Wanting to build a stitch and glue gheenoe-like craft

Uploaded with ImageShack.us

Here's my current plan. Shaded areas are optional plans for benches/storage/and flotation.

thoughts?

 

Put some rocker in the bottom, say 3" up front and at least 6" in the back, it will go much better with a trolling motor if the transom is out of the water and not dragging.

Also, you're not going to get 3-5 years out of Lowe's lauan, it will delaminate way before that. Best 1/4" cheap non-marine ply I've found is pine BC, if they have that in Texas, but it's heavy.

 

So this is a three piece nesting skiff? It looks good for the first pencil sketch. Shorter and beamier than I thought you wanted at almost 5 ft. One of the slickest things about these is when they fit inside vehicles, flat if possible -does it fit in yours?. The least attractive thing about them is the weight -not good car-toppers. If it needs a trailer for transport it's not so clever.

I wonder if two benches fit comfortably in less than 5.5 ft. I think the forward bench should be for someone facing aft to share legroom. You might want to design for rowing and consider what that would do to your dimensions. If it was mine I would try to make the forward bench be a solid mount for the battery and an efficient rowing seat so that the boat is in trim with one person operating (sitting aft). You could go the other way and mount the battery aft and control from the center but that is harder. It would also be slick if you had wheels that would attach to the rear gunwale at the mounts and store away when you get to the waters edge.

When you have the shape of the waterline and displacement you can add rocker (bow at waterline, transom a little above and enough belly volume for the displacement).

 

The 4' beam shown will provide plenty of stability for one person to stand up and move freely around. A little less than 4' at the bottom suits the 4' ply width.

Outbord motors - including I assume trolling motors - are usually designed for a slight outward rake in the transom, about 12-15 deg is usual.

Have enclosed volume for buoyancy in every section. I notice you placed the breaks at seats which is good as it takes the joint above the waterline.

The midships depth of 12" is very shallow. You need more freeboard for safety, at least 16".

You should provide for backup propulsion in case the trolling motor battery goes flat; oars would be my choice. You can find info on the web on dimensions for rowing. 16" is probably bare minimum but, depending on seat height, much less than about 20" can be tiring as the handles have to clear your knees when the blades are out of the water.

1/4" ply sounds a bit thin , OK for the sides (sheer planks) but the bottom should be thicker, you can get by with 3/8" by reducing the span: the simplest way to do that is glue a wide doubler piece (keel) down the center; outside is easiest as it doesn't require notches in the frames. The transom will need to be thicker to accommodate the motor clamps, 3/4" is minimum.

Flat-bottomed boats can be a challenge to keep running in a straight line, especially when rowing. This is called tracking. I suggest adding a small skeg, although It may complicate nesting.

It is wise to make a model of the boat; this will ensure the sections nest correctly and the boat looks good, it will also allow you to determine if the design makes best use of the standard 4 x 8' ply sheets and if it will fit in available storage space when nested.

BTW you labeled the frames "gunwales" but in a boat of this type the gunwales are one or more reinforcing piece(s) of wood along the top edge of each sheer plank. The oarlocks are attached to these.

 

Don; You did not put dimensions on the section views. Let me presume that you will use the whole width of a ply sheet for the bottom. (48 inches) If, and only if, your total weight including boat, trolling motor,, battery, fishing gear, PFDs, and people.......Then the boat will float in just a tiny bit less than 4 inches of water. You will want to get the transom up above the waterline as mentined by ACP1934 above. Use about 3 inches of rocker at the bow and about four and a half to five inches at the transom.

You have shown the sides at 12 inches above the bottom. That is not enough height. At 4 inches of draft, you will have only 8 inches between you and the wet stuff. Fourteen or fifteen is a better figure and will bring the boat into more normal dimension range. Sixteen inches is not enough at the bow. Go for 20 +/- above the baseline which will make the stem 17 +/- inches tall on account of the three inch rocker you have put in. Fourteen inches of transom height is the minimum to contemplate for the load you have mentioned. Another inch or two is a better deal for a modicum of safety. I'd use at least 15 inches above the waterline, which makes it 15 + rocker = nineteen and a half to twenty above the baseline..

You need enough freeboard to stand some waves or wakes. You will have something on the order of 400 pounds of movable weight in the boat. Movable weight = people, dog, etc. The boat, the motor, and the battery will likely come somewhere near 200 pounds.

I am not picking on your boat design, just trying to help prevent your disappointment or worse.

P.S. You said the the motor was free. If it came with a good battery, that's great. If not, figure on nearly 100 dollars for a deep cycle battery. You will also need a charger......$20 up. Another important hint. Locate the battery in the center of the boat and by all means have it fastened down so that it cannot slide around.

 

Wow guys! First of all let me say thanks a bunch for the input! This rookie builder/designer is very grateful!

acp: Thanks, I will definitely add rocker to the bottom. I had another design that incorporated one and don't quite know why I didn't draw one here... As far as the ply wood goes, I know that my choice is not ideal by any means but I am limited and have also seen boats made of cheap ply last for more than a couple of years with proper maintenance.

skyak: It is indeed a 3 piece but I'm still trying to decide on making it a skiff or square back canoe if there really is a difference. My final design will be small enough nested to fit in the back of my Matrix (hatchback) with the back seats down. You also bring up good points concerning the seats. Its hard to tell from my crappy sketch but the entire front section will be one large compartment with a lower seat facing the rear. As I mentioned, the other two benches pictured are optional and I will just have to see how the spacing works after the boat is built.

ancient kayaker: similar to the point about neglecting a bottom rocker, I failed to incorporate an outward angle at the transom. Next plan will surely incorporate an angle. And the freeboard advice is great. I think I'll change it to be 18" at the transom, 16" in the middle, and 20" at the bow. I think those heights should work ok. I will also build a skeg on it to aid with tracking. And I will have to look at more options concerning the thickness of the plywood.

messabout: I really appreciate the advise and do not interpret it as picking. I will have to buy a battery but have found a shop in Ft Worth that sells refurbished deep cycles for about 40 bones. And I was planning on mounting the battery in the bow compartment.

Thanks again guys! I'll try to update my plans and post them for more thoughts in the next few days.

 

This board gets very 'helpful' when they see a sketch of a new boat.

Measure your car. I don't think what you drew will fit and when you get the right widths it will look more like a canoe. Squareback canoes have transoms to make it easy to mount and float heavy powerful gas motors. If your boat will never have a gas motor you don't need a heavy wide transom.

When you have the waterplane shape you can consider where you can carry mass. Check different conditions, 0, 1, 2 and 3 people. Where do the masses have to be to be in trim? More seats than passengers is undesirable and a movable middle seat will help trim. A negative of having the battery in the bow is when there is nobody in the boat it will float bow down and the battery will not contribute much to lateral stability. The other point is safety/recovery. When you consider freeboard it is not for what looks right next to the dock. What you need to consider is yourself and all your passengers floating along side your swamped boat trying to bail it out enough between waves to get back in. This situation concerns me more because the low bulkheads between sections act as holes until they are above water line. The empty boat needs to float higher than these bulkheads when swamped which won't happen with the battery in the bow. My first thought is a float and battery box in the middle section so with the boat swamped you can make it dump water by forcing the bow down but that will not work because of the bulkheads. The only way to dump is to the side. At any rate this picture will make you much more conservative about flotation and freeboard.

 

The section joints can be located next to the seats with the volume under the seats filled in to create buoyancy chambers, ensuring each section has it's own chamber. The chambers can be part of the structure to minimize added weight and would have more than 400 lb buoyancy in the unlikely event of the boat becoming totally swamped and should keep boat and folk afloat.

If the battery box does not go in the stern section the motor wiring will need heavy duty connectors or it will have to be rolled up: the battery could go in the last buoyancy chamber to simplify wiring.

Load distribution could be worked out using a free design aid like Carene2008 which can be downloaded at
http://www.epoxy-resins.co.uk/Carene2013/carene.htm - it probably has the least learning time of any. The attached file is an approximation of your design, you can load it into Carene2008, adjust it, and use the Hydro tool to place loads and check the trim. Carene has a built-in design when it runs, ignore it, just load this file from the File menu.

 

The only issue I have with building in volume under the seats is that I wouldn't be able to nest the stern section in the middle section. I have drawn up a modified plan that I'll post here soon that only has floatation installed in the bow and stern compartments.
So am I to understand that a bow battery placement is a bad idea? I would think that the weight of the battery up front and me and the motor in the back would balance okay.

And btw the new drawing will show taller sides

 

I'm not saying the bow battery placement is a bad idea, just that the wiring is thick and connectors will need to be hefty. The longer the wiring the thicker it must be or losses will increase. If wiring is removable to avoid connectors at the section joints, it will have to be flex (stranded, not solid) so it withstands frequent handling.

Afterthought: do you really have to put the battery at the bow to balance the boat? I would have thought a battery with adequate capacity would be several times the weight of the motor and wouldn't need to go that far forward, unless you can afford lithium . . .

 

Small boats need to be concerned with meeting big waves. To do this well mass wants to be toward the rotational center and in a wide buoyant part of the boat. When you meet a wave you can go over it with a light buoyant bow, you can split it with taller topsides, or it goes over you -refer to above section on recovery.

The battery is likely 50 lbs or more and needs to be handled gently and upright. I think you want it in a separate box anyway because it makes the nested boat too difficult to handle. The center bench must be removed to nest so why not make it the battery box?

Connections and cables exist for reliable power but are not cheap. By the way this is intended for fresh water is it not?

 

Ok well those are some good points about battery placement. I guess I'll design the middle bench as an inclosed compartment for the battery + floatation.
This boat will only be used in the small waterways and lake coves in the DFW area.

Also, I've come to realize that my plans must involve 4 sheets of play instead of 3. Big bummer to me but its the only way to get the dimensions I desire

 

He is all freshwater up there - about 6 hours to the Gulf coast.

The lakes in that area never get any chop ....



A small gas motor would be easier to manage than a trolling motor wouldn't it?

 

It might be easier to manage, but I got the trolling motor for free. Its a minn kota 42 lb of thrust which should work great I'd think...

 

Freemium always has those little catches ....

Have you thought about ordering Okoume Ply to cut down weight? Or, are you trying outdoor ply?

PS your thread gave me some ideas to digest.