AC120 new build Philadelphia

Tomorrow is October 1, I will begin my AC120 build. I have seen lots of Kiwis on the forum, so I have decided to build the America3 from Claudio’s design. I am not sure if it will take me two months or two years, but I will post my progress and my questions, which will be many. Thanks to those masters out there for supplying great knowledge and wisdom to us newbies!

1st day, 1st question:
Can I use 1/4 inch hard wood for the shadows as anything thinner is tough to procure locally?

Thanks for choosing America3
1/4 is probably too much, 1/8 is probably too thin, try to find something close to 1/6

I am now waiting for the UPS man to deliver my 48" balsa planks. Some of the interior cuts on the shadows were a little rough as they do not need to be as accurate as the exterior edges.

Hi Matt,
to make the job easier, I would suggest to use plain balsa bloc from shadow ‘0’ to the ‘bow’
The stripes will be used only from fram ‘0’ to frame ‘10’.

A provisory* shaped balsa bloc between ‘10’ and ‘11’ to manufacture a skirt support.

  • Skirt will be glass laminated on the provisory shaped balsa bloc covered with packing tape. Later the laminated skirt will be bonded to shadow ‘10’.


Thank you Claudio. I understand your comments regarding the bow, but I do not understand your comments regarding the area between shadows 10 and 11. It sounds like I should make a provisional balsa section that fits between shadows 10 and 11 and laminate it with glass. Why would I do this instead of just using balsa strips between the two shadows and laminating the whole hull with glass to complete my male plug?

This is what should look like the stern of America3

and this is why I was suggesting to use a balsa bloc from shadow 10 to 11.
Practically shadows 11 do not exist !


i now understand, thank you

My fingers are covered with super glue, but I am done.
Time for epoxy on the inside, followed by light sanding of the balsa, then fairing compound and more sanding.
Once that is complete. I will put on a coat of paint and sand again in order to find the high and low spots, then more sanding.
Claudio, is there a certain type of paint i should use or avoid that could cause issues with my first layer of fiberglass?
Traveling for work this coming week, so no updates until next weekend.

Hi , Pp
personally I would had applied the glas epoxy before in order to get a stronger support.
Further brushing Epoxy resin inside that also produce a wood protection against water when sanding with wet paper.
Polyester filler on top of glass/epoxy lamination and sanding until the surface is mirrored.
If you wish to make a female mould, then do not use Polyurthane paint and Wax, there is a risk of uncompatibiity.
Check wax manufacture instructions and eventually make a sample before.

I have not started sanding, so I will follow your directions.

Hull came right off the mold using the packing tape method. Weight of the hull is 235, but I used 4 layers of 2.5oz cloth (70.9g). I am hoping it will be strong enough. Lots of sanding to do.
Claudio, one question regarding the profile/width of the fin and rudder. Your appendage.pdf file shows two percentages, 8.0% next to the fin and 10.0% next to the rudder. What do these numbers mean?

Generally the Fin profile should stay in the range of 7% to 8%. Thinner profile, while may offer slightly better performances, could present loss of strenght to flexural effort.
The Rudder are instead thicker to compensate for the risk to enter in stall . Stall = loss of speed. Common profiles in the range 10% to 12% are suitables.

Thank you

After sanding and more sanding, I discovered too many imperfections due to my inadequate fiberglassing technique. I am scraping hull #1 and will be making a new one this weekend.
The build goes on.

Claudio, I was planning on purchasing a bulb for my fin, but here in the US there are no suppliers that have AC120 bulbs. I now plan to pour my own. Do you have templates (wood patterns) for your bulb? I can draw my own, but would rather borrow from the master craftsman himself.

I found the bulb calculator and printed out the templates myself.

With the bulb calculator you can manage to select the profile and define the paramerts for a bulb weight of 2850g.
I do not have templates, but you can print directly as bmp image.
My bulbs are not poured but rather assembled with multiple layers of lead sheet of 1mm thick. This material is used in France by the carpenters.
Technique described in various tread open by myself like Footy, RG65, AC100, etc.