Wharram Builders and Friends

A Photo & Discussion Forum for Wharram Design Enthusiasts

     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

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Something like this? Its a "long tail" boat in Thailand, similar hull shape to a wharram but with stern sponsons. I think intended to help with squatting at speed, but should also prevent pitching. Though is the hobbyhorsing/ pitching a bad thing? 

See the source image

I'm sure I have seen some Wharrams fitted with sponsons aft, but a long time ago. I'm not sure if I will be able to find the photos, and I'm not sure how successful it was, otherwise I'm sure the idea would have caught on.

I'll see if I can find any photos.

I was envisioning a foam and glass pod streamlined into aft end of the hull beginning just at the waterline creating additional displacement.  It might begin just aft of the last bulkhead... or a bit further forward.  The idea being to increase displacement at the stern much more rapidly was the stern pitched down.  It would be inboard only.... to avoid a strange appearance. Perhaps sponson is not the proper word.  There was / is a photo somewhere on the net of a racing trimaran where the owner added sponsons to the forward end of the amas to prevent submarining.  The classic trimaran flip is on the diagonal when one ama buries itself in the back of a wave while at high speed...usually flying the opposite one. 

I HATE GOOGLE SEARCH............. It increasingly gives the results it thinks I should be looking for rather than what I'm actually looking for.  It places the junk on top, and what I want deeply embedded in irrelevant garbage.  2.5M search results in 1.3 seconds is useless when they are mostly irrelevant and it would take years to dig through to what you want.  I'd love to have a tool that would allow me to quickly and easily sift the garbage out using various criteria easily......... I know exactly what it would look like, and how it would operate, but I lack the skills or patience to build it.  If anything the problem is getting worse.... Unless you are looking for the same thing as 50 million mindless sheep.  

                                                       D.W.

Interestingly the above photo shows a Thai longtail as an example.....  I have toyed with this idea for catamaran propulsion with the ending mounted forward of the pod in the middle of the foredeck (assuming a pod).  done right, this would allow the tail to be raised up to the bottom of the deck, the prop would be aft of the cockpit.    The advantage is that a 2GM10 or some such could be used, and it would be housed under a cowliing easily accessible, and would allow the engine to perform functions other than just driving the boat.  For example driving a watermaker pump, a decently powerful alternator, a hooka, etc.....

I love not broaching surfing big waves with my canoe sterns. For racing and inshore sailing, I'd rather be on a Nacra, but for cruising out there alone, the canoe sterns are the shit. 

Have you people considered the stress those sponsons will transmit to the hull in a seaway? And what about slowing recovery after submersed? I wouldn't like that.

Dean, if you'd prefer transom stern you might look at Richard Woods designs ( http://www.sailingcatamarans.com/ ), he used to work with Wharram and then started his own designs, some of them are similar to Wharrams but with transom sterns. You might like the Windsong or the Surfsong for example.



Ricardo Aráoz said:

Have you people considered the stress those sponsons will transmit to the hull in a seaway? And what about slowing recovery after submersed? I wouldn't like that.

Dean, if you'd prefer transom stern you might look at Richard Woods designs ( http://www.sailingcatamarans.com/ ), he used to work with Wharram and then started his own designs, some of them are similar to Wharrams but with transom sterns. You might like the Windsong or the Surfsong for example.

Good point....... I  notice that all the modern transom stern cats are breaking up  ;-)

                                                     H.W.

Sorry, I must have messed up my meaning.

My objections were not directed against transom sterns but against transom sponsons as illustrated in the picture Levi Verwoest posted.

Dean Wilkerson said:



Ricardo Aráoz said:

Have you people considered the stress those sponsons will transmit to the hull in a seaway? And what about slowing recovery after submersed? I wouldn't like that.

Dean, if you'd prefer transom stern you might look at Richard Woods designs ( http://www.sailingcatamarans.com/ ), he used to work with Wharram and then started his own designs, some of them are similar to Wharrams but with transom sterns. You might like the Windsong or the Surfsong for example.

Good point....... I  notice that all the modern transom stern cats are breaking up  ;-)

                                                     H.W.


I misunderstood your reply obviously..... What is shown in the photo has no resemblance whatsoever to what I was suggesting, which was a sort of pod steamlined, and added onto the inboard side of the  stern above waterline in such a way that displacement would rapidly increase when the bow pitched upward, trying to pitch the stern down.  It would be likely be built of foam, glassed over, and glassed to the hull........ it wouldn't be especially pretty no matter how nicely you shaped and streamlined it, but the whole problem seems to be that the sterns are very fine.   It would be a retrofit for an existing boat.  

                                       DW


Ricardo Aráoz said:

Sorry, I must have messed up my meaning.

My objections were not directed against transom sterns but against transom sponsons as illustrated in the picture Levi Verwoest posted.

Dean Wilkerson said:



Ricardo Aráoz said:

Have you people considered the stress those sponsons will transmit to the hull in a seaway? And what about slowing recovery after submersed? I wouldn't like that.

Dean, if you'd prefer transom stern you might look at Richard Woods designs ( http://www.sailingcatamarans.com/ ), he used to work with Wharram and then started his own designs, some of them are similar to Wharrams but with transom sterns. You might like the Windsong or the Surfsong for example.

Good point....... I  notice that all the modern transom stern cats are breaking up  ;-)

                                                     H.W.

D.W. I take it your sponsons are to retrofit an existing boat rather than a new build, in which case I'd be inclined to put a swept back horizontal foil on the bottom of the rudder skeg set parallel to the waterline, such a foil would not only dampen sinking sterns but also plunging bows without substantially negating the positives of a canoe stern.

However on a new build I'd use a small transom mostly to facilitate boarding the boat from the water or dinghy. 

Ruben:

      The sponson added to the inboard side of the hull could incorporate boarding steps, functioning like a transom with steps.  It's main function being to add displacement above the WL, and secondary function to facilitate boarding.  It would be faired into the hull forward at least half way between the two bulkheads forward of the sternpost.  

     IMHO, the stern compartments offer very little of value, so if you were to chop the boat there during construction you would be gaining more than you would be losing......... except for one factor.   If you put a transom in.... say at the aft bulkhead, or slightly behind it, your transom would drag, and proper transoms should be above the WL at rest.    That's a standard bit of engineering on modern cats.   So adding a transom would involve putting  triangular bottom piece of ply in to transition from the deep  V to a flat or rounded bottom........ Not a big deal, but something to remember if you contemplate doing this.    To do this on an existing boat would be a challenge to say the least!!  One would  have to remove the last too bulkheads and the stern post in order to achieve a smooth shape transition for a more full stern, as well as adding freeboard, though that could be done as part of the coach roof.   All of these considerations apply to a new build.      I see zero real benefit to the canoe stern personally.... it's a styling statement IMHO and not much more than that.  

     Here is a photo of some rather elegant inward facing transom steps...... I like the idea.... Obviously is is not directly applicable.   The general idea of adding to the inboard side right at WL makes more sense to me in terms of the simplicity of being able to keep the canoe sterns.  This is by no means what I have in mind, but it was the inspiration...... Nothing says the steps have to be right at the transom.

DW

Hi Dean,

My understanding of the rationale behind the canoe sterns is that these designs are primarily for tradewinds sailing, i.e., downwind, as I understand it the idea is that the canoe stern sinks into the following wave rather than be lifted by it which could then stuff the bow in. 

It's my observation that as soon as modern cats increased transom volume to avoid pitching they then went rapidly to plumb bows and higher prismatics to stop the bows digging in, this now creates more volumous shapes and higher building complexity.

Back to Wharrams, if your going to add volume to the stern you might need to plumb the bow and now your back to building from scratch and it is really no longer a Wharram.

As for the canoe stern it's not entirely useless, it adds waterline length and lowers hull beam ratio which decreases wave making, I'm sure you know all this.

Boat designs are an assemblage of compromises so I guess it's up to each individual to work out which design fits the bill.

I crew on a Corsair trimaran, as a yacht its a beauty to sail, fast, nimble, exciting, but no load capacity, flops from side to side at rest, probably over canvassed, especially the asymmetric so not relaxing.

I'm not a rich man, just a humble handyman, I may, if I'm lucky, get a small used Wharram to potter up the coast in, every style of catamaran I have looked at, Wharram, Woods, Simpson etc gets a mental redesign, thinner ply to torture some extra volume maybe, Paulownia timbers, better foils. At the end of the day, without doing a boat design course, its better to just accept the compromises.

And hence why I suggested the horizontal foils. KISS.

Reuben

Ruben:

    I like the foil concept... I've toyed with the idea before.   The useful range of angles of attack for the foil you are using, if I read the chart right is about 8 deg either side of being in line with the fluid flow, after which the drag rapidly increases, and the foil will stall.... that's fairly significant in the real world, especially as water has a very high  Reynolds number compared to air, and a fairly small foil will create considerable force.  If the foil is aligned with the static water line, then when the boat is moving forward the aft mounted foil will produce a lifting force when the stern drops, and a downward force when the stern lifts, which is exactly what you want.  If it has enough authority, it will considerably dampen pitching, and if the foil projects outboard it should also dampen roll, which though roll is hardly an issue, would also be a positive contribution.   The airfoil should actually be a symmetrical foil, and if I were doing this, I would use the top profile on both top and bottom.   The further aft the foil is, the better, and an endplate / foil on the rudder would be pretty ideal, as the foil would have the max authority there, and at the same time increase rudder authority by blocking the flow around the bottom of the rudder.

       Hobby horsing seems to be a problem only in very specific conditions, but foils would seem to be beneficial all the time, and even lowering the outboard nacelles would probably have a useful dampening effect... far more than the drag they create.

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