Own design "kayak" plan

[labr@]

Have been looking at kayak plans for several months now trying to figure out what to build next. Initially wanted something a little longer than my Laker and in a cedar strip design but there are very few in the length/beam range I was looking at.

After many moons of indecision I finally decided that my next build would be a paddle craft primarily for fishing and that I would design it myself. So the brief ended up something along the lines of -
Short
Stable (very)
Square stern – my idea was that this would make for easier re-entry when tipped out
Light weight (a heavy boat won't get used)
Speed not important but try to maximize ease of paddling at lower speed
Reasonable tracking
Low enough not to catch the wind too much but enough knee room for an inflexible paddler

So, using the Kayak Foundry program I have done the iterations looking for the best compromise between speed/stability etc and come up with a design that I think will work. Printed the form shapes out on the weekend and they are all ready to stick onto chipboard.

Before I go ahead and actually spend money / use up materials it seemed like a good idea to run this past some people that actually have an idea about such things. Not expecting anyone to make decisions for me or give an in depth analysis but there’s this feeling that I may have missed something obvious.

To give an idea of scale, length is 11ft, beam 25.5 in, height at front of cockpit is 13 in.
The hull forms into a small skeg at the transom to help tracking.
Have tried to keep reasonable volume in the rear so I can climb up over without it going under.

Here are line drawings of plan, profile and section views. Any comments?

http://img267.imageshack.us/img267/1674/planprofileoutlinedwgs.jpg

http://img695.imageshack.us/img695/2870/offsetoutlinedwgs.jpg

[Boatmik]

Howdy,

I like the boat ... very cute.

The most significant problem I can see is that the transom is immersed with a big wide flat area below the water. This will make the boat quite draggy. I would go for something around the same transom width but keep the transom above the water except for the little finlike shape at the back of the boat.

That finlike bit might be difficult to plank, but not impossible.

A bigger problem is the way the sides of the boat fair into the transom with a radius. That will be way too tight for any timber strip to be able to make the bend. Better that the juncture around the edges of the transom to the rest of the boat is square.

The side view rocker makes it look like it has a little bit too much depth forward compared to depth aft - your instinct to create a little fin at the back to counteract this is good. If there are problems with the steering becoming unstable when there is wind and sea from behind you can add a timber skeg or a rudder.

I do like the profile.

Sometimes a longer boat can be easier to get up on roofracks, but that is not a big problem.

It is a nice looking boat.

Best wishes
Michael

[labr@]

Thanks Mik, I think you've spotted the major problems with uncanny accuracy!

The drag induced by the transom is part of the reason for the amount of rocker - I wanted to get the rear to come up as close to the water line as possible to reduce the effect. It looks worse from the side profile view because this follows the skeg line but I agree it will still have some effect. It's a trade off I was prepared to accept but now I might go back and play with the stern a little and see how much the drag changes with the transom right out of the water. If it's significant it would be worth doing.

As for the transition from sides to transom I thought I would square this off by planking straight across and keying in to each side by alternating the overlap - a bit like doing the bow but at 90 degrees. The software is limited in it's ability to handle such shapes so I put in a bulkhead at 2 inches from the transom to use as the final form and plan to run strips across past the end of the stern form.

Off to play with stern rocker now :D.

[b.o.a.t.]

G'day Bob
nice lines
I'm a sucker for a wineglass transom, but being a lazy sod, I'd do the stern
rounded & just glue a skeg on. Concur with MIK about lifting the stern as
close as reasonably possible to the waterline. Hard to do as in such a short
boat, you really want all of it to be carrying weight. The kick down in the last
300mm is a good idea. Should help to keep her nicely trimmed whilst being
dragged all over the Gulf by a big snapper.

I don't know how wide a kayak needs to be to be stable enough as a fishing
platform. The commercial ones seem to be around 30+ inches, but being
sit-on-tops, the seats are set quite high.

What happens to the numbers if you go a few inches wider?
Am guessing that the weight won't go up much, but the stability should
improve significantly. Also, being wider might lift her up 15mm or so, & save a
redesign of the rocker aft?
cheers
AJ

[labr@]

whilst being
dragged all over the Gulf by a big snapper.


The way my fishing's been going lately I'd be happy to be dragged onto the rocks by a leatherjacket :(

Thanks for the input. General concensus seems to be that the main problem is the transom in the water. One thing I forgot to mention is that the waterline shown was for a weight of around 240lb - I think that unless that elusive snapper does come aboard (with some company) the all up weight is going to be closer to 200 so it should be a little higher than shown in normal circumstances.

Stability is something I'm not going to try to improve. Maintaining the beam so far back gives heaps of righting moment (hey I used a technical term!) so both the initial and secondary stabilities look really good. In fact compared to my Laker I think it approaches "extreme" and I can sit and fish from the Laker without problem in flat conditions. If it feels as stable as your Blue Boat it will be good - any better than that will be great :D.

[Off topic: We had a microwave combustion event last night :~ - if anyone has knowledge of good ways to reduce the smell of very nasty smoke in a house (through carpet, clothes, furniture etc) please PM me. Thanks.]

[anewhouse]

I am surprised that I haven't seen your design posted on the forum associated with the software you used to design your kayak. There are people there who have designed a lot of kayaks and who understand how the Kayak Foundry program can be used to get the design you want.

Neither of my browsers shows your plan or profile view, so there is a limit to how much I can help. However I believe there are some issues with your design that you could improve.

You have a hull shape that doesn't try to maximise primary stability. For a fishing kayak, that might not be ideal.

You suggest that you want a lot of volume in the stern to make it easy to re-enter in the water. It is easier to climb onto the back of a kayak if it sinks low in the water.

There are ways to maintain the tracking without the very real construction problems associated with you current design.

It would be helpful if I had the .yak file produced by the program to see what might be done and the effect of some of the changes I think would improve the design.

[labr@]

Thanks Allen,

Will have a go at putting the yak file on the KF site tonight.
After seeing it you may find it is more stable than you expect - but then again maybe not :rolleyes:.

Any methods of achieving the end with less work are welcome.

[anewhouse]

I'll keep an eye out for it. :2tsup:

[labr@]

Well I've had some discussion over on the KF forum and decided the standalone skeg is the way to go. So that's one issue out of the way.

It's also been pointed out that CLA should be behind LCB but I'm having trouble finding out what CLA means. "Centre" is easy enough, as is "area" but what does "lateral" actually mean in this term? To me it suggests a horizontal section through the hull at waterline level - but it doesn't seem to make sense. Can anyone supply a definition of the term 'cos I have done several searches and am none the wiser. Once I have a definition I can then work on gaining understanding and maybe sort this little sucker out.

Thanks for your input

[b.o.a.t.]

G'day Rob
As far as I can make out, Centre of Lateral Area is a crude way to guesstimate
whether the boat will behave as if the skeg is at the front or the back. And by
how much. Will it docilely go where it is pointed, a if a small tail skeg were
attached, or will it fight the paddler for control every inch of the way, as if
tripping over a large bow skeg.
cheers
AJ

[anewhouse]

Here are a couple of explanations of some of the terms relating to kayak design.

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

Hydrostatics of kayak designs, form coefficients, volumetric and waterplane properties (http://oneoceankayaks.com/smhydro/hydro.htm)

It is possibly easier to explain CLA and LCB if we talk about arrows rather than kayaks.
While it is not a perfect analogy, it is probably easier to understand.

If you add feathers to the back of an arrow, it moves the CLA back and it will tend to go straight. If you add weight to the front, it will move the LCB (centre of gravity on the arrow) forward and it will tend to go straight. So with an arrow, the further the CLA is behind the LCB, the more it will want to go straight.


The CLA on the kayak is the centre of the underwater area you see if you look at the kayak from the side. The further back it is, the more it is like an arrow with feathers in the back.

The LCB is the point of balance fore and aft of the kayak. The centre of bouyancy is a bit like the centre of gravity. The further forward it is, the more it is like an arrow with a heavy arrowhead.

Generally speaking, a kayak with the CLA a centimetre or two behind the LCB will tend to have good tracking. If the CLA is in the same position as the LCB, an experienced paddle will still find it has good tracking and a beginner will probably find it acceptable. If the CLA is in front of the LCB, an experienced paddler will probably like the liveliness of the handling, but a beginner will have trouble keeping it straight.

There are other factors that affect the handling, but that may give you a rough idea of the importance of the CLA and LCB positions.

[labr@]

The CLA on the kayak is the centre of the underwater area you see if you look at the kayak from the side.

THAT's what I was looking for:2tsup:. Thanks Allan.

This implies that if I eliminate the wineglass effect it won't really matter if the CLA is in front of the LCB in the design because adding a skeg later will effectively move the CLA backward significantly.

While playing with the design this morning I found that accentuating the wineglass effect at the rear (to simulate a skeg) moved the CLA back relative to the LCB which tends to confirm this.

The way forward now looks clear - but if anyone else would like to comment please feel free. :)

[anewhouse]

The skeg will certainly move the CLA back, so what you say about it being OK to be a bit forward on the plans is correct.

What you said about accentuating the wineglass shape changing the relative positions of the LCB and CLA is correct, but probably slightly misleading. As well as increasing the area at the back, you are probably reducing the volume at the stern, so the centre of balance or the LCB moves forward a bit. It just depends how you made the adjustment. However, that isn't really important. It just illustrates the fact that almost every time you adjust one part of a kayak design, it has an impact on another aspect of the design or even several aspects.

Anyway the important thing is you now seem to have a clearer idea of what you want and how to achieve it. There was a suggestion earlier that you should lift the transom as high as possible to reduce drag. The problem with that is that you will be increasing the rocker at the stern and that has the effect of significantly reducing the tracking. If you do that, you will notice that the CLA moves forward. You will need to be sure that you don't finish up having to fit an enormous skeg to get the tracking back under control.

When it comes time to add the skeg, one option you might consider is a very long, but not very deep skeg. On kayaks with a traditional stern, several builders, including me, have just extended the external stem that most people fit. If it goes down and forward, you get a skeg that doesn't look as if it was added on and it can easily be reduced in size or even removed without affecting the appearance of the kayak.

[KJL38]

Hi Bob, I agree with what the others have said about raising the transom above the waterline.

As stability is an objective I would suggest a flatter hull in cross section with harder chines. A design you may want to look at for inspiration is the Rockhopper Rh340 Rockhopper 'Coastal' TEK Sport (http://www.teksport.co.uk/Rh340_Rockhopper_'Coastal'.htm)

With regard to the relationship between CLA and LCB it depends on how twitchy you want it but as you say you can adjust it with the size of the skeg. Kayaks that need to be very maneuverable have the CLA a long way forward, examples are slalom and polo kayaks 11110 K1 Kapsl 36 (http://www.vajdagroup.com/products/slalom-racing/slalom-racing-k1/342-11110-k1-kapsl-36) Kayaks Plus Content (http://www.kayaksplus.com.au/vampire.htm) With practice it is possible to compensate for a very twitchy hull but the skeg should make that unnecessary.

Kelvin

[anewhouse]

Competition boats like those benefit from being very quick to turn. Beginners often find themselves going around in circles with no hope of going straight.

Another way of explaining the "twitchy" feeling of a kayak with the CLA too far forward is that the instant you give your attention to the wildlife, the scenery or just to the thought of how wonderful it is out on the water, the kayak will turn sideways. Often it will turn so violently that several correction strokes fail to get it back on line before you come to a complete stop.

Some kayaks will even turn beyond the point of no return if you hesitate for a split second between paddle strokes.

Such kayaks do exist and in the hands of a capable paddler, they can be made to do very exciting things in white water. In the hands of a novice or someone who would rather be fishing, they can be impossible to control.

[labr@]

... in the hands of a capable paddler, they can be made to do very exciting things in white water. In the hands of a novice or someone who would rather be fishing, they can be impossible to control.

Yeah, the first time I paddled a Dancer I did very exciting things on flat water :-

Kelvin, that Rockhopper is surprisingly close to the concept I had in mind at the beginning of trying to design this thing. Do try to avoid posting links to sites showing boats made of that stuff that melts though OK? :D As for stability I'm happy that what I have now is going to be more than adequate, partly from what the software is telling me and partly from experience.

Have been back into KF today and smoothed the stern out so the transom should sit just on the waterline. Took the time to smooth a few other spots out too thanks to bits of info I picked up on the KF forum over the last 2 days - thanks for pointing me in that direction Allan. Things like the exaggerate function and showing the control points in the side view help to create fairer lines.

Hope to reprint tonight.

[anewhouse]

I tried to put this comment in your other post about how to rip the boards into strips, but as that post is over 113 days old, I couldn't reply to it.


I imagine that you will have already ripped your strips by now. I missed your post when it first appeared.

However, my comment might be useful for the next time you build a stripper.

Strips for kayaks and canoes are almost always quarter sawn. That is the growth rings are parallel to the short, 6 mm face. Since they are normally ripped from a 19 mm board, that board starts off back sawn. That is the growth rings are parallel to the wide face.

Most people consider that it looks better that way and you have a lot less trouble planing and sanding the kayak if the strips are quarter sawn.

I am not suggesting that they have to be cut that way. One of my kayaks has several strips that are not quarter sawn. However the vast majority are, for the reasons I mentioned.

That is an extra factor to consider when working out which way to get your strips out of big planks.

[labr@]

Thanks bloke!

Yes I have already cut the strips - and they are relatively rough but will have to do.

They are cut from recycled WRC so there was not much choice about how to orient them within the planks. Many are very straight grained and bend/twist superbly but some have a lot of figure and I wonder if they will be as strong.

Do you have any comment about the differences shown in this photo?
http://img695.imageshack.us/img695/2757/wrcgraincomp.jpg

For future builds the plan is to use the rest of the WRC in combination with new Paulownia but for now I have to make do with what I have. These strips are also almost 30mm wide so some will have to be split down to around 1/2" - but I will try to use them in the wider form as much as possible

[anewhouse]

Having the strips almost 30 mm wide will create two difficulties in some places.

The first problem will be that it will be harder to get them to conform to the curve of the forms. I'm sure you already know that as you have mentioned ripping some in half for the difficult bits.

The other problem is that they will be very hard to bend across the wide face. Fortunately your sheerline is fairly straight. If you had an upswept bow, it would probably be impossible to bend the strip up enough at the end. However you don't have that problem.

There will be places as you work your way down the hull towards the keel where you will find the curve required is just too much for the wide strips. The sooner you can start stripping the football on the bottom of the hull working out from the keel, the easier it will be because you will be using straight strips. When you get to the point shown in the photo of the bottom of the hull, you need to be adding straight strips. Your 30 mm or even 25 mm wide strips will not bend around that curve on the outside. Remember, a strip that is twice as wide is not twice as hard to bend. It is more like four times as hard, or something like that.

You might consider instead of ripping some of the strips down the middle to rip them so that you get something like 18mm and 7mm wide. I am assuming 28 mm to start with and 3mm lost as sawdust. Those 6 or 7 mm wide strips can be useful. On the deck of my last sea kayak in the middle photo, the outline curves were a piece of cake with strips about 8 mm wide. Trying to do the dark bit with a couple of normal 19 mm strips was right at the limit of what I could bend even with the heat gun.

Thin strips can also be used to outline areas of different colours, like on the TK1. I really like that effect. Maybe you don't.

It looks in your photo as if the top strips are quarter sawn and the bottom ones are back sawn. You may need to be a bit more careful planing and sanding the bottom ones. Maybe try to use the ones like the bottom two in places where they don't need to bend much, like filling in the football on the hull.

Do you have enough of a selection of each type of strip to consider something like a panel on the deck of back sawn strips with carefully matched grain, outlined with some thin, contrasting strips and surrounded by quarter sawn strips? If you think having a few different strips will look out of place, don't try to hide them, make a feature of them and draw attention to them. :)