JP´s model catamaran

Introduction

As you all know, I am planning to sail and cruise one day on a balestron rigged catamaran. I have studied the subject and realized that one of the most difficult parts of this type of design is to get the necessary "bury" height to anchor the unstayed rig, about two meters on a 40-45 foot cat. In place of fighting to have this height and structural reinforcement on the center of the platform, I suddenly thought why not locate the mast in the hull, not two masts as some other design, but one mast in one of the hulls. This leads to an assymetrical design not unlike a proa, it is still a catamaran as the hulls are the same size. The majority of the comments that I received were that it would not work! So I decided to give it a try building a model. This page describes and illustrates this effort.

Design aspects

The design is based on the Crowther SP40 (Design No. 274) for which I have the study plans, I used the deck plan as a starting point, and a side picture for the hull shape. The scale is 1/20 which is the scale of the plan. I had to guess the shape of each station and it went relatively well, so I have a decent hull shape. I would recomend anybody to obtain the real offsets as it would be much easier. Each hull is built using five stations which are glued onto a backbone along the centerline (normal model building technique, obviously impracticable in real life). The platform beam was slightly increased and a swinging water surface piercing board was planned along the mast hull. The mast is based on the Aerorig design and has a balestron or front boom. It is freestanding and two roller bearings allow it to rotate freely.

The model boat is 65 cm long, the mast is about one meter long (92 cm above the deck), and the boat weights 2,2 kg (nearly 5 pounds).

Building the model

The hulls and deck are wood (balsa and some plywood), which are then covered and reinforced with epoxy and fiberglass. I do not have pictures of the actual building of the model (it was too messy!), but here are one or two photos of when the boat was in trials before being finished and painted. The model is controlled by a two channel Futaba basic radio control (less then US$ 100 anywhere), the right hand controls the steering (right means right), the corresponding servo acts on the axle of the starboard hull rudder and this rudder is connected to the other rudder in the other hull (see picture below):

The other control handles the sails (in fact one sheet controls the rig, that is the intended idea of the Aerorig) and the board. When the sail is in, the board goes down, when sailing off the wind, the sail is out and the board goes up.

Board in the up position Board in the down position

The mast and boom are carbon fiber tubes and the sails are made of polyester (50 microns). The cabin was made to cover and protect the electronic parts. The batteries are located in the starboard hull and their weight counter balances the weight of the mast in the port hull. The boat was then painted (couple of spray cans). Scale wise it would have been more logical to make the cabin a couple of centimeters higher. The cockpit area should have been more open but I had to cover the link between the rudders.

The board and rudders are Naca profiled and end shapes are eliptical design, rudders are slightly compensated.

Tank trials

Here in São Paulo we are lucky enough to have a special place to go for model sailing. It is in the Ibirapuera park and is not too far away from where we live. So here are the first pictures with one of the first sets of sails, which did work but were not as smart as the real thing.

Close hauled on the port tack, check the board in the down position in the picture at right.

On the starboard tack Some nice wave patterns

Obviously, she likes the downwind ride!

Everything worked as planned. If the wind is too strong, there is a risk of the boat capsizing (already happened!). I suppose I should have a second mast with a smaller set of sails. At the beginning the boat had some difficulty to tack, I realized that the boat had too much weather helm, so some adjustments of the angle of the board and the shape of the sails did the trick.

Sea trials (Ilhabela, Brasil, December 2000)

I took the little model to try it in the sea, more to have nicer pictures then anything else, but the exercise was fun. The model tacks in a bigger angle than normal and maybe the size of the board should be increased. This will be the next project!
Also when the waves are dead ahead the boat does not have sufficient inertia to pass them well, it tends to stop too easily.

It nearly looks like the real thing, it even surfed down the waves!

Conclusions

Does a freestanding rig located off-center in one of the hulls work? Yes it does! There is no noticable difference on how the model behaves from one tack to the other.
Does a freestanding rig have to be located in the hull? No! There are already plenty of examples showing this type of mast located conventionally in the center of the cabin. The question remains, is it worth the trouble of reinforcing the cabin and build a structure that will diminishe the space and visibility from the main saloon? Imagine the view from a inside steering station located on the starboard side in a design such as above. Nearly 160 degrees of free panoramic view in front of you, no mast to obstruct it! The structural reinforcements needed in the hull are much more logical and would go along with existing structures.
Will I stubbornly insist to have my boat built like that? Probably not on my own, but if a naval architect would agree to modify one of his designs, then I would probably give that solution all my attention.

Updated 01.01.2001