Gunwale Design Question

Hey there WBP,

 

Does installing gunwale blocks for the inside gunwale have a structural function?  I've seen boats that have attached the inside gunwale directly to the hull without the blocks.  Must save quite a bit of time but wondering if this compromises anything?

 

I am thinking about doing a quick and cheap drift boat build and it seems like doing a simple gunwale is an attractive thought.

 

Thanks for any ideas on the matter.

-Kelly

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Permalink Reply by Sandy Pittendrigh on May 2, 2011 at 2:47pm

The strongest gunwales have no blocks inside or out.  You can glue one gunwale directly to the inside, and then on the outside sandwich a tapered strip between the plywood and the outwale.  The strip would be about 36" long, 1-1/8"  thick and tapered at both ends, with a few saw kerfs cut almost through in order to let it bend--with 5/8" oarlock holes bored into that strip in approprite places. If you putty-up the saw kerfs and clamp and glue all of that together as a unit--and then wrap glass tape over top the whole works, you get an absolutely immovably-stiff gunwale.  

 

An alternative to that is to glue and wrap the gunwales together with no tapered oarlock spacer. Then you build movable, bolt-on oarlock holder that hangs over the outside edge.  

 

Gunwales made from laminated 1/4" strips are even stronger yet. The Wood Products Industry Engineering manual includes various differential equations that quantify the laminated strength increase over any solid wood of equal dimensions. Some stitch and glue boats tend to be a bit flexible at the gunwales. Hence many stitch and glue boats rely on a side-to-side rower's seat to keep the boat from flexing inward (but not necessarily outward).

 

The following is a laminated gunwale on the outside only (glued up from 5 1/4" fir strips) with a 3/8" by 4" horizontal plywood arc on top, overhanging to the inside, with movable oarlock blocks.  That gunwale is really stiff. Glued. Laminated and wrapped in glass, it would take a huge force to move it.

 



 

 

 

 

 

Permalink Reply by Kevin C. on May 3, 2011 at 7:36am
nice boat sandy...add a couple ,few jets and it would make a nice spa too!:)
Permalink Reply by Brad Dimock on May 3, 2011 at 7:13am

We built the Hindman Double-ender with transom out of Roger's book for about $200 in lumberyard materials. Took us a week. It's amazing how fast and easy it is to build without the damned epoxy and glass. The boat has a couple San Juan trips, a lower Colorado trip, and a few lake excursions on it and is still in great shape so no need to burn it. I'd think that would work fine for the Lower Salmon, though you might end up having to bail a bit in Snow Hole or some of the other big-wave rapids.

 

I do like the blocked gunwales though, for holding on to, tying the boat down, carrying it, etc. They truly are a good deal stronger that way. And with the double-ender with transom, the ribs are close enough you don't need blocks anyhow.

Permalink Reply by Don Hanson on May 3, 2011 at 8:44am

  "Is too....is not"  I was just gonna keep quiet and not start a dumb argument and/or an engineering debate among non-engineers, but...I couldn't..

  Solid gunwales, if you make them the same dimensions as a blocked gunwale structure...yes, they will probably be just as stiff, but much heavier.  Like cored panels are lighter and stronger than solid panels, the same goes for gunwale-structures made with some sort of spacers, be the spacers frame heads or simple blocks.

  Think of it this way:  A couple of sheets of 1/8 plywood bonded over a honeycomb, baltek, polyurethane or  even a styrene core will certainly be a lot stiffer than a  single solid sheet of 1/4 plywood. (similar weight, equivalent amount of wood)  For discussion, make 'our' hypothetical core 1/2"...you'd have pretty light, stiff and strong (except in the case of a direct blow to either face) panel in a 3/4" thickness. 

  Sure, an equivalent  sheet of 3/4" plywood would likely be just as stiff as 'our' 3/4" sandwich panel, but it would also be much heavier.

  Gunwales..The Don Hill boat I have in my shop right now, the gunwale "structure" is 3 1/2" thick-1 5/8" deep.  If I laminated or steam-bent 3 1/2" x1 5/8" of white oak, solid, it would be very heavy...close to twice the weight of this blocked gunwale structure, for no real reason besides perhaps a bit easier to maintain.

  If you started 'skinnying-down' the scantlings you'd soon find flex.  If you consider weight vs stiffness, two thin (this old Don Hill boat's inwale and outwale are about 5/8 thick, not especially thin, but...) gunwales connected into a single truss-like structure on the sides of your boat are going to be lb for lb., significantly stiffer...In my humble opinion.

  Don Hanson

 

 


Permalink Reply by Lawrence E. Long on May 3, 2011 at 7:23pm

Don:

Right on.  This old coot managed to kick the hornets nest again.  I still like my boats to look like a boat not a bathtub like a Hyde or Clacacraft.

Permalink Reply by Kelly Neu on May 3, 2011 at 2:04pm

$200... not bad at all.  Hmm you got me thinkin' though... Salmon river, wooden boats, how about a sweep scow!  I'm not sure about the ethics of building the thing then sticking my best friends in it but sure would be fun for a little while.  Might be pretty cheap too.

 

I agree those blocked gunwales sure are handy.

 

How did you fasten the Hindman Double Ender?  Screws?

Permalink Reply by Brad Dimock on May 3, 2011 at 3:02pm
Gold deck screws from the lumber yard. Quick, easy, relatively cheap. Galvanized carriage bolts for the gunwales.

Re a scow: it only took a day and a half to build, but did cost about $500 in rough cut lumber and plumbing hardware for the thole pins. If you've run a sweep rig on the Middle Fork, you could probably get a scow down the Lower. Make your friends wear helmets and two life jackets though. And take out travellers insurance.
Permalink Reply by lhedrick on May 3, 2011 at 4:09pm

I can't let that one go.  

 

This is 2011, time to leave the sweep scow in the past.  Glen and Bessie most likely left the colorado because someone laughed at their ugly horse trough with the oars on the wrong sides. Brad, did you ever consider that possibility?

 

I know what's coming, only a sweep boat can X Y Z.  Yes, if you need to get a 30 foot boat down the MF with 5000 pounds of logs on it than it's about the only thing which will do it.  But,,,,,,, look at those things, hundreds of piles of junk which are only support for raft groups.  It's time to get those pigs off the river.   I almost got ran over by one of those things last July.  If it won't fit one a 16 foot oar rig it doesn't need to be there.  Get ride of those things and force the comercial dudes to operate like everone else and we private boaters might not need to wait 10 years for a MF permit.

 

Like Martin Litton said "sweep boats are ugly, dories are beautiful"  well, something like that.  

There I said it.  I feel calmer already.

 

Kelly, I think you should build one with a killer paint job like the big Z.  We could put 10 fly fishermen/woman on the thing and run it down the Green and haul in every fish in the river.  Think of all the beer we could bring and how larger your next wine rack will be.

Permalink Reply by Kelly Neu on May 3, 2011 at 6:45pm
Awesome. We could build a fish weir into it which could shoot the fish directly from the stream bed into our mouths while at the same time powering our beer refrigeration unit. Hehehe.  You got to admit a scow would make for a sweet bonfire though.
Permalink Reply by Brad Dimock on May 3, 2011 at 7:24pm

But from a boatologist's point of view, I have to say, running a sweep boat on the Middle Fork was the most exhilarating thing I've done in a boat north of the Colorado. They stink on the Colorado, but up in Idaho--wow, what a fun and novel craft. I learned so much running that thing that is applicable to rowing, and that's the only way I think I'd have learned it.

 

To the row-boaters up there, well, sweeps are just one of the inherent things you have to watch out for on that stream, like hippos on the Omo or crocodiles on the Zambezi or cottonwood logs on the Green and Colorado in flood month. Sweeps are fast and have no brakes. You gotta watch your back. Although I agree with lhedrick that the purpose they are used for is way to safari-ish for me--hauling too much crap for service-oriented dudes--I'll vouch for the craft itself. They predate rowboats in the Salmon drainage and do have a place. And they are a total blast to run.

Permalink Reply by Joel Sauder on May 3, 2011 at 8:21pm
I have to say, after watching Brad build Edith, I got at wild hare to do something similar here in Idaho. Now building a full size sweep boat built from rough lumber and running it down the Salmon is on my bucket list, as is the awesome bonfire at the end.
Permalink Reply by Brad Dimock on May 3, 2011 at 8:29pm

You'd have to get the top lumber burning hot in order to get that soggy bottom wood going. Might get a bit smouldery--so you might need to drink a fair amount of liquid smokescreen. 

My old wood scow was on its way down the Salmon when it disappeared. Long story, but I think Glen and Bessie Hyde came back and reclaimed it.

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