My first attempt at a kayak design. Review please!

[waxenate]

A little back ground to help with getting feedback. I'm pretty much a novice so don't laugh at my design. I am @200lbs and would like a kayak that I can use on tidal rivers and inter coastal water ways and on the harbor. I live near Charleston SC to give you an idea of what I mean by harbor. It can get choppy at times but mostly pretty calm. I would also like to be able to make a run out to one of the small barrier islands also. No real sea trips. Basically I like the look of the guillemot that schnade designed. I would like something relatively stable and with a decent track line and the potential to be quick. (straight line)

So please feel free to comment and make suggestions.

Thanks

Mike

[anewhouse]

You will need more stability. Your maximum righting moment is less than 9. You need something closer to 15.

Your sheerline is too low. It is already touching the water even without leaning the kayak.

Move the waterline up until the target displacement and the design didplacement are almost the same.

[waxenate]

Thanks! I did a little more playing with an earlier design i tried. I have it at about 13 on max righting angle. Take a look?

Thanks

[anewhouse]

The 5lb of cargo weight you have is making your stability look better than it really is. When you don't have that ballast on board, your maximum righting moment is less than 12. That isn't really enough for an inexperienced paddler in slightly choppy water.

Try doing most your your design work with no cargo weight. Otherwise you can get an unrealistic impression of the stability.

Your hull cross section doesn't help the stability much. A slightly flatter bottom with a shallow V is usually better for a general purpose recreation kayak.

Your sheerline is still very low. Having water running up over the deck in choppy conditions can make the handling a bit unpredictable for a novice paddler.

Right click in the plan view ans set the exaggerate to 5. You will see that the curves are not smooth.

Your kayak is 17' long, but the waterline length is only about 14' 9". Quite a bit of the length is just sitting up in the air. It looks fast, but actually costs you some potential top speed.

That amount of sweep up at the stern might look sexy, but it doesn't help the performance and it makes building a bit more difficult because the strips have to bend so much.

[waxenate]

I really appreciate the input. Starting to think maybe for my first build I should just go with an existing design. This is fun though!

[woodman]

Here is a good explanation of how stability works in a design...
viewtopic.php?f=2&t=1630

[flyinglow]

Don't give up, everybody starts somewhere, and this is a great place to start. I had no idea how many little things go into a good kayak design starting out, and this forum(and Anewhouse in particular) helped me out a ton. Check out some of the other design reviews in this forum, you can learn a lot from what others have done before. Mine went through weeks of revisions, but i ended up with a boat i'm really happy with.

general design ideals: smooth curves, Smooth lines from hull control lines, appropriate stability, solid tracking.

Personally, i don't think you're too far under the stability you need, but a SF of 95 or 100 would be more comfortable for a beginner. If you put your mind to it, eventually you'll get comfortable with just about anything, but the idea is to make the process fun instead of intimidating.

A bit of overhang at the bow and stern can help with bigger waves, but in flat water long overhangs above the waterline just shorten your waterline, effectively shortening the boat and not really doing anything in terms of performance. Basically, the more of the boat that's in the water, the faster it'll be. You want enough angle on the bow to shed weeds, and keep the bow above water under most conditions. Too much overhang will make the boat slower in flat water, and cause rough handling in extreme conditions. For a 17' boat, i'd try for a waterline ~16' long, it won't look as fast, but will paddle faster. My 16' boat has a waterline of 15'9", and outperforms most 17-18' boats on flat water, simply due to its waterline length. It's only what's in the water that counts.

Sheer line: Should really be a few inches above waterline, this enhances secondary stability, and adds volume/load carrying capacity. Too much volume will increase sail area(problems in windy conditions), but 2-2.5" AWL at the cockpit might be a good place to start. As Allan said, once the sheer line drops below water, the boat becomes much harder to predict. maybe 15 degrees of tip before the sheer line goes in would be a good baseline(look under stability tab)

Tracking: CLA should trail 2-3" behind LCB, and lcb should be around 52-53% as a general rule. There are exceptons to most of these rules, but starting out, it's best to follow the rules until you understand them and make a conscious decision to move outside of "normal"

Keep with it, Designing and building a boat is totally worth it.

[woodman]

I have a question..where is the percentage of the LCB listed in the hydrostatics?

[anewhouse]

You have to expand the centroids sections to see it.

[waxenate]

ok...tinkered some more. Cleaned out some "Designs" I had tinkererd with to keep my brain from scrambling lol. Review this?


Thanks

[anewhouse]

Right click in the plan view window and set the exaggerate to 5. What you want is smooth curves. What you have is a series of fairly straight lines with sharp turns.

With the CLA so far in front of the LCB, the kayak will want to keep veering off line. An exaggerated explanation of why that is a problem is that it makes the kayak behave a bit like an arrow with the feathers at the front and the weight of the head at the back. It won't want to go straight.

While you work on the design, keep moving the waterline up or down so that the target displacement and the design displacement are almost the same.

[waxenate]

WOW! NEver would have figured that one out! lol I did the exagerated mode and made some changes. Took your advice to keep adjusting the waterline. Ended up with a huge improvement in stability..or at least i think i did.

LCB? What is that? Couldnt find it.

thanks

Mike

[anewhouse]

WOW! NEver would have figured that one out! lol I did the exagerated mode and made some changes. Took your advice to keep adjusting the waterline. Ended up with a huge improvement in stability..or at least i think i did.
No. You only have about half as much stability as you had previously.
LCB? What is that? Couldnt find it.
If you expand the list of centroids in the hydrostatics, it is the first one in the list called the Centre of Bouyancy.
thanks

Mike

You still have the LCB behind the CLA. Think of the LCB as being the centre of gravity and the CLA as the centre of the resistance to being pushed sideways. If the CLA is behind then the kayak will tend to go straight and if it is in front, it will want to turn. Of course in real life, it isn't really quite that simple and it is only a tendency, not a guarantee.

There are a few other things about this design that are less desirable than some of your previous ones. Have a read of some of the comments that people have offered about other people's designs and look at those designs. Then try to work out which features of other designs would suit you and try to incorporate them in your design.

[waxenate]

You would not believe how disgusted I was when i woke up and read your reply! Hahahah! I uploaded the wrong design! Apparently I didn't delete enough plans yesterday! lol

So have a look at this please!

I'm pretty sure I have corrected the problems you pointed out. So if The LCB is slightly ahead of CLA what effect does that have. Is it better to be slightly ahead of or behind the CLA?

Thanks again for all your input!

Mike

[woodman]

You have to expand the centroids sections to see it.

WOW this type of info regarding tracking and stability should be pinned ..