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Thread: Scaling up a design?

  1. #36
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    Default Re: Scaling up a design?

    Reading the responses from the NA's, I think you are getting some honest and very well intended advice. mmd, TR and Welsford are set up to produce good looking and good performing designs in a short period of time, and at reasonable cost. To walk you through the design process from understanding the basics to final design of all the bits and pieces is not something they are going to do. And being called an amateur is OK. It means that you are doing it for the love of it, not as a professional. If you are called a layman then the connotation is either neutral or bad.

    I know you are trying to understand the basics at this point, and responses to that end are valuable to you and others reading this thread. Similitude is a fascinating engineering subject. Download "How Wooden Ships are Built" and see what structure it takes to build 285' wooden ships at the upper end of wooden boat construction. And search for "Mallows Bay" to see where this last generation of wooden ships came to rest.

  2. #37

    Default Re: Scaling up a design?

    Quote Originally Posted by Skiff Man View Post
    Reading the responses from the NA's, I think you are getting some honest and very well intended advice. mmd, TR and Welsford are set up to produce good looking and good performing designs in a short period of time, and at reasonable cost. To walk you through the design process from understanding the basics to final design of all the bits and pieces is not something they are going to do. And being called an amateur is OK. It means that you are doing it for the love of it, not as a professional. If you are called a layman then the connotation is either neutral or bad.

    I know you are trying to understand the basics at this point, and responses to that end are valuable to you and others reading this thread. Similitude is a fascinating engineering subject. Download "How Wooden Ships are Built" and see what structure it takes to build 285' wooden ships at the upper end of wooden boat construction. And search for "Mallows Bay" to see where this last generation of wooden ships came to rest.
    Skiff Man, I know you are trying to mend a gap that may have been created by some folks comments and my response, but it wasn't that I took their responses personally. I was asking that constructive input be contributed instead of attempts to intimidate others or efforts to squelch the discussion.

    Speaking of the topic. Thanks for the response where you provided some formulas in your earlier posts. I haven't applied them in any way but I'll probably get back to them though and use them to see how the numbers work out.

    In that same response though you mentioned that my sail plan scaling is off. I used your number of 77% more sail area from one of your earlier posts to compute the SA/D. Was I wrong in my computation somewhere? I figured I'd done something wrong when the SA/D ratios were so close.

    As for revising the sail plan, I'm tempted to loft out the design with an overhang so that it creates a smaller transom and enables the backstay to be connected at a point farther from the mast where more room will be provided for including additional square footage in the main sail. It wouldn't take much effort to modify what I've already done. And moving the forestay out to where its connected where the stem and sheer come together. There's a design K. Aage Nielson in Worthy of the Sea that looks a lot like a Nordic Folkboat done just that way, but without the overhang I mentioned adding. It's a sloop called Decision built back in 1960 and the numbers work out close to the numbers I came up to for the scaled up Nordic Folkboat. Unfortunately, I've heard I can't get my hands on any of Nielson's designs. The family won't sell them to folks like myself who want to build my own boat. Nielson had actually requested that his work be destroyed after his death. Fortunately it wasn't. That's a good example of why some folks like myself would maybe resort to doing something like this, out of lack of availability, and lack of trust to work with designers who do not want to discuss the details for fear of losing their coveted "knowledge". And, I'm just having fun going through this process anyhow. Although, it might just put me in danger of becoming a little bit of a "rocket scientist", lol.

    The thread has kind of seemed to cool down after the comments by Jim and my response back to him. I was afraid that would happen. Hopefully, the wheels in folks heads will start turning again and they'll contribute some more comments.

  3. #38
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    Default Re: Scaling up a design?

    Quote Originally Posted by George Ferguson View Post
    Paul, I think your comments stating, "this Forum also has lots of professionals that would be happy to be paid for their services, be it Design, Building, Repair, Sailmaking, Rigging, Painting/Varnishing, etc." provides substance to my thoughts as to why certain individuals appear to feel threatened by technical conversations on this forum that "professionals" may feel infringes on knowledge that provides them with their livelihood.
    George,

    I'm not a designer and have no ambitions in that direction, but have had the chance to interview many of today's small boat designers (including John Welsford) and review their designs for our host, WoodenBoat. I can tell you that you are almost certainly wrong to assume the advice they are giving you is motivated by feeling threatened by amateurs wanting to learn. In my experience, designers who post on this Forum are some of the most generous contributors here, and they regularly share their knowledge and experience (and a good bit of their time) to encourage all builders, professional and amateur, without any expectation of payment. It bothers me a lot to see a relative newcomer jump in and start accusing some of the most experienced posters we have of trying to diminish anyone's efforts.

    I can also tell you that good designers feel a responsibility to see things done correctly and to keep builders safe from their own inexperience. Ignore their advice at your own peril. They are only trying to help you by giving you honest feedback basked on LOOOONG experience. They are not trying to intimidate anyone--they are trying to save you lots of time and trouble.

    Quote Originally Posted by George Ferguson View Post
    So if you don't have anything productive to say that would be helpful to others, I'd rather you just didn't say anything.
    Sometimes the most productive advice that can be offered is to show someone why what they are considering might not be such a great idea.

    And this comment:

    Quote Originally Posted by George Ferguson View Post
    Rather than respond in such a condescending way, why not try and provide some constructive input if you have any?
    is way off the mark, as is this one:

    Quote Originally Posted by George Ferguson View Post
    I know that its hard for some to embrace new ideas, or think they are above benefiting from the contributions of others thinking
    John is not being condescending, and he is nothing if not open to new ideas. But he has been around long enough to know which new ideas have a decent chance of success, and which might not.

    Tom
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  4. #39
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    Default Re: Scaling up a design?

    Quote Originally Posted by George Ferguson View Post
    In that same response though you mentioned that my sail plan scaling is off. I used your number of 77% more sail area from one of your earlier posts to compute the SA/D. Was I wrong in my computation somewhere? I figured I'd done something wrong when the SA/D ratios were so close.
    SA/D stays the same as you scale up. So you did it right, although no calculations were needed. Your error I was pointing out was in scaling up just the sail area to match the increase in stability. You also needed to scale up the moment arm (mast height). Thus scale up the entire rig not just sail area.

  5. #40
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    Default Re: Scaling up a design?

    If you like double enders, how about a Venus 28? Plenty of headroom in the house. There is a thread on here with the lines and offsets, if you build one, you owe Paul Johnson some dosh. It is set up for strip plank and i see no advantage in carvel myself.




  6. #41
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    Default Re: Scaling up a design?

    George it seems to me you should be evaluating lots of existing designs based on the specific parameters you believe you want in a boat. Some boats will measure up and other will not. After you narrow the field you can sort through them using any pros and cons.

    I'm certain there would be plenty of lively discussion.

  7. #42
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    Default Re: Scaling up a design?

    I know about as much as most people about scaling up a design beyond which things are linear and which are cubed. (Nothing.)

    Having established my level of expertise, I did manage to google up the Alberg 30, a boat of a similar size that was based on the Folkboat that seems to have worked out. http://www.alberg30.org/boat/A30vsFolkboat/ You might want to compare it to the scaled up Folkboat.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  8. #43

    Default Re: Scaling up a design?

    Quote Originally Posted by MN Dave View Post
    I know about as much as most people about scaling up a design beyond which things are linear and which are cubed. (Nothing.)

    Having established my level of expertise, I did manage to google up the Alberg 30, a boat of a similar size that was based on the Folkboat that seems to have worked out. http://www.alberg30.org/boat/A30vsFolkboat/ You might want to compare it to the scaled up Folkboat.
    Why does everyone seem to want to claim that every boat designed after the folkboat was influenced by it. As if no boats before or were long keeled, transom boats with a bow overhang? That direction bs many of the boats that came before and around the time of the folkboat. There are some valid folkboat derivatives (Contessa 26, Folkdancer etc) and some designed as competitors to the Folkboat (e.g. Stella) but so many of the boats people claim were inspired by the folkboat share none of the characteristics beyond those that were shared by all the boats of the period.

    But maybe I'm a grumpy old man and the only only one annoyed by it.

  9. #44
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    Default Re: Scaling up a design?

    Quote Originally Posted by Jackett View Post
    Why does everyone seem to want to claim that every boat designed after the folkboat was influenced by it. As if no boats before or were long keeled, transom boats with a bow overhang? That direction bs many of the boats that came before and around the time of the folkboat. There are some valid folkboat derivatives (Contessa 26, Folkdancer etc) and some designed as competitors to the Folkboat (e.g. Stella) but so many of the boats people claim were inspired by the folkboat share none of the characteristics beyond those that were shared by all the boats of the period.

    But maybe I'm a grumpy old man and the only only one annoyed by it.
    I wasn't aware that anyone would make such a foolish argument about Folkboats. They weren't built untill 1942. Folkboats are certainly an iteration of earlier designed boats. William Fife was designing similar boats well before Folkboats came along.

  10. #45
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    Default Re: Scaling up a design?

    "Why does everyone seem to want to claim that every boat designed after the folkboat was influenced by it."

    The highest form of flattery is imitation?
    Hope for the best, but plan for the worst.

  11. #46

    Default Re: Scaling up a design?

    Quote Originally Posted by MN Dave View Post
    I know about as much as most people about scaling up a design beyond which things are linear and which are cubed. (Nothing.)

    Having established my level of expertise, I did manage to google up the Alberg 30, a boat of a similar size that was based on the Folkboat that seems to have worked out. http://www.alberg30.org/boat/A30vsFolkboat/ You might want to compare it to the scaled up Folkboat.
    Thanks Dave! I checked it out and it sure looks similar looking at the info on SailboatData.com. It doesn't look scaled up as much as I'd been attempting to do though. I was also looking at the Contessa 32 yesterday and the stats on that boat are very close to the stats I come up with when I scale up the Folkboat by 1.33. The Alberg 30 reminds me of how the Folkboat would look if the transom was moved back onto an overhang. In order to get the Folkboat transom shape when I lofted the design, I extended the lines back to another station I added, made sure the shape was fair, and then went back and lofted out the Folkboat transom where it was suppose to be and everything came out great. It worked so well that I think I'll do that with any project in the future that I loft that has a transom.

    Something else I've noticed about the Contessa 32 and the Alberg 30 that you just recommended, is that the SA/D for both of them are under 16. That would imply they are both under powered according to one article I read. The reason I'd checked the stats on the Contessa 32 is because I'm listening to a book on Audible by John Kretschmer that is titled Sailing A Serious Ocean, and he really thinks a lot of the Contessa 32. By the way, that is a great book! The author has spent most of his life on the water in sailboats making deliveries and voyages all over the world. He gives a lot of good advice and analyses about different boats.

    Thanks again for bringing the Alberg 30 to my attention!

  12. #47

    Default Re: Scaling up a design?

    Quote Originally Posted by Skiff Man View Post
    SA/D stays the same as you scale up. So you did it right, although no calculations were needed. Your error I was pointing out was in scaling up just the sail area to match the increase in stability. You also needed to scale up the moment arm (mast height). Thus scale up the entire rig not just sail area.
    Skiff Man, I think I see what you are saying. I may go back and add in the rigging to the profile view I've created and post it on here for you to look at. Lately, I've been trying to read more about stability statistics and how to analyze a boat by compiling the different stats. I'll probably stick with the scaled up Nordic Folkboat when I'm attempting my analysis using those different stats. Maybe once I get it done I can post it all on here and get some really lively conversation coming out of the woodworks! All of this has inspired me to do something I should have been doing long ago. Learning how to analyze a boat in all these ways is very valuable, in my opinion. Folks who go out bluewater cruising, or just sailing in more lively conditions, should probably need to understand all this stuff. Just the things I've been learning about so far can really make a big difference on how a boat performs. Some folks may be ok sacrificing stability for speed, some maybe not. I also think it would also make a difference in placement of stores on a cruising boat.

  13. #48
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    Default Re: Scaling up a design?

    George, after you finish evaluating your desired boat via similitudes to other, similar, boats, you should turn your attention to calculating the specifics of your boat. You seem to be a smart guy and capable of assimilating the mathematical concepts, so this task should not be too onerous for you. Evaluate the structure by calculating scantlings as promulgated by Dave Gerr in his book "Elements of Boat Strength". Skene's book on yacht design will be a good guide to rigging & mast scantlings, and calculating required sail area. Once you have done that, I strongly urge you to do a detailed weights & centres calculation to determine displacement and centre of gravity of the boat. Set of a spreadsheet to do this, be careful not to generalize too much (detail is your friend). Then do stability calculations for the design. I cannot emphasize this enough - a new design (even one created from extensive similar-ship analysis) may have stability eccentricities that could get you in trouble in the right conditions. If you go offshore - especially with friends and family aboard - the safety of the ship and its complement is entirely in your hands, both as a captain of the ship and as the designer of the ship. In your research and reading of design topics, you may be interested in a paper published by ISO entitled "Small Craft - Stability and Buoyancy Assessment and Categorization, Part 2 - Sailing boats of hull length greater than or equal to 6m". You should be able to find this on-line and download it.

    Also, as you design your boat you will have to process and evaluate a great deal of information, which can get confusing and if not organized well, you may miss something which will bite you in the ass later in the project. if you are not familiar with the concept of the design spiral, google the term and become familiar with the concept. It will help you greatly.

    Finally, and probably not welcome, I urge you to at least have your finalized design drawings and calculations reviewed by a professional yacht designer or naval architect. Errors creep in all such work, and a trained and experienced second set of eyes is, on the whole, rather cheap insurance when compared to the cost of building in an error, or building a yacht that does not perform well or safely due to an engineering error.
    Hope for the best, but plan for the worst.

  14. #49
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    Default Re: Scaling up a design?

    Quote Originally Posted by Jackett View Post
    Why does everyone seem to want to claim that every boat designed after the folkboat was influenced by it.

    But maybe I'm a grumpy old man and the only only one annoyed by it.
    **Sigh** Nobody ever reads my links. Maybe it is because:
    The direct link to the A30 was the Canadian Folkboat Association's committee of NYC members going back to Hansen and challenging him to come up with a larger successor to the Folkboat; Hansen went to Alberg and our A30 was the response.
    As for being a grumpy old man, use it to your advantage. Folks was taught to respect their elders, now that we are grumpy old men, we can use that training to our advantage.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

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    Default Re: Scaling up a design?

    Quote Originally Posted by mmd View Post
    Finally, and probably not welcome, I urge you to at least have your finalized design drawings and calculations reviewed by a professional yacht designer or naval architect. Errors creep in all such work, and a trained and experienced second set of eyes is, on the whole, rather cheap insurance when compared to the cost of building in an error, or building a yacht that does not perform well or safely due to an engineering error.
    The best engineers I worked with were the ones who were most willing to have someone review their work and listen to their criticism.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  16. #51

    Default Re: Scaling up a design?

    Quote Originally Posted by mmd View Post
    George, after you finish evaluating your desired boat via similitudes to other, similar, boats, you should turn your attention to calculating the specifics of your boat. You seem to be a smart guy and capable of assimilating the mathematical concepts, so this task should not be too onerous for you. Evaluate the structure by calculating scantlings as promulgated by Dave Gerr in his book "Elements of Boat Strength". Skene's book on yacht design will be a good guide to rigging & mast scantlings, and calculating required sail area. Once you have done that, I strongly urge you to do a detailed weights & centres calculation to determine displacement and centre of gravity of the boat. Set of a spreadsheet to do this, be careful not to generalize too much (detail is your friend). Then do stability calculations for the design. I cannot emphasize this enough - a new design (even one created from extensive similar-ship analysis) may have stability eccentricities that could get you in trouble in the right conditions. If you go offshore - especially with friends and family aboard - the safety of the ship and its complement is entirely in your hands, both as a captain of the ship and as the designer of the ship. In your research and reading of design topics, you may be interested in a paper published by ISO entitled "Small Craft - Stability and Buoyancy Assessment and Categorization, Part 2 - Sailing boats of hull length greater than or equal to 6m". You should be able to find this on-line and download it.

    Also, as you design your boat you will have to process and evaluate a great deal of information, which can get confusing and if not organized well, you may miss something which will bite you in the ass later in the project. if you are not familiar with the concept of the design spiral, google the term and become familiar with the concept. It will help you greatly.

    Finally, and probably not welcome, I urge you to at least have your finalized design drawings and calculations reviewed by a professional yacht designer or naval architect. Errors creep in all such work, and a trained and experienced second set of eyes is, on the whole, rather cheap insurance when compared to the cost of building in an error, or building a yacht that does not perform well or safely due to an engineering error.
    Mmd, thanks for the info. If I follow through with building a scaled up design with modifications I will definitely have a professional review everything first. This whole exercise has turned into a real learning experience for me. I'd seen the Elements of Boat Strength on Amazon but I haven't bought it yet. I think you just help me make that decision.

    I'd also like to add that I greatly appreciate your input, and I've always felt that way, from you, other professionals, and any others on here who like to contribute to these discussions. This forum is a great way to share information, socialize with like minded people, improve, learn, and a whole bunch of other benefits too numerous to list.

    EDIT: I forgot to mention, I've also got my eye on some of the designs I seen in the recent WoodenBoat Magazine by Ed Burnett. The little pocket cruiser, Panacea, sure is a beauty that can make a man dream.

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    Default Re: Scaling up a design?

    George,

    For the life of me, I cannot see how how you felt I was being threatened by your original question, or was condescending to anybody or any idea. I have no pretensions of being a Naval Architect, I am following this thread with interest.

    You asked for some design help in the feasibility of scaling up an existing(successful) design. All of the responses you got back were that this was not a good idea, would lead to a bad(un-successful)boat, and various reasons and data were given as why the idea would not work.

    If your goal was to pursue a design larger in interior volume than the Folkboat and avoid paying for a new design, than you have gotten good advice...donít do it.

    If your goal was to understand coefficient this or ratio that, in order to better specify your dream boat, then you will have gotten a quick overview of some design criteria and will be able to communicate your vision to a Naval Architect. In other words you were being advised to get out your check book.

    That was really the gist of my original post...there are a lot of people who make their living in Naval Architecture who would be glad of this commission. And many of these very people have responded to your question with free advice, and that advice was universally that the original idea would not give you a good result.

    That is my most cheerful, productive, and helpful advice.

  18. #53

    Default Re: Scaling up a design?

    Quote Originally Posted by mmd View Post
    Evaluate the structure by calculating scantlings as promulgated by Dave Gerr in his book "Elements of Boat Strength".
    Mmd, I downloaded the Kindle version of this book last night and I've read chapters 7-8 along with the introduction and chapter 1. I'm very glad you recommended it and hope others read it as well. A very good book.

  19. #54
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    Default Re: Scaling up a design?

    George, Mr. Gerr has created a very good book in "Elements of Boat Strength" but, as with everything else in life, you have to recognize its strengths and weaknesses, too. This is your responsibility and no one else's. (This is where the pro's have an advantage - they have read dozens of such texts, and seen hundreds of boats, so have a broad base of knowledge upon which to rationalize what they see.) Dave's book draws heavily on Herreshoff's and Nevins' compiled scantlings formulae, and while these are no doubt unimpeachable sources, they are also geared toward lightweight (for their era) bay and coastal racer-cruisers, so by extrapolation they all lean toward lighter scantlings. Perfectly fine for the boats that they are directed at, but maybe not so much for more boisterous sailing. If I were to design a boat for, say, a trans-Atlantic voyage or sailing in northern waters, I would like to have a bit more meat on the bones. When you complete your scantlings calculations per Gerr's formulae, don't assume that it is the be-all and end-all of the process. Look back at some of the classic designs you have collected that came from rougher places than Long Island Sound or the English Channel - Colin Archer comes to mind, or boats specifically designed for offshore passages - and compare the scantlings of those boats to what you have in-hand. If there is a significant difference, first ask yourself "why is that?", then maybe consider upping your scantlings a bit if you plan on going offshore.

    If you like spreadsheets as I do, and have enough data to populate them, a good tool to have available is a spreadsheet of data on, say, frame scantlings vs. LOA and run a regression analysis with graphical representation of the relationship between the two. From this you can pick the spot on the graph where you want your design to live ("I want a 32-ft boat built 10% heavier than average...") and determine from your ra what the scantling should be for your boat. Sorry to say this again, but this is also where us pro's have an advantage - we have been collecting and evaluating this sort of data for years (or decades, in my case) so have a yuuuge database upon which to draw. You can never have too much information - the challenge is to file and collate it so that you can find it again. To quote Harry Benford, "Someone has said that a naval architect is no better than his file. (But) there is no point in owning a fact that you can't find."
    Hope for the best, but plan for the worst.

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    Default Re: Scaling up a design?

    Scaling factors
    In order to overcome the unwanted effects of linear scaling of all dimensions and thus arriving at a vessel with completely different characteristics compared to the original design, differentiated scaling factors are proposed in German literature ( Sharping, Barkla) :
    Length of vessel and Rudder:linear factor L
    Sail area: factor L^1,85
    Width, depth of hull body, freeboard: L^0,7
    Keel ( draught, area?) L ^0,7
    Authors claim that by application of differentiated scaling factors to the power as given above, sailing vessels of similar characteristics ( e.g. heavy displacement cruiser, light displacement yacht) will be created.
    Scaling should be limited to factors L<1,5
    Regards
    Detlef

  21. #56

    Default Re: Scaling up a design?

    Quote Originally Posted by mmd View Post
    In your research and reading of design topics, you may be interested in a paper published by ISO entitled "Small Craft - Stability and Buoyancy Assessment and Categorization, Part 2 - Sailing boats of hull length greater than or equal to 6m". You should be able to find this on-line and download it.
    Mmd, I haven't downloaded this paper yet that you'd recommended, and it may provide what I'm looking for at this point. Can you refer me to a book that provides a detailed explanation of how to compute metacentric height? I've read a little about this off and on lately but last night I worked on it for a couple of hours and I'm still having a hard time visualizing how a hull will react to different forces. I've got the center of gravity and center of buoyancy concepts fairly figured out. And I understand how a longer distance between the center of gravity and metacenter provides more stability, but I can't figure out how to compute the location of a metacenter. What's helped me so far is to visualize a boat that is literally shaped like an upside down floating triangle with squared off ends. By visualizing a section of that "hull" I can understand quickly how the center of gravity is 1/3 the distance from the "deck", and the center of buoyancy is 1/3 the distance from the waterline to bottom of "keel", and the center of both are at the center of the "hull" vertically in the longitudinal view with the distance from the bottom of the hull being the same as in the sectional view. The first one I get from basic physics. The second one I'm assuming is correct because I read somewhere that the center of buoyancy is at the center of mass of the volume of water that hull displaces. Assuming that statement is correct, and there is no ballast "keel" being used in the hull, and that the weight of the materials used to create all three sides is uniform, then the center of buoyancy would be where I supposed it is, 1/3 the distance below the waterline. Thinking about all that, if I'm thinking about it correctly, is interesting to understand how the location of center of gravity and center of buoyancy are important to keeping the "hull" upright. Then I start thinking about things like taking away the squared off ends and folding the sides together so that a uniform vertical "stem" is created on each end of the "hull", making the hull shaped like a canoe. Then, if the hull has a mast and it can be moved to different places along the center of the "hull", how will the forces acting on the mast from the sail cause the "hull" to heel? I know the answer come from understanding where the metacenter is, but I can't visualize that. I'm assuming the center of the force acting on the mast will be approximately 1/3 the distance from the bottom of the sail to the top of the sail but I don't know that for sure. I haven't read anything that documents that thinking. But the reason I am trying to figure that out is because, if that is correct, the height of the mast will have a big impact on the heeling affect of the wind forces reacting on the sail. I'd think that effect needs to be understood so that a person can understand how much force the hull can be subjected to from the wind before it heels over so far that it capsizes. And depending on where the center of the forces from the wind reacting on the sails is being transferred into the hull, relative to the hull's longitudinal center of gravity, it will cause the hull to move differently in direction and the way it heels, if the force of the rudder is disregarded.

    I've got to admit, for all my desire to build a boat, and dreams of going cruising, I'm not an experienced sailor. If my "visualizations" I spoke about seem odd, that is probably why. I am somewhat analytical though and really like to understand how things work in such a way that i can visualize what is going on as things change. In a boat, I think this is especially important since how a boat hull is loaded can have an impact on how it reacts just as how the wind on the sails and waves against the hull create different reactions in the boat. Also, if I can understand these things, it makes me more proficient at choosing a boat design that best fits my needs.

    EDIT: Reading back through my statement, I'm not sure that a ballast "keel" would make a difference anyhow other than to lower the center of gravity to some point less than 2/3rds the distance from the bottom of the keel to the deck, which would make the distance longer between the metacenter and center of gravity, making the hull stiffer. But that brings another question up which has to do with how stiff is too stiff. I've read that can be the case but I can't understand it since I haven't experienced it other than spending months on a construction barge offshore in the Gulf of Mexico watching the horizon through a doorway and seeing it disappear above and below the opening as the barge reacts to the waves. And very much disliking the movement of a crew boat under my feet as we sit and wait for a moment to safely approach the barge. The crew boat is probably the best scenario of a stiff boat since they are designed to carry loads on deck for transport out to the offshore sites. A lot of ballast would be needed to offset that. But a lot of time they only have personnel on board they are transporting and are underloaded relative to their design, which probably causes the undesirable motion. A construction barge doesn't usually have that much of an undesirable motion. Even though they heel a fair amount sometimes, they are stable enough that cranes can be successfully operated on them (another story).
    Last edited by George Ferguson; 04-18-2019 at 09:11 AM.

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    Default Re: Scaling up a design?

    George, I am about to leave the office for an errand so don't have time for a complete answer right now, but will get back to this. In the meantime, the text that I used in Nav Arch school on ship stability was D.R. Derrett's Ship Stability for Master and Mates, Stanford Maritime Limited, London, ISBN 0 540 01403 6

    Hope for the best, but plan for the worst.

  23. #58

    Default Re: Scaling up a design?

    Quote Originally Posted by mmd View Post
    George, I am about to leave the office for an errand so don't have time for a complete answer right now, but will get back to this. In the meantime, the text that I used in Nav Arch school on ship stability was D.R. Derrett's Ship Stability for Master and Mates, Stanford Maritime Limited, London, ISBN 0 540 01403 6


    Thanks! I found it online in a pdf version.

    https://www.azoresuperyachtservices....%20Derrett.pdf

  24. #59

    Default Re: Scaling up a design?

    Quote Originally Posted by MN Dave View Post
    **Sigh** Nobody ever reads my links. Maybe it is because:

    As for being a grumpy old man, use it to your advantage. Folks was taught to respect their elders, now that we are grumpy old men, we can use that training to our advantage.
    I read you link. Despite what it says, I don't believe it can be claimed that the A30 was based on a folkboat. Yes, it was commissioned by some folkboat owners, but to be based on a folkboat or even inspired by a folkboat, to my mind it needs to exhibit some of the characteristics of the folkboat that set it apart from the average yacht of the times. I can't see any in the A30. A nice boat, but to my mind a very different beast to the folkboat.

  25. #60
    Join Date
    Jun 2000
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    Pleasant Valley NS Canada
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    16,830

    Default Re: Scaling up a design?

    Jackett, are you limiting your comparison to only hull shape? If so, have a look at the design ratios, things like sa/displ., prismatic coefficient, etc. Sometimes remarkably different looking boats can be surprisingly similar in non-dimensional comparisons.
    Hope for the best, but plan for the worst.

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