Building Logs

Under done

Posted by Paul

Take 2. The site went down a short while (hours or minutes not sure) after I uploaded this post, then 2 days later when the site was finally available again, after showing all sorts of weird stuff including someone else’s blog, this post was gone. This is a complete rewrite of that blog, so if you think you have read some of it somewhere before, you may have, or if you read that and re-read this and it seems different, it is. I usually am able to post from memory so whilst I might change the order or emphasis, the content should be the same but perhaps including stuff I thought of later.

We left off with the rudders with foam and filler on top that looked like cake frosting to get the little bit of extra height I needed at the shaft (from memory another 5mm needed from the filler, down to nothing at the trailing edge). Next task was to sand all of that roughly applied bog to shape. That shape and the extra height required is dictated by the angles of the rudder and hull shape. For a while I was baffled as to why the rudders, with the additional height added, cleared the hull by 20mm at the shaft and 10mm at the trailing edge but would hit the hull when turned. Its because the angle of the rocker of the hull which is about 75 degrees from the vertical shaft compared to the almost flat (there is the slightest of V shape) hull shape side to side. This hitting of the hull initially concerned me, but as I grew to understand what was happening and that it was actually a benefit once I shaped the top of the rudders properly.

The additional height added to the rudders is to close the gap between the rudder top and the hull, mostly at the shaft end of the rudder. But when the rudders are in the fore and aft position (when the boat is steered straight ahead) the rudders are not doing much work, the rudders come into play once you decide you want to steer off straight ahead. So the gap between hull and rudder is not important in the straight ahead position but the smaller it is when turning, and to extend the logic, the smaller it is the harder you want to turn, the better. And of course, that is exactly what is happening. If you look at the original rudder top angle and its distance from the hull as a result of the rudder tube to hull fairing you can see that the original rudder was made to match the hull angle exactly.

(As an aside, I also realised how lucky I was with the measurements I had made, because they were made in complete ignorance that the rudders would hit the hulls and that if I did not have the rudder tube fairing on the underside of the hull to make that gap, the rudders would never have been able to turn enough before hitting the hull, it turned out, purely by chance, that I had made the fairing big enough for the rudders to turn enough before hitting, lucky break. The bearings were poured onto the shafts with the assumption of the tubes protruding through the bottom of the hull the amount they are, and the clearance for the tillers etc and for the thrust plate on the rudder shaft to be just above the blade top, etc, etc,  all oblivious to the geometry of the angles, and it still all worked out. The solution had it not worked out would have been in where I placed the thrust plate on the rudder shaft but this would have meant that the poured bearings would not have been perfectly aligned inside the uhmwpe bearings, they still would have worked but full surface contact is better than not I guess.)

original rudder angle and gap

Because of the way the rudder hits the hull when turned it would need to stay this distance from the hull in order for it to be able to turn enough before hitting the hull (enough being about 35 degrees either side of centre, any more than this causes the rudder to become a brake rather than a steerer of the boat and the rudder “stalls” meaning water flows equally over both sides and it no longer steers the boat, just slows it – there’s more to it than this but I neither understand it nor does it really matter to me). The problem with leaving the rudder some distance from the hull is that the rudder is somewhat more effective if water cannot flow (and escape) over the top of it. The water that does escape over the top causes turbulence in the water flow reducing the effectiveness of the rudder and in turn causing drag on the boat slowing it marginally. I also figure that if I leave a gap between the hull and the rudder top it becomes a place where ropes can foul or barnacles can grow which could one day impede the rudders, having said that, the wider the gap the easier to clear I guess. The problem with the matching rudder top angle to hull angle is that when the trailing edge hits the hull in a turn, because the gap is now closed at the hull edges, the gap at the rudder shaft (on the hull centreline) is still some 50mm.

Because most steering is not full lock and because the gap narrows the closer to full lock it turns out that the ideal angle for the rudder top does not mirror the hull, making the gap less at the shaft than at the trailing edge. So, that is what I set out to do with the measurements I wrote on the rudder top in the last post in order to add some height to the top of the rudder at an angle that allows the rudder to turn enough but closes that gap as much as possible so that it is at its least (just touching the hull) when turned to just beyond about 40 degrees and I will ensure that the rudder cannot turn enough, with the use of bump stops on the tillers at 35 degrees, to ever touch the hull.

Starting with the new rudder top angle at dead ahead and then the rudder as it touches the hull on each turn, here are the pics of what it looks like now:

port rudder fore aft straigh ahead

port rudder hard over hitting hull inboard port rudder hard over hitting hull outboard

And at the angles of turn shown above, about 45 -50 degrees, the matching tiller angles at those positions, showing what a close run thing this was, any less turn before hitting and the steering would have been compromised:

starboard tiller position when rudder hits hull starboard tiller position when rudder hits hull 1

I also learned why the starboard rudder is a little tighter to turn than the port rudder. You can see in the picture below, that with the top bearing out the “native” resting position of the shaft at the top is still not exactly centred. The top uhmwpe bearing can still be inserted, although it needs to be tapped down, or if in before the rudder is lifted into position the tightness causes it to be pushed back out. But the fact it isnt centred is a little baffling since it seemed to be when we poured the bearings. The only thing I did not allow for (or make marks to check for) is perhaps the uhmwpe bearings are not back in the tubes in the exact rotational location they were and the shaft milled into it is not exactly parallel to the outside meaning if the uhmwpe bearing is rotated it moves the position of the shaft at the top bearing and that if both top and bottom are out the same amount and happen to be placed in line to cancel each other out all works fine but one out of alignment and it tightens up some. I might try to experiment with this trying to rotate just the bottom bearing to see if the shaft moves.

reason starboard rudder is harder to turn

Having said all of that, I am not sure that being loose is better than being tight. It might just be that, given both rudders can be moved by hand anyway, that the tighter one is actually the better one. Under the load of the boat being steered, with hydrodynamic pressure being placed on the rudder (resistance to the pressure of the hydraulic ram pushing the rudder to turn) that the tighter bearings helps in resisting that hydrodynamic load placing less stress on the ram? (The line on the pic above is the fore and aft position of the blade).

As I was sanding the foam and filler rudder top I ended up sanding too much off the trailing edge of the starboard rudder. The problem with filler over foam is that the foam is so much softer than the filler and the trailing edge had little or no filler as it did not need any extra height other than the foam added so that as I sanded the filler was hard to move, the foam not so much and ended up rounding over the edge and taking too much of it off. No biggie. It is so easy to work with this stuff. I simply took even more off to make a bigger platform and glued another piece of foam in and re-shaped it. It was only the last 40mm or so and only about 3mm too low (as seen in the before and after shots below) as  could have left it but it only took half an hour to fix.

fixing starboard rudder over sanding starboard rudder over sand correction foam filler

starboard rudder over sanded on the end 100 mm sb rudder second reshape 1

The other rudder work needed before I could glass was to strengthen and fill the thrust plate to the shaft. I had glassed a web in place fore and aft along the rudder top to hold that plate in place but it would need to be much stronger. Because the rudder bottoms protrude slightly below the lowest point of the hull the boat if deliberately beached would sit on the rudders, and although the bulk of the weight would still be on the hulls where they ground, there will still be some weight resting ultimately on the thrust plates. So I effectively filled the sides with rolled uni to form a solid glass filler then glassed with double bias to seal that in then filled the rest of the shape with filler to create a fair rounded surface so as to minimise turbulence at that point. Unfortunately I was not able to take pictures of that work. But the later pics of the way it all fairs into the shape of the rudder tube fairing is very pleasing.

rudder to thrust plate fairing rudder to thrust plate fairing 1

So with the rudder shapes settled, numerous dry fits to be sure they turned enough before hitting, and closed the gaps where needed I could finally lock that shape in by glassing them (the dry fitting assumed another 2mm would be added with glass and fairing). I used 200gm plain weave cloth for 3 reasons. First it moulds around corners and curves so much better than the 450g double bias and the very end tip of the rudders are very tight curves/edges. Second it is thin so does not add as much bulk and finally it is not as strong as double bias and that is the intention. The rudders are carbon so very very strong. The hulls are also strong but I dont want to test them by pitting them against the rudders in a dual to the death. Should I strike something with enough force to push them up into the hulls I would prefer neither to “give” so the added foam is somewhat sacrificial. If it compresses or even rips completely off the rudder, that is preferable to breaking the rudder or tearing/smashing the hull. I may even be able to remove the sacrificial part at sea and still sail home to replace or repair later. So it is made deliberately as the weak point. The only place I used double bias was over the flared fairing to the underside of the thrust plate, this I do want to be as strong as possible. I put 3 layers of glass there.

rudder top glassed and peel ply rudder top glassed ready for fairing

Unfortunately because I was only using thin glass, I ended up sanding through the glass into the foam/filler below, on the trailing edge of the, yep, you guessed it, starboard rudder, the one I had already repaired once. Thankful that it was just the one rudder, and doubly thankful this stuff is so easy to repair, I simply de-cored under the glass as far as I could then filled it with glue as I bogged the rest of the rudder, it would set as the bog set, and then when I faired them later this just faired right out. Below is a before and after pic.

hole in rudder repaired stb rudder trailing edge and corner1 repaired stb rudder trailing edge and corner

Next step was bogging them ready for fairing. A fairly simple process of trowel-ling bog on and screeding it out to nothing past the point of the glass onto the already faired original rudder surface. It is near impossible to do this without layering, just try to be sure that you are too high rather than too low, too high you can always just keep sanding, too low is going to require another fill, then wait for it to set (a second go over is inevitable but this avoids it being 3!).

rudder top bogged 1 rudder top bogged

Guess what comes after bog….yep….sanding by hand. I again cheated a little by taking the tops of the bog off with an orbital sander, but the bulk of the work is done with a sanding board. This is especially important on the rudder faces and trailing edges because you need them to be fair along their entire length. So you are fairing the added work to the existing. Considering I have added about half a millimetre in glass and half a millimetre in bog to the height that the rest of the rudder is at, you would say that to be impossible, but once you have sanded it all down the change is imperceptible, the glass edges disappear in the faired bog. I have just a couple of small divots in the shaft plate flare to fill, which I am hoping to do with extra highbuild when I get to that stage and the rudders are fair again (they were faired when I got them). The sanding process took 2 goes, first to get it almost fair. But the first of bog layers inevitably leaves pin holes (they are invisible until you run a blower of them, then bingo the dust is blown from them and they suddenly all appear), so the second go over is to fill as much of that as possible with a runnier bog (less chance of more pin holes in that). You cant use the runnier bog to start with as it sags and runs too much and is much harder to sand (higher resin to filler ratio). You can then fill the final ones you miss with highbuild, but you do have to get them before you paint. Probably not so critical when applying copper epoxy with a roller as that will fill those pin holes, but better to get them than not.

port rudder spot filled starboard rudder spot filled

Whilst it annoys you to roughen up with more filler a job you just spent an hour sanding smooth, the next sand is very quick, 10 -15 minutes to knock the excess bog off the surface leaving the filled pin holes fair to the rest of the surface. Each sand I use a finer sand paper. First sand to shape I initially used 40 grit. Then first bog sand I used 80 grit first then 120 grit. Then final sand I used only 120 grit then when I eventually sand the highbuild I will use 240 grit.

Some shots of the rudders ready for highbuild. Note how its hard to tell that the rudder addition was repaired twice. Also note the shape of the hull tube fairing and how it blends into the rudder thrust plate fairing. Nice.

sbd rudder sanded and trailing edge hole repaired sbd rudder faired ready for highbuild 

port rudder faired trailing edge sbd rudder faired trailing edge

both rudders ready for highbuild

Feeling quite chuffed with myself over how well I did on the fairing of the rudders, the time it took (a lot less than I thought) and the end result, I am perhaps not quite so fearful of the massive fairing job the topsides present. But to get to them I have to finish the rest of the under hull work and get the boat back down.

The last part of the under-hull work is the final glassing, bogging and fairing of the hull extension including my bottom step inboard overhang. I have coved the underside of that overhang (with a large radius cove, about the size of a 2 litre coke bottle) and glassed that, and glassed the rest of the joins of the hull extension as well as the edge of the step overhang. Next is bogging that and sanding it ready for highbuild and copper coating the underside of the hull. Same goes for the rudders, highbuild, sand, epoxy coat then 2 or 3 copper coats, re-install them and lower the boat.

port rear step edge glassed sbd rear step edge glassed

I am shopping around for a deal on 20 litres of highbuild. I only need a fraction of that for the rudders and hull underside but I will need all of 20 litres and more for the topsides, and buying it in 20 litre lots is way cheaper than buying 4 litres at a time. From memory 4 litres runs at about $150 and 20 litres at about $600, so its like getting $150 off or 4 litres for free. Hopefully I can find a good deal.

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