Subject: Re: [harryproa] 60' Trailerable Proa
From: Dennis Cox
Date: 7/27/2010, 10:16 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Hey Mike,
 
Thank you for taking the time to help me out.  I value anyone's actual experience over my rationalizations.  I've never seen a Proa in the flesh, much less sailed one.  Here are some of my rationalizations... please point out anything that is wrong.
 
In my brief, I state two configurations... racing and cruising.  The idea being... if I was trailering to a race, I would unload the "cargo" in the lee hull.  Similarly, if I was cruising around and "found" a race to play in, and if I sized up the competition, I could unload the "cargo" at the dock and go have some fun.  That leeward cargo includes, most of the potable water, all fuel, portable generator(s), some of the battery bank.  I'm looking at electric motors so the battery weight is sizable.  It also includes toys like kayaks, bikes and general "junk" that any couple gathers in the attic.
 
In race mode, we'd be on top of it all the time so getting caught aback shouldn't be an issue.  In fact, in light airs (in a race) I'd plan on running as a Pacific and freek out the competition by tacking and/or shunting at will.  In Pacific "mode" as you suggest, I'd more readily fly the long hull (old lee hull) and remove its surface drag.
 
When running conservative... in cruise mode, the leeward righting moment is 170,000 ft-lbs.  To windward, its 57,000 ft-lbs (33% of leeward).  So I would have had to be running pretty hard (more than 33%) before the shift to be an issue.  Also, its my understanding, that the EasyRig weather vanes in an aback situation.
 
Although I have not decided, I am also looking into doing wings instead of an EasyRig.  In that case, they will not use sheets, but will have attached rudders controlling their angle of attack.  They should wheather vane and eliminate any possiblity of going aback.
 
I am concerned that I'm missing something fundamental.  Besides the possible lighter weight of the leeward hull, is there anything about the sail placement (HarryProa vs Atlantic) that makes aback more sensative on an Atlantic?
 
Dennis


From: Michael Gehl <mike@vail.net>
To: harryproa@yahoogroups.com.au
Sent: Tue, July 27, 2010 9:16:55 AM
Subject: Re: [harryproa] 60' Trailerable Proa

 

Dennis, I note one comparison point that's missing - being caught aback.


In your no-mission-other- than-flotation leeward hull in the Atlantic configuration, if the sail(s) were caught aback, the Atlantic proa would suddenly become a Pacific proa, the lightweight leeward hull would present little righting moment against capsize, and the lever arm of the sails would be acting nearly in line with the roll center through the windward hull.

In the Harryproa configuration, being caught aback would a) provide a much lesser moment about the roll center, b) need to lift a significantly heavier, more complex leeward hull, and c) once the boards were lifted clear of the water, would tend to shove the windward hull sideways across the water.

Been through enough sudden 180 shifts in the tropics that I can't imagine feeling comfortable without total vigilance 24/7 while sailing a what may become a Pacific proa in seconds ;-)

Mike

On Jul 26, 2010, at 5:17 PM, Dennis Cox wrote:



Hey Mike, et al.
 
I have the greatest respect for Robert's HarryProa.  I am certainly not implying any superiority of the Atlantic design.  In fact, most of the reasons are not performance related.  Also, its interrelated with the steering leeway resistance issue.  I am hoping someone here might take pitty on my fence riding and give me something definitive that tips the scale either way for good... but hear me out, because these are the things that am pushing me toward the Atlantic.
 
Non Performance Reasons
These are reasons that have to do with the trailerable aspect or for me personally.  They have nothing to do with one design being superior to the other.
  • When on the trailer, the first thing, I'd step the masts on the windward hull.  As you suggested, I was thinking of a relatively small "crane" based on the cheap car engine hoists.  Once up, these can be used in conjunction with the boats winches to lift the windward hull up and over and down. 
  • I've been watching a lot of Rick Willoughby's comments.  If I keep the leeward hull simple (without) mast and leeway prevention structure, I can make it quite cheap.  The one I'm making now should come in complete for less than $2000 and somewhere around 800 pounds if I don't get sloppy.  The point being, I could very easily make a second one using the squared off sheet hulls that Rick's research suggests may have a superior performance edge.  It may go for less than a $1000.  IOWs, I can play Mr. Potato Man. 
  • The lee hull is lighter - for getting off the trailer and moving it around. 
Structural
IMO and for a schooner rig, one design has no significant advantage over the other.   
  • In a HarryProa or an Atlantic Sloop the mast loads have to be transferred from the mast base to the cross beams by torquing the hull.  In an Atlantic this would make things far more difficult.  It fully justifies Robert's statment about being able to make the unstressed HarryProa windward hull far lighter than the Atlantic.  It would be clearly lighter.  However, in a HarryProa or Atlantic Schooner there is no torque in the hulls.  If the masts are mated to the cross beams all moments from the masts can be directly transferred into cross beams.  My contention is that both hulls could be made lighter in a schooner rig and frankly there would be no advantage of one over the other (that I can imagine yet). 
  • The largest moment in the beams of a HarryProa (and thus largest cross section of the beam) is at the base of the leeward hull.  In an Atlantic, the largest moment in the beams is at the base of the windward hull.  It is helped out by a significant structure... the bridge deck.  The beam (in an Atlantic) as it goes into the lee hulls has nearly no moment... just shear.  It can be much smaller and thus offer less resistance to waves and submerging.
  • I hear what has been said about leaning the mast out and keep the boat from going past 70 degrees.  I can easily see that in Robert's smaller designs.  I'm having a little trouble with it for a 10,000 lb boat.  Mast tend to come away in roll overs.  It is said any mast that doesn't... is way over built and way too heavy.  In my minds eye, a boat traveling 20+ knots and has flipped at a rate that the helm couldn't save must have significant angular velocity.  When the mast and sail slaps the water, IT will come to a dead stop while the linear and angular inertia of the rest of the boat (27 feet up in the air) is certainly going to continue... result (I believe) is a snapped mast and flipping of the hulls.  Arguably, the HarryProa having a little chance is better than no chance at all with an Atlantic. 
  • This may be a cardinal sin, but, I'm debating about putting the rudders on the windward hull also... seehttp://au.groups. yahoo.com/ group/harryproa/ photos/album/ 1001384692/ pic/461401281/ view?picmode=&mode=tn&order=ordinal&start=1&count=20&dir=asc  Structurally, I can tie all the major force producers into one structure.  See more in fluid dynamics section.
Fluid Dyanmics
Out of my comfort zone, but...
  • I guess the heeling moment does get larger with an Atlantic when flying, but only as a result of exposing the bottom of the hull in addition to the sail forces, while a HarryProa windward hull shields the sail somewhat.  But in this size of a boat, I don't really thing exposing my bottom is really in the game plan... unless I've unloaded all the soft personnel and am racing.  And even then, I wouldn't be flying... just kissing.
  • The main one that drove me toward a Harry Proa is the significant decrease in COE by lowering the base of the masts to the lee hull.  At first this was clear cut.  HOWEVER, strangely enough, if I go to wings or oversize wing masts (still debating that one also) the heeling moment is far less than a sail.  My calculations show I can not get the lard-ass, windward hull up so I can get to the 25+ knots Michlet says the lee hull could do if unburdened.  So in this strange case, its better to put the masts up on the windward hulls to actually encourage heeling.  Yes, I know, I could put larger wings/sails on the leeward hull, but that adds to the trailering issues. 
  • If I put the rudders under the mast bases (on the windward hull) all that splashing and spray as seen on the all the videos, is under the bridgedeck and we have a cleaner view from the bridgedeck overlooking the lee hull.
  • With the rudders on the windward side, flying the hull reduces leeway prevention and thus lets it slide.

I think that's all, but I'll bring up more, when they hit me.  HELP!  Stop the voices.

 

Dennis

 
 

From: Mike Crawford <jmichael@gwi. net>
To: harryproa@yahoogrou ps.com.au
Sent: Mon, July 26, 2010 5:07:09 PM
Subject: [harryproa] Re: 60' atlantic proa

Dennis,

  That is extremely cool.  Thanks for pointing out the video link.  You could probably do this with a purpose-built trailer and a stub mast in a separate socket (1/3 full mast height, used to lift the primary mast at its center of gravity) , no crane required.  

  Now I *really* want to see you build it.  I've said many times that i want the largest trailerable boat I can get, and this would certainly qualify.  Actually, I'd be willing to put up with a 12' width, but fitting into 8.5' is even more impressive.

  I had actually brought up atlantic vs. harryrproa in a post right before the one you responded to, but for some reason the post didn't make it.  (thus my comment about ignoring my question about how the boat goes on the trailer).

  I'm partial to the harryproa design for the following reasons:  a) more space in the windward hull,  b) heeling moment decreases, or remains the same, as you fly a hull, while heeling moment increases with the atlantic design,  c) stresses are in line with the hull, while the atlantic has to translate rig stress through the beams to that long leeward hull, and  d) the possibility of popping back up after a knockdown.

  As far as handling the sails, a leeward design might offer more safety in a schooner configuration (from the tramp), while a single mast would probably offer safer sail handling with the atlantic (from the cockpit).

  But I'm sure you're familiar with all that.  Your design has more than enough space to deal with masts in the windward hull, and the taller leeward hull needed for a rig there might get in the way of your trailering setup.  As long as you don't sail on the very edge, there shouldn't be any issues.

  I wouldn't put on a pod for safety as cavalier suggests.  If you're sailing an atlantic proa hard enough to fly a hull, in anything other than flat water, there's a good chance you'll be going fast enough to trip over any pod small enough to justify its own weight penalty.

  What are you planning for steering and leeway resistance?

        - Mike

 
On 7/26/2010 2:43 PM, Dennis Cox wrote:

 
Hey Mike,
 
Did you see the video... on post #9.  It might clear things up.  Unless you've already seen it... :)
 
Also, I'm still bouncing between Atlantic and HarryProa placement of the sails (windward, leeward hulls).  I've got pros and cons on both sides and the tally list is still balanced.  Fortunately, the hulls and internal configurations don't have to be changed either way.  So... I've got PLENTY of time to decide. 
 
Dennis



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