SOF -Design Critique Wanted

-- Last Updated: Oct-14-05 6:16 PM EST --

I'm finally beginning construction on my SOF. Looking for some general comment/impressions of what I'm planning.

Rather than following the traditional "design as you go approach" based on body measurements I have a specific design I am going to follow (I'm a designer - can't help it). It's not set in stone, so I can take advantage of adjustments along the way (body measures/logic taken into account), but there is a specific design intent beyond the simple joy of building.

Specifically, a fast touring SOT (that is still optimized for rolling). An open water distance at speed boat. Not a flat water go fast boat, not a play boat.

Images show the basic lines, intended waterline, a basic frame plan, and overlaid comparisons with several other kayaks.

Some of the comparisons are to scale (you'll see length/beam don't match and can get an idea of the real size of my design) while others are just comparing lines (and have been scaled to same length). Boats selected for comparison are random based on those I could find line drawings for - but conveniently provided a mix of good rollers and fast tourers/racers -including the hull I'm most familiar with, the QCC 700.

The way my lines ended up comparing to the professional designs makes me think I'm on the right track.

Approximate Specs (subject to change during build):

Length overall: 600 cm (19' 8")
Overall Beam: 48 cm (18.9")
LWL* - 550 cm (18')
BWL* - 46 cm (18")
Foredeck** (measured outside - minus coaming): 28 cm (11")
Aft deck*** (outside - minus coaming)): 17 cm (6 5/8")

And depending on actual sheer (the one thing I can't figure 2D an will have to bend the gunnels):
Bow depth (and end of keel) - 10"
Stern depth (at end of keel) - 7.25"
Rocker - TBD - min 2" forward, 1.25" aft

* LWL/BWL is a 12 to 1 ratio (how slow can it be?)
** Minus keel, ribs, and masik - say 8-9" inside. I have pretty big thighs and don't want sardine can fit for touring - bout not too loose for rolling.
*** Inside dimension from top of keelson to top of coaming should be about the same. Top of ribs to top of deck beam - 1" less.

I realize QajaqUSA is SOF central, and KayakForum is the builder's Mecca - but there is a method to my madness in asking here. I'm looking for less builder and more paddler targeted impressions that I might not get on the specialized boards where every project/SOF is great as the building is often the focus with performance more broadly defined - except rolling.

Really I'm just thinking out loud to verify my plan. If bored by all this qajaq/kayak stuff - please go back to all the B&B, poster etiquette, and Jet ski threads!

Hmmm… On Quick Perusal
if speed is what you after, I think the hull needs to be more rounded and more deadrise from the center keel. BTW, there is nothing to stop you from using multichine ( 4 stringers instead of 2) to get a more rounded shape. It is still a SOF. Not quite Greenland but not quite baidarka either.


I want one!

Mixed use
A little clarification. Yes, could round it more, but this isn’t to be a dedicated race boat. I want to be able to do more mixed paddling and not be hammer down all the time.

I think an 18 foot waterline and round hull would be happier with a rudder too (yet the baidarkas manage…). If I want rounder I think I’d build a baidarka or just spend time on my dusty surf ski L (Actually, my ski has a bottom that’s far from round. I should use it as a rib form!)

Look at the cross sections comparisons. Even though I am limited to flat planes - I think it compares OK with even the two race hulls (though finished product may vary considerably).

I’ll take the the small drag penalty of the flatter squarer hull for the extra stability, tracking (no rudder/skeg), and ability to carve turns when edged and chines become keel. I also just like the aesthetic of Greenland hull (though I still violate some design elements).

If I was limited only build a more traditional 17+ foot Greenland design I’d do it (almost was as I didn’t want to scarf long pieces). I’m just trying to add some variation to that basic design based on characteristics of other boats I’ve enjoyed and to match what I want to do with it.

Compromises, compromises…

Again, just thinking out loud.

one question
What kind of freeboard are you expecting at the aft deck?

1" or so
Just behind the cockpit I’m shooting for about an inch or so. 1.5" OK. 2" not a disaster. Below 1" I’d start qualifying for submarine pay. Arcs up from there, with stern tip 4-5" out of the water.

An earlier rough 3D model of similar size/shape was in this ballpark range. I suspect it will ride higher than that model indicated, but hard to say without spending computer time. Rather get building. I’ve gained weight since I ran those early number - so unless I lose it while building I’d better hope it’s higher. I’ve seen photos of paddlers my weight in smaller qajaq - so I hope I’m not off to far the other way - but easier to adjust volume down than up if way off. If really off on the high side I can shorten ribs. If too low - I diet and exercise more! Hmmm, I should remodel it and run new numbers and make it low on purpose…

If I get it where I want it I expect the aft deck to be awash a good bit in beam/following stuff. Quite a bit of volume aft (relative to typical pinched end fishform SOF) so the stern will lift and be a tad drier and surf better.

Once the frame is done and I can do a plastic wrap test I’ll see where I’m at and if I need to play with the stringers (or more) to adjust this, the stability, etc.

Will not open up for me …cannot see.
Greyak, do those rib and stringers in the Divincell foam with a little carbon over them like we talked about a while back…

Remember you can heat form these things perfectly. Cut all parts with a razor blade … no sanding ! : ) Ribs will be flatter too for more skin contact / shape holding.

Alternate image links
Individual image links below.

Comparisons are with Winter’s QCC 700, Björn Thomasson’s Njord and Black Pearl, Nick Shade’s Mystery, and the new KayakPro Vampire. Only 700 and Vampire are compared same scale (roughly), the rest are just to compare lines at same length.

Of these the Njord’s intent is closest to what I want functionally overall, a LV open water fast tourer. It derives from Greenland designs. The Black pearl makes for a nice comparison with a more traditional LV rolling machine (Check out Björn’s site if you haven’t: ), the QCC serves as a familiar baseline, and the two race boats - just because…

BTW - Foam composite construction still interesting (I could just use my Mark 1’s hull as a form), but I’m going to try to stick to natural composites for this. Don’t really want yo deal with epoxy here in my bedroom/living room. If I break too much of my oak I may reconsider…

I Personally Wouldn’t Go For A SOF
optimized for “rolling” unless I thinking of doing a Greenland comp. I think a boat that is almost awashed will be a bit of a pain in conditions in open water. I don’t mind getting wet but to constantly have waves go over the deck and slam into my body is not my idea of fun on a long haul, unless I am surfing.

My SOF started with a flatter hull. Was out on a trip in wind and choppy 3’ foot seas coming on the beam. It took a bit to stay upright and I actually went over at one point. I have since modified the SOF to give it more deadrise. It handles beam seas much better and surer. I favor more secondary than initial stability for conditions.

But hey, you can also do the same. Go flatter first and see how you like it. If you don’t, you can always gorrilla glue and peg another wood strip onto the keel to give more deadrise (and even rocker) if you want. It does involve wasting a skin.


All things relative
When I say optimized for rolling - I’m not talking a comp boat.

You have to realize I am currently rolling a boat with a 10" rear deck height. A layabck was impossible before I moved the seat forward. Even now it’s just a partial layback - except as a flat water trick with a lot of arch and coming way off the seat and raising the COG too much.

Compared to that, a very ordinary - even a bit voluminous Greenland qajaq would feel optimized to me.

The ability to adjust frame is a big reason I’m not doing a S&G!

On my last S&G I shot for 2" and I’m
glad I did. 2" seems to be the sweet spot for all-around rolling and touring. On my surf boat I wanted a 1\2", mostly so I would have more rail in the water but it sucks for going out into and around surf, but for being on the waves it’s perfect.

Anything under 19" is optimized
for capsize (at least for a #180 male). At nearly 20’ and single hard chines this design will have a large learning curve. One of the reasons I say that is a bit of lean will be needed to get a decent turn out of it and the narrow single chine hull will have little secondary. I’d keep a bit of freeboard to help with leans.

But I say build it. It’s an interesting mix and I don’t think anyone will really know how it paddles until it’s built. I’d personally build two boats instead of the two-boats-in-one design, but that’s just me. But with that said I caution you to not get very extreme with any design aspect without trying it first, i.e very long, very low V, or very narrow, or even very lightweight. You’ll kick yourself for building something you don’t like to paddle.

Oh, but it does look very nice!

What software are you using?
You should get some data from you design by using a Marine Design software. At least, run it through KAPER for resistance predictions.

Look at it this way
Take a QCC 700. Add 6" to the waterline length. Cut 2" off the beam. Lower the foredeck an inch or so. Lower the aft deck 3". Add some overhang for aesthetics and adapt to Greenland style SOF construction.

That’s not what I did, but that’s how I project the performance/handling.

As for stability, I’ve been in a few S&G hard hard chine LV boats with 19" and they felt pretty solid. Surprisingly so. My extra length should only add more footprint.

Maneuverability may be a bit dicey - but I’m already used to almost that LWL and not looking for a playboat experience. More concerned about weatherhelm.

Drag predictions and other specs
LOA: 6m 19.685’

BOA" 48cm 18.897"

Total Volume: 301750cc / 10.66 cu ft / 79.7 gal

LWL: 550cm / 18’

BWL: 46cm / 18"

Draft: 10.14cm / 4"

Wetted Surface: 22.57 sq ft

(Design Waterline - total displacement 233 lbs / 29.12 gal / 3.893 cu ft / 110239.73cc)

Prismatic Coeficient: 0.585

Block Coeficient: 0.4386

Midships Coeficient: 0.748

ADratio: 8.69

Drag Estimates (lbs/knots):

3.50 - 4.00 - 4.50 - 5.00 - 5.50 - 6.00 - 6.50 - 7.00

2.55 - 3.36 - 4.45 - 5.91 - 7.88 - 9.81 - 12.01 - 14.2

Aft volume
One thing that I’ve noticed with a stern that is narrow and low volume is that when surfing you can lean back and the stern will react more to your leaning back and pull the bow up more and help the bow from diving. It really works well.

Sounds good
It seems like your interested in a similar type of sof that I built last winter. It rolls easily, surfs great and is faster than my previous kayaks. I’m not sure if the speed is related more to the design or the weight (31lbs.) of the kayak. I was very surprised at how well the kayak reacted to leaning back when surfing. The bow seems to lift up very easily. I gave it 3" rocker in the bow and 2" in the stern. I weigh 165 lbs. and have about 1 1/2" of freeboard. It is a wet ride in rough conditions and I love it. It balance braces by itself and has made learning greenland rolls easy. Ever other kayak I have paddled since getting used to this one feels like a barge. I have enjoyed it so much that I’m building a cedar strip replica of it. I’m currently putting together a building log of the replica. If your intersted you can take a look at the log I made of my sof.

Your picture will be helpful and I’ll refer to them along the way.

Quite similar - though I may be crazy to go as much longer as I am. Had to experiment somewhere.

My original rocker estimate was same as yours: 3" fore 2" aft- but is now less in the drawings. Final will be by eye and will likely go back up a bit. I’ll need it at that length.

Those Kaper numbers look really fast. The kayak I designed and built and just tested today looks like this for Kaper:

2 knots .96

3 knots 1.99

4 knots 3.67

4.5 knots 5.08

5 knots 7.32

6 knots 12.64

And it feels pretty fast. Yours should fly. Do you have any stability predictions?