Thwart-less rowboat possible?

[Russ Kaiser]

I have been flirting with a slab-sided rowboat, sort of dory shaped, that would be my winter project. The boat is primarily for me, but I might want to occasionally take one of my kids along to row with me. Since the length is short, 4 meters, and that length is dictated by transport and storage constraints, setting up three possible rowing stations with traditional thwarts seems like it will make this small boat's interior impossibly crowded.

I have seen small boats with a longitudinal bench, and I thought I might adapt that construction technique on this project by making a center box section. Then I could make modular seats to move along its length, possibly secured by dowels in the bottom of the seat dropping into holes along the top of the beams surface. The seats would be just for the rowers butt and would not extend to the sides.

My main concern is the stiffness of the sides. I have a couple of pictures here roughing out a proposed framework. I think the height of the beam's top needs to come down, but you get the picture. The ribs are on 150 mm centers and I was thinking of using 50x20 mm yellow pine. In fact, as much of this boat as possible (exluding the sheet stock) will be yellow pine.

Not shown would be some type of laminated sheer clamp also of yellow pine. I don't know if I would make it an inwal, tying the tops of the ribs into the structure or go to the outside. Also not shown are a breast hook, rib gussets, etc. The bottom and side skins are 6 mm plywood.

The boat length is about 13 feet (4.0 meters). The framework you see drawn works out to about 17.5 kg using 500 kg per cubic meter, so this boat could be a bit on the heavy side when all the details are finished.

Do you older, wiser, more experieced guys (or gals) think this type of construction would yield sides stiff enough to take oars?

[Squidly-Diddly]

material like plywood at roughly horizontal and deleting(or scaling back) the ribs would stiffen the sides better.

I'm thinking about 4" wide of 1/2" plywood gunnel and the whole boat built "stitch and glue" without ribs (or add some ribs latter if needed).

I like the longitudinal single bench idea with multi position stations.

[NoEyeDeer]

It's massively over-framed. You could get away with frames every three feet.

[Russ Kaiser]

It's got a lot of sticks, I'll give you that.

Mainly the number of frames were to support the bench. I could make the bench completely out of sheet stock, a true box section. And then make the sides and bottom out of 9mm Ply, just putting in some corner gussets between them. Maybe I will sketch that out, it would certainly be a quicker build.

As for the the plywood gunnel, Squidly, that could be incorporated too.

With the initial plan I was only going to glass the outside of the hull and tape the inside seams.

[waikikin]

Originally Posted by Russ Kaiser

It's got a lot of sticks, I'll give you that.

Mainly the number of frames were to support the bench. I could make the bench completely out of sheet stock, a true box section. And then make the sides and bottom out of 9mm Ply, just putting in some corner gussets between them. Maybe I will sketch that out, it would certainly be a quicker build.

As for the the plywood gunnel, Squidly, that could be incorporated too.

With the initial plan I was only going to glass the outside of the hull and tape the inside seams.

Russ
9mm is pretty thick for a 4 meter boat 6mm is good, I like the longti seat esp if its a box for floatation etc, My thought would be to make the gunnel area quite stiff by spacing the inwale inboard on blocks- maybee about 60mm/2&1/2", combined with the gunnel/rubbing strake that gives an I beam effect, the spacer blocks rather than being cut square on the ends look smart if cut with a holesaw using a jig- to radius the slots left between, also stategically some blocks can be of hardwood with rowlock holes & wear plates for adjustment of rowing position dependent on passenger/load. Looks like a great project & all the best from Jeff.

[river runner]

I just finished my river dory (for lack of a better name). I originally designed it with five frames, a stringer and two seats that spanned the width of the boat. As you can see from the photo (hopefully there is a thumbnail)it ended up much simpler than that. I haven't had it in the water yet, hope to this week, and there is some chance it will simply fold in half, but I kind of doubt it. I used 9mm Okoume and quite a bit of glass. I think it would be possible to build a small boat with no thwarts or frames of any kind. I think the key would be a little thicker plywood than you might normally use, multiple layers of glass and make really beefy gunwales. Most of the professionally made wooden drift boats and river dories have a sturdy inner wale spaced out from the hull. This gives you a good solid place to put the oarlock sockets and strengthens things up quite a bit.

[rasorinc]

If you go with side frames, side battens will stiffin light plywood with out being heavy. Even 3/8" thick by 1.5" will do wonders and if you have 2 of them it will be very stiff (using epoxy of course) Yellow pine is a great wood but very dense and thus very heavy for a row boat. Consider Cypress, it is avaiable most every where. Very rot resistant and quite light.

[PAR]

Some basic understanding of engineering principles here, would go a long way to addressing the various and quite speculative concerns. There are a lot of ways to get sufficient athwart stiffness and yes, you need it. Sawn frames on 12" centers, is just so far off base, that it doesn't merit further comment as does using no frames or inwale as an other example shows.

An inwale or sheer clamp/shelf arrangement can eliminate much of the need for frames. This requires some inward space to accommodate, making a thick rail that you can sit on an install oar locks. 9 mm plywood on a 4 meter boat is a little heavy, but a few frames would also be helpful, depending on the SOR. You could employ a pure monocoque structure and skip the frames, but you'll need to triangulate things together some where. This could be as simple as little fore and aft decks, boxed to the hull shell or you can venture out onto an engineering limb with a centerline box and a movable set of thwarts.

Personally if it was me, I think you're over thinking the crap out of it. I'd run a seat stringer down each side, full length. At each end would be a fixed thwart and in the middle of the boat would be two or three adjustable thwarts, all locking into pin holes or I'd probably use a notch in the stringers. It's light (the 9 mm makes it heavy enough), simple and adjustable for different crew situations. The thwarts need to positively lock into position, not just rest on some dowels. I'd probably use a "dog ear" and wedge arrangement which can be changed quickly when necessary.

[Russ Kaiser]

I am trying to distill these responses, especially yours PAR.

1. I don't need so many frames but a few are a good idea.

2. Try and use 6 mm (1/4 inch) plywood.

3. I can choose to stay with the center beam, or not, but a strong inwale is necessary and more important to provide the stiffness for a rowing vessel than a multitude of frames.

Additional thoughts:

To "box in" the beam and tie it into bottom of the hull the geometry is much easier if there is no dead-rise. In the initial hull profile I was trying to make this little boat as efficient to row as possible, but I might flirt with the typical dory style flat bottom too.

The thing that really appealed to me about putting in a center beam was that I could easily modify things such as seat height and position which in a traditional boat would be fixed. I could also build a sliding seat sculling rig that I would not have to use all of the time.

As for the seats, I am sure I can come up with something to make them both quickly movable and easily secured.

Russ

[Easy Rider]

If Russ was to build w stitch and glue and no frames but w a lengthwise side seat (not to sit on and all plywood) glassed in wherever it contacts the hull w straight and parallel edges inbd instant rowing seat adjustment would be had as well as great stiffness of the entire boat. The "seat" would be like the hiking out side seat on a small sail boat but it would be just a large lightweight angle to provide a place for the adjustable rowing seats, great stiffness of the hull and space for lots of flotation. And if there was any concern about the stiffness of the gunwales small struts could be installed between the gunwales and the inbd part of the "seat" described above. I'm putting such struts in my aluminum skiff attached to the seats.

[Russ Kaiser]

Easy Rider,

I have read this a couple of times and I'm sure it's me, but I can't visualize what you are suggesting. Can you scan and post a pencil sketch or something?

Thanks,

Russ

[waikikin]

I use a Pacer dinghy to row & sail, it has side bouancy tanks similar to what EasyRider has described although the inboard edges aren't paralel, heres a link to some pics http://www.pacersailing.org.au/gallery/fitout the pacer I have now has a skeg fitted aft to help tracking.Regards from Jeff. You may like to check out Michaels Storer's plans for "goat island skiff & row boat"

[PAR]

Russ, you're making assumptions about framing and scantlings, which can only lead to an overly heavy, complicated boat. Do yourself a big favor and purchase Dave Geer's book "The Elements of Boat Strength", which will offer some insight, though I still think you'll need a better grasp of the concepts involved.

[Russ Kaiser]

Par,

I will seriously consider your book suggestion. Other than a couple of tiny boats for my kids, I haven't built or designed anything.


Now I get it EasyRider, thanks Waikikin.

I don't think this boat (as first envisioned) is quite beamy enough to use side tanks with parallel inboard edges to make a platform for universal seating. It isn't double ended but close enough that trying to incorporate construction like that would give it a strange look since the boxed sections would be relatively skinny before they merged into the front and rear decks.

Not to mention that unless you are sculling, it's nice to spread your feet apart a bit and side tanks would narrow the available bottom.

In the long run, the boat may just be too darn short too, the more I look at two people rowing in a 4 meter boat, the sillier it looks. I might have to just build bigger and buy the darn thing a trailer. In reality, the amount of sheet stock I am going to buy for this project would build this boat about half a meter longer.

I will continue to fiddle, I don't need to start for a couple of months yet.

[BATAAN]

Dories traditionally don't have thwarts because they were designed to nest like paper cups on the deck of fishing schooners. Here is a real dory after a long life.
Several frames joined with riveted steel clips in the corners, pine dory-lapped planking, no chine log and a hefty sheer rail made a work boat that could be hauled aboard daily and carry 1000 lbs of fish.
Here's a free plan for a nice small old school dory that can easily have removable parts, and also sails pretty well if you want to put in the centerboard.
Scroll to bottom of page for free PDF print outs of all important parts.
http://www.classicwoodenboatplans.com/about-the-plans/