Understanding influence of underwater shape on reversing

[NigelCraig]

I struggle to understand the physics of reversing keel boats under power - my interest is trying to understand how a particular shape of keel/rudder will influence things. For example the Nic 32 is apparently v. bad in reverse, and that has a long keel with the rudder hanging off. Westerlarly Fulmar has a fin and ( I think) a spade rudder and is apparently reverses well. Can I assume that the separation of the rudder from the keel is what makes the difference, or the fact there is no skeg. Will a Contessa 32/Sadler 32, which have rudders hung on skegs be closer to Westerley or Nicholson? Or is it more complicted than that - presumable type of propellor must have sone influence

 

[oldbilbo]

This'll be good.....

Perhaps some good soul will post a selection of representative underwater profiles, which we can AGREE represent the majority of types under discussion. Here's a start....

Here's the profile of a Rival34 of my acquaintance, which many - including an RYA Instructor/Examiner - agreed was a pig to reverse....

Let the fun begin!

 

[johnalison]

One problem with keel-hung rudders and a prop in a "hole" is that when the rudder is put hard across, a burst of forward prop slides past the rudder and doesn't give the kick needed, since the rudder is now outside the path of the stream.

 

[alant]

One problem with keel-hung rudders and a prop in a "hole" is that when the rudder is put hard across, a burst of forward prop slides past the rudder and doesn't give the kick needed, since the rudder is now outside the path of the stream.

Understanding influence of underwater shape on reversing !

 

[Tidewaiter2]

Understanding influence of underwater shape on reversing !

Same result only backwards??
The long keeler owners i know prepare for unpredictability, aka toss a coin as to which way it'll go this time, and usually have at least one long, long warp for getting out of U ends, where marinas seem to enjoy putting them( cos everyone has a bow thruster(s) now, innit?).

 

[jimbaerselman]

The key influence is directional stability - or instablity when in reverse! A dart is very directionally stable. It will only travel point first. That's because its centre of gravity is well in front of the feathers - the area where air pressure acts.

The same with boats. If (under water) centre of pressure is well aft of centre of gravity, the boat is strongly directionally stable going forward. Great for holding course going ahead, good cruising characteristic since it takes a lot to knock them off course. But now try going astern . . . a small disturbance, and you quickly have to act to correct it to stop the boat from pivoting right round. Such a boat is strongly directionally unstable when when going aft.

Boats built with racing in mind have to be manouevrable, and in modern marinas that's also helpful. So too much directional stability isn't such good news for them. So the water centre of pressure is moved forward, closer to the centre of gravity. This reduces directional stability when going forward, and also reduces directional instability when going aft. So less effort is needed to manouevre - both going forward (less helm) and when going aft (the boat's not always trying to spin around).

It's difficult to fiddle with the C of G and centre of pressure of a long keel boat. As a genus, they tend to be strongly directionally stable, and therefore very demanding to steer when going astern (if not impossible when prop wash doesn't run well over the rudder!). Fin keels can be put wherever you want . . . so they can be placed to give the degree of stability you want. And skegs can add more stability.

So it's not so much shape, as the fore and aft distance between C of G and water centre of pressure which determines how difficult it's going to be to steer a boat in reverse.

 

[NigelCraig]

The key influence is directional stability - or instablity when in reverse! A dart is very directionally stable. It will only travel point first. That's because its centre of gravity is well in front of the feathers - the area where air pressure acts.

The same with boats. If (under water) centre of pressure is well aft of centre of gravity, the boat is strongly directionally stable going forward. Great for holding course going ahead, good cruising characteristic since it takes a lot to knock them off course. But now try going astern . . . a small disturbance, and you quickly have to act to correct it to stop the boat from pivoting right round. Such a boat is strongly directionally unstable when when going aft.

Boats built with racing in mind have to be manouevrable, and in modern marinas that's also helpful. So too much directional stability isn't such good news for them. So the water centre of pressure is moved forward, closer to the centre of gravity. This reduces directional stability when going forward, and also reduces directional instability when going aft. So less effort is needed to manouevre - both going forward (less helm) and when going aft (the boat's not always trying to spin around).

It's difficult to fiddle with the C of G and centre of pressure of a long keel boat. As a genus, they tend to be strongly directionally stable, and therefore very demanding to steer when going astern (if not impossible when prop wash doesn't run well over the rudder!). Fin keels can be put wherever you want . . . so they can be placed to give the degree of stability you want. And skegs can add more stability.

So it's not so much shape, as the fore and aft distance between C of G and water centre of pressure which determines how difficult it's going to be to steer a boat in reverse.

Thats very interesting thanks for posting - so most if not all the boats I am considering would fall into the "dieectionally stable" category e.g nicholson, contessa and potentially be a pig to reverse. It's not always easy to be on the helm, fend off and warp out with one pair of hands!

Just in passing, there was a Nicholson 345 in Lyme Regis harbour last weekend coming in for repairs - for some reason she came up to the wall by the HB office and aborted - but reversed out all the way I thought very handily, especially with a howling easterly blowing across the harbour. ( The rest of their visit not so happy - broke away from visitor mooring and nearly beached behind the Cobb - lifeboat managed to drag her off JIT)

 

[Rossynant]

Some aspects are getting omitted in this.
First thing is a prop - normal prop is much less effective in reverse, but some can be better, and especially feathering props which act same way backwards. Sometime just installing properly designed prop is enough.

Another: back of keel. Some longkeelers reverse very well, certainly motor fishing trawlers are very good, despite being long keel. It's the shape of keel and the place where propeller race lands - as water goes forward, the stream lands on back face of keel and on hull. Sailboat designer seldom know anything about naval architecture, or do not think about it (some seem not to think at all...)
Provided keel is streamlined to prop and the stream is not going upwards to land on wide hull section, boat is handling nicely. If this stream of water lands on flat surface of keel back or hull surface - it's deflected sideways, any old how it happen to go, but mostly goes sideways. Not much use...

Ruder is another thing. First: rudder stock should be about vertical, but on old-fashioned racers it's on angle, very bad for effectiveness. This was common under Thames Measurement, so traditional British boats are known to be hard to steer Then this idea of cutting part of rudder off to place the prop there - naturally this results in no rudder anymore behind the prop. Don't ask why so many boats are made this way, as this was allways considered wrong. Obvious thing. Nic 32 is classic example. It's a sailing boat, ye know, not a MOBO...

 

[HinewaisMan]

I always find prayer is a good starting point when going astern in Hinewai, our long keel Roberts Maurtius, keel shape akin to the Cutlass above.

I then explain kindly to her where we would like to go and why, but always, and this is important, reassure her it is of course totally up to her where she goes.

Sometimes, bless her, she then even goes where we want.

 

[ianat182]

A couple of non-scientific design aspects occur to me that may have some bearing.
Deep keel yacht hull lines are so deep the influence of the flow from the propellor is deflected upwards against the the hull, so taking much more time before the flow is sufficient to draw water across the rudder blade .
The rudder blade section is working in an opposite direction for which it is designed, tapered section becoming the leading edge and possibly diverting the flow outwards as opposed to the forward drive which is the opposite foil section to increase flow over its surface. Perhaps too the angle of the shaft and generated thrust are being directed against the hull upper surfaces rather than directly along the keel form. This is maybe the reason for the Rival 34 's better but not perfect reverse performance.
When I sailed one, some 20 years ago now I was told to let the engine create a flow before quickly releasing the last warp, which worked but that was only on a couple of occasions I had the opportunity; once started she could be steered with narrow tweaks of the tiller/Throttle.




ianat182

 

[NealB]

This'll be good.....


Here's the profile of a Rival34 of my acquaintance, which many - including an RYA Instructor/Examiner - agreed was a pig to reverse....

Interesting.... I did a pre-yahtmaster practical course, with Southern Sailing in December 1989, on their Rival 34 'Bluenose', and found her to be quite handy in reverse.

Maybe it depends, in part, on the engine and prop?

Our very old wooden gaff cutter, with absolutely full length keel, was decidedly challenging when she had her original, low revving, Lister air cooled engine, but was absolutely transformed when we replaced it with a 30 hp Yanmar.

 

[boguing]

I've parked upwards of a hundred different boats. With one exception they all behaved predictably in reverse when given a chance.

Make it easy for you and the boat. Start reversing at least fifty yards from the berth. Point the boat downwind, engage reverse and slowly increase rpm until you start moving backwars (watch the little specks in the water to judge) now keep the helm central until you have a Kt or two.

It will now go where you ask..

Unless it's a Southerly 28. Effing binary rudder. Same advice though, with a twist. Start reversing, steer. If it goes the right way, carry on. If not, repeat until it does.

The almost universal mistake is in not playing enough while reversing, which is the only way to get to know a boat. Equally common is to start manouevering far to close to the berth.

 

[Poignard]

Make it easy for you and the boat. Start reversing at least fifty yards from the berth. Point the boat downwind, engage reverse and slowly increase rpm until you start moving backwars (watch the little specks in the water to judge) now keep the helm central until you have a Kt or two.

It will now go where you ask..
.

Not always easy to find a berth that is upwind and has a 50 yard space in front of it

 

[Poignard]

The long keeler owners i know prepare for unpredictability, aka toss a coin as to which way it'll go this time,

No no laddie. There is no unpredictability about my boat's path astern. She will head unerringly for the boat with the shiniest gelcoat.

 

[Pleiades]

Donald Rumsfeldt guide to long keel boat handling

Donald Rumsfeldt gives us a clear explanation and answers Nigel's question perfectly in so far as it refers to long keeled thingies like my boat. Going backwards is simply an unknown unknown - something that we don't know that we don't know about will cause her to wander off wherever she fancies in astern. Accurate estimation of tidal flow, wind direction and strength, using prop wash and kick, bursts ahead/astern, wiggling the wheely thing, a quick prayer - all might help, maybe, but not definitely - no idea why she wants to go some directions some times. So one does one's best and savours one's falibility and the unknowns. No matter where she goes the trick is of course to make it look deliberate cause if it is a tricky one a crowd is sure to have gathered.
Robin
Pleiades of Birdham
MXWQ5

 

[dunedin]

I've parked upwards of a hundred different boats. With one exception they all behaved predictably in reverse when given a chance.

Make it easy for you and the boat. Start reversing at least fifty yards from the berth. Point the boat downwind, engage reverse and slowly increase rpm until you start moving backwars (watch the little specks in the water to judge) now keep the helm central until you have a Kt or two.

It will now go where you ask..

Unless it's a Southerly 28. Effing binary rudder. Same advice though, with a twist. Start reversing, steer. If it goes the right way, carry on. If not, repeat until it does.

The almost universal mistake is in not playing enough while reversing, which is the only way to get to know a boat. Equally common is to start manouevering far to close to the berth.

So how does this advice of starting pointing downwind and 50metres away work ..... when reversing out of a marina berth with limited space (like we had to do a couple of times last weekend).

Re the OP question, a fin keel and separate rudder, like the Fulmar, is generally much easier to reverse predictably under motor (and faster under sail) - so IMHO frankly an all round advanance over old long keel designs for typical coastal cruising. Long keel was designed to suit wooden construction techniques of 100 years ago and things have advanced, so no longer relevant except perhaps for some blue water needs.
Run for cover .....

 

[Rossynant]

Long keel was designed to suit wooden construction techniques of 100 years ago and things have advanced, so no longer relevant except perhaps for some blue water needs.
Run for cover .....

Better run
Naturally finkeelers can turn around the finkeel, rest of hull having no side resistance. So they are easy to drive. No special skills required. That's all about it.

Well, not being good at motoring meself I looked closely when experienced friend took the boat from very tight berth in Gdańsk. Boat being 15 metre, 18 ton wooden classic (don't even think of scratching the varnished mahogany...) we had 17 metre distance from quay to fort brick wall and needed to turn around... Boat had to make 180 turn in place, no moving forward or back, while parked tightly between others.
Took three bursts, changing forward/reverse and a bit of patience. He said it would not be possible on light finkeeler, steadiness and mass makes boat more predictable.
The guy is an instructor, so I'm listening

But may I add that such manoevers, including backing, I was taught to make under sails? Whoever have seen square rigger backing into a berth under sail would never say anything against a 100 years old long keel. Bavaria cannot...

 

[Nina Lucia]

+1
We always ask for easy berth if possible...
hardly ever got one, than I cross my fingers and Go for it!

 

[mjcoon]

So how does this advice of starting pointing downwind and 50metres away work ..... when reversing out of a marina berth with limited space (like we had to do a couple of times last weekend).

Isn't that supposed to be the classic Irish directions: "Oh, I wouldn't start from here..."? ;-)

Mike.

 

[KellysEye]

Fin keelers will generally reverse with little no prop walk because you have clear water both in front of the prop and behind thus the the prop bites immediately. Long keelers, such at the Cutless 27 shown in a previous post, only have clear water behind so the prop takes control of the boat. Leaving a pontoon use a spring to stop the boat going back and use 1200 revs for about three minutes, that gets water going over the prop and you go out straight. If you want to back into a pontoon blip the throttle and the boat will go straight. We had a long keel with cutaway forefoot and it took me a while to work that out.