Improving Flattie "Sea-worthiness"

[MakoShark]

It's something of an academic question (since I'm a land-locked sailor), but looking at the ease and simplicity of building flat-bottomed sailboats, I find myself wondering how one might go about designing one to increase her "sea-worthiness".

Flatties have a lot of great characteristics and - designed well - are good-looking boats as well. The only serious draw-back I find to them is that once capsized, they rarely have much self-righting capabilities. Now, there have been sea-going flat-bottom sailing craft and some of them quite famous. However, I've not seen to many discussions about what makes one more (or less) seaworthy. Pretty much everyone advises a full-keeled boat with lots of weight in her bottom for real blue-water sailing.

I've seen a few designs intended for real foul-weather durability, but they are rather like slapping a V8 engine in a VW Beetle so you can haul a trailer. Fixing the wrong problem, in a bad way, in other words. One design effectively provided righting moment by making the boat fairly narrow and attaching a 400lb iron keel and flange on her bottom. Defeated the purpose (and simplicity) of the little boat completely, had it been intended as a sailor. The author got around this by saying it was more intended as an motor-sailer, with the sailing rig only intended for the right sort of weather.

So... thoughts on this? What can one do to a flattie to make her less of a death trap for anyone crazy enough to venture (a little) further from shore? A well-thought out deck, excellent flotation tanks, and low windage are high on my list. I would try to plan her with an easily un-stepped mast that could be lashed flat to reduce her profile in the wind even further, possibly with a special "storm rig" to help her heave-to if at all possible, and some heavily reinforced for attaching a sea-anchor.

EDIT/NOTE: Seaworthy here having the meaning that her crew could reasonably expect to survive a particularly nasty storm, should they be caught in one. NOT meaning that she would wander into such storms intentionally.
EDIT 2: Also, interested how having a leeboard rather than centerboard would affect any answers.

[Zane Lewis]

What sort of size are we talking. Sailing or motor?
Boats evolved from flattie to dead rise to round.

Decking over helps as does a cabin, same as going deeper and adding ballast. Making it narrower will reduce sail carrying power but will also make it less stable when tipped passed 90°. Every thing is a compromise, you gain in one area and you lose something else.
Z

[Peerie Maa]

What sort of size are we talking. Sailing or motor?
Boats evolved from flattie to dead rise to round.

Decking over helps as does a cabin, same as going deeper and adding ballast. Making it narrower will reduce sail carrying power but will also make it less stable when tipped passed 90°. Every thing is a compromise, you gain in one area and you lose something else.
Z

^ this.

[Craic]

Waterballast.
https://www.semainedugolfe.com/pr%C3...se_bateau_25=&
https://www.semainedugolfe.com/previ...056_085_1_.jpg

[Breakaway]

I'll be the curmudgeon of the day. Sea-going boats developed for that specific purpose. And even the captains of those boats don't relish getting caught in a storm.

A boat for protected water is just that. Rather than redesign, I suggest staying within sight of land and and paying very close attention to weather. Forecasting is quite excellent in the 21 st century.

Kevin

[FF]

Make a sea anchor. And have a mast that you walk upright instead of having a simple round hole in deck or thwart. Lee boards give you room in the boat where you want to lie down and pray. Frank

[Peerie Maa]

Depoends on what sort of flat-bottomed hull. Main Scows and Thames barges are definitely sea going forms.

[MakoShark]

On the size; 18ft-36ft, for the sake of discussion.

Good point on the ballast. What kind, though? I'd assume you'd want fixed, to prevent damage and/or injury. Maybe water ballast tanks on the sides?

To the self-designated COD I'm of much the same opinion myself, but the thread was started out of curiosity; I see tons of references to old time designs and designers mentioning flat bottom ocean-goers, but little useful detail on what really made one.

[Autonomous]

May I suggest you are making more work and creating more complexity for yourself by trying to force a hull to go beyond its design envelope. Designing a boat is a careful balance of many elements. Clumsily overcompensating for one aspect can doom the entire project. Witness that failure you referred to in your first post. I'm pretty sure I know the boat; its profile looks salty on paper 'till you actually work out its dimensions vs. a real human.

If you can build a hull with sides and a bottom, you can build a boat with another chine or two that is actually designed for your intended use.
In the scheme of building a boat, the hull is only a fraction of the whole job. Another chine or two is only a small fraction of the hull. 1/3 x 1/10 = 1/30 more work. That is more than it actually is, IMO.
Certainly, less bother than guessed at and untested barnyard modifications.

[Yeadon]

I wouldn't mess with the shape but I could see adding in light bulkheads and hatches to create air chambers at each end of a flatty hull. Enough to displace enough water that you can easily climb back, start bailing, and get sailing again. Can't hurt. You could also use those chambers for gear storage when you're out and about on a daysail.

[stromborg]

"Autumn Leaves" is at the far end of what I would consider a "flattie" and it is intended for coastal cruising as opposed to out of the sight of land sailing.

[Peerie Maa]

"Autumn Leaves" is at the far end of what I would consider a "flattie" and it is intended for coastal cruising as opposed to out of the sight of land sailing.

I thought that was a Bolger Box. I know that it works, but a bit of flare would be nice.

[MakoShark]

"Autumn Leaves" is at the far end of what I would consider a "flattie" and it is intended for coastal cruising as opposed to out of the sight of land sailing.

How is she at far end of the definition?

[stromborg]

When somebody says "flattie" I think of this:

A simple single chined open boat. (This is my Steve Redmond Flapjack Skiff, 14' LOA)

Autumn Leaves is just a bit more complex.

[ahp]

I suggest that you considerCommodor Munroe's "Egret". He claimed that she handled well in rough conditions. She is a doubled ended sharpie, 28 feet LOA, flat bottom with a lot of flair. Plans are available from Parker Marine Enterprises, and reduced plans are shown in Rauel Parker's "The Sharpie Book".

[MakoShark]

@stromborg Ah, fair enough. It's certainly the sort of thing that springs to mind, though when I wrote the original post I was mostly thinking of just the flat-bottom and single-chine, though I suppose that is rather broad.

@ahp Exactly the sort of thing I was looking for, sir! I shall consider her thoroughly. Interesting that he transitioned to her after previously building more traditional hulls...

[stromborg]

You might look at Iain Oughtred's "Haiku" too

[Canoeyawl]

On the size; 18ft-36ft, for the sake of discussion.

The difference in "size" between an 18 ft boat and a 36 ft boat based on a 3 beam model with proportional draft is about 8 times.
There is no relevant comparison
If you want to go to sea get a larger vessel, they increase in "size" by the cube

[John hartmann]

That volumetric relationship always astonishes me when I see it in the real world. Here are a few images that show how dramatic the volume differences become with not so large increases/decreases in length and width.
on the left, a Precision 23, on the right a Vivier Jewell.
9F1594F7-33C1-4E59-BE15-C7BA41B425AB.jpg
and under way:
5EB19CF1-E6ED-4666-8C17-60A40B89F0E0.jpeg
0F863048-568D-44A7-8238-38179AEF875B.jpg
and the data for each, which look on paper like pretty similarly sized boats:
P23. Jewell
LOA. 23.4. 20
beam. 8.5. 7.5
displ. 2450. 1650
SA. 248 ft sq. 235
SA/displ. 35. 29. (Includes inside ballast)

The difference in the boats while towing/trailering, raising or striking rigs, and launching is equally dramatic—the P23 is an order of magnitude “more” boat. The numbers don’t convey that nearly so clearly to me, but side by side, or when below decks, the differences are much more obvious.

[MakoShark]

I'm not much interested in "MORE" boat, either in hypotheticals or reality. As the original post said, it's mostly academic; I simply want to learn what makes a seaworthy flattie. I'm quite familiar with the concepts that make up a seaworthy full-keeler, but there's much less info out there on flatties.

Found this thread while researching Egret:
https://www.boatdesign.net/threads/e...y-curve.18924/

Sound like Mr. Parker has an idea of what he's talking about. A few of his points (narrow bottom and duck stern) I had wondered about myself while looking at Egret's lines-plan. The duck stern is usually considered a bad thing on full-keelers; on a flat-bottom, it would seem to take advantage of the flattie's "bob-like-a-cork" nature. Any waves coming astern would be even more likely to lift that duck-tailed rear up and out, rather than inviting themselves aboard. That's the sort of design feature I'm interested in.

Going bigger does usually mean more seaworthy (all other things being equal), I agree up to a point, but it's much easier and cheaper to build a small, strong boat (all other things being equal) than it is to build a big, strong boat. The same math that gives you significantly more volume for a given increase in length also significantly increases the stresses on the boat, surface area to be planked, strength required from stays, etc. Anyway, all other things being equal assumes both are well-designed, well-found boats. You have to know how to design such a boat before you discuss size. I've seen plenty of reports (and seen some of the boats personally) of sailboats of fairly considerable size that were (in design alone) about as seaworthy as your basic cardboard box. I've also read accounts of very small sailboats that were either designed (or rebuilt) to be far more seaworthy than almost any production boat currently made, and went on to prove it in the worst conditions imaginable. And then there are accounts like that of Egret. Ran the mail in Florida. There were definitely more conventional boats around, but the mail went on her. Hm...

@John Hartman, in terms of elbow-room, the bigger boat always wins hands down. However, if that Jewell had a cabin and cockpit designed closer to the one on the P23 (smaller and better enclosed) I'd far rather be in the Jewell than in the P23 in foul weather, especially if I had built her from scratch.

EDIT: Thats probably unfair of me to the P23.... I'm heavily biased against the modern, overengineered production sailboat. Anything that needs space age materials to repair every portion of her worries me.

[Craic]

... I simply want to learn what makes a seaworthy flattie. ...

: Stability while the boat is upright, and INstability while the boat is upside down. The former is achieved by a low point of gravity, and the latter by a low point of gravity in combination with much buoyancy in the superstructure, preferably over both bow and stern. Think of a banana.

No need for a ballast keel, in-hull ballast and even waterballast can do the job as well. Think of the design of the early british rowed lifeboats. Or of all modern ocean rowing boats.

In-hull waterballast has two special benefits over fixed ballast: It can be taken in and pumped out "on demand" at sea, and the "free surface effect" of the water ballast (i.e. the sloshing about inside a part-filled ballast tank) can cause further instability of the upturned hull.

[Nicholas Scheuer]

Both 21' Dovekie and 28' Shearwater by Edey & Duff have flat bottoms with slightly rounded bilges and topside flare.

[MakoShark]

That makes sense. So, plenty of rocker and heavy curve in the shear would help a lot? And my small deck-house/cabin would hurt rightability, even if it helped reduce windage...

On the water-ballast... in this sort of boat, would double isolated tanks be better, or would something like a free-flowing pipe between the two help? I've heard of the free-surface effect in videos on sea-keeping in much larger vessels (ferries and liners) but not enough to have a good understanding of what the effect would be in a small boat.

I've been reading this, as well:
https://archive.org/details/sharpiebook0000park
It's quite fascinating!

[TerryLL]

Jay Benford designed a whole series of bluewater sailing dories.

Screen Shot 2022-11-30 at 9.34.18 AM.jpg

http://www.benford.us/dories/

[stromborg]

On the water-ballast... in this sort of boat, would double isolated tanks be better, or would something like a free-flowing pipe between the two help? I've heard of the free-surface effect in videos on sea-keeping in much larger vessels (ferries and liners) but not enough to have a good understanding of what the effect would be in a small boat.

Small Craft Advisor has recently run a series of articles on water ballast. They just went 100% digital so accessing their archives should be pretty easy.

[Peerie Maa]

On the water-ballast... in this sort of boat, would double isolated tanks be better, or would something like a free-flowing pipe between the two help? I've heard of the free-surface effect in videos on sea-keeping in much larger vessels (ferries and liners) but not enough to have a good understanding of what the effect would be in a small boat.

Free surface effect is independent of boat size. It destroys stability in all sizes and types of hull.
Simplistically, water ballast must fill the tanks completely, so that the water acts as a solid, or be kept in separate wing tanks without cross connection.

[MakoShark]

@stromborg, Oh, cool! I'll go check that out. Water ballast intrigues me, as a trailer-sailor. I definitely love the idea of free ballast and the benefit to gas mileage even more! I'm now interested in how dumping/loading might affect the handling of small craft, too.

@Peerie Maa, I think I must've misunderstood Craic's post, then; I was under the impression he meant it could be a useful thing, rather than a problem, which is what I'd previously understood it to be. The only context I had for it was in documentaries of sinkings by the Youtube channel Casual Navigation; he mentions it with a brief explanation of why it aggravated (and in one case, caused) sinkings and capsizes of large commercial vessels.

[Craic]

Again, you want great stability when the boat is upright, and maximum instability when the boat is inverted upside down. With clever design you can have both.

P.S.
Here a link to capsizing and recovery of a modern design waterballasted boat.: https://www.youtube.com/watch?v=yYrFXsHzvcg

[rogypop]

Other than Egret type flat bottom hulls Matt Layden's Paradox comes to mind as a seaworthy flat bottom boat, much smaller though. This french design also looks quite promising in that department.

https://www.boat-et-koad.com/plans-d...s-de-7-50m.php