Space Cadet Cat

I currently have a 16’ Hobie with a pretty messed up hull. I am building two new hulls and going to use the full tarpaulin -frame, rigging and sails off the Hobie. I purchased a set of plans from an architect that is well established. I do respect this architect and his designs. I know that his design through his technical abilities and extensive experience was optimized. However, I am a very curious and opinionated engineer (are there any that aren’t?) I wish to strike off on my own tangent. I realized I will likely have a less efficient design. I may even have a less safe design. I here state for the record that I take full and complete responsibility for my design decisions for myself. Anyone gleaming anything off this thread… should do likewise. Now, that we’ve got that happy horse-legal **** out of the way…

His plans are for a 16’ V hull catamaran with small skeg keels. The design I am currently building will be 20’ long. The hulls will be slightly wider and the total beam will be slightly wider. I plan on rounded bilge hulls (closer to the Hobie Fox) in concept. I will be axing the skeg keels and am currently looking at blade keels that rotate out of the way like the Hobie’s rudders. I am obviously not joining the hulls together via his plans (eg horizontal mounting of an aluminum tube), but through the Hobie’s vertical tubes descending from the tarpaulin. Basically the only thing I am using out of the plans are:

• the techniques (stitching)
• the basic structural factors (scaled where appropriate) eg hull panel thicknesses, fiberglass thicknesses and locations and so forth
• series of steps used as a checklist

Which brings me to the question… should I reference his work or if it was your design would you rather distance yourself from such a clearly divergent work? On his web site he seemed quite testy about people lengthening his designs. I didn’t ask his permission. I can’t imagine anything I’d ask of him for technical support because his plans are extremely concise and logical.

 

Altered States Kitty

From where I sit, you're on your own, Bubba. Leave his reasoned work out of this and do your own R&D.

Perhaps you fully understand the beach cat idiom and perhaps you don't. It sounds very much like you will be making beat tester #1 to that end and so.... let the journey begin.

The only real good way for you to understand his design choices is to make your own and see how they work in the real world of sailing your boat. Efficiency under sail, ease of fabrication, expense associated with the build, functional utility of the design under actual conditions, legal aspects of the trailered hull, etc., will all come clear as you follow the path of enlightenment, Grasshopper.

Let us see the pictures and read the report.

Chris

 

your design

Don't reference his design in your design without his permission and co-operation.

 

Overview

I’m fine with that. The saga begins…

So this thread will now highlight my progress and what I learn and maybe people will poke and prod and steer me in some good directions. We’ll call this SC20. And R&D is the whole purpose of this project. I don’t think it will be over any time soon. For those who have seen the fiberglass mast thread I participated in, you’ll eventually see two of those on here. Two you say… … it’ll be a biplane, but not really the biplane you're use to, it’ll be a little different. Who knows, there may even be foils on this before it’s all said and done.

Here is the ten thousand foot view. Besides the damaged hull, the other big issue I had with the Hobie 16’ was the limited load capability. One good beer drinking buddy and I on it, out in the ocean (not drinking beer at the time… safety first) well… felt so-so. Three people on it and pig would describe the scene. (Now, be nice) Also, the sailing here in Atlanta is, shall we say… boring! The best wind this past summer (short of going out in a thunderstorm) was 10 knots. I did manage to tip it over. It was my first day out and I hadn’t sailed in a year or three. I didn’t think there was enough wind and I had the main sheet in the chock. Anyway, in the first iteration, it’ll just get the Hobie 16’s sails, however it’ll probably get more sail area as time goes by. I want something that I can tool around on with four and go like stink with a 400 lbs crew of two. I will probably put a forward rope tarpaulin to carry bodies under the jib.

Feel free to comment…
I don’t mind negative criticism, but try to substantiate it so I can learn something. I appreciate this forum a great deal!
Thanks to all ahead of time!

 

Side Hull Panels

The design uses a stitched plywood hull calling out 4mm marine grade plywood. Finding marine grade plywood even here in Atlanta was becoming impossible. The mail order places wanted $45 plus shipping. I’m sure I can trust the product, but shipping a small quantity of plywood sounded both expensive and damaging. In one of my many outings to Home Depot (HD from now on), I ran across the 5.2 mm Lauan plywood for $10.95. I bought a piece and took it home. Researching on the web showed that some people have used it in boats and others discourage it. Several key things to check for were noted.

Type of Wood - I noted Lauan was a hardwood and quite a bit stronger than the typical fir based marine grade plywood material.

Waterproof Glues – I cut some samples. Some I weighted down in an outdoor pond. Some I boiled to death. Some I ran through the general household dishwasher for a week or two. In no case did anything delaminate. Since this boat won’t be left in the water, these tests seemed harsh enough… considering, it’ll be covered in epoxy/glass anyway.

Ply Thicknesses – The web noted that often the plies are considerably smaller on the outside since these are cosmetic. Being an aerospace engineer specializing in composite materials, I understand the importance of ply thickness and orientation. Sure enough, one batch at HD had this problem. The inner ply was about 80% of the thickness leaving a mere 10% for each outer ply. However, my batch the plies appear to be about in thirds. Which works perfect for my project. The zero degree direction outer plies will be taking all the loads of the structure. The inner ply, being smaller than normal, allows it to be easier to bend in that direction which allows me to round the bottom of the hull more. Like the Guinness commercial… Brilliant!

Ply Gaps – This is the only concerning aspect. Although there are no gaps on the outer plies, the inner plies do have gaps. I’d estimate they make up about 2 to 3%. As the design is more cigar shaped than say a flat bottom boat, most of the loads will be in the axial direction… mostly in bending of the hulls. Compression on the top sheets and tension near the keel will be the highest loading conditions. However, shear will be present in the sides. The gaps will reduce the shear strength some. Running some G/E at a 45-degree bias should solve that issue.

The net result, this plywood will be stronger (5.2 vs 4 mm and hardwood versus fir). It will also be heavier. Weight carrying capability is one of the main design goals. 20’ length and rounded bilge should solve that nicely.

 

Basic Dimensions

The basic dimensions are based on maximizing the usage of the plywood to meet the nominal design. I need five sheets of the Lauan and I have HD cut them in half lengthwise. They have a far better rig to cut 4’x8’ sheets of plywood than my table saw. Once set-up it was real consistent.
This defines the height of the side walls being nominally 24”. The nominal length of 20’ will actually be defined by 2½ of these 24” wide sheets minus the scarf joints.

Scarf Joints
I hate scarf joints… I turned to the web again… and the common thread, I’m finding, I need a power planer. Now since I have no desire to ever make a second boat in this size range, I didn’t really want to go out and pay to buy a power planer. Using a hand plane sounded more like self abuse. So I get a new carbide blade for the table saw and turn it on its side using the rip fence as the table to support the edge of plywood. I made a little frame out of 2”x4” material and two rollers to shoot the wood through the saw. Built the frame, cut and sanded the 16 scarf’s in about two hours. My scarf joints turn out to be 2” long and with the 5.2mm thickness, this comes out to be 10:1 joints. The web assures me this joint will be stronger than the wood. Also, since there are two joints per length, the panel lengths are now 19’8”. Another important point about making the scarf joints… You want to make sure that “as the water flows” you do not want the scarf to “catch” and open up. See figure.

Cutting the Outline
Before cutting, I stacked and C-clamped all four hull sides together so they’d be as consistent as possible. At first, I thought I would have to use a jigsaw to cut the curved hull lines. But once I drew them out and saw how little curvature there really was, I just took a regular circular saw to it. The following pictures are after this cutting process.

 

Sc 20

Hi SC,

I'm kinda curious, in the desire to develop your own 20' cat to haul around a gang of excitable boys, what did you use to actually develop the hull form, the hydrostatics of same, the displacement capacities for the adults and all-up weight of the boat, the rig area, righting moment and stability curves, etc., etc.?

Is this going to be a trailer legal cat when loaded flat on the trailer or are you going for a wider beam and one of the tilting rack setups as seen on Tornado trailer rigs? Perhaps you're doing one of the many solutions to a demountable arrangement? Maybe you have come-up with a totally unique and righteous method for getting beam without making it a hassle on the launch ramp?

Are you making your own beams, as you suggest in the clue regarding the bi-plane mast setup, or are you getting it on with off-the-shelf spars from a known supplier?

I would have to guess that you have all this sorted as you've started cutting hull parts already, so.... what gives?

I agree that criticism that is offered in a mean spirited fashion does nobody any good. You will not see that from me as you move down the roa on your exploration. If I have something contrary to say it will be based on experience from many years of beach cat sailing or the pragmatic aspect of being a guy who builds the prototypes of his own designs. It will, of course, be completely up to you as to how you use the info.

I really hope your work takes you into a much greater understanding of the multihull form and that you share your discoveries with all of us.

Best of luck,

Chris Ostlind

 

Hello Chris,

I would like to tell you I did some high-tech, AE sounding analysis. My background was in structural analysis. Cut most of my teeth in your back-yard as a matter of fact… West Jordan, UT with a little company called Hercules. I was at their HTS building. My background is a little more rudimentary when it comes to fluid dynamics. The truth is, I'm kind of winging it... to a degree.

Maybe some more experienced members can confirm my reasoning is sound or, at least, so-so… or point me to some URL’s or books that might flesh-out my understanding. “Principles of Yacht Design” (POYD) by Larsson & Eliasson or “Theory of Wing Sections” (TOWS) by Abbott & Doenhoff are about as analytical as I’m prepared to go. This is a hobby… not a career.

Hydrostatics & Stability
As the baseline, I am mainly relying on the architect of the plans analysis for the basics. My first premise is that if I use his exact bulkheads and relative positions and just stretch those from 16 to 20 feet, I will not change the basic hull characteristics. The weight carrying capability should theoretically go up linearly. In pitch, if anything, the oscillations should go down just because of the length increase, but the general characteristics should be the same. In roll, I am relying on the simplicity of the catamaran concept… its dependent entirely on the displaced hull volume with a moment arm between the hulls or in the case of a small cat like this… by where the bodies are located (out on trapeze, etc) relative to the leeward hull.

My deviations from the baseline’s V hull will be subtle. The curving ability of a stitched hull will only let me diverge so much. Once I stitch the sides together, I will be using the stock bulkheads and working from there. I’ll use shims to see how much additional rounding I can get. Before committing the design to glue, I’ll take measurements and run them through FreeShip and BioDesign’s Hydrostatic program (both are freeware) for a reality check. I feel this seat-of-the-pants is of low risk because: (1) most designers evolve new designs from old designs. (2) In this size class balance is largely where you place the bodies. You are probably more familiar than me on this, but on the Hobie, the bodies are always on the windward side… usually as far out as possible. As the wind picks-up those bodies tend to shift from just behind the mast to near the rudders. So adding mere cubic inches (balanced about the desired C.G.) should be of low risk.

I’m going to a round bilge because of the lower wetted area hoping to improve my light wind speed here in Atlanta… and I wanted to play around with various wing shapes/foils for the keel anyway.

Beam
In its first incarnation, since it’s using the basic Hobie tarpaulin frame, rig and sails, its only going up from 7’11" beam to the full trailerable legal width of 8’6”. I may play around with something more foldable once I start playing with rig design and go with the modified biplane.

I don’t know about righteous but some of the design goals for my eventual replacement rig are:

• Two masts
• ~10-15% chord rotating wing mast made out of fiberglass (see Epoxy/glass mast for a cruising yacht?)
• No shrouds, but not cantilevered either.
• Easier mast raising. Raising the Hobie’s 26’ mast is not really that easy by myself
• More sail area without more tipping moment

POYD talks about how everything is dependent on everything else and how a change in one causes changes to be required in another. I can fully understand this for a large monohull. However for a small catamaran things are more decoupled. I don’t believe the hulls really need to be modified fluid dynamically as long as I maintain the centers of sail thrust, keel location and CG. They only need to be modified structurally if I change the mounting locations… eg. If I increase the beam, but use the same mounting points the hulls don’t need to be modified. However, I will be placing reinforcements in locations planning on future design changes. So (I am hoping) that I can evolve the design in stages. e.g. Make the hulls; place them on the Hobie platform/rigging and go do some sailing using the same trailer. During the sailing season, I can start building the next phase, and do a weekend swap by disconnecting the old and mounting the new. So to answer your question… no, the whole thing isn’t designed and I was hoping to introduce the segments in this thread as I go and see how response might drive me to a better solution.

I need to get back down to the basement and make some sawdust.

 

Testing and Comparing Modifications

Does anyone have any suggestions on how I can chart improvements?

As describe above, in the first stage, I want to replace just the hulls. I’ll still be using everything else off the Hobie. Are there any techniques to quantify its abilities now so I can compare them after the change? Recommended boat headings, how to take data, how to account for variability… whatever crosses your mind.

 

Easy but not very scientific ... Sail the Hobie in as many different conditions as you can with a handheld GPS unit. Keep a log of the sailing conditions.

After each sail download the track data from the GPS and put it into a spreadsheet. You can get SOG and heading information for that days conditions. If your sailing area has current, you can use the tide/current information to correct the SOG from the GPS track to SOW. (get the true wind direction from the heading on each tack)

It won't be as good as instrumenting the boat but if you sail the boat in the same area, try to duplicate the track as much as possible. Set the GPS tracking function to store about 2 hours of data. My Garmin 12 can store 1024 data points. Set to 10 second intervals gives 2.8 hours of data. You could set up a routine 15 min upwind on each tack in 5 min tacks, 10 minutes beam reach each way, and downwind tacking back to the origin.

A handheld windspeed meter would allow you to label each track with windspeed data. Since the track is timestampes with Zulu time. You can note the time of the wind observations and correlate them with the track information.

I'm using this to evaluate rig changes on my dinghy. It's not great, but its better than nothing. 10% changes in performance are easy to spot, less than that gets harder unless you can make 100's of runs.

 

Can you make any recommendations on models of GPS and/or some kind of wind speed measuring. I would think wind direction would also be useful to help correlating relative aspects. Being able to store points would be vital, being able to download all to a PC would be nice. On the cheap would be a major decision point also.

 

As far as I'm concerned GPS stands for Garmin Positioning System.

IIRC most of the hand-held units except the very low end e-Trex have NMEA and data transfer to PC ability. You can probably find one for $100 or so.

Davis makes a nice little hand-held wind speed meter for under $50 the last time I checked.

For wind direction, you compare heading on each tack and halve the tacking angle. If the track is 45 on port and 315 on starboard, the True wind direction is 0. You can deduce the angle from the boat's performance (assuming equal sail trim on each tack).

The only caveat is that in current the track is COG not COW so you have to know the speed and direction of the current to get real boat data.

If you want to minimize the effect of current set and drift, plan your testing times close to slack water and make notes in your test log of the time and direction of predicted current.

Test 1
Date
Time
Wind Speed at dock
Time +/- of slack current
Compass heading on port tack
Compass heading on starboard tack
True Wind Angle (port heading - starboard heading) x .5
Beam reach Headings, TWA +/- 90

If you spend 5 min on each tack, then 5 min on each beam reach and 10 minutes downwind, you should be in the same bit of water for each test and can update your log with windspeed information every 30 minutes.

The difference between your compass heading on each tack and the GPS track heading is a combination of leeway and current.

After you have some data you should be able to see a pattern for your local area compared to the current predictions and adjust your results to compensate.

If possible include a repeat run of a base configuration every now and then to see how accurate your data is.

It's time consuming, but I can tell from your posts that you are a numbers geek too. I keep a data book of every race on my Cat 30. After a period of time I get a pretty good picture of the wind and current in my local water.

 

She's having a bad hair day...

I used the scrap pieces of 5.2 mm thick plywood that were leftover from cutting the curves of the sides. I pieced them together with more scarf joints to create the bottoms. As you can see, I decided to use the plastic wire ties instead of copper wire for stitching per gonzo and chandler’s suggestion in my “Upside down stitching technique” thread.

No, I’m not making a flat bottom keel…

After I glue in the bulkheads, finish-off all the interior stuff and close up the hulls, I’ll turn them back over and use cedar to strip up a curved bottom. Its on my to-do list!

Next Phase - Spreading the sides with bulkheads. I want to see if I can get a little more curvature than normal. Are there any downsides to spraying the sides with water before bending? I have also heard of using amonia/water mixture... but I've seen this mainly in furniture woodworking.

 

Wide load coming...

As I mentioned above, I wanted to more or less evolve the Hobie 16' into my own SC 20' with an odd-ball bi-plane rig. While trying to design the bulkheads, I realized I wanted certain structure in places to support the old Hobie Tarp and structure for the eventual design. I finally realized I’d have too much unused structure in both cases; so, I’m just going to bite the bullet and head toward the final design in one fell swoop. I’ll use the rudders and blocks… but that’s about it.

I’ve been doing a great deal of research on the Tornado (thanks to farjoe: Tornado) and some detailed reading on sail design, rigging, balance using both Theory of Wing Sections and Principles of Yacht Design. I’ve decided to approximate the Tornado’s width (120”) which brings me to…

Could you shine some light on this? I’d like to some input on the problems associated with trailering this… e.g. whether to hard mount the beams to the hulls and have a tilting rack or have dismountable beams and keep everything flat. The former would be stronger and the latter would have to have bolt heads sticking up… so I’m leaning toward the former. But real-world experience would be greatly appreciated.

To answer a question in yours… Yes, I’ll be making my own beams.

My biggest hangup was dealing with mounting the tarp... until Tom Speer's post made that an elegant no-hole solution. I hate holes in composites!