BOX KEEL
Okay, Idea in my head.
I’m sure most people in here are familiar with Bolger’s ideas on box keels and plumb sided sharpies with matching curves on the bottoms and sides.
I’ve been doing the math on a VAKA where bother the B/L ration as well as the draft are @ 16:1
If one were to take an 8’ sheet and rip it into 16” wide strips for the bottom, and then 3 more sheets and rip them into 24” strips for the sides of this “box keel”, it would have a really high prismatic and displace @1700lbs at a 16” draft, leaving 8” clear to a knuckle formed by whatever habitable leepod/windward pod structure you place on top of it. This of course would require some thought as to how it meets the waves.
realizing of course that this is not the “optimum” speed shape, I’d be interested in hearing how people here think it would perform and whether or not this would provide enough lateral resistance for windward work or if a dagger board would still be needed. I don’t think pounding will be an issue with a botton this narrow. Bolger stylke curvature would have 8” of rocker, meaning the bottom is 8” below the waterline at static rest.
Tom
I think there is some merit in a long skinny slab sided hull, but question the goodness of a box keel at these L/B ratios.
http://wikiproa.pbworks.com/w/page/31152409/Bionic Broomstick
Is my take on a slab sided approach, though for a different boat altogether. Your slab sided boat will probably go to weather with out a daggerboard but would point higher and sail faster with one. Only way to find out is to try.
I’m hoping to build the Bionic Broomstick soon for an early spring splash with a bidirectional cambered junk sail.
cheers,
Skip
Skip,
I’ll be really interested to see how that works out. It’s basically what I’m doing at a MUCH reduced scale, right down to the cambered junk. I was planning on exchanging rudders as the sail CE went from end to end, and possibly a pivoting leeboard/daggerboard if that didn’t quite cut it for CLR. I suppose if I tried it on a 16 footer first I could pull the junk rig outta my little tri.
Tom
Hi Tom,
I’m hardly qualified to answer your questions, but I found it, er, “kismetic.” The reason is that I happened to be reading Bolger Boats when I checked in here. I actually had the book sitting just to my right. I checked it out of the library today because it has a proa in it.
Bolger’s design features what looks to be an equilateral triangle cross section made of two planes only—not counting the deck, giving the boat long, rakish bows. The drawings in the book are small, but it looks like the boat had a huge, side mounted, leeward rudder, so apparently Bolger though it would need that, even given the fat vee hull.
BTW, this proa could easily be built of ply. It certainly would not have the high prismatic coefficient you are looking for, but it uses material economically. I’m pretty impressed by Bolger’s general sense of aesthetics. He was a highly innovative designer who yet always seemed to have an eye for tradition.
Best,
Rick
I’m sure your narrow box section hull would go to windward - how well is another question. As well as a Hobie? Better? The thing to remember about a hull that you are relying on for lateral resistance is - where exactly IS the CLR? Most likely forward of center, maybe quite a bit forward. So the rig should reflect that, with a fwd position of the Center of Pressure. Otherwise the boat will have serious weather helm and in a worst case scenario won’t even be able to gather way after a shunt. This actually happened to a fellow in Hawaii (can’t recall the name) who built a lovely asymmetrical hulled proa and coupled it with a junk schooner rig. The hull’s CLR was so far forward that the boat had a lot of trouble after the shunt, turning head to wind before the boat could gather way. The lesson being that proas are tricky bastards!
I’ve already come to the conclusion I’ll need a deep foil way toward the ster for CLR/CE balance. I’m playing with the aspect ratio of the rig and how much protrudes forward fo the mast to minimize that. taller narrower rig far forward on the mast would have a much more “centered” CE form tack to tack than a low aspect rig mounted farther back.
Russell’s rudder setup with a large skeg is looking like a pretty good option. One thing you’ll notice on any classic junk rigged monohull that works well is a really large rudder.
Another interesting dimension is where the CE is located side to side. On a normal proa the drag of the ama is balanced by the very forward CE of the crab claw. the closer I can bring the CE of the rig to the ama, the less windward twist I’ll see from Ama drag, which exascerbates the far AFT CE of the junk. One of the early posts here got me thinking about that one.
I did the math for a Bruce # of 1.6 with a 1.6 aspect ratio, and I end up with a 324ft2 sail 14’ on the boom. If I put the mast at the 15% point, that puts the geometric CE @ 6ft aft of center on my 23ft waterline, or @ 25% of LWL in from the “Now” stern. Right where the rudder normally goes. Becasue of the aforementioned tendency of the Ama to give weather helm, I either need to bring that CE closer to center or mount the rudder/skeg farther back.
I could go to a centered sail ALA the Bolger ARYS rig with dedicated windward and leeward sides, and I think the rigid batten/soft camber in the fabric would allow it to luff/depower better for safety. what bothers me there is that moment in shunting where you’re swapping leading edges and you got the sail broadside to the wind.
Tom
I could go to a centered sail ALA the Bolger ARYS rig with dedicated windward and leeward sides, and I think the rigid batten/soft camber in the fabric would allow it to luff/depower better for safety. what bothers me there is that moment in shunting where you’re swapping leading edges and you got the sail broadside to the wind.
Tom
Not quite so, if you head up just beyond close hauled then change tacks either the Bolger way or with a sliding car similar to an inside out drafters parallel bar, you are immediately on a very broad reach.
Be aware that you are also standing at the portal to the Dark Side; your Editor and yours truly have already passed through and I for one have not yet found a way to get back.
cheers,
Skip
Phil was a smart guy.
I remember reading about his alternative shunting methodology, and of course be both now understand why.
As to being at the portal, I’m seriously considering a junk sail on a proa with a Bolger box section hull. I think I’m way down the rabbit hole flicking my Bic to navigate 😉
Tom
Well, I need to check my math better.
“I did the math for a Bruce # of 1.6 with a 1.6 aspect ratio, and I end up with a 324ft2 sail 14’ on the boom. If I put the mast at the 15% point, that puts the geometric CE @ 6ft aft of center on my 23ft waterline, or @ 25% of LWL in from the “Now” stern. Right where the rudder normally goes. Becasue of the aforementioned tendency of the Ama to give weather helm, I either need to bring that CE closer to center or mount the rudder/skeg farther back.”
It’s actually only @ 240ft2, and that’s at max displacement for the high bruce #. Looks like a 10ft boom keeps the mast shorter than the boat….the CLR ends up at the 33% of LWL position…...
Damn this just might work 😉
Tom
[/quote]
Sorry I sent this post two times ... go on reading under this
http://www.youtube.com/watch?v=5AsOjx_Twtw&feature=related
No, thats no hidden hint its just to smile a little
Michel
Okay, Idea in my head.
I’m sure most people in here are familiar with Bolger’s ideas on box keels and plumb sided sharpies with matching curves on the bottoms and sides.I’ve been doing the math on a VAKA where bother the B/L ration as well as the draft are @ 16:1
realizing of course that this is not the “optimum” speed shape, I’d be interested in hearing how people here think it would perform and whether or not this would provide enough lateral resistance for windward work or if a dagger board would still be needed. I don’t think pounding will be an issue with a botton this narrow. Bolger stylke curvature would have 8” of rocker, meaning the bottom is 8” below the waterline at static rest.
Tom
Hi Tom,
for some days now I think about your box keel aproach of your design and I saw your sketch in the flying applecrate thread. To be honest I don’t like it and I think it will not work satisfying ... sorry to say it that clear end in no way I want to offend you. But I have some reasons to say so:
Bolger was not the brilliant allround designer producing only succesfull designs and concepts. In my opinion he was a guy between genius and madness and he had very inspiring solutions for various problems of boats.
But he also was a man who liked it to offer good hydrodynamics for a radical oversimplification.
Particular his multihull designs were more than questionalble. Think of the 3m Multi or his Singlehander Cat ... they were simply not working very well. I don’t know if ever a Bolger Proa was built and sailed succesfully. Bolgers best sailing boats were the conventional ones with more traditional lines like the blackskimmer, jinni, teal and junebug ... to stay at the sharpies. And this boats all had a slight flare in their sides
Having flared sides has a lot of advantages, especially in a multihull with accomadation. The rising volume above the waterline will help your boat to reduce the hobby horsing effect if you going in short waves or if you go very fast with strong power from the sails. Lower wetted surface is another effect. Spray will be deflectet away from the hull, the boat will sail dryer. The entry angle with your pods will be smoother (even more if you make a rounded fillet with epoxy bog), so less pounding.
If you want to have a boat with a flat bottom its good to have some constant rocker in it and to keep the forefoots near the waterline. This is a design detail you will find in every succesful flat bottomed boat and will will be important in a proa too.
Its very difficult to have enough volume in the Vaka of a proa unless you want to have ultralight beach proa without a cabin. Your 18:1 LWL/BWL ratio is far to radical for your desired 1700 lbs of displacement in a 24’ hull. With a ratio of 12:1 you will have a better balanced boat in every way. This will help your boat to deal with multihull specific problems like hobby horsing and nosediving.
Ease of construction and material effiency ist not only based on a simple boxy form, it is the result of a intensive design process. Even if it takes a week longer to build a hull because of some flaring sides and some more lofting work, it is worth every hour that you are sailing.
I hope you will not damn me for my thoughts.
By the way .... you asked for sailing without daggerboards. Have you ever thought of adding anti vortex panels? Bernd Köhler is using them very succesfull on his chined cat designs.
Michel,
Aboslutely no offense, one of the reasons for forums like this is to get input of every type. There was a very good discussion of lining up the thrust of the rig on a proa that has me working on ways to get my preferred freestanding mast to windward with enough structural support. sticking in the Vaka eascerbates the weather helm issue if you can’t pivot your rig way forward like a traditional crab claw.
Phil was the first guy to admit that not all of his design were winners. I would like to submit that some of his most popular such as Micro, Long micro, martha jane, the AS19 and AS29, and many I’m forgetting all had the plumb sided “box” shape. His arguments being a sharper “V” when heeled meeting the waves as well as the most bottom and Bouyancy for a given total width. anything extending past the bottom is extra weight that the bottom cross section has to carry. As to rocker, this has a cople of uses. on a tacking boat, it puts most of the lateral resistance in the center of the boat to pivot around. that’s why dagger cboard or fin keel boats tack so much more crisply than something with a full keel. More turning resistance out at the ends. It’s also to get enough Bouyancy when you lift the ends up near or just above the waterline. the disadvantage is the deeper final draft. generally the height of the wake is proportional to the depth of the draft on a boat, and a tall wake is wasted energy. If you look at his fast, low power flat bottomed power boats such as sneakeasy or wyoming, there is essentially NO rocker. almost no wake either. VERY low horsepower requirements.
As to the Box keel concept, it was used very succesfully on several designs, the one comming immediately to mind is his “Fast Motorsailer”. These boats are known for good speed on low horsepower as well, the one drawback being the pounding of the “wings” in a short steep chop. I’m hoping to alleviate that with the long fine entry. I’ve essentiall got a 1:6 LB double ended sharpie stacked on top of a 1:16 double ended sharpie. as far as Monohulls go, the 1:6 ration is an extremely fine entry. This step also takes the place of the flare on a dory for reserve bouyancy, and will make a fine spray rail.
I love Dory hulls. If you’re building a stitch and glue, they would be very fast to assemble. They are actually deceptively complex if you’re building on a frame with chine logs and such because everything has a bevel. If I wasn’t curious about the box keel scenario, I’d be designing a very narrow dory hull much like P52.
I’m hoping to find the time and space to do a quick and dirty 16’ version to test the concept. This would be almost perfect for the existing 78ft2 junk rig I’ve already built for my 3m trimaran (which has a dory central hull by the way). We’ll see how it goes 😉
Tom
Hi Tom, just a few comments.
Rocker: Turning ability is just one effect of various rockerlines. In any flat bottomed boat the rocker is a major factor of trim and performance. Too straight and the bottom will just go flat thru the water sticking paralled to the water flow, this is ok as long as your speed is moderate ... if you are going much faster than hullspeed and you devellop a bow down trim, the bottom will stick its bow right right into the next wave. I think thats what Michael meant when he spoke about “running like a freight train” in another thread.
If your rocker ist curved too much ... the bottom will “stick” to the passing water like a laminar airfoil does and your bottom is sucked into the water which causes much waves = drag. Did you ever wonder why this funny PDRacers or Optimist dinghys are unable to get into real planing mode while other scow type boats just a little longer are planing like mad? There is the answer ... too much rocker.
If you have the right rocker shape the bow will rise the forefoot out of the water and the forward bottom will create some dynamic lift. At the stern the rockerline will suck the back part of the boat into the water so that the aft bottom is parallel with the flow of the passing water. It leaves the hull with nearly no resistance. The boat will get a stable trim with a slight bow up tendency.
This is not restricted to planing monohulls, fast multihull that sail far beyond hull speed behave equal. Even if they are not planing in the way as monohulls do.
I dont know if the shapes of sailing boats and motorboats are comparable, but sneakeasy and its 2 times scale up wyoming are pure flat water planing machines with lots of planing surface at the optimum flat angle of attack. They are pushed by their motor “up the hill” all the time as every planing motorboat does. So there is no use for an aft rocker.
There low hp reqirements are based on that huge planing surface and their light displacement. I think the “box keel and flats for planing” hull shape is one of the genius devellopments of Phil Bolger, but its not only the box, the planing surfaces are in the water to develop the dynamic lift for planing. I dont know if this shape is transferable succesfully to sailboats and particular multihulls. Is the fast Motorsailer good performing in sailing mode?
I believe that your 1:16 / 1:6 Vaka with the pods will work, but do not design the lower hull thinking as a box keel, please design it as a hull with a proper shape in it.
To plane a bevel into a chinelog is no time consuming or difficult operation. In a sharpie hull I would need around 1 1/2 hours for 15 Metres of bevel (At the bottom of a 7.2m proa for example). With your design you will have a total of 4 bevels to plane if your pods stay right angled) It’s for sure faster to do than to finish the equivalent sitch & glue epoxy and glassfibre panel joints ready for painting. And I mean to finish them, not to glue and laminate them
Lateral resistence: My Opinion is that lateral resistance (LR) should not be provided from the hull alone. Some additional help from some kind of foils is my way. I think the only boats that get away with hull LR alone are the “deep V” boats, but even James Wharram is now drawing small keels or daggerboards into his newer designs. “Madness” also.
Best Regards, Michel
Okay…..First 3d rendering ever.
Only had to go from hulls to turbocad to cutePDF to a Jpeg converter…....
The learning curve is still damn near vertical 😉
This is my box keel idea as built in hulls.
And another view
Congrats on your first 3D render! The learning curve must have some slope, because before you couldn’t yet and now you can. Soon you will even better.
Best,
chris