The Simplest Proa?
This is actually the road my mind has been going down lately. I got off the trawler and got into a house with a large garage last June, so now I have building space. I just have to decide what to build for when it gets warm enough to glue here in another month.
I’ve been leaning towards a Bolger sharpie hull and fluctuating on a chine runner in the center 1/3 of the hull. I’m hoping this keeps the CLR for the hull more centralized. Then a pair of junks with the mast @ 25% to 33% in from the ends. I was going to go to the trouble of building a split junk with a central sail and rudders, and step the mast to windward of the vaka. the split junk allows you to have the mast @ 35% of the sail chord, and the CLE should end up VERY close to the mast, meaning a very neutral helm on all points if I get the mast in the right place. standard issues of mast screwing up the middle of the boat as well as supporting it.
With the schooner, just rig a standard cambered junk sail so the one in the bow is to leeward of the mast and the aft “rudder” sail is the one with the mast screwing up the camber. The I could put the masts free standing IN the vaka out of the way as well as have my @35% of chord mast placement on the sail, and a bunch of problems solved at the cost of one extra sheet to play with.
Being junks and easily reefed, I could go stupid high on sail area, as well as potentially partially reef the aft sail to help with the balance. If I have the euphroe blocks on travelers, there’s no reason it couldn’t be a tacker as well with enough ama volume, and go wing and wing downwind.
In pratical terms, there would be a possible performance improvement and a big safety improvement, as it would be next to impossible to capsise it, and might not compromise the simplicity of the boat… very tempting!
My two imediate concerns are leeway and sail performance… might need a daggerboard as the main hull (in it’s current form) would be heeling the wrong way to resist leeway, and probably a better sail (have one nearly done that might do the trick…). The ama probably would need a re-design as well, and maybe for the sail to generate more vertical lift it should shunt on the ama, keeping the mast where it is? Mast back stay would be impossible then, but if my improved system works I might soon have a free standing racking mast anyway.
Don’t just turn the boat around, take it apart and reassemble it inside-out. Swap the hulls’ positions, but keep the same side of each hull to leeward. You might consider building a little inflatable hull for the ama—with this geometry it will be lightly loaded and light weight is an advantage.
The advantage of your rig with the kite is that the kite is always high up, so it will always pull the ama up, but with a sail, as you sugest, it would probably be much easier to control.
Much easier. “Real” kite elitists call your rig “kite-on-a-stick,” yet more and more of them are actually using it… Ideally, there’s just enough power from the rig to optimally angle (cant) it to raise the lee hull just out of the water. If you’re overpowered, you cant the rig a little bit more and the ama zooms up to point, rigidly, at the center of the sail—and sits steady as a rock, regardless of rig power. When the wind’s too light to fly the ama, bring the rig back to vertical and use all the energy for driving the boat. There are very few compromises here, once you’ve committed to the kite-on-stick, which you have already done. 😉
Dave
This is actually the road my mind has been going down lately. I got off the trawler and got into a house with a large garage last June, so now I have building space. I just have to decide what to build for when it gets warm enough to glue here in another month.
I’ve been leaning towards a Bolger sharpie hull and fluctuating on a chine runner in the center 1/3 of the hull. I’m hoping this keeps the CLR for the hull more centralized. Then a pair of junks with the mast @ 25% to 33% in from the ends.
Congrats on the new digs. Everyone deserves a workshop.
Since you don’t yet know how far the CLR will migrate, and since by all reports everyone who does it this way is shocked at how far that is, I strongly recommend you put the two masts as close to the ends of the boat as humanly possible. It will be much easier to move them in later than to move them out later, I’m thinking.
Being junks and easily reefed, I could go stupid high on sail area, as well as potentially partially reef the aft sail to help with the balance. If I have the euphroe blocks on travelers, there’s no reason it couldn’t be a tacker as well with enough ama volume, and go wing and wing downwind.
I have seen many boats woefully under-rigged. Un-fun slugs. So I’m partial to too-much sail area, too. Again, always easier to make it smaller than bigger. Technically, a tacker isn’t a proa (I know, it’s just semantics. But sorry, in my universe “proa” means it shunts) OTOH, being *able* to tack is a very valuable thing, so long as the boat also shunts… 😉
Dave
Dave,
yeah, That CLR is the biggest bugaboo. Admittedly it’s a hope and a prayer that the heavy rocker combined with the chine runner only in the center third would hold it significantly far back. It would be easy to get the sheeting angle I need on the rear sail with bow sprits that could demount for trailering and storage.
There’s a scrap yard near me where I can pick up 20ft aluminum tubes of various diameters and wall thicknesses, and I’m only charged by the scrap value in pounds. Allowing for hull bury, a little deck clearance and what’s needed at the top for the halyard, I could easily set a pair of 15x8 sails on a 3 sheet boat, which would be 240ft2 and leave me a whole lot of sheeting room before I get to needing the bowsprits. 65% of 8ft is 5.2ft of radius for the swing of the boom(s). Ideally the Euphroe would hang @ 2ft beyond that, which puts me damn near the 33% mark with no sprits for mast placement. a 2ft sprit lets me step @ 22% in.
If the maximum camber on the sails is at the same 35% point as the masts, and they’re stepped as far to windward as possible in the hull, The CE on that front sail doing the hard driving should be very near the center of the Vaka as opposed to well leeward like with a standard jib headed sail or crab claw. It’s one reason I’m so interested in Slieve’s split junk. His forward third has more camber and operates clear of the mast compared to the aft 2/3. I would think it has proportionately more pull, dragging that CE even farther forward that a basic cambered junk.
Primary mode of transport would of course be pacific shunter, but with that much spread and such short chords, a sudden need to go atlantic would be a breeze. There would be no such thing as back-winded.
Sounds good on paper, eh? Worst thing that could happen is I would have to do what Skip did on P52 and add some rudders. If the schooner setup is too much hassle, it would be easy to leave the steps and build in a third central one, and I’d have a scaled up Brett Proa, which is no slouch.
Once I hang rudders the whole simple thing goes out the window, but what the hell.
One way to find out for sure.
Dave,
yeah, That CLR is the biggest bugaboo. Admittedly it’s a hope and a prayer that the heavy rocker combined with the chine runner only in the center third would hold it significantly far back.
Somebody here posted a very nice rendering of a hybrid hull—it had what amounted to a mini-keel in the center 1/4 of the hull, then round-bottomed shallower sections out to the ends. Not really a mini-keel as it had significant interior volume (I think I’d try one of Tom Speer’s reversible proa sections); the important things were that it was not much deeper, overall, than a sharply asym hull is amidships, and the whole boat would still respond normally to weight shift—but maybe without the big CLR migration. If this works, it’s a bit of genius, as it takes one of the weakest attributes of asym, weight shift hulls and chucks it out the window.
I wonder, is Gary Dierking monitoring this forum? He’d know if it’s been done—and if it works!
Why don’t we have more modelers on this forum? This stuff isn’t hard to build in miniature, and the geometry of the setup isn’t effected by scale.
Dave
Somebody here posted a very nice rendering of a hybrid hull—it had what amounted to a mini-keel in the center 1/4 of the hull, then round-bottomed shallower sections out to the ends. Not really a mini-keel as it had significant interior volume (I think I’d try one of Tom Speer’s reversible proa sections); the important things were that it was not much deeper, overall, than a sharply asym hull is amidships, and the whole boat would still respond normally to weight shift—but maybe without the big CLR migration. If this works, it’s a bit of genius, as it takes one of the weakest attributes of asym, weight shift hulls and chucks it out the window.
Are these the ones you were talking about? I like where you are going, but what is the weakest attribute of asym that this solves?
I was just trying to take the “cutaway forefoot” thing to its logical conclusion (Herreshoff style!). I guess another way to think of it is like a Dart or Hobie Wave catamaran. The wave is a surprisingly good little boat.
Hugo put a lee keel on Proud Mary with big pointing improvements.
Are these the ones you were talking about? I like where you are going, but what is the weakest attribute of asym that this solves?
Might have known it was you. 😉 The thing we are looking to “solve” is the large migration of CLR with asym hulls (ie: w/o daggerboard(s)) and weight shift steering.
One solution is to use a hull with little ability to produce side-force (round sections, shallow overall). Another *might be* using a chine runner, again in a hull without strong side-force generation.
Question is, would the “mini-keel” you show also significantly reduce the CLR migration? If it could, it’d be a nifty method for doing so—not requiring deep draft or moveable foils. I have a feeling the answer is “yes.” I am less sanguine about the chine runners Tom plans on.
Why is the large range of CLR migration a bad thing, Dave?
Doesn’t that effectively give you more precise control of weight shift steering?
What Dave is referring to is that the CLR moves a long way forward of midships. A hull hull with a long horizontal keel line will have a larger CLR migration than a hull with a centralised short, deep keel. What this means is that the rig also has to be moved a long way forward on each shunt. If the CLR could be kept close to midships on each shunt you would have a greater choice of rig styles to choose from, i.e. rigs with the mast fixed at midships.
Don’t just turn the boat around, take it apart and reassemble it inside-out. Swap the hulls’ positions, but keep the same side of each hull to leeward
I understand that, but what I’m refering to is the flat bottom of the hull will be sligtly raised on the leeward side, making it dig deeper the windward side. Im I correct in thinking this is not as good to resist leeway as the oposite, digging deeper to lee, as it does do now?
Question is, would the “mini-keel” you show also significantly reduce the CLR migration? If it could, it’d be a nifty method for doing so—not requiring deep draft or moveable foils. I have a feeling the answer is “yes.” I am less sanguine about the chine runners Tom plans on.
Munroe’s first proa had something similar to this, a semi-circle board/keel, I don’t think it would have been foil shaped, and a fixed mast on the center, but on is second proa he did away with it and got two dagger boards to balance the CLR… Don’t think the “Mini Keel” worked well there.
I did try a “mini keel” on Proud Mary, but it was just a board, no shaping or volume at all. As Chris mentioned it seems to have improved pointing, but I think it just acted as a very low aspect dagger board. Unfortunately the times I got it on the water weren’t the best for propelly test it. I’m quite sure it improved leeway, can’t really be sure on anything else…
PM is now equiped with a chine runner, 1/3 of the hull. I’m just trying to get better leeway resistence, and I don’t think the CLR migration afects PM in a bad way because of the rigg.
I only have a few little jobs left to do before getting PM wet again, when that happens I’ll report on the chine runner.
As a disclaimer, I’m not an experienced sailor or boat designer/builder, so do take my reports as my impressions, not scientific facts. PM is my first boat and I just like to have fun trying this and that!
Cheers
hugo
Don’t just turn the boat around, take it apart and reassemble it inside-out. Swap the hulls’ positions, but keep the same side of each hull to leeward
I understand that, but what I’m refering to is the flat bottom of the hull will be sligtly raised on the leeward side, making it dig deeper the windward side. Im I correct in thinking this is not as good to resist leeway as the oposite, digging deeper to lee, as it does do now?
You are correct. Sorry, I thought the main hull was more asymmetric.
I did try a “mini keel” on Proud Mary, but it was just a board, no shaping or volume at all. As Chris mentioned it seems to have improved pointing, but I think it just acted as a very low aspect dagger board. Unfortunately the times I got it on the water weren’t the best for propelly test it. I’m quite sure it improved leeway, can’t really be sure on anything else…
PM is now equiped with a chine runner, 1/3 of the hull. I’m just trying to get better leeway resistence, and I don’t think the CLR migration afects PM in a bad way because of the rig.
In that case, I wouldn’t change the chine runner. Only one thing at a time, or you’ll never really understand what changed!
As a disclaimer, I’m not an experienced sailor or boat designer/builder, so do take my reports as my impressions, not scientific facts. PM is my first boat and I just like to have fun trying this and that!
I have sometimes found that it’s the guys who don’t know precisely what they are doing—thus don’t know what “can’t be done,” that make the big breakthroughs. Carry on. 😉
Dave
On my AmbiProa model I used a hullform which went some way towards the mini keel idea, but without actually having a keel as such. The hulls (vaka and ama are similar) have a deep vee midship section, the bow sections are a shallower vee. All in an attempt to reduce CLR migration.
Very nice.
Its sort of a symetrified (ww/lw) classic proa shape.
I wonder what the handling characteristics of this shape would be relative to the marshalese profile that has the flat spot in the keel and lots of asymmetry.
Less migration of CLR in shunts? More or less sensitive to weight shift? Rig shift?
On my AmbiProa model I used a hullform which went some way towards the mini keel idea, but without actually having a keel as such. The hulls (vaka and ama are similar) have a deep vee midship section, the bow sections are a shallower vee. All in an attempt to reduce CLR migration.
Understood Mal, but I wonder… T2 from Gary (below) is pretty similar, and her CLR migrates a great deal, by most accounts. I’ve a feeling that, unless there is a discontinuity in the underwater shape (a knuckle or crotch), the CLR migration is going to be more like the hull-as-chord rather than daggerboard/keel/deeper hull as chord.
An interesting question, though. I’d love to see both T2 and AmbiProa tested under similar conditions to answer this. And speaking of which, this might be a perfect place to consider a Bruce Tank towing tank. Bruce rigged up a set of falling weight to propel models the length of an ordinary swimming pool. Gravity gave constant force and he measured leeway (course) and velocity, rather than as most towing tanks using constant velocity, then measuring leeway and drag. I’ve done this for comparing models which are grossly similar and it works quite well. As does his yoked towing system, which tows each model from opposite ends of a yoke, which can then be balanced to determine to great precision what fraction of lift or drag or resultant a test hull yields, compared known hull.
This sort of test isn’t very sensitive to scale factor (except for wetted surface, which can be dealt with), so seems to work OK with relatively small models. Simply measuring—or even just observing—two nearly identical hulls for difference in CLR migration should be very easy to accomplish.
Dave
Very nice.
Its sort of a symetrified (ww/lw) classic proa shape.
I wonder what the handling characteristics of this shape would be relative to the marshalese profile that has the flat spot in the keel and lots of asymmetry.
Less migration of CLR in shunts? More or less sensitive to weight shift? Rig shift?
This hull was symmetrical because the boat can both shunt and tack, hence ‘AmbiProa’.