Friendship modifications

Discussion in 'Boat Design' started by member 69256, Oct 9, 2021.

  1. member 69256
    Joined: Mar 2020
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    member 69256 Junior Member

    I am currently exploring the Phil Theils Friendship and Joli designs. I believe the hulls to be the same with exception of the motor well in Friendship. I have input rough dimensions from published materials the Freeship but receive no calcs as the block coef. is beyond the limits of the software.
    I wish to modify the hull by adding an extra station in the center of the boat bringing the length to 26' from 22.75' . I would prefer the friendship's covered pilot station and remote controls but wish to reduce curvature of the main roof to become a deck and the pilot house roof accommodate solar panels. I would like to know if adding length to affects power requirement or stability. Further if the existing roof structure can be flattened and used as a deck or suggestions as to modifications required to do so? My intentions are to use the boat as a weekend get away in the thousands of small glacial lakes of northern Ontario ( in fact there are 653 lakes within our city limits). Speed isn't a concern only that a 9.9 outboard on a motor bracket will be adequate to make headway.
     
  2. Tops
    Joined: Aug 2021
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    Tops Junior Member

    [​IMG]

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    Interesting boat. I don't see 20% more length making this horribly unstable or under-powered at 9.9hp. I am a non-professional with one box-shaped boat (PDR).
     
  3. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    A rounded roof provides an "additional" stiffness caused by its shape. Converting it to a flat deck means to weaken it. The rounded roof is not intended for people on it in normal use. If you prefer an even deck you probably intend to use ist as space for crew and or passengers. That would mean a significant rise of the center of gravity (CG) which makes the boat less stable, maybe unstable. The recognised rule of thumb for stretching a design says maximum is 5 to 10 per cent, more is considered a different boat.

    You shouldn't do it without consulting an expert/professional.
     
  4. Tops
    Joined: Aug 2021
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    Tops Junior Member

    Danke Guenter. I did not think about a large load on the new upper deck and what that would to to stability.
    I have been on houseboats where the upper deck was usable but there was far more mass and machinery below.
    This is an interesting topic, I hope it get explored.
     
  5. Tops
    Joined: Aug 2021
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    Tops Junior Member

    friend_stab1.jpg friend_stab2.jpg Can stability be seen looking at an IGES file (into a software) of the basic shape, before and after the 4' of extra length?
     

    Attached Files:

  6. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Stability cannot be seen, it must be calculated. With those files that you show us, plus the complete relationship of the weights and their centers of gravity, the stability of both boats could be studied.
     
  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The immersed volume you show is about 79 cu.ft. or about 4900 lbs. I used 22x8x0.75 max draft x 0.6 correction factor.
     
  8. TANSL
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    TANSL Senior Member

    "... 0.6 correction factor....", what is it that corrects that coefficient? Or is it what the designers call the block coefficient? In any case, a little low, don't you think?, I would speak of a block coefficient of 0.75
     
  9. Tops
    Joined: Aug 2021
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    Tops Junior Member

    According to Inventor...Gonzo was pretty close!

    upload_2021-10-14_17-52-8.png
     
  10. TANSL
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    TANSL Senior Member

    Not according AutoCAD if we talk about block coefficient.
    I cannot comment on the correction factor as I don't know what it is intended to correct.
    But the matter is not to give much thought.
     
  11. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    This type of hull has a ridiculous amount of initial stability. And for this boat's likely use, this is the kind that matters. The boat won't be going much over six knots, and it's not likely to encounter any waves big enough to push it past its initial stability limits.

    Lengthening the boat increases its displacement. If it's weight doesn't increase by the same proportion, it will float slightly higher in the water. The added length will increase the initial stability.

    The problem with making the roof flatter is that doing so increases the effective profile area above the WL. This adds more effective area for the wind to push against.

    Also, making that area a sun deck increases its structural requirements which usually means more weight. I don't see this as a stability issue as much as I see it as a towing over the road one. This longer boat will likely be 6,000 lbs, and of course it will need a trailer. Combined with the trailer, the total towing weight will likely be around 9,000 to 10,000 lbs. Can your towing vehicle handle that?

    The added profile windage could spell trouble for the little 9.9 hp mill. On a windy day, you could have real control issues. The poor little thing would be much like the grinch's poor dog towing the over loaded sleigh.

    How many stunners you allow on the roof at one time can really matter. Three or four would probably be OK.

    I suggest you make the roof with a two foot bevel on each side. This would leave your sinners a four foot wide strip to lay out on. It would discourage them from getting too close to one side of the boat, as the bevel would be about 45 degrees. Doing this would reduce the effective profile area too, which would help with the windage issue.
     

  12. Tops
    Joined: Aug 2021
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    Tops Junior Member

    Here is what happens when a goofball tries putting this into Delftship Free...
    friendship1_refaced.jpg friendship1_refaced_plan.jpg dhr1.jpg dhr2.jpg
     
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