Canopus and Sculptor

[IanD]

Meanwhile if you have no concept of where the channel is likely to be and can't read the clues the boat constantly gives you regarding your speed and position, you'll still struggle.

 

Bottom line, your boat's a moving hole in the water. If there's not enough space around you for the water to get past and refill it, all calculation and theory is irrelevant.

 

It's not irrelevant, either making the hole smaller (lower displacement) or better shaped (hull form) will still help make the best of a bad thing...

[IanD]

For people that are stuck to (their) old ideas, all the changes that have been done over the years, and that took a lot of time to be accepted i.e.:

 

The change from wooden to steel (iron) boats, with some hesitation at first still using a wooden bottom.

 

Welded instead of rivetted construction, welds were something that many boat people had difficulities with too.

 

In the future we may see more "Shilling" rudders being installed on narrowboats, for the time being, I only know of one.

 

It all takes time, but maybe one day we will see a new kind of narrowboat arriving, with less draft and better underwater shape, as they don't have to carry cargo anymore for which the traditional shape was developped.

 

Peter.

Controversially, I suspect this would also show that the double-curved Josher-style bows so beloved of many people are a waste of space and money (though pretty). If they worked then similar shapes would also be seen on fuel-critical hulls like tankers and container ships, but they're not -- their bows are always quite bluff because on slow-speed vessels this doesn't increase drag or make a bigger wake. The reason is that the water in front of the bow stagnates and forms a "virtual bow" that the water then flows smoothly around. It's the same reason even high-speed fish and submarines are blunt at the front...

 

The stern however is a different story, here they take great pains to get a smooth flow of water past the hull and over the prop and rudder because any sudden changes of cross-section (or rate of change i.e. sharp edges) cause turbulence and drag. Most narrowboat hulls are terribly designed in comparison, and don't even follow the basic "good-practice" rules for things like skeg/rudder shapes and prop clearances. It's true that a canal is very different to the open sea, but the same rules of getting smooth water flow apply equally in a shallow/narrow channel -- however the optimum hull shape could well be different.

 

Trouble is that as was pointed out, due to fitting overpowered diesel engines nobody's really interested in doing anything about all this -- most boaters don't understand anything beyond how "nice" a hull looks, most hull builders have little or no knowledge of issues like this, and even if they did almost nobody apart from a few geeks like me would care anyway :-(

Edited December 31, 2016 by IanD

[GUMPY]

I believe that a cheap and dirty alternative to a Schilling rudder is to leave the rudder flat add the top and bottom plates to the rudder and a piece of angle to the trailing edge of the rudder.

If someone could do experiments on this and report back I would be grateful.

Edited December 31, 2016 by Loddon

[Dalslandia]

Meanwhile if you have no concept of where the channel is likely to be and can't read the clues the boat constantly gives you regarding your speed and position, you'll still struggle.

 

Bottom line, your boat's a moving hole in the water. If there's not enough space around you for the water to get past and refill it, all calculation and theory is irrelevant.

If there is no space for the water to pass the boat, well it is corky

I believe that a cheap and dirty alternative to a Schilling rudder is to leave the rudder flat add the top and bottom plates to the rudder and a piece of angle to the trailing edge of the rudder.

If someone could do experiments on this and report back I would be grateful.

 

That's don many times, an angle as a V or a T bar at the end, some even an O at the trailing edge of the rudder, The best is to wrap the leading edge also, giving a profile that is 15-25% thick of the total length, and end the wrap at 60% chord of the length.

[ditchcrawler]

If there is no space for the water to pass the boat, well it is corky

 

That's don many times, an angle as a V or a T bar at the end, some even an O at the trailing edge of the rudder, The best is to wrap the leading edge also, giving a profile that is 15-25% thick of the total length, and end the wrap at 60% chord of the length.

Saw this on a ships lifeboat in Norway on a Hurtigruten ship. Not very clear photos sorry

[Guest]

I believe that a cheap and dirty alternative to a Schilling rudder is to leave the rudder flat add the top and bottom plates to the rudder and a piece of angle to the trailing edge of the rudder.

If someone could do experiments on this and report back I would be grateful.

 

I'd save your weld... there's Gurney flaps and flettner strips, but benefits are only felt at low angles of rudder deflection.

 

For a device on the trailing edge to work then you must have an effective boundary layer attached to the rudder blade. A flat rudder blade will stall quickly, leaving only turbulent flow past the rear of the blade.

 

The Schiiling however stalls considerably later, and still produces high lift at larger deflections than any other, and out performs all NACA sections when astern. It's not the trailing edge alone that gives the Shilling it's high performance.

[twbm]

 

It's not irrelevant, either making the hole smaller (lower displacement) or better shaped (hull form) will still help make the best of a bad thing...

 

...... however the debate is about the behaviour of a fixed hull shape.

Edited December 31, 2016 by twbm

[GUMPY]

 

I'd save your weld... there's Gurney flaps and flettner strips, but benefits are only felt at low angles of rudder deflection.

 

For a device on the trailing edge to work then you must have an effective boundary layer attached to the rudder blade. A flat rudder blade will stall quickly, leaving only turbulent flow past the rear of the blade.

 

The Schiiling however stalls considerably later, and still produces high lift at larger deflections than any other, and out performs all NACA sections when astern. It's not the trailing edge alone that gives the Shilling it's high performance.

What do you mean by a boundry layer?

[mark99]

Look up boundary layer wrt fluid mechanics. It's a fundamental.

Edited December 31, 2016 by mark99

[MtB]

What do you mean by a boundry layer?

 

 

I suspect Mr Paws is talking about laminar flow, where the water in direct contact with the containing wall (i.e. the rudder) is stationary, and the further from it you measure, the faster the flow.

 

As opposed to turbulent flow, which is what it sounds like.

[WotEver]

What do you mean by a boundry layer?

A boundary layer is where the water (or air in the case of aircraft, cars etc) is 'attached' to the surface and flows along that surface.

 

Wikipedia explains it In a lot more detail: https://en.m.wikipedia.org/wiki/Boundary_layer

[Dalslandia]

Boundary layer is the closest layer of fluid neer the surface, the transit from standing still closest to the surface to moving along with the free stream.

 

the boundary layer will be first laminar and then turbulent layer, the turbulent layer contain more energy and can hold on longer on the back side then the laminar flowing layer.

 

when the boundary flow can't follow the surface anymore because of big angles it is said to stall.

Edited December 31, 2016 by Dalslandia

[Guest]

Have a look at this vintage NACA footage, between time points 01:30 and 02:00 for example:

 

 

Imagine that you are seeing a rudder blade section from the top, this section is a low lift symmetrical section, our simple plate rudders aren't as good as this. Also, if our plate rudder leading edge isn't rounded then it will be even less effective as the flow creates leading edge vortices, disrupting the flow further.

 

As the rudder angle increases you'll note that the flow stops being smooth on both sides and a turbulent flow begins. The angle at which this occurs is the stall angle (...of attack). The further we can turn a rudder before this happens then the more useful the rudder is to us for manoeuvring.

Edited December 31, 2016 by dpaws

[Dalslandia]

The NACA symetric 00xx airfoil is not a bad airfoil for thin keels and rudders, like 10-15% thickness, perfect for keels and rudders on standard sailboats

[Guest]

 

The Thistle/fishtail rudder follows the same principals as the Schilling, but is typically a thinner NACA 00xx section, with a V flare on the trailing edge.

 

There's a photo of one here:

 

http://www.boatdesign.net/forums/attachments/boat-design/71435d1339512375-schilling-rudder-question-img_0086.jpg

 

The rudder can be fine tuned in the water to some extent by changing the angle of the V'd tabs.

[Guest]

I've just picked up a comment on a different forum regarding a problem with steering control at low speed:

 

"My first instinct is that the current prop is a couple of inches bigger and lower pitched than the original. This may have improved fuel efficiency, but it could also have created a problem for manoeuvring. If you originally had a smaller prop, when you blipped the throttle to effect a turn, you got a lot of rudder and a little thrust. With the bigger prop, you aren't able to blip the throttle because the thrust is now twice what it was for the same amount of steering. (philSweet)

 

The situation for Canopus is exactly as described - a 2" bigger prop & finer pitch. It also describes the technique of "blipping" the throttle to provide some momentary flow over the rudder. This is something that comes naturally when you have a Morse lever in your hands, but the classic speed wheel doesn't lend itself to "blips" - well, not yet in my inexperienced hands.

 

My "blips" at present are far too slow, and consequently 'Pus launches forward, directly towards the object that you're so desperately trying to avoid!

Edited January 1, 2017 by dpaws

[GUMPY]

 

The Thistle/fishtail rudder follows the same principals as the Schilling, but is typically a thinner NACA 00xx section, with a V flare on the trailing edge.

 

There's a photo of one here:

 

http://www.boatdesign.net/forums/attachments/boat-design/71435d1339512375-schilling-rudder-question-img_0086.jpg

 

The rudder can be fine tuned in the water to some extent by changing the angle of the V'd tabs.

That is pretty much what I was thinking about but with a flat blade.

I realise in theory its not perfect however the stall angle can be no worse than a normal flat blade.

Realy only musing as Loddon doesn't need it as already turns on a sixpence (big blade, big prop and 50hp)

As for speed wheels and speed I bypassed mine when doing tug of war, cable direct to the governer lever on the JP, pull cable and almost instant full throttle only limiting factor was speed of the flywheel. Similar could be used to blip throttle

[Guest]

That is pretty much what I was thinking about but with a flat blade.

I realise in theory its not perfect however the stall angle can be no worse than a normal flat blade.

Realy only musing as Loddon doesn't need it as already turns on a sixpence (big blade, big prop and 50hp)

As for speed wheels and speed I bypassed mine when doing tug of war, cable direct to the governer lever on the JP, pull cable and almost instant full throttle only limiting factor was speed of the flywheel. Similar could be used to blip throttle

 

It sounds like you have a well balanced set-up on Loddon that behaves despite the big prop - is there a side-on photo of your stern swim, prop & rudder anywhere? I'm curious as to why yours does and mine don't! You mentioned both 50hp and a JP - is it a 4 pot?

 

You're right - the flat blade is not so bad for our type of boat, but the prop needs to be close to the rudder, certainly closer than on 'Pus, which means having to remove that vertical post in order to get the prop on and off.

 

The trailing edge strips would actually increase lift a little, delaying the stall about 5° apparently. Trailing edge "wedges" should be angled 20-30° to the main blade but short, typically less than 5% of the chord (rudder length).

 

One thing to watch - when you add the V to the tail the centre of pressure for the rudder moves aft - this means you'd need to extend the front edge of the rudder blade a little to compensate, otherwise you'll find the steering has suddenly got really heavy.

 

http://www.boatdesign.net/forums/attachments/boat-design/44372d1276699827-rudder-design-split-flap.jpg

 

I'll be fitting a cable control for now, short term - that'll overcome the speed wheel woes and allow me to understand further what's me and what's the hull. All the linkages will be left in place so when I feel comfortable I'll convert back again...

Edited January 1, 2017 by dpaws

[chubby]

Jesus Christ

 

There ll be formula one teams looking at this and thinking " sod that for complicated " .

Its just a narrowboat FFS . If you ever put your manuals away you might learn to enjoy it

[Tonka]

Jesus Christ

 

There ll be formula one teams looking at this and thinking " sod that for complicated " .

Its just a narrowboat FFS . If you ever put your manuals away you might learn to enjoy it

 

I second the above. Well said. Surprised it has taken so long

[Guest]

Jesus Christ

 

There ll be formula one teams looking at this and thinking " sod that for complicated " .

Its just a narrowboat FFS . If you ever put your manuals away you might learn to enjoy it

 

The enjoyment will follow the controllability, and besides, it's interesting

[chubby]

I d happily donate body organs to own a boat like that . If i did own it i sure as shit wouldn t sit around hypothesising about its faults id be chugging up fhe cut thinking " what a lucky bastard am i ?" with a grin from ear to ear .

Untie it , turn it on & go for a burn .

Edited January 1, 2017 by chubby

[Guest]

id be chugging up fhe cut thinking " what a lucky bastard am i ?" with a grin from ear to ear .

 

Pride comes before a fall... or an unscheduled excursion up the far bank in my case... I only have time for grinning when stationary!

[bizzard]

For learning your rudder, I suggest cruising everywhere in astern gear for the first 6 months, but you'd need to give the gearbox a rest every 20 minutes.

When I taught dinghy sailing I showed them how to sail the boats backwards, awkward, but it can be done, we used to have sailing backwards races, they really learned their rudders and sail adjustments.

Trouble is you might find it difficult to steer forwards after it.

Edited January 1, 2017 by bizzard

[GUMPY]

The JP was on a previous boat there is a link to Picture earlier in the thread

As for Loddon its got a beta 2203 with a 19x14 prop

https://www.dropbox.com/s/i5ni6z3vobipm5f/2012-07-24%2010.56.35.jpg?dl=0

https://www.dropbox.com/s/ljvhhk78jsl0net/2012-07-24%2010.43.21.jpg?dl=0

https://www.dropbox.com/s/2o9o64l9h24bbu2/2012-07-24%2015.11.44.jpg?dl=0