Motion in a Seaway

[Vega]

Originally Posted by Tim B

I would suggest that full sterns actually make the boats rather uncomfortable in adverse conditions, and probably more prone to broaching.
Tim B.

About comfort, it would not depend only on the transom, but on a larger number of factors, not to speak of sailing positions.

About broaching, I disagree. The 40 class boats, like the one I have posted photos above, can be sailed at planning speeds (or lesser speeds) under autopilot, in all sailing positions and with winds on excess of 50k. What kind of boat with a traditional transom can do that ?
I believe that the large transom and fine entries is just what gives these kind of boats the directional stability to do the trick.

[Tim B]

Modern boats have a lot of rudder area. They also have very large driving forces from the sails. This gives high speed and a lot of control. The full stern certainly promotes planing (which is why they keep appearing), but if you consider comfort with no forward motion, I wouldn't like to be sat in a following sea.

It is worth noting that the boat's moment of inertia will play a massive role in seakeeping, but so will the sectional area curve. Perhaps someone could do a seakeeping analysis on two boats with the same MMI but different hull shapes. I'm too busy to do it at the moment, as I'm working on a yacht balance research project.

Cheers,

Tim B.

[Guillermo]

Interesting document to the purposes of this thread.
Cheers.

[charmc]

Originally Posted by Tad

Or is this the answer....

http://www.youtube.com/watch?v=z2TXkmX--wc


Tad

Or possibly this....
http://www.fireflate.no/Bourbon_Orca...anuar_2007.wmv

It looks like Ulstein has simultaneously reduced the wet ride and added buoyancy forward with the extraordinarily high sheer of the X-bow.

If the video's intro is taken at face value, the X-bow vessel is able to cruise comfortably at 20-30% higher speed than a similar size vessel with a flared bow design in moderately heavy seas. It looks to me that the X-bow ship would be fairly dry except in the very heaviest seas.

The Damen axebow design might give more speed, but the photos I've seen look like those vessels will bury the bow whenever seas are more than "moderately rough". The initial point about this bow design contributing to pitchpoling in high seas seems valid.

[Man Overboard]

Tim B
It is worth noting that the boat's moment of inertia will play a massive role in seakeeping, but so will the sectional area curve. Perhaps someone could do a seakeeping analysis on two boats with the same MMI but different hull shapes.

I would be very interested in this also

[popie]

Relatively small and fast vessels, such as patrol ships and fast crew suppliers, can have serious problems in heavy seas. The waves push the vessel up with such force that its nose clears the water before dropping back down onto the waves again. This can result in the crew becoming seasick or sustaining injuries and it can also damage the cargo and the structure of the vessel. Dr. LexKeuning of the Delft University of Technology experienced the dangers of these levels of vertical acceleration for himself in the early 1980s when he seriously injured his knee during trials with a high-speed vessel when the vessel fell back into the sea with a huge impact after a gigantic wave had lifted it out of the water. This incident caused him to dedicate a large part of his professional life to improving the sea-keeping behaviour of high-speed vessels. Keuning’s latest solution is the axe bow. A high, straight bow in the shape of the blade of an axe enables .....................more on http://www.tonistasworld.blogspot.com and label Silni movie

[safewalrus]

And again, the worlds Naval Architects argue about shape! Interesting to note that as I read the first post in this thread I thought - "Turbinia" and lo and behold in the next post jehardiman gives us a nice picture of ..........the "Turbinia" at speed in the Victorian era (end of 19th Century!!!)

OH well, as the saying goes .."thre's nothing new under the sun!"


If you don't want to move in a seaway, stay ashore!!!

Your all very clever boys (and girls?) but your reinventing the wheel!

the Walrus

[Willallison]

Walrus,
It is indeed rare that something truly new comes along. Though there's very little in common between the shape of Turbina and the Axe Bow...
It is the marketing department that would have us believe otherwise - whittness Volvo's IPS for instance...

[safewalrus]

Too true, Will!

Incidentally some clever little person who is apparantly an abject coward has just 'docked' me points for posting on this thread! Will this be the seconf forum I 'walk off' today! so come 'dock me a few more' but pray have the guts to say who you are!

[charmc]

Hey Mike,

Always good to hear from you. I enjoy hearing your opinions on boat/small ship design, since they're from experience at sea. Mine is a lot less than yours, but comes also from years of operating a number of boats of different types. Mostly they work as designed, but sometimes the design is modified because it doesn't work out as planned. I don't like that taking away points stuff, despite what that new guy "Frosty" says (could he be jack frost's great-grandchild?). Always glad to listen to the voice of experience. Besides, my mom told me I had to!!