Question about Viper 11 Thwart Width (UPDATE)

Post by **sbroam** » Fri Jun 10, 2011 3:47 pm

http://cboats.net/cforum/viewtopic.php? ... er+thwarts

Clarification: meant to stress that the *gunwales* were attached, so it wasn't just the plastic warping at the point I had the temporary thwart attached. If I ever re-gunwale that boat (i've regunwaled MEs 4 times at this point so the odds are good) I'll perform this experiment again and take *pictures*.

clarion · Post by **clarion** » Fri Jun 10, 2011 3:52 pm

Hmmm. Craig, could it be that basically flat-bottomed canoes don't lose rocker when pulled in whereas more old-school designs do? I know the latter is true. But I'm starting to see how flat bottomed, hard chined boats would resist this tendency.

Post by **sbroam** » Fri Jun 10, 2011 4:17 pm

I could definitely see that - the hard chined, flat bottomed boat would tend to bend along the crease that is the chine; a shallow arch would tend to bend along the radius of the curve, deepening the boat at the point where it is getting pulled in and not at the ends - thus lifting them.

ezwater · Post by **ezwater** » Fri Jun 10, 2011 5:25 pm

These observations about individual boats are interesting, but as I said, I lengthened thwarts and the rocker increased. There may well be something about the Ocoee and the L'edge that prevents the rocker from changing, but on other boats, pulling them in must reduce rocker, and widening thwarts must increase rocker.

All we've got so far is "In these few instances it does, and in these other few instances it doesn't."

Craig Smerda · Post by **Craig Smerda** » Fri Jun 10, 2011 6:22 pm

ezwater wrote:These observations about individual boats are interesting, but as I said, I lengthened thwarts and the rocker increased. There may well be something about the Ocoee and the L'edge that prevents the rocker from changing, but on other boats, pulling them in must reduce rocker, and widening thwarts must increase rocker.

All we've got so far is "In these few instances it does, and in these other few instances it doesn't."

a canoe (generally speaking) has four profiles... the top, the bottom and the sides.

if you pull the edges of the top closer to each other it alters the angle of the sides and may create more crown on the bottom of the hull from side to side... it will also make the top edge of the sides higher and expand (stretch if you will) the overall length of the boat a touch... but it shouldn't change the rocker profile profile as the rocker profile is dictated by the general shape of the sides of the boat and not by the pushing or pulling of the top of the boat.

(some of this depends upon if the boat has no gunnels at all, if one is just adjusting the center thwarts or center and end thwarts. the boats we adjust have no end plates and we do adjust the end thwarts as need be.)

you can alter the shear line (top of the boat) somewhat just by moving the thwarts in and out.

there may be an ever-so-slight alteration in the rocker but I feel pretty confident saying it wouldn't even be visible to the naked eye if there's much at all.

the premise of the thread is basically asking how much one might want to pull in the center thwarts on a Viper (see picture below)

regardless of all the other conversation going on... by pulling in the thwarts it will have the greatest effect on the angle of the sidewalls of the boat. by pulling them in you make the boat less stable on it's secondary stability (more noticable when transitioning from primary to secondary), give the boat a harder edge due to the change of angle and you make it a bit narrower overall. by changing the overall flare of the sidewalls the boat will sink a touch further into the water... which does increase your engaged waterline and potentially make it a touch faster... also due to a narrower width.

my suggestion... get some cheap pieces of pine and cut a few different lengths of test thwarts to figure out which pull works the best for you and your style of paddling. you'll know when you've pulled it too much.

(fwiw... the reason I cut the entire deck off the L'edge I modified was so that I could adjust the entire boat from end to end... if I had only pulled the center thwarts (as found on a factory open model) I could have only adjusted the center of the boat.)

philcanoe · Post by **philcanoe** » Fri Jun 10, 2011 7:47 pm

It would be my guess...

... that modifying the same canoe's width, could indeed vary as to not only the state of having gunwales or not. But also as to placement of the thwarts, the number of (1,2,3), such as the use of mini-thwarts, the type of saddle (stiffness/depth/length), the length of deck plates, and even potentially the type (stiffness) of deck plate. It would also be highly logical to assume differences with materials, such as between ABS, HDPE, and FRP type hulls. And maybe a potential difference between R-lite and standard ABS, with-in the discussion of gunwale on or off. Especially considering that laminar flexing would be different in each case of gunnel'ed, ungunnel'ed, and/or material choice. I would guess it would vary for instance from Kevlar Viper -to- ABS (or Ocoee's).

It has been my experience that changes do occur, in rocker and edges. Changes that were measurable on a flat concrete floor, when simply spread. As well as in the feel on moving water, when performed with C-lamps.

However as with most every thing I've ever done personally, or seen written by experienced boat builders (naval architects).. any change(s) is likely to have unanticipated results. Every mod, change, alteration, of even iteration of different layup will have variances... it's like some application of Murphy's Law or Robert's Rules of Order. If something can change, it will. Every naval architecture article is most assured on this.

I believe each and everyone of us, in their own fashion is simply re-validating this. Through actually noting a real change in rocker, or no change in rocker. I'd guess you would have to try to see your result? Honest - I believe you'all

!!!!

Craig Smerda · Post by **Craig Smerda** » Fri Jun 10, 2011 8:09 pm

how the same exact boat reacts/responds to a person that is 5' tall, 5'-3", 5'-6", 5'-9" or 6'-2" and/or weighs 100lbs, 130lbs, 150lbs, 170lbs, 190lbs and so on is always intruiging for me to watch... sadly I've never found a group of people that all have the exact same identical skills and techniques to watch at one time... in the same boat.

life is so subjective...

ezwater · Post by **ezwater** » Fri Jun 10, 2011 8:26 pm

Craig, you're looking at the effect of thwart lengthening on the sidewall contour. But the more direct mechanism is that the top of the sidewalls cannot lengthen or shorten, so pushing the sides out via thwart lengthening *necessarily* will pull the ends of the boat upwards, though it may also just straighten the gunwales in front or in back of the thwarts.

The sides go out, the ends go up, and the rocker increases. At least it does in my boats.

pblanc · Post by **pblanc** » Fri Jun 10, 2011 8:41 pm

I have pulled in the gunwales on a few boats that had a shallow arch hull contour.

I found that pulling the gunwales in about 2 inches had only a very slight effect on rocker (reducing it ever so slightly) but a more significant effect on cross-sectional hull profile.

I did not think that the miniscule reduction in rocker affected handling, but I believe the change in cross-sectional hull profile changed the secondary stability and, perhaps, the dryness of the boat.

Mikey B · Post by **Mikey B** » Fri Jun 10, 2011 9:40 pm

I've pulled in the gunnels on quite a few flatwater solos and pushed some out (Bell Flashfire) and even though everyone states the rocker profile would change...I couldn't see a measurable change in the rocker, but could notice differences paddling with the side profiles changed. From my experience, moving in or out an inch or two didn't affect the rocker much if at all.
Mikey

ClassFive Boats · Post by **ClassFive Boats** » Sat Jun 11, 2011 1:04 am

People really shouldn't make statements they know nothing about.

As I stated , I have personally changed the viper11's rocker by 1 1/2 in total in by changing the width of the thwarts. It can certainly be done if the deck plates are removed,, Just because it doesn't work in Ocoee doesn't mean thats true for all boats.

ezwater · Post by **ezwater** » Sat Jun 11, 2011 1:41 am

I suggest a Thwarted By Smerda Armada. We all bring boats and spare thwarts, and the one who changes rocker the most gets an Esquif L'edge for a prize.

For those serious about increasing rocker, I suggest not just lengthening the center thwart(s), but lengthening those nearer the ends (if any) or adding thwarts nearer the ends to push the sides out.

If that doesn't do it, run a steel cable and turnbuckle between the bow and stern tips. By sitting on it, you can increase rocker on the run.

Larry Horne · Post by **Larry Horne** » Sat Jun 11, 2011 2:29 am

so glad I paddle a decked boat.

kx250guy · Post by **kx250guy** » Sat Jun 11, 2011 7:56 pm

Mr Smerda,
Changing rocker by movin thwarts around or getting different sizes
has been around for years. We did it when Sunburst l and ll came out in the 80's and it worked well. I believe Psycho did some mods to his SB l back in the day. Gotta take gunnels off these newer canoes though. My current boat is a beat up outrage and I narrowed the gunnels to pick up some speed. not a huge effect but it worked. All these boats were royalex BTW.
Come to think of it, I think Psycho bent his sunburst around at tree as well .

Sir Adam · Post by **Sir Adam** » Sat Jun 11, 2011 8:16 pm

Easy folks... lets not get personal. Like a lot of aspects of boating there seems to be two camps. Although I have a limited amount of open boat knowledge, I think each is correct to a certain degree...depending on the boat.

It WOULD be interesting to get a bunch of boats and play with different thwart lengths and see what the the new measurement (rocker and otherwise) are. Changing the thwart length will have other impacts... and it is always interesting to see what they are.

To make the 7' kids boat I designed more stable I had to increase the thwart length back to the original plug length. Made a HUGE difference - not so much in changing the rocker (I didn't measure it), but in flattening out the hull (more stable side to side), which is what I intended it to do. It also slowed the boat a bit. Not a problem for kids.