Design Modification Questions and Thoughts

Discussion in 'Boat Design' started by CatBuilder, Dec 9, 2010.

  1. CatBuilder

    CatBuilder Previous Member

    I am looking at making a couple of modifications to the stock plans for the Kurt Hughes catamaran I'm building.

    I have completed half of the female mold and am now at the point where I have to decide if and also HOW to do these modifications.

    First Modification - Change to square, plumb bows from "bullet bows" or "drop bows."

    Kurt has designed better bows that most. He has drop bows or bullet bows. What this means is the most forward part of the deck is lower than it is amidships. There is a constant curve that takes the deck level (and bow height) down as you get toward the stem. I presume this has many design advantages: 1) Less windage, 2) Less weight at the end, so as to provide more kindly motion in heavy chop etc...

    Personally, however, I like the look of a strong bow line on a catamaran... a nice, plumb bow with decks at a uniform height equivalent at bow and amidships.

    Here is a picture showing the modification I'd like to do:

    [​IMG]

    My questions are: 1) What do you think about the modification itself? Is it worth a performance hit to have a boat look sexy? Many boats have these bows I'm talking about. Kurt's bows, while technically better, don't look as good, IMO. What do you think?

    Modification 2 - Forward Cockpit

    This one's a real doozy. I prefer a forward cockpit. My boat has an inside helm in the design already and I prefer to have my inside helm and my outside helm very close by so I can actually sail from either, especially in bad weather. I live at sea on my boats. I do not spend time on land, own no house and am underway frequently in all kinds of weather. I need protection from that weather, but I want to keep the lines of the boat looking good. I have thought about moving Kurt's deckhouse back 3 feet (a meter) and instead of a center window on the forward part of the deckhouse, putting in a door to a forward cockpit.

    Here is a picture of the modification (compare to the first picture, where the deckhouse is in the designed location).

    [​IMG]

    What do you think of this modification? Will I be messing the boat up by putting too much weight aft? What do you think of the effect on the boat's lines?

    Additional question:

    Ok, assuming I can't move the deckhouse back and make a forward cockpit, does anyone have any ideas on how to handle all the sails from the inside helm? Lines cannot come inside the boat, as they are moldy, wet and bad for the interior. Somehow, one would have to be half inside and half outside the boat. Any design ideas there?
     

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  2. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    It is very hot inside a wheelhouse...I like the way you boat was originally drawn...space for a small sun top, dodger to keep the top of your head from burning up. People like to sit in fresh air in the windshadow of a wheelhouse...not on foredecks. Also think of ergonomics... everyone trapped in a wheelhouse, claustrophobic for many people.
    , plus...your ease of movement from cockpit to deck 100 times per day. . The only advantage I see is that you get the mast out of the wheelhouse window. Mast partners thru wheelhouse windows must be a difficult detail to get right and watertight.
     
  3. CatBuilder

    CatBuilder Previous Member

    Very good points, Michael.

    I may mot move the deckhouse at all. If I did, I would probably have to make it smaller to preserve the cockpit.

    Speaking to the inside helm, the boat will have continuous air flow through hatches while underway and also air conditioning available. I would like to get the helm out of the cockpit in the aft because it takes up too much room back there. It is a charter boat, so I need outdoor dining and lounging areas in the aft cockpit area. That's why I was hoping to move the cockpit forward - like this. It already has the inside helm in the plans. The outdoor helm is a second one. I wanted to take it out from the aft cockpit and move it forward like this:

    [​IMG]
     
  4. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    I wonder why an inside steering station ? Ive never sailed a yacht with an Inside steering wheel. Modern autopitos drive the boat from the inside. The outside steering is for guests to enjoy steering , you docking and heavy weather when you must be outside to control the situation. Id "minimize" the inside steering station and transforme it into a desk, reading, internet emailing, relaxing area for sailing. Guests are always looking for place to sit..watch and read cruise guides, fiddle with video cameras, write post cards.......
     
  5. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    ...one thing I see immediately is the angle of the forward cabin and the mast...almost impossib;le to work at the mast with this sort of design. try a portugese type front in a drawing to see if you like it, that would get direct access to the mast, increase cabin room and provide glare free windows inside.
     
  6. michael pierzga
    Joined: Dec 2008
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    Location: spain

    michael pierzga Senior Member

    Not a bad idea...I hate window glare and those are probably plastic windows so not optically pure so difficult to see thru at night when salty.. Difficult to clean the inside of high angle windows and the wheelhouse overheats because its difficult to devise a simple curtain arrangment. Do cats sail with a boom vang ? Monos. need that window angle to clear the vang.
     
  7. CatBuilder

    CatBuilder Previous Member

    Land Lubber. I'm very interested in this Portuguese type front. I noticed the same thing about the angle of the forward cabin and the mast. What the other boats with forward cockpit do is steepen that angle up quite a bit to give more room for the forward cockpit:

    [​IMG]

    What is the "Portuguese type" front. It sounds good, but I've never seen one. Any pictures or a description?
     
  8. CatBuilder

    CatBuilder Previous Member

    All windows are Bentlgass real glass... no plastic forward... that's bad for an ocean crossing boat. :D
     
  9. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member

    Plastic is plenty tough but easily scratches. Glass is heavy, you cant bend it at home...but nice to look thru...also can be treated to keep the heat down. Make sure to design some nice pull down window curtains...
     
  10. yellowcat
    Joined: Feb 2007
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    Location: canada

    yellowcat Junior Member

    I am late, for what its worth. I was very inspired by KH CM system, i bought the vhs and the dvd .
    His construction phylosophy reaches alot mine.
    1 - raising the bow height is what we see in our racing beach cats. We even see reverse. We will likely see if not already the cheese knife bows soon. The idea is to pierce the waves. With equilibrium being in my view the most important factor for speed and safe handling (taking into account lightness, etc) , if you travel in areas showing steep chops , definitely i would add height, but it will add weight so you have to counter balance aft, the imerged volume will increase too.
    On a different note, if you add a kite in the front end of the cat, at 1000 oclock up, it will raise the bow, it lifted my cat 3 feet up until we reacted. You dont need much of a kite to add interesting power, a flysurfer speed 15 for example is fun at low-end winds.
    I agree that aesthetically, a plumb bow is better looking, this is partly what made the Nacras so cool looking. I have a friend who added a stern platform behind a sort of classic motorsailor, "catastrophe" a 8000$ mistake, but it did its purpose though.
    2 - i am and advocate of forward cockpits. But here are few notes:
    2.1 - Where mats usually fall in a storm ? and where things fall from a mast from 40-60 ft up ... F=GM.
    2.2 - If you don't want people to fall MOB it is a good idea to use a central access forward. Is this a gunboat ?
    2.3 - if you want to talk to you kite "mousse" you will be near him.
    2.4 - there is always that wave ... have a door closer !
    2.5 - i rarely see charter advertisers showing bad , cold, misty, ... weather, it seems more often we want to go where the wind is from. The call of sirenes perhaps.
    2.6 - Are you proaned to have soar necks looking up ? install a couple cameras aft looking up.
    There is more good and bad, but overall more possibilities. In my bigcat, i plan to have 2 masts just aft of the aft crossbar, and a front enclosed cockpit (the kiter will be right at my back) . A boat is a compromise, in architecture, we say if it looks good, it probably is.
    "with epoxy, when you heat it you can solve mistakes, but not if you heat your wife"
    By the way have you heard about the new epoxies with no toxic VOCs ?
     
  11. CatBuilder

    CatBuilder Previous Member

    Going back to the first question for a minute...

    I would like to "square" off the bows, I think. My wife likes it and well... I think she's right, despite the small difference in performance. I think the squared bows will give me better resale someday with a better looking boat.

    So... how do I do it??

    How do I take my hull lines plan for the sloping bows and make them into a straight line?

    When I was doing the plywood version of this boat it was easy: Just cut the plywood square instead of rounded at the bows. Now, I'm doing the female mold with fiberglass and foam core. The female mold is an entire half hull (half side of a standard hull lines plan), so I need to figure out how to make the change on the lines plan without disrupting the fairness anywhere else along the topsides.

    Here is an example of what the drop bows look like.

    [​IMG]

    The red line is the line I'd like to keep the bows and everything in between them at - the maximum height of the hull. The blue line is the hight of the current bows. See in the hull lines plot how they slope?

    I am cutting my female mold shapes from full size plots of the hull lines like this:

    [​IMG]

    SO... how do I change the ever-decreasing lines on the hull plot to make the bows (and everything in between them) stay at the level of the red line??:confused:

    I'm not sure how to do it. :( I'm hoping to get some advice because my build is stuck at the moment until I can figure this out. I have half the female mold built (the stern half) and can't move on until I figure out how to make the bows a straight line.
     

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  12. yellowcat
    Joined: Feb 2007
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    Location: canada

    yellowcat Junior Member

    Quickly, have you considered adding the front end as you want it and then attach bungies all the way back, tie and tighten them , this will create a pulled line. This is what you expect from a plumb bow pontoon. You will create the top part , it doesnt exist yet, you increase the height of it. try a 1 1/2" to a foot and end up with a flat or pinched top . With this , you will see what it will look like , stop at the water level perhaps. It is important you have a nice pulled hydrodynamic line. The junction with the existing kept lines will be hard to do or to accept, the lines of the existing will likely be different era looking. Or not. Once you like what you see , you can opt for various ways to do it. You can even buy fiberglass fibers and wrap the pontonns "amas" cut at junction once glued. Fill spots at stringers locations a hand planer and patience will be key for the first ones, the others will be mirrors or almost because it is a CM hull right ?. When you like what you see, you call your wife and ask her to get you epoxy , the one you prefer depending how fast you work, i like 205 WS , then , you add fiberglass matts how much pain ! Yes, pvc or i prefer Corecell by Gurit for the stringers and behind at impact places. They even sell epoxies. Add fiberglass and laminate when shape is done.
    You can consider the front end in wood or even a combination wood corecell for wood i would consider Meranti.
    For finish, ask about Alexseal system.
    What do you think ?
     
  13. CatBuilder

    CatBuilder Previous Member

    Quickly answering: No, this is not a CM boat anymore. I tried CM and it didn't work for me. I'm not a wood guy. I started over with foam/glass (Core Cell).

    I am building in a female mold with foam/glass. There is no wood involved in this particular build.

    I will try to envision/understand the bungee method you suggest tonight.


     
  14. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    Hi Sean,

    Yellowcat has it close to correct, although I would not use bungie cords. You need to establish the pattern at the bow that you want, and use battens which have some stiffness in them to fair the topside shape back to the tangent station. All the intermediate stations are going to look more like the biggest station in your bow view but with decreasing radius toward the centerline.

    Eric
     

  15. CatBuilder

    CatBuilder Previous Member

    Excellent! Thank you Eric (and Yellowcat).

    I am getting to work on this bow modification right now...
     
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