Building 3 PD Racers at Duckflat
Well, I've just had a big day up at Duck Flat Wooden Boats doing some work on the 3 PDRacers they are building for the nationals at Goolwa in March.
I'm not the only person working on them. Robin Badenoch has been in and more or less finished the masts and is on his way with the foils (centreboards and rudders). He had also made a good start with precoating the plywood sheets with epoxy.
Duckflat staff are also helping when they can steal the occasional moment from the rough and tumble of commercial work.
Just to rewind a little - sorry about the duplication of the mast pics but I just want to get everything in the one thread.
The experimental area in the masts is that they are quite thin walled compared to conventional hollow timber masts. Generally it is recommended that the wall of a timber mast not be less than 20% of its overall thickness - though "with care" it is supposed to be OK to go down to 15%.
My feeling is that to go down less than this in the "good old days" would have been to put the gluelines under too much stress - but glues have improved enormously in the last 30 years. So we went quite a bit smaller to see what would happen with our Mk1 masts. They worked OK but were a little bit too bendy. So we increased the outside dimensions a tad to increase stiffness but retained the same wall thickness.
So those are our Mk2 Masts which are the ones in the plans.
So to the photos below
1/ It can be a bit tricky to make up a hollow mast. In the Oz PDR case it is a two stage process. The masts are made up of four pieces of timber hte length of the mast to produce a box section. So there are two narrow pieces of timber ( narrow staves) and two wide ones (wide staves to produce a box section that is tapered.
The first stage is to make up a "ladder frame" from the two narrow staves with blocking between. The first two photos are actually from Rob Thomson's (Dopey Driver's) PDR.
There are a number of blocks between the narrow staves to space them the right distance. At the base of the mast is a long block that takes the loads right where the mast passes through the deck - the point of maximum load.
2/ After the ladder frame is made up the wide staves are simply glued to the faces of the ladder to make it a long box. We avoid needing lots of clamps by using brown packaging tape to clamp things up. Starting at one end we clamp the assembly together until the glue just starts to ooze out the joins. Then we wrapped brown packaging tape right next to the clamp - which meant that the clamp can be moved about a foot up the mast and the process repeated.
And so on until the whole mast is taped up.
3/ The tape is removed and the mast is cleaned up by sanding off glue ooze and planing or routing the excess width of the wide stave down so they are flush with the faces of the ladder frame.
4/ The corners are routed to a radius equalling the thickness of the staves - any larger radius and the glue joins will start to be cut away.
One of the interesting thing about square masts is that they can be made a lot lighter than birdsmouth masts. Purely because the square section is much stiffer for the same weight.
The booms are solid - it is too much trouble to make them hollow and they don't weigh much anyhow.
5/ This is a pic of me upsetting people. The onlookers weren't as confident as I was - but I already had seen what our Mk1 masts could do.
These masts are designed to bend - a lot!!!
That's how we can manage to carry sails about 30% bigger than everyone else. The masts bend and flatten out the sail which is cut to match the bend of the masts. It is the same system as hot racing boats but here it is very, very cheap compared to their expensive carbon fibre or exotic varieties of aluminium alloy.
Remember our original two PDRs cost us $300 each with some scrounging of materials. A commercial aluminium mast for a PDR would be around the $700 to $800 mark just by itself - or a carbon one between $1000 and $1500. Things that racing sailors take for granted.
Much rather have two PDRs on the water for the same money as a commercial mast or one PDR if you have to buy all the bits.
Michael
http://www.pdracer.info
3 x PDRacers coating, sanding and cutting panels
I have to back track almost precisely two weeks to explain about cutting the panels.
Duckflat had decided to precoat the ply sheets which saves a lot of messing around later - trying to get epoxy on inaccessible places. It also means that the epoxy goes onto the sheets in a very controlled way and because work is done horizontally there is little or no risk of runs in the epoxy.
1/ My job on the one day I was up at Duckflat that week was to sand as many of the panels as I could. So I got stuck into it. I was using 120grit paper and it was taking about 15 minutes and three discs to do a sheet.
I've done a lot of production work in my time including some big areas on big boats which are a real mental challenge - it can seem endless. So I mark a small area to concentrate on and when it is done mark a new area. I used the sander here to mark the area, but sometimes I use a pencil. It's the only way to survive as instead of always heading toward a goal that is so far away I'm continually meeting smaller goals - so it feels quite different.
2/ The marked area is now sanded and I'm about to move onto the next. I got about half the sheets done for the three boats in the time I had available. They were finished off by Geraldine when she came in from school for work experience at the beginning of the following week.
Almost all the panels of the PDRacer are rectangular which keeps the labour and complication of building the boat to a minimum. Almost every angle is right angle which can easily be marked off the ply sheets which are already square.
The shape of the curved bits is simply marked out on a grid as we have detailed on the PDRacer.info site here
http://homepage.mac.com/peterhyndman...arkingout.html
3/ As I was building three boats I decided to go more towards a mass production scenario and cut three pieces at a time in a stack. I cut them a little oversize then used the templates that Duckflat have made for for the PDR to router them down exactly to size. I couldn't route and take pictures at the same time but here is Pat from Ducflat posing with the end results.
The side panels are to the the left and the sidedecks are on the right
I think that if you were building one or two boats it would hardly be worth it to go to the router method - unless you finished one of each successive panel down to accurate shape then used that as a template to run the router round. Because the templates were on thin ply I had to put a number of chipboard screws through the stack I was routing to keep them all tight up against one another.
4/ There are only the two side panels for the hulls, the two side decks and the two faces for the buoyancy tanks that have much of a curve at all as can be seen in the ply layout from the plan.
I managed to do the cutting out for all the panels for three boats in around 3 hours of steady work.
Problems with splitting timber
Plywood almost never splits because it is made up of timber with grain going in different directions.
However very occasionally the self tapping Plasterboard/Drywall screws will split the timber framing.
Before pulling the offending screw out we put a bit of glue in the split. Then removed the offending screw. A new screw was put in at a 90deg angle to the original to hold the two sides of the split together - see attached pic.
Then the section could be clamped in place - or another screw put in further back from the end of the piece of timber.
Once the timber has plywood glued to two faces in the finished boat there will be no possibility of any further splitting as the plywood will prevent it.
The reason the timber is protruding past the edge is because the drawing tells us that it needs to be set up this way to allow for a bevel later.