SailNet Community - View Single Post - Modern Hull forms and Motion Comfort
View Single Post
  #194  
Old 10-31-2013
skygazer's Avatar
skygazer skygazer is offline
Senior Member
 
Join Date: Sep 2011
Location: western Maine
Posts: 382
Thanks: 2
Thanked 4 Times in 4 Posts
Rep Power: 4
skygazer is on a distinguished road
Re: Modern Hull forms and Motion Comfort

Quote:
Originally Posted by Sasha_V View Post
Sailhog, plumb bows are difficult to get right for the cruising boat, in part because they "work" or don't work in a tighter band then swept bows. This means if you overload or badly trim up a boat with plumb bows, it is going to be an uncomfortable and possibly dangerous ride.

At issue is usually downwind performance. Plumb bows tend to dig in and pig-root around in the wave, meaning that the helm has to perform exagerated and frantic corrections...that sometimes don't work and the wave runs away with the bows and the boat broaches.
even when it is not broaching, it makes for an uncomfortable "swirly" ride with an extra twist to the boat motion as the bows bite and get scewed sideways for a little bit of every wave.

There are good solutions to this available...but they are not bog standard for production boats that are basically floating caravans.
My favourite simple solution is to sweep the below-water-line hull upwards to the plumb bows so that the bow is sharp, and vertical...but not very deep. The boat can thus carve into and through waves when going up wind, and sit back a little and get its nose out of the waves entirely when going downwind. No more biting in and pig-rooting around....But it takes a lot more engineering and design know-how and it all goes for nothing on the day that some new owner decides to mount 120kg of 'something" shiny and nifty in the bows.

Sasha
I did not see anyone address or comment on this post, which interested me.

Reading through this site (my favorite) has made me more interested in modern designs. I'm more used to old wooden full keeled boats with deep hulls that are part "submarine", compared to GRP boats with tall topsides which (at first) looked to me like plastic fishing bobbers on the surface of the water. I still feel my heart skip a beat when I see a deep full keeled boat.

But I've been influenced by the many wonderful people here. Two years ago I bought a more modern -'86, so 27 years old, not that modern - Hunter 23 with a flattish shallow bottom, relatively sharp entry, shoal draft wing keel, fairly wide aft section and slightly reversed transom. I was surprised at the way it cuts through waves with less hobby horsing throwing the wind out of my sails, I like sailing it. I'm amazed at some of the speeds it has reached. I do hate looking at it, straight as a stick, ugly! But I can't see it when I'm in it. Sailing or at the mooring, I see the beautifully kept up wooden sailboats all around me. They have to look at mine.

Last year I had an experience that I still occasionally wonder about. I had just crossed a deep water shipping channel (two way traffic), sailing basically downwind against a strong tide. A fast deep draft boat, a kind of private ocean liner, approached and passed well behind me, at right angles to my direction.

This boat towers over all other boats in the area, it's big. It threw up an enormous wake, which I thought would speed me on.

But when the wake reached my boat, it lifted the stern and pushed it ahead and sideways. The wake seemed to keep coming and coming, and perhaps the period between waves caused some kind of harmonic with the motion of my boat. Each wave pushed my stern more and more over, requiring larger and more difficult rudder corrections. I could tell that I was in danger of losing control of my helm, in danger of broaching. That is a feeling I hate! I couldn't even look back, I kept asking my wife, "how many more waves?" On the brighter side, I believe I went quite fast against the tide!!

So this great expression caught my eye "bows tend to dig in and pig-root around in the wave, meaning that the helm has to perform exaggerated and frantic corrections..."

"Pig-root around", gotta love that description!

Now, my boat does not have a plumb bow, but it has quite a sharp entry. My anchor is light, with only 20 ft. of chain, the rest nylon rode. With no overhanging bow, overhanging fine stern, or wineglass profile, it is much closer to a modern shape than I'm used to. So I'm wondering if the bow was pig rooting and the stern was trying to pass it due to the greater flotation aft. Or was it just that an unusual artificially caused wave train hit a harmonic that my boat's motion resonated to.

Anyone have any insights or care to comment?

Last edited by skygazer; 10-31-2013 at 10:31 AM. Reason: clarity
Reply With Quote Share with Facebook