My first plywood cat.

I've just abandoned another thread (link) regarding a cheap "tube-cat" as it was not worth it (for me).
I quickly realized that I could make a much better craft with plywood as suggested by messabout and tom28571. The cost will be around 10% higher and it will require a little more effort, but in my opinion it'll be worth it.

About me:
My name is Tom aka nakamoomin or naka and I am an Industrial Design Engineer from Norway with a newfound passion for catamaran sailing. I have extensive experience in SolidWorks, Catia etc but I found these cumbersome with regards to calculations, on-the-fly optimization of hulls etc. I've downloaded CARENE2008 and FREE!ship to help me with design of the hull shape and hydrostatic analysis. I have also downloaded Sailcut CAD to help me with sail-design later on.

My sailing experience is limited, but motivation is sky-high. I've got some experience with Laser (racing dinghy, mainly for fun) and my classmates from Uni arrange an annual cruise along the coast of Norway and Sweden (30'+ monohulls, mainly leisure). My family has had powerboats during my entire upbringing and I am accustomed to the sea. However I've always been fascinated by sailing and I really got Cat-fever this Easter while sailing some Hobie rotomolds (Wave/Getaway) in Florida. Also, my mentor for my master thesis, mr Guy Lonngren has also been a great inspiration. (examples of his designs at http://www.guydesigngroup.fi) (PS: his wavepiercing designs for Kvaerner Massa are at least 10-15 years old)

Design brief:
Mision: Design and build a handy-size (5-6m), safe, fun, affordable sailing catamaran made from plywood (no tormenting, though).

Vision: A platform suitable for recreational cruising (fun), but also racy enough to fly a hull and be interesting to an experienced crew (more fun!).

Construction: Plywood (covered with glass/epoxy?)
Crew: 1-2 (racing) 3-4 (cruising)
Sail: TBD
Daggerboards/Skegs: TBD
Cost: 1000-2000 USD (excl upgrades)

My intention/expectations:
A first-build that I can refine/develop as my sailing abilities evolve. Therefore I'm focussing on the hulls for now. I'll have a rig (of course) but I'll probably go cheap/underpowered until I get some experience and get to test the performance of the hull under different conditions. I can always improve mast/sails later on. For starters I've got a mainsail size of 10-12 m2 in mind.

My proposal:
I've read almost everything I have come across on the web (for free) on hull/boat design. (This forum is one of my greatest sources of inspiration.) Since I abandoned the "tube-hull" idea, I've searched the web for designs I want to compare to. I've been looking at the assym Prindle 18's, The newer Nacras, the Tornado (albeit a little on the large side). I've evaluated benefits of narrow/wider transoms @ below/above hull speed, skegs vs daggerboards vs assym hulls, fineness ratio, Prismatic coefficients, etc.

This is what I have come up with so far (iteration 5 of my "NaCat"):

Design: Symmetric, non-tormented plywood
LOA = 5.63 m
B(hull) = 0.423 m
Fineness >= 13.3
Cp >= 0.62

Hull weight = 50kg (6mm ply hull and deck, 24mm bulkheads)
Draft (400kg) = 0.28m

I've taken design-cues from the Nacra F18 (slight drop at bow, overall dimensions). I'm not sure if I'll implement the "reverse sweep" bow (wavepiercing). As I see it it'll be a minor modification anyhow.

Now for some pictures:

Any feedback is greatly appreciated!

 

It looks like a good start. Pay particular attention to stress points(cross-beams meet hulls).

 

Cats not my line at present but I usually ask the same question if nobody else does: what kind of sailing conditions do you expect to encounter? Little lake, big lake, sea, ocean etc.

 

@ancient kayaker
Small/medium sized lake. Not much sea. However I'd like to get feedback on seahandling as I'll probably use this project as

A) a template for later builds
B) a project to learn how the different aspects of the design affects seaworthiness, speed, etc

@all
I've designed a similar hull with a smaller transom (40% smaller). This has a prismatic coefficient of 0.68 and will not compute in FREE!ship.

How will a narrower/shallower transom affect seaworthiness/speed/etc?

Tom.

 

When I looked at your design that was precisely what it seemed to be for, but again I caution you that my knowledge of cats is almost non-existent. however the information will be useful if the experts get interested in this thread. It is also worth your while to search the forums, especially the one for multihulls, for other people's ideas and comments already made, if you haven't already done so.

That's what I have been doing with my canoe designs/builds and more recently a small sailboat. Learning from books is something I don't seem to do well any more, I forget what I have read, so I learn from my mistakes instead: the lessons seem to stay with me longer that way.

I am not aware of any reason why it would not compute in Free!Ship. However Free!Ship will not perform any hydrostatic or hydrodynamic calculations on a hull that will not float, and it is easy to leave a gap somewhere, usually at the transom. Perform a leak check (Tools, Check Model) and ensure leak points are above the waterline, if you have not already done so.

 

Cp at 0.68 is a bigger number than I would expect from a hull of this type. You must have used a pretty small rocker dimension....or it is unusually fat somewhere near the ends. Be sure to use enough rocker or the boat will be difficult to tack. Cats are already a bit stubborn in that regard. I once built an Australis, at that time the A-cat equivalent. It was a double ender, wicked fast and a real dog to maneuver. Damned thing did not have much rocker! The next cat I live with will have a transom of something like 2/3 the width of the max beam. The transom will clear the water by a small amount. If you get the forefoot too far submerged the boat may have a tendency to root or oscillate. Not good. Too much foreward immersion also makes it more chancy off the wind where burying the lee bow is a natural tendency, dangerous if you are moving fast. On a small cat you can control that problem to some extent by moving the skippers weight around. But you have to be vigilant. At times, in a brisk wind, you'll find yourself perched on the rudder stock of the windward hull.

Some of the cat afficianados do say that one or two mid stations should have equal sections. That would tend to make th ends proportionately fatter. Troll around the multihull section of the forum and see what you can find.

 

@ancient kayaker
Thanks for the tip on leakage points! Never thought of that! I'll look in to it.

@messabout
Thanks! All good advice, I'll check the mulitihull section.
What do you think of the rocker as indicated on the pictures? I think I'm pretty close to the Nacra F18 in terms of overall proportions. (which, for all I know, could be a dog to tack as well)

Rocker is far more pronounced in the rear, but I guess I could try and raise the keel at the bow a bit. As long as this does not affect wave-piercing abilities in general.

Thanks guys! I'll get back with more info once I've checked out the forums.

T.

 

OK, so here's what I've done so far:

Read the forums
Read different articles on stitch-and-glue
Sourced material from local providers
Experimented with hullshapes in CARENE and FREEship


What I have learned:
- Rocker eases handling? Mixed opinions, daggerboards seem to have a greater impact
- Rocker increases hobbyhorsing? Probably, due to less "righting" moment longitudinally
- Deadrise eases ride? To an extent, but also gives less of a planing surface at higher speeds
- Narrower transom eases ride? To an extent, due to less buoyancy aft.
- Narrower transom reduces drag? At low speed and when submerged, at the cost of less planing surface aft
- Buoyancy: center ideally placed at ca 60% (from bow) (2.0 - 2.5m from transom for 5-6m LOA)
- Mast: placed at 3-3.5m from transom (slightly forward)
- Crossbeams: I'll use 100mm Ø 3mm Aluminium pipe. Placed at 0,5 from transom aft and at 55-60% forward (depending on rig).

My conclusions:
As the F16 and F18 cats have very little rocker I assume that I'll manage with the same amount. (how much is a guestimate based on images)
I guess I'll go for daggerboards instead of skegs. As I'm using stitch/glue I can build a box for daggerboards. This would ideally be placed around midship, right?

Questions:
Deadrise - Flat, 5 or 10? 5 provides best sim-results, 10 will give smoother ride (i suspect) and flat will be easier to build (one cut/stitch less). Difference in drag is neglible, though.

Daggerboards - where do I place them? Some boats have them at about 60% aft, while others have them at midship. Is there a way to simulate optimal placement?

Should daggerboards be asymmetrical (providing more lift in one direction than the other) or symmertrical?

Canting of the hulls - Nacra Infusion has symetrical hulls canted at 4deg. This is supposed to give better downwind performance and reduce pitch-poling. Are there any drawbacks/pitfalls to avoid here?


On a more practical note: Is there anyone here with experience with stitch/glue that can give me a guesstimate as to how much epoxy I'll need?

 

I suspect a cat planes rarely although I may get corected on this by folk who know. A cat gets its stability from hull spacing rather than hull width and flies one hull at speed, so the hull in the water may not have enough area to plane. This does not limit speed to "hull speed" since underpowered boats with long narrow hulls such as canoes and kayaks can routinely and easily exceed their theoretical hull speed by a large margin. If that is correct, the comments relating deadrise to drag have no relevance. More significant would be minimising the wetted skin area.

Daggerboard fore-and-aft placement is related to the center of area of the sail rig, same as a monohull, with fine trimming of the rig after launch to tune it. You want the rudder lifting slightly in the same direction as the daggerboard, referred to as weather helm: the usual reason given is safety (to boat heads up into the wind without helm) but it is also related to performance.

 

OK! Thanks!! The wetted surface will be less with 10 deg deadlift as this makes the shape more "round".

Is there a way to get SailcutCAD or other free software to project center of area or will I have to estimate this (by my trusted "eye meassure")?

T.

 

Have you considered a transverse asymmetric hull form?

The asymmetry should provide a little more pointing ability, which might be more fun.

Michael

 

Questions, questions, I feel your pain.

Deadrise: I presume that you are referring to the angle of the beveled chines. Deadrise, as ordinarily understood, refers to vee bottom angle with respect to the keel, not beveled chines. The chine angle can be any thing you like. I would think that the intersection of the chine with the sides ought to be a little above the load waterline.

The beveled chines planks (ply) will be bending in two directions at one time. The ply will bend in a cone shape without much trouble. You will make the build easier if you anticipate that fact. Lay out a body plan. That is where you stack all the section views onto one centerline. The midsection will be the probable control point. Now project a line down from the mid section chine. The projected line will intersect with the center line. That will be the control point or focus of the conic section. Use that point to project back upward onto all the other sections. The angles will change but the focal point will remain fixed. Fiddle around with this deal and relate it to the locations of the bottom curve and the curve of the sides. You will also find the right location for the upper chine joint when viewed in elevation or side view. Do this with pencil and paper along with a good sized eraser. The paper and pencil method will reveal far more about this odd ball geometry than any computer program. Pencils are very good practice tools for budding designers. Computer programs are marvelous tools but they do not expand your intellect very much.

Ride quality. Try not to get choked up on that. A small cat is not a Mercedes. More like a motocross bike. Live with it, enjoy the ride.

Recognize that your flat bottomed boat is not the same breed of cat as a Nacra, Prindle, Hobie or similar fancy boats. The water that exits the back of a flat bottom does not behave quite the same as if the bottom were vee, round, eliptical or whatever. I suggest that your bottom should taper upward in a near straight line, from the point of maximum immersion all the way to the transom.

The boat is not going to plane. The quarter beam aft buttock angle, something like 6 degrees, is so large that planing in anything short of a half gale is not going to happen. That does not mean that the boat will be slow. It is a catamaran not a planing dinghy.

Canted hulls: go for it if you like. But I fail to see how a small cant angle would mitigate a pitch pole situation. Vigilant helmsmanship and correct skipper weight placement is the major determinant in very windy conditions. Downwind the boat is pretty near level and the canted hulls might even be a disadvantage. The cant might be favorable when the boat heels but that is the case mostly when going to windward or reaching.

Boards: Dont get fancy just yet. Save that for another day. Plain old symetrical boards will do fine.

Stitch and Glue: Lord deliver us from evil. Glue yes, stitch no! Save a ton of time, profanity, punctured fingers, and sweat by avoiding the wire stitching. Build a form with ribbands at the intersection points of the ply panels. You can trace the ply panels exactly by placing the ply over the form and simply marking at the ribbands. Damn sight easier, faster, and more precise than fooling with elaborate gridwork layouts on the floor. When you are ready to stick the panels together, put waxed paper over the ribbands to protect them from the glue (epoxy for sure). Give the joints a light coat of glue and temporarily staple or tack the parts in place. After planing and fairing the joints put a ribbon of glass over the joints. It;s beginning to look like a boat and you dont have to fool with a bunch of miserable wire ties.

Board placement: Well that is going to be a problem that may defy simple solution. As a generality, the sail center of effort should be located in the same vertical plane as the center of lateral resistance of the composite of the hull and the board. Some people include the rudder in the calculations. Nice generality but it does not work quite so neatly as that. We will almost always use some "lead". Not led, leed. That is, the CLR will be ahead of the sail CE by some amount. Usually somewhere between 5 and 10 percent of the waterline length. Think of a sail that is sheeted out for a run or broad reach. The sail CE has moved forward and sideways too. Even hard on the wind the sail is out a way and the Ce is....is....well who the hell knows where it actually is? The geometric centers of the sail is not necessarily where the center of thrust is. Depends a lot on the cut of the sail, length of the boom, and too many other variables. Designers usually go with their experience, make an educated guess, and hope for the best.

Now that we are into sail talk, here is nearly a sure thing for advice. Get a professionally built sail. Dont even think of building it yourself. The difference between a good sail and a not so good one is so remarkable that it is almost frightening. Sure you can make a sail that will propel the boat. You'd have to be incredibly lucky to build a sail to match a professionally built one. Save money for a decent sail, or you might get a good used but proven one from another cat.

 

Great to get some feedback here! Greatly appreciated!

@mcollins
Yes, I've considered asymmetric hulls, but I've rejected the idea for a number of reasons:

a) Ease of build: Symmetrical means that there will be 3 main shapes to cut (upper board, centerboard, bottomboard) apart from the stations. This saves me some time in cutting

b) Ease of design: I can not design asymmetrical hulls with CARENE (which is what I use for this plywood design). I could tweak the design in FREEship but I have no way to simulate the effects of the asym hull at speed.

c) Safety: This is my biggest concern. How much asymmetry will be to much. A hydrofoil so "thich" and long as this hull will be able to generate loads of lateral lift. The deeper it sits in the water the more lift it will generate. I foresee that this is a potential hazard when the boat is pushed hard, as the more the hull is submerged the more it will point to windward. I believe this could lead to broaching.

I've tried to look at some asymm cats, but i have no plans or accurate drawings, so I really have nothing to compare with. I won't risk making a huge blunder with tech that I don't fully understand on my first hull. I'll probably consider assymetry on my next build though.

T.

 

@messabout

WOW! That was a long and very informative post.

By "deadrise" I mean the V shape of the bottom planks (it's what it's called in Carene). The angle of the remaining planks are 45 and 90 deg. The way I see it flat bottoms will be the easiest to build, but probably a bit "slammy". The deadrise rounds the bottom a little without adding to much wetted surface.

Canting: I see your point! I can not get my head around why this would reduce pitch-poling (other than possibly on a broad/beam reach, where it would keep the hull more "upright"). I must have misunderstood the Nacra leaflet (or it's just marketing-talk) However, it could be a good thing going upwind (more upright hulls). I'm still undecided, but a compromise would be a two-position system (0 and 4 deg). It'll add some weight, but as this is a first-build it will be a platform for me to experiment on..

Boards: I just thought I could kill two birds with one stone here. By shaping only one side of a ply board and then rounding off the leading edge I imagined that I'd have a board with a little more lateral lift and at half the work (lazy bugger, I know).

Board placement: I guess i gould make a strap-on kind of board to evaluate placement and then put a box in when I find a suitable placement.. It'll add some drag, but it'll only be preliminary.

(no) stitch-and-glue:
I get that you have had some bad experiences with wire and sore fingers in the past? I'll certainly look into your technique when it comes to building, but for now I'm more focused on cost and the bill of materials. How much epoxy will I need for this build? A gallon per hull? more? less?

Sails: I can probably get second-hand sails off anything from old hobies to modern F18s that wil outperform self-built sails by quite a margin. I recon the cost would be managable as well. If I'm lucky I might get an entire rig from an old cat.

Thanks for your insights! It really helps to get some pointers.

T.

 

Simplest way to find sail center of area is to cut out the pattern in card and find the balance point.