16m Cruising Proa by James Brett

03 November 2011    Editor    0 Comments

Aukland based marine designer James Brett as turned out a tantalizing rendering for a proposed 53’ (16.2m) offshore cruising proa.

The bridgedeck cabin includes a double sea berth, full galley and saloon, while the main hull contains two double cabins, and the head. The workings of the boat are the same as my Free Radical design, using the same well proven rig and rudder systems. Construction will be in plywood with ply/balsa laminate in the bulkheads.

 proas / new designs 


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05 Nov - 08:06

John

This is an intriguing design and a very nice looking proa, but there’s a serious problem of misalignment between power and drag.

The mast needs to be moved closer to the lee side of the deck house (as drawn)! Exactly how much depends n the characteristics of the sail.

Here’s why:

With the sole exception of when you are flying the ama, the lateral center of hydro drag will vary and be somewhere between the two hulls, while the Center of Pressure, specifically the “Center of Forwards Power,”  will usually be to leewards of the mast. This creates torque between the center of drag, the CFP, and the rudders, as follows:

{Drag (in ft/lbs or metric equivalent) X lever arm between CD and C-Fwd-Power} / lever arm to rudders.

This causes an extra lifting load on the aft rudder, leading to higher drag and earlier stall, and larger (heavier) rudders to compensate.

Since this is a cruising boat it is unreasonable to expect that the ama will be flying constantly, and since most cruising is done on off-wind courses this constant drag and torque (and positive feedback circuit) will be maximized as the ama will be in highest-drag mode, supporting much weight. There can also be a pretty drastic positive feedback mechanism between the bow board and the stern board any time the boat yaws seriously to windward: front board lift will increase at a rapid rate, trying to spin the boat even farther to windward, while the aft board stalls while trying to maintain course.

Keeping loads off the aft board is important on off-wind courses, since the effects of circular water molecule motion as each wave front passes will mean the rudder is located in a cyclical low-lift environment for several seconds at a time (since boat and wave are traveling in similar directions), which can lead to aft rudder stall and a broach.

Back to the equation: it’s obvious that if the CD and CFP are aligned, then torque on the rudders is reduced to zero. This is the point of moving the mast more to leeward.
This gets you (A) more effective rudders that will be less likely to stall; (B) improved directional stability, and; (C) better maneuverability (due to reduced constant loads on the rudders).

This is also why Russ Brown’s proas have worked so well, where many others haven’t. He’s got the forces aligned! Several other proa designers have NOT paid attention to this, and their boats have had continual “rudder issues.” Now you know why… grin

This problem will NOT show up on a small-scale prototype! Reason: on a day-sailer/camp-cruiser size boat, mobile crew weight makes up a majority of the windward “ballast,” and when you move off-wind this weight is naturally moved closer to the main hull as overturning forces decline.
Therefore the center of drag also moves closer to the main hull, and the out-of-alignment effect is mostly eliminated.

On a cruising-sized boat this crew movement, if it happens at all, will account for much less change in the position of CD, so force misalignment is NOT reduced.

This is an unusual rig and I know the designer has experimented with it on a smaller scale. I’m sure he has a good idea of where the CP is, and will be able to adjust mast position and bring CD and CP into alignment, especially for off-wind courses. It appears the mast is free-standing and so needs strong support, but it is not difficult or heavy to do this. If he can tie mast support into the deck house, this likely will result in both a reduction in weight and an improvement in static stability (as well as freeing up prime real estate inside the hull).

Also, it appears the mast has a slight windward cant, and this will also help solve the out-of-alignment problem.

05 Nov - 08:14

John

Damn… that sentence should read:

“This is the point of moving the mast more to WINDWARD.”

Ouch…

07 Nov - 15:06

Tom

I would think that the fron board would be up most of the time.  this puts the CE forward of the aft rudder, tending to counteract the drag from the Ama.  The junk style rig shown will move the CE to the aft half of the boat on each shunt.  As this is a free standing mast, there should be no reason why the rig can’t be rotated OVER the Ama side putt the CE rather close to the combined CD if the 2 hulls.  I’ve actually been playing with a much smaller version of this basic layout.  I actually intend to be able to “Tack” the boat when necessary/convenient, not to mention being able to achieve the downwind balance just mentioned.  a leeboard MAY be necessary, possibly fore and aft pivoting to assist the rudders on the lateral resistance.

08 Mar - 15:52

Tom

John,

After playing with a whole bunch of rudder designs, I like your analysis better.  I am now playing with ways to get a properly supported, unstayed junk mast to windward of the Vaka.  I find myself arguing your point….