Double ender on a Crusing Cat

[fburton]

A warm Hello to all for the first time.

I’m doing my best to design a large cruising Cat and the passage of water around the stern intrigues me. I’m drawn to the idea of a very slender stern double ender I reason this allows a more gentle keelson upstream that lowers wave making and would even improve the viscose resistance as the displacement is more rounded. The transom is small, round and submerged Cp is 0.615.

Of course I realize this is not a contemporary feature and it worries me not to fall in with my betters.

I see popular designs favour keelson/diagonals of about 9 degs with a short overhang
Larson says diagonals should be 18-30 degs erring to the lower limit, he uses 9 degs on his YD-40 (not a Cat).
MM notes ‘transoms are designed to float well clear of the water at rest’.
Shuttleworth does advocate a fine stern and has just the vestige of an overhang.

I can see the positive in what I want but am I trashing the boat? is a double ender on a Cat a nonsense? am I sacrificing planing? does a pocket sized submerged paunch-like end agitate the wake?

Cheers
Fred

[Seafarer24]

The general idea is not to drag the sterns in the first place. Also- most cats have transom-mounted rudders (to promote kick-up rudders and their shoal-draft ability) which need a suitable mounting surface.

[marshmat]

I'd think you would be sacrificing planing to go pointy-stern here, but my guess would be you'd also have better low-speed efficiency. My temptation would be to fair the rudder right into the stern lines and have the hull taper off like the trailing edge of a foil. But this isn't too common these days, partly because it's hard to get it working smoothly, and partly because you sacrifice some interior volume and load capacity compared to a transom.

[fburton]

Thank you Seafarer24 I do welcome the dialog.

I would add that if I dont drag the stern then I dont optimise the length.

Yes if I sink the stern I will impact (pun intended) the rudder. I would tentatively suggest that a transom-mounted rudder is most common on the shorter cats.

Much of my talk tends to be theoretical and I do appreciate any practical observations.

[Skippy]

It probably depends a lot on how much you load the boat down. A racer will definately get dynamic lift and benefit from a square stern, but a party barge might not.

[fburton]

Marshmat

I like your temptation, I think your describing a cod's tail with a tab-rudder, a modern day historic cat would be spooky. No I need to keep the displacement well aft to keep the bow up.

Sacrificing planing is my worry but I dont ken the dynamic trim involved

[nero]

I must have misunderstood Mr. Shuttleworths articals on his web site. From what I understood, he says the problem with most modern cats is the stern is too wide. I finer stern with sides that angle in less than the bow's flair (as I understood it) is supposed to give a pleasant sea going ride.

In 2 or 3 years when I get this boat into the water, I'll have some real advice. smile

[fburton]

Nero

I concur that's how I understand Mr Shuttleworth's articals. I started the waterplane with a foil shape in reverse so the fine foil nose is the stern, 'with sides that angle in less than the bow's flair'. (The resistance is good and even better going in reverse).

However as I see it Mr Shuttleworth does not use a foil nose shape for the keelson profile that would give a double ender; even though he moves towards it far more than most.

This is the nub of my query. I can see the benefits, lower resistance, better seaworthiness, even helps with heave. But its not a common profile on a Cat.

Some have already remarked on the practicalities, its more complex to build and there are rudder issues.

Bonne Vitesse with your boat.

I apologise for missing you, I've just caught your earlier posts and realise you are proposing a double ender.

My hull sub surface is based on a NACA65A in reverse. The bow turns out to be nearly a straight line (30% of the length) delaying separation for as far as possible. The 65A then gives an elegant nose as the stern. I can see in simple speed situations that the wing will move quicker the right way round (boat going in reverse).

I'm tempted to use the same foil nose as the stern profile and diagonals but what are the effects? It would push the Cp up to your 0.65. At the moment I've shied away from the full profile keeping Cp 0.62 but I'm in limbo over it.

Just for the hell of it I've put your proportions (L/B = 13.85) on the foils 64, 65, 65A and 66. 64 puts displacement more aft and the stern is slightly swelled. 65 gives a convex bow. 66 would give a shorter bow. Resistance needs to be rechecked but I recall 65A wins.

PS. Your 9cm nacelle point gives a wing bridge clearance of 6.5% of LOA. I'm sure you know the MM52 uses the same ratio for ocean voyageing. 'A generous figure … at half payload…. compared to production multihulls'. I like this number too. Mr Shuttleworth is a windage guru but doesn't seem to sacrifice wing clearance.

Dread I can't upload the attachment, must be my popup blocker. Next time.

fred

[Seafarer24]

I'd recomend contacting Hans Geissler, the designer and producer of G-Cat and G-Force sailboats. He's designed and built 16'-36' catamarans with deep-v yet very efficient hulls. My G-Cat 5.7M is practically a double-ender, as the tiny transom very rarely sits in the water. Still as fast as a Hobie 18 though, with it's more typical hull shape.

[fburton]

Seafarer24

Thanks Seafarer24

www.g-catmultihulls.com

Just the info that interests me.
Published sections, thats very nice.

This is very literally a double ender; No keel and trailing V sections!
Is that allowed?

fred

[nero]

my cat is not to race. It is a live aboard ... someday. I want a smooth ride before I get into speeding around.

For the rudder it complicates the swing up a bit. Other than that, I have a bulkhead about 30 cm in front of the rudder post. I can triangulate off the hull bottom/hull sides and this bulkhead.