As a newcomer getting a general feel for the information on this site I have noticed that there have been several comments on proposed designs recommending keeping the mast inboard to achieve a desired balance by more closely aligning the forward thrust from the sail with the overall centre of drag. Further Russell Brown’s designs seem to be mentioned as providing proof of this theory. 

With absolutely no disrespect to Russell whose exploits and proas I admire greatly, I may be wrong and I am now somewhat of an armchair admiral not having played with sailing proas for 15 years. I believe that there is a flaw in this reasoning which may limit the creative juices of readers of these pages whose thoughts may otherwise take them to a different configuration.  That reasoning may be a good “rule of thumb” for a proven Russell type proa with a fixed central mast a big main sail and a front board which is lifted each tack to bias the centre of lateral resistance rearward but in my view it has nothing to do with the reason this arrangement works and works well.

This reasoning appears to completely ignore the placement of the non-productive side force generated by the sails for which we all use a centreboard to negate.  This side force is about 3 times greater than the forward thrust from the sail when sailing to windward. Thus its longitudinal position has a far greater effect on balance than the transverse position of the sail thrust.

In simple terms, but slightly more complex in reality, if the centre of effort through which this side force acts is forward or aft of a centrally located centreboard this sideways force will generate a torque or turning force about the board in one direction which for balance, must be opposed by a corresponding and opposing torque produced by the forward thrust from the sail.  Whether or not these forces line up with centre of lateral resistance is irrelevant.  The real question is does the sail arrangement tend to cause the boat to bear up into the wind or bear away or is it neutral, that is balanced.

For comparison, think of a keelboat, as they heel this misalignment increases dramatically and yet there are plenty of them that sail OK when heeled. Also look at an aeroplane wing it produces lift and drag and a pitching moment or torque which tends to cause the aircraft to tumble.  This is why they use an inverted airfoil at the rear, the tailplane, to counteract this torque. This pitching moment is also working on your centreboard so clearly the size and efficiency of this board will also be very relevant to the balance.

I don’t claim to be an expert sailboat designer but I hope that I understand the basic forces involved and so far a consideration of their size, direction and position has paid off for me. I would be interested to hear Russell’s view on this. I suspect he did not position his mast in the first instance so that the forward thrust from the sail close hauled would be as close as possible to the centre of drag but for far more practical reasons.

Copying a successful design makes sense and Russell’s certainly is one but it is clearly not the only one and to ascribe some geometrical significance to that layout ignores the important fundamental forces that are at work in the sailboat.  In my view such rules stifle innovation in design and layout which I believe most proa enthusiasts want and appreciate.

As I understand it, the forward sail thrust close hauled will remain the same irrespective of the sail’s position, variations thereof will only change the tendency of the sail(s) to cause the boat to round up or bear off.  Thus provided the boat is more or less balanced the force producing the speed remains the same.

If the sail can be positioned to cause the craft to have a slight tendency to bear off and there is a spare front rudder which can counteract that and at the same time create lift to windward more efficiently than a fixed symmetrical centreboard and which will have to be countered by the rear rudder also providing lift to windward I will take that every day.

For proaliferation,
John Pizzey