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  #1  
Old 12-10-2008, 02:29 PM
archeryrob archeryrob is offline
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small boat stability

Preface: A newbie thinking out loud.

I have been playing with a boat design for my own personal sneak box. I have been interested by the flat bottom hybrids being built but they have no ability to track and are basically just plows just like an old jon boat. This boat is for layout boat hunting in rivers and mild brackish water only.

I was looking for something like a Chesapeake skiff in the back, a canoe/kayak on the bow and and barnegat bay sneak on the top. The idea is for a low profile for hunting and the ability to track if paddled. My plans so for are for a shallow 5 degree mod V bottom and a 55 degree chine that rises 11.5" above the bottom line. I was hopping for the stabilty and tracking with an almost flat bottom and pointed front like a canoe.

Does the Mod V help with tracking any or would it just be better to have a slight round or flat bottom in the back? Is the bow enough or would tracking V or lines help?

Basically I want the stability of my jon boat and the shallow daft and tracking of my canoe, but with stability.

OK, fire me up or point me with some links. I have thick skin so fire away.
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  #2  
Old 12-10-2008, 04:38 PM
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rwatson rwatson is offline
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"but with stability" is a real open ended question. My .8 metre wide canoe has "stability", but it will tip over if you get in wrong. How much stability are you after? "Foot on the gunnel fighting a big fish stability" or "stay seated in the centre stability" ?

The V on a hull has a huge impact on tracking. My first impulse is to suggest that you do a small retractable skeg - so you can do shallow water stuff, or paddle in a straight line, and still have a flat bottom for superior stability.
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  #3  
Old 12-11-2008, 07:52 AM
archeryrob archeryrob is offline
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I am looking to make the boat foot on the side stable and being able to track.

I want the boat to be able to be paddled or run with a small motor on it. Nothing but like a 5 horse mud motor or something small like that.

Being a newbie, in design, and canoeing my whole life I know my flat bottom canoe tracks well and is center stable, but losses stability to the side. It also track well until one man in the back make the bow come out of the water and then it fails to track at all.

I wanted t read up on adding tracking lines, the bows effect and mod-V's effect and the cons of adding one for tracking and still keeping stability.
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  #4  
Old 12-12-2008, 12:38 PM
Virtech1 Virtech1 is offline
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If one took a jon boat that normally drew 2" of water, and raised the center third of the hull 3" from bow to stern, the boat would now draw 3" of water, but it would be a bit more stable, slide through the water a bit easier, and track better, as the hull now would have four hard chines to fight sideslipping.
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  #5  
Old 12-14-2008, 02:33 PM
messabout messabout is offline
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Why not use the traditional lines of the Barnegat Bay sneakbox ? They have evolved over many many years to be a most functional boat. Though not a duck hunter schooled in this sort of thing, I am told that sneak box users are more likely to scull from the supine position than to paddle. The better to sneak.

If all you are after is stability, very shoal draft, and decent tracking, then a simple flat bottomed skiff will do the trick. Way easier to build than a sneak box too. A skiff will track very well when fitted with a skeg that does not extend below the lowest part of the underbody. With a name that suggests archery, are you going to take ducks the hard way?
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  #6  
Old 12-15-2008, 05:52 AM
archeryrob archeryrob is offline
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I had considered something like the Barnegat Bay sneakbox but most are very heavy. I am wanting to make something small enough to drag, stable enough to put a dog in and still track like a canoe. I know having that much profile in the water is going to slow it down, but getting wet in 20 degree air and 34 degree water is a major concern.

Now for a keel you definition would be like the aqua pod as shown here.


how is a keel not lower than the lowest part of the boat better? Why would not a keel that ran full length be better? Similar to a tracking line down the center of the boat and then becoming a full keel?

No the archery thing is that I have make Indian type bows for hunting. That is just how I have been identified for ever on the internet. See here.
My bows
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  #7  
Old 12-18-2008, 11:02 AM
messabout messabout is offline
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The Aquapod looks like a slalom kayak, sort of. Both ends of the boat appear to be above the waterline. That boat will turn on a dime but will not track well. The skeg in the back is a band aid appurtenance that will help aim the boat in a straight line. The skeg will help, but not conquer, the tracking issue. Not the kind of boat I would choose if my dog is to be my companion. An excited 75 or 80 pound lab could cause havoc in a boat of this size and design.

Stability of a very small boat is a function of width at the waterline and to a lesser extent length. Also the shape of the cross section of the boat plays into the equation. A generality is that a hard chine boat (square chine) is likely to provide the best initial stability. So the flat bottomed boats that you mentioned are utilizing that reality.

Tracking is a matter of getting some cross sectional area at, or near, the ends of the boat. This boat will be primarily propelled by oar, scull oar, or paddle. It needs to have the bottom, at the transom, a bit above the loaded waterline. If not the boat will drag an unwanted wake and be more difficult to row or paddle. Put the fore foot somewhat below the loaded water line. That will provide the needed lateral area in at least one end of the boat. Add the skeg to the back of the boat, like the Aquapod, and you now have a boat that will go pretty straight and have some resistance to cross winds. Alas, the boat will not turn as easily so we should not overdo the lateral area thing.

If the boat is to have an immersed fore foot then it will need to be pointy at that end. All things considered, we have described a simple flat bottomed sharpie or flattie type boat. I would think it a good idea to build some side decks into the boat. The reason for that is mainly to keep the dog nearer the center of the boat. The side decks can also provide a covered space for guns, bows, or other long things. The side decks also contribute to the stiffness of the boat without adding a lot of framing, sheer clamps, etc.

Part of the structure of such a boat will be an inner longitudinal plank along the centerline of the boat (a keelson). Add another plank on the outside bottom that mirrors the inner one. That one will help tracking as well as act as a skid shoe. If you are going to drag the boat, the skid shoe will be a practical element.

Weight will be a factor of course. I think that you could build a very useful, and reasonably safe flattie in the 55 pound range. Say 132" long by 42" wide. Use 6mm Okume for the main body and 4mm for the side decks. The sides can be plumb (straight up and down). Plumb sides make the building process very easy and except for appearance there are few valid objections. That construction will make a boat that is adequate but far from indestructible. Boat design is always a series of compromises.



.
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  #8  
Old 12-18-2008, 12:34 PM
archeryrob archeryrob is offline
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Is the wake reduced on a boat like this? (flattie) and does that wake on a flat stern slow it down?


Is a slight taper like the Flattie ok or can it get longer like a canoe with stern?
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  #9  
Old 12-19-2008, 12:58 PM
messabout messabout is offline
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The flattie will produce some wake, yes. Just about any thing that moves through the water will produce some wake and the dimensions of the wake will be a function of the speed of the object through the water. A better underwater design might resemble a Whitehall or Adirondike Guide boat. Neither of those are particularly well suited for your purpose. In addition the build would be far more difficult, time consuming, and probably more expensive.

Designing a boat for a specific purpose is always a series of compromises. If you want speed with oars or paddle, the design will likely be long and narrow. The narrow boat will propel easily but you will sacrifice stability. You want stability... the boat will be wide but you concede easiest propulsion. When you want tracking, you use little or no bottom rocker but the boat will not turn as easily as you might want.

For simplicity, practicality, economy, I like the flattie. It is not the best in any department but it does combine several modest virtues. The difference in what I might dream up, for a boat of your stated purpose, and the little boat in the picture is: The duck boat will not have floorboards, no thwart (seat) because you will be sitting, kneeling, or lying on the bottom. The boat will have small side decks running the length of cockpit. If your chosen propulsion method is the paddle, the sides need to be pretty low. The side decks and the fore and aft decks will have considerable rise from the gunnwale toward the centerline. Viewed from the side, the bottom will curve up toward the bow a small distance and it will curve up toward the stern a larger distance. The bow can be perfectly straight up and down. we are trying to get the longest waterline that we can have while remaining a short boat. We made a short boat, not because it is best, but because we must conserve weight and keep the boat small enough to hide in the weeds or blind or whatever. In addition there may be transport and storage constraints.

A little boat that is 11 feet long and 42 inches wide at the widest point on the chine, will draw about 3.25 inches of water when loaded to an all up weight of about 320 pounds. That is the total weight of the boat, the occupants including the dog and whatever gear that may be carried. The boat could carry a bit more weight if needed. At 4 inches of draft it would displace about 390 pounds. If we anticipate 390 pounds the bottom of the transom will rise, perhaps four and a half to 5 inches above the deepest point of the bottom. The bow end will rise about two to two and a half inches. You could expect to row such a boat up to 4 miles per hour and not much more. Paddling would get maybe 3 MPH. A little Honda two HP motor would likely push the boat as much as 5 MPH. A larger motor would not do much more unless you are willing to pull a huge wave train behind you.

If you made the boat in such a way that it would plane, then the 5 Hp motor would push it much faster, maybe 13 or 14 MPH. If designed to plane, the boat will be a beast to row or paddle. A planing boat would have zero rise at the transom bottom. Then the transom would drag when not on a plane, and it will take Herculean effort to row at 4 MPH if it was even possible. So you see that there are compromises and you just have to decide what is more important and do the design work from that viewpoint.
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Old 12-19-2008, 01:44 PM
archeryrob archeryrob is offline
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Well, the design I am looking for is light, stable and being able to be paddled. So stability and slight tracking would be the most desirable traits. It may get a transom so it could mount a trolling motor or weed whacker motor.

My problem is I hunt and fish where there are rocks. Where I would use the boat is in areas my jon does not easily travel without taking damage to the outboard. The rocks have me scared on using glass so I was thinking of trying to secure polyethylene sheets or some plastic ply sheet and vacuum them over a wooden model. Make a deck with cockpit with channeled J ledges around the cockpit to stiffen the deck and glue/weld with Methyl Methacrylate where the deck and hull meet. This should handle the rocks.

I am just thinking on the hull design. I like the flattie, but I might add 30 degree hard chines and a sweping up bow and sweeping down deck to the bow, like a kara hummer bow, but a little higher.

i just wonder about the rowing. I don't think the joined polyethylene body would handle rowing locks or the strain of them. I know my old Town Gunnels would but they are an extruded plastic tube for rigidity.

I guess some of this post has gone past stability and into design.
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  #11  
Old 12-20-2008, 05:53 PM
messabout messabout is offline
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Stability is inexorably linked to design.. Tracking ability, loaded draft, wind loads, and about every other behavior is too. If you bevel the chines the boat will need somewhat more draft and initial stability will diminish some. Probably good cosmeticly. Additionally there are some useful structural advantages. ( the flat plate area of the bottom becomes smaller and is less subject to deflections). You trade one thing for another.

Plastic sheet is questionable as a building material. HDPE is surely more able to withstand abuse from rocks and oyster shell than the majority of alternative materials. Unfortunately there is the matter of stiffness. If you put enough thickness in the plastic bottom to handle the loads, the boat will be much heavier than a wooden one of the same size.

Seems to me that the aluminum Jon boat would be about as good as you could do within reason. I dislike the ungainly things but they will stand hard use and will survive most encounters with rocks unless you are going awfully fast when you hit 'em. Electric trollers need to be run pretty deep to get decent propulsion. They could be raised considerably if they are fitted with a cavitation plate.

Good luck with the plastic.
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Old 12-20-2008, 09:07 PM
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rwatson rwatson is offline
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For the weight addition and difficulty of fastening, of heavy plastic, I would be inclined to use Dynel and Epoxy over ply or timber. Together with a few rubbing strakes, and just be prepared to bog up any damage from rocks from time to time.

Dynel wont add much cost, but will give you plenty of robustness, and hull protection.

For the little bit of after trip filling, with a quick slap of paint, I would expect you to enjoy a much lighter and faster ( from smoothness) trip each time.

I think you will find a composite plastic/timber approach will produce more weight and maintenance issues than you need to put up with,
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Old 12-21-2008, 07:17 AM
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You guys scare me with your small boats. Manie is into these small things as well, do a search I' sure you'll find his... er... baby bath.
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  #14  
Old 12-24-2008, 07:37 AM
FoggyBottom FoggyBottom is offline
 
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That is sure a lot work
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  #15  
Old 12-24-2008, 08:07 AM
archeryrob archeryrob is offline
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My problem is I don't think glass over wood is going to be great on contact with rocks. I have a old town discovery 169 plastic canoe that handles the rocks great, I was just trying to make something of the same type of hull but more stable, could paddle lightly or motor with a small motor or even double row.
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