long keel rudder mod?

Lazy Kipper had a 40% increase in her rudder area to cure weather helm which was just silly - a bowsprit and proper sails cured it - but it left us buying a boat with an oversized rudder - what we find is that hard over in ahead is good, and not much more than half over in astern.

Then we had enough and fitted a bow thruster. It's great, we now have a degree of influence, like calling a dog 30 metres away we have a chance of getting it to do what we want in astern, but it's only influence - on a long keeler you've swapped marina parking ability for a whole load of other benefits. The answer is always prop walk, we have so much it's like having a pair of stern thrusters, then the bow thruster comes into its own, you start the turn with the prop walk and carry it on with the thruster - very efficient. If the marina berth is the wrong side for your prop walk then ask for a swap, it's what we did in our last two places and they were very happy. We can tuck the stern in to starboard in ahead with ease, add some thruster and we go sideways - brilliant. But there is no relaible way of tucking it in to port at slow speed.
 
Hi,
My liz 29 is very si
milar to Ken's old Twister, and I can echo his comments. One thing I didn't notice mentioned is the wind, the boat will weathercock with her stern into the wind, so if it's strong this needs to be remembered. I try and work out, between prop walk (she turns much better to starboard than port due to LH prop) and wind, just what the boat wants to do. Then fit that into what I want her to do. We have always got into and out of a berth, eventually, although rarely conventionally.

One good thing in your favour is a large prop aperture: my problem is that the prop tips are very close to the edge of the aperture which is not good for motoring.
 
The rule of thumb is that a rudder should be 15% of the underwater area, but like all such rules you will find many boats deviate from that. It may well be that your rudder is too small, although you would notice it more if control was poor when sailing normally. As many have said, the rudder has very little influence on control in reverse because it is no longer a foil and the major influence on where the boat goes is from the prop interacting with the keel.

You have already identified another major drawback to your design of boat - as TK explained earlier - the aperture for the prop. You need 15% of the diameter of the prop clearance from the tip of the blades, so for 15" prop the hole needs to be at least 20" top to bottom. In practice, if you have a raked rudder post you need even more clearance otherwise the blades can hit the rudder - probably why your boat has rudder stops.

The suggestion to increase blade area forward of the pivot line is a good one, but usually impractical on long keeled boats because the rudder stock is hard up against the back of the keel. I expect your rudder is elliptical shaped and you may find changing the shape by increasing area lower down may help lighten steering loads. If you can see the rudders on a Tradewind 35 compared with the earlier 33 you will see the difference. However, it will have little effect on steering in reverse for all the reasons given.

As a matter of interest when I redesigned my long keel boat with a 9" deeper keel, I redesigned the rudder to give it a higher aspect ratio - that is retained basically the same width but increased the depth. My rudder has balance because the keel does not come right to the stock, but I lost some balance area because I now have a feathering prop, so need extra clearance when the rudder is hard over. The new rudder improved sailing performance, mainly in reducing weather helm, but did nothing for reverse. Like others I have learned to go with the flow, knowing it will only go to port from standstill.
 
Beware of increasing the area in front of the pivot - this can help with balancing but a local Colvic Watson overdid this and made it almost unsteerable - a small turn of the wheel would turn the boat 180 degrees.
 
Richard D said:
One thing I have noticed is that the prop hole seems enormously big to miss the prop, is there any dimension stated anywhere to say how far away it should be or should it be just enough to not foul on full rudder as if as mentioned by someone the main problem with long keelers is hole in the rudder for the prop, maybe reducing this whole could be a benefit.
Regards Richard
Click to expand...

My long keeper had a cut-out behind the prop which later boats did not have, so filled it in. The result is a better balanced rudder and noticeably better steerage forward and reverse. The rudder on mine is angled, so at that depth it was adding mainly to the front end. Didn't seem to affect any prop performance even though the 'books' say leave some space. If you do want to increase the size of the rudder then filling in the cut out may well be a good place to start - you can always cut some back out again if it isn't right.
 
Richard D said:
I know evertyone says long keels dont do reverse but feel mine is worse than normal.
Click to expand...

With a long-keeler, 'normal' = very bad!

Anyway, good luck with your proposed modification but I reckon that if long-keelers could be made to steer astern simply by enlarging the rudder then most of their owners would have had it done by now.

It is worth noting that the owners of Twisters, which are among the worst 'offenders', tend to keep them for a very long time so there must be compensations. I must confess that I have sometimes found myself in embarassing situations during the last 16 years when I have muttered "Sod this for a lark - I'm going to sell this bloody thing". However, I can't remember ever having damaged anything, except my dignity, when manouvering astern. :)
 
Interesting comments. The previous owner of our long keel Roamer 111 did exactly that.................increased the size of the rudder by some 40%. Don't think it was to increase steerage going astern, but if it was, and it helped, then I cannot image what it was like before :D Going fwd we turn more or less in our own length. Going astern, well, she decides which way she is going to go. Give her a burst of power, back to tick-over, then put the rudder over which way I want (intend) her to go. Sometimes she responds, sometimes she does not. In which case I take the power off and start again with the rudder on the opposite side thinking she will then go the way I want. Sometimes she responds....................................................
and sometimes she does not :D Oh joy of joys :D
 
Richard D said:
My rudder will only go to 45 degrees each way, is this normal and bearing that in mind does that affect the comments about if you put the rudder on full ,lock that it causes the flow to stall?
Click to expand...

I believe 30 to 35 degrees is the accepted maximum rudder deflection. Any more and the rudder could be working against you ie stopping you rather than steering you.
 
Hopefully you spend much more time going ahead than astern. Be pleased she's a long keeler at least you are less likely to get caught up in the Lobster Pot thread over on Scuttlebutt :)
 
It depends on a number of things. I have a long keeler, 28' with the rudder hung on pintles on the transom with a notch out of it for the prop. Reverse is tricky and you have very little steerage if you try to reverse any distance. I find that the best way of adjusting the angle of the boat is to use the propwash on the rudder in forward. So, you put her in reverse until you have a little movement (you can try to steer a bit with the tiller) then you put her into forward for a quick blast with the rudder hard over. In essence this works as a kind of stern thruster. With this method you can turn the boat on a sixpence without actually moving anywhere. It becomes a bit more tricky in a wind but then long keelers are slow to blow off. I would think very carefully before adjusting the rudder. If it is behind the prop, like mine, then any extension will give you enormous force on the tiller when under motor and will increase your weather helm.
 
I am bit confused with some people saying the gap around the prop should be small, others saying a small gap improved all aspects of rudder control and another saying that a close prop is not good for motoring. Hope Evadne can reply to say how the small clearance effects motoring and in what aspect, more revs for given speed, not as much control as when under sail, more fuel used.
I must say that in some circumstances when tacking the rudder has failed to make the boat turn, so maybe the rudder is too small to control the boat all the time.
Some say a bigger rudder helped weather helm , others it made it worse.
Think I am seeing a bit of light and pick and mixing ideas.
The good comments about eliptical rudders unfortunately does not stand as my rudder is basically square with a bit on the top that follows the contours of the hull.
Think my first step is to make as good an estimation of my hull area as a ratio to my rudder size and then make a decision on a bigger rudder and maybe close up the prop to rudder clearance.
Be pleased for more comments.

Regards to all contributors Richard
 
45 degrees of rudder movement is plenty. (Our long keeler's is the same or perhaps a little less.) You'll usually want to use less than that, particularly going forward.

I disagree (from experience on our long-keeler) that putting the rudder hard over is always ineffective. It is necessary to experiment to see what works and when. As i said before a long keeler's rudder is not an independent foil, and the normal rules don't always apply. (+ A sail with the wind flowing over it at a shallow angle generates more lift than one at 90 degrees to the wind (all other things being equal), but the latter stilll generates significant lift.) On a long keeler at low speeds in reverse, where you can't get a smooth flow over the rudder, and in any case the flow is over the combination of rudder and keel, not an isolated rudder, you can get a marked and helpful effect in the right situation from having the rudder hard over.

I would encourage you to try enlarging the rudder, but suggest that extending the depth over as much as possible of the length of the rudder in preference to increasing the greatest depth (i.e. if the rudder aft edge is like a semi-circle, extend the top and bottom to square off the shape a bit rather than simply having a deeper (fore and aft) semi-circle). While a deeper 'unbalanced' rudder (i.e. one pivoted at it's front edge) will tend make the steering heavier, I doubt that it will have as marked effect as some might suggest because (i) with a bigger rudder you'll not need to put it over as far for the same effect, and (ii) the sideways lift from the rudder is about a third of the way back from the leading edge, so a increase in the (average) depth (fore and aft) of the rudder of, say, 6 inches will only move the centre of lift back about 2 inches.

With regard to the cut-out for the rudder, I believe that a sideways clearance of 15% or more of the diameter of the prop is the usual rule of thumb. But before you fill in the cut-out....

Another key factor in the manouevrability of a long-keeler is the power of the engine. The amouint of prop-walk and prop-kick you can get, and how much distance it takes to get up to a speed where the rudder will work 'normally' in astern is directly related to that power. Unfortunately traditional long-keels tend to come with traditionally modestly powered engines. You are unlikely to want to change your engine just to improve your astern manourvrability, but it would be worth checking out (a) whether your propellor is well matched to your boat, engine and gearbox, and even if it is (b) whether the engine/gearbox (and space in the keel and rudder cut-outs) could allow you to swing a bigger prop which would give you more bite at low speeds (albeit at a cost of more drag under sail). Also if you have a two-bladed prop consider a three bladed one for similar reasons.
 
Richard D said:
I am bit confused with some people saying the gap around the prop should be small, others saying a small gap improved all aspects of rudder control and another saying that a close prop is not good for motoring. Hope Evadne can reply to say how the small clearance effects motoring and in what aspect, more revs for given speed, not as much control as when under sail, more fuel used.
I must say that in some circumstances when tacking the rudder has failed to make the boat turn, so maybe the rudder is too small to control the boat all the time.
Some say a bigger rudder helped weather helm , others it made it worse.
Think I am seeing a bit of light and pick and mixing ideas.
The good comments about eliptical rudders unfortunately does not stand as my rudder is basically square with a bit on the top that follows the contours of the hull.
Think my first step is to make as good an estimation of my hull area as a ratio to my rudder size and then make a decision on a bigger rudder and maybe close up the prop to rudder clearance.
Be pleased for more comments.

Regards to all contributors Richard
Click to expand...

Yes, there are a whole load of conflicting issues here. The slowness in tacking is not necessarily due to the rudder being too small - may be just the long keel or not enough sail area forward. So a short bowsprit moving the sail area forward may be useful.

Motoring performance can be limited by a small aperture because the prop wash is working in a confined space. A wide keel and poor waterflow into the prop may also be an issue. You will often see the back of the keel deadwood scalloped away to improve flow to the prop. A large diameter 3 bladed prop is better than a small 2 blade. On my boat I swing a 15" prop with a small hp engine by having a bigger reduction ratio. The more normal 13" prop for the engine size would be partly masked by the wide keel.

As others have said, you need to get away from the idea that the rudder is a major factor in steering the boat astern as it is not - and more angle reduces its effect rather than increases it. Think of it more as a trim tab on the main foil, which is the keel. Things are very different on a spade rudder boat where the rudder is a free standing foil, usually at the aft end of the waterplane and the boat pivots around the high aspect fin keel. Once there is waterflow over the foil it can be used to steer the boat.

Be wary of increasing the area by reducing the aspect ratio - that is widening the rudder as you can easily end up with a heavy helm.
 
A larger rudder will not help the heavy weather helm that occurs when the boat heels a lot but it can give more power to avert a round up. I increased the rudder area on my little boat to give more authority when running a shy spin in strong winds and it helped a lot. Witha canoe stern I( wonder if OP would consider an additional transom mounted rudder a bit like a self steering rudder. This could be ideal for an autohelm to operate on a tiller. And of course if he doesn't like the additional rudder then it is easily removed.
The 12metre Americas cup yachts always had a rudder at the aft end of the keel and they called it a trim tab. Then a real rudder further aft. I wonder if windward performance can be improved by use of this trim tab to get the keel to give lift. (like a wing flap)
I noticed in the link given earlier that in fitting a spade rudder they filled in the old rudder space on the long keel. I would like to have tried the old rudder as a trim tab.
Anyway for a final mad suggestion. I think it is possible to buy or build a variable pitch prop which can go to reverse pitch. if you fitted one of these. I think the old Sabb engines used them then you could use you existing engine and gearbox to give Rh or LH rotation (forward or reverse) then select the direction of travel you want from the prop pitch. ie selectable prop walk to port or starboard when going backwards. good luck olewill
 
Current PBO reprints a story from ages ago about Denny Desouter modifying a rudder to better balance the boat (not specific to going astern) but which is worth a read on general principles.

In your case, as the boat is a one-off and not from a recognised yacht designer, it does sound as if there's a good chance the rudder arrangement was 'best guess' rather than being based on any recognised formula, or refined from trial and error (aka the customers' research department) with previous models, so why not try some mods?

However, to revert to being dogmatic, don't ever expect it to steer like a Mini Cooper when going astern, and it's certainly not worth messing up ahead performance (99.99% of the time) for astern performance.
 
Regarding prop size that is certainly something that surprized me when I saw its size, it is a quite small 3 bladed affair, I did measure it one time but have forgotten now, would estimate at 12 inches, that said the boat will happily cruise along at 5 knots at 1500 rpm with my Perkins 4/107 but must admit that it takes a while to get up there from a standstill giving it the 1500 rpm from rest. The comment about the wide keel could be a factor as my keel ballast weight is about a at the front bottom edge and tapering to about 8 inches at the rear a freeing of mine has taken photos and is going to put them on the site, as am away in the wilds, hence not being able to reply as much as I would wish as internet is not good where I am, so would be grateful for comments after the rudder and hull shape can be judged.

Richard
 
Expect the prop is small in an attempt to limit its effect on sailing performance and reduce the size of the aperture. Another of those trade offs. A cutout in the leading edge of the rudder to clear the prop has a significant impact on the effectiveness of the rudder. Imagine the hole created when the rudder is turned and the loss of a clean leading edge. If you look at old style long keel racing boats in the days before engines were fitted, you will see relatively small rudders because they were efficient without a cutout for the prop. When engines were fitted it was common to use props coming out of the quarters, partly to preserve rudder efficiency. Such boats did not go backwards either - only in circles one way!
 
You have to appreciate the relative effect of the rudder. If you are not tacking well then it is more likely due to unbalanced sails. We had this before I bought a new genoa as the old one was just a second hand mismatch. The sails and keel will always win against the rudder. What you need to do is ballance the boat and then determine the effect of the rudder. It also depends on how your rudder is hung. If there is some rudder forward of the pivot then the tiller can sometimes snatch away from you increasing the angle of a steer. This is much more obvious under steam as the prop forces water onto the front of the rudder. Normally the opposite will occur with the rudder wanting to return to a neutral location. If it is too large then, especially under steam, it will do this with force.

I suspect you may be expecting too much of your rudder although as a one-off it could be slightly out (I expect most boatbuilders would have researched the style). On a long keeler there is no reason why you can't manoeuvre very well but you have to use various 'tricjs' rather than expect it to respond to the rudder like a laser. I looked into all this when my tiller snapped due to rot and weatherhelm and found that it was the ballance above water that was the biggest factor.
 
There's an article somewhere in the Rustler Owners' site describing how someone increased the size of a Rustler 36's (iroko) rudder, apparently with good effects in controllability astern. On my own 36, I discovered that the prop type has quite a big effect. She tends to go straigher for longer with the current prop (a three-blade Darglow Featherstream), though this would be a costly approach unless you were replacing the prop anyway. I also have a bowthruster, which I seem to use about twice a year but which is highly effective if I can't solve the problem any other way. A light hand on the tiller, lots of practice and plenty of fenders and warps to hand will usually enable you to get where you want to be.
 
Don't know what happened to last post but words have not come through. What I was saying about the keel was it is 12 inches wide at the front. A "friend " of mine is going to post photos of the rudder and keel. Regarding the rudder shape as said before it is squarish and its top edge is some 12 inches plus below the water line and totally unlike all long keelers I have seen, which all have elliptical rudders which go from sea level to the keel.

Richard
 
You must log in or register to reply here.

Share this page

Top