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Old 18-05-2016, 12:08   #31
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Re: Fat or Thin Hull for Offshore

"The wider the beam, the greater the initial stability. Initial stability is resistance to initial heeling. A boat with higher initial stability will heel less initially for a given force applied abeam (like wind or a wave). This makes them faster (can carry more sail), ..."

I don't know that this is necessarily true.... yes it resists heeling a bit more, but requires more sail too with more wetted area. Nothing says a narrow boat cant sail when well heeled with similar sail area. Right?
If you looks at some of the modern racing designs that are beamy, many of them are very narrow boats at the waterline....
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Old 18-05-2016, 12:09   #32
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Re: Fat or Thin Hull for Offshore

Thank you very much for all the input and replies! My boat is very much the shape of Lantau's Pacific Seacraft with a big rake on the stem, a canoe style stern, and it also has a relatively heavy displacement and 3/4 keel. However, as pointed out it does indeed have much less accommodation compared to a modern production boat (mine is an old Robert Tucker design) although this is not a problem as I don't need much of it.
I will now have to sit down and seriously study some of those technical curves etc that you have kindly posted... I like my boat as she is very much, but it is all of great interest...
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Old 18-05-2016, 12:18   #33
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Re: Fat or Thin Hull for Offshore

On very wide boats slamming can be a real issue. Particularly upwind into a steep chop these boats can be brutally uncomfortable. The only way to mitigate this is to heel the boats over far enough to get the windward chine out of the water, which again can be very uncomfortable just because of the significant heel angle. It's a tricky balance, very much akin to flying a hull on a multihull. Keeping the windward side high enough to reduce slamming but still as flat as possible. The other option is to either slow down, or bear off a little bit (5-10 degrees). Which also reduce slamming, but at the cost of vmg.

Of course the trade off is that as soon as you turn them downwind the boats are far more comfortable, and livable, and depending on design much faster.

Don't get me wrong, I am not opposed to narrow boats, or don't see their value. My objection is simply to a screening formula that isn't predictive of the real world. I am still pretty sure that Beowulf may be the best world cruising boat ever built, and she is about as long and lean as they come.

I very broad generalities there are two ways to make a boat of a given volume faster:

The first is to make the boat very narrow and very long (the longer and narrower the better) at around 10:1 L/B wave making resistance stops being a major hurdle and only skin drag limits the speed of the boat. But there are practical restrictions here, an 80' x8' boat would be a little nuts (which is where catamarans come in to their own). A practical limit for monohulls seems to fall in the 4.7:1 range, but compared to the 3:1 that is more typical of monohulls it is still a huge advantage. Up until about 20 years ago this was the only option for designing a fast passage maker.

The second option to increase speed is to move to a wide planing hull like the Pogo 12.50. Here the goal is to get up on the water and start planing. This is only reasonable off the wind, but in the right conditions can far exceed the speeds possible for a long narrow hull.

With either option there are serious advantages and serious disadvantages that have to be born, but they can each provide a boat that far excedes the sailing capabilities of a traditional hull.

The OP's boat clearly falls into the first catagory. Long, lean, and not terribly weight sensitive. She should be faster on a beat than most boats her length, and pretty quick on a reach compared to anything but a modern planning hull. She should also be reasonably sea kindly at all points. But these are generalities, not absolutes, an under canvased narrow boat will still be slow.
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Old 18-05-2016, 13:34   #34
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Re: Fat or Thin Hull for Offshore

I am baffled and can't figure out why a wider beam would have any effect on Single-Handling or sailing in weather????

A wide beam shouldn't make any difference in Single-Handling or Heavy Weather...

Wouldn't the Length (LWL) of the boat have more effect on the ride in heavy seas more than anything else?

As far as Single-Handling... That is about how the boat's rigging is set up.

For instance on my boat (Catalina 47) I have roller furler main (in-Boom) and Jib. Power primary winches, a power halyard winch and a Bow Thruster... Also all lines are lead to the cockpit. It was set up that way so either my wife or I could Single-Hand while cruising

The only troubles I have Single-Handling is getting on the dock in winds over 15 MPH...

So with all of that said... I don't think fat or thin matters.
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Old 18-05-2016, 13:43   #35
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Re: Fat or Thin Hull for Offshore

And I have always thought that some of the beamier boats don't do well when heeled over because you are now trying to shove a big beam down into the water, which, in order to maintain the same buoyancy, will be pushed deeply creating a less streamlined shape (more rocker) trying to plow its furrow through the sea. Now if you have a flat yet narrow boat with a deep fin keel you'll really have something! (something that will beat the brains out of anyone who has to try to sleep forward of the keel when going to weather...)
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Old 18-05-2016, 16:23   #36
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Re: Fat or Thin Hull for Offshore

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Originally Posted by Don C L View Post
And I have always thought that some of the beamier boats don't do well when heeled over because you are now trying to shove a big beam down into the water, which, in order to maintain the same buoyancy, will be pushed deeply creating a less streamlined shape (more rocker) trying to plow its furrow through the sea. Now if you have a flat yet narrow boat with a deep fin keel you'll really have something! (something that will beat the brains out of anyone who has to try to sleep forward of the keel when going to weather...)
It really depends on the design. Far more than long/narrow boats wide transoms are a slave to the hull design. If it's done well then you can reduce wetted surface when compared to a traditional hull form. If the design is done poorly it the boat can be far more draggy than normal.

Below is a picture of a mini 6.50 heeled over very moderately. But notice that the keel is out of the water, as is a good portion of the leeward hull. Because of this the effective wetted surface is low, the entry angle is low, and the boat will actually go up wind pretty well.

Take a look at the Pogo 12.50 for instance, when heeled to go upwind the ideal is for only the curved chine to be wet, and the entire hull to stay dry. In effect the boat sails like a multihull with the windward hull lifted. A very fine (effective) entry angle, low wetted surface, and narrow beam. Ideally the wet portion of the boat reached a L/B ratio almost as fine as a racing multihull. Allowing very high S/L before wave form drag plays a part.

So why do these hulls have a reputation for slamming? Because they are going faster than a more traditional monohull can, and they have huge amounts of flat exposed hull for waves to slap into. It may be fast, but it isn't quiet or comfortable.
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Old 18-05-2016, 16:28   #37
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Re: Fat or Thin Hull for Offshore

By comparison here is a J-35 heeled to about the same angle. But note that she is in no danger of of showing her keel. The windward side doesn't lift, the boat just rolls.

It's also this 'lifting' that adds so much form stability. Getting that windward hull out of the water takes a huge amount of grunt that the J-35 has to make up for with a heavier keel.
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Old 18-05-2016, 20:00   #38
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Re: Fat or Thin Hull for Offshore

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By comparison here is a J-35 heeled to about the same angle. But note that she is in no danger of of showing her keel. The windward side doesn't lift, the boat just rolls.

It's also this 'lifting' that adds so much form stability. Getting that windward hull out of the water takes a huge amount of grunt that the J-35 has to make up for with a heavier keel.
Ok I am seeing that, but those are two pretty light displacement boats. Tell me about heavier displacement beamy compared to similar displacement narrow boats. I know it is kind of hard to draw a straight comparison. I am also curious about whether there are videos out there of various scale models in tank tests for demos of these phenomena. I'll start googling

edit check pp 69 to 72 in this link. Makes me appreciate my skinny long keel boat even more.
https://books.google.com/books?id=pe...apsize&f=false
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Old 18-05-2016, 21:17   #39
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Re: Fat or Thin Hull for Offshore

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Originally Posted by Don C L View Post
Ok I am seeing that, but those are two pretty light displacement boats. Tell me about heavier displacement beamy compared to similar displacement narrow boats. I know it is kind of hard to draw a straight comparison. I am also curious about whether there are videos out there of various scale models in tank tests for demos of these phenomena. I'll start googling

edit check pp 69 to 72 in this link. Makes me appreciate my skinny long keel boat even more.
https://books.google.com/books?id=pe...apsize&f=false
As always it depends. A narrow boat with a low center of gravity (G) and a lot of ballast (say an old maxi sled) is going to be difficult to capsize and easy to reright if it does capsize. A narrow boat with a lot of ballast and a high G may be easy to capsize and difficult to reright. The same way that a wide boat with a deep G will be easy to right while the same boat with a high G may be all but impossible.

Thirty years ago it was incredibly time consuming to generate a GZ curve so it was rarely done. And thus there was a need for simplified rules of thumb to make it possible to compare boats. These days it is trivial to have the computer kick out a GZ curve as part of the design process, and thus the need for these simplifications has been minimalized.


I am not aware of any really wide heavy displacement boats to use as an example. But these two factors alone simply aren't enough to predict how the boat would respond. How deep is the G, how is the ballast carried, what's the cabin look like, how much freeboard is there. Basically all the things you need to create a GZ curve. Then you would plot the curve, at which point some intelligent things can be discussed.
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Old 19-05-2016, 05:25   #40
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Re: Fat or Thin Hull for Offshore

one point against the "fatsos" (even though my preferences go that way):
loading them up for cruising (overloading them...) moves the cg aft which makes for less directional stability under windvane (ce stays the same as unladen): any wave-induced yaw is accentuated
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Old 19-05-2016, 05:45   #41
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Re: Fat or Thin Hull for Offshore

...& what we probably should not forget: time moves on, boats considered all-out-racers, totally unfit for cruising 50 years ago - are outdated now & considered lame ducks even for cruising (but of course there is always the "seakindly double-ender fanclub"..."...parts the following sea like the hand of moses the red sea..:")
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