Hollow Masts for Small Sailboats
A solid sailboat mast from a single 2x4 board make excellent masts for puddle ducks.
The idea of hollow masts was created to save weight on boats that used a 20' or 30' mast that was held up with stays, so they could save on weight aloft.
Hollow masts are significantly weaker than solid ones because they have less material, and more glue joints which create weak spots.
The reason you can use a hollow mast with stays, is because the stays convert the force into compression. But the key is that the stays must keep that mast in column. Think of a soda straw. If you press down on the top of the straw, it can take a lot of weight. If you push the straw from the side a little bit so it is not standing perfectly straight, it will collapse with a lot less pressure. This is how a stayed mast works, the straw must remain as straight as possible to retain it's strength.
In contrast, pdracers typically use un-stayed masts, which rely on the deflection strength. So take that same straw, hold one end of it it in your fist, and start bending it by pushing on the other end. The straw will collapse because it's hollow structure is not very strong. Do the same test with a solid stick of the same material & diameter, and it will be significantly stonger than the hollow tube.
There are many ways to make hollow masts, which are sometimes called:
faux birds mouth
hollow box mast
Another one of the hyped features of hollow masts, is to build them with the goal of saving weight. So lets explore how much weight you can save with a hollow mast. Here is a diagram I got from a ducker, it came from the book The Sailing Yacht by Juan Baader The diagram shows how much weight you would save by making various types of hollow masts.
A mutton sprit sail uses a 16' tall mast, the following examples are making masts that are cylinders (not tapered).
Using a solid mast such as figure A, you will make a mast weighing about 22 lbs.
Using a hollow mast such as figure B, you would save 6.8 lbs (making a 15.2 lbs mast).
Using a hollow mast in figure C, you would save 10 lbs (making a 12 lb mast).
So you can see, making a hollow mast has the potential of saving 12 lbs compared to a solid cylinder of the same size. However those are cylinders. If you build a solid tapered mast which has the large diameter at the bottom where the stress is applied, it will weigh about 15 lbs (thats what mine weighs). Most importantly the bulk of the weight in that mast will be in the lower 30% of the mast. That also puts the bulk of the strength near the mast partner where the majority of the force is being applied.
While a hollow mast is slightly lighter than a solid one, it is definately not stronger. The logic is rather simple: If you remove material from the mast, then there is less material to resist it being bent and broken.
Now compound the problem that typical duckers use huge sails and really push them to the limit plus our hull is very stable, so we will see double the stress a regular boat this size would have.
The picture to the right says it all, it was a hollow mast. Ironically the mast was built exactly as specified in a set of purchased plans. So just because information is presented in a set of plans you have to buy, doesn't mean that information is any good.