Catamaran Design

Hello all...

I ran this through the multihull forum and was wondering if this forum would contribute comments on the design. I am enclosing the study plans for perusal and would be happy to answer any questions if it will evoke opinions. Comments of the study plans format are also welcome.

Thanks
J

 

Anybody? Anything?
63 views and no comments? Good or Bad...I'll take them.

J

 

half the views opened the study plans, me beeing one of them
allready said you made a great effort, made some suggestions, pointed out we ( i ) have to work harder
compared with my ladybird wildcard only sean had a "has potential" remark on your doing good

 

polls, pre-elections, politics, comments, geeez, where is trouty

 

Hello Yipster...

Well okay.

Thanks to you and to Sean whomever that is for the "potential" remark on doing good.

Geez... 111/46 views and total silence. Don't rightly know how to take that.

J

 

My only comment is that study plans are usually sold by designers. I hope you aren’t infringing any copyright laws by publishing them on the internet - unless of course, you are the designer.

 

JCD,

A few comments from here.

- Study plans usually consist of a lines drawing, profile and general arrangement, and some excerpts of the construction drawings. What you have posted here would be better described as a design narrative or outline as it contains plenty of description and lots of tables, but few drawings.

- You mention bow berths. I hope these aren't too far forward as the motion right in the forepeak will be much more violent than the motion closer to the centre of gravity, especially on something this light.

- You mention that the vessel should be capable of ocean work. While it is possible to cross oceans under sail alone, you seem to have spec'd about half the power and half the fuel of a typical cruising cat this size.

- "Storage is moderate..." (pg.4) - Everyone I have ever talked to who has done serious cruising has wished for more storage.

- "dual fighter stick steering" - What does this mean? I can't think of any flight-stick system that gives sufficient leverage to handle a boat's rudders, not without power assist.

- "A happy medium was reached between high end aeronautical construction methods and low end strip plank woods with skins. The construction method is to be vacuum bagged composite sandwich foam core with glass skins" - Many people are surprised to find that strip plank wood/epoxy construction is actually one of the most structurally efficient systems there is, and it's awfully hard to beat for one-off construction (no moulds!). The construction technique you describe would seem to require hull moulds. Foam coring, especially below the waterline, can be a risky business if the workers are not experts in how to do it right.

- The table on page 9 says the hull skins are to be 0.032 in, or 0.81 mm, thick both inside and out of the 0.59 inch / 15.0 mm foam or balsa core. While such a panel may well be stiff enough and strong enough for your purposes in terms of bulk properties, I would bet that I could put the claw of a good hammer right through the hull with little effort. (I have done so on several occasions with sample/mockup panels of similar skin strength, even with carbon/kevlar.) You need to consider impact resistance as well, and the extra weight of beefing things up at high stress points like crossbeam mounts, chainplates, rudder bearings, jib track, etc. Have you factored this into your scantling and weight calculations? If not, you may be in for a shock when you do.

- Lastly, the above points likely read like an angry critic's review of a mediocre movie. If you have encountered my posts elsewhere on here, you are likely aware that I set my standards of scrutiny according to the apparent intent of the poster whose work is being scrutinized. Thus, for a case like this where it appears there is some serious thinking and preparation going on and a reasonable chance that the project will move from the study phase to the detailed design phase, I am going to increase the level of criticism accordingly. Likewise, when fanciful never-gonna-float stuff pops up, I will back off for fear of offending the poster. In your case, JCD, I can see you've done your homework and I will be relentless in encouraging you to find ways to improve it, because I know you can and will.

 

Trevlyns,

My study plan is not for sale. It is for everyones use as is the design itself. I'm designing it for me but choose to put it out on the open in the community. It is my design so a copyright issue does not apply.

I can't very well call myself a designer in the pure meaning of the word because there is no way I can top their experience, nor do I kid myself, but I learn and design for myself. What I don't know, I ask.

Now...what do you think about the design?

Thanks
J

 

I like your design.

There is a lot of intelligence to make the hulls completely closed like you have. Speeds up build time. Perhaps, make the inside area between the beams a bit more squared into the hull area. This would give more space in the main part of the hull area.

Really like the idea of having less solid bridgedeck.

 

Hello Marshmat,

Now that's what I'm talking about! Medicine is bitter and we have to take it to get better. Cherry flavor didn't get added until some rich kid started to cry and whine. I'm not a kid and I'm definitely not rich, so continue to offer without worrying about caution because I don't cry or whine. Thank you for your candor and know that I don't censor thinking or speech. I will offer my thinking to your comments.

Now. The plans are not complete but as you have pointed out, they do appear to be a brief. I wanted to provide the plan, profile, section, deck, sail and arrangement plans from CAD drawings which I'm working on with some setbacks, but they will be overcome. These will eventually lead to a construction manual as work progresses.

The bow berths I thought about and it was brought up already. I know they are the worst place they could be. Having said that, the start of the berth is 11 feet from the bow. I know that is only 1/3 of LOA. I wanted to trade them up with the head at the stern, but thought that maybe I may want to remove one entire head and set up a sitting area in one of the hulls. Then there was also the motors aft which would be somewhat noisy, so I settled for bow berths. There are other places where one can lay down to rest and if it must be at night while under way, reducing canvas will make it a bit more tolerable. The reduction to keep her at say hull speed would make it safer in any event.

Yes. I designed her blue water capable. I know that I reduced the fuel and hp for the motors, but I didn't think it was half. For me, motors and fuel are a necessary "noisy and stinky" evil and I would probably not go to motors under any condition except a desperate situation. The horsepower to get her to hull speed has been doubled as redundancy so there is plenty of HP there. At 1GPH consumption with 80% throttle, it can be expected that the motors could be run for 24 hours and at hull speed of 7.6Kn's, I can expect to get 184 miles under the keel. I consider that quite a bit of distance that can be covered and anything above that can be anticipated with extra fuel.

Storage is as you said, always in high demand. I have rejected it right from the start. One of the laws of physics, for my wife anyway, says "if storage is available, it will be filled, stacked and wrapped". One cabinet in the head, nav station, galley, bow berth and cockpit and that is it. There is plenty of space under the berths and cockpit seating for extras, but keeping her weight down will start with the least possible storage available. On a positive note, with easily driven hulls, faster passages are expected so less will be needed.

The dual fighter stick concept is ahem...well, you know, the kid in me. But, it does have a purpose. Space conservation. Acting just like a tiller but in a vertical direction it will in my opinion be an excellent tool for the rudders. With some gearing, blocks and a centralized drum with linear auto-pilot, I believe I can reduce loads to the point where power assist, whether electrical or hydraulic may not be required. This is still theory because I have not worked out the different ratios for the gear etc., but I think I will be able to do it.

The Green Lantern began as S&G because of the simplicity. Yes, strip plank cedar is excellent. However, it is super intensive in labor with all the cutting and fairing and gluing and cutting and fairing not to mention the mess. There is a weight penalty. As a combat veteran I sustained injuries that although not crippling, they act up more and more the older I get. I realize my limitations and know that to go to that method would be certain failure, so yes, I redesigned for moulds. The moulds will be hard to get completed, but it will be hard one time and then 95% of the associated issues with strip plank are eliminated. They will not be like some of the professional moulds. They will be lined with formica. No, they won't get 100 hulls pulled from them but even 5 wouldn't be a problem. With caution and vacuum bag, I think I can overcome poor workmanship below the waterline.

Generally, yes, the skin and core is as you pointed out, for a total of .655 inches overall width. I also bet that you could put the claw of a hammer through the skins fairly easily but it wouldn't be so easy if I was there.
Beefing things up is already incorporated in the design and all connecting and insert stress points have as much layers as the ring frames which can be as much as 20. Also, insert areas and backing plates will either go to an H200 core or solid plywood.

Plank 1 (keel) and plank 2 (waterline plank) will have the following callout from outboard to inboard. Gel coat (white and sea green), mat for printthrough, dynel for smoothness, structural skin (triaxial) twice, the core, structural skin twice, dynel and formica for interior finish. These are the only hull panels with double structural skin and, overlapped at the keel line, they provide 8 layers of structural skins. That is a total of 23oz*8 at the keel for a total of 184oz per sq. yd. This does not even include reinforcement. I know a whale whacking me is still possible, but with all other things considered should I still worry about the scantlings?

Now, knowing the above thinking and plan of action, is there anything else I should consider or worry about?

I appreciate that. It has been rewarding but difficult. I have always gone to great lengths to better and improve my own little accomplishments with the design instead of trying to outdo someone else. I will continue to welcome your input and would be grateful if you popped into the thread every now and then and threw a wrench into things so that it keeps me open minded and honest. I do intend to float this thing and what I do now may be the difference between sharing a room in Davy Jones locker or retiring to the world while walking on water. If you would like to "scrutinize" the scantlings call out let me know and I'll produce it for review.

Thanks
J

 

Thanks Nero...

I though about squaring off the inboard area as you pointed out, but it took away from the form and the moulds wouldn't be symmetrical, so I gave it up.
Yes...less deck. I love that.

Have you followed the progress? Here is the link to the thread. Any comments would be appreciated.

http://www.boatdesign.net/forums/showthread.php?t=20302

J

 

This was getting down to the bottom of the list so I brought it back up.

Anybody else? No comments? There is no way that it can be that perfect or bad. Anything?

J

 

pragmatic prismatic

Yeah its a nice boat and anyone can see the art that's gone into it. Indeed I recognise much of what you explain but since you push for comment I'll find something to carp about. But please excuse my emphatic manner I'm only making a point.

Your CB seems a little too central. How about 54/55% a lot of respected cats put it there. This hits another thread, debating if a hull is an elongated tennis ball or a log; I suggest the aft should be the tennis ball and the fore should be the log. I'm sure I read references about this (maybe Joseph) a hull shape to attenuate pitching before it starts. I suspect your vertical sided stern comes from this advice.

Michlet makes you pay dearly for moving the CB backwards by 5 points, a 10-20% hike in wave resistance and 1-2% penalty on the v coefficient. I guess its the price you pay to not spill the wind.

Lastly your high cp 0.64, so cool for cats often quoted to mitigate pitching. But I wonder how you achieved this. With a wave-piecing stem and moderate stern you must have pared down the mid section. This was probably forced on you because of the V shaped freeboard as opposed to Shuttlecock's signature light bulb.

For a fixed length, fixed beam and fixed draft I can see a higher cp gives a lower resistance. But adversely for a fixed displacement (and weight carrying is what its about), then a fuller mid section could be superior. It gives a higher area to girth ratio hence lower fictional resistance; it will slash your draft, reduce the diagonals with better run out at the stern, all mean lower wave resistance. Yeah your cp will be less than 0.6 but i suspect your Michlet figure per ton will be lower.

So nice to see figures published. Best Fred

 

Hello and thanks,

Here are my thoughts.

Everything I have read regarding Catamarans states that the CB should be dead center and directly under the CG. Obviously, this is while the vessel is at rest. Therefore, when waves come up from ahead or astern, they pass a distance under the keel before they begin to lift, thus lifting from the center of the boat and eliminating the potential for pitch-pole, or hobbyhorsing. Also, since the bows have reserve bouyancy, when depressed, the leeward hull will allow the CB to move further forward and reduce pitching further. Boats with broad transoms have the center of buoyancy farther aft and as you have already noticed, the aft sections are a flat run with vertical sides to keep the CB forward as much as possible, for as long as possible. I'm certain that many respected designs place their CB as you have pointed out, but I think that it could be a consequence of a "broad" stern.

Vertical sides aft, as you have pointed out, keep the CB from shifting aft. The stern, not being too broad, is purpose designed to reduce any further drag when the hull is depressed. I'm not sure whom I read that from since I have read a lot from many and remember may saying the same thing, but to me, it just made so much sense, that I consider it irrefutable.

I'm not sure how to interpret your observation about Michlet. I have been kicked several times by Rick to learn these programs and I'm procrastinating. I will, but can you just explain a little more about what is the value of a "point"? Also, I think you mean "v" as vertical? Why would such a penalty occur and how is that penalty good for keeping the wind you catch? Sorry, I'm confused.

Yes. The mid section was kept slim to make the hulls more easily driven. I am enclosing an attachment to show you what the waterline looks like so that you can see how pared down she is. It's a little hard to see the color. The high CP is better for light wind performance since there are 2 hulls. I'm not really sure what my Michlet per ton is or would be, or whether it is good or bad, which now makes me want to find out.

I think I went as far as I was willing to compromise and probably could have had better B/T and Cp etc, but I feel confident that I have improved the design considerably and definitely to the point where I am comfortable with the "mathematical" performance results. I never set out to make a perfect design. That is usually within the capacity of the pros and I suspect that they would encounter significant difficulty.

You like figures? have you had a chance to look at what I have provided for perusal? Read my thread and feel welcome to offer any opinion as nothing is meant to be authoritative.

http://www.boatdesign.net/forums/showthread.php?t=20302

Thanks
J