Please Sanity Check My Catamaran Design

[rwatson]

Do those flatten at the stern though?

No, not at all. I don't get the "flattening of the stern discussion" at all. No sailing catamaran of any type leaves a flat transom in the water. Its just so obvious how badly flat immersed transoms drag, and no one EVER should choose that option.

 

[redreuben]

Do your homework Watson that’s just nonsense.
See Richard Woods just for starters, Kendrick etc.

 

[rwatson]

Do your homework Watson that’s just nonsense.
See Richard Woods just for starters, Kendrick etc.

You didn't read the details - no square transom is underwater on sailing cats !!!! Plenty of sugar scoop upper transoms - none in the water flow.

Obviously.

 

[upchurchmr]

Depends upon the size.
A daysailor which depends upon crew weight shift would work well, if sailed correctly.
A big roomeran wouldn't work so well.

 

[fallguy]

No, not at all. I don't get the "flattening of the stern discussion" at all. No sailing catamaran of any type leaves a flat transom in the water. Its just so obvious how badly flat immersed transoms drag, and no one EVER should choose that option.

Well, some of Richard's sailboats are hard chined; sadly I do not know them well enough to speak to his sterns; so I was truly asking how the vee hulled Wharram you showed exits if it enters as a vee. It surely isn't a double ender eh? [Post edit: the Wharrams are indeed double enders, thank you RW]. That boat you pictured appears to have pretty wide hulls, but it could just be the angle of the pic.

How does it exit then? Truly?

Where is the kettle stirring emoji?

 

[upchurchmr]

The Wharram catamarans I'm familiar with are all double enders.
There might be a later model with a modified stern, but I don't think so.

Search Wharram catamaran and look for "images".

 

[fallguy]

All the ones I saw were double enders.

 

[BigCat1950]

Double ended catamarans are noted for pitching and slow speeds. Also, a little separation between the LCF and the LCB reduces pitching. If we are talking about sailing boats of any kind big enough to have accommodations, they are pretty much all alike - pointy 'v' shaped bows, lower deadrise and greater beam amidships, and flattish sterns that are somewhat narrower than the point of maximum hull beam, that have very little transom, if any, under water. Recent racing sailboats may have very wide transoms, and very long, flat runs, but those are meant to heel a lot to pull a lot of that immersed transom and beam out of the water. Deep 'v' shaped hulls like Wharrams have a lot of wetted surface, which reduces speeds in lighter winds, and the sharp sterns have a lot of resistance when going fast. The only real benefit of Wharrams is ease of building.

 

[rwatson]

Double ended catamarans are noted for pitching and slow speeds. ......

Not at all. You are lumping double ended designs all in the same basket. You can't assume all double ended hull designs are built like Wharrams

Like James Wharam says on his website - "A double ended catamaran hull, is just a wider sterned cat hull with the natural lines extended."
So, double ended hulls actually have more buoyancy to prevent pitching, than "blunt ended" ones, in active sea states.


There is nothing slow about double ended displacement cats - IF you put big enough sails on them. Wharram cats are low tech, low aspect, low sail area.

The long aspect ratio, double ended displacement hull IS the fastest you can get, but not at all practical for docking and crew accommodation, hence their scarcity.

The proof is in the shaping of the most non-planing, energy efficient hulls on the market - rowing sculls and Surf Skies. Ever seen a flat transom on them ?

 

[CT249]

You didn't read the details - no square transom is underwater on sailing cats !!!! Plenty of sugar scoop upper transoms - none in the water flow.

Obviously.

Well buried lee transom

shows a well buried transom at 52 seconds, 2:19, 2:45;

Deeply buried lee transom

Deeply buried lee transom, with the deck barely above water level;

 

[CT249]

There is nothing slow about double ended displacement cats - IF you put big enough sails on them. Wharram cats are low tech, low aspect, low sail area.

The long aspect ratio, double ended displacement hull IS the fastest you can get, but not at all practical for docking and crew accommodation, hence their scarcity.

?

So why did Irens, VPLP, Crowther, Shuttleworth and so many other designers of outstanding racing boats not use these superior shapes? Funny, none of them seemed like fools........

Why did the Cunninghams, Rudy Choy and Hobie all abandon double-ended hulls? Are they all as stupid as VPLP, Crowther, Shuttleworth, Woods and Irens must be?

Or is it possible - just possible, mind you - that the world's top racing multi designers are actually extremely smart at their jobs, and not idiots as they must be to ignore the supposedly faster double ended hull?

 

[redreuben]

A double ended hull is lots of wetted surface with very little displacement.
Obviously

 

[upchurchmr]

Watson,

You are confusing apples and oranges.
Anything with a paddle or oar can't really meet the requirements of high speed sailing, nor is it intended to.

CT249, nice pictures, my favorite is the Tornado. It clearly show what I could see from the skippers position in heavy air.
A Hobie 16 is the closest thing to double ended I know of with significant sail area, and my very strong memory is a whole fleet of them pitching fwd and aft heavily in unison.

 

[CT249]

Another slow boat with a dragging transom - Decision 35

Look at this for a stupid sailor on a stupid boat with a silly cut-off square sectioned transom.

No wonder the design only won something like nine world A Class championships, and the guy silly enough to sail it has only won about 14 world championships and an America's Cup.

PS- Cheers Upch. The T is a lovely, lovely boat, and it's interesting to see how flat the sterns in the more modern boats are compared to the T. The Viper 16 renderings show it very well - rather ugly in some ways compared to the Tornado, but certainly effective. I wonder how Brett and Greg will take the news that their Viper stern shape is actually all wrong?

Here's an interesting shot from a Formula 18 CFD run, showing the transom tip about 15cm under water level.

F18 Open Project update Feb 2013. | Catamaran Racing, News & Design https://www.catsailingnews.com/2013/02/f18-open-project-update-feb-2013.html

 

[upchurchmr]

Chris,

What is the first picture? Do you have an overall picture?
I'm quite behind the times.

Marc