Balsa Wood Thickness
Yesterday I downloaded your boat plan, PT 109 model boat.
It will be my first adventure in the world of model boats. I'm writing to ask a technical advice, if I may...
For balsa wood, what is the best thickness to use in modeling?
There are several factors that determine what balsa wood thickness to use. Off the top of my head, some are:
- Weight concerns - performance
- Size of model
- Density of the Balsa
- Method of sealing the hull
Weight Concerns - Performance
The PT boat on the plan is the same size as the Lindberg PT 109
. I built that kit many years ago and converted it to RC.
The biggest flaw with the Lindberg kit as an RC model (and all plastic kits I've seen converted) is that they always seem to end up too heavy - especially top-heavy. As a result they handle poorly and look out of scale.
With the Lindberg experience in mind, I put a great deal of emphasis in the design phase trying to keep the model light. In addition, being a planing boat, any weight saved translates to higher speed and extended endurance, both of which is highly desirable.
Some design decisions were also done to make the hull relatively easy and fast to build - such as the chine shelf construction etc.
With this in mind, I settled on using 1/8-inch (about 3mm) balsa for anything structural - the bulkheads, chine shelf, transom, shear stringers etc.
Size of Model Boat
The norm is usually to use thinner material for the planking than the bulkheads, so for the PT 109 I used 1/16 (about 1.6mm). In hindsight, this was almost too thin. I have bumps and irregularities in the planking where the bulkheads are.
These thicknesses are fairly typical for this size model. The PT 109 is about 30-inches (about 750mm) long. Obviously a smaller boat can use thinner material, and vice versa. I wouldn't use balsa for a model over 48 inches (about 1250mm) long. Simply because balsa tend not to last over time as well as more rugged hardwoods. Also, the cost is starting to shift in favor of common commercial materials, such as plywood etc.
The hull is indeed light, but I don't think a slightly thicker balsa planking would have hurt too badly.
Here in the US I can get 3/32-inch (about 2.5mm) which would have been better. I believe 2mm thick is available in Europe as well, and would make a good choice.
Density of the Balsa
An important factor is the density of the balsa at hand - it can vary widely. I order my balsa online, which is not ideal. The best is if you can hand-pick what you need.
It doesn't matter as much if you choose light or heavy - most important is that each sheet is even in texture and density and that all sheets used for the planking are reasonably close in density.
So, if you choose lighter balsa, it's easy to justify using a thicker material without any gain in weight. The added thickness will help achieve a smoother hull.
As I order online I tend to buy several extra sheets just to compensate for the possibility that a few may be sub-par.
Sealing the hull
I sealed my model boat with tissue-and-dope
. This is in old-school and time consuming, but probably the lightest way to seal the balsa.
Most people today probably use epoxy and light fiberglass. The result is stronger, but also heavier, in most cases.
If using fiberglass, the thinner base material can be selected without much sacrifice in strength. However, you're still stuck with the uneven surface.
Some may suggest using Bondo or other filler to fix a bumpy hull. I would if weight was not an issue, such as with a tug boat or other displacing hull. With a planing boat, I'd rather live with the uneven hull, but optimize for performance.