Rudder Design for a Small Schooner

[Harbormaster]

Two basic questions.

I've got a little old schooner, and I think I need to make a new rudder for her. What do you think'll make the best design? Since the water comes rushing up the hull following the run, under the counter, does it make sense to have the biggest part of the rudder up high (like #1)? Or, since the water is effectively "thicker" the lower one goes, is something like #2 more effective? Or is a coaster style rudder like 3 better, but does that create too much wetted surface area? I've never sailed her, so I can't justify a bigger rudder to balance the weather helm, etc. and then of course sometimes she'll be under power....

Question 2: How is it best to attach the wooden rudder to the Bronze rudder post? The old one was originally bolted all the way through and they added some stainless straps around the front of the post and back partway on the outside of the rudder.

If I make the new rudder a little thicker on the forward edge I can cut dadoes (grooves) for the strapping into the wooden rudder, but should I replace the old ones with bronze ones that are brazed to the post? And skip the through bolts?

[paladin]

I'd go something like number 2, with the leading edge a bronze tube/bar stock, with a flat heavy plate to the rear to laminate the wood around the poust, shaped, followed by multiple layers of xynole fabric (not glass), trailing edge of rudder left square by 3/8-1/2 inch, then start the line outlining the rudder foil shape about 6 inches in front of the rudder, using the stock as the widest point, and shape the foil to the trailing edge. I could lay out the foil if I knew the dimensions.

[Ian McColgin]

What is wrong with the old rudder? Mechanical problems aside, how well does the old rudder steer her? Different boat shapes require different rudders and without some understanding of the existing dynamics of rudder and hull a rational answer is not too likely.

I don't understand how the rudder is attached now. I'd not rely on bolts through the rudder stock but rather strapping around the stock and back, through bolted strapping through the rudder, brazed around the stock.

I like rudders built up of oak planks, drifted on a classic pattern, as about the strongest you can make but plenty of good rudders have been made of plywood glassed over.

I'm not convinced that you need to let the straps into the rudder, though that will look nice. Given the way stresses work on a rudder, I don't think you need to worry about weakening the rudder by the trough for the strapping.

[rbgarr]

He said he hasn't sailed the boat yet.

[Ian McColgin]

You are right and I'd read carelessly. The question remains, why the change? Also, someone has sailed the boat, maybe even someone living and discoverable. I'd be loath to change a major design feature without considerable understanding as to what's happening now and what might happen with the change.

[Harbormaster]

Paladin, I plan on laying it up out of oak planks, I'm not a fan of laminations and glass, though it may make for a smoother rudder, and if I didn't let the straps into the rudder, it might be easy to design it so the blade could unbolt from the post. I suppose I could glue it up in plywood and glass that. I don't know that I want the rudder glassed around the post.

Ian, I want a new rudder because the old one is ugly? Splitting and coming apart. The shape maybe okay, but I don't know that the builder was a great designer, and may have just shaped the rudder the way it was because that's just how he did it. I'd like to let the straps into the blade, fewer bumps for the water to have to run over.

[Roger Long]

Rudder number 3. Don't worry about wetted surface unless maybe being 10 feet farther ahead after a 10 mile race against an identical boat is a consideration. You could make this rudder twice as big and never notice the difference in speed sailing without a trial horse. It's nice to have lots of rudder maneuvering in light winds and tight quarters. If it's too big, you just carry a little less helm angle in the same conditions which isn't bad.

The profile at the top of the coaster style rudder will let the hull work a bit like an end plate and help trap keep that high pressure region from spilling over the top of the rudder. This won't be important most of the time but, if you are ever trying hard to fall off in front of a squall with the rudder working to the max (think "Pride of Baltimore") , you'll be glad it's there.

There is so much flow separation and boundary layer by the time you get to the rudder of most traditional craft that nothing is very critical from the drag standpoint. Straps flush or proud are strictly a question of how you want people to feel when you are showing the boat to them out of the water. The conventional bolts running through the stock are one of those wooden boat things, like caravel planking, that just should not work from an engineering standpoint but do. Engineering could prove that 95% of the wooden boat rudders ever built fell apart the first day.

Instead of straps, I would put welded cheek plates at top and bottom. The upper ones I would just put on top of the wood for maximum strength at this high stress area. The sternpost may swell out there a little anyway. The lower set could be let into the wood just so you'll feel proud when looking at the hauled boat. With that first plank held top and bottom and the cheeks transmitting the torque, the through bolts holding everything else together will be working properly, like keel bolts, after the rudder swells up.

[Peerie Maa]

I agree, keep to the original profile. That said I don't like 2 at all, 1 looks best, then 3 . As to the straps, change to bronze, get rid of dissimilar metals. I would use the through bolts to attach the straps to the stock, that is how the local boat yard built their rudders.

[wizbang 13]

I've never had a boat that I did not goose the rudder in some way.
One thought, build the fwd part very strong, no reason you cannot make it the same thickness of the stern post.
As the blade goes aft, strip plank it of thinner, and lighter wood. taper the strips thinner as you move aft.
In time, as years go by, you can lop off, or add on, blade area, without mucking up the super strong forward part.
The trailing edge of the rudder can be very thin, strip planked.
(not too cool on the use of oak).

[Harbormaster]

I don't know that I can get the post out (or in) if I really weld/braze straps or cheek pieces to the post. It slides straight in from above, not up from below. I'd need to disconnect the rudder blade from the metal post and have nothing left on the post if I want to get it off. Can I bolt on the cheek pieces to the post too? Or should they be more like straps?

[Paul Pless]

are there pics of this schooner anywhere?

[wizbang 13]

I mis read / speed read, too. You are not a fan of laminating.
These old fashion rudders are the REASON I jiggle with mine.
Running straps and bolts and drifts all the way aft is a recipe ( imo) for disaster.
How small a boat? Smaller boats need proportionatly larger rudders , cuz they are slower.
If you build the rudder with those bolts like that, you will be locked in to that . Future modification will be tougher.
A strip planked rudder will be stronger, more stable, easier ( possible) to modify.
Picture your boat , offshore in a gale, hove to , sliding slowly backwards. A whopper wave comes by, the boat gets pushed astern, the rudder takes a huge load, from astern. The rudder as drawn will survive 100% or break off at the post. Well,maybe it will just bend.
A strip planked boat, as I describe, will have a" breakaway" part, ( aft 1/3 approx). One can make it back to port, ( or around the world) with 2/3 of a rudder.
This opinion is 'specially relevent cuz you say the schooner is small.

[johngsandusky]

Two thoughts: If the rudder is dry oak, previously saturated, you'll be surprised how much it swells when wet. Two of my old boats would dry til you could put your fingers in the gaps, the swell up smooth and tight. The other point is the waterline. My current rudder comes right under the surface, with a similar shape to #1. waves tend to knock it around, making for hard steering in a sea.

[Harbormaster]

Here's a picture of my little girl - she's about 33 feet:

[Ian McColgin]

Love to see a profile shot of her stern. Looks like a nice little darling.

I have a prejudice against chain bob stays. For a give weight they are weaker. They are more prone to elongation followed by surprise failure. And every link is two major sites for electrolosis, corrosion, or both. I personally go for simplest approach with no turnbuckle or toggles or such. Carefully size the bobstay such that you can just get the jaws over the eyes at each end if you steve the bowsprit down a bit with a few hundred pounds of weight. That way, when you tension the jib stay it'll come out about right. But that's just me.

[Sayla]

In professional boat builder mag (www.proboat.com) David Gerr did a sequence of articles on steering systems, including rudder design (Issue 98,99,100) - very authorative - I used to be able to access it for free online, but that seems like it may have changed at the moment - maybe I just can't find the right button again - great articles if you can get hold of it

Design wise you standardly get more bite from area down lower (the more distant aspect), but, if you have the benefit of a hull above the rudder then that traps flow (& pressure) as well and so that's effective also - if the hull above the rudder is indeed in the water, in which case No.3 is looking good, but if the hull doesn't trap the upflow of pressure then No.2 is more effective - that all being said, if the rudder is big enough it'll work anyway, but optimum design is always better than drag/wetted surface etc

Sayla