jon boat type- flat bottom or V?

I'm new to boat building but not new to woodworking and composites, so I'm building a boat and in need of some opinions- I'll be building a jon boat that is car toppable (logistics of how i already understand and will be posted in a different thread when i start my build, but it'll be plywood stitch and glue).

the specs:
LOA: 12'
beam; 42"
floor width: 30"
transom height 16"

the question is which would be more suitable as a crabbing vessel- rivers only, so should only have to handle minor chop and passing wakes, but the ability to move around easily under low power (3-5hp motor max) is nice. V or flat?

 

The flat bottom version will be more stabile, but the v bottom would get less pounding.

 

the V bottomed boat would also be just slightly heavier. but if it'll net me a better/ more versatile boat i'm more than willing to sacrifice a pound or two.

 

Is that step intentional?

 

tdt,

I would suggest that you create a hull bottom that is about halfway between the two examples you have shown. This mid-point deadrise angle will give you some chop cutting when looking to attain some speed and have virtually all the side to side stability that you'd get from a pure flat bottom.

I would also suggest that you reduce the amount of bow ramp just a bit so that going forward when alone will not be as much of an adventure.

I've designed a couple of boats (Swamper and Swamp King) that are near to the one you are developing. http://www.lunadadesign.com/swamper-and-swamp-king-jonboats.html Help yourself to any elements that might trigger ideas for your own efforts. This isn't a hustle to sell you any boat plans. Just providing a possible source of stimulus as you work your way through your project.

 

A 12 foot boat with a 30 inch chine beam is going to be a bit tiddly. With the small power that you mention, you will not lose a measurable amount of performance by going either way. Flat bottom is the most practical for such a boat. It will have a somewhat better initial stability. Sobriety is also a good game plan in this short and skinney boat.

Why do need 6 inches of flare on the sides? If you make the boat 42 inches wide then why not make the chines wider than you have proposed? You do not need all that flare unless eye candy is important to you.

 

Flat would be good. To help it track, put a little keel on it.

 

what step are you referring to?

i was thinking of having that much flare in order to 1. cut down on splashing, and 2. make it easier for crabbing since i'd be either pulling up pots or running a trotline along the edge a flare would give me easier access to things where a flatter side would make me hit the side of the boat more often. If there's not much difference in stability and ease of maneuvering i'm not opposed to widening it, although the max beam shouldn't change since it needs to fit on top of my car. I'm also not opposed to shortening the boat if that will make it more proportional, although the ability to carry 2-3 plus gear is a must.

 

Your model v hull looks like it has a stepped hul.

 

ah ok sorry i don't know all the proper terminology yet. It was intentional, however i'm not stuck on it, it was mainly to facilitate ease of building and to maximise floor space. I'm now leaning towards a modified V hull, combining the two

 

Suggesting that you'd like to carry 2-3... along with their gear, is probably asking for trouble. Better to have a bit more boat than you will need, than to have a bit less.

 

Chris has told you the gospel. Here is the reality. The boat that you have suggested will probably be OK for one person who is willing to sit on or very near the floor. With three people, the boat will sink into the water nearly six inches. That will leave 10 inches of freeboard. Succinctly, the boat will be dangerously overloaded. Moving around in such a boat with that load will not be wise. In fact you will need to exhale carefully.

The chine beam is very influential for the initial stability of the boat. The 30 inch chine is more like the dimensions of a canoe than that of a crabber. Make the boat longer, perhaps fifteen feet and increase the chine width to 36 inches more or less (mostly more). Now you can go crabbing with a couple of friends and have reasonable hopes of avoiding disaster or a wet *** at the very least. If you build with plywood you will need to scarf the panels to get to 12 feet. Since you have to scarf anyway, just make the boat longer. If you have a couple of friends to help, the added weight of the extra boat length will not be a problem for loading or unloading. The extra length will not add an awful lot of weight anyway.

 

Not only will you need to sit on the bottom of the little V bottom 30" beam boat, you will also need to have your hair parted in the middle. These are the dimensions of a one person rowing boat, not a crabber.

No good reason to have the bottom anything less than 48" beam and flat over the aft part.. Very practical and beamy enough for a small boat but still not overly so for crabbing.

Look at the boats Chris refers you to for more practical data.

 

optimism has probably been clouding my judgment. i have no problems revising my plans to be a bit more realistic, lengthening the boat to 14' and upping beam to 44" with a chine of 36-38" is about the max i could go though.

there seems to be plenty of small johns in the 9-12 foot range that are made to fit 2+gear, plus i've seen quite a few cartoppable rowing skiffs/dories/etc out there, so while i know it isn't ideal, that's the max i could do, since my boat needs to be car toppable and stored inside (it's the no place to park it that would keep me from trailering)

by my calculations:
me+2 friends+boat +motor=715, call it 750 for extra room. (and i doubt this boat'll ever be used by 3 people, i just want to design it for the load of 3 it'll most often be used solo, sometimes 2 people)
(180*3 +100+75)

750/0.03611 = 19385 cubic inches of water to displace. lets say we want the boat to draw 4 inches.
19385/4=4846.25 square inches.

meaning the bare minimum i need a boat with a bottom area of at least 33.6 square feet, or 10'x33.6".

although my preliminary designs fit this requirement, they're very close, so adding a few inches of width will do wonders to improve the boat's stability as well as displacement (as a 1 inch increase in width nets me a square foot or 144 square inches of displacement) i think i'll have to choose function over form and make the hull a bit wider with less flare. also, crabbing gear for 2 people is less than 50 lbs.

 

All true, but normal crabbing calls for standing on one side of the boat and hauling a crab pot off the bottom. I don't see that happening with such a narrow beam and small foot print unless the crew is very agile. At height of crabbing season, there may be several dozen blue jimmies in there also.