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Old 07-12-2019, 11:53   #1
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Flat bottom hulls?

Besides the bow portion of a sailboat monohull being V-shaped, the majority of the bottom of the hull is flat. Is this good or bad, thoughts?
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Old 07-12-2019, 12:10   #2
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Re: Flat bottom hulls?

Flat bottoms can pound/slam going to windward w/heavier waves. The flat bottom will give it more initial stability, but may not be able to right itself if knocked down. It can also carry heavier loads.
If you are going to use it in the Chesapeake you'd probably be ok.


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Old 07-12-2019, 12:41   #3
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Re: Flat bottom hulls?

When you say flat, would 'canoe' shape be a better description? if so and with a nice deep fin keel they will be fun, fast, responsive and easy to park in a tight marina. I think you would be pushed to find any other new shape for sale in any quantity in Europe.

There is one of these parked in our marina, I keep trying to get an invite on board for an afternoons blast.

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Old 07-12-2019, 12:59   #4
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Re: Flat bottom hulls?

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Originally Posted by EllisElectric View Post
Besides the bow portion of a sailboat monohull being V-shaped, the majority of the bottom of the hull is flat. Is this good or bad, thoughts?
Your premise is incorrect. this is true of some sailboats but a long way from the "majority"

Buy one that suits your purpose or desires.
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Old 07-12-2019, 13:25   #5
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Re: Flat bottom hulls?

Great initial stability, lousy ultimate stability. Drier. Think about the other extreme: The Flower class corvette of WWII had a round hull cross section. Would "roll on wet grass." Water came down the passageways. Went though every Northern North Atlantic winter storm between 1941-1945. Lost a few to torpedoes, but not a single one to weather.
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Old 07-12-2019, 13:52   #6
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Re: Flat bottom hulls?

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Originally Posted by Bill O View Post
Flat bottoms can pound/slam going to windward w/heavier waves. The flat bottom will give it more initial stability, but may not be able to right itself if knocked down. It can also carry heavier loads.
If you are going to use it in the Chesapeake you'd probably be ok.

Bill O.
Most of pounding while sailing is from the flattish topsides forward (flat sides aft of the bow). A heeled over boat, even with a flat bottom, presents a "V" shape to the water with the chine but the topsides forward are flat and hit solidly against waves.

But when motoring, which the vast majority of cruisers seem to insist upon, the bow can rise enough to present the flat hull shape to the water when dropping onto the next wave, so yes.

And the ability of one's boat to right itself when knocked down (when does anyone actually experience that?) is due to a lot more than the form stability of the hull. You'd have to consider in the first place how hard will it be to heel the boat to an extreme angle anyhow. For example my boat has flat bottom sections from the mid bow to behind the rudder, yet the limit of positive stability is 120 degrees, and we have yet to experience a knockdown, ever.

Basically, in my view, if you are sailing, the shape of the bottom affects many other characteristics, such as ability or not to plane when sailing off the wind, and overall, there are many possible shapes of boat hulls which have with potential impacts on the behavior in seas besides the flatness of the bottom of the hull.

It is not so simple.
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Old 07-12-2019, 14:08   #7
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Re: Flat bottom hulls?

Ovni, Garcia and Alliage- French designed metal boats- have a totally flat bottom plate. As you describe, the bows V flows into a flat bottom a few feet aft and continues to the stern.

I've asked this same question to more than a few owners of these boats, but none have said it's an issue at all. Unfortunately, I do not have enough experience sailing these designs to give any other insight besides what the owners have described.


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Old 07-12-2019, 16:24   #8
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Re: Flat bottom hulls?

Pete and Annie hill sailed to Greenland and the Falklands in a flat bottom 34í Benford dory. I have had the opportunity to sail a sister ship a few times. Once we had a wind against tide situation and I thought we would pound the fillings out of our teeth. But instead she just parted the waves and kept on sailing.
It encouraged me enough to build my own plywood version of an Ovni. The reason we went flat bottom so we can beach her where we want.
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Old 07-12-2019, 17:47   #9
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Re: Flat bottom hulls?

I sailed a Jeaneau 49 quite a bit in San Francisco Bay, and the surrounding coastal waters. A pretty typical example of the modern "flat bottom" boat designed to maximize interior volume.

In the bay it was a true delight to sail. She was fast, easy to handle, weatherly. In the ocean, it was AWFUL. Trying to make progress upwind in a steep chop would knock the fillings out of your teeth. To save wear and tear on the humans onboard, it wasn't at all unusual to have to crack off 10 or 15 degrees, or more, from close hauled to avoid the worst of the pounding. Really uncomfortable. I saw no evidence whatsoever that "the chine cuts the waves" in any meaningful way.

Another problem... there is no useful bilge! Any time the boat heels over, ANY and ALL water in the hull runs up into the storage lockers. A total PITA. Heeled over 15 degrees you could have many dozens of gallons of water sloshing around before the centerline mounted bilge pump could get any of it out of the boat.

All in all, a boat very good for what it was designed to due: Ferrying charter customers around in relatively protected water. Not at all a boat I would want for long term cruising in blue water.

"Sea kindliness" is a feature of boats that has long since gone by the wayside in so many modern mass market designs.
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Old 08-12-2019, 08:12   #10
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Re: Flat bottom hulls?

Quote:
Originally Posted by billknny View Post
I sailed a Jeaneau 49 quite a bit in San Francisco Bay, and the surrounding coastal waters. A pretty typical example of the modern "flat bottom" boat designed to maximize interior volume.

In the bay it was a true delight to sail. She was fast, easy to handle, weatherly. In the ocean, it was AWFUL. Trying to make progress upwind in a steep chop would knock the fillings out of your teeth. To save wear and tear on the humans onboard, it wasn't at all unusual to have to crack off 10 or 15 degrees, or more, from close hauled to avoid the worst of the pounding. Really uncomfortable. I saw no evidence whatsoever that "the chine cuts the waves" in any meaningful way.

Another problem... there is no useful bilge! Any time the boat heels over, ANY and ALL water in the hull runs up into the storage lockers. A total PITA. Heeled over 15 degrees you could have many dozens of gallons of water sloshing around before the centerline mounted bilge pump could get any of it out of the boat.

All in all, a boat very good for what it was designed to due: Ferrying charter customers around in relatively protected water. Not at all a boat I would want for long term cruising in blue water.

"Sea kindliness" is a feature of boats that has long since gone by the wayside in so many modern mass market designs.
A few thoughts billknny: Most likely it wasn't the flat bottom sections but the flat topsides which were banging in the steep chop, as you would have been well heeled over. Makes no difference to the occupants of course, still knocking fillings out. Any boat I've ever sailed on would bang if sailed into steep chop unless it is a hull with a very deep forefoot sailed nearly upright (how does one do this? Oh, the motor.)

Cracking off 10-15 degrees eases the impacts a bit and if the boat is weatherly the resulting angle is still pretty good for VMG.

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All in all, a boat very good for what it was designed to due: Ferrying charter customers around in relatively protected water.
It seems to me that Jeaneau designed that boat (don't know which model) to sail well in all conditions, that is, to go any direction in any wind and sea, and to appeal to the owner who really enjoys getting about under sail. A performance oriented boat, in other words. It will be lighter and have the keel, rudder, rig and sails to do that. It is going to react to the wind and sea conditions, including banging around a bit (or a lot) going upwind in steep chop. BUT IT WILL BE GOING.

One could get a concrete bridge pontoon which will be very stable and comfortable in a steep chop, however it won't go very far. Between the two there is a continuum.

I chose a vessel probably very much like the Jeaneau. I dislike pounding to windward as much an anyone, but we need to do it rarely. When we have to get to weather in rough seas, we can. Mostly however, we enjoy a smoother ride and get to our destination well-rested, and first.

As to the lack of bilge and water ingress to the storage lockers: Big problem! If the lockers are open to the bilge this is what you get. This is a design/manufacturing defect, in my opinion. To remove water in the bilge we use a Whale Gulper with a hose and put the hose where the water is.
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Old 08-12-2019, 08:37   #11
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Re: Flat bottom hulls?

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Originally Posted by Pete7 View Post
When you say flat, would 'canoe' shape be a better description? if so and with a nice deep fin keel they will be fun, fast, responsive and easy to park in a tight marina. I think you would be pushed to find any other new shape for sale in any quantity in Europe.

There is one of these parked in our marina, I keep trying to get an invite on board for an afternoons blast.

Pete

OMG! Did you'all watch this video?
No roller furling, no spinnaker sock, no in-mast mains sail furling, no boom brake, no preventers, no solid dodger (hell, no dodger at all!) no MFD at the helm. Not even the requisite dog on board!
They should never show this type of thing on Cruiser's Forum. What will the newbies think?

This sailor needs to get off the boat and attend a few more cruising seminars on how to equip his boat.

And stop having fun for cris' sake.
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Old 08-12-2019, 09:17   #12
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Re: Flat bottom hulls?

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OMG! Did you'all watch this video?
This sailor needs to get off the boat and attend a few more cruising seminars on how to equip his boat. And stop having fun for cris' sake.


You missed one, whilst flying a cruising chute single handed left the autopilot to do the steering and maintain a proper look out.

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As to the lack of bilge and water ingress to the storage lockers: Big problem! If the lockers are open to the bilge this is what you get.
Why in a modern GRP yacht is there water in the bilges? Ewwww

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Old 08-12-2019, 09:35   #13
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Re: Flat bottom hulls?

Flattish, wide aft sections are popular on modern boats for reaching power etc. Aft is not a problem for pounding. I know of no real flat section sail boats forward or mid except like dories or sharpies.
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Old 08-12-2019, 09:55   #14
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Re: Flat bottom hulls?

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Flattish, wide aft sections are popular on modern boats for reaching power etc. Aft is not a problem for pounding. I know of no real flat section sail boats forward or mid except like dories or sharpies.
All IOR race boats from the late 70's and the 80's and thousands of designs influenced by those race boats, have a flat triangle shaped area directly forward of the keel which extends from just aft of the knuckle to the keel. The flat area of the bottom continues aft past the keel, and it is often 4-5 feet in width. Most of them also have flat sections, similar to the Omni pictured above, between the bottom and the chine. These shapes were a reflection of the IOR design rule, not created for any sailing or seakeeping reasons, however they did result in very moderate and easily driven shapes, which also have good seakeeping, but they are slower than modern designs.

Most fin keeled cruiser or racer cruiser boats have more rounded and fuller sections.

Many modern boats, while lacking the angular shapes of the old IOR boats, have broad, gently curving shapes which encourage planning. They also allow bigger areas inside the hull.

The most modern even have hard chines aft. Again, this is a copy of the current trend in high-speed racing machines.

The overall progression is to broader, flatter, lighter boats which will sail faster but, unfortunately, are going to pound more while shortening passages.
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Old 08-12-2019, 10:07   #15
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Re: Flat bottom hulls?

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All IOR race boats from the late 70's and the 80's and thousands of designs influenced by those race boats, have a flat triangle shaped area directly forward of the keel which extends from just aft of the knuckle to the keel. The flat area of the bottom continues aft past the keel, and it is often 4-5 feet in width. Most of them also have flat sections, similar to the Omni pictured above, between the bottom and the chine. These shapes were a reflection of the IOR design rule, not created for any sailing or seakeeping reasons, however they did result in very moderate and easily driven shapes, which also have good seakeeping, but they are slower than modern designs.

Most fin keeled cruiser or racer cruiser boats have more rounded and fuller sections.

Many modern boats, while lacking the angular shapes of the old IOR boats, have broad, gently curving shapes which encourage planning. They also allow bigger areas inside the hull.

The most modern even have hard chines aft. Again, this is a copy of the current trend in high-speed racing machines.

The overall progression is to broader, flatter, lighter boats which will sail faster but, unfortunately, are going to pound more while shortening passages.
Yeah, sailed many of them. The lack of wide more flattish sections aft is exactly why those boats roll so badly, and probably why so many designers went to those powerful aft sections after the IOR phase in sailboat history fortunately faded!. A small flat area directly on the bottom doesn't qualify as a "flat bottom sailboat" to me I guess! A Dory does though!
Here's a Peterson IOR hull, while it does have a narrow flat section on the bottom, It doesn't tend to inspire me call it a "flat bottomed sailboat", in fact it's pretty round bilged.
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