> Has anyone had any experience with translating single plans into
> plans for a double? I am thinking that scaling would be a safe way
> to do it, 10-20% bigger with everything. But I'm not sure how much
> is too much or too little. I am not sure if the boat would perform
> the same with the larger size. How would increasing the dimensions(length
> and width) by 20% affect the volume or displacement?

Chad:

My best references on this come from two books Gil Gilpatrick's book on strip building canoes and George Putz's book on canvas covered kayaks. I'll summarize, but you might want to add these books to your library, anyhow.

Putz has plans in his book for a 17 foot kayak- a single - and also for an 18.5 foot kayak with the same lines, but scaled up slightly. The bigger one is for big people as a single, or as a double.

Gilpatrick writes about a student who was working on a canoe, and mis spaced the forms. Instead of placing them 18 inches apart, he placed them 20 inches apart. He ended up with a canoe that was 18 inches longer than planned, and it worked just fine.

My reading of an old Grumman catalog, and a few other catalogs give a bit more info. The old Grumman aluminum canoes were all 36 inches wide. The only difference was the length, which affected the weight, the displacement, and the carrying capacity.

Lets tie this into your question.

When converting a single into a double (or going the other direction, using a double with a single paddler, which is more common) you have two main considerations. One is balance. You need to put the paddler or paddlers in places where the boat floats level. The second is carrying capacity of the boat. Sometimes this is called displacement, or maximum load. Different designers rate their boats' carrying capacity in slightly different ways. More on this in a minute.

Lets assume some generic boat is designed so that the bottom of the hull (sometimes called the keel, but not all boats have a true keel) is 4 inches below the waterline when it is loaded with a 150 pound paddler. This is called the draft, or people will say ``the boat draws 4 inches when carrying 150 pounds''. Lets also assume this boat is 16 feet long and 22 inches wide. If you have 2 kids who each weigh 75 pounds, then they match the weight of the solo paddler. You can ignore changes in draft. All you have to do is move the seats an equal distance forward and back from the current solo seat. The weight matches, and this seat placement will be close enogh to ideal that you'll just need to fine-tune it a bit to get perfect balance.

to get those seats into these positions, though, your boat will need changes to the cockpit. If you put the seats far enough apart you might find that you will be better off with two separate cockpits. If they are close enogh you could make do with a large singel cockpit, but you hit the problems of paddle blades knocking together, (particularly when the paddlers are not in sync) and the difficulty of making a sparayskirt for two.

Now. let's suppose that your paddlers are heavier. Check the specs on your boat. On this generic one I'm using, let's say the draft increases by an inch for each added 100 pounds, and the maximum capacity is 450 pounds. So, if I have a 150 pound paddler with 300 pounds of gear, the boat will be filled to capacity. The boat will sink deeper into the water as more is loaded on it, until the additonal 300 pounds have lowered the boat 3 additional inches into the water. The bottom of the boat will now be 7 inches under the water line.

If the boat is well designed, it will be a bit slower to paddle because of the slight addition to it's resistance ( greater wetted surface of hull, and probably larger cross section) Its stability will change. If you go beyond the 450 pound maximum load the boat will sink deeper into the water and stands a greater risk of capsizing.

Now, if you have two 200 pound paddlers and you put them in this boat you will have a slightly more sluggish craft -- but all you will need to do is change the seat positions, and cockpit shape as mentioned above. Of course, you would also carry less than 50 pounds of gear.

Now, let's work on eliminating that increased sluggishness. We can assume that the deeper the boat sinks into the water the more sluggish it will be. We can also assume that the wider it is, the more sluggish it will be. These are generalities. You can find lots of info on hull design on the web. Check some links from older posts on this BBS. But before you do, bear with me.

In checking a lot of catalogs and spec sheets on canoes and kayaks it becomes clear that for a given hull width, the carrying capacity increases with the length. Exactly how much it increases is variable, depending on the design, but increases of 100 to 200 pounds per additional foot are common.

Lets assume that I can increase the carrying capacity of my generic boat by 200 pounds for each additional foot. In this case changing the length from 16 feet to 18 feet should give me a draft of 4 inches with a load of 400 pounds. That is 250 pounds more than the first example of a 150 pound paddler. I chose those numbers because the math is simple. Now, I have an 18 foot boat, with a width of 22 inches and a draft of 4 inches with two 200 pound paddlers. I round numbers, with the additional length the total cargo capacity would be somewhere around 850 pounds, and the boat would sink an addtional inch for each addtional 150 pounds.

With this longer hull you'll have more room to space out individual cockpits for each paddler. Just position them to balance the boat.

Now, with a double you may want a slightly wider boat for slightly greater stability. The more forgiving the boat is, from a stability standpoint, the easier it is to avoid capsizing when one paddler zigs and the other one zags.

Assuming you have a boat designed to carry two paddlers. Let's say it has a draft of 4 inches with a load of 350 pounds. If you have a single paddler who weighs less than this then the boat will float higher. Since this is probably a longer or a wider boat when compared to most solo designs (and it would have to be to have the additional displacement) a solo paddler would have a few additional challenges. By floating higher the boat might be more affected by wind conditions, and some wave conditions. Adding weight for ballast can cure these problems. The added weight means the paddler has to paddle harder to accelerate or slow the boat. The greater length might make turning more difficult. With or without added ballast, the difference may not be a lot, but it is there. In cruising you might never notice it. In a race it might be critical.

As an amply proportioned paddler, my personal choice is to build a boat designed as a double, but with a single seat. Second choice is a long cockpit design so I can take one of my kids with me, and send them forward far enough to balance things. You seem to be toying with going in the other direction, so let's get back to you.

Imagine an isosceles triangle with a short side of 2 feet for a base, and 2 long sides of about 9 feet going up from the base. It would look something like an 8 foot tall tipi. Now imagine another triangle just like that, but pointing down. If you join these triangles so their bases overlap, you'll end up with a diamond shape about 16 feet long, and 2 feet wide at the middle. (Substitute inches or centimeters for feet and you can draw this on paper if you can't picture it)

Lets use this diamond shape as a rough outline of a kayak's shape. This boat would be symmetrical, with a length of about 16 feet and a width of 2 feet. The entrance angle would be a pretty sharp point. Now, let's try stretching this boat to about 18 feet. I have three proposed ways to do it.

The first way is to simply lengthen the centerline to 9 feet. The sides would grow, and the entrance angle would get sharper. The base would remain at 2 feet wide.

The second way is to separate the two triangles by 2 feet and then re-connect them with straight lines. If you do this your picture will look like an up-pointing triangle on top of a box on top of a down-pointing triangle. The base of each triangle and the width of the box will be 2 feet wide. In this case the entrance angle does not change, but you are stretching the midsection of the figure, kind of like pulling taffy.

The third way is to extend the long sides of the triangles by a bit over a foot. You still have two connected triangles, and the entrance angle is the same, but as the length has grown, so has the width. Each triangle has gotten 1/8th taller (growing from 8 to 9 feet) and the base has gotten about 1/8th bigger too. It would grow to about 27 inches from 24.

Now, let's look at how each of these ways of stretching my triangles would apply to stretching the plans for a kayak. I'm assuming you are making either a skin on frame, or a stripper. (stitch and glue designs do not seem to be too user friendly for people making changes.)

In the first case we stretched the center line. If you were to do this with a kayak you would make the frames for the short boat you like and increase the space between them when you mount them along the length of the strongback. For example, if your design has a bow and a stern piece plus 15 frames located a foot apart you would set the ends 18 feet apart, and fill in the space between then with the frames set 13.5 inches apart.

It is easy to do the math on the new spacing if you just deal with one half of the boat at a time. Work from the middle of the boat. That is where frame 8 sits. For a 16 footer the bow or stern piece goes 8 feet from the middle. For an 18 footer those pieces go 9 feet from center. The frames for the front, which had to fill an 8 foot space before, now have to fill a 9 foot space. You have 7 frames to put in there, so split the distance and space out the frames accordingly.

Lets look at plan two. Still assuming the frames are set 1 foot apart, in this case you would make two additional, identical copies of the middle frames and put them at the middle of the boat.

Assuming the 16 footer is symmetrical, if the frames were numbered from front to back, these frames would be identical: 1 and 15, 2 and 14, 3 and 13, 4 and 12, 5 and 11, 6 and 10, 7 and 9, and 8 is the middle piece. What you want to do is make 2 more copies copies of 8. You can call them 8A and 8b, or you can re number your frames from 1 to 17 instead of 1 to 15. If you renumber the frames, your new center frame will be #9, and it will be a dead ringer for frames 8 and 10.

If you can't follow this in your head, draw a sketch.

In practice, when you set these frames on your strongback you may want to adjust the spacing slightly. Put on a few full length strips and see if the lines look good

Notice that for these first two methods of increasing the size of your boat you have NOT had to scale any pieces. Not a bit. No way, No how. You have either duplicated pieces, or spaced them out differently.

Prospect number three changes thing a bit. For this plan you will have a new width at the midpoint. For that you need another frame. MAYBE. (that's a BIG `maybe').

Let's take a look at our triangles again. In the proposal each triangle had a base of 2 feet, and those bases overlapped. In our kayak, the front half of the boat goes from the bow to frames 1 through 8. Frame 8 is at the midpoint of the kayak and would represent the base of our triangle. The back half of the boat starts at the stern and then goes from frames 15 through 8. We are back to fram 8 again. Now, if we wanted to keep the spacing of the frames at the desired 1 foot interval, but we wanted to add 2 feet to the middle of the boat, we would have to separate the front from the back and fill in the middle. To do this, we start with needing another copy of frame 8.

In the first two examples we established the middle, and the ends, and then we filled in the space in between with frames. In this third case we start at the ends and work towards the middle. The bow and stern pieces are set up to be 18 feet apart (tip to tip) working from the bow you would set up frames 1 through 8 at 1 foot spacing. Then you go to the back of the boat and set up frames 15 through 9 That takes care of all your original frames, and you have a 3 foot gap. You take the duplicate of frame 8 ( lets call it 8A) and place it a foot from frame 9. Noe the front half, and the back half of your boat have the same width (whatever the width of frame 8 is) and there is a 2 foot space between 8 and 8a.

You have two options here:

Either construct a new middle frame, which will go on your strongback halfway between 8 and 8a (it will be a foot from each) and be slightly larger than frame 8.

Or, don't bother constructing a middle frame. With a bit of care your strips can easily bridge the distance between frames 8 and 8a (it is only 2 feet). They will naturally bend to give a smooth line to the center of the boat, with a slightly wider midsection. If you use bead and cove strips, they should mesh well enough to support each other over this gap. If you use straight edged strips, you can use 1/4 inch staples to keep them together. If you are working with strips that are not full length, fill the center of the boat first, bridging this gap, and put the smaller pieces and the joints closer to the ends.

If you insist on making a center frame you will have to decide how big it should be. You can do this with a fancy computer program, or you can do it by hand, which may be faster and easier. I'll describe how.

You mount your frames to the strongback as described above, with a gap where the middle frame will go. Mount a rectangluar piece of plywood at the point swhere the middle frame will go. This piece of plywoow should have pencil lines drawn on it which are horizontal, parallel, and 2 inches apart. Frames 7, 8, 8a and 9 should be marked at the waterline, or the top chine, (gunwale?) depending on which your design aligns on. The should be a horizontal line drawn on the frames connecting the waterline on the left side of each frame to the same height on the right side of the frame. measure up and down from these marks, and draw parallel lines 2 inches apart.

Take a straight strip (it can be scrap) which is at least 6 feet long, and staple it to the frames at the middle of your strongback, so that the top edge of the strip touches the waterline mark you made on frames. Put a similar strip on the other side of the boat. The top of these strips now will define the waterline for the form you will make. Clamp a straight strip (or a yardstick or meterstick) to one of the straight lines you have marked on the rectangular piece in the center and move the entire board to align it so that the strip on that board lightly touches the waterline strips. Measure the distance from the inside of each strip. Write it down.

Now go down and up in 2 inch increments. Tack strips to the forms to line up with the marks you have there, and measure across the distance between them, using the marks on the rectangular piece (that is mounted where the frame should be) as a reference.

It may be convenient to cut a strip of cardboard or thin plywood which is 2 inches wide, and just use this as a gauge. If yo are working in metric. Use 50 mm.

If you have enough scrap strips, leave them on.

Get a piece of cardboard or thin plywood which is roughly the size of the frame you will make, draw a grid on it, with 2 inch squares ( or 50mm) and plot the numbers from your measurements. Using a thin strip to connect the dots, draw out the shape of the new frame. It will probably be wrong. Cut it out and try it in place. Stretch as many strips past it as you need to determine high spots, and low ones. Sand or cut down the high spots, shim up the low ones. When you have it right, transfer this pattern to a more substantial piece of wood and cut out your new center frame.

Measure down from the water line to find your new draft. Measure across the waterline to find your new width.

Strip away. Send pictures when you are through.

You have not scaled a darn thing, but your boat is larger.

If you want a boat that is 3 feet longer, or your plans are for a 17 footer and you want a 19 footer --> Same process, different numbers.

sorry to go so long, but there is probably enough info here to help you and many others build something, and I didn't want to stope once I got rolling.

Hope this helps. Wish I had pictures. It would have meant fewer words.

Paul Jacobson