Stern designs and trade offs

[Cruisingdad]

THis is a carry over from another thread so as not to take over another thread.

The question was raised as to whether (and why) some boats have more of an issue in a following sea than others. I tern this, "The tail wagging the dog". In effect, it is when you are running with a following sea either at the quarter or directly astern and the stern is seriously "pushed" with the seas. THis is more prominent with the steeper seas than flat seas, but I imagine many people have been exposed to some aspect of it.

I am comparing two boats. One boat is the traditional, offshore type of boat like a Tayana 42. The other is the more modern type of boat like a Catalina 400. The Tayana, when viewed from the bottom and stern (ignoring the canoe stern for a moment) has a soft chime and is fairly rounded on the bottom. It is very narrow at the stern and has very little wetted area exposed to the following sea. THe other boat is the Catalina 400. Its bottom is fairly flat and has a very wide stern.

I can attest, personally, that the Tayana handles better in a following sea than the wider, flatter boat. Why? It is my position that the stern of the Catalina exposes more wetted/hull area than the stern of the Tayana. Being that it is also flatter, it is harder to displace the force of the water. THis is especially apparent when you consider any pitching.

I have tried to illustrate it here. It is a bad illustration. I would apprecaite others comments as design is not my strength - at all. I am viewing this from first hand knowledge of time aboard the two boat types and a basic common sense approach.

- CD

[sailaway21]

From the other thread:

From Principles of Yacht Design, Lars Larsson and Rolf Eliasson, third edition, 2007, pp 95:

"A final point to mention is the balance between the forward and aft halves of the hull. Many modern yachts have very full stern sections, while the forward sections are very sharp. This may be good for the surfing abilities of the hull, but it is not good for the course stability when rolling. When the hull heels over, the centre of buoyancy moves much more sidewards in the stern than in the bow. The force required to move the volume of water sidewards comes from the hull, which by law of action and reaction is affected by the same force from the water, but in the opposite direction. The stern is thus affected much more than the bow, and the hull changes its course in the heeling direction. This happens, of course, both to starboard and port, and the hull becomes difficult to keep on course."

[Idiens]

I read somewhere that the effect that Sway describes leads to the broad sterned boats broaching more dramatically than the narrow sterned boats.

I guess with breaking seas astern, the tendency to broach is increased and the sugar scoop bathing platforms may be more wave-welcoming than high narrow rear decks.

[Cruisingdad]

Regarding the canoe stern, I have already metioned that it should be excluded. However, the design of the hull under that is what I am talking about. It is very rounded or pointed. It sticks out abover the water.

Another example would be a Mason. Sorry, I have no pics of them rafted up with me since none of them want to!!!! Some people have no class!! At any rate, the design under stern and the hull exposed while pitched is minimal. It allows for a rolling (or even breaking) sea to split around the boat with less force than the other.

Here is a pic:

[Idiens]

Tristan Jones was very emphatic (as usual) that canoe sterns let over just as much, if not more, chasing waves as any other stern. He claimed the big benefit of a blunt transom was more buoyancy for the same length.

[painkiller]

I'm going to open a bar with a mini wave tank so gentlemen can discuss such matters over a pint while proving their ideas with miniature models. Who wants to invest?

I may add a mini wind tunnel for testing sail design and trim ideas.

Whaddya think?

[Jeff_H]

I think the original question on the other site dealt with tracking in waves. My current understanding on this issue is that tracking in waves is more an issue of the bow and less about the stern. A boat with a full bow and deep bow sections will tend to root, by which I mean that the bow digs in, built resistance and tries to slow the boat relative to the forces trying to puch the boat forward. Conceptually if the bow is full, it adds has little or nothing to directional stability allowing the ends to begin to veer off to one side. This is the reason that is generally cited for displacement powerboats broaching in following seas and why tradtional cruising boats generally do not do well going dead down wind in big seas without towing a drougue as a way of keeping the loads on the bow to a minimum.

If you take a full stern and couple it with a full bow, as Catalina tends to do, then you have the worst of all worlds, because the full stern is exerting even more force on the bow causing the boat to want to swap ends, (think of it as an arrow flying backwards).

But when you look at the better IMS/IRC style designs, the forward end is quite fine and sterns are not all that broad, and great care has been taken to avoid longitudinal trim changes. As a result they do quite well in a following sea, certainly better than the more traditional boats that I have owned.

As to the quote from Principles of Yacht Design, Lars Larsson and Rolf Eliasson, third edition"
"This may be good for the surfing abilities of the hull, but it is not good for the course stability when rolling. When the hull heels over, the centre of buoyancy moves much more sidewards in the stern than in the bow. The force required to move the volume of water sidewards comes from the hull, which by law of action and reaction is affected by the same force from the water, but in the opposite direction. The stern is thus affected much more than the bow, and the hull changes its course in the heeling direction."

I think this is a little bit dated. It is my understanding of the current design concept of fine bow design runs counter to the thinking that is being described above. My understanding of the current thinking is that it is not the fact that the center of buoyancy shifts outward on the hull that causes roll steering but the potential for assymetry of the underwater hull form. In older or poorer wide sterned boats, there was little regard for the heeled shape of the hull, and in many cases the heeled hullform produced a form that looked like a boat with its rudder hard over. That was also the case with many more traditional, narrower designs as well because of the wide proportion of waterline beam to waterline length. The better designs of today are carefully modeled so that their heeled hullform promotes tracking rather then turning, and careful roll and pitch dampening reduces excitation rolling and thereby roll steer.

Respectfully,
Jeff

[painkiller]

Jeff,

What are some good books for reading up on this subject? You mention that Larsson and Eliasson might be a bit dated. Can you recommend a more current book?

Thanks!

[ardoin]

Some Mason owners have class!
We will overlook that comment. The wetted area argument is correct for the initial swell in that the Mason doesn't lift it's stern as quickly as other boats. In a very large swell it was my understanding that the balance between the reserve buoyancy between the front and aft is a key factor to stability.
There were comments from Brewer on this in net posting some time ago. If I find the link I'll send it.

[Cruisingdad]

Originally Posted by ardoin

Some Mason owners have class!
We will overlook that comment.

Ahhh, I wondered why they sail upwind from me!!! Love the boat, btw.

- CD