My F16-class pontoon design (comments/suggestions?)

Discussion in 'Multihulls' started by kerinin, Jun 20, 2009.

  1. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    I've been working on the design for a F16-class catamaran, and I was hoping to get some feedback from the (incredibly helpful) community here. I started out using the typical Blade-style wave piercing hull, but the more I thought about it the further I got from those designs.

    Last week I decided to start from 'first principles' and see where I ended up. I started by doing a series of airfoil drag simulations using Xfoil to get a rough idea how different mass distributions would affect the drag performance. From those simulations, it started to look like a profile with a 'fat' trailing edge (which is required to mount the rudder) needs to be roughly elliptical in shape, so I did a quick hull design which was essentially an elongated sphere. I'm fortunate enough to have access to Solidworks, so I ran comparison drag simulations on the elliptical design and the 'typical' blade-piercing hull I had been working on to start with. Fully submersed (Solidworks doesn't deal with free-surface wave effects) the ellliptical hull had around 20% less drag, so I decided that was a good starting point.

    From there I narrowed the bow to reduce leading edge drag and to reduce wave resistance. This turned out to have about 5% less drag than the original elliptical shape. About this time, I ran across Retired Geek's LR2 design which has a very similar design; roughly elliptical section, symmetric about the waterline, rounded gunwales, etc. The LR2 is in its second incarnation, and they recently discovered that the large rocker was creating suction under the bow when sailing downwind. They recently finished some hull modifications and should be testing it out this weekend. I haven't seen their hull modifications yet, so it will be interesting to compare their response to mine. In any case, what I did was to drop the bow 12cm and use asymmetrical ellipse sections for the top and bottom line of the bow. I've tried to maintain a circular hull section below the waterline to reduce the wetted surface.

    The final design move was to take shift the max beam aft 50cm. This moves the LCB aft (it's currently at 4%), effectively decreases the hulls curvature and makes the whole thing look much cooler (very important ;) ).

    I've attached a couple images. The image that's cut in half is showing the DWL fully loaded and flying one pontoon. The pretty colors are a curvature plot.

    I'm curious what people think about the design's performance (I'm trying to to design for aesthetics). One thing I'm very curious about is the pronounced reverse bow. My hope is that this will reduce wave drag, but I'm not sure what the impact of water washing up the bow will have on drag. Would it be better to use a straight bow up to the DWL?

    Thanks for any input!

    PS - I'm a big believer in the opensource philosophy; my opinion is that more information sharing helps everyone including the person doing the sharing. If anyone is interested I'd be happy to send the digital file - Solidworks, STL, whatever.
     

    Attached Files:

  2. TTS
    Joined: Jul 2007
    Posts: 112
    Likes: 3, Points: 0, Legacy Rep: 28
    Location: New Hampshire

    TTS Senior Member

    You should talk to either Retired Geek or John Lindahl, designer and builder respectively. The have done some pretty serious tests with both the LR2 and LR3 learning quite a bit along the way. I know that the LR3 is currently being modified due to too much drag going downwind. You can email me at tsiders@hopkintoncc.com if you would like John's email.
     
  3. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    Retired Geek was actually kind enough to give me a few pointers over email already (although that was on an earlier iteration of the design). I thought I'd let him enjoy his weekend - I hear they're sailing the revised LR3 today.

    I tried to contact John Lindahl a few weeks ago but haven't heard back - maybe he's busy. Probably for the best really - I originally was planning to just pick some plans and build from them and I've had a lot of fun (and learned an enormous amount) designing the hulls, even if the end product ends up being bad.
     
  4. bad dog
    Joined: Apr 2009
    Posts: 155
    Likes: 8, Points: 0, Legacy Rep: 93
    Location: Broken Bay, Australia

    bad dog bad dog

    I have a few thoughts, although I cannot model or drag simulate them, so they tend to be theoretical and anecdotal. Your model looks like a bloody ripper, so the aesthetics are spot on in my book!

    Your use of semi-circular sections below the DWL should give optimal performance in light air and smooth water. I'm not convinced that it is the best shape for all conditions - a flattened vee-biased shape should give better strong wind performance.... it's that question of balance, eh! Then there is the old chestnut of angle of heel affecting the shape as well - are you planning on canting the hulls?

    Other thought has to do with the raked bow. Wave drag and hull speed at displacement speeds would indicate a straight stem to DWL - but then you end up with something like The Tool A Class - fast but not pretty. [http://www.thetool.com.au/Welcome.html] The 70s Volvo of the catamaran world - "They're boxy but they're good!" (Sorry Wayne - eye of the beholder and all that stuff - I'll wave elegantly from my ancient but oh so elegant Mk IV as you rip past me). But The Tool does the ugly bow trick for a good reason. I wonder if curves could be used to the same affect?
     
  5. bad dog
    Joined: Apr 2009
    Posts: 155
    Likes: 8, Points: 0, Legacy Rep: 93
    Location: Broken Bay, Australia

    bad dog bad dog

    ps - I just had another look at the Tool website. I still think the angularity of the bows jars with the lovely subtlety of the curves in the rest of the hulls, but the sheer functionality is obvious. They really are purposeful. The photos don't really show them at work properly - in fact, in smooth water, they bury the straight stem only to about 2/3 of the way up the straight bit - thus maximising DWL.

    Question for the gurus on this thread: assuming perfect control of reserve bouyancy, would it be better to have all of the hull below water - torpedo style? Is wetted surface more of an issue here than wave resistance?
     
  6. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    Thanks for the comments bad dog. What causes the better strong wind performance of flattened-v profiles? I've seen some discussion of prismatic coefficient and how deeper hulls have reduced wave drag, but from what I can tell this is due to reduced hull curvature. I should mention that thee will be daggerboards in the final design if that makes any difference.

    Thanks for that link too - you're right that the hull is a little ungainly. So I'm guessing that the benefit to the vertical bow is that the bow wave is pushed directly to the side, rather than to the side and over the bow?
     
  7. TTS
    Joined: Jul 2007
    Posts: 112
    Likes: 3, Points: 0, Legacy Rep: 28
    Location: New Hampshire

    TTS Senior Member

    From the LR2/LR3 site:
    After sailing the LR3 at Lake Hartwell we found there were issues with the boat going downwind. It just wouldn't go wild as quickly as Flyers and A3's. It also had a tendency to pitch more than the other boats. Since the LR3 and the LR2 have a lot of rocker we suspected that to be the problem. And we got that verification when the hulls got a design evaluation at Crain Technologies. There was actually a suction being created that added a lot of drag when going downwind. So we got out a long batten and drew a new rocker line on Ian's LR2. The alterations were evident to those of you who saw the boat at Ft. Walton. Even with all we have done to his boat by adding bow, stern and hiking rails the boat is still 5-6 pounds under weight.
    The LR3 is in process now. We are taking an inch of rocker out at the transom and 3-1/2" at the bow. Should make the boat much more stable on those downwind runs and keep it's excellent upwind ability. We did fit our boards and rudders on Matt's Flyer for the NA's and he reports they work real well.
     
  8. Guest625101138

    Guest625101138 Previous Member

    Reverse bows are effective at picking up any flotsam that happens to be in your waters. It is annoying to have a plastic bag or weed lodge on the bow and stay there. It does not look pretty either. Easier to remove when caught on the rudder and cannot be seen.

    The reverse bow has been discussed before and others have commented on the hazard it poses to other boats because a heavy collision will likely result in a hole below the waterline.

    I would be interested in an .igs file of the hull if it is a simple file export function.

    Rick W
     
  9. CTMD
    Joined: Dec 2007
    Posts: 198
    Likes: 9, Points: 0, Legacy Rep: 117
    Location: Melbourne, Aus

    CTMD Naval Architect

    Kerinin,

    before I make too many comments I'll ask a couple of questions....

    1. Is the boat being designed for one crew, two or both?
    2. What is the displacement of one hull on your designed water line.
    3. Measured from the transom where are the LCB and LCF
     
  10. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    Rick:
    Flotsam - there's something I hadn't considered. I'm running some simulations on the model now but I'll upload and IGS when it's done.

    CTMD:
    1) The boat is being designed for 1 or 2 person crew, that's one of the reasons I was attracted to the F16 class in the first place.
    2) The displacement of a single hull is .248m3 (.254 tonnes) give or take - I'm basically designing for a 104kg boat weight and assuming 80kg for two people).
    3) The LCB is 2.77m from the bow (so 2.22m from the transom - it's a 5m hull) and the LCF is at 2.83m from the bow (2.17m from transom)
     
  11. CTMD
    Joined: Dec 2007
    Posts: 198
    Likes: 9, Points: 0, Legacy Rep: 117
    Location: Melbourne, Aus

    CTMD Naval Architect

    I'm assuming thats on the dwl. How much reserve bouyancy do you have?

    Quick Weight estimate.
    Item. Weight LCG LMMT
    Hulls 40kg 2.2 88
    Mast 16kg 2.5 40
    front beam 5kg 2.5 12.5
    back beam 5kg 0.5 2.5
    steering assembly 10kg 0.2 2
    prod, kite, etc 5kg 4 20
    sails 10kg 2 20
    boards 4kg 1.8 7.2
    misc (tramp etc) 9 1.8 16.2
    Total 104 2.0m 208

    your LCG 2.77

    required crew location 3.27m fwd of transom.

    The above numbers are very rough. but if you hash them out in more detail you'll be able to calculate the required LCG. For your initial calcs set your crew's front foot on the front beam (so crew's LCG is say 300mm behind beam and skippers is a further 600 behind that)
     
  12. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    Here's that IGS file. The shape has changed a tad since those images, but nothing major.
     

    Attached Files:

  13. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    Yes, that's on the DWL. I have roughly 100% reserve buoyancy - there's .243m3 above the DWL.

    ? I'm not quite sure about the LCG - I don't have great info on the mass distribution of the mast, sail, rigging, etc. I could figure out the rough LCG for the pontoon, but I suspect mast placement will make that number irrelevant.
     
  14. kerinin
    Joined: May 2009
    Posts: 31
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Austin

    kerinin Junior Member

    Disregard that last part - the post was clipping some of your content.

    Those numbers help a ton, and I don't particularly like the idea of being that far forward, so thanks for pointing that out! Looks like I need to shift some buoyancy around.
     

  15. CTMD
    Joined: Dec 2007
    Posts: 198
    Likes: 9, Points: 0, Legacy Rep: 117
    Location: Melbourne, Aus

    CTMD Naval Architect

    Kerinin,

    Now that you've got a preliminary hull you need to start a weight estimate. Set up a spreadsheet and list every item on the boat. Using the net you should be able to calculate or find the approx. weight of each item. For example the F16 rule has a minimum "mast tip" weight, double that and you have an approximate mast weight. The sails will be something like Apen 09. so if you sketch up a sail plan and then find what that weighs per meter you'll have a sail weight. The F18 (yes i know you're doing an F16) rule has max weights for the foils this weight would do until you've actually designed them.

    The idea is to work on what we called a design spiral. You've done a hull so now incorporate that into a general arrangement/sail plan. Using that info do a preliminary weight estimate and determine where your LCB need to be. Then go back and modify the hull shape. Then design the structure. Then update the weight estimate then modify the hull shape etc. etc. etc.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.