suggestions for a new rudder

so im at the point in this build where the aftermost bulkhead is to be tabbed in. its located just in front of the rudder tube (solid fiberglass from hull bottom to deck - about 38"). im just planning delrin/UHMWPE sleeve bearings top and bottom for the solid stainless 1 1/4" shaft. tiller steered.
the rudder that came with the boat was poorly glassed so ill grind off the shell and inspect the grid and welds. im wondering if there might be a better shape/foil section when i re-glass it. elliptical seems to me less important than area and balance to make the tiller as light as possible yet maintain a powerful helm. so what percentage of the area should be ahead of the shaft to achieve this? other than catching kelp and crab pots, any concerns with a more squared-off shape? foil section recommendation? and while im at it, any ideas how to incorporate rudder stops under the deck with a continuous tube...and for that matter, an arm for the autopilot?the boat will sit on her lines at about 18,000lbs.
thanks as always

 

I agree that the plan form of that rudder could use some updating, though I question the need for this and the other changes, unless you have some sailing experience with the yacht and are looking for improvements. Personally, I've always disliked that style of partial skeg arrangement. They're easily damaged and unshipping the blade is a pain in the butt.

The amount of area can be calculated to a degree, but she appears to have sufficient area. The amount of balance is a bit of black magic, though some very general rules can be applied. The same can be said of the sectional shapes employed, though these are often governed by the performance envelop expected from the boat.

It's also likely the yacht received "model run" changes during it's production run. This would be a common thing to do, if a yacht was experiencing steering and/or control issues or if the designer felt she could use an improvement.

Is this your 40' cutter project?

 

hi par,
yes, my boat. its never been in the water and ive only sailed on one other and that with a wheel. aside from the heavy disconnect in feel induced by the small wheel, she handled well...suprisingly well for the long keel. what little info ive been able to get from other owners has been positive as to the control and resposiveness of the helm. all to my knowledge have been wheel steered. also, the vast majority of the yorktown 39s where center cockpits, only 9 in the aft cockpit configuration and none of them with tillers that i know of.
i cant say im trying to cure any particular problem. but before i fair and re-install the rudder im going to have a look inside, and figure why not optimize if its warranted. as i intend to have a tiller, seems like particular attention should be paid to lightening-up the helm pressure as much as possible. i was hoping there was a somewhat standard ratio of area ahead/behind the post to help me zero-in on the new shape?
ive attached pics of 2 other Y39s who have modified their rudders, both c-cockpits w/small wheels, both done for strength rather than performance, both report no noticeable difference in the helm. i dont think it would be ideal in my situation with a tiller.
the partial skeg has got to be more secure than a spade, no? three bearings and all, and the bottom bearing could be left exposed for easier service. im more worried about the gap catching floatsom and jamming. any thoughts on that issue, maybe a guard of some sort?
thanks as always

 

10 to 20 percent of the total area is typically used in the balance portion, with 15% being fairly common, but again, it's a crap shoot, as to actually how much you need. It's unlikely you'll know, until sea trials, so the wise thing might be to give it 20% and chop off what you want, until helm feel is right. It's easier to subtract than add, in other words. I don't think you'll need any balance honestly.

 

I'd like to challenge your assumptions a bit here. Why do you think force-balancing the rudder is a good thing?

Have you ever tiller steered a boat this size in open water? You don't need to worry about balanced feel so much as having a long enough tiller and still having some feel. For heavy conditions, 1/4 inch chain passed around the tiller three times and secured to the secondaries works for a tiller stop. A tiller-minder autopilot can be fitted to the tiller, as can vane steering. Nothing in the boat should be capable of damaging the tiller. Figure the breaking load of the genoa sheet and design the tiller to handle it. I've seen a tiller strain out two crew and a pair of full propane tanks and an outboard motor during a swamping and near knockdown. I'd make it about 3 1/4 dia at five feet out from the stock and shaped like a banister railing.

On tiller-steered boats, a balanced helm is a pain in the butt. You have to push and pull on the thing. It is much easier if you can sit on the low side with your feet on the tiller pushing hard, then not so hard, then hard again rather than trying to push, then pull, then push.

For cruising, I think you need to look for ways to unbalance the-on center force. A better way to say this is you want the torque to be sensitive to smallish changes in sail trim. Racers with racing sails hate this because they trim the sails every wave. Cruisers with cruising sails might think about trimming sails every couple of days or so. They trim sails more for helm management and lowering power draw of the autopilot or so the cook can work.

So the cruiser wants an unbalanced helm. It is sensitive to small tweaks in sail trim, aids in sailing the wind as opposed to sailing a racecourse (particularly with vane steering), and lets you set the helm up for different conditions more easily. You can balance the weight of the tiller at different heel angles, for instance. That is important if you start talking about 7-8-9 foot tillers. Trust me, having to pull on a tiller even a little bit when you are on the high side at 25 degrees heel will kill your back.

In short, you want all the positive feedback that you can get. You can trim the sails to control the on-centerline load, but only if the rudder strongly responds to this. Otherwise, you have to trim the sails to the point you lose drive. This ain't a dingy. It will work you to death if you don't make it responsive to small sail trim changes. This positive feedback is what allows you set the boat's helm up for different conditions and still sail reasonably well. Basically, if you do the opposite to what the weekend racers do, you will cruise with less effort. The goal is to go 90 percent as fast with 2 percent of the effort in any conditions.

 

outstanding. this is a perspective i hadnt even considered and the reason i love this forum.
ive steered plenty of big (relatively) boats offshore in all kinds of weather with a tiller, but they have all been raceboats. i have in my head that the lighter the steering forces the LESS work it takes for man and machine, but now completely agree with your assessment. when i think back, ive rarely had to stay at the tiller for more than an hour, all the while a bunch of guys grinding away keeping us full, and yeah a bit tired and that with a neutral helm!
this boat will cruise. vane and tiller pilot, 5' MAX tiller. of the two pics, which rudder shape would you chose for a full skeg ?
and thanks again both.

 

I've some big boat tiller time and helm balance is key for them to work. One 37' boat with a deadwood hung rudder was a bear to use in heavy weather, easily overpowering the skipper. Another with a similar arrangement but a 2' longer tiller was very manageable. Most that I've sailed have had some balance in the blade, though also were spades, with reasonable aspect ratio and good length tillers. My point is, tiller length and blade balance is a matter of some speculation, unless you calculate loads and leverage. On that rudder, I'll bet you'll need some blade balance with a 5' tiller. How much, well this will be a guess without some number crunching. Phil does have a point about a stout tiller and blade balance, but it is a set of compromises that have to be juggled, to get an acceptable helm feel, without burning down the skippers arms. This is why I suggested a good bit of blade balance to start, which can be trimmed down as you find an acceptable helm feel, given a limited tiller length.

The aspect ratio on the current blade suggests it'll need some leverage. Since you're tiller length is limited, aspect ratio and/or blade balance may need adjusting. One of the modified blades shows it's plan form and aspect ratio have be increased. It appears to be the same area, just taller, which would improve efficiency, easing helm pressure and drag. I don't think the elliptical plan form is best in your application, mostly because you lose a fair bit of area, just where it's most effective, below the skeg. I'd opt for trapezoidal tip shape, increasing area low, probably with some balance and yes picking up some tip drag (eddie making), but likely overcome with the increased lift from the extra area there. Playing with sectional shapes on the tip can also reduce some of the turbulence associated with a fatter tip.

The sketch shows about as much as I'd do. The removed area aft is now at the bottom and in the balance, improving aspect ratio and offering balance to be hacked off, if necessary. The plan form will be less likely to cause much tip turbulence too. Lastly I've closed up the gap at the top of the blade, which should be as tight as practical.

 

thanks again par, thats about what i had in mind. after some reading, im assuming naca0012 section over all, including much of the skeg. ill have to fill quite a bit as the skeg itself is quite thin. area, shaft dimension etc per gerr.
so ill have to fab a custom bottom (split) bearing. should the shaft pass through and continue into the full-foil lower section of the blade to provide support, or possible terminate at the bearing and allow for a 'break-away' section in case of collision or hard grounding? any trouble fabbing rudder post, bottom bearing and propshaft from the same alloy...and should it be 316L, aquamet or something else? just trying to keep all the underwater metals similar. thruhulls will be composite. and curious about the water that gets past the middle bearing and lives in the rudder tube...is enough of it recirculated that corrosion of the SS shaft is not a concern, or do i need to open up the gap at the middle bearing to increase the exchange?

next time im in Fla ill paint your house...

 

You do not have to use the same foil section from the root to tip to have it perform well. If the longer section is thinner, say a NACA 0008, and you transition to the 0012 where the cord length is shorter, it will work just fine-better in fact. It will have less drag and be more stall resistant than having 0012 from root to tip, and save you the trouble of having to build up the longer cord. All it needs is a smooth transition. This is commonly done in many applications, including aircraft wings and rudders.

On your rudder tube you will want different alloys where there is relative movement between the parts. IOW, where you have a wear surface you must have a large hardness difference between the two parts or you will get a lot of wear and galling. A Rockwell hardness difference of about 10 points or more is generally considered acceptable. That is why you will always see a soft metal as a bearing material and the rotating part being much harder steel or cast iron. Use bronze bearings and stainless steel shaft for long life and minimal problems between moving parts. You might also check out the corrosion potential between dissimilar metals, many very different metals are fine in contact with each other, and others will trigger pitting and corrosion almost right away. there are alloy compatibility tables you can find on the internet and in machine design texts.

good luck with it, it looks like a worth while project.

 

thanks petros. as for bearings i plan UHMWPE or visconite as the wear surface holding the metal bits apart, so the thinking was to have all the alloys identical, eliminating the metals talking to each other through the water. what is the preferred alloy below the waterline in salt water?

 

Monel, bronze, 316L stainless, 316 stainless, in descending order.