Lee wants a new trimaran design...

[Lee.007]

Dave,
See the picture below for the numbers you were asking about. Dimensions are in inches(apologies to the metric purists out there).

The Ama center of buoyancy trails the Vaka by a little over 3 inches. I think I understand what you were saying about bow down or bow up. And if I understand correctly, seems like I would want the Ama centroid (center of buoyancy?) in front of the Vaka centroid. Correct? So when the boat heels, the Ama's center of buoyancy will want to raise the bow up? If that is the case, I need to move my Amas forward, or make them longer. Moving them forward will be easier at this point, but I haven't cut any frames yet so I can still make changes.

The Amas have a volume of 14215.9756 cubic inches = 61.5 gallons, multiply that times 8.35 pounds / gallon for freshwater = 513.5 pounds of buoyancy (Seawater is about 8.6 lb/gal, but most of my sailing is probably going to be freshwater). From what you are saying, the Ama does not have enough volume to support the entire boat + people + cargo all on its own. Looks like I am about 200 pounds short. Are you saying I want it to do that? Should it have enough to support the entire load? If so, then I will need to make it longer / wider. I would prefer to make it longer instead of wider to maintain the 9.5 LWL to BWL ratio.

The forward Aka is 54 inches from the bow, and 15 inches above the waterline. I have not been on the ocean sailing yet, but in the lake I have seen 3 to 4 foot waves with very short periods coming at me when the winds are about 15mph and higher and there are a lot of power boats on the lake churning up heavy wakes for the winds to push higher. Do you think that will be far enough back, or should I move the forward Aka further aft? If so, how much would you suggest?

Like I said, I have not cut any of the frame / skeleton forms yet so I can still make changes. And I am not a naval architect, nor am I arrogant enough to ignore sound advice or constructive criticism. I have been in the Army 19 years, so I am pretty thick-skinned. I want to make this as good a boat design as I can before I build a boat that is totally a mess. I would really like this thing to make 15 mph before it starts to bury the Amas and slams the breaks on. With my current boat, I can't get much more than 7mph before the Amas go submarining, but they only have about 100 pounds of buoyancy. Good advice from you an others on here is what has gotten me this far, so I am all ears!

Numbers.jpg

[Lee.007]

You could simply glue one of your stripped panels to the underside of your aka housing outside of the gunwales, you only need 12" or so of width, and it won't interfere with your folding mechanism.

Yeah Matt, that's part of the plan to get some tramps / benches attached to the Aka housings. I just haven't decided exactly how I want to do that yet. If I put them on top of the housings, I can make them 18" wide, but they will be several inches above the gunwales and look kind of awkward I think. If I mount them underneath, I can only get 12" of width before the folding akas crash into them.

[Woxbox]

Lee -- If you haven't had a chance to read the link that Klepper posted, that would be job one! It's the best comprehensive review of ama design I've seen. To your specific questions, yes, the ama needs more volume, and most of it should be added forward.

If the volume of the ama equals total buoyancy, you can still drive it under. (I've done this.) The weight of the boat and everything in it is one factor trying to drive it under, and then there is a significant down force from the sails when heeled, plus momentum. Note in that article that W17 skippers are advised to ease off on the sheets when the lee ama is pressed down to a certain level. The less volume you have in the amas, the sooner you have to back off.

There are no hard and fast rules. Some designers have gone to 200% of displacement in ama design for a high level of safety in ocean-going trimarans. The F27 I used to sail had 100% amas, mostly so the boat could be folded up for road travel. (And this is why I was able to drive them under from time to time.) 150% is a good compromise. But you might find that volume hard to fit into your other requirements. One big thing in favor of smaller trimarans is the ability to use crew weight to compensate for lower inherent stability in a design.

There are similar tradeoffs in the placement and height of the akas. Sooner or later they'll be catching the wavetops as the seas pick up. But the more you can move the forward pair aft and get them higher, the less often this will be an issue.

Frankly, Lee, by the time you're done refining your design the boat will look so much like the W17, you'll be wondering why you didn't build that one in the first place. The designer knows his stuff.

[Lee.007]

Hey Dave,
Well, I read it again just now. I kind of skimmed over it the first time. But, I did cant the amas out a little bit so the outside of the ama did not become the deeper side when it is heeled to help avoid pitchpoling, but I didn't make it shorter like it is on the W17. After re-reading it, I understand now that I need more volume up front, and less in the back, but it can't look like a fish tail in the back. I kind of used the same idea from the drifter where the back is a triangle shape instead of a squared off like the Slingshot. You may be right, by the time I get done it may look like the W17, and I may build one in the future, but I want to see if I can't get this thing tuned in enough to make it a decent craft to build less expensively. And, I want to keep the cedar bright finish look too. It would take a lot of cedar I think to do a W17 with all of its surfaces. I don't expect Olympic performance, but I do want to do better than my kayak/sailrig. If I can keep her together in 20mph winds with the sail reefed, and if I can get a top speed somewhere close to 15 mph without capsizing, I will be happy.

I will re-design the amas a bit to make them longer and get at least 150% of the Vaka displacement in them. I can stretch them out so they are almost as long as the Vaka similar to the Slingshot.

Speaking of Vaka displacement, if I use the entire displacement/buoyancy of the Vaka before water comes over the gunwales, its like 2500 pounds. But I figured that is un-realistic, so I went with the displacement of the volume that is submerged at the design waterline. Which means I have a little bit of extra, but I want to work with the designed water line. Is that normally the way it is done? Using the submerged volume from the waterline down as your number to work with?

[kleppar]

It seems to be a couragious thing to do - buy a design, and then alter it. I took another approach for my small trimaran - first I ruled out the type of trimaran I did not like, i.e. the canoe type. Then I decided for performance, rather than pottering; then comfort was an important factor - I wanted a roomy, self draining cockpit for a dry ride. Finally, due to restrictions regarding launching, I needed to decide on a quick folding system that served my purpose. I ended up with two designs that I liked - Chris White´s Discovery 21 and W17. Both had over the main hull folding systems, which I did not like, and the Discovery was too big and heavy for my purpose (dolly launching by hand). In the end I made use of Mike´s swing beam folding, originally developed for a larger trimaran.

I wonder if you receive any assistance from the designer of Slingshot, as designer support was a decisive factor for me when choosing the design. Mike´s website with very comprehensive documentation of all things related to small trimaran building and sailing, and his personal support during the two year build process, was invaluable.

Best regards from snowy and cold Stavanger, Norway

[Lee.007]

I made use of Mike´s swing beam folding, originally developed for a larger trimaran.

I was not aware the w17 had a swing beam folding? I thought it was folding over the main hull? I will definitely check out his website. Thanks for the link.

[kleppar]

Officially, W17 does not have a swing beam folding system, but the ramp I use is too narrow for launching W17 unfolded, and with the current over hull folding, the narrow main hull will not float unsupported by the floats. My W17 is the first with this system, so it took some time to figure out how to make it. It has one pivot bolt and one lock bolt for each hinge.

[Woxbox]

Is that normally the way it is done? Using the submerged volume from the waterline down as your number to work with?

Yes, that's the number that goes into the calculations. And when at rest the amas really only need to touch the water enough to stop the boat from flopping back and forth when at anchor. What you show in the diagram is probably good. You wouldn't want them any lower.

[Lee.007]

Thanks Dave, I figured that was correct, cause it just made sense.

Ok guys, I have another dilemma. I am playing with the sail plan. I want 150 sq feet. See below for pics and numbers. In order to get that out of the mast height I want to stay at, and keep the mast position at a manageable spot with the clew clearing the backstay and such, I am either going to have to fly the jib as a genoa, or make a bowsprit that will extend 37 1/2" past the bow. Other than wear and tear on a genoa during tacking, which is the best option? Also, if I move the clew of the headsail back and make it a genoa, do I need to recalculate my center of effort?? I think I should, but not sure. This might be a Todd Bradshaw question. I don't know, maybe I should just raise the mast height another foot or two. Right now it is at 21 feet and 8 inches. Suggestions?

Sloop Rig.jpg
bowsprit.jpg

[upchurchmr]

Where are you going to attach the shrouds?

Eliminate the backstay.
Move the mast back, shrouds go to a bridle to the ends of the ama's
Put the 51 sq ft into the main, with additional height on the mast.
This is about what you would get with a Hobie 16 main.
Now you can add a jib later when you want more performance.

Last time I'll harp on the Hobie sails.

Looks to me that the sail ce lead over the hulls/ boards is too much. But you don't show any numbers.
Where are the boards? They will completely overshadow the effect of the hull on CLR.

[Woxbox]

This is about what you would get with a Hobie 16 main.
Now you can add a jib later when you want more performance.

Last time I'll harp on the Hobie sails.

Lee -- Upchurch is right about looking at the Hobie rig or something similar. Sail area alone won't push a boat over the 10 knots you're looking for. A big roach, fully battened sail is far more effective, both for drive for sail area available and to stay flat and well shaped in higher airflow. The multihull designers got away from those pinhead mains decades ago in favor, first of the big roached Hobie type mains, and now it's all about the square topped sails.

Check out the Weta rig -- now here you have a fully developed high performance trimaran. It can reportedly get into the high teens. I believe this, but it does take one slick engine to drive it that fast.

[Lee.007]

Put the 51 sq ft into the main, with additional height on the mast.
This is about what you would get with a Hobie 16 main.
Now you can add a jib later when you want more performance.

Last time I'll harp on the Hobie sails.

Lee -- Upchurch is right about looking at the Hobie rig or something similar.

Ok, ok, Im listening. I checked on Sailrite's website and they have a kit where you can sew your own Hobie 16 main for $495 and a kit for the jib too for $288. The mainsail kit comes with everything you need except the battens, but I can make a set of those just like I did for my kayak trimaran laser sail. I will look around on the net and see if I can find a used mast with a sail for same price or cheaper. If I can't find a mast, I will have to figure out a way to carry a nearly 30 foot mast, or make a two piece mast.

Dave, I really like the square top mainsails. The square footage just jumps up there so quickly when I add a square top instead of the "pinhead" as you called it. Calculating the CE is more difficult, but I can do it. How long can the square head be before you have to rig it like a gaff or lugsail? I really don't want to have to rig a gaff or lug.

upchurchmr, I really would like to get all the square footage into the main. I remember you saying don't make a folding mast, but what about a two piece mast held in the center by an aluminum pipe sleeve? From what I am reading, the hobie is a two piece carbon fiber mast. any objections to a two piece birdsmouth mast?

EDIT TO ADD: Oh, and the numbers are on the pictures in post #141. I put that sail CE leading the Vaka CLR by 15% of the LWL. My books and the internet said 13% to 17%, so I split the difference.

One more thing. If I eliminate the back stay, how do I keep a jib from pulling the mast forward? Is there some trick I am missing?

[Lee.007]

How do you like me now? I drew this up based off the Hobie 16 mainsail dimensions, and added a square-top head. The head is 24 inches long. I figured if I sent this up to Sailrite, and have them design me a sail, it might be close to the same price of the Hobie kit, maybe even a little cheaper since its not a "hobie" sail. But, I was right about the Mast being a monster. Its going to have to be two pieces for me to get this thing on a trailer.
HobieStyle Sail.jpg

[mattglynn]

It seems like you have two paths you could take here. First one is canoe trimaran style, Gary Dierking's Ulua was mentioned on the first page and meets your criteria in the first post to a tee. PRO Strip planked, light weight, capable of being paddled, and a short unstayed mast. The high length to beam ratio, light weight, and long waterline will make it pretty quick. The unstayed mast you can self build, as well as the sails. The build time would probably be around 150-200 hours. For trailer sailing this is an extremely fast setup, just put a hinge on the aka's. CON The lower buoyancy amas means at high speeds you have to hike out to keep the ama from submerging. Generally they are not as comfortable as a tri with a nice cockpit. It will also be a wet ride at high speeds https://www.youtube.com/watch?v=Obda...SamuelLanghorn .


The second path is the one you're headed down now. You'll add buoyancy to the amas, realize that to keep them narrow enough you'll have to make them deeper, so you'll raise the freeboard on the vaka to keep them from dragging. The added forces from the bigger amas will make you beef up your akas, the beefed up akas will mean you need more structure in the vaka to take the load. A cockpit would be nice and comfortable, so you have to ditch the leeboard cause it gets in the way of your seating, and add a centerboard. In order to drive all this extra weight you'll end up with a 28' mast you have to step everytime, and lots of sail area. By the time you're done you'll have a coat hanger abortion of a W17, Strike20, Cross 18, and seaclipper, all of which are great boats, but is this style really the best fit for you?

I just wanted to raise a couple questions because in the year since you started this thread it has drifted pretty far from your original statement of requirements. The bigger, heavier, more comfortable boats are great, I built one for myself, but is that really the best fit for you? If you primarily trailer sail in warm(ish) waters in Georgia, is getting wet a big issue for you? A twenty something foot mast can be difficult and time consuming to step, is that something you want to do everytime you want to sail? Do you mind running from side to side everytime you tack, or would you rather be able to sit wherever you want and stay there?

[upchurchmr]

Lee,

A Hobie 16 mast is 26'. Last time I dealt with one I was 65 yo. It was so easy I can't understand what you are concerned with. That was just connecting the base to the boat and walking it up starting from the rear cross beam. I was shocked at how light the mast was compared to the Tornado cat of my youth.

There were only 50K+ of these sold and trailered with the mast on the trailer. It can't be hard if all the first time sailors who bought these succeeded at it.
If you have a problem raising the Hobie mast, you can always set up a "gin" pole. And use a trailer winch to pull up the mast. Takes a little longer but it is easy, and won't threaten your newly fixed back.
The best thing about a Hobie mast and sails is that the mast and sail were designed and developed to work together. Remember all the trouble you had with a wood mast and some second hand sail not made to work together?
Same thing about making battens if you get a Hobie rig. It is not worth the fact that it will take you a year to get the home made battens to work right - and probably require rebuilding them 3 times.

If I knew of a sailboat class where you could buy a used square top rig, I would be all in favor of that option. Someone else might know of an option, I don't - not one reasonable.

Study a Hobie rig. It and all other day sailing cats have the shrouds angled backwards about 1.5 feet (I don't remember exactly). The angle takes the place of the backstay. Nobody uses a backstay on a small boat for at least 50 years.

Please, Please, Please don't make a 2 part mast. Modern sails depend on bending of the mast and proper sail design to get good performance. A 2 piece mast will have a heavy stiff joint in the middle, changing the bend characteristics. And then it will usually be a significant source of potential breakage unless it is well engineered and built. 2 piece connected with a pipe joint will introduce big stress points at the ends of the pipe (assuming you are making a wood mast).

Please study the Weta drawing Woxbox provided. Ignoring the centerboard/ leeboard will give you a bad balance. The board (whichever kind you want) provides 90+% of the lateral force.
Did you get 15% lead from articles about old monohulls? Not the same thing. Weta, Tornado, Nacra will all be in the same range of position and much better guideline. Woxbox's tri he talked about might be even better, its a proven solution for a Tri.

I guess I broke my promise to not harp on Hobies.

[upchurchmr]

Lee your attachment for the hobie rig on your boat did not work.

By the way, the latest hobie masts are aluminum, with an upper section of carbon fiber (I'm not sure its fiberglass instead of carbon). This was because stupids kept raising the all aluminum mast up into power lines.
The point was to get a non conductive section in contact with power lines - carbon fiber is conductive, so I don't think this is right.

It is a pity the slingshot designer has not worked out. I'm a little confused, when he originally brought his first boats out, I contacted him a lot, he was very nice, and we had good conversations, even though we disagreed some of his design ideas.

If you look at any used mast off a boat, look very carefully at how straight it is. Turn the mast until its leading edge is upright, and sight down it. Don't accept any degree of bend, it will always cause the sail to set wrong, and you will sail differently on port and starboard. Actually there have been people who have straightened masts which do not have any dimples in the side walls. Good luck if you try. I wouldn't try unless it was free.

[Lee.007]

[QUOTE=upchurchmr;6770665]Lee,

A Hobie 16 mast is 26'. Last time I dealt with one I was 65 yo. It was so easy I can't understand what you are concerned with. That was just connecting the base to the boat and walking it up starting from the rear cross beam. I was shocked at how light the mast was compared to the Tornado cat of my youth.

Well, I didn't think of stepping it and walking it up. With my current boat I have to stand it up, then lift it up to where the foot of the mast is even with my waist and set it down in through the partner into the step below. Its not so much that its heavy, its just the huge amount of torque that it takes to right it if the top starts to lean while I am trying to drop it in the partner.

If I knew of a sailboat class where you could buy a used square top rig, I would be all in favor of that option. Someone else might know of an option, I don't - not one reasonable.

Now that I have my pic fixed in my post above, what do you think about that one? Basically took the dimensions from the Hobie sail on Sailrite's website, added a square top, and there she is. Of course I will contact Sailrite, explain the construction of my mast I intend to build, and see what they recommend.

Study a Hobie rig. It and all other day sailing cats have the shrouds angled backwards about 1.5 feet (I don't remember exactly). The angle takes the place of the backstay. Nobody uses a backstay on a small boat for at least 50 years.

I noticed that on some of them. But I didn't know if moving it that small amount backwards would be enough force to keep it from flopping some like my current boat does. But if everybody does it that way, then I guess there is a reason.

Please, Please, Please don't make a 2 part mast. Modern sails depend on bending of the mast and proper sail design to get good performance. A 2 piece mast will have a heavy stiff joint in the middle, changing the bend characteristics. And then it will usually be a significant source of potential breakage unless it is well engineered and built. 2 piece connected with a pipe joint will introduce big stress points at the ends of the pipe (assuming you are making a wood mast).

Roger that! All good points, I was actually wondering about the stresses at the end of the metal pipe myself. I was thinking it might just be enough of a sharpish edge to shear off the mast.

Please study the Weta drawing Woxbox provided. Ignoring the centerboard/ leeboard will give you a bad balance. The board (whichever kind you want) provides 90+% of the lateral force.
Did you get 15% lead from articles about old monohulls? Not the same thing. Weta, Tornado, Nacra will all be in the same range of position and much better guideline. Woxbox's tri he talked about might be even better, its a proven solution for a Tri.

I did get it from monohulls and also page 35 of my Sailmaker's Apprentice book. But, I was questioning it. I didn't think of monohull vs multihull, I was thinking sloop rig vs bermuda/marconi rig because they don't say anything about a bermuda/marconi rig when they talk about sail balance and lead. What is the general rule of thumb for a single main sail?? Also, I get it that the centerboard/leeboard lateral force should be taken into account. I got some of my data from this website: https://www.diy-wood-boat.com/sail-balance.html
They mention flat bottom boats, do we consider multihulls to have the same charcteristics as a flat bottom since the beam (including the amas) is so wide and it is generally a shallower boat? But there is no mention of multihulls on the website, nor is it in my book either. The book barely gives lip service to multihulls. The website didn't give any clue as to where to put the centerboard/leeboard.

I guess I broke my promise to not harp on Hobies.

Yup, ya did. But if you have good advice, I aint worried about it.

[upchurchmr]

The issue of stepping a mast as if it is unsupported by shrouds is exactly why a freestanding mast is mostly impractical for trailer boats, IMHO.
Cats which use a hinge at the base of the mast are relatively easy, but it is difficult to keep the mast from leaning to the side up until you are pushing against the shrouds. You can set up very short "shrouds" temporarily to control the side ways issue. These need to be anchored on the amas directly across from the mast base.

I had seen some recommendation on sail balance vs center of lateral resistance for multihulls which said lead could be zero. This seems a little radical, but I believe multis will need less. Since I have no concrete number, and some designers differ (from when I was interested in anything multi) I would suggest looking at successful boats/designers. Realize that these recommendations are generally aimed at sailing to weather. I.E., going as hard upwind as reasonable. When you are going on a reach, the driving force from the sails is angled much farther forward. Where that line crosses the boat is significantly different from when you are on a tack.
The thing that allows it all to work is the rudder. It also provides lateral resistance, and the position of the total lateral resistance can be changed by changing the position of the rudder blade.
When sailing down wind the whole thing doesn't match any lead suggestions, since the sail force is going directly parallel to the long axis of the boat. Lead is infinite???? There is a turning force if the resistance of the main hull and the in water ama is not directly in line with the sail force. But the rudder can counter that turning force (if the rudder is big enough).

You should ignore comments about flat bottomed boats, since monohulls will be 6ft or greater width and are not going to act like a 1-2ft wide multihull (each individual hull). A flat bottomed multihull ( both main hull and ama) will develop some lateral resistance, but the amount is vastly inferior to the same hull with a dagger/centerboard/leeboard. So much so that you really ought to just consider the board and rudder to be all the resistance (since neither one of us has good quality estimates of actual values). One point is that the bow on any boat will develop noticeable lateral resistance, if it is well buried in the water. But I can't suggest a value.

Actually I would prefer to not harp on hobies, since I don't consider them to be some of the better sail rigs.
Tornado, Nacra, and others sail faster and better upwind, but you won't generally find used pieces for sale since they weren't made in the same quantities.

I haven't looked at your hobie picture yet.

[upchurchmr]

Looked at your "hobie" with square top.
It gives me the creeps.
I think your square top has little relationship to the original Hobie sail plan. The Hobie has a noticeably long boom. If you do this you have abandoned any benefit from using the Hobie rig.

Look at any commercially designed tri.
The mast is approximately in the center of the boat.
The amas are much farther back.

One problem with a narrow hulled multi is the depression of the bow of main hull or ama when reaching (possibly running).
Your mast position is going to put its weight far forward, starting the boat depressing the bow(s) with no wind.
Putting the mast near the middle allows 1/2 of the hull to be depressed by sail force, keeping the bow from going under water and stopping you rather rapidly.
This is a fundamental fact of life for all multis.

The only reason Drifter gets by with it is that it has a low / small sail area. And the hull (main) is wider than desirable, unless you don't care to go fast.
That is a perfectly reasonable choice, if that is what you want.

Personally, the greatest reason for a multi is to go fast or even faster. Drifter is probably designed to never have enough sail force to "fly" the main hull. That will avoid heel - but by itself that makes you go slow too. I don't know why you would go to all the complication and not go faster.
But that choice has been made by some designers. Generally they no longer design boats.

If you are ok with slower, then you might look for a Hobie 14 rig. You would get all the benefits of a professionally designed/ developed rig, intended for a multi. And reduced stress on the amas/ akas and connective structure. I would still suggest the fwd aka be farther aft and the mast mounted on top of it.

Are you intending to sit in the main hull? or will you use nets between the akas to have room for passengers to sit and balance the sail force. We talked about this on your previous boat - but you were severely limited there. That decision will drive your choice of how much sail area and potential speed.

[Lee.007]

Well, I don't mind sewing one together. Sailrite has some nice looking kits, and the Hobie 16 sail is almost as much square footage that I want. And Sailrite didn't have any other trimaran sails. Does it matter if it is a trimaran sail? Or can it be any fairly high performance sail?

I will look into the others. Maybe I can find the sail plans for Tornado, Nacra, or Farrier somewhere and estimate the dimensions if they are not readily available. That Weta looks super fast on their website, but their video shows the crew getting pretty wet. But they do have some of their sail data on there. Let me know what you think of my "hobie" square top sail.

Edit--You and I must have been typing at the same time! I do intend to have room for maybe one extra passenger besides the two sitting in the main hull. I just havent figured out how I want the trampolines or benches to be mounted yet. And yes, I want to go fast too. Where should I put the leeboards? Inline with the main hull clr? Or does it matter?

[Todd Bradshaw]

Back in the beginning when Sailrite was trying to pay for their computer system they were plotting sails for other sailmakers for a per-yard charge. They plotted some of the sails for our 20' Farrier Eagle and probably still have them on file.

fd6533a9.jpg

[upchurchmr]

Lee, I'd listen to anything Todd says. And I always do.
The Farrier 20 looks good, I don't know anything about the mast size / section shape to see if it would match well with the Hobie 16 mast.

[Lee.007]

Dave, Marc, Todd,

I have reworked the amas and made them longer and wider, raising the buoyancy up to 660 pounds. I think I am going to stop there or they are going to be almost as big as the Vaka. I will contact sailrite and see if they have that Farrier 20 design still on hand. Take a look at these drawings. I am worried that my Amas are too far forward. Oh, I also moved the akas back a little. I am only showing the housings cause I haven't reworked the beams yet. The bow of the ama is only a little over 2 inches behind the bow of the vaka. I moved it that far forward to get the ama center of buoyancy in front of the vaka center of bouyancy. Do y'all think that will cause any issues? Also, added the leeboard in. I have its centerline right on the vaka CLR. What do you guys think?

Amas reworked.jpg
Amas reworked iso.jpg

[Woxbox]

Lee -- It's looking much better to my eye. There is no negative I'm aware of to having the bows lined up. It's more about the relative positions of the centers of buoyancy.

Now, about that leeboard. It's not about the location of the board by itself. It's about the board plus the hulls plus the location of the sail(s). It all needs to be figured together. Because this whole venture is experimental, it would make sense to build it in such a way that you could adjust either the mast or board position after some test sails. If you review board positions on various trimarans, you'll find that different designers have different ideas. Look at the Weta -- it seems awfully far back. But then look at the F27 below. It's well forward. And I can tell you from experience that this board is in exactly the right place for the boat. It even sails well under main alone. It's my understanding that getting this right has always been a real bugaboo for yacht designers, and adjustments are often made after the first test sails.

[Lee.007]

Thanks Dave. I had a hairbrain idea to actually mount the leeboard on a track or rail system, attach it to a lead screw with a handle crank, and then just crank the handle under sail to see where the sweet spot is for max speed. But then I was like, nope. Cool idea I think, but too much mechanical crap to build when I can just drill a few holes in different locations and move the leeboard fore and aft while making a few test runs at the lake. Yeah, I agree I need to engineer some flexibility into the setup.

[Lee.007]

You know, the more I look at that picture of the Farrier, the more it looks like I almost matched the position and the vaka to ama proportion pretty damn closely. But the Farrier has a lot more rocker. I remember our conversation about that from before.
Most of what I read about Lee board position has only one real hard and fast rule: put the CE of the leeboard inline with the CE of the sail plan. Other than that, the opinions vary. I think I agree with the school of thought that says to put it at the clr of the vaka, or slightly aft of it. Most say the leeboard becomes the pivot point of your tack, and I figured the closer to the rudder it is, the faster the bow will swing around. But I am not sure what other things it will affect. I kind of feel like moving it aft of the clr may induce some lee helm.
I am at my in-laws this weekend and can't do any drawing, so I'll research leeboard position more.

[upchurchmr]

If you put the leeboard (any board) CE too close to the rudder, you won't be able to get it to go upwind. Too much turning moment between the sail CE and the centerboard and the rudder won't have enough power. Won't be much fun

[Lee.007]

If you put the leeboard (any board) CE too close to the rudder, you won't be able to get it to go upwind. Too much turning moment between the sail CE and the centerboard and the rudder won't have enough power. Won't be much fun

That's exactly what I was thinking Marc. The closer the board and sail are to the rudder, the less torque the rudder has to turn the boat and sail against the wind.

[upchurchmr]

OH NO! I didn't phrase the last comment well. What I meant to say is that the board cannot be too far back from the sail CE.

The sail and board need to be close.
The rudder is plenty far back to turn the boat, in any catamaran, where the mast is 1/2 way back from the bow ( or more).

You need to accept that the tens of thousands of boats built like this work well, or they would not of sold.

The rudder just needs to be not too small.

[Lee.007]

No worries Marc, I pulled that picture of the Farrier 27 from above and put it into my AutoCAD. Then I traced the outline and estimated proportionally where the leeboard most likely is. Looks like they put the trailing edge of the leeboard / centerboard right at the Vaka hull CLR. So I have re-drawn mine to match. With the CE of the sail directly over the centerline of the leeboard.

I also downloaded a plan drawing of the Hobie 16 sail and traced it as well. I think I like this sail. It is kind of shaped like a sprit sail with the clew higher than the tack, with the foot angled upward going aft, and that gives me more clearance for my head so I don't get boom-whacked. I think 140sq ft is enough for this boat, and if not I can always add a jib. Sailrite has kits for both the Hobie 16 main and the jib. I think since they were probably designed by Hobie to work together, they should be good. I will create a second step and partner location to allow for the shifted center of effort sailrig in the event I decide I need a jib.

So here is a question I have for the sail designers out there. Since the Hobie 16 sail is not a true triangle, how do I locate the center of effort? Cause I don't think it qualifies as a true 4 sided sail either since the "4th side" is at the head and is so small. It also has a pretty significant roach. I will post a drawing of all this later this evening that shows how I lined everything up.

There is going to come a point pretty soon where I will have to stop drawing and start cutting wood. When I was working as an electro-mechanical designer, the production guys would tell me to get it good enough and stop splitting gnat hairs so they could start building the thing before the end of time. I want to start building in January. I think I am really close. I just need to tweak the size of the rudder, and re-draw the akas now that I changed the size and position of the amas.

[Lee.007]

Ok sports fans,

I think I am done optimizing the amas, the leeboard, and the sail location. Take a look at my drawings. Am I calculating the CE of this Hobie 16 sail correctly? Now I gotta get to work on a rudder, and optimizing the akas since I changed the ama a little. By the way, I got my amas up to 665.36 pounds of buoyancy.

Hobie Sail isometric.jpg

Hobie Sail portside.jpg

Hobie Sail portsideZoomed.jpg

[Woxbox]

Lee -- I think you're right in going with the Hobie 14 sail, with the option of adding a jib. In fact, in light airs you'll want an even bigger headsail, but that's a better option than dealing with an overly tall mast all the time. As you have the boat drawn now, you could very easily fit a bowsprit and fly a sizable asymmetrical spinnaker or similar.

A couple more thoughts on the leeboard. My hunch, and it's just a hunch, is that it's a bit too far aft. But the solution as we've said is to make it adjustable for the trial runs. Second point -- boards are less efficient when they are surface piercing as opposed to housed in the hull, and it will throw up water as you go faster. You've must have seen this in your current trimaran. Also, at higher speeds you want to make sure you're not creating some sort of a dam between the board and hull. Ideally, it should fit very tightly to the hull or be set off some.

There is an analog method to finding the center of a shape -- cut it out of a piece of cardboard and find the balance point. I read somewhere that this method has actually been used in boat design, way back in the dark ages of slide rules, pencils and erasers.

[Lee.007]

Second point -- boards are less efficient when they are surface piercing as opposed to housed in the hull, and it will throw up water as you go faster. You've must have seen this in your current trimaran. Also, at higher speeds you want to make sure you're not creating some sort of a dam between the board and hull. Ideally, it should fit very tightly to the hull or be set off some.

Yup, I did see a lot of water thrown up with my current trimaran. When she gets up to 7mph and higher, the leeboard starts rooster tailing a bit, and it throws water against the hull which just splashes up on me. If there are big waves (2 to 4 feet) then I get a shower.

How close is "very tightly"? with the angle of the gunwales if I mount it right on the hull the leeboard will be angled in a very wrong direction towards the hull. It would be best I guess to make it part of the hull, but then it turns into a "twin center boards" condition and I don't really want to build a bunch of trunk housings. I will keep it adjustable like you have suggested and see where the sweet spot is during sea trials. I will test the fore and aft positions as well as snug or further out from the hull. I am also going to angle it in a bit. I have been doing a lot of reading lately and found some really interesting data. Pointing the leading edge of the leeboard about 4 deg towards the bow / centerline of the vaka is supposed to induce some force that helps counter leeway, as well as making the inside surface convex, and the outside flat.

Cant wait to start cutting some wood!!

[Gary Dierking]

http://outriggersailingcanoes.blogsp...-leeboard.html

[Lee.007]

So today I got the bright idea to deepen the vaka and round off the bilges. I think this should slide through the water very efficiently. My waterline displacement is now 1213 pounds. Thats plenty to carry 2 to three people and a decent amount of camping gear. Since I am going to strip build it, the curves will be easy to fair. I have not estimated the boat weight yet, but I am swagging its probably going to be 150 to 200 pounds. What do you guys think?

New Hull.jpg

New Hull transom.jpg