long keel rudder mod?

If you are going astern blip the throttle to get water flowing over the rudder then the boat will go straight, put power on and it won't because as you move the water hasn't started flowing over the rudder and the prop walk will take over turn the stern around. If you are leaving a pontoon going astern use a centre cleat on the boat to a cleat level with the aft end of the boat for a spring, run the engine for four minutes at 1,200 revs then release the spring, that gets water flowing over the rudder and you will go straight. It's easy when you know how, unfortunately it doesn't seem the be common knowledge.
 
One thing no has metioned is the shape of the rudder at its rear. I was reading some highly technical stuff, way above my head with all sorts of massive rocket science formula on rudder shapes to hull form etc and they all suggested the rudder should tapper to practically nothing whereas mine is the same 2.25 ins thick all the way to the end with a modest 3/4 in scallop on either side, in no way could it be considered shaped, any ideas.

Richard
 
Yes, there are well established NACA profiles you can use - although to be honest with a boat your shape the difference from what you have now is likely to be small - unlike a modern spade rudder running in clean water. There are often practical structural difficulties with profiling rudders so perhaps not a good idea to start carving lumps off what you have. No reason though why you could not build a new rudder with a better shape and profile.
 
To the OP

Post some clear photos and it will save an awful lot of wasted speculation perhaps..?

If you had a windvane self steering on the very aft end, eh, all that extra leverage to harness with ingenuity and clean water flow..follow my drift?
 
Blueboatman said:
If you had a windvane self steering on the very aft end, eh, all that extra leverage to harness with ingenuity and clean water flow..follow my drift?
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I was sort of wondering about something along those lines. Treat the existing rudder as a trim tab, and fashion a new, tiller-steered, transom-hung rudder hung from the stern post.
 
Richard D said:
One thing no has metioned is the shape of the rudder at its rear. I was reading some highly technical stuff...., and they all suggested the rudder should tapper to practically nothing whereas mine is the same 2.25 ins thick all the way to the end with a modest 3/4 in scallop on either side, in no way could it be considered shaped, any ideas.

Richard
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Ahem!

LittleSister said:
The rudder also looks to me very thick and poorly profiled. Tapering the thicknes of the aft two thirds to nearly a point would make it more effective, I'm fairly sure, even if the area stays the same.
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Not sure that will do anything for Richards problem. Seems apart from being a bit slow in stays there are no problems with the rudder when going in the right direction. If you look at the pictures the heel of the keel is a long way forward and there is not much of a stern post to hang a rudder from - although a vane gear would fit. However, that will not help with his problem of lack of control going backwards, neither will a more efficient foil shape for the existing rudder as if one can get enough speed up for the foil to become effective, control will be better anyway. The simplest thing is to increase the blade area by filling in the aperture, but as has been noted earlier a bigger diameter, finer pitch prop would probably have more impact on reverse performance - at the expense of some sailing speed.

The best way of dealing with the prop is to fit a 15 or 16" feathering prop, and then reshape the aperture to fit that. That was my solution on my boat with a similar underwater profile and rudder, although I do have the advantage of a small balance area in front of the stock. Still prefers to go with the prop walk, but very predictably and the big prop helps with the bursts of power to get it straight. Once straight and rudder central it stays straight, provided you hang on tight!

Sorry! Response to TK rather than LS.
 
I owned this boat for 20 years. Yes as with all Long Keel boats reversing can be a problem. I am afraid Yachtsmen today do not get the training that the sailors of old received.
The way to get this boat out of a berth in a straight line is to warp out. Attach a running line to a convenient cleet on a pontoon or some other convenient object. This way the boat can be forced away from the natural kick to Starboard. In a position where the bow can not be pushed round such as in strong winds, use a bow fender and put the bow gently on a pontoon or some other object and gently rotate.
The size of the prop on this boat does look small. It it was any larger ( as it once was ) the power becomes uncontrollable. As soon as the drive is engaged the boat tries to do 3 knots. With the prop the size it is you have sufficient control to "ferry glide". This is the true sign of good control.
With this boat she can be turned on a sixpence. A very good technique for turning in tight locations. The method is first engage full starboard rudder and engage reverse. increase the revs to the point where the boat starts to move in reverse. Deselect drive count one, two and spin the wheel to Port and select forward drive. When the motion starts to go forward deselect drive count one two again and select reverse and starboard rudder. By doing this the boat will spin on the spot. Of course it doesn't work when the wind is very strong as the wind will hold the boat or push it due to the windage.
 
Quite a lot for the OP to have thought about - presumably after two years he has researched and experimented or chosen not to. There are a few confusions in some of the posts as there are two issues addressed which are completely independant. Weather helm is a function of the sail balance and heeled hull waterline shape. With a hard dinghy going out solo to the moorings, sitting in the stern to steer the outboard lifts the bow and the dinghy slams badly. Moving forward to the rowing thwart means I can no longer reach the outboard, but move weight to starboard and the boat turns to port and vica versa. This is the same as a boat heeling under sail and tending to round up - weather helm. Note that the rudder is not involved here, it is the boat's balance requiring a rudder input or a rebalancing of the sails to negate the effect.

There are many good books on foil design and theory, some of the simplest and best explained being those on self steering design as they are often backed up by experimental data, demonstrating the effects of individual design criteria. Changing the foil section to a suitable NACA profile will have less effect on a keel hung rudder as it is effectively a trim tab to the keel, but it will improve its performance toward the extremes of its abilities as the profile will encourage laminar flow across the blade and delay stalling. None of this, of course will have any effect on reverse handling. A low aspect ratio rudder like the OP has, already has the centre of effort quite a way behind the pivot, so can be expected to be heavy (probably masked by the wheel steering) and increasing its chord will move the centre further aft, the resultant increase in lever making the steering heavier still. I cannot see any way that the hull shape allows a modification to balance the existing rudder. To incorporate balance area, you'd have to move the rudder post aft and maybe fit a new skeg hung rudder of use the Chuck Paine concept referenced above. Either would have the added bonus of extending the blade toward the waterline, resulting in a higher aspect ratio, already requiring less balance! Weather helm has no influence on how heavy the helm is - a heavy unbalanced rudder is heavy under motor as well - it just means you have to use it more.

Rob.

P.S. Last month's PBO had an article on just such a case study and that was a Van de Stadt prototype, so even well respected designers have got it wrong sometimes. Maybe that's why he almost always used a balanced spade rudder later in his career?
 
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Pretty sure the OP sold the boat very soon after these exchanges. His issue was not about hull balance, it was inability to steer astern. It was he first boat and he found it a handful - as well as requiring a lot of work. It was a one off and from what one could see of it from photos there were serious misgivings about its design, particularly the small poorly shaped rudder. However, just like many of its basic type there is little one can do to make it go backwards reliably. As rogerball says, it was overpowered and with the correct diameter prop it was even worse, so a small diameter prop had been fitted to reduce the prop walk.
 
Twister_Ken said:
Trouble is, it's not basic physics, it's advanced hydrodynamics. In the case of a boat like the Twister, the prop is in an aperture in the rudder. Going astern with the rudder centred creates a waterflow down both sides of the keel, more to one side than the other, depending on prop rotation direction. However, once some sternway is made, and the rudder is put over, more of the aperture is presented to the waterflow, and some flow squirts through the aperture from the high pressure side of the rudder, down the low pressure side of the keel, being accelerated by the prop and thus cancelling out some or all of the turning moment the rudder is inputting*.

With the Twister, the only thing I could be sure of was that she would turn, but I was never sure which way. The 'secret' was to arrange things so that whichever way she went, I had good options. Part of the 'arrangement' might have been a doubled line left ashore so that I could restrain her, or haul her round, or it might have been a matter of unrolling a few square meters of genoa, and sheeting it hard aback, so that the wind would dominate turning-proceedings, rather than the rudder.

*at least that's my theory!
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+1 as a once Twister owner and it would be similar with my Tradewind if I didn't have a bow thruster.

When owning a long keeler you soon learn how to deal with its habbits.

As long as the rudder works well when going forward sailing and motoring, then I wouldn't enlarge the rudder.

If a small amount of rudder gives no appreciable steerage to balanced sails then perhaps a larger rudder blade would help.
 
Tranona said:
Pretty sure the OP sold the boat very soon after these exchanges. His issue was not about hull balance, it was inability to steer astern. It was he first boat and he found it a handful - as well as requiring a lot of work. It was a one off and from what one could see of it from photos there were serious misgivings about its design, particularly the small poorly shaped rudder. However, just like many of its basic type there is little one can do to make it go backwards reliably. As rogerball says, it was overpowered and with the correct diameter prop it was even worse, so a small diameter prop had been fitted to reduce the prop walk.
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First point it was not a one off. Three boats were built. One was a little shorter and one a little longer but all to the same design.
Second point. The prop walk was no worse in face it was considerably better with the smaller prop. The problem with steering in reverse is one of the water being pushed down the Port side of the boat and thus pushing the stern to Starboard. Once thee boat is going at a reasonable speed aft the rudder becomes more effective. The problem for a novice sailor is getting the boat moving in the first place.

On this particular boat weather helm should not be a problem. As stated this is a product of balance and centre of effort. The fore sail and the Main are fully adjustable ( foresail and in mast ( behind mast add on) If any weather helm is felt taking in the main a little was always the easiest solution. It even helped increase the speed a little.
On the point of the engine being over powered. There is no such thing as an over powered engine. What matters is what you do with the power. I would much rather have an engine which will get the boat into somewhere like Portsmouth Harbour when the water is ebbing at near 5 knots than have to wait outside for the slack at low water, as some do.
Any first boat is always a very steep learning curve. This Palmer ( she was called Chase of Mylor when I owned her ) took some learning but when you did get to grips with her she looked after you very well. I was so sorry to sell her. I sailed her single handed most of the time and with a bad hip I was finding sailing becoming too much. THe problem was me not the boat.
 
I would be very interested to know who is the proud owner now.

know she was on Brokerage at Largs with David Cook. David Cook has now retired so I wonder where she is. I do hope she has found a new loving owner.
 
I suspect there are two points which at first are not obvious. The rudder would appear to be hung some way aft of the Prop' The transom is in the water. The boat which the thread is referring to is a Canoe stern, this means there is no "hull in the water above and behind the rudder. The Bilge plates will also help with control in reverse. It looks a very good design.
 
rogerball30@gmail.com said:
On the point of the engine being over powered. There is no such thing as an over powered engine. What matters is what you do with the power. I would much rather have an engine which will get the boat into somewhere like Portsmouth Harbour when the water is ebbing at near 5 knots than have to wait outside for the slack at low water, as some do.
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Important to remember that it is not the engine that moves the boat but the propeller. That boat was a good example of not being able to use the power because of the restrictions of the propeller. Speed is limited on a displacement boat so putting in excess power does not lead to better performance. The dilemma when powering boats of that age was the gap in the available engines which led to overpowering because there was not a viable alternative. As you discovered the downside is the boat can become a handful at low speed so you have to prop it so it does not use the power that is potentially available. An engine in the 25-30hp range would have been more than enough for the boat and it would still have achieved hull speed.
 
The engine was in fact 38 Hp. The 4-107 was built as a static engine and used on either a pump or generator. Perking were not certain. The first prop the boat had was a two bladed large diameter prop. I think it was the prop which had been fitted when the boat was built. It had a Stewart Turner petrol engine. That was later changed for the Perkins. It was raw water cooled. When one of the liners developed a leak I changed the engine to indirect cooling. The engine was rebuilt and is still running after more than 20 years. ( I assume it is still running). The advantage of the indirect cooling is it gives hot water for the domestic use on the boat and reduces any salt water corrosion.
I agree with what you say regarding over powering, however as I said it is nice to know you have an engine which is capable of driving the Prop easily. It never struggled it was always there when it was needed.
It should be noted the boat was built as a Motor Sailer. The wheelhouse was removed by the second owner who had her on the Helford River. I have seen boats like Fisher's which have had quite small engines fitted which have ended up being quite unreliable due to being over worked. I know a lot of people say Diesels like to be worked hard, but not thrashed.
You say it is the Prop which moves the boat. That is correct as far as it goes. However it is the engine / gearbox which drives the prop. To get the equation spot on is a very technical thing and most yachties don't know the first thing about it. There used to be a very good Prop man in Southampton who ran a company Power Propellers The other person who I would listen to on marine Engines is a Mr Bellamy at Lansing Marine.
 
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