Personal Offshore Catamaran Design

[JCD]

Originally Posted by rayaldridge

Well, the question is not whether the box will float. The question is whether the box will float on its lines. I'm guessing that the thing that struck Chris about the drawings is what struck me: how small and shallow your hulls look, below the waterline, compared to any other design I've seen. Presumably your hulls are designed to be extraordinarily light for their enclosed volumes, particularly considering the amount of structure in your bridgedeck, but I have to wonder why other designers have not discovered how to do this. Can you tell us the weight of your boat, unloaded?


Ray

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

[JCD]

Originally Posted by fhrussell

Finally got them opened! Thanks Chris! ( ...had to go to the darkside and use a pc )

Hello Frank...glad you decided to chime in...was looking forward to engaging someone that has direct hands on experience...not to offend anyone else...because I don't know if anyone else has hands on experience too. I had to figure out a way to deal with these quoting and answering without missing anything...so from now on, I will quote and respond to specific issues in red...in between the quotes...hope it works.

Nice looking design and with interesting and well thought out features.
I like the main traveler arch, but wonder if it can be extended further across the beam of the boat, ...and is it high enough so as not to be a nuisance when moving to your aft bench (nice!) Speaking of the aft bench, I am concerned that your displacement aft may be a bit tender for more than one person in that position in addition to a helmsman. It "appears" she may 'squat'. Perhaps fuller sterns? Also, how will she powered? Unless you drop an outboard through the bridge, you will likely be adding more weight in the aft sections.

Thank you for your observation. The traveler arch or targa or radar platform etc...is a bi-product of military thinking. I wanted to keep the cockpit finished neat and clean without clutter and had to figure out a way to do it without dropping the mainsheet to the deck or placing it on the aft bench and having a "rope" all in the face. It originally started out outboard to the ama centerlines but the sheeting angles and the extra costs and weight penalties didn't justify the span so I brought it inboard to the point where the sheeting angles just cleared the helm comfortably, but it can always go back.

On the high aspect rig with "short boom", the targa is situated so that it wraps around and hugs the back of the seat facing the bow by the dining table ...you would love it if you see it. I'll try to attach a partial with some items you can use for reference on the drawings you have now. The clearance from seat to the bottom of the targa is 40 inches...just enough to bend and sit in order to reduce windage.

You're abolutely right. I have always been concerned with the stern squatting and I'll be the first to admit it. Thinking that some squatting would be good to keep the bows up when pressed I tried to move the biggest amount of mass as far forward as possible to try to teeter the aft load...but even with the bridgedeck that far forward, I still believe she will squat because the bridgedeck is designed for lightweight strength instead of mass. On the credit side...movable ballast such as stores or water could be stored forward in the bridgedeck. Also, one could always go to heavier tackle, but that would just be overengineering and a weight penalty is imminent.

I have overpowered her because of her light weight and windage. The engines attach to sliding brackets underneath the helm seats that are dropped for motoring...not fully aft, but far enough that it always drew a concern for me. I'm cutting and pasting some of the mechanical recommendations below for review.

Suggested Mechanicals
CID/HP Ratio = 1:1.20 or greater for inboard or outboard.
1Hp per 1K#'s * 50% Reserve = 9HP/2 = 4.5Hp
Recommend (2 motors of 8Hp each x 4 stroke)
Gas Outboard installed inboard beneath helm seats.
100#'s Max each

Bilge Pumps w/sufficient battery amps to allow 4 hours pump time.
2953#'s/8#'s * 7.5Gallons = 2768GPH
2768GPH/60 = 46.1GPM
2 - 3000GPH pumps per hull = 27.7 Minute de-water rate
3 - 3000GPH pumps per hull = 18.5 Minute de-water rate (Recommended Primary)*
Bilge suction from engine intakes (Recommended back-up) for operating engines.
18.5 Minutes / All pumps * .75 Volume = 6.9 Minute de-water rate.

Initially, I feel she will likely draw more water than designed as is common for us optimistic designers. Also, it would be helpful to know what your building materials and/or laminating schedule will look like. I think the drawings are decieving though. According to your spec's, you have a waterline beam of 2.31. This is certainly adequate. The question will be "How light (heavy) is your bridgedeck and topsides/ crew/ supplies/ rigging?" It just appears to be a lot of topside structure for the amount of wetted displacement you have spec'd

I also believe that she will draw more than the designed Lwl because there are so many variables. If I end up building her, I will definitely put a weight management system in place and I will do everything possible to continue to convince myself that the engineering is accurate no matter how small something may "look". You have to trust to fly by the instruments because looking outside for orientation or confirmation can cause serious spacial disorientation. If a yard builds her, there is no question that she will draw more and I might have even anticipated that for amateur builders although I don't recall if I compensated for that...maybe I did subconsciously...ergo the "shallow' draft?

The design started as plywood/glass and then I advanced it to foam/glass. I'm not prepared to present the scantlings or schedules because I have not received word from the consultant yet...but I will in the future.

I'm not sure how the drawings are deceiving... the waterline beam of the ama is 2.31 feet as you have observed and as denoted on the plans. It increases with the draft but I have quoted lwl/bm ratios for 4,5 and 6K#'s. No deceit there.

I won't comment on the rigging as it has been addressed earlier.

Okay...but don't forget that there is more than 1 for the design.

Again, your bridge clearance may be a bit low at the stern. For a cat with this DWL, I like to see a minimum of 2', although so many cats these days get away with much less and on larger waterlines, so who knows? ...my preferance ... there is certainly nothing wrong with getting that bridge as high as possible. I do believe that anything less than 2' will yield some discomfort when beating. You did mention a concern for having sitting headroom in the berth.

I trust when you say that the clearance may be low aft, as a practicality. My study on wave cycling, excluding tsunamis, calculates otherwise, but as stated earlier, the deck can be lifted in you don't mind losing the headroom in the bridgedeck berths by lofting for a thinner naca foil. A radical move would be to keep the berths as they are, end the cockpit floor at the bridgedeck and raise the entire cockpit to a happy distance.

Take a look at this design:
http://www.ikarus342000.com/P86cat.htm

The design parameters are similar in that they have similar DWL's and similar accommodations, and hull & bridgedeck cross-sections (windage). I think you'll find that the P86 does have quite a bit more boat in the water than your design.

Every single time that someone does that I kick myself. I kick myself because while designing, I tried not to bias my design and delay my progress by looking at other designs. That is a very nice and very similar design.

I ran some numbers on it...limited by the information given, and I have some disturbing news that isn't published but is easily calculated. I don't mean to bust the bubbles and I definitely will not discredit the design but as a comparison, the design, unless proven, will not reach published speeds and the windage that everyone is raving about with my design, is at least 25 to 27% for that design!!!

27% for sitting room? Holy mackerel...1 foot of windage for every 3.7 feet of LOA. That will translate to 8.1' on my design or another 1.85' air draft above my current height! With the air draft in the bunks of 4' added to the 1.85', I would be .15' short of another standing deck!!!

Ofcourse the draft had to suffer the penalty on the design...it would be a box if it wasn't! And, if you do the comparison I did as a percentage of the wetted area you will find that...well, never mind, the points made are enough. By the way...the design was done nicely, but those slab sides could have come out worse for modified V's.

Come on...you guys have to admit that the 20% is not very high and is in fact optimized for perfect air draft on my design.

The daggerboards and rudders are very efficient shapes for most speeds, particularly the envelope in which she'll likely spend her sailing life (5-15 knots). My only concern is when you are powering though a tight anchorage at 3 knots; will the rudder area be adequate?

Naca 15's in the rudders for structural reasons and increase in stall speeds 12's on the daggers for better pointing. Wow, I like your confidence... 15 knots will probably be tops while surfing, but I hope I'm wrong and look forward to it because all foils are engineered for 20 knots. Yes, the rudder area will be adequate. The rudders will rotate to a high degree and still produce lift...but you can always lock them on the centerline and use your motors to spin on your own axis with some practice and one eye on the wind direction.

I'll admit that my comments and opinions are not mathematically backed-up. But, as a builder and sailor, I can give 'real-world' advice and opinion. We built a 44' catamaran sometime ago with a surprisingly similar hull crossection from the shear down to the keel. She has nice flair forward, relatively full stern sections, plum bow, and a slim waterline. When I looked at her lines, my gut feeling was that she'll sit lower than designed and possibly squat, which all were the case when said and done. She sat 3 inches too low and her transoms were a good 4 inches under too. It was the designers first catamaran design and he is a VERY well known NA, with hundreds of boats of all shapes and sizes on the water and he knows his stuff.

Math for a design is a consequence of the mind. Old shipwrights, way back when, would get a 2d, plan, profile and section view of a vessel, drawn on some parchment and they knew exactly what needed to be done and how to size their scantlings. They couldn't read or add but their experience advanced the field. I respect your opinion and will listen closely and with extreme thought to anything you recommend. The bottom line is that a design no matter how mathematically correct, needs to be translated into the actual, representative working machine.

Thank's...

I'll take a look at the new set you sent! Thanks! Looking to further the discussion......

Frank

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[JCD]

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

[JCD]

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

[fhrussell]

Originally Posted by JCD

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

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

[fhrussell]

For $50 + shipping!!!

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

[sailsocal]

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.

[catsketcher]

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

[JCD]

Originally Posted by fhrussell

For $50 + shipping!!!

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

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.

[fhrussell]

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 ........

[JCD]

Originally Posted by sailsocal

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.

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

[rayaldridge]

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

Originally Posted by JCD

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

[JCD]

Originally Posted by catsketcher

Hello all,

Hello Cat...

Welcome aboard. I'm glad that you are here and looking things over as I am looking for shoulders to stand on.

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).

Thank you for your observation...I'm humbled by your recognition. Your experience is of concern to me and I hope that I will be allowed to pick your mind as I hope everyone else in here will allow me to do so. There's no sense in being dishonest or withholding...so I share.

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

The panels are not unsupported per say...support is incorporated into the chine connections and there is joinery...just not as most perceive it to be. I was going to go with standard cabinetry at once, then I designed the whole interior to be modular for removal, then...well, designed it so that it was light, held things together and could fold up or be taken out...it was an evolution sort of speak. The furniture is designed to "decrease carry on storable non necessities" because they are for 4 of this and that...everyone else, bring your own plate, glass and bed and then take it when you leave! I have noticed from my own observation that there is a law in "person physics" that if a cabinet there exists, the cabinet must be filled!

Slab sided? Are you looking at my design?

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.

Worth more than 2 cents...it's all true. I have all kind of data, small sample below, I hope it pastes okay...

Property Unit H 45 H 60 Balsa LD7
Nominal Density kg/m3 48 60 90
Shear Strength MPa 0.56 0.8 1.6
Shear Modulus MPa 18 22 96

Property Unit E – Glass Carbon Fibre
Young’s modulus GPa 72 230 / 40
Density of fibre kg/m3 2600 1710
Poisson’s Ratio- 0.2 0.25

Property Unit Polyester(isopthalpic) Epoxy Phenolics
Density Kg/m3 1210 1200 1150
Young’s Modulus GPa 3.6 3.0 3.0
Poisson’s Ratio- 0.36 0.37 -

...in order to solve as closely as possible, (I think I even made allowances for hair from rollers...I know, it's sick) but I still think that she will come in heavier than anticipated. I can calculate the immersion rate, but it won't do any good when it comes down to getting her on her lines. I have been on several types of cats in order to study them as you have surmised...I even saw one splashed without the waterline and boot...then the guy climbed in a tender with a foam and pencil and traced the waterline around her...the owner sailed it one season, put her on the hard and the guy came back and painted on the dwl, boot and undercoat. If that don't beat all.

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.

This is the one thing that I still have not done because I didn't think that there were actual experienced boatbuilders near me...but...it turns out there is, so I'm waiting for an invite...from somebody in here...someone who's name starts with Frank...

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.

Hot damn...been waiting to hear that. Had foils that were deeper and greater in area by 45% but I was told by the consultant that they were too big and with a reduction it would be sufficient. I still have them drafted if interested parties would be interested. Excellent observation...excellent!

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 agree. The design does not further any other design disadvantage. I have studied as much as I could, from as many as I could with complete humility and took off from their strongest points... not to improve on their flaws but to instead design and incorporate what I beleieved to be their greatest strengths into my design. Now, I stand here, in this forum, with just as much humility to stand on their shoulders and learn from them...and hopefully, be able to reciprocate the favor by teaching them something...or possibly assisting them in their endeavours.

I know that the volume was designed to give a feeling of openess and there will always be the skipper that is going to fill it and there will always be the skipper that is conscious of the design and not just the hole. But, we forget one more skipper...the dockside skipper that lives aboard and goes out into sheltered waters just to smell the ocean. He should be allowed to have the comforts of home without the restrictions imposed by designers and, one day if he wants, he can empty it and cross the sea and fill it again...just because it is his perogative. It may not seem like it, but I have tried to design with a wide spectrum of small compromises for me as the designer, but not for the skipper...which will be me too.

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.

The problem with large hulls is called parasitic drag. For some reason or another, it is overemphasized by and sacriligious to most designers that want to sail at the equivalent of Mach on the water...can't be done just yet...and there has been little progress in doing it in the last 5000 years...but the bottom line is that it is not "that" critical. You can run as fast as you want, but you cant run between the raindrops...did that make sense?

Don't know too many cats like this...unique and not inhibited by accountant designers...whoops, I hope I didn't kick the pig. But, it will be affordable to all.

Question...I did a cost out 7 months ago and came up with $9.95 per pound to build, without motors and about 20% as a bid-out or labor for 2 helpers. What is your current opinion on cost to build per pound and percentage of hired help or job bid? The build was for top materials...since I figured the labor saved can be carried into the design with quality materials for a high level of finish and, 17% of the cost went to medium quality, overengineered hardware above the waterline...rig, mainsheet etc.

Do it yourself, dawn to dusk, high level of finish with top of the line everything, worked out to about $6.19 per pound, sail away, without motors.

It includes everything but the foil shaped carbon mast, which I received an actual seperate quote on for $7,600.00, soup to nuts, delivered, so don't guess on this.

Thank's


cheers

Phil

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[JCD]

Originally Posted by rayaldridge

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

Yeah...what Ray said.

[JCD]

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