Kayak design help needed

[cjbfisher]

I have identified the need for a small sit on top kayak, and have drawn something up on Carlsons Hulls program. It is 9ft 8in long and 30in wide. I've designed it by looking at the lines of other kayaks, and from ideas that I have in my own head.
I've tried to attach a pdf with the drawings, but I'm not sure if it's going to work.
I'm after a bit of feedback about the design, as I have no design experience.
The cockpit is going to be between the stations at 762mm and 2083mm, and I intend to fit a Hobie Mirage drive for propulsion.
I know that 9ft 8in is a small kayak, and that it's not going to perform all that well, but it is only going to be used in small creeks/canals and only over a relatively short distance. Besides I already have a 4.0m plastic SOT and a 15ft S&G under construction.
I am thinking that I might make this one a strip S&G out of WRC.
Over to you guys with a bit of design experience to give me your thoughts.
Cheers,
Chris

[labr@]

Given the length, ie very short, I'd say the critical thing will be location of the drive unit as this will determine where your weight will go and if it is not very close to the longitudinal centre of bouyancy then the boat will either be nose up or nose down. A longer hull would be more tolerant and if you were using a paddle you could just move the seat but in this case you are more constrained.

Not being familiar with the Hulls program I have to ask - does it give any hydrostatic predictions such as LCB, CLA etc and also what about drag and stability indications?

I notice the shape is Swede form which is considered good for lower drag but since you already accept that this thing will be slow then a bit more width toward the bow might help with stability. Remember that stability is not just about width - more fullness in the bow and stern will give more righting moment. B.O.A.T. designed his 12 foot kayak with Hulls (I think) using this principal and although narrower and shorter than my Laker it is significantly more stable. Yours may be OK as is but if you need more then this is something to consider.

[cjbfisher]

Bob,
I read the thread regarding your design this afternoon. Everything I know about hydrostatics came from there, so my knowledge is very limited. I simply designed with my eye and tried to consider weight distribution to keep it stable.
Hulls does give some hydrostatic info. It definately has the LCB and CLA. The LCB is roughly 3in behind the centre station and the CLA roughly the same distance behind that.
As for drag and stability, I couldn't tell you.
I downloaded Free!ship on the weekend and imported the design into it. The hydrostatic info is available there, but it means nothing to me. It's just a bunch of numbers:?. This is why I'm asking for some help with the design and to understand what all the numbers mean.
A few more things that I haven't considered until tonight, is that I haven't allowed for the weight of the hull and Mirage drive in the displacement:doh:. Adding these to my own weight will push the waterline up higher than the bottom of the transom. Also the seating position will be slightly aft of the LCB, so I may have to lengthen the kayak slightly. I was hoping to sneak in under 10ft, so I must admit that dropping it down to 9ft 8in was probably going to be a bit of a challenge. Extending it out to 10ft will just about put the LCB right under my bum. I assume that this would be desirable.
Any further thoughts or guidance is much appreciated.
Thanks,
Chris

[labr@]

Chris,

Have a look at this site for some explanations of design terms and principles. There is a page of terminology and another on stability among others.

Kayak Design | Guillemot Kayaks (http://www.guillemot-kayaks.com/guillemot/information/kayak_design)

In most cases I would think that the centre of a paddler's body mass would be just a bit forward of the centre of the seat because of the weight of the legs sticking out forward. With a Mirage drive I understand that the position is semi reclining which would tend to move it back a bit - but then you have the weight of the drive up where your feet are to move it forward again. If the seat is "slightly aft of the LCB" as you say then it is probably about right.

If I had to choose one or the other I would rather have the nose up a little than the stern.
Presumably you will fit a rudder so the relative positions of the CLA and LCB are not so important as long as the CLA is not miles in front.

Do either of the programs give you either a stability factor or a stability graph? If so, try to post an image here. One thing that is important to remember with SOTs is that you are sitting much higher than in a normal kayak and even a slight lift can reduce stability significantly.

[cjbfisher]

I'm now up to version 3 of my kayak design.
Bob, thanks for the link to the design page. Some good info there, and it helped heaps.
I've been playing around with it in Free!ship, and learning more each time. It is now 10ft long and just under 31in wide. Displacement of 254lbs with just over 4in draft. LCB is 1404mm from transom, and CLA is 1367mm from transom.
I reckon that I've just about got it right, although a few tweaks here and there will probably eventuate.
I've attached the linesplan, stability curve, and resistance graph from Free!ship.
Anyone got any further ideas/comments?
Cheers,
Chris

[labr@]

Well it looks good.
And the drag figures are as expected - it will be slow, but you already knew that and it doesn't really matter for the application.
What worries me still is the stability. I am not used to that type of stability curve so don't have a feel for what it means - especially the dynamic curve. One thing I can say is that I think the static curve shows you will be back to zero righting moment at a bit over 36 degrees heel. This is a long way over but I would have expected something of this design to be closer to 50 degrees. Maybe the length is a factor, but I'm really only guessing.

I think you need some other input.

[PAR]

I think you should consider shaping the hull more appropriately, as both drag and stability aren't what I would consider acceptable. She's initially stiff, which is typical of this set of shapes, but then her stability falls off a cliff and fairly quickly too. In aggressive paddling, in rough water or crossing wakes, etc. I can see you approaching max RM and having to struggle with the boat's stability, rather then focusing on propulsion or steering. This is fine for fast, low stability shapes, intended for quick propulsion and manuvering in relatively smooth water, not so much for more stable, truncated shapes, where just getting sufficient volume for usefulness is a primary concern.

The basic problems you've identified, so it really depends on what you want and applying the appropriate shapes. For example, if you insist on a boat this short, you can drag some of the hull volume forward, which will decrease the entry's fineness, but will also permit a much better exit, so resistance will be better, as well as stability (probably). To gain some of the entry's fineness back, you can increase deadrise in the forward sections. In fact, I'd use a wholly different set of shapes if it was me. I'd employ a more "U" shaped forward section, with the relatively flat midship sections you have used and couple these with the now diminished volume aft, into steeper "V" shaped sections there. You'd get a fairly fine entry, firm and comfortable midship sections and a "kinder" exit, which will make it easier to paddle and likely a stiffer boat too.

[cjbfisher]

PAR,
Thanks for the reply. It’s great to get another perspective, especially when it comes from someone with your skill and experience.
Firstly, I’d like to explain the way that I came up with the hull shape that I did.
Being such a short craft, I never expected it to set the world on fire with its resistance figures, so I tried to concentrate more on its stability and ability to carry the required payload (200lbs). However, I didn’t disregard the resistance completely. I tried to keep the entry fine so that there was less water being “pushed”, and I kept the transom above the waterline and the bilge panels folded up into more of a skeg shape to eliminate drag. Basically, those were my thoughts regarding the resistance.
As far as stability goes, I tried to keep the maximum beam as close as possible to the seating position so that the LCB would not be too far towards the bow or stern. In my mind, this would keep the trim fairly neutral. The hull shape is reasonably flat through the midships for initial stability, and I’ve tried to widen it above the waterline for secondary stability.

Now for the questions.
Given that it’s a short craft, and there isn’t much scope for moving the seating position fore and aft, wouldn’t moving the maximum beam forward create a problem with the trim and lower the transom below the waterline. I realize that then changing the shape up front to more of a “U” shape would tend to compensate this by reducing the volume of the bow, but then I start to get concerned about the ability to carry the required payload.

Are my initial thoughts for the design reasonable?
Would the changes in shape that you have suggested change the hulls behavior as I’ve outlined above?
I think I understand the static stability. Could you please explain the dynamic stability?

Thanks,
Chris

[PAR]

I'm not sure what to tell you Chris, other then to do some studying. This is a great place for tips, tricks and techniques, even explanation of some technical aspects, though not so much for a dissertation on the complexities in making yacht design decisions. Simply put, your questions are of a nature, that you'd be best served with one (or more) of several texts, rather than piece mealing your way through. I'm not trying to be rude, but there are so many questions. For example, where is your CG (with you on the boat) compared to the CB? What is your MT 1" (or cm), Where's the CB located and do you think this is suitable for it's intended use, Froude number, etc. Generally we look at a number of things when contemplating a design, which usually forces our hand in the decision making process, thereby making the shapes similar in most craft of this configuration.

Considering the cost of a build such as this, you certainly could just give it a shot and see what happens. You'll learn a lot, maybe generate more questions than answers, but a true learning experience, none the less. If on the other hand, you'd like to nail it down pretty close, than some study is in order.

[anewhouse]

Have you considered asking for help on a site dedicated to designing and building kayaks?

There is an excellent free program available here:
Blue Heron Kayaks (http://www.blueheronkayaks.com/kayak/index.html)

It is easy to use and is specifically intended for designing strip built kayaks.

There are also probably more people on the forum there who have actually designed and built something like the design you are working on.

There are certainly people there who can interpret the hydrostatic as they apply to kayaks and there are people willing to offer whatever help you might need.

BTW, your first post mentioned building this as a "strip S&G". Strip and S&G are two different things. Which one did you mean? The shape you have created with multiple hard chines does not lend itself well to strip construction.

[PAR]

KayakFoundry is an interesting bit of software, though not especially impressive. The problem with software is it doesn't tell you if the shapes you've employed are the ones that will suit your SOR. What software will tell you is all sorts of stuff about the shapes you've created, but unless you understand how these affect your desires and needs (an SOR), then it just a bunch of numbers.

[anewhouse]

One of the advantages of using Kayak Foundry is that there are a lot of people on the forum who have used it to design a wide range of kayaks.

As they have actually built and paddled the designs, they are able to convert those meaningless numbers into helpful comments about what those numbers actually mean when you get out there on the water.

At first glance, it might not look as impressive or as versatile as some of the other more sophisticated programs out there, but the number of people from beginners to very experienced builders who have used it to produce some very impressive kayaks speaks volumes for its usefulness.

I would certainly recommend it to anyone starting from scratch with no experience of design software. The support from the community has often helped people who clearly started with no idea of what works and what doesn't.

[labr@]

Allan, a number of people are now building S&G designs out of flat panels made by gluing strips together. This way they get something that looks a little better and is possibly lighter because they can't obtain or afford good quality marine ply. It is known as SS&G - strip, stitch and glue. If I ever build another S&G design it will be SSG using paulownia and cedar strips and I think this is what Chris is referring to..

As for Kayak Foundry, I agree with Allan and personally am very impressed (possibly because I know nothing about boat design:U). A combination of the software and discussion on the right forum enables people to design and build some kayaks that are very successful at meeting the user's requirements. Like most things it has its place.

Chris, if you have any S&G experience it might be worth considering a strip build. Some aspects seem harder but I think they are just a bit more time consuming, and if you really want to design your own then KF is worth a look.

[cjbfisher]

Thanks for the reply guys,
Unfortunately my wife has been in hospital for the past few weeks, so the project has been shelved for the moment. Hopefully i will be able to continue in the near future.
Chris

[Terry Haines]

Thanks for the reply guys,
Unfortunately my wife has been in hospital for the past few weeks, so the project has been shelved for the moment. Hopefully i will be able to continue in the near future.
Chris

Hi Chris: I hope your wife is well now. Are you planning to get back to the boat anytime soon? I have a rotomolded kayak that has the same overall dimensions and it is a remarkably stable little boat, though hardly a speedster. It's principle advantage are it fits inside the minivan and it keeps my legs warm in cold-weather paddling! The design would be of interest to you I think, as it has a very successful compromise of qualities.

It has a narrower entry and exit which makes for good tracking and low drag at casual speeds, with the waterlines swelling out rapidly to achieve the midships waterline beam for primary stability.

The bow and aft portions have considerable flare with the deck overlapping the underwater portions which makes for remarkable secondary stability without incurring additional drag.

Midships above the waterline the side decks have considerable tumble-home which helps prevent the paddle slapping against them, which can be a problem with such a wide beam.

It would a difficult shape to replicate with either ply or strip construction, but thses commenst may give you some ideas of what to aim for. You may find the transom noisy and/or draggy if it becomes immersed. It would be crucial to ensure she sits on her designed waterline. It would help us evaluate your design if you could show the waterlines and/or stations.

One note about asymmetrical designs with the max beam set aft: they tend to slew sideways when you are coasting, so it can be hard to creep up on waterlife for photography purposes for example, although they generally track fine while you are paddling. Some of these boats benefit from a skeg or rudder.

[PAR]

Welcome the the forum Terry . . .