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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction > Modern Hull forms and Motion Comfort
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Thread: Modern Hull forms and Motion Comfort Reply to Thread
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Topic Review (Newest First)
10-31-2013 09:29 AM
skygazer
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?
04-20-2013 01:55 PM
bobperry
Re: Modern Hull forms and Motion Comfort

Out:
True. Steve is a class act and his shop is the cleanest and best organized I have ever seen. We are very fortunate to have PSC building this boat and the choice of yard was the client's.

In the photo of the three of us pondering the drawing ove the deck plug the fellow in the white shirt is Tim O'Connel. I met Tim through Sailing Anarchy. He is an extremely experienced offshore sailor and racer who combines sailing skills with an accountants' brain for detail. I get Tim involved in all my new projects. He makes me look smart. And he plays guitar well, has a good singing voice and puts up with me.
04-20-2013 01:45 PM
outbound
Re: Modern Hull forms and Motion Comfort

Had some limited dealings with Steve looking to do bigger PSC. Didn't work out but clearly a gentleman who builds great boats. That ketch is a work of art. Congrats.
04-20-2013 01:18 PM
bobperry
Re: Modern Hull forms and Motion Comfort

Res:
I'm not gone. I'll stick around.
If you go to Sailing Anarchy (I know it's a scary place) and then go to the Cruising Anarchy forum you will find a thread on CATARI. I also have posted stuff here but I don't know where it is or how to search for it here.

After a long day at the yard we went to Steve Brodie's fabulous house (owner of PSC), feasted on boiled local shrimp and then sat around the campfire and played music well into the night.
04-19-2013 09:00 AM
Resolute_ZS
Re: Modern Hull forms and Motion Comfort

Sorry to hear you're backing out of this thread, Bob, but I can understand. If you're still reading - do you have a link to your ketch? I'd love to see what you're building. (I've been lurking in the Sliver thread on SA too...)
04-18-2013 09:59 AM
bobperry
Re: Modern Hull forms and Motion Comfort

One:
WPA is not a two dimenional way to look at loading. It would be if WPA was the only component but you multiply the WPA by 64 and divide that by 12 to get lbs, per inch so it most definetely is not "two dimenional" as used.

I have no interest in getting into a pissing match here. My only interest is seeing discussions of yacht design elements be kept accurate. I have just returend form NC where my 60' ketch is being built and having read the posts that were posted while I was gone I'm not sure I want to stay on with this thread. People see what they want to see and this thread is a good example of that. I have no inclination to try to change that. I'm just a yacht designer, not a shrink.

I'll think about Wolf's project and correspond privately with Jeff.
04-16-2013 09:10 PM
outbound
Re: Modern Hull forms and Motion Comfort

thanks- I stand corrected. Would think given less weight even less load so even more durable. Knew the restriction on carbon was asking a more general question which was answered directly by stumble and also included in your post.
04-16-2013 08:45 PM
Stumble
Re: Modern Hull forms and Motion Comfort

Outbound,

From the best data I can find, carbon/epoxy actually has a flatlining S-N curve, like steel, not a liner one like aluminium. So as long as the design is done properly fatigue life can be controlled to near infinite levels. Even a more conservative assumption would place it at an order of magnitude better than aluminium in terms of fatigue properties.
04-16-2013 08:36 PM
PCP
Re: Modern Hull forms and Motion Comfort

Quote:
Originally Posted by outbound View Post
Given this generation of boats float on the water instead of in it but still need to cope with a perturrbed water/air interface one would think aspects of the structure would be subject to frequent loading/unloading cycles of a severe degree. Kind of like a Donzi skipping over chop versus a Kady Krogen. Given weight reduction and stiffness are at a premium to enhance speed with many of the the construction elements non plastic ( engineering wise not chained hydrocarbon deriviatives) and brittle one wonders about usable life. Surely they survive more hours sailed and more miles in a single ocean race than even a cruiser decades old but cruising boat's structure is rarely loaded anything close to it's point of designed failure. Furthermore, carbon cracks when oveloaded and cored structures with load on skins fail catatrophically. Metal and even GRP is more plastic in these settings. Wonder if this presents another design difficulty for th N.A.s of the pogo's and like vessels. Although this hull form has been popularized for ~a decade wonder if there is any feedback on how these boats fare down the road given the abuse a cruiser endures.
There is some confusion here: The Pogo is not a Carbon boat. The Class 40 racers are not also Carbon boats, Carbon is specifically forbidden by the rules. The idea of the Class is to provide a fast and inexpensive racer.

Regarding the fragility of the cruising Pogo, that use the same hi-tech building techniques used on the Class 40 and mini racers, there is nothing to fear.

They have been making race boats for the last 26 years and the boats are on the water, racing or cruising after many transats and some circumnavigations. In fact the Pogos are known to be incredibly though.

Look here and you will see how much:



Pogo is among the builders with more experience building light and strong sailboats. It is a passionate firm that started with doing mini Racers. His boss won in the 1999 mini transat race on the series class, on a Pogo, of course.

He is not young anymore but last year, just for the Fun of it, made with the owner of one of the first Pogo 50 (cruising boat) the ARC in the racing class and they were the third boat to arrive after a 40class racer and close to a Swan 80, being faster than a lot of much bigger boats

Of course they did not won anything (they were 5th on Handicap) because as I have said the handicap of the boat is horrible. The boat is not made to race (it has a swing keel) but to cruise fast and that they have done, crossing the Atlantic in 8 days and 20 hours. A Pogo 12.50 on the same ARC was also among the first to arrive crossing in 9 days 21 hours.

Regards

Paulo
04-16-2013 07:37 PM
outbound
Re: Modern Hull forms and Motion Comfort

Given this generation of boats float on the water instead of in it but still need to cope with a perturrbed water/air interface one would think aspects of the structure would be subject to frequent loading/unloading cycles of a severe degree. Kind of like a Donzi skipping over chop versus a Kady Krogen. Given weight reduction and stiffness are at a premium to enhance speed with many of the the construction elements non plastic ( engineering wise not chained hydrocarbon deriviatives) and brittle one wonders about usable life. Surely they survive more hours sailed and more miles in a single ocean race than even a cruiser decades old but cruising boat's structure is rarely loaded anything close to it's point of designed failure. Furthermore, carbon cracks when oveloaded and cored structures with load on skins fail catatrophically. Metal and even GRP is more plastic in these settings. Wonder if this presents another design difficulty for th N.A.s of the pogo's and like vessels. Although this hull form has been popularized for ~a decade wonder if there is any feedback on how these boats fare down the road given the abuse a cruiser endures.
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