another rudder/tiller project

[Murray Campbell]

hi

there have been a few threads lately about rudders and tillers...hoped i could ask some advice too, while we are there

-boat....william atkins, colin archer flavoured with norwegian rudder

-current rudder is plywood and delaminating

-i had planned on epoxy laminating some sheets of marine ply, sheething in glass, using stainless steel plate for the cheeks and making a hardwood tiller...not sure what the best combo of plywood thickness / # of laminates is best (fewer better?)

-should i be thinking about piecing one out of solid wood? any advantages in strength or cost? ...i don't have a jointer or a lot of cash but i want something solid i can sail hard

-also i want to save the carving on the end of my old tiller and i was thinking of scarfing it onto the new tiller, maybe one of those scarfs with a notch in it...scary??...

thanks!

[Art Read]

Bump... (Seems too good a question to ignore. For what it's worth, I think it sounds like you're on the right track. Laminated, encapsulated plywood certainly seems to be a popular approach, though drifted hardwoods have served well for a long time. Seems to me an odd number of laminations, (3?) makes the most sense for stability issues. And I'd go ahead and scarf on your fancy tiller head, what the hell? But I'd also carry along a "beefy" spare that can be installed QUICKLY... just in case and just on general principle.)

[ 12-10-2003, 05:07 PM: Message edited by: Art Read ]

[Noah]

I built a ply (two layers 3/4) rudder a couple of years ago and glassed it all. This was for my Nordic Folkboat. It was only in the water 1 year with the new rudder, but it seemed great, and after 6 months there were no problems. I built the Cheeks out of 1 1/4 Mahog and drifted them together.

Good luck.

[Jack Heinlen]

What is the thickness of the rudder?

I'm not sure the number of laminations in the ply would make much difference one way or the other, but I'm wondering what thickness ply you are going to use that has only three plys. I'd want to use as few pieces possible to get the desired thickness. Good marine ply manufacturers are (usually) better at laminating than you are.

I don't have any experience with this, but my pants seat says more plys in the sheet goods would be better. But I don't know.

[Bruce Hooke]

As a general rule, higher quality plywood has more layers than lower quality plywood. Increasing the number of layers reduces the impact of a defect in one layer, and reduces the stress each layer exerts on it's neighbor. I think you may know this and may have been asking about the number of layers of plywood overall. There I would certainly agree with you (and Jack) that as much as possible you might as well leave as much of the gluing as possible to the plywood manufacturer.

Both plywood and solid wood have been proven as viable solutions for rudders. I will leave it to others to discuss the relative advantages of each. Metal cheeks are also a proven solution. So, it seems like you are on a reasonable track.

That said...rudders and tillers are highly loaded parts that are critical to the safety of the boat (imagine yourself sailing to windward of a rocky shoreline in a strong wind and suddenly having the rudder fail :eek: ). As such I would be inclined to either:

1. Follow the existing design closely except in places where weaknesses in the design have become evident (e.g., not sheathing the plywood led to it delaminting).

2. Or, where changes are necessary or desirable carefully engineer the changes, taking into account the loads involved and either calculating the correct dimensions and materials for a given component or determining the dimensions and materials via close comparison with similar designs and documented standard practices (for the most part reference to standard practices and past designs is the more viable route). Changes like sheathing the rudder are obviously a good idea and not a big deal. Changing the material used for the cheeks may well be a good idea but needs to be planned carefully to avoid the risk of failure at a critical moment.

3. Or, dig up what the designer originally specified and follow those plans.

As to cost, this will probably, to a significant degree, depend on factors like how efficiently the shape fits a sheet of plywood (e.g., can you get everything out of one sheet or do you need three sheets a lot of which will become scrap), and the cost of the possible materials in your area. Some quick calculations should tell you the relative cost -- it's not like we're talking about a lot of material here. Don't forget the cost of the epoxy.

[Murray Campbell]

thanks for all your thoughtful replies...i'm leaning towards minimal layers of good marine ply sheathed with glass and a million dollars of epoxy

i will also be fashioning a bracket for a self-steering trim tab...the gudgeons for the tab will be stainless, i am picturing long straps on either side of the rudder, through-bolted...i'll try to get some epoxy in the edge grain at the bolt holes...it's kind of hard to imagine how heavy these have to be, but i imagine there could be quite a bit of torque out there

guess its better to overbuild

( ...might be the wrong forum for this, but does anyone know what percentage of a trim tab's area should be forward of the axis to give it a good balance?)

m