The perils of edgy design offshore

Discussion in 'Sailboats' started by CutOnce, Jul 18, 2011.

  1. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

    Here are two pictures that seem to show that quequen has it about right.
    Nice work quequen!

    click on image:
     

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  2. daiquiri
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    daiquiri Engineering and Design

    Doug, can you explain what do you see in those pics? Boat's displacement? Volume of wings? Position of the CoG? Tiny details can give 10°-15° change in AVS. You cannot evaluate them just from those pics...

    However quequen has indeed done an excellent modelling work. :) He just needs to correct that displacement figure and verify the congruence between cross-sections of wings on his model and those given by the linesplan.
     
  3. Doug Lord
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    Doug Lord Flight Ready

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    I haven't seen a lines plan I don't think. Quequen's drawing appears to me to represent the wings and their intersection with the hull correctly-I think one can make that judgement, fairly closely, based on the pictures.
    All I've seen so far is this-and it is not accurate. There is a great deal more volume at the intersection of the wing and hull on the actual boat than is represented in this sketch. I think Quequen nailed it......

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  4. Paul B

    Paul B Previous Member

    Not really a big foiling moth type, is it? Actually more in common with your Sid than a foiling moth I think...
     
  5. Paul B

    Paul B Previous Member

    To be fair, I don't think OHR added the wings to this design for style. He added them to move the human ballast a greater distance from the CB while under sail, therefore increasing the lever.

    So the wings were part of the "science" (physics if you like) of the design, rather than the "art" of the design.

    We don't know if the designer ever did calculations regarding the overturning possibility in 6 foot waves and 40 knots of wind. We also don't know what his opinion was of the suggested use of the boat for a Cat 2 type race.
     
  6. Paul B

    Paul B Previous Member

    Nice to see someone make this sort of effort.

    The volume of your wings where they intersect the hull seem to be more than the model shown in the report. I would hope Jim Teeters worked from the actual plans.

    As soon as you noted the Cp was so low you should have wondered why. I would imagine the Cp for this boat should be in the ballpark of 0.55.

    Did you include the rig weight and deck hardware in your VCG calculations?

    Why do you think your displacement is so far off? I would assume a lb/in. immersion for this boat to be maybe in the 700 pound range. Maybe the LWL you chose was a bit off?

    Also, from sailing the boat I can tell you the wings hit the water before 35 degrees of heel.
     
  7. quequen
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    quequen Senior Member

    Guillermo, Daiquiri, Doug, thanks for your comments!:)
    Guillermo, block and prismatic are from the hydro report of Delftship, it takes submerged areas of the boat totally right up standing. But as this boat is almost rounded (like an "habano") from -30 to +30 deg of heel, I don't think it will vary between that angles. Should be really fast upwind, I think.
    Daiquiri, wings looks well to my eye, considering the lack of info (I agree with Doug on this). They start copying the profile drawing (dash line) and being tangent to the hull, you can check this at the picture showing a k35 without his wings. VCG comes from hydro report, considering a total disp. of 1.3 tons with a ballast of 0.499 tons. That's why VCG is so down. I agree that 1.3 seams strangely low for a 35' sailyacht, even on kevlar, so I may be wrong on this. I've taken data from the attached image. If disp is 1.8 then my results are totally wrong. I'd appreciate any new confirmed data (Total disp, BWL or Draft could be very useful). Anyway, pay attention to the image showing a guy over the second crosshead at the top of the mast, and three guys standing on their foots over deck. I think this boat has a good inicial GM, and being a rounded thin hull, it may have an acceptable hull/ballast relation.
     

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  8. Doug Lord
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    Doug Lord Flight Ready

    The Perils of Edgy Weather in Any Boat

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    Quequen, on the first page of this thread I came up with the same figures you did. I don't think they reflect the additional keel weight added later which was 300lb. or the 100lb.(45kg) taken out of her rig for a net gain of 200lb.(90kg). Therefore, her total displacement would have been(w/o crew) 3050lb(1386kg)-w/o crew) with 1400lb.(636kg) in the bulb.
    The report says* her "approx. displacement was 4000lb"(1818kg) and her "approximate bulb weight was 1400lb"(636kg) . They may have included crew weight in total displacement but if they did the average crew weight(8 crew) was 119lb(54kg) according to their "approximate" figure which seems low, but maybe within their definition of "approximate".
    Again, thanks for your effort --very well done!

    * from page 15 of the report attached below:
     

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  9. daiquiri
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    daiquiri Engineering and Design

  10. Paul B

    Paul B Previous Member

    There is something wrong. Have you included the Keel in the model used for this calculation?


    I think you are correct about the profile, but have not used enough radius in the underside of the wing. (see attached suggestion)


    If I recall correctly, the boats came out much heavier than the design. I know the one out here was heavy. That was normal for ULDBs back in those days. For example, the Olson 30s had a "brochure" DSPL of 3600 lbs. They were all something like 10% heavier than that. That was before the boats had any running rigging, sails, or other fitout weight. The 4000 lb estimate for the Kiwi is probably the "PHRF" weight, which would include running rigging, safety equipment, etc. Their PHRF measurement might even include sails (ours does not).
     

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  11. DCockey
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    DCockey Senior Member

    How does the your GZ curve compare to the one in the report, Figure B-2 on page 36?
     
  12. DCockey
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    DCockey Senior Member

    I assume Paul's suggested section shape is based on the figure in the report on page 4.
     
  13. quequen
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    quequen Senior Member

    DCockey, sorry, I didn't read the report, my mistake. At what page is the link?
    PaulB, to my eye, your profile is right slightly after and before the middle section of the wing, but not at the center. I think this is the point that really matters, so having a better profile could be important. I'll make a wing as you say to compare, but think it will destroy max GZ and RM.
    Spar and sails are included on CG, sails replace the rig (modelling rig is difficult). Spar weights 37kg, may be light for alloy, well for carbon. Deck and Hull are included (deck could be a little bit too heavy?). Boom is not included, nor batteries or water tanks. Crew is Not included.
    When calculating hydros, I must include keel and bulb to take accurate data on the CG. Maybe that induces error on the block and prismatic, Martijn could help here...
    Taking the values from Daikiri, I came to almost the same results, even a little bit more conservative. AVS, initial GM, VCG, etc. are a little bit better.
    Having the canoe Draft and/or de BWL, could change this results.
    Sail area from report is diferent than the drawing, from 44 to 54. I did't change this, since I don't have the real Sails drawing.
    Attached the files.
     

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  14. DCockey
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    DCockey Senior Member

    Doug Lord posted it a few days ago here.
     

  15. Doug Lord
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    Doug Lord Flight Ready

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    I posted it again tonight-post 413 just above here.......
     
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