Thwart Knees

[Pernicious Atavist]

Not sure if this is the right place, but....

Okay, referring to my flatiron skiff I'm building (the uh, "Florida Redfish Skiff" for we Southerners) Traditional skiffs have knees atop the thwarts, and hanging knees below the stern sheets. Why are not hanging knees used for the thwarts as well? Seems they would take up less valuable space that way. :cool:

[Stephen Hutchins]

This is an exellent question. It seems to me hanging knees would be better for thwarts in most circumstances. This will take some thought... I'll get back to you. I'd be most interested in others thoughts as well.

[Pernicious Atavist]

mechanically, it seems to make sense, but i wonder if we're missing something. could it be the weight of the thwart would bow the sides inward with additioanl lateral bracing?

[Bruce Hooke]

Maybe because by going up the thwart knees do more to reinforce the gunwale than they would by going down. In the stern sheets, with the transom nearby, reinforcing the gunwale is less of an issue. Also, the stern sheets are generally narrower than the thwarts so there is more of an issue with needing to leave enough space to comfortably sit. Also, hanging knees in some cases may end up being part of the support structure for the stern sheets because, of course, the planks that make up the stern sheets often run fore-and-aft so some sort of support needs to be arranged at the forward end. Obviously hanging knees alone would normally not be enough, but they might be part of the overall system. In my experience, many boats do not have any knees around the stern sheets...

[Pernicious Atavist]

i like the hanging knees, so they stay, but i also like the idea of having a clear space for my rump on the thwart while having a drink...since i plan on mounting the thwart higher in the boat--thus reducing the space between rail and thwart--do we reckon hanging knees will suffice? frames will, of course, be used as well.

[Bob Smalser]

The whole idea of standing thwart knees is to protect the structure from racking stress. Mechanically, that is best done from the corners ala a sheer brace in a stick frame wall. The upper "corner" of a boat hull is the gunwale, and the thwart is a near-perfect equivalent to a wall's sole plate.

A floor-frame standing knee wouldn't have the same strength for the weight of wood used the knee, nor would a frame-thwart hanging knee.

You can lam your knees to make ample room for two fannies. Note that the rowing thwart knees in the pic are shaped differently from the mast thwart knees...not as strong, but strong enuf and leaves much more room on the thwart.

http://pic3.picturetrail.com/VOL12/1104763/7372002/99237485.jpg

[ 06-04-2005, 03:38 PM: Message edited by: Bob Smalser ]

[Thad]

I agree Bob about reinforcing from and too the rail with the standing knee, but I also think that, grown or bent, the 110 degree knee is more common and stronger than the 60 or less degree needed for a hanging knee.

[Pernicious Atavist]

again, bob.... ;) :cool:

[Stephen Hutchins]

My opinion is that they only need be used in a boat with a flimsy sheerline. or to put it nicely: very light construction. In most cases, I think it better to use hanging knees. If you look at bob's dory pics , you'll see a frame next to those knees, Making the knees to be of little use. In fact, they seem to just be adding weight, up high at that.

[Stephen Hutchins]

Hmmm, I wonder If I'm mixing up standing and hanging: Hanging knees would "hang" down below the thwart right? Standing knees would "stand" on top?

[mmd]

Consider this:

In a traditionally built rowing boat of yesteryear - when all of our ingrained habits began - frames were mostly small, close, and stem-bent. Transoms were mostly rather narrow because our boatbuilding predecessors had discovered that narrow waterlines aft meant easier rowing. Such boats were frequently banged against wharves, piers, and bigger boats so strength on the sheer amidships was required. Finally, such boats were workboats and their work was often carrying passengers who had to be kept as comfortable as possible.

So what does this have to do with hanging or standing knees?

In the stern sheets, if the supporting knees were to be standing, they would take up a lot of the space needed for the passenger's bum, forcing her or him to partially sit on the knee legs which would have been mightily uncomfortable. Also, the sheer just ahead of the transom is made quite strong by the additional support of the quarterknees, so added strength to the sheer by standing knees is not necessary.

However, the sheer at midships takes a beating every time the boat is pulled alongside at the intended destination. It can use the extra strength of the fitting of standing knees. Happily, the thwarts in this portion of the boat are wide enough that there is ample room left on the thwart to seat the rower comfortably without his having to sit on the knee legs.

Simple evolution of details as directed by practical considerations, IMHO.

[Bob Smalser]

Originally posted by Stephen Hutchins:
1) My opinion is that they only need be used in a boat with a flimsy sheerline. or to put it nicely: very light construction. In most cases, I think it better to use hanging knees.

2) If you look at bob's dory pics , you'll see a frame next to those knees, Making the knees to be of little use. In fact, they seem to just be adding weight, up high at that.1) That thwart also is mortised into a very large centerboard trunk and the knees are needed for trunk longevity...flimsy sheerline or not.

Give those trunks any chance at all to flex under the considerable sideways pressure of the centerboard...and they will....and then they'll eventually leak.

2) If the frame wuz somehow strongly connected to the thwart, I might holler uncle. ;) But it ain't....and I won't.

[Stephen Hutchins]

Point taken Bob. I wouldn't be me if I didn't present an argument ;)

[Bob Smalser]

You could soundly connect that frame to the thwart...

...it'd take a knee to do it, tho. ;)

[Bob Smalser]

Originally posted by MIke:
I like this arrangment for the limited area of seating midship.
There's a terrific idea.

I'm gonna do a book some day entitled "details" with no words....just pics of all these little treatments the boatbuilding tomes leave out.

[Dave Fleming]

ACES Idea, eyrster!

[Paul Girouard]

Excellent all around discussion smile.gif Learned a lot , the pic,s with comments sure help sort out the boatie terms :cool: The forum at it's best ;) PEG

[Pernicious Atavist]

thank you ALL for your input! this is the reason i'm here! huzzah! :cool: