Personal Offshore Catamaran Design

Hello Ray...

Here we go again...enough so that I have actually started to consider redesigning the draft so that we can move onto a diffrent subject.

I have already posted a clear...clear to me...way that I was able to eliminate a lot of the extra weight associated with the usual design and other designers have already figured out how to do the same and practice it religiously. There is really not that much mystery, but I will give a small example for the bridgedeck since it has been mentioned.

The span is 15' at the ama deck, approximately, and the span is 9.7' at the bridgedeck sole, approximately. The lower span also acts as the cockpit sole and is framed with composite studs at 12" centers. This is plenty of support for that span and length. There is a longitudinal bulkhead supporting the deck stepped mast reducing the span of the bridgedeck at the ama deck level. This reduces the span to 7 feet, and no, it is not a "floating" longitudinal because it is connected to tranverse fore and aft frames. I'm not sure if anyone is able to see the bulkheads that run through the bridgedeck and transfer loads into the hulls but that is probably why the area seems so big and unsupported. It is like a box with a longitudinal running down the center and I will try to post it if the 2mb limit is not exceeded.

This is the part where Frank gets to correct me if I say anything that is considered suspect. I wont get into their properties of balsa, honeycomb and foam...suffice it to say, they are strong...pound for pound because of their cellular design. With the sandwich method, that span can be engineered to be as strong as you want and lighter than a feather, area for area. Bottom line is that you can sandwich anything, but the concept is to increase the stiffness and compression for the least amount of weight. The lightest by far is foam...relative to density and whether it is structural or not, and it is not suceptible to rot, water absorbtion etc...in other words...it is the best material out there...but, be careful with the word "best" because there are many types of foam and not all are equal or applicable to the designed use.

In the high densities and excluding all other disadvantages, end grain balsa will beat out almost everything in shear and compression hands down and high cost aeronautical aluminum honeycomb doesn't even come close.

It is always easier and cheaper to change the geometry and inertia in such a way that the gains in strength far exceed changing or re-engineering the material properties. For example:

(The below is written from a study on foam by DIAB and credit or discredit is given to them.):!:

Fiberglass Skin = x
weight = 1
strength = 1
stiffness = 1

Sandwich Core = 2x
weight = 1.06
strength = 3.5
stiffness = 7

Sandwich Core = 4x
weight = 1.09
strength = 9.25
stiffness = 37

A sandwich of 1 ply, 24 oz. WR and 1 ply 1.5 oz Csm on both sides of a measly 1/2" thick pvc foam of 3.7# density will weigh 1.53#^2 foot and has a flexural rigidity of 17,227#'s compared to a fiberglass layup with flexural rigidity of 16,720#'s and a weight of 4.28#^2 foot.

Now, imagine that same layup with carbon of even the lowest caliber and a structural foam of greater density and realize how small those spans really seem.

Thank's and keep it coming...waiting to hear from the man down the block from me...well, 3 counties over from me...Frank.

PS...I just realized that noone has answered any of my questions or requests...what is up with that? Help me to help you to help me!

J

 

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Shipwright History

Hello FranK:

Just wanted to give you a little food for thought and a little history to be proud about...don't know if you or anyone else knew this...but here it is.

Sir Anthony Deane, a SHIPWRIGHT, is credited with conceiving and giving birth to “modern shipbuilding”, which is defined as the fusing of nautical art and nautical science, when he published his beloved treatise the ‘Doctrine of Naval Architecture’ in the year 1670.

Wish I had a copy...anyone have it or knows where to get it:?:

 

Wetted Area

Good Morning to all...

I have solved for the wetted area. After much research and finding too great a difference between calculations that are "rough" estimates, and others that require too many solutions to get a "less rough" estimate, I decided to expand the panels since I'm going to offer the capability for full size patterns and a nesting plan.

The figure is +/- 7.9% for errors that might manifest from not getting the waterline expanded perfectly by superimposing on panel 3 and, or other radii for the chines. At this point, I'm not sure that this percentage is a bad thing because deep research has shown that designers protect this wetted area figure like gold is protected at the Federal Reserve Bank. I suspect it may have something to do with racing and skippers looking to get every millisecond out of every knot or perhaps is has to do with the "pro" designers looking to perpetuate the mystery.

I will however, publish this number at +7.9% because I really want to belabor its significance as used in ratios and because it may be that...ahem...maybe I might have been, thinking about the adamant concerns from... oh damn... there is a slight possibility, infinitely small possibily, that I may revisit this issue on the drawing board as it has been pointed out by Frank, Chris and Ray. There I said it. . In order to lessen the humiliation to my ego...the justification must be quantified empirically or otherwise. That, I leave up to the knowlege here in.

The envelope please...looking left and right hoping Frank, Chris and Ray did not attend the event...opens the envelope with wet shaky hands...tightens trembling knees from the dramatical drum roll...and,

The wetted area per hull is:
76.2 sq. ft. +
10.02 sq. ft. per dagger board fully down +
4.85 sq. ft. per for rudder =
91.09 sq. ft. per hull =
182.18 total WSA.

The wetted area for the dagger board and rudder equals the sum of both sides with some error for leadind edge radius etc.

So what will it mean for a catamaran...or will it mean the same for all forms beneath the waterline?

Now for some solutions...

Sail Area to Wetted Surface Area Ratio
SA/WSA = 401/182.18 = 2.20.
In light air most of the drag is due to friction. A design SA/WSA ratio is a parameter for light air, therefore it is related to performance. The lower the number the worse light air-performance. The "number" is for static conditions on flat water with no regard for a "hull in flight" or any percentage therof.

Water Surface Area to Displacement Ratio
WSA/(Volume^.333)^2 = 182.18/(62.5^.333)^2 = 11.56
This is important because low wetted surface means low resistance. A design WSA/Displacement ratio is a parameter for resistance, therefore it is related to performance. As stated earlier the lower the ratio on hull (x) the more it can carry than hull (y) for the same wetted area...or conversely, equal drag/friction for more displacement at the same wetted area.

Does anyone have any comparative ratios for the above solutions for a catamaran? To me, and if I'm interpreting the above correctly, with nothing to compare apples to apples with:

1. The WSA is very high...maybe because it is 2 hulls instead of 1,
2. The SA/WSA ratio is low...indicative of poor light air performance,
3. The WSA/Displ. ratio is high...indicative of poor light air performance and poor carrying ability.

Unless my interpretation is inaccurate, the above solutions are contradictory to all others that point to excellent performance for a wide spectrum of operation for air and displacement in the design. I hope I can get some feedback on this with some references to study.

In the meantime, I will take a good hard look at the "hull WSA" (76.2 sq. ft. x 2) and try to calculate loads to see if I can find a relationship for the displacement/lateral area issue that Frank, Chris and Ray have so gentlemanly -like pointed out. In simplified form (6K#/142.4sq.ft) the WSA must support 42.13# sq.ft. at 6k#'s static. Lift a hull in static and the WSA must support (6K#/76.2sq.ft) 78.74# sq. ft.

Time to go sailing.
Thanks

 

That IS amazing! Wish I had a copy or transcript! Will have to start looking up that one....

 

For $50 + shipping!!!

http://www.tomfolio.com/bookdetailssu.asp?b=8593&m=106

 

PDF plans?

Would someone consider posting a picture of this design in a non-proprietary format, like jpeg or PDF? I'd like to follow the discussion but it's difficult without seeing a design sketch.

Thanks.

 

Stopping weight increases

Hello all,

I was lurking through the discussion and firstly I must take my hat off to JCD for being so honest and forthright with his design. My observations come after building a few multis and having owned them for about 20 years (designed a couple too).

I am unsure about the idea of unsupported flat panels being used in the scantlings without any internal joinery to help reduce panel size. The interior cupboards and furniture are very useful in modern slab sided multis to reduce span of unsupported panels. If you halve the span you reduce deflection by eight times so furniture is great in a lightweight cat. We had a death on a Crowther 10 m cat that has subsequently circumnavigated when it was being delivered without its internal furniture and had a structural failure

I will put in my two cents worth about the volume - it may seem all right on paper but I bet in two years the boat will be way under water. It is a sad fact that boats and people put on weight as they age and it takes a zealot to keep it not so. I am always amazed at the things I can forget to include in my caclulations when building a boat - extra resin and high build primer, incredible quantities of fibreglass tape, even after a few builds I still do it. The best way to work out how much a boat will weigh in the cruising sense is to find a similar sized cruising boat and weigh it or if not see where it floats and then using the design data see of it is under or over weight. If you know the immersion rate you can get a pretty accurate figure.

Otherwise get down to a building shed and ask the builder how much stuff he bought and then used this to calculate. Work from real data, not just the computer and I think there will be fewer problems.

I also favour much bigger foils. I know that multihulls are supposed to go fast but big foils are like knobbly tires on a car. They are really nice when it gets rough or when you are going slow and coming out of tacks. Going through moorings and tacking up rivers monos do really well because they have such huge foils and low windage. Multis have high windage so a cruising multi should have large foils if yuo want low speed or bad conditions control. In my cruises I almost always slow the boat down so the loss of performance is not a worry.

My final point would be to be careful with numbers. All of the geat designers of cruising multis - Piver, Jim Brown, James Wharram, Lock Crowther, Prouts, CSK et al all were builders and keen observers of the scene around them. Look at Aikane and then at a Crowther Spindrift and you see the family resemblance. Browns Searunners were great development of Piver's tris and I am certain these guys didn't have numbers for everything. What all of them had was building nouse and lots of sailing miles. My question about this design is - What problem with what boat does this design fix? I feel that you really should stand on the shoulders of the giants before us and be careful not to commit the mistakes of the past. I feel that this design if built would end up having all that volume storing heavy stuff.

I can't see the problem with a larger hull. When we cruised a Crowther Twiggy (fast and thin 31 foot racing tri) we went slower that in our 38ft cruising cat because the Twiggy was dragging her bum and was uncomfortable so we slowed her down. Our Robin Chamberlin 38footer is a delight offshore and is so dependable she is given a free rein and has a higher average than the Twiggy. On the subject of averages some real life data - kankama does 6.8 to 7.2 knots on the GPS on our cruises. That is the average for whenever we move - meandering and fanging. We have sailed 160 miles in 16 hours ( what a blast !) but usually go much slower and still have been passed by only one other cruising boat in 800 miles. They really don't go that fast when cruising and under autopilot.

Well that has ended up as a rant but JCD I would urge you to sail on as many cats like this as you can, work out what is and isn't good, talk to their owners and builders, look at them on the slips and the boatyards to start your journey. Of course you already have looked at lots of boats but I always think if Isaac Newton had to stand on the shoulders of giants then us mere mortals had better do the same.

cheers

Phil

 

Hello Frank:

Thought you might like that!

Are you gonna get it? They have only one you know. I don't know how you found it because I looked for something when I posted to let you know where you can get one, or me too, and couldn't find anything. Good job.

 

I'm not sure yet, but I did find some more copies on other sites. If anyone is interested, go ahead and grab it! I think my library has this book, too. I'll investigate ........

 

Hello Sail...

Welcome aboard...

Wish I could help but I'm not microchip literate...maybe some of the guys in here can help. I had an insane time just trying to get it on here through copy and paste onto a document.

Can't see why you can't retrieve it though. As Nero said to me...keep trying.

Thank's

 

I think the problem is that you posted the drawings as a .doc file, a proprietary Microsoft Word format. Some people don't have Word. I run Linux as my operating system, but I have Open Office, which can open most .doc files. There's a version of Open Office for Windows, and it's free.

Ray

 

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Yeah...what Ray said.

 

Ghost Town

Hello One...hello all:

This place has become like a ghost town. It seems there was a lot of inquisitevess about the design and little help or recommendation to give clarity about my concerns. Maybe I gave out too much info and advanced the agendas of others while creating a detriment to my own. We grow and we learn...well, not all questions asked were answered, but I learned anyway.

Anyway...as a final post and as a courtesy to those whom did contribute, I offer the following evaluations. Windage, performance and clearances. If you wish to help further, I welcome it and probably need it, but, do it through e-mail. You are welcome to contact me anytime if you wish to further explore the design with me. I have been doing some reading on the forum and found quite a few interesting, forthcoming fellows in here, so I'll be reading in the background, learning.

Frank and Ray...I reworked the hull below the waterline, without anything else being analyzed or changed and came up with the following percentages in (-) as a negative change to the design and (+) as a postive change.

Diplacement remains unchanged, LOA remains unchanged.

Draft increased 5.5% to 1.175' (-)
Lateral area increased 13.6% to 28.29 Sq. Ft. (Very -)
LWL decreased 1.9% to 28.83' (-)
CP increased to .60 (+/-/Insignificant)
D/L increased to 37 (+/-/Insignificant)
S/L decreased 1.8% to 2.68knots (-)
D/L Speed increased 2% to 14.39Knots (+/-/Insignificant)
Windage decreased 2.25% to 17.75% (+)
LWL/BM ratio increased .006% to 12.81 (Insignificant)
PPI = 590#...this amount increased from the last amount of 575# so I gained some flotation at the stern where she looked tender. (Very +)
Headroom remains unchanged in the ama since the windage was reduced through the increase of draft.

I increased the aft clearance as you suggested from 1.8' to 2' and the 2.4" was hardly noticeable on the calculations although the calculation showed a 10% increase in vertical clearance. It worked out to something like 1.2" for a 15 foot wave over the 30' LOA...don't hold me to this as it was just a rough guesstimate. It could be done and the sit room would still be excellent in the bunks. I would probably raise fore and aft clearances equally to maintain the slope of the line oppossed to more horizontal.

I also re-drafted the bridgedeck (roughly) flush with the ama and it looked very lean and streamlined almost like a plate for the sails, but there went the sit room in the bunk area and windows now have to be placed on the hulls. It is definitely pleasing to the eye and someone that wants to live on the boat instead of in it could be very happy, but the amount of natural lighting to be lost does not justify the change.

It is my own personal view and opinion that further streamlining the area above the waterline and below could still produce a very nice design...if that is to be the design...however, the changes as compared to the design itself are not justified because many of the performance parameters are effected negatively in proportion to positive change.

The idea of raising the bridgedeck clearance has definite merit because it reduces weight, increases clearance above the waterline and allows the helm better angle of view at little if any expense to the interior of the bunks Even then, the mattress can go from 6" foam to 4" of foam to make up the loss.

Well...that's everything that was open...I think. Best of luck and hopefully we will "write again".

Fair winds and following seas to all...

Thank's