Hello all Proa fans, I have been cruising the site for a while but can’t find specific info re mast position, Not sure which is more efficient or creates less rudder loads, mounting on cockpit beam as Jzerro or hull edge as Bieker design. I have read Russel talking about the great windward performance being due to the large gap between jib and main giving a clean slot. But in his latest collaboration mast is mounted on gunnel edge, leaving a much narrower slot. Any ideas or explanations appreciated, or suggested calculation method. I will be using Bermudan rig as I am familiar with it and love windward performance!

A little about me as my first post.
I am from Australia and built my first multihull model at 10yrs old, unfortunately I then was side tracked into the land of the monohull and am only now finding my way back at the age of 46. I will be building a 13m voyaging Proa as soon as possible. The design is underway and I will no doubt have many questions as I go. Not deviating too far from the concepts Russel has been using.

Thanks for reading,
Jo

in case you haven’t seen the design below, it is not mine but Brown/Bieker and has most of the elements I will be using.

Hi Jo,

Where abouts in Australia are you? I think I can safely say that all of us proanuts are eagerly awaiting this boat being built!

Regarding your question, there is of course no simple answer. Having the main and jib closely coupled causes the sails to act together more or less as one slotted airfoil. You get higher maximum lift at the expense of a lower lift/drag ratio (L/D). Decoupling them (moving them further apart) allows each sail to function more independently, resulting in a higher total L/D, but the total maximum lift will not be as high.

Which way you choose to go would depend on the L/D ratio of the hull, the windage of the hull and how much time you will spend sailing to windward. A draggy hull and/or a boat with a lot of windage will not benefit much from improving the rig L/D because the rig drag is only a small part of the total drag. If you can’t benefit that much from the improved L/D, then the extra lift from the close coupled rig means that the rig can be smaller, which lowers the centre of effort, countering some of the negative effects of the lower L/D.

There aren’t all that many boats that can greatly benefit from high L/D rigs, usually boats that travel at high windspeed/boatspeed ratios such as high performance catamarans or foilers. In the case of your boat I would tend to think that a close coupled main and jib would give the best results, but the only way to know is to either try it both ways or do an extensive analysis that takes into account all of the major variables.

Mal.

HI Mal,
thanks for the fast and informative reply. She won’t be a foiler but certainly slippery with low windage. currently working on 1.6m beam (at the deck) over 13m with lightest displacement possible, building in cold molded ply over frames/stringers.
I have been inclined towards the bieker/brown mast position, to enable enlarging the cockpit width for 2 seats. From what you have said I don’t think there would be a huge difference either way. And I will still have some separation of the foils which will probably be the perfect compromise, if it reduces back winding of the main that would be great. Watching videos of Russels boat the sails are setting so cleanly!

I won’t be doing any major analysis prior, not having the money or patience and I don’t want to get bogged down in detail. No matter which way I go I don’t think I will be disappointed with the performance. It has always seemed to be that when you really want to get somewhere it is to windward, hence my love of boats that drive well in that direction. If it means I or the crew can’t take much with us, well all the better.

I am in the NSW Sth coast, currently landlocked having sold my yacht to start the transition to multihull land. No one wants this boat built sooner than me, but realistically I think it will take me 18mths from here at best. It will be a simple boat with the ability to trailer! which has also dictated somewhat the max beam.

I have my hopes it will be sooner.

Thanks again for the reply and more comments or ideas welcome.

Jo

Hi Jo,

I’m in the NSW Southern Highlands, so not too far away!

Cheers,
Mal.

Jo - 18 February 2014 03:35 PM

Not sure which is more efficient or creates less rudder loads, mounting on cockpit beam as Jzerro or hull edge as Bieker design.

With drag to windward of the lee hull (unless you fly the ama), there should be less rudder load from moving the mast to windward. 

You also have one structural and one spatial constraint.  The further the mast is to lee and the closer to a line connecting the two stays, the greater will be the compression load if you are caught aback with sails up.

If you try to optimise for that structural constraint, then you may not be able to sheet in the main sail closely enough, unless you choose a masthead rig without roach in the main.

The Bieker design is inteded for racing.  Mast placement may have been influenced by the need to sheet the main more closely than on a cruiser.  Russell is the person most likely to know.

Regards

Robert Biegler

Hi Robert
Thanks for your response, I am a little unclear of a couple of your points.
I understand that rig loads will increase if caught aback with mast further to lee, though I feel that reefing correctly (ie early) should take care of that issue, and with a slight cant of the mast to Lee it effectively widens the base.

you mention optimising for the structural restraint of having mast further to lee and suggest I may not be able to sheet main closely enough unless I choose masthead with no roach.
Then the following paragraph seems to contradict this re the Bieker design “mast placement may have been influenced by the need to sheet the main more closely”. So I may be reading it incorrectly, I am proposing mast position similar to Bieker and they have fractional rig and main with large roach

If you could clarify and expand further on “optimising for structural restraint” that would be appreciated. I assume you mean transferring mast loads through to the hull?

I imagine Russel does have the answer to these questions but I am hesitant to approach him so soon after joining as I imagine he is asked more questions than he cares to answer.

Kind Regards
Jo

I think there is a very practical element also figuring into the lateral placement of the mast.  In my case, co-ordinating the structural support needed with proximity to a ring frame, while maintaining enough room to access space forward of the ring frame is a driving factor.  Since interior space is at such a premium in proas, placing the mast offset also buys you a usable companionway where a centreline mast support post would be.  Weight is also optimized if you can double up usage of structure eliminating single purpose load bearing structures.

I’ve noticed Mr. Brown’s design’s are using small lateral sheeting spar(s) to extend the sheeting position to windward as well - allowing centreline sheeting, even though the now “bow” is to leeward of the offset mast.

I’ve noticed Mr. Brown often does what he feels is right (and will arrive at the solution he wants) - even if he breaks convention with what others might expect.  This has led me to very closely inspect anything in his designs that doesn’t seem conventional - because I’m pretty convinced everything done on his boats is for a reason.  It’s hard to give up on bi-lateral symmetry after a lifetime of being quite convinced by the likes of Kate Upton and Olin Stephens that it is perfection.

On a similar note, I’m pretty convinced that Mr. Brown’s designs are based greatly on his experience level and venues as well - I don’t think that he fears getting caught aback as much as people like me - and he’s designed his boats for optimal structural loads when sailed properly.  HIs boats are also designed for offshore venues and long, autopilot-controlled tacks - he’s not twitching about like me in rivers and lakes with channel widths that can be measured in feet.  I suspect this is also why he does not actively promote his own designs for others - he has based his calculations on his own abilities and may not want to put someone like me at his whipstaff - for fear I’d hurt myself or the boat.

—
Bill in Ottawa

Jo - 19 February 2014 03:12 PM

you mention optimising for the structural restraint of having mast further to lee and suggest I may not be able to sheet main closely enough unless I choose masthead with no roach.
Then the following paragraph seems to contradict this re the Bieker design “mast placement may have been influenced by the need to sheet the main more closely”. So I may be reading it incorrectly, I am proposing mast position similar to Bieker and they have fractional rig and main with large roach

A masthead rig without roach in the main would likely be a less efficient than what is on the boat.  So I think they had three basic options:
1) a fractional rig with low loads when caught aback, meaning large lateral separation between mast and stays, but an angle of the stays that prevents close sheeting of the main
2) masthead rig with no roach that allows large separation and close sheeting
3) fractional rig with close sheeting but higher loads when caught aback

They chose the last option, which I believe to have the highest loads when caught aback, but the greatest aerodynamic efficiency.  If you design a racing boat, then the trade-off between arriving first and risking damage that forces you to abandon may well be skewed in favour of increasing your chances of winning.  Besides, one generally assumes racing skippers to be exceptionally skilled. 

Dashew tells of discussing the dimensions and price of a rig for one of his designs with a company that offered to build the rig.  He spotted a factor in the design spreadsheet that made up to 50% difference in the weight of the rig, and they didn’t want to tell him what it meant.  He refused to give them the contract unless they explained.  They said it was the bozo factor, their subjective assessment of the seamanship of the customer.

I have sailed in places where gusts changed wind direction up to 90 degrees in about a second, and I have had the occasional involuntary gybe even without such quick changes in wind direction.  I would want to make allowances for that in any rig designed for my use.  This may be less important for you.

Regards

Robert Biegler

Thanks also to Bill for your input.
I did some more searching and there are older posts with much good info about this topic, they are in this forum and listed below.
“how much mast offset “feb 2014 and “the need to get aerodynamic and hydro dynamic forces aligned” (2011)

It appears that there is a good case for having the mast further to windward regarding minimising rudder load but I am still unable to ascertain why the new bieker/brown boat will have it further to Lee than Jzerro, they don’t seem that dissimilar in shape to warrant such a large change in mast position. Anyone with more knowledge than me, especially about the Jester class (I think that’s what Brown/Bieker call it ?)  might like to try explaining fully.

I want to get the balance right, an unbalanced helm destroys any sailing craft, have been there and not keen to go back!

Also does canting the mast slightly to leeward move the CFP (centre of forward power) further to lee? I think it will a bit.

Thanks, Jo

Jo - 20 February 2014 03:50 AM

It appears that there is a good case for having the mast further to windward regarding minimising rudder load but I am still unable to ascertain why the new bieker/brown boat will have it further to Lee than Jzerro, they don’t seem that dissimilar in shape to warrant such a large change in mast position. Anyone with more knowledge than me, especially about the Jester class (I think that’s what Brown/Bieker call it ?)  might like to try explaining fully.

Strictly from an engineering standpoint, Jzerro’s mast placement opts for the mast base to be out on the Aka beam, rather than the edge of the Vaka hull.  This brings in a lot of loading and Aka beam strength requirements, as there is no dolphin striker or waterstays.  If you placed a mast on the beams between the hulls of a catamaran there would be a lot of structure supporting it.

The Jester design places the mast in-line with the edge of the Vaka hull, transmitting compression loads straight downward, reducing the beam loading demands.  If the GA drawings are correct, there is a ring frame that joins the hull sides at the exact loading spot of the mast as well as the boat sides.

Although people seem really concerned with the loads generated when the rig is caught aback, the actual staying base width on a proa is almost as wide as a monohull - on the normally lee side.  By offsetting the mast to windward, the offset twin “fore/back” stays on the “lee” side are more than sufficient to protect the boat if it does get caught aback.  The real problem with getting caught aback is the forestay (now backstay) getting caught on the large roach of the square topped main.  This leads me to believe the first action on getting caught aback would be releasing the main halyard - to drop the excess roach below the “backstay” enough to let the pressure off the trapped main.

Jzerro and Madness (a Brown collaboration with John Harris) opt for a more beginner-friendly wider staying base via the mast placement on the Akas - where the Jester opts for the structurally sounder compression loading approach that has a somewhat reduced tolerance for getting caught aback.  Given the Jester is a racing hull and both Madness and Jzerro are high performance cruisers, it seems appropriate.

Mr. Brown’s designs seem to evolved from huge on-topic experience and mad genius in equal parts.

That’s my armchair analysis - I’d love to hear from Mr. Brown to see how far I am off the target.

—
Bill in Ottawa

Bill S. - 20 February 2014 05:07 AM

Jzerro and Madness (a Brown collaboration with John Harris) opt for a more beginner-friendly wider staying base via the mast placement on the Akas - where the Jester opts for the structurally sounder compression loading approach that has a somewhat reduced tolerance for getting caught aback.

Bill,

Is it not the reverse of what you are saying??? Jester’s mast is closer to the line between forestays and therefore (slightly) more tolerant to getting caught aback in terms of sail trim? Jester’s mast compression loads caught aback of course will be greater.

My guess on mast location is simply that the Jester mast is located where it is to allow quick and easy access to below from the cockpit whilst also allowing the sailor to get a better view of the mainsail and work more safely at the mast facing to leeward away from the wind and spray.

Rob

One point, with the fat-head sail snagging the back stay, it will not happen when reefed. So in strong winds, when you really would not like being aback, you would be ok
Another,  it is with a big genoa that getting aback is bad news,  with a small jib, the main is working to push the boat back into the wind.
So when going below the sails can be set safely, and of course, compared to most, you will still be flying along.
Mark.

Mark - 20 February 2014 12:09 PM

One point, with the fat-head sail snagging the back stay, it will not happen when reefed. So in strong winds, when you really would not like being aback, you would be ok
Another,  it is with a big genoa that getting aback is bad news,  with a small jib, the main is working to push the boat back into the wind.

Yes and other thing I forgot to point out is that with Jester’s mast being further to leeward and the genoa car tracks still well inboard on the pod, Paul Bieker is obviously looking to go down the closewinded, narrow slot route.

Rob Zabukovec - 20 February 2014 11:25 AM

Is it not the reverse of what you are saying??? Jester’s mast is closer to the line between forestays and therefore (slightly) more tolerant to getting caught aback in terms of sail trim? Jester’s mast compression loads caught aback of course will be greater.

My guess on mast location is simply that the Jester mast is located where it is to allow quick and easy access to below from the cockpit whilst also allowing the sailor to get a better view of the mainsail and work more safely at the mast facing to leeward away from the wind and spray.

Rob

Rob:

From one perspective you are right.  There is a little more space before the backed sail fouls the stays.  From another perspective the wider spacing between the mast and the line drawn between the forestays in Jzerro and Madness mean the mast is better braced by a wider staying base.  In reality both designs easily could run into trouble getting the large roach up high fouled on the “backstay”, creating a high pressure barn door that is tough to depressurize.

It’s pretty simple - getting caught aback isn’t a good thing.  Real world though, if the wind is high enough to make getting caught aback a real problem, you should probably be reefed down and the extra roach area is already beneath the backstay and can’t foul.  I suffer from most armchair analyst’s biggest problem - theoretical extrapolation doesn’t come close to experience.

I talked with John Harris and he indicated that Madness tolerated blown shunts (causing the boat to get aback) pretty well (in lighter winds).  Everybody starting in proas has a lifetime of “unlearning” to do, and we have to develop the same amount of respect for avoiding getting caught aback that we already have for sailing by the lee and causing unexpected crash gybes.  I haven’t been caught sailing by the lee for decades - and I expect that once I get “good” at proa sailing and develop some muscle memory I won’t get caught aback - or so I hope.

Cheers,

—
Bill in Ottawa

Thanks all for the well thought responses.
I am personally not worried about being caught aback any more than I was that my keel would fall off, probably less!
I am still wondering re the drag issue and coupling moment difference between Jzerro and Jester class. Having the main further to lee accentuates the distance between drive force and centre of drag. Is it just a matter of dropping the front board/rudder to minimise the imbalance as I can’t see anything else that would help. Whereas Jzerro would have the forces more in line giving excellent helm balance.

It seems that Brown/Bieker have probably optimised the ama for light skimming to minimise the far out there drag component? and therefore can locate the mast on the aka. Any agreement with that?

As a few have pointed out locating the mast on Aka gunnel is a very solid approach to spread loads. I don’t like the companionway setup though, diving in seems the best method so my boat will have a hatch offset (currently), though it does weaken the area It will have an A frame and lateral tie through cabin top, so will in my opinion be strong.

Jo

Jo,

2 basic comments so far:

1) 13 metres is long and a bit of a handful for a trailersailer, even in Australia. Think about making it 12 metres so you can at least stuff into a container if trailering ends up being too much of a hassle.

2) I don’t like central hatchways on proas either, basically because the best space on the boat is taken up with just getting below and they get in the road of the mast and lots of other things. Use 2 hatchways below….. and use the aft one when it gets rough to keep more water out of below. You need another hatch for cross ventilation and escape, so just make the most out of it.

Rob