PLUG CONSTRUCTION. By Stuart Calvert

My article here is of course not the only way to do it, but I wanted to impart some knowledge I gained on how best to and more importantly how NOT to build a plug. I built 2 plugs. The first one ended up being sawn in half so it would fit in the rubbish bin… it was an expensive mistake.

The overall quality of the plug was OK and the shape generally fare but the problem lay in not enough forethought and a lack of experience. I’m not entirely an amateur when it comes to this sort of thing. I have built a model airplane (which incidentally crashed on its first flight – badly, I then came across this hobby and decided to build something that was designed to crash or sink). So I have a reasonable amount of model making experience. I have also built a 2 man racing yacht with my brother and helped restore our family’s 35ft wooden yacht. I know stuff about fibreglass, resin paint etc… anyway I thought I knew what I was doing…

HOW NOT TO BUILD A PLUG

The first plug was made by photocopying the sections from the plan, enlarging them to the appropriate size. The plan was in 1/144 scale, but to get the correct beam (ship width) I had to enlarge the sections 1.48 times. Before I copied the sections I drew a 10mm wide section down the middle of each. This was to make sure that the mounting position of all the sections was the same. The plan sections were cut out of paper and glued to 5mm Medium Density Fibreboard (MDF) and cut out with a jigsaw then filed and sanded to the final shape (I should have just used the jigsaw more carefully). For the straight parts of the hull (bottom and sides) I used a tennon saw.

I then got 2 pieces of the straightest pine I could find at the hardware shop, "teed" them together to stop them warping and mounted the sections along it. I measured and re-measured the distance from the first station (A) to all the other stations. DO NOT measure the distance between consecutive stations - this will produce an error.

With the sections in place I "planked" the whole thing with 1mm thick balsa (quite enjoyable because it started to look like a ship then. This done, I filled it with expandable polyurethane foam. The type you get in an aerosol, it turned out that I needed 3 cans at about $30 a can! DON"T USE THIS STUFF!!!!! The idea was great, but because it is rigid foam (intrinsically full of air) it expands and contracts with varying temperature and humidity. Because there are also different concentrations of air in it, it is IMPOSSIBLE to get fare.

The bow and stern of the plug was shaped from solid balsa. When done and fared using light modeling shaper (had some left over from the plane) I sprayed the plug with "spray putty" which is great stuff. Automotive repair shops use it as a spray on bog. I used a litre and built the entire plug up by about 2mm all over, concentrating on the bow, stern and edges. This was sanded and then top coated with black lacquer. Dark colours show up imperfections really well. She looked great. The next day however (which was warmer) made me want to scream! Bulges all over the thing - albeit small ones. I put the plug in my work office for a few days adjusting the air conditioning until it looked marginally acceptable. I rushed it home in the weekend (having waxed it 4 times – NOT ENOUGH!!!) and proceeded to make my mould. No gel coat (mistake I think) and no ‘lip’ or anything to stop the resin sticking to the deck level.

What I had in the end was a $300 piece of foam, paint and fibreglass that vaguely resembled a ship. I couldn’t get the mold off… eventually jumped on it and when that didn’t break it I had to use the saw.

I took some pictures of this attempt as I went. I’ll see if I can dig them out and scan them.

ONE WAY TO BUILD A PLUG

3 months later…

I managed to pluck up the courage to try again. I got my plan again and drew a 5mm grid in autoCAD which I printed out on transparent plastic OHP material. I choose a datum on the sections, which was the highest point of the hull – the tip of the bow and called this (0,0). I drew all the sections in autoCAD – well only half of each of course and I then mirrored then along the ship’s centre line. This was the best thing I could have done.

Sections H, I and J were all supposed to have the same beam. They do in MY sections now, but the plan varied by about 4mm once scaled. This occurred when the sections changed from one side to the other (about half way down the ship).

I printed these out and checked the deck level on the sections with the elevation view on the plan. Some of them didn’t match up correctly, so after a bit of guesstimating I had the best sections I think I could come up with. Yes it was a bit of work, but well worth it in the end! (For those who know something about CAD, I used Solidworks and them exported them into autoCAD as a dxf file. I think the easiest way to do the curves is in Solidworks with a new sketch for each section and use a spline and enter points of the grid you put over the plan as co-ordinate references).

I put a 3.25mm line offset each side of the centre of each section so I could mount the sections to a 6.5mm thick piece of angle iron ($NZ25). I wanted to make sure that this thing wasn't going to warp! All sections had another line 80mm from the datum so they would all be in the correct place when glued to the angle iron.

Figure 1. You can see here the slot cut for the angle iron up the centre of each section. when printed out there was only 1 section on each piece of paper. also note the horizontal bit sticking out at deck level. The "termination" parts were flush with these.

These were all printed out and cut out of paper. I used 3M spray on glue to stick them to the 5mm MDF (left over from my previous attempt). Cut them all out carefully with a jigsaw. I used a tennon saw for the mounting slots in the middle of each one and did the end of the mounting slots with the jigsaw – worked well.

The 100x50mm L shaped angle iron I got cut to the exact length of 1227mm. This was the distance between the first section (A) and the last section (M). I then measured the distance from A (first section from the bow) to each section on the plan and marked then on the angle iron with a file. I placed each section on the angle iron and using 2 tri-squares (for horizontal and vertical squareness), glued them in place with Cynoacrylate (CA) glue and a spray on kicker so I didn’t have to hold them there. You may need an extra set of hands for this. My flatmate helped me out just fine.

Figure 2. The sections cut out and glued square to the L Section. It wasn't going to warp!

There was no way in the world that I was going to use expandable foam again. On the advice of Arthur Buckland I used kiln-dried polystyrene. Kiln dried polystyrene is what surf board manufacturers use – easier to shape and doesn’t shift as much. It cost me NZ$10 for more than enough to do the plug. I cut the polystyrene with a normal panel saw to fit between the sections on the angle iron and also a rough cutout in the middle so they could fit over the angle iron. I glued them to the angle iron and sections using 2-pack epoxy resin. CA glue is too expensive to use for this I think (you need a bit of glue). It will also melt the polystyrene!!!. If you cut some of the polystyrene too short and it doesn’t touch the sections, just put some thin (I used 3mm) balsa in there and glue it each side.

Figure 3. Gluing the roughed out polystyrene fillers with resin.

Figure 4. After shaping the polystyrene.

At this stage, you have nothing that looks like a ship. A bit of very rough shaping with the saw again and then more with a course wood rasp and then sandpaper and voila! An hour later you have most of your mold. Easy! Don’t do this inside – even if it is a rainy day. The stuff goes everywhere – just take a look!

Figure 5. Shaping polystyrene sure makes a mess.

I don’t know if the next step is recommended, but I painted the whole thing with epoxy. I did this for 3 reasons. To harden the outside of the polystyrene, I knew the epoxy wouldn’t melt it, and to give my faring compound a good base to stick to. About 6 hours later (when the resin had just about fully hardened) I mixed up some more resin, added lightweight faring fibres and went to work. To cut a long story short, this worked well, but maybe I went a bit over the top with the faring. It was quite hard to sand, but in the end it was it was a good thing because it provided a good surface for final faring.

Figure 6. The plug after adding the "termination" bits between the sections and some faring with epoxy resin faring compound. I've also added the small skeg down aft. This was tapered slightly from scrap balsa and glued in place with CA glue.

I added the balsa for the stem and stern. I used 9.5mm laminations. This was excellent. It was much easier to see the contours of each lamination as I shaped it to ensure that it was symmetrical each side (first with a panel saw, then a wood rasp, then sandpaper). In the end, there’s nothing better then just doing it by eye. If it looks right – it probably is.

ALWAYS use a sanding block or a longboard!!! If you don’t have one, make one. I made one in about 5 minutes with some STRAIGHT pine (thick MDF may be better), 2 pieces of 25x25mm pine and some CA glue. I attached the ends of the sandpaper to the TOP of the board and went to work.

I then added sections of MDF in between the sections at deck level. This will give the mold somewhere to terminate when finished. Filler needs to be applied where this termination meets the hull. These termination pieces were up and down by about 1 mm in parts (noticeable after sanding with a longboard). This was corrected with more filler and the original sections in autoCAD were changed.

The last thing to do was to measure the correct height at the stem and stern. I measured from the plan, converted to 1/96 scale and measured from the bottom using a straight edge and a tri square. I cut these using a tennon saw to the height of the deck level at the next station, rasped it down a bit and glued on my "termination" pieces of MDF with CA. Applied some more filler where hull meets the termination bits and that was it.

The plug needed a little more faring. This was done and sanded again. I set up the spray gear in the weekend and thought it’d be a good idea to wipe it with thinners (paint solvent) before painting. DON"T GET THIS STUFF ANYWHERE NEAR POLYSTYRENE!. Just a little bit made huge holes where the resin hadn’t completely sealed the polystyrene. No painting that day then. I filled the holes with more 2 part resin faring compound and went out to visit Arthur for the rest of the day. The next day I sanded again and painted. There were still some gaps in the resin – this is what happened…

Figure 7. Solvent based paint melts polystyrene!

I couldn’t believe it! After 2 coats I stopped (Spray putty dries pretty fast) and filled the holes with Carl Goldberg model airplane filler. This is water based and dries in about 10 minutes.

I sanded again and then painted the rest. You can see in the picture that the formers are showing this was because the plug had been in the sun and the polystyrene expanded in the heat. I put it in the shade to cool off before painting. Note when working with ANYTHING that must be fare, try as much as possible to keep it in a constant temperature and humidity or it will continually shift/warp/move.

Figure 8. Filling and holes in the polystyrene. You can also see the lines of the formers. This was due to the polystyrene expanding and the MDF sections staying put. DON'T work on your boat in the sun!

I noticed as I went that several parts of the plug were dipping again (they appeared to be melting just a little). I just more paint on these parts which was totally the wrong thing to do. The solvent in the paint was soaking into the airplane filler and melting the polystyrene beneath it. Things went from bad to worse and in the morning there was a dip of about 5 – 6 mm (1 / 4 inch).

Figures 9 and 10. A couple of pics of what happens when solvent from paint gets into polystyrene. Note also the big dip in the side of Fig. 9.

I cursed and swore, let the paint dry properly and then filled it with some ordinary water based plaster board filling compound. I used "Selleys mirror finish" It worked a treat and at NZ$12 you can’t go wrong. However it sands VERY easily and shrinks a little when it dries. Several coats were needed. I got the plug fare again and sprayed the newly painted bits again. Several light coats this time with plenty (15 – 20 minutes between coats) of time between coats.

Figures 11 and 12. Final sanding. Reasionably fare I think... You can also see the 2 steps in the deck quite well. One is near the stern and the other near the bow. (I had to get one picture of me doiong some work in there somewhere)

I sanded the plug very carefully with 240 wet and dry paper with water and fished it with 600 grit paper. There were a few imperfections especially around the deck line but I figured these could be filled with wax. So I let it dry properly and waxed it that night. Once I waxed the thing, more paint or filler won’t stick to it – I did that on purpose, just so I couldn’t tempt myself.

Now when I make the mold, I’ll be sure to use PLENTLY of wax and probably a gel coat. After all this work AGAIN I’d launch the thing through the nearest window if it got stuck – watch out neighbors! Actually I found a friend that works at an airline in the composites division and he has offered to make the mold and hull for me. I immediately took him up on his offer.

Figures 13 and 14. The finished plug after a few coats of wax. I have sanded through the paint in a few places but once waxed, it wasn't that noticable.

A few comments to finish with…

If I could do it all again (my next plug maybe) I’ll either use the "bread and butter" method if the plan has the right information, or sections along a mounting piece again. If I use sections again I won’t use polystyrene or foam (2 part foam may be OK though). I’m sure if you did it following Bob Pottle’s article you could do it fine (btw I think Bob’s method of a proper keel and how he gets the deck shape is better than mine. I just had a deck with 2 steps and I thought my way was better – possibly it wasn’t…

Next time I’ll use strips of thick (9.5mm) balsa between the sections. Not totally filling between sections (would cost a fortune but gluing the balsa in between the section so that it stands proud a little then sand it down with a long board. This won’t shift as much, and will be much better to paint than polystyrene.

If using water when sanding make sure that all the timber and other porous material used are well sealed (I did but water still swelled the sections and cracked the paint in parts – not to badly though and the cracks were sanded and given a good waxing).

NEVER put the plug in the sun to dry, or work on etc… it will shift and if painted it will probably crack. Just look at Fig 8.

Spray putty is good stuff but anything that is thinned with a volatile solvent WILL melt polystyrene. I’d definitely use spray putty again if I make a mold with balsa or MDF.

When building a plug, it is essential to build on a building board (or fit a flexible base at deck level if the deck is curved – see Bob’s article) or have some sort of termination at deck level as I did (in hind sight mine was too narrow. I’d recommend at least 6 inches as well).

 
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