Question resizing a kayak length - pre build

Builder's Forum
1

Hey all,

I have a question about modifying digital kayak plans, and I’d love to get some input from those with more experience.

I have a set of digital plans for a 17-foot kayak. I’m considering building a slightly shorter version—around 16 feet—for my tween/teen kids, who are just getting into paddling. They’re not full-grown yet, and I figure a 17-footer might be a bit much for them to handle right now. That said, I’d still like something they can grow into for a few years.

Here’s what I’m wondering:
Assuming the plans are designed with forms spaced every 12 inches on center, could I just compress the spacing slightly to make a 16-foot version instead? For example, instead of 12" OC spacing, I’d tighten it up to fit the same number of forms within a 16-foot length.

I did the math: 16 ÷ 17 = ~0.941, so that would mean each form would be spaced at approximately 11.29 inches instead of 12 inches.

Is this a reasonable way to scale down the kayak? Are there issues I should watch out for (e.g., hull performance, rocker, symmetry, tracking, etc.)? Would this approach throw off the whole shape, or is it a valid method for a minor adjustment like this?

Alternatively, do you think I’d be better off buying a different set of plans specifically for a 16-footer designed from the ground up? I’m not sure if I’m overthinking this or underthinking it.

Appreciate any insight—thanks in advance!

2

Buy the 16-footer plans.

Yes, you can scale boats. Changing them less than 10% is usually pretty safe, but…

Kayaks are a bit finicky compared to a skiff, for instance. Changing only the length will change the weight and buoyancy distribution so getting it to be balanced may be difficult and balance is critical for kayaks’ performance and control. Changing the length also changes the center of lateral resistance. It could end up not tracking well. And finally, shortening the length will reduce the stability.

Leaving the widths alone while scaling the length will also change the hydrodynamics. It’ll be a fatter and stubbier boat and will not slice through the water as well as the original boat. It will push through and have more drag and probably a lower speed.

If you scale all dimensions by 94%, you’ll end up with 83% of the volume. Since the wood thickness will not be scaled, the weight of the wood will be 88% of the original. So proportionately it will be a heavier hull with a reduced payload volume. And the stability will go down.

If you are an experienced naval architect or some other kind of engineer, you could probably finesse most of these problems within the 6% reduction range, but why take a chance with a special gift for your kids. A poorly performing, unstable kayak that won’t track right could put them off boating for life. Get a set of plans designed for the size you want. The Chesapeake 16LT digital plans are only $79 and I believe that there’s a special deal for building a second one from the same plans as long as it’s for a family member. Check with CLC on that.

Laszlo

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Might be better to scale all the dimensions. But have look at Nick Crowhurst’s Shrike and Vember hulls at cnckayaks.com. The Vember is 16’ with a cedar strip hull and plywood deck. Fantastic British style boat. I have built two. The plans are free and Nick provides the DXF files so that you can have someone with a CNC cut the forms for you. Over 500 Shrikes (plywood stitch and glue) hulls have been built.

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based on research i have done in the past, i was left with the strong impression that you can safely adjust a 17 foot boat to a 16 foot boat through the simple act of decreasing the spacing between forms. i don’t have the sources in front of me presently, but i did extensive research on the topic at the time including talking to nautical engineers on the topic of ‘reductions’ and worked with a naval architect on translating a 17 foot design down to about 15.5 feet for my wife. (Frej special – Howard Spira | Björn Thomasson Design) my research indicated you did not want to exceed 10% reduction in spacing if you wanted to try this technique.

while laszlo mentions some of the factors that come into play in a reduction like this, my research indicated that these concerns may be a bit dramatic with respect to what you would actually experience going from 17 to 16 feet.. The reason that was explained to me that you don’t want more than a 10% variance is that if you exceed that tolerance, those factors could become signficant….but 17 feet to 16 feet is well under that tolerance.

in your post you did not mention a design, and of course, i would agree that finding a design at your target length is better than ‘reducing’ a design to your target but sometimes there is a design that catches your fancy and the designer only drew one version of it…..so you have no choice if you want that design in a smaller size to do some type of reduction.

fwiw, i also recall that simply taking a design and reducing all dimensions by x percent, was not something you wanted to do without a naval architect involved…..the reasons was a bit complicated and i can’t explain it…..which is why my wifes boat had the designer involved in the project….because that project involved more than simply changing the spacing between forms.

all that said, i would highlight that there are a number of high performance kayaks under 16 feet that paddle really well. so while we traditionally think of a 17 foot sea kayak as standard, that candidly is a bit of old news and there are strip build designs like the petrel play (14ft) and petrel sport (16ft), and microbootlegger sport (15.5 ft) that will outperform most traditional 17 foot sea kayaks and be a great, easy to handle platform for the teenagers through adults.

so my gut would tell me to do the reduction of changing spacing if there is a design that is really special…..as long as you can keep the change to less than 10%. if you are not wed to a design that requires a reduction, explore some of the newer, modern boats at 16 feet or less.

h

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The reasons are what I mentioned - the fact that the volume would be scaled in 3 dimensions and the surface area and weight of the build materials in 2. That would set up square-cubed imbalances that affect strength, moments and centers. These in turn affect performance, maneuverability, stability, etc.

You get exactly the same issues in a reduced form just reducing the length. How dramatic they are depends on the design that you start with. It sounds as if you had the happy combination of a tolerant design combined with careful research and the input from a naval architect to help you avoid the pitfalls.

Phil Bolger once wrote that anyone was welcome to try scaling his designs, but that if they did it would be a new design, no longer a Bolger. Who am I to argue with Bolger?

Laszlo AKA The drama queen :slight_smile:

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Thanks so much to everyone for your thoughtful replies. It’s great seeing the discussion and hearing the range of perspectives.

This would be my second kayak build. My first was a Chesapeake 17 from CLC, but I forwent the plywood deck and instead made my own cedar strips from red and yellow cedar I had around the house. So I ended up building a hybrid, and I loved the process. Prior to that build, I had watched a lot of Nick Schade’s videos, and they’re really what inspired me to start the whole journey of kayak building in the first place. I wasn’t ready to jump straight into a fully stripped kayak at the time, so the Chesapeake was a great stepping stone. But now I feel confident enough to take on a full strip build from scratch.

That’s what drew me to Nick’s new design — the Runner — which I was excited enough about that I went ahead and pre-purchased and downloaded the plans. I started thinking this would be a great project to build for myself. But the more I thought about it, I realized I probably don’t need two full-length 17-footers. What I would love is to build something my kids could enjoy over the next several years — and maybe even something my wife (who’s around 155 lbs, versus my 200) could use comfortably as well.

My son is currently about 85 lbs soaking wet, and my daughter who is a year older and near 100 lbs. They’re both growing fast, and I know they’ll fill out soon, but it still feels like a 17-footer might be too much boat for them right now. My hope is to paddle with them around our local waters as they grow up — maybe even do some kayak camping trips. So I’d like whatever I build to be something that fits them well, feels responsive, and isn’t intimidating or overly sluggish.

That’s why I was curious about scaling down the Runner — just slightly — by adjusting the form spacing. I understand the implications of changing buoyancy and trim, and I was hoping that a ~6% reduction (from 17’ to 16’, adjusting spacing from 12" to ~11.29") would be minor enough that it wouldn’t drastically affect performance, especially for a lighter paddler with a relatively light load.

I wasn’t necessarily planning to pause the build based on the responses, but the comments on weight distribution, volume, and stability definitely gave me more to think about. I appreciate the insight.

I may end up holding on to the Runner plans and building a 16-foot Petrel or similar boat for this next project. I’m still leaning toward a touring kayak and not a short playboat. If anyone has more thoughts or suggestions, I’d love to hear them.

Thanks again — this has been a really helpful discussion.

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You could ask Nick directly. He’s good about answering these questions and there’s nothing like getting the word directly from the designer.

Laszlo

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Jesse, you may want to do some more looking on the Guillemot web site. When you do, you will see that Nick does not recommend the Runner for either beginner or small paddlers. Definitely not something that I would put a kid in. The Auk 14, Great Auk or MicroBootlegger Sport would all be better choices. Honestly, for a kid I would look strongly at the Shearwater 14 or 16. All of those boats will be a little big now but kids grow quickly. Picture shows my baby and I introducing her baby to kayaking.

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Seems Nick weighed in on the topic of scaling many years ago.

By:Nick Schade
Date: 5/31/1998, 3:51 pm

In Response To: Re: Re Scaling plans (Roger Tulk)

The thing about children is they tend to get bigger, so a big advantage of building them a scaled down boat is you will eventually need to build a bigger one. If you have to justify your building projects, this could be good planning.

Regarding scaling boats: Changing the scale in one direction will change the performance, in some cases significantly. But, as long as the thing looks like a kayak, it will probably perform pretty well. If you are not trying to make race boats, you probably don’t need “perfect” performance. I don’t recommend changing a proven design without good reason, but if you have a good reason - go for it. It is hard to make a truly bad kayak, whatever you make will be usable.

>

> I am considering more than one boat, as my daughters want a kayak,

too, and they are about 2/3 my size. Thirteen feet was a compromise,
as they thought they would like an even smaller boat, but I thought
anything less than 13’ would be too small for practical touring. They
would like to repeat a trip they did last year in canoes in Algonquin
Park. (and if I’m good, they may take the old man along too. They
just aren’t sure I can keep up with them. :-} )

10

from first hand experience, on the list mentoned, i have a petrel play and petrel and a microbootlegger sport. i would describe all of them to most people as touring kayaks. that said, the petrel and petrel play happen to be quite comfortable in more dynamic water….which sometimes happens when you are touring. (fwiw, there is nothing more uncomfortable, candidly, for me then being in a traditional go straight touring kayak when it gets rough)

for speed, they all keep up with most of my friends in standard 17 foot touring kayaks…becuase my friends when touring don’t tend to push that hard…. but the microbootlegger and petrel are definitely faster if pushed.

none of the boats above are overly tippy and they have a nice blend of stability and maneuverability and go straight capability (skeg is important feature for all of these)

just to round out my thoughts, i have done 20 mile plus days usually inshore/near shore in all of these boats. the microbootlegger sport has the most volume but all three are a fine touring boat if your idea of touring is a long day trip and your main thing is to ensure you have food and water for the journey.

if your touring means several days of unassisted camping and you need to take a lot of gear….i would probably pull out my night heron (18 feet) or my shearwater 17 becuase they can simply accomodate the kind of stuff you need when you start overnighting.

so as you think it through….i would encourage decomposing ‘touring’ into a real mission profile….and what you need to carry and the type of conditions you might be encountering (open water crossings, near/inshore protected, surf landings, rock gardens..etc). that’s what you really need to focus on to sort out what kind of boat you want. i find that folks often end up with a lot of boat and a lot of length and weight to manage….becuase they have simply defaulted into touring boats focused on a mission profile that is not related to what they are really going to do…

h