Design Displacement vs Construction Displacement

Discussion in 'Sailboats' started by SmokeyBear, May 6, 2020.

  1. SmokeyBear
    Joined: May 2020
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    SmokeyBear Junior Member

    Hi all, I have a question that I am hoping some of the designers or anyone else may be able to answer for me.
    I am looking to build a sailboat between 33 and 36 feet long, I know all the arguments for buying and refitting etc. I just want to build one, I realistically have 20+ years before I can sail away so I can afford to spend a lot of time making it really nice. Anyhow, think I have found a suitable design in the Van De Stadt 34, I ordered the study plans and from what I've been able to find I think I'll like what I see. My real question is about the displacement numbers given for the design. For the three construction methods given (steel, aluminum, wood/epoxy) there are two different displacement figures with the aluminum and wood/epoxy naturally being lighter than steel. Being that I can't weld aluminum and I'm iffy on welding the steel myself I want to use the wood/epoxy. My question then is on load carrying capability, we all know that as a boat approaches it's displacement value it can suffer negatively in safety and sailing ability. In two boats of an identical design, one built in steel and the other built in wood/epoxy and therefore 1 ton lighter, does this mean that the wood/epoxy boat is then capable of carrying an extra ton of cruising goodies due to the design displacement, or would it mean that the heavier steel hull shouldn't carry as much to start with? I'm pretty sure that I'm right in thinking it can carry more and I know that using different materials and such to make weight savings typically means that you can carry a little extra, but I haven't really been able to find a definitive answer for something as drastic as using a different material for the hull entirely. Any help would be appreciated as will any thoughts on doing lots of cruising and hopefully a circumnavigation in such a boat.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Smokey.

    For reference here is a link to the Van de Stadt 34.
    Van de Stadt Design - Van de Stadt 34 http://www.stadtdesign.com/designs/stock_plans_sail/van_de_stadt_34

    Re your question about displacement, re steel vs wood / epoxy - I think that you will find that the wood / epoxy version has a finer hull form re her displacement being a tonne less than the steel version. She also has a 'round bilge' hull form, as opposed to the steel 'multi chine'.
    Both boats have been designed to float on their design waterlines, which basically means that you should not think that you can load up the wood / epoxy boat with an extra tonne of goodies to get her down to the same displacement as the steel boat.
     
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  3. DCockey
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    DCockey Senior Member

    A good design should be designed for use at it's design displacement and not "suffer negatively in safety and sailing ability."
    '
     
  4. gonzo
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    gonzo Senior Member

    I think that is a question for Van de Stadt. They are one of the most reputable naval architecture firms in the world, so it shouldn't be difficult to get an answer.
     
  5. SmokeyBear
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    SmokeyBear Junior Member

    I guess I should have said that as you approach a payload equal to the displacement weight it starts to negatively affect it.
     
  6. gonzo
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    gonzo Senior Member

    You would never have a payload equal to the displacement weight on a smaller sailboat.
     
  7. SmokeyBear
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    SmokeyBear Junior Member

    Thanks for the welcome! If its the case that the round bilge is a finer hull form then would it be worth it to improve my welding skills and build the heavier boat to take a greater payload?
     
  8. SmokeyBear
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    SmokeyBear Junior Member

    That's what I was thinking which is why I was wondering about the weight difference in the lighter boat. The other thing is that on their website they don't list a different displacement for the round bilge vs multichine. I don't know if there will be a difference listed in the study plans and I was wondering if theres some accepted rule about such things.

    Thanks for the suggestion of asking Van De Stadt, I sent them an email Monday and I'll probably ask them when they reply, I just have a mental thing about verifying anything "official" independently and didn't want to bother them with a question that seems like it should be simple and is frustrating that I don't know the answer.
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Smokey, you said "The other thing is that on their website they don't list a different displacement for the round bilge vs multichine."

    Yet in your original post above you mention that the multichine steel boat is approx 1 tonne heavier than the round bilge wood / epoxy boat.

    The 'payload' of the steel boat is probably about the same as that of the wood / epoxy boat. The design displacement of the steel boat has to be heavier, because the building materials are heavier.
     
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  10. SmokeyBear
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    SmokeyBear Junior Member

    Sorry Baja I was also looking at the specs for the FORNA37 they designed and it has a round hull version listed for steel and aluminum and the same displacement for both hull forms and I was thinking they had the same listed for the 34 for some reason.

    I guess I just assumed that the hull form would be roughly the same and hadn't considered the heavier steel means a heavier frame and so forth
     
  11. Alan Gilbert
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    Alan Gilbert Junior Member

    I wish the answer were straight forward. Many years ago as part of a paper I wrote on genereic yacht characteristics, one of the terms that often lacked clarity is displacement or design displacement. A smallish sailing yacht's displacement (weight) can vary as much as 30% from lightship (totally empty) to full load (sort of leaving the dock on an extended passage). So the question is, for which condition did the naval architect base their design on? Having messed around designing yachts for over 55 years, I have always used the 1/2 load condition, that is when 1/2 of the consumables were used up. I felt this was an reasonable average operating condition.

    Often designers will shoot (often too blind) for a displacement, develop the hull lines and then estimate the weight of the result. Consequently as for example, I have seen boats with their anchors (in hull side wells) contacting or almost contacting the water. I would hope that this wasn't the original intent, but rather a gross under guess at the actual flotation weight, and the list could go on and on.

    The boat in question has two displacements one for aluminum/wood and the other for steel varying just under 20%. The best source would be Van de Stadt.

    With respect to the strength of the various materials they should all be structurally designed to withstand the same loads. However, there are often physical and practical limitations to the choice of plating or shape. For example, for strength reasons a very thin thickness of steel might be required, but it might not be possible to weld such a thin piece without too much distortion or burn through. So a thicker piece might be specified, hence more weight. On the other hand this would make the plate stronger, thus the frame spacing could be increased which would save some weight in the framing, and so on and so on. Or as aluminum is about 33% as stiff as steel, the hull plating might be increased to improve impact resistance (you know when the mate isn't paying attention to the piling but rather the people on the dock). In short the structure is a system with the parts integrated with each other.

    With regard to adding weight if it turns out to be appropriate. Where the weight is added becomes important. For example, if you add weight to the keel which would make the boat stiffer (greater righting moment) this would put a greater load on the rig/mast which should be beefed up, thereby adding some weight aloft. Or if you added weight inside at the extremes, it could put the boat out of sailing trim? The trend in today's sail boats is to have very wide transoms, and narrow bows. As a consequence when the boat heels there is a tremendous amount of buoyancy gained aft and very little additional forward, which would suppress the bow. Often the entire crews moves as far aft as possible to maintain sailing trim. And, in an extreme sea state it minimizes the chances of burying the bow and worst scenario driving the boat under.

    In short the principal characteristics, strength, stability, performance, hull form, choice of material are an integrated system, and changing one impacts the others. There are tons more I can add to the above. It is not my intent to discourage you from undertaking this build, but I just wanted to sensitize you to the importance of making well informed decisions.

    And again Van De Stadt would be the best starting point for your questions. I hope this is of some help
     
  12. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Alan!
    That is a wonderful first (or second) post above - you have summed everything up very clearly and concisely.
     
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  13. SmokeyBear
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    SmokeyBear Junior Member

    Thanks for the info Alan, that is basically what I was tying to figure out. I did contact Van De Stadt and sent them a list of questions I had. They seem very friendly and professional and responded to me yesterday. From their email they said that "The load carrying capabilities were the same for all versions" which I take to mean that the wood core and epoxy version will carry just as much as the steel version but faster. That being said I am getting clarification on that before i just start loading it down.
     
  14. Alan Gilbert
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    Alan Gilbert Junior Member

    I am pleased to hear that my diatribe was helpful. In rereading it I do have one important correction. The 5th paragraph, at the end, discussing bow down trim should read, "And in extreme sea states it increases the chances of burying the bow...", and not minimizes. Sorry for any confusion, but it is my dyslexia working overtime.
     
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  15. Ad Hoc
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    Ad Hoc Naval Architect

    I am so pleased to read this.
    I too am afflicted in the same way - it often gets me into trouble as I don't realise I have miss-typed or miss-spelt a key word, until it is pointed out to me!!
     
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