Hobbyhorsing

[Dean Wilkerson]

     Any boat will hobbyhorse in some conditions, and you can't always change course to get out of it.  Factors such as length compared to wave period, rocker, weight distribution, and longitudinal distribution of  displacement with greater displacement aft than forward, and of course size matters.   The modern catamaran hulls have benefited from many years of experience, and trial and error.  The transom stern with the bottom of the transom right at or slightly above the waterline is said to offer significant benefit in pitch dampening due to the rapid increase in displacement as it is pushed down.... Or that's how I read it.  

      Wharrams have always been double enders.... supposedly for ease of build, though that is hardly significant compared to what is lost.   You only build the boat once, and hull construction is the easy part of boat building....  It looks dramatic, and feels like a huge accomplishment, but like building a house, when the walls are up and the roof on, it looks "almost done", but in reality is only around 10% done.

     I've included a drawing showing the bulkheads on a 28' flat bottom plywood catamaran to show that the difference is one panel, not significantly more complex, no compound curves or anything tricky....an easy build.   You can see how much faster the displacement just above WL increases at the stern than the bow when the boat pitches.  When the bow pitches up, the stern cannot pitch down very much, and of course the sharp bow will slice more than pitch

       With this in mind, I wonder about the addition of stern sponsons just above WL on the inboard sides of the hulls designed just for pitch dampening..... a foam and glass "afterthought".............

                                                        DW

[Reuben Filsell]

And it could possibly be used as a step for swimmers. Win win win.

[Allen Bosely]

Dean have you seen this thread? 

http://wharrambuilders.ning.com/forum/topics/adding-winglets-to-my-...

Cheers, Allen 

[boatsmith]

If you chop off the stern of a Wharram and install a transom you will have a bastard child of no value. The boats do pitch more than many other designs. It also won't get shoved into a broach by large following seas as easily as a transomed boat.  The double ended design also adds to the slippery nature of these boats which move quite well considering the tiny rigs. Like others have said, don't try and build an orange from apple plans

[boatsmith]

https://drive.google.com/open?id=1Nr49NPWi6LW2FkIwm-P-nQrQxzPcYoXv  

https://drive.google.com/open?id=1ikgKUNCytENwhfPX8hvf996DOo-KDBYr

https://drive.google.com/open?id=1EUN2Yg24xcE1V919CIOuYhnSpL2eyGym

[Dean Wilkerson]

I agree that chopping the stern for a transom wouldn't work well........... But that's because you will have a dragging transom.  For a mod like that to work, it would have to be recontoured so the transom was at or slightly above waterline... I disagree about the "slippery nature"........   I think that's absurd.   Being open deck, they tend to be lighter than many other cats.  A properly designed cat with a transom should allow the water flow to rejoin at the stern just as well as a double ender so long as the transom is not dragging.   The problem with cats in general is that they have little load capacity, and are often if not usually overloaded.   This results in a dragging transom, which would be a huge liability in terms of streamlining.   It's pretty difficult to compare Wharrams to other cats as a generalization.    The  deep V hull if anything is a liability in terms of performance.  It has long ago been abandoned by virtually every other designer of cruising multihulls...... for good reason.    I think you would find that a Richard Woods catamaran with open deck, comparable length, and loaded weight would outperform a Wharram with the same sail area.   He uses mostly round bottoms for just this reason.   He does retain some deep V designs just for simplicity of build.  How many people want to strip plank a round bottom hull with a chine??

Has the lateral fin shown in the photos been sailed yet?   What effect does it have? I can see where if the AOA is neutral with the boat level, as the stern pitches down or the bow up, it will get some positive angle and provide some lift.......  I'm wondering why the trailing edge isn't feathered like an airfoil?  Seems to me that an airfoil shape would probably be worth the effort  unless it's just an experiment.......but having an aviation background, I tend to think this way.  

                                                        Dean

[Reuben Filsell]

Immersed transoms are only a penalty at very low speed, if the boat is moving quick enough and the flow breaks away then there is no consequence. What that speed is for various hull forms I'm not so sure about. From my observations of a few local cats if there is enough breeze to press the hull in, there is enough speed to free the transom. If it was a serious liability why would the majority of designs use it ?

My thoughts on transom mods for a Wharram would not be to cut it off but to bring a more forward frame back which of course can only realistically happen on a new build.

Wharram has actually done this on the Tiki 25 "race" boat here.  http://pca.colegarner.com/SeaPeople-001.pdf

I agree with Dean, that foil hasn't had much thought put into it. But I do like the idea for a stock Wharram, I would have gone a bit bigger with a swept back foil section rather than a Delta, and deeper, on the bottom of the skeg. However on that boat the situation is complicated by the shaft and prop.

Anyway, I have already mentioned these thoughts on page one.

Merry Christmas people.

[Dean Wilkerson]

     Ditto the Merry  Christmas to all!!    I will be spending it alone as I have for many years.  Family  is 800 miles away, but I prefer a quiet Christmas.   I'll call my mother (87) and brother, and I'll light up  my Komodo grill and smoke cook a special treat.   Making up some mini pecan pie tarts with my own sort of puff pastry shells....... a creme cheese layer that settles in the middle.   Pie filling made with real maple syrup instead of corn syrup, and bits of crystal ginger in it.   Sound good??      Sounds really good to me, as I will be coming off the second of my annual winter fasts on Christmas Eve.   5 days of water fast, following a single eating day after a 3 day water fast....... It's  hard not to obsess about food under the circumstances ;-)  

I don't think most cats are designed to have a submerged transom...... rather they are overloaded.   I totally agree with framing  back during the build rather than chopping a finished hull off.  The big benefit of the transom stern is more space where it would do the most good.... a decent aft berth.......... where a berth should be....... widen the aft portion of the hull rather than sucking it together for a Vee.    Boats have been cut off and spiced back together many times, but that is an extreme solution.  Here is a Utube of someone adding 5 feet in the middle of a Woods Gypsy Catamaran.    I'm  not advocating anything like this, but it's a very impressive project.    As this is about hobbyhorsing, I think displacement pods, fins, and foils are far more viable solutions.   The chopped hull wouldn't do much if anything in and of itself.   It really calls for a rounded or flat bottom and more aft displacement to do the job I think.  

   

                                                              DW