Transom vs. V Shape for Catamaran Stern Hull

What is the difference between a cat stern with a transom:

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And a cat stern with a v-shaped, kayak-like stern:

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===qqqqqqqqqqqqqqqqq<-- Like this but smooth and symmetric
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Does one result in more or less drag?

If you're in a catamaran with one or the other, is there any difference between them if the hull cuts through a wave?

If the rudders are the same, does one turn different than the other?

Any other differences you can think of? Even things you think are irrelevant to my questions might help me out with my design.

 

How about this

http://www.multihullsmag.com/magazine/articles/cat%20hulls/performance_of_cat%20hulls.htm

Are you talking about the difference between transoms on displacement cats and power planing cats? displacement=canoe transom..........planing =flat transom

 

Right. At low speeds, the flow behind the square transom is turbulent, behind the canoe it's smoother. Going faster, the flow breaks cleanly from the square transom, but stays attached to the canoe, adding drag.

 

It will be a sailing catamaran, sorry for the lack of clarity there. It'll be for 6 people, so I doubt it will plane unless there's a tropical storm. It might be able to fly a hull in a moderate breeze if the crew moves to the lee hull, but I'm not far enough into design to tell. It's going to have to be able to be launched from the beach through the waves. I guess it will be similar to a Hobie 16, but a feet feet longer and wider. It will also be heavier, since it's going to be made out of wood and have 6 people on it.

That link about hull types looks informative, I'll look through it. Thanks!

 

Theoretically, the minimum wave drag shape has a sharp stern. But that's only for straight-ahead motion in flat water, and neglects the buildup in the effective shape due to the boundary layer.

The sharp stern was typical of early generation mulithulls. It has problems with pitch damping, leading to hobby-horsing, and is not a good load-carrier.

The modern practice is somewhat different than either of the options you've presented. Instead, the beam-depth ratio is greater at the stern than for the canoe-stern, much like the transom stern, but instead of having an immersed transom the lines are swept up out of the water so any transom lies above the design waterline. If you terminate the lines right at the waterline you end up with the transom just kissing the water. If you continue the lines aft some more, you get some overhang. Often the stern is finished in an open "sugar scoop" which saves weight and makes for a swim step.

The wider sections at the stern keep the boat from squatting too much when loaded. They also damp the pitching motion much better. Most boats tend to have their center of gravity aft of midships, so the wider sterns are the way to go. Finally, the flow on a sailing boat does not come from straight ahead, but from an angle. The wider, flatter sections at the stern allow this cross-flow to exit the hull with less separation than might be expected at a sharp stern.

One good way I've found to design such a stern is to create a generic cross section shape and smoothly vary the beam-depth ratio along the length of the boat. Then design the cross sectional area distribution independent of the section shape. This will determine the buoyancy, center of buoyancy, and largely determine the wave drag. Then scale each section so that it has the area given by the cross sectional area distribution for that station. Since the section shape and area distribution are smoothly varying, this results in a fair hull that has exactly the characteristics you specify. The rocker is determined automatically by the beam-depth ratio of the sections. Likewise for the center of flotation.

You'll also want to read the articles by John Shuttleworth on multihull seaworhiness and topsides shaping.

 

tspeer: Theoretically, the minimum wave drag shape has a sharp stern.

Tom, what are you holding constant in that comparison? If you create the transom by chopping off the end of a long canoe, that certainly sounds worse. But then you're comparing very different lwl's. What about boats of equal length? The canoe sides would be more curved to reach the centerline. Would that still generate less wave drag than separation from the straighter sides with a transom? For a fairer comparison, you could hold something else constant, like the area enclosed by the waterline, so the canoe is only somewhat longer.

tspeer: But that's only for straight-ahead motion in flat water, and neglects the buildup in the effective shape due to the boundary layer.

Speaking of the boundary layer, I have another question that's totally unrelated, but I've been wondering about for quite a while. I've read that for an object with a blunt trailing end, you want to encourage turbulence before the laminar flow creates too much drag. The example I saw in a fluid dynamics textbook was a bowling ball dropped vertically into a container of water. It had a rough horizontal band all the way around, somewhere between the leading point at the bottom and the diameter halfway up. The induced turbulence thickened the BL as it rounded the ball, greatly reducing its drag.
My question is, is that a good thing to do with a boat hull? Would it be good to polish most of the hull, but have one intentionally roughened area, possibly somewhere aft of max beam, to improve the flow past the stern?

 

It sounds as if it's closer to an F18 in dimensions than any other development cat class; they are 18' long and weigh a solid 180 kg IIRC and carry 150kg of crew (anyone lighter has ballast and/or smaller sails) so they are quite a bulky boat but fairly fast.

F18s have a transom. With much respect to Tom (who seems to be writing about larger cats), I'm sure that F16s and F 18, Tornadoes and IIRC A Class cats DO have their transoms immersed at rest with crew aboard. People like the Cunninghmans had Vee or canoe sterns for years but decided that transoms were better. Transom width can be a delicate issue on small boats IIRC, wide sterns are good at times but too much lift aft creates nosedives.

Perhaps the most important part of a small cat, say the designers I've spoken too, are the bows. They must be low drag even when the boat is nosediving; look at the F18 shapes.

Do a Google for Formula 18s, F18s, and builders and designs like the Hobie Tiger, AHPC Capricorn (very fast) and Nacra F18 and you'll find lots of shots of succesful stern shapes.

 

Yeah, it's more like an F18 than a Hobie 16, but it will have to displace about 400 kg of crew, and over 200 kg of its own weight. What should I change? The main thing I'm worrying about is being able to launch the boat from the beach through 2-4 foot waves. It will have to go through about 7 meters of waves like that until it gets out to the calmer part of the ocean, then it will sail along, about parrallel to the coast. I know it's tricky to get a kayak out through the waves on a windy day, so I'm mostly concerned about getting thorugh those waves. I figure there's no point to having a fast boat if it can't get on the water. Is there any type of cat similar to mine with the same priorities? Thanks for everyone's help.

 

It's not really relevant to a boat hull because the transition from laminar to turbulent flow will happen not far from the bow, so there's not that much laminar flow to trip in the first place. If you trip the flow earlier, all you're going to do is add a little more skin friction drag.

If you look at tank test models, you'll see a row of small studs sticking out of the model just behind the bow and the leading edge of the keel. These artificially trip the laminar boundary layer at a similar location to where it will transition on a full scale boat.

In the bowling ball example (and the infamous dimples on a golf ball), what you're dealing with is laminar separation. The flow around a sphere (or circular cylinder) reaches its maximum speed at the mid point. Aft of that point the flow is slowing down and the pressure is increasing. The increasing pressure will back up the slow-moving flow in the boundary layer, causing it to separate from the surface. A laminar boundary layer is easier to separate (requires less of an adverse pressure) than a tubulent boundary layer. In the bowling ball example, the boundary layer trip ensures the flow in the increasing pressure region is turbulent, so it penetrates deeper into the adverse pressure region before it separates. This results in a smaller wake and less drag than for the earlier laminar separation.

You can use the same trick anywhere you get laminar separation in a specific location. If you look at the underside of a modern sailplane wing, you'll often see a line of zig-zag tape placed just ahead of the concave cove near the trailing edge. These airfoils have decreasing pressure back to that area, promoting laminar flow over most of the lower surface, but then have a steep pressure increase, possibly followed by a decreasing pressure again to the trailing edge itself. The pressure distribution on this supercritical airfoil is somewhat similar because of its aft loading:

It can benefit from a boundary layer trip at about 60% of the chord. This can prevent the formation of a laminar separation bubble in the aft-cambered region that will increase the drag.

The point is, boundary layer trips are a cure for a very specific disease: premature laminar separation. If you don't have the disease, taking the cure won't be helpful.

So you need to know what the pressure distribution on the hull is doing, and where transition occurs. Here's the pressure distribution around an AC boat hull:

Red is high pressure, blue is low pressure. The pressure is decreasing from the forefoot to the keel, which you could deduce yourself by looking at the wave pattern. The pressure will be high at the crests and low at the troughs. After all, that's why the hull produces waves! So the pressure is generally decreasing from the forefoot to the keel region.

Water had a kinemetic viscosity approximately 1 centistoke (10^-6 m^2/s). At a boat speed of, say, 6 kt, this means the Reynolds number will be something like 3 million per meter. The boundary layer will be thinned by the favorable pressure distribution and this will delay transition past a typical value of critical Reynolds number of 500,000, so let's be generous and say the critical Reynolds number is a million. This still puts transition about a third of a meter behind the bow for this case.

So you have transition occuring way ahead of midships, in a region with a favorable pressure distribution. And no laminar separation. A boundary layer trip is definitely not called for. And putting roughness aft of max beam is only going to add to the turbulent skin friction there.

 

tspeer: It's not really relevant to a boat hull because the transition from laminar to turbulent flow will happen not far from the bow ... If you don't have the disease, taking the cure won't be helpful.

Wow. You know Tom, that idea would have rattled around in my head forever if you hadn't cleared it up for me. Thank you.

 

I think they are designed so that the transom is just clear of the water, as can be seen in the views below



Gareth
www.fourhulls.com

 

Hmmm, I may be wrong about that, but I'm still not sure.

I see that the plans of the Taipan 4.9 (F16) show the bottom of the transoms at water level, but I'm sure in practice they are partly immersed at rest, and I know they are immersed at sailing speeds to a beam of 10-12" or so.

The Melvin A in the bottom pic at

http://home.planet.nl/~dwars000/id59.htm

seems to have its transoms at water level without crew weight aboard. Similarly, the Capricorn F18s in the enclosed pic have the transom well immersed. Sure, they are lounging between races but I regularly see Caps at that attitude and the crew isn't TOO far aft.


The Flyers spend a lot of their time sitting around between races with the bow knuckle out of the water, unlike the drawing above. Obviously crew position has a huge impact, so maybe we are all worrying about a situation that never happens in real life?

 

CT249,

I think your observations are correct, you are not going to get the trim that you see in the pictures I posted unless the crew is sat on the front beam in the centre of the boat, as the static centre of buoyancy is around 2.5 m in front of the stern.

Those views are of the A cats statically loaded with 75kg crew, both hulls in the water. Although they are really designed to sail on one hull, in that situation the stern will be in the water.

Gareth
www.fourhulls.com

 

Ya, I'll have to have a decent look between races this Sunday. Unfortunately our cat club's fleet has dwindled this year so we probably won't have any world-class Flyer As there but Nacras, Capricorn, Auscat 5 A, Tornadoes and Taipans should give me an idea.

I've been trying to find that sketch of A Class hulls, I lost it a while back. Can you give me a url for it? Ta.

Having taken damage in the vulnerable part of the topsides from a Flyer's ram bow, I wouldn't be altogether happy about sharing a course with the Nils 2001 I must say.

 

Would adding 'rocker' (I don't know the correct term, is it sheer?) to the freeboard help with forward-back-rocking stability? What do you think the benefits and drawbacks of the attached (rough) hull design are? What should be changed?

I think the bow might get totally submerged, therefore displace more and float more, if it gets hit at the right (or wrong ) time by a wave. Also, when getting through the surf, the stern might be submerged is the boat is hitting the waves a certain way. Maybe I'm wrong and there's enough displacement so they won't get submerged much above the DWL. This boat will only have to cut through waves for around 10 meters, should I worry about surf-performance so much? Also, I haven't really been thinking too much about leaving the surf (from the ocean to the beach), which will most likely be done by paddle, and by riding the waves in.