rocker or no rocker?

[3pepper]

hi

i have read on several posts , here , that explain the need for rocker on human powered craft and the need for no rocker on a boat expected to plane out with an outboard motor .

so this may be a dumb question ... i have seen the plans for the ozark jonboat , and i may build one to get on the water while i save up the money for a 4 stroke outboard .

on the ozark , i plan to have 2 marine batteries nested in a center seat box and power it with a 50 lb thrust trolling motor that i already have .

so ... on a trolling motor powered craft should i have rocker or no rocker ?

also , can i increase the beam to 4' and keep the length at 16' ?

thanks
stephen

[Bruce Hooke]

In general, a low-powered craft, whether human-powered or motorized, should have some rocker so that the transom is not dragging in the water. So, your boat should have some rocker.

That said, if the design you've picked does not have rocker or if the design you've picked was not intended for something reasonably close to your intended use, I'd be very cautious about trying to modify the design by doing things like adding rocker. Rocker is just one part of a complex set of decisions that need to be made about the design of a boat.

I am not familiar with the "Ozark Jonboat" design, but adding a little to the beam is not likely to be that big a deal. However, I would not go very far with this -- a few inches would be reasonable. Narrow means easily driven, so it is very easy to take a nice design and make it into a pig that shoves half the river in front of it and drags the other half behind by adding to the beam. What is the designed beam? Actually altering the design to add beam could end up being tricky, because it could mean a good bit of re-figuring and redrawing. Length is easier to add because you can simply increase the space between the stations/frames a bit.

[paladin]

I assume the boat is flat bottomed to begin with, and is narrower than 4 feet and you intend to increase the beam to 4 feet....under those circumstances I see no problem...essentially it is a flat bottomed pram to begin with. You may need to add a little stiffness athwartships...

[Woxbox]

The flat bottom is only a benefit when the boat is planing. There's not much chance of that happening in a battery-powered boat such as the one you're planning. In fact, for a boat moving at discplacement speed, too much rocker is better than too little. In other words, it's better to have the transom(s) above the water than down in it.
But I would second Bruce's comment -- if the design doesn't fit your needs, keep looking. There are so many small boat designs out there, there's at least one, probably a dozen, that will fit your needs almost exactly, not just closely. Sounds like you're going fishing? If you describe the uses the boat will be put to, you'll get plenty of suggestions.

[Cuyahoga Chuck]

You are trying to boil a lot of things down into a nice little soup. The features of a boat design are ALL compromises of some kind. You give up something to get something.
Ideally a rowboat bottom will be both rockered with a vee or rounded bottom with transom(s) that are placed above the perceived waterline. This gives the waterline the shape of a long skinney football which is a good shape for low power operation. With a flat bottom the below-water shape is not going to be as good but rocker will help if it keeps the transoms, front and rear, above the water with a normal load. If either of the tansoms go into the water from a heavy load or bad trim that ruins your underwater shape and creates unwanted drag. Things like where the pilot and passengers sit make a difference.
If you rocker the bottom, the hull becomes a "displacemant" hull and it will not plane. It will cruise with low power at any speed up to a theoretical point called "displacement speed". Usually 5-8 mph for a small boat. If you try to go any faster your motor will work it's guts out but your speed will only increase slightly.
The above is a statement of a lot of generalities but it's the best I can do with the small amount of information you posted.
Boat plans don't usually give you an option of how the bottom is shaped. That's usually decided before the designer starts to draw the plans.
Next, most of us wouldn't know an Ozark jon boat if we found one on the front porch. If you could give us a name, picture,web site or a quick decription ,with dimensions, it would help a lot.
I'm sort of struck by your desire to "increase" the beam of this boat to 48". The pram in my avatar (jon boats are prams) is only 8' long but it is 48" wide. And I can pull up the plans for longer prams/jon boats and they are also all 48" wide or more. If your design is narrower, how narrow? And is there a reason for this?

[Steve Paskey]

Jim Michalak has a ply version of an Ozark jon boat. This is from his write-up.
=============================
http://www.duckworksbbs.com/plans/jim/ozarkian/ozark.gif
The traditional Ozark boat is in some ways better than the aluminum canoe. For one thing it carries a huge load, being 20' long, 4' wide, and having a dead flat bottom. It is a drifting barge. Godsey gives the basic use: load with fishing guide and "sport" and gear to float 125 miles from Galena to Branson, Missouri, (first home of the mythical Jed Clampett), in five days. No motor, just a paddle for the guide to steer with. At Branson it all got shipped back to Galena (just 24 miles away by land) by rail for the next trip. In the old photos the amount of gear carried is enormous. The boats have seats in the ends but the photos show the people sitting very comfortably in canvas director's chairs.
http://www.duckworksbbs.com/plans/jim/ozarkian/ozark1.jpg
Construction in 1956 was what one could expect. You start with something you might not find at your lumberyard today - five "clean" pine boards 1" x 14" x 20' long. Three boards make the bottom with tongue and groove joints and one board makes each side, so these boats aren't too big on freeboard

[Cuyahoga Chuck]

If it's made from dimensional lumber it must weigh a ton. And, with the dirth of railroads today you may have to find alternative ways to cart that baby around.
How do they handle going upstream?

[Thorne]

It sounds to me as if Stephen is asking what sort of rocker to build for this boat that will be used both with the trolling motor and later with the outboard. So in that case, he'd want very little rocker once he saved up enough for the outboard, right?

As above, find a set of plans that give you what you want and build to the plan -- modifying them can cause all sorts of performance and safety issues, not to mention possibly wasting $$ and time.

Good luck and keep us posted!

[Lewisboats]

If you wanna plane with the OB, don't put any rocker aft. If you want the the most out of your trolling motor you will need the rocker, just to reduce drag from the transom for the maximum displacement speed. If you want the most out of your OB and it is enough to get the hull to planning speed, I would sacrifice displacement speed so you can upgrade.
" You only need to tolerate slower speeds in order to go faster, it is impossible to go faster in order to exhalt slower speeds". In other words...design to go faster if you wish, because designing to go slow only lets you go slow, even if designing to go fast goes slower at first.
A planning design will go slower with a trolling motor than a displacement design, but will go faster with a 4 stroke motor designed to plane the boat.
SO...the main question is: do you want the first boat you build to be the second also or are you willing to build another to a different spec. The optimum boat you build for a trolling motor will not perform much better with another (more powerful) motor but the boat you build for the more powerful motor will certainly not perform to optimum with a trolling motor!

Steve

[Lewisboats]

Rereading the original post...a trolling motor approximately replaces human power...so keep the rocker if you wish to replace human power with batteries and low power electric motors...you will go as fast as humanly possible but not as fast as possible with another design and another motor.

Steve

[bainbridgeisland]

Your view is too restrictive when you only look at rocker. Obviously many fine rowboats have very little rocker. Two examples are rowing shells and whitehalls.

The amount of rocker needed is dependant on other features of the boat and is also very sensitive to the planned cruising speed.

I believe you are correctly interpreting rocker as a measure of distribution of volume forward and aft. However, rocker is only one part of the picture. Section shape (which infers waterline half breadths) is just as significant. The reality is that you need a specific distribution of volume fwd and aft to minimize power required at a given speed. This volume can be had by using full section shapes fwd and aft along with plenty of rocker. It can also be had using fine section shapes fwd and aft with no rocker.

The measure of volume distribution fwd and aft is the prismatic coefficient. The optimum coefficient varies with speed. This may present a problem to you because even a modest trolling motor has more power than a rower (however the prop will limit the transfer of this power in many circumstances).

Here are some optimum prismatic coefficients based on speed to length ratio (S/L). This ratio is defined as the speed in knots divided by the square root of the waterline length in feet.

S/L = 0.8: PC = 0.53
S/L = 1.0: PC = 0.52
S/L = 1.2: PC = 0.58
S/L = 1.4: PC = 0.64
(From Taylor's Standard Series)

You can see that optimum distribution of volume varies greatly in the speed range you are interested in. Most rowboats operate around the S/L ratio of 0.8 to 0.9. Most low powered motorboats operate in the S/L ratio of 1.2 to 1.34. Most likely, an electric launch would operate between these two examples, probably in the S/L ratio of 1.0 to 1.2.

This means boats designed to be rowed or motored are not likely to have optimum volume distribution fwd and aft for best efficiency under electric propulsion. A good rowboat would likely squat and have higher drag than optimum. A good motorboat would likely have more wave making drag (and probably more wetted area drag) than optimum.

Your best bet is to choose a design intended for the amount of power you want to use. If well designed, it will not matter if it has rocker or not.

David Mancebo

[boylesboats]

A mud motor will work quite nicely on that....

[3pepper]

thanks for the replies ,
i found the ozark jon boat in http://thelibrary.springfield.missouri.org/lochist/periodicals/bittersweet/wi73b.htm
and i guess my question was more general to give me a better understanding of hull design and power requierments .

i do plan to fish and hunt from this boat in a river and lake . the catawba river in s.c. and several of it's lakes . jon boats and bass boats are the norm here .

i liked this design because its built with simple plank construction , no plywood and no epoxy . i have access to high quality yellow pine at a very low cost , .40 /brd ft. and i don't care about weight as long as it beats the current with a trolling motor . this boat will live on a bass boat trailer i already have .
i would like to be able to stand up to cast and bowfish .so i will add a deck to the bow .nothing huge just 2 1x12s set on the shear .
the design in the link is 3 feet wide and 16 feet long , i would like to increase the width to 4 ' if possible , but not mandatory . and maybe use 1/2" thick lumber instead of 3/4 .
i have built several scale models just to give me something to look at in 3d and to satisfy my cumpulsive need to build .

this is not the only boat i will build , i have built several wood kayaks , both stitch and glue and strip built . i also built a 12' 6" flat bottom jon out of 3/8 ply , and fished out of it for 5 yrs untill a pine tree smashed it during a thunderstorm .
my tastes have changed over the years . i am a professional wood worker/carpenter and i want a simpler more primative approach to this boat . the pleasure and rewards for me now come from the research , education , and building prosses . and the boat's use is iceing on the cake . and if i don't like it's looks or performance , i will give it away or take it apart and build cabanets with the wood and not feel any guilt or loss .

i hope this clarifies what i seek . i do plan to build a more refined design next year when money and time are more abundant , but right now i just want to explore the simpler and more primative approach and make use of what i have on hand and not layout so much cash.
thanks for all the help , i love this magazine and forum
stephen

[Ron Carter]

Rocker!!!

[Thorne]

Wow -- 35" wide -- that is NARROW!

http://thelibrary.springfield.missouri.org/lochist/periodicals/bittersweet/1i2p11b.jpg

The plans look very trad but if you want to stand to fish, I'd try to find alternate plans that have the width you want rather than modify these.

The weight of the solid-wood boat will of course require trailering, and you'll have to forego any sort of portaging, but it will be nice and stable in the water when compared to a ply boat.

Good luck!

[boylesboats]

Wow -- 35" wide -- that is NARROW!

http://thelibrary.springfield.missouri.org/lochist/periodicals/bittersweet/1i2p11b.jpg

The plans look very trad but if you want to stand to fish, I'd try to find alternate plans that have the width you want rather than modify these.

The weight of the solid-wood boat will of course require trailering, and you'll have to forego any sort of portaging, but it will be nice and stable in the water when compared to a ply boat.

Good luck!

A "Pole Boat"

[boylesboats]

Jim Michalak has a ply version of an Ozark jon boat. This is from his write-up.
=============================
http://www.duckworksbbs.com/plans/jim/ozarkian/ozark.gif

The traditional Ozark boat is in some ways better than the aluminum canoe. For one thing it carries a huge load, being 20' long, 4' wide, and having a dead flat bottom. It is a drifting barge. Godsey gives the basic use: load with fishing guide and "sport" and gear to float 125 miles from Galena to Branson, Missouri, (first home of the mythical Jed Clampett), in five days. No motor, just a paddle for the guide to steer with. At Branson it all got shipped back to Galena (just 24 miles away by land) by rail for the next trip. In the old photos the amount of gear carried is enormous. The boats have seats in the ends but the photos show the people sitting very comfortably in canvas director's chairs.
http://www.duckworksbbs.com/plans/jim/ozarkian/ozark1.jpg

Construction in 1956 was what one could expect. You start with something you might not find at your lumberyard today - five "clean" pine boards 1" x 14" x 20' long. Three boards make the bottom with tongue and groove joints and one board makes each side, so these boats aren't too big on freeboard

Welcome to Missouri........:D

[Bruce Hooke]

At 35" wide this boat is akin to a canoe. The down side of that, of course, is that the boat is not as stable as a wider boat. I am quite comfortable standing up in my canoes, especially if I have a pole in my hand (a pole used for moving the canoe, not the fishing-type pole), but many people are not so comfortable standing up in a canoe, and I should note that I pretty much only stand up in my canoes in relatively narrow, shallow rivers. Out in the middle of a lake, with waves and motor boat wakes would be a VERY different matter. Also, with a pole in my hands I have an additional balancing aid and a "third leg" whenever the pole is on the bottom. On the other side of the coin, a flat-bottomed boat may be a bit more stable than a round-bottomed canoe.

NOW, the obvious way to address the issue of stability is, of course, to make the boat wider. This pretty much turns the boat into a jonboat-type craft. The problem with this is that anyone who has rowed a jonboat knows that they take a lot of work to move through the water. A small motor is going to be much less happy pushing a wide boat against a current than a narrow boat. Of course a key question is how much current. A slow, small, lazy stream is obviously very different from a faster, larger stream. Note, as well, that as streams grow into rivers the current speed often increases. This can be less than obvious if there are no obstructions in the channel to create rapids, but try pushing upstream on a large river and you will see the difference very quickly.

It should be noted that on the drawings posted above it looks like the 35" dimension is the length of the center thwart, not the beam, so the actual beam will be wider.

[Bruce Hooke]

i would like to be able to stand up to cast and bowfish .so i will add a deck to the bow .nothing huge just 2 1x12s set on the shear .

There are two problems you need to watch out for with this:

1. Since this boat has rocker, moving too close to the bow will depress the bow and lift the stern out of the water. This will make the boat much less stable.

2. If you are actually hoping to step up onto this bow deck, forget about it unless you have the balancing skills of a ballet dancer. Keeping in mind #1 above, you may be able to get away with putting one foot up on the bow deck, but that's about it. One foot up does not raise your center of mass that much.

Of course all of this also comes back around to the issue of width and lateral stability, because if the boat is too unstable to stand up in then this is all a moot point...

[Cuyahoga Chuck]

So, it's a paddleboat. If you would have posted this up front we could have cut to the chase in a hurry.
A few comments. It may set your heart on fire because it's "traditional" BUT, as a paddleboat it's going to be a slug. And it will weigh a ton even tho' it's only 35" wide. If you build it in the traditional manner, with nails and cotton caulking, it will loosen up and leak in a hurry. I notice the practicioners in the article are using some kind of modern caulk and resin glue which is only slightly removed from epoxy. So much for tradition.
To me this boat is a celebration of old timey Missouri boatbilding but as a versatile water conveyance it leaves something to be desired. I would not want to paddle this baby any distance against the current and I wonder how it would handle dodging snags going downstream.
Converting it to power and making it wider is easily done but how it will work out can only be guessed at.
If I wanted to go traditional I woud choose "White Duck" ( the plans are at
http://www.svensons.com/boat/ ). It a traditional cross planked bottom rowing skiff 14' x 48" that has been around for a couple of hundred years. It's commodious, stabile and has a transom to hang a motor on. And it's wide enough for oars if you wanted to go that route.

[mcdenny]

I'd make it 4' wide too. At a 4:1 BL ratio its still finer than most of whats at boats-r-us. I'd change those angled bottom pieces fore and aft to make the bottom one smooth curve (there's your rocker) with the middle 8' feet of the bottom being almost flat and the radius of curvature increasing as you go to the end so as to rise to the same height as the angled segment in the design. I'd angle the fore and aft transoms outward about 15 degrees so they knock a wave more down than up.

That's aboat meant for calm water and very low power. One or two hp will drive it as fast as its ever going to go. It's a good design for a simple electric trolling motor.

Besides, who here ever saw a design he / she couldn't 'improve'?

[Bruce Hooke]

So, it's a paddleboat. If you would have posted this up front we could have cut to the chase in a hurry.
A few comments. It may set your heart on fire because it's "traditional" BUT, as a paddleboat it's going to be a slug. And it will weigh a ton even tho' it's only 35" wide. If you build it in the traditional manner, with nails and cotton caulking, it will loosen up and leak in a hurry. I notice the practicioners in the article are using some kind of modern caulk and resin glue which is only slightly removed from epoxy. So much for tradition.
To me this boat is a celebration of old timey Missouri boatbilding but as a versatile water conveyance it leaves something to be desired. I would not want to paddle this baby any distance against the current and I wonder how it would handle dodging snags going downstream.
Converting it to power and making it wider is easily done but how it will work out can only be guessed at.
If I wanted to go traditional I woud choose "White Duck" ( the plans are at
http://www.svensons.com/boat/ ). It a traditional cross planked bottom rowing skiff 14' x 48" that has been around for a couple of hundred years. It's commodious, stabile and has a transom to hang a motor on. And it's wide enough for oars if you wanted to go that route.
I disagree.

No, it's not going to be a racer. It's not meant to be, but I don't think it will be a slug, at least not if the width is left as is.

Given any sort of reasonable care it will not loosen up and leak in a hurry. Furthermore, using a few modern poly products, if that is the preferred route, is not the same as making an epoxy-based boat.

Oars (as in White Duck) are, in my experience, pretty much useless for narrow, winding rivers. This is the place for paddles, or better yet poling.

By the way, for reference, this is White Duck:

http://www.svensons.com/boat/?f=RowBoats/WhiteDuck/WhiteDuckA.jpg

[Bruce Hooke]

I'd make it 4' wide too. At a 4:1 BL ratio its still finer than most of whats at boats-r-us.
Every bit wider is going to make it that much harder to push through the water. Those boats-r-us boats are designed for an outboard large enough to get the boat up on a plane, so I'm not so sure they are a good reference point.


I'd change those angled bottom pieces fore and aft to make the bottom one smooth curve (there's your rocker) with the middle 8' feet of the bottom being almost flat and the radius of curvature increasing as you go to the end so as to rise to the same height as the angled segment in the design.
Ummmm...Why? He wants to be able to stand near the end. Keeping the bottom dead flat out to near the ends will make this easier to do because the boat will have less tendancy to cock up.


I'd angle the fore and aft transoms outward about 15 degrees so they knock a wave more down than up.
A good idea, but this will also make it much harder to mount a motor on the boat.

[Lewisboats]

Personally I would increase the beam making the bottom 4 ft wide and the ends correspondingly wider. With the flare you should come out with a max beam of about 54-56". That will give you decent initial and reserve stablility...certainly enought to stand up and do some moving around with comfort. As to the deck forward... Bruce is right... this isn't a Bassboat and you don't have the beam or the buoyancy up front to stand on the deck with any kind of safety. Built out of 1/2" planks, this will be quite heavy, enough to make one-manning it a nice workout. The weight will help stabilize things tho when you move around. The trolling motor will move it along provided there is rocker aft to get the transom out of the water to reduce suction, and of course you will want enough rocker forward to keep the nose up out of the water. Send me an email...I have pictures and offsets for a similar boat in ply that could be adapted to plank construction.

Steve
numbaoneman@aol.com

Steve

[3pepper]

thanks again for the input .
a consolation would be to have swivel seats mounted on the 1 x 12 seats and only stand up when stable . i have 2 left from the smashed 12 footer . at 12 ' long and 35 " wide and 150 lb ( i way over built her ) it only drafted about 3" with me and fishing gear . i could easily paddle up stream with the hydro dam running and the current just slipped under her . but with a second person the transoms were in the water and she did not want to move at all . i could not only stand up , but you really had to fight to try and turn her over .


i need to estimate the weight of the finished boat options and that will help decide if the extra width is worth the extra weight . standing up would be nice but not mandatory . and i am sure the narrower lighter hule will eqate into longer battery power and easyer controle with a paddle when going down stream .

i noticed the builder glued the bottom to the sides with resin glue . would it be better to use a bedding compound and ring nails ?

thanks again
stephen

[Bruce Hooke]

By swivel seat do you mean something that is basically at the level of the seat, or one of those seats that lifts you up a foot or more above the seat? The former should be no problem. The latter create pretty much the same issue as standing up, plus you add the weight of the seat, which further reduces stability...

[3pepper]

these are light molded plastic with a lazysuzan and mount directly to the wood seat with no riser and they weigh only a pound or two
s

[Bruce Hooke]

Sounds good. One point to keep in mind is that at least with a boat that is only 3' wide, you won't be able to do a whole lot of swiveling without your legs hitting the gunwales...