How much deadrise and rocker for kayak?

[Brian Fredrik]

I am designing a hardchined Plywood kayak for slow moving river touring. It will carry no more than 200 lbs. Good directional stabilty and ease of paddeling (read speed) are important and I am shooting for about 12 ' loa and 2 ' B max. The kayak will have reasonably high freeboard as windage is not too big a factor but keeping dry is. I am searching for what a moderate deadrise angle should be at the middle station and how much rocker I should incorporate into this design.

[JEM]

depends a lot on the shape of the kayak, how many panels, span of the panels, etc.

[Brian Fredrik]

I assume that panels are the total peices of plywood. If this is correct than I am shooting for four with a single chine and a "v" shaped hull.

[JEM]

Just as a general guess, I'd say stay somewhere between 5° and 11°. 5° if you want more stability. 11° will be fast, but twitchy.

Also consider that the more the angle you put into it, the less you'll be able to displace. 12' x 24" isn't going to displace a whole lot without sinking the hull deep into the water. That will kill your speed.

[Raggi_Thor]

If you know the length and the displacement you can estimate the midship section area, then you camn play with it, adjust beam and draft.
If Lwl=4m and Displ=0.15tons,
then with a Cp=0.5 you get an area of 0.075m2 or 0.8 square feet.

[Brian Fredrik]

Thank you both very much. I think that I may alter my plan to increase LOA as I am scarfing two peices of ply anyway. I will also give 8 degrees of deadrise as I am moderate kind of guy. As for rocker, I believe I going rather flat. Other kayaks of this ilk seem to have this trait. This is my first project so I'll write back in the spring with my results. Thanks again.

[Thin water]

You won't need much rocker for turning ability with that short of a hull but it can't be very fast at only 12' long. JEM's web site has some nice sit on top and conventional designs.

http://www.jemwatercraft.com/categories.php?cat=5

[Chris Ostlind]

Hi Brian,

Like anything having to with hull design issues, there are a combination of ideas at work here. These ideas have to be brought into some sort of relational balance for the whole thing to work nicely.

It is possible to put a boat on the water in which the various design elements would look to be fighting with one another and still get it to work for you. The details, however, when combined nicely, will actually enhance one another for your benefit.

Before you start cutting wood into pieces, take a look at the work of John Winters at Green Valley Boat Works http://www.greenval.com/ Look at the rocker profiles the bow-on sections and the general feel of the boats shown there. John is considered a Master in the world of canoe and kayak design and it would do you well to absorb some of his approach on your path to your own boat.

John has also written a fairly comprehensive set of articles having to do with the design of hulls of this type. Take the time to read them. http://www.greenval.com/jwinters.html

[Brian Fredrik]

The Jon Winters articles are a great read on design. Thanks. I did also look at the rocker of many boats. I still cannot find any measurement of rocker expressed as ratio, formula or any other quantitative value. Does one exist?

[JEM]

It depends on what your goals are and how you prefer to paddle. Much more depends on the hull design.

12' boat won't track too hard but then again you'll have hard chines. Generally, if you have a little more rocker up front than stern, you can track pretty good and still be able to turn the boat in a reasonable distance.

Open mostly calm water: use less rocker.
Tight streams or bigger waves: use more rocker.

This is GENERAL advice/input. There is no straight forward A+B = C.

[Chris Ostlind]

Hi Brian,

You've walked squarely into one of the issues of boat design in which there is an extreme lack of concensus among designers; How do you measure the rocker of a hull? There are at least five different methods that I know of.

For boats of this type (canoes and kayaks) I like to rest the boat on a smooth level surface. I measure the distance from the floor to the keel line at the point where the horizontal line of the keel makes a definitive curve upward to become the stem. On many boats, this is a judgement call, as the curve can be very long, spending a good deal of time reaching forward as it sweeps up to the bow tip.

Look at the Current Designs Solstice GT model for a better visual understanding of what I mean by this problem.
http://user1039303.wx15.registeredsite.com/

As you can see, the issue can be complicated. Just where does that bow sweep become more vertical than horizontal?

Some designers like to measure the rocker at a point 12" from the waterline intersection with the bow and/or stern stem. They are still working from a baseline relative to the lowest point on the hull. The suggestion is that this method accounts for the weird and the radical forms that exist out there as all boats have to have a waterline. Others say differently, so the discussion continues.

Matt's comments above are accurate in that the rocker of the hull is closely tied to the desired performance of the boat overall. Too much rocker and you will give the boat extreme turning tendencies, as well a give away waterline length which will rob the hull of speed potential. Too little rocker and the boat will likely be a real pig to turn, but more than likely fairly fast for its length.

Most designers of modern hull types like to give their boats a good amount of rocker forward and less aft as previously stated. They typically couple the added rocker forward with an asymmetric hull form (widest sections aft of the CB) By doing this, the boat will want to track due to the asymmetric shape and it will also tend to turn responsively to paddle and heeling input due to the enhanced rocker forward.

Perhaps the best way to get a better handle on all this is to paddle boats of differing shapes and rocker profiles. In this way, you can get a real feel for what is going on with the hull as it moves through the water and begin to draw your own conclusions as to what types of forms yield what types of results. Keep in mind that canoes and kayaks, being very light boats, are extremely sensitive to trim, so maintaining a steady trim relationship from boat to boat will help to keep that variable out of the mix.

When I was learning all about the topic (as if I'm not still coming to a more complete understanding each day) I took pages of notes about each boat I paddled and sat back later, digesting what was going on. Very small changes in form can sometimes yield very big changes in behavior.

I'm guessing you'll have to build and paddle a few boats of your own before you start to form a really solid picture of how it all works together. May I suggest that you re-read the Winters articles once you have built your first boat design and tested it on the water. I did that and this really huge light bulb went on for me. It cast just enough light for me to see my way to my next design and built boat.

I owe a great deal of gratitude to some really talented folks in the design world such as Jon Winters, Jim Antrim, Steve Killing and Skip Izon. Without their patience, suggestions and willingness to share with me, I would have been back to the "two kids on a raft in the local pond" scene very quickly.

If you find passion in the process, stay with it.

[Brian Fredrik]

A set formula for rocker is tough as it may change arc from station to station. I suppose the sheer has the same predicament. I will take your advice and be sure to re read those articles after I build. The important part being..to build. For me, analysis may lead to paralysis. I'll post what I build and how it works out. Thanks again

[JEM]

Sounds like you're on the right track. You get to a point where you find so many questions that it's best to just take a stab at it and see how it goes. Let us know you're progress.

What software are you using? Plyboats is a nifty little program.

[Brian Fredrik]

I will be using a program latter on perhaps. For now, it's a ruler, a pencil, a wooden spline, a T square, maybe a calculator and lots of erasers.