thinking ahead to my rudder project

this fall im getting back to the cockpit build that was put on hold for want of a new rudder. itll roughly follow the shape suggested by par in the attached picture. three bearings and tiller steered. the bottom bearing id always envisioned to be the typical split assemble through bolted at the bottom of the skeg. im wondering now if G10 would be a possible substitute for bronze in this application. ive got room to oversize the assembly if i need to for strength, and id still use a UHMWPE bushing for the wear surface. i like the idea of eliminating as much metal as i can below the waterline, and ive got several large blocks of G10 looking to be machined into something useful. any ideas on the relative strength of G10 vs sillybronze ? any suggestions as to how much id need to plus up the dimensions if using G10 for a bottom bearing ?

 

I like the idea of UHMWPE for bearings and G-10 can be useful too. As to dimensions, well this is a real crap shoot, without a lot of data you probably don't have.

Truck it on out here, we're get her squared up and good to go

 

thanks for the offer, but im just having too much fun building this thing myself. a quick search turned up tensile (yield) strength of silibronze = 60kpsi and G10 = 38kpsi. of course metals have much more elongation before rupture than glass does, but for a bottom bearing that might be a good thing. id think given a choice, id want the bottom bearing in a three bearing system to break rather than bend out of column possibly jamming the rudder.
like i said ive got room to oversize the part in G10, maybe up to 1" around the shaft butt and as tall as i want, say 6" or so. but i cant say ive ever seen G10 used in this application...

 

Dont know what G10 is. Simply research how much it swells in warm seawater then machine the bearing . Both inside and outside.

My last bottom bearing on a skeg rudder was delrin. It achieved 200,000 miles of service before it wore egg shaped and was replaced.

Bottom bearings wear fore and aft due to prop thrust.

A proper skeg bearing handles downward thrust caused by the weight of the rudder...the bottom end of the rudder shaft is machined convex . A machined convex insert of the same rudder stock material is then placed inside the bottom of the bearing shoe

 

hadnt thought about delrin, at a tensile strength of just 9kpsi and relatively flexi, good to know it served and it gives me a frame of reference. got a pictore or two of the bearing assembly?

G10 is epoxy/glass cured under heat and pressure. heavy, but lighter than aluminum and similar in tensile strength. shouldnt absorb water. ive got a couple dozen large chunks - 2"x5"x18". intending to machine cleats, standoffs for hand rails, maybe stanchion bases, and epoxy directly rather than through bolt. its tough on tooling though, and the dust is trouble.

 

Im at sea at the moment and cant post images. I make landfall, Montenegro in 4 hours. Ill post the drawings....if I can find them ?

I have posted the drawing before on Boatdesign net. Perhaps search "rudder bearing"

 

UHMWPE doesn't absorb water, nor does G-10. UHMWPE is a common bearing material in this application.

The only problem with G-10 tooled dust is as a particulate. It's inert, so a dust mask is all you need.

Side loads will be your major concern in the bottom bearing, as the stop collar on the shaft, above the "port" will handle the weight and any down ward loads. A simple cup will work, though I'd probably just use a sleeve arrangement, so it can freely drain. A really bullet proof setup would be a milled G-10 assembly, with a UHMWPE insert to take the abuse. The G-10 will prevent distortion and the UHMWPE, being self lubricating, will accept the shaft.

 

yep, thanks par. thats what i was thinking. does 1'" of G10 around the shaft seem sufficient?

and the next step... building the new rudder and fairing it into the skeg. NACA0012 tapering to 0008 at the full section below the skeg. but as you can see from the pic, cord increases dramatically as the skeg meets the hull. im assuming the section is continues right up to the hull and the rudder breaks where it breaks? how to fill and fair the skeg? could be an inch and a half thick in some places. carved foam with epoxy/biax over? slather on structural filler? (that doesnt sound good) how does one fill relatively large voids like this below the waterline?

TIA

 

It's difficult to understand your uses for G-10 and your fairing concerns. Can you post some sketches of what you plan for your new rudder build?

 

yeah sorry, i really do need to start drawing this stuff up. very low tech operation here, no scanner no cad etc. ill free hand a sketch and stop at kinkos soon.
the G10 bottom bearing is straight forward, just a materials substitution. the fairing issues im anticipating stem from the fact that the skeg as laid up from the factory is relatively thin and flat in section. it will have to be built-up and shaped to form the the leading edge of the foil profile. so the question is what to use to bulk it up?

if the attachment works, it should be a crude sketch of my existing skeg (roughly) in red and the new foil shape (naca0012) im going for. hence the need for fillers. how 'carve-able' is divinycell in this application? ive got access to some H80 scored stock i could epoxy to the skeg, shape and then biax over... or something else?

 

I'm still not clear what the G-10 is for, as I'm assuming the skeg is still in good condition, unless I'm forgetting portions of this thread (not unusual).

The blade (my assumptions again) should have a lower bearing, likely just a cup, the rudder shaft rests is, possibly with a zerk if sealed. At the top of the blade, just before where it enters the hull a possible second gudgeon or bearing and lastly a rudder port where it passes through the hull. Depending on what is done inside the boat, you only need an upper and lower gudgeon/bearing assemblies. The upper naturally would be the port.

From a construction point of view you can do this several different ways, from an encapsulated wood approach to a laminate. I'll assume (there I go again) you want all laminate.

Leaving the shaping issue aside for now, the rudder shaft should have an "armature" attached, so the blade laminate has something to grip. This could be as simple as a plate of shaft material, weld to is aft side, possible with some holes in it to lighten it and offer more mechanical "keying" for the laminate.

The leading edge of the blade should be "tucked" into the trailing edge of the skeg, so you have a continuous foil section, without a gap. A semi circle on the skeg's trailing edge will do, with the rudder shaft tucked in behind it.

As to adding shape to the skeg/blade assembly, foam is the logical choice. A 5 pound foam is fine, of course sheathed with enough fabric to make it tough enough for the job. Remnants of the old blade should give you an idea of laminate thicknesses to tolerate the loads and biax as the bulk of the fabrics just makes sense too.

The sections you use are up to you and they'll change considerably from top to bottom. The top of the skeg will want as thin of sections as you can live with. As you progress down the skeg/blade assembly, you'll transition into proportionally fatter sections, as the shaft comes to play with section thickness. After you pass the lower bearing, you can again transition into a thinner section, possibly leaving the 00 series choices further up for one more likely to ease tip turbulence.

You may even want to consider a break away lower section, below the lower bearing, so in the event of a grounding, it'll break off, but you'll still have a blade to work with as you limp home. This can be accomplished with a bond line parallel to the LWL, at the bottom of the skeg on the blade. Use just a fairing compound to bond it, instead of a structural bond. I'd just make the blade whole (sheathings and all), then cut the bottom off and re-bond it with a some balloons and goo. It'll hold, unless it take s a good shot, where just this tip would break off clean (ish).

 

the G10 would be used to fabricate the bottom bearing in lieu of bronze. it would have to be split in two pieces for removal/installation of the rudder. the rudder shaft itself enters the hull inside a fiberglass tube that extends all the way to the rear cockpit seat top (well above the waterline). so there are three bearings... the bottom of the skeg in G10 with a UHMWPE sleeve, another middle sleeve inside the bottom of the rudder tube, and a top bearing where the tiller head is attached. i like the solid tube arrangement rather than a stuffing box below the waterline.

ive got the rudder assembly figured out as you described. the filler is just to transition from the original flat sided skeg to the foil profile. what i was assuming was a continuous section tapering from top to tip, but it doesnt sound like that is ideal. i guess im having trouble picturing the profile shape as it changes from the top of the skeg/rudder section, down past the bottom bearing, and on to the rudder tip. do all the 'max camber' points end up in a straight line or curve with the leading edge of the skeg?

 

par - sent you a pm.

 

I don't often feel I have to openly disagree with comments but UHMWPE does absorb water as do most composites to some degree, it can be small but this is still an important part of working out the clearances (it is for us and we supply one of the lowest moisture absorbing bearing materials)

The second (very important) factor if calculating the clearances yourself is the thermal swell (this will be significantly greater than moisture swell) and finally you need to account for the material of the carrier and the shaft as they will also have different thermal expansion rates. Consider that a vessels may see temperatures from 0C to 35C

The latest hard (not elastomeric) bearings have performed very well under rudder simulation tests, but for many smaller boats Polyacetal (Delrin) will be good enough. It does swell in water and it expands quite a bit with heat so do your sums.