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  #1  
Old 07-16-2010, 12:28 PM
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Wynand N Wynand N is offline
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Which hull type is more stable?

Which boat would be more stable say up to 15 degree heel - hard chine flat bottom or hard chined V bottom?
Hypothetically, let's assuming both has the the same WPA, LWL, TWB, displacement and are similar fishing boat hull types about 16ft length overall.

I would venture to say that the V shape may have a little bit more stability than the flat bottom. What would you say?
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  #2  
Old 07-16-2010, 01:24 PM
jehardiman jehardiman is online now
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Static or Dynamic stability? Emerged or immersed chine?

For Static, V bottom has less if the chine is immersed, because, all else being equal, KB is further below KG which reduces GZ. If the chine is emerged, then the CoB shift is greater and waterplane inertia changes greatly so it will depend on the deadrise angle relative to the beam.

For Dynamic, V bottom will always have a greater added mass adding to the impression of "stability".
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  #3  
Old 07-16-2010, 09:05 PM
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You have problems Mate..................

V feels more comfortably, but less "rugged", the rest is calcīs, as you know.....
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Old 07-16-2010, 09:42 PM
ancient kayaker ancient kayaker is online now
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It's mostly about how high the CoG is above the CoB. In general a Vee bottom adds bouyancy low down without lowering the weight, but if that allows a significantly weighty object to move down, such as an engine, stability might improve. In my early canoes I started with a Vee bottom, very unstable, my bum wasn't the right shape to move down, so I transitioned to a full-width flat bottom, better but it felt awfully tippy. Finally I settled on a 5-plank design with a half-width flat bottom and bilgeboards - ah! Stability and a good safe "feel" at last.

The last configuration still allowed me to get just as low in the boat as the flat-bottom but reducing low-down bouyancy also moved the CoB up. The righting moment continues to build at an increasing rate as the heel increases, whereas with the flat bottom boat it peaked quickly then fell off, providing the impression it was going to flip any moment.

But it doesn't always work that way: my small flat-bottomed saiboat feels almost as stable as a dock despite only being 10' long. Theoretically it would be even better with a bilge board design BUT a sailor doesn't sit snugly in the bottom like a paddler does so that might not be true. I admit I haven't quite figured the whole thing out yet, but there should be some threads in the Stability forum with some information.
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Old 07-17-2010, 11:06 AM
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With the same beam and displacement, the flat bottom boat should be more stable.
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Old 07-17-2010, 12:01 PM
Submarine Tom Submarine Tom is offline
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Stable, tender or seaworthy...?

-Tom
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Old 07-17-2010, 03:35 PM
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He is asking for initial stability.
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Old 07-18-2010, 05:02 AM
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Most of my fishing is done on local dams where I have no problems launching my boat.
However, we have a local river with some giant bass lurking in those waters but the problem is that access to the river is very limited due to reeds etc and most of the river flanked by farms and not so friendly farmers. Impossible to launch with a boat trailer due to the river banks.
I need a lightweight boat that can be loaded on the pickup and off loaded onto the river by two persons

I plan to build a light weight stitch and glue plywood fishing boat of about 3.0 meter LOA with a beam of about 1.5m max and 1.2m BWL. Displacement should be about 280kg to LWL (full load).
The preliminary design is already done on FreeShip and the weight of the boat about 41kg for a flat bottom hull. Having no stability curve function, (and me to lazy, and recently short tempered to do it the hard way paper and pencil method) to compare the same boat with a V bottom I asked this question as there are many experts on the board.

It goes without saying that the boat need to be as stable as possible within the design parameters. Planing is no issue as she will be driven by a Suzuki 2hp outboard engine.
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Old 07-18-2010, 06:10 AM
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Go with the flat bottom. A vee bottom will increase the draft for the same displacement and increase the amount of wetted surface for no appreciable benefit or gain. If you want to increase your initial stability...go wider on the bottom, if you want to increase the ultimate stability...add more flare to the sides so it stiffens up as it heels when the weight is shifted in the boat. To ease the load on the motor...put enough rocker aft to get it out of the water...or not...up to you. Give yourself enough freeboard so you can sit right on the wales and not get your bottom wet...say about .3-.33m. I mean freeboard...not total depth of hull which would also include your level draft.
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Old 07-18-2010, 05:18 PM
ancient kayaker ancient kayaker is online now
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Definitely flat-bottomed for that size and application.

My small flattie sailboat is only 2.5 m x 1.2 m at the waterline and is stable enough to support my entire 90 kg on the side deck when I climb in after launching from a beach. At only 27 kg in its rowing configuration it is easy to car-top. I can stand up and move around freely provided I remember it is basically a hydraulic skateboard

A boat that size could likely handle an outboard with heavier construction and maybe ballast to hold the bow down while boarding.

My own boat rows so effortlessly even an electric would shove it around with no problem.
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Old 07-19-2010, 04:58 AM
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Quote:
Originally Posted by Wynand N View Post
The preliminary design is already done on FreeShip and the weight of the boat about 41kg for a flat bottom hull. Having no stability curve function
But Freeship does have a stability curve function. You can find it in "Calculations" --> "Cross Curves and SSD".
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Old 07-19-2010, 07:47 AM
MikeJohns MikeJohns is offline
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Quote:
Originally Posted by Wynand N View Post
...........we have a local river with some giant bass lurking in those waters but the problem is that access to the river is very limited due to reeds etc and most of the river flanked by farms and not so friendly farmers. Impossible to launch with a boat trailer due to the river banks.
I need a lightweight boat that can be loaded on the pickup and off loaded onto the river by two persons........
Wynand You need a punt (and yes the flat bottom is the most stable form) you can walk around the gunnels of a good punt. They plane easily. There are some very thin alloy ones sold here which are very light and strong.

The fishing sounds interesting. Is it a deep slow river? you need a river boat for rapids.
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Old 07-19-2010, 10:56 AM
ancient kayaker ancient kayaker is online now
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Quote:
Originally Posted by daiquiri View Post
But Freeship does have a stability curve function. You can find it in "Calculations" --> "Cross Curves and SSD".
I find that FreeShip provides a righting moment based on displacement and bouyancy. I am not sure how it factors in the height of the CoG, but it calculates that based on hull material density and area. Is there a way to tell FreeShip the CoG height of the load the hull is carrying? If not, how should the righting moment be corrected for the true height of the CoG?
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Old 07-19-2010, 12:27 PM
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Quote:
Originally Posted by MikeJohns View Post
The fishing sounds interesting. Is it a deep slow river? you need a river boat for rapids.
The river depth varies between 1.5 to about 4 meter but on average about 2.5 meter. The width of the river from about 30 meters to 4 meters averaging say 10 meters.
The river only flows when we have heavy rains that over fills it - on a distance of about 8 km river there are 5 dam walls across it built by farmers to trap the water in dry season for irrigation purposes.
In reality, the river is a series of "dams" most of the time and only flows after heavy rains that lasts for days. This is the reason for this light weight boat - when going upstream, she can be taken out of the water and carried up and around a wall.

I had seen those tinny punts in Perth when I visited your beautiful country a couple of years ago. Never thought they were that stable.
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Old 07-19-2010, 12:46 PM
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Originally Posted by ancient kayaker View Post
1) Is there a way to tell FreeShip the CoG height of the load the hull is carrying?

2)If not, how should the righting moment be corrected for the true height of the CoG?
Hi Terry,

1) Please find attached a sample dialog screen of Freeship's SSD command. I've circled in red the dialog box where you input the Zcog, and the graph where you read the calculated curves for various Zcog.


2) Regarding your second question, it can be corrected manually too - and it is not so difficult. The formula is:
GZnew = GZold - Dz sin(Heel)
where:
- GZold is GZ at the initial vertical position of the CoG, for a given heel angle
- GZnew is GZ for new vertical position of the CoG, for that heel angle
- Dz is the vertical shift in CoG, positive when upwards
- Heel is... the heel angle.
The effect of a horizontal (transverse or beamwise) shift of the CoG is evaluated in a similar manner:
GZnew = GZold - Dy cos(Heel)
where Dy is the transverse shift of CoG. It is positive when contributing to the heeling moment, negative when contributing to the righting moment (canting keel, for example, or a sailor on a trapeze).

Hope it was clear enough. Cheers.
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Which hull type is more stable?-freeship-ssd.jpg  
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