a flow past the rudder to give steerage the sideways thrust takes over, that is the theory anyway. However, what I cant work out is the sideways and indeed the top and bottom and the other side thrust must be equal and cancel each other out. Any physicists out there that can enlighten us? 

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I believe the answer to be that the blade of the prop at the

lower point of the turn is in 'thicker' water where the blade

at the top is in 'thinner' water, the frothy part. For the same

reason you don't want to follow in the wake of the boat in

front unless the water has settled back to 'thick' water as

you will get less 'bite' from the prop. 

Happy to be proved wrong of course :default_beerchug:

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All very interesting and the point is made clearly how much 'prop walk' occurred when I was mooring at Ranworth  in my video posted on a different thread.

The actual conditions there was that there was a constant breeze as shown by the red arrow which would push the boat to the left - see my example below:

Ranworth.png

The problem I have found with a 'bath tub' is there is very little towards the bow that is under the water - in  way like a planning boat, so I have found the bow does seem to wander more that perhaps a boat where a more pronounced 'v' bow with more under the water.

As I came to moor the first time I tried to counter the wind direction pushing to the right, by lining up with the bow of the Hampton:

Approach.png

But, as you can see here clearly (but from the helm of Bronze Gem I could not) 'all is lost' because of the angle generally of the boat. While the wind will help push me to the right, and while the prop walk will help bring my stern to the left - the angle is too acute and the thrust from her prop not great enough to 'kick' the stern to the right and straighten her up.

The only thing that can happen is the wind will win, the bow will drift and the boat will become sideways on to the quay.

On my second attempt having turned (with great help of that same prop walk) astern, lined up to the Staithe I then cut power - and then any steerage way I had and gave another hand to the breeze. So you see, it is a case of knowing just enough to be dangerous but not enough to be proficient lol.

In all seriousness, should you wish to give this a go in your boat, see what affect putting a couple of cameras on the boat makes when you want to do things - sometimes (more often than not actually) it goes very well, and I like to begin some distance out to stop any ideas that I do things a few times and use the best looking take to include in the final edit.

I really like the Princess 30  DS - she is the mould used for Belmore and Belmore TC - 30ft long but with some beefy power under the floor and a good pitch prop you can really use that thrust to good effect, not to mention the final touch of a bow thruster as I experienced here after cruising up the Chet and arriving at Loddon Staithe (videos begins at the mooring stage)

While some might say that thruster as just toys and a good helmsperson need not use them, I actually think they are very handy additions. You can use them to make very slight corrections when mooring, along with using them should you need to make a turn to cause the boat to turn in a far tighter space than without the thrusters  assistance. Of course knowing their limitations helps - in the main they are feeble against a strong current or a gusty wind.

 

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Just to reply that some old woodies actually steer quite well backwards due to the location and size of the rudder, finding the "balance point" is something of an art and don't let go or some fingers or a wrist may suffer the consequences.

 

To the OP an excellent post with an elegant and simple explanation, love the diagramatical skills not scribbles at all. For a boat that weighs in at over ten tons Malanka is quite a maneuvrable old lady. Turning in almost her own length is fun and surprises loads of folks when they see it. (only to the left of course .. he he)

 

When I was in my early twenties I cut my wooden boat teeth on Silver Jubillees and Juliettes, on one ocasion we reversed out the whole length of Horsey Staithe. Something we hadn't planned on and had no idea how to do at the time.  With lines from the stern to both banks we needn't have bothered the prop kick or bias or walk was minimal she just chugged and steered and it was fun in the end...

 

Mastering Malanka's prop walk is another thing all together. If you don't keep on top of it and then add too much throttle to correct you could easily have someone off the back and into the water the stern jumps so much. Which reminds me I must go get some new actuators for the life jackets the current ones are two years old now.

 

Amazing how my mind wanders, I'm supposed to be finalising stuff for work as I'm going on holiday, what am I doing, typing here and thinking about next week. Get a grip man.

 

M

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2 hours ago, Malanka said:

 

 

Amazing how my mind wanders, I'm supposed to be finalising stuff for work as I'm going on holiday, what am I doing, typing here and thinking about next week. Get a grip man.

 

M

Your priorities look pretty spot on from here!

I would defy anyone about to go boating to honestly say that they were concentrating on work this close to the event.

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Brilliant post Vaughan, most informative.

There is also something called ground wheel effect, which in essence is something you only ever come across in shallower water i.e. around a foot to 18 ins below the prop.

This is where the pressure differential caused by the centrifugal throwout of the prop, causes the prop to act like a driven wheel on a car and can really slew the stern over. Best method to counter act this is by using slow, short bursts of power. Do that and the ground wheel effect will be minimised in shallower water.

cheers

trev.

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I am pleased about this thread since it has opened a lively discussion and a sharing of experience, which I really hope may be of use to those, reading this forum, who sometimes wonder why their boat seems to have a mind of its own?

Robin, I have realised, seeing your post, that what I saw on your video, was not the whole story! It seems that it was not the prop kick but the wind, that blew you away!

Buffalo Bill is getting into the realms of cavitation but we had better not go there, except perhaps on a different thread?

Trev, your comments are bordering on SQUAT EFFECT. I put this in capitals as I know Jenny Morgan always enjoys it when I rattle on about squat effect! I will come up with another scribble (which Malanka is kind to call a diagram) and a couple of photos tomorrow, to try and explain squat effect as it has a big influence on boat handing in narrow waterways.

Meantime someone may like to talk about "canal effect", when taking tight bends in tidal rivers? The photo that Peter posted recently on another thread, of a coaster aground on Whitlingham bend, is a classic example of canal effect.

 

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3 hours ago, tjg1677 said:

Brilliant post Vaughan, most informative.

There is also something called ground wheel effect, which in essence is something you only ever come across in shallower water i.e. around a foot to 18 ins below the prop.

This is where the pressure differential caused by the centrifugal throwout of the prop, causes the prop to act like a driven wheel on a car and can really slew the stern over. Best method to counter act this is by using slow, short bursts of power. Do that and the ground wheel effect will be minimised in shallower water.

cheers

trev.

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?

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