Guy's Wooden Drift Boat Project

Hello Everyone,

 

My name is Guy and this is my first post in this forum.  I am here because I want to build a wooden drift boat and, undoubtedly, I will occasionally need advice of the pool of experts and experienced drift boat builders on this forum.

 

In preparation for this project I have read and studied three books: Drift Boats and River Dories by R. L. Fletcher, Boatbuilding with Plywood by G. L. Witt, and Covering Wooden Boats with Fiberglass by A. H. Vaitses.  I have also developed my own set of construction plans based off of the "Original McKenzie Double-Ender with Transom" in Fletcher's book.  I used and MS Excel spreadsheet to calculate all of the dimensions, cut angles, compound angles, and bevel angles of all frame components and I used Pilot3D software to calculate the as-cut dimensions of the plywood sides and bottom.  At this point I am pretty comfortable with the mechanics of construction and I think that I am just about ready to start purchasing lumber.

 

Presently, I plan to use Meranti Hydro-Tek plywood; 1/4-in. on the sides and 1/2-in. on the bottom.  I found plenty of places to purchase these materials, but they are all far away from Idaho Falls, Idaho, and as a result shipping costs more than the materials.  Are any of you aware of a business within a few hundred miles of Idaho Falls that sells this plywood?  I have a few requests in to the local lumber companies, but I have yet to talk with someone who has heard of this material before.

 

I also plan to use Port Orford Cedar (CVG) for the straight frame sections and White Oak (quater sawn) for the bent frame sections (chine logs and sheer rails).

 

Thank you, Guy

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Permalink Reply by Guy Fredrickson on January 19, 2012 at 4:55pm

Randy Dersham, Dave Z, Dave Branes, and Rick Newman,

 

Thank you.  I am taking all of your points into consideration.  I am in the process of working the curvature of the boat so that the widest beam is front of center.  This requires two Catenary curves: Curve 1 describes the bend curve from the widest beam to the transom, and Curve 2 describes the bend curve from the widest beam to the stem.  The point of the widest beam is also the lowest point of the rocker.  By moving this forward, I can create a hull where the center of gravity sits behind the widest beam by what ever amount I choose.  I will post some screen captures soon.

 

And the sides did look a little too steep.  I will give them a little more flare and I will reconsider the transom angle.  Thanks again for all of the good insights.

Permalink Reply by Guy Fredrickson on January 19, 2012 at 6:41pm

Here are some screen captures to illustrate the original problem and the proposed solution.  These pictures show the "bend curve" and not the sheer or chine edges.  The "bend curve" is what I use to shape the side panel before it is rotated into position.  But they are all related.

Dead center (DC) is a line located at a point equidistant between the transom and the stem. And the widest beam (WB) is a line located at the widest beam, which is also the location of the lowest point of the rocker bottom.

This first picture shows the original approach.  The single Catenary curve was symetric about the WB.  But because of the way the transom intersected the curve, the DC was forward of the WB.  The concern was that this design had too much weight forward.

This second picture shows the next iteration.  There are now two Catenary curves involved: one describing the bend cruve on the transom-side of the WB and one describing the bend curve on the stem-side of teh WB.  I manipulated the two curves to align the WB with the DC.  Notice the increased curvature of the front and the decreased curvature of the rear.

This effect is exagerated even more in the thrid picture.  Now the WB and DC have changed positions.  In each of these iterations, the center of gravity moves toward the transom.

In all three pictures, the length of the WB is constant (approx. 3.3 ft. half-width) and the distance between the transom and the chine is 17 ft.  This was just for illustration.

 

Permalink Reply by Guy Fredrickson on January 19, 2012 at 8:03pm

I said above that "In each of these iterations, the center of gravity moves toward the transom".  More accurately, the center of gravity relative to the WB moves toward the transom.  In each iteration, I think the boat becomes less nose heavy.

Permalink Reply by Randy Dersham on January 19, 2012 at 8:26pm

I believe that the boat will want to rest on the rocker front to back on the widest beam of the bottom unless it is overcome by other center of gravity factors. Looking at this from the top perspective it does appear that the widest beam would be a fulcrum that the boat would pivot on and you have slightly more surface area on the transom side of the WB; however there are also higher sides near the stem than on the transom.  You can't see that from the top perspective. They add to the stem side weight.

I like the center boat shape a lot and think it is balanced.  There is some magical math that happens when the widest beam matches the center of the LOA and the center of gravity. There are a lot of things that affect COG more than this shape so that calculation is a very dynamic part of your design.  I recommend that you print this out, scale it down and build a 1:12 model.  That will give you confidence in the basics of the hull COG and where it wants to trim when at rest.

This is a great set of drawings to show how moving the widest beam along the stem to transom center affects the boat shape.

Your bottom boat shape reminds me of later Don Hill boats that were 16x48 meaning a 16 ft panel and a 48in widest bottom beam. In these boats Don moved the widest beam forward to give two passengers in the front more shoulder room.  This resulted in a stress point on the chine batten where it is forced to make the sharper turn.  A big rock hit there tended to break the chine batten.

Permalink Reply by Dave Barnes on January 20, 2012 at 10:20am

As usual, Randy is spot on with the real life stuff. In your middle picture above, the rower would be at near the -1.9' mark and the most common point of rock impact will be at somewhere near the +2.3' mark. As you go into rapids at an angle, you are exposing that area of the chine to the greatest potential for whack-ation in bony water so you do not want the high stress bend right at the same point. If you want two passengers up front, putting the bench at about the +4' mark should be plenty wide enough with a 56" beam at center.   I think.

Permalink Reply by Nick N on January 20, 2012 at 3:12pm

Guy, 

Like you I'm new on the forum and have been hunting for an Idaho supplier of marine grade plywood.  I just spoke with BMC West, http://www.bmcwest.com/locations/default.aspx?area=25&location=313, in Boise, ID and they quoted me 22.50 for 6mm Hydro-Tek, 58.60 for 1/2" fir.  

The rest of the wood is much easier to locate, but in Idaho Falls I would call Intermountain Wood Products, http://intermountainwood.com/idahofalls.htm, as they usually have the best prices.  BMC did quote me 5.00 BF for 4/4 white oak.  

You may also want to contact Hyde Drift Boats and see if they have any used trailers left.  I picked one up last weekend and while it needs some new springs and hardware, I'll still be under the cost of fabricating one from scratch.

Permalink Reply by Guy Fredrickson on January 20, 2012 at 3:47pm

Thank you Nick.  BMW West is in Idaho Falls and I'll give them a call about the Hydro-Tek.

Permalink Reply by Guy Fredrickson on January 20, 2012 at 4:38pm

Here's new "bend curve" for the design.  Centerline length is 16 ft.  I will post a new set of rendered drawings when I get that far.  Incorporating this new design strategy into my spreadsheet will take a little time.

 

 

Permalink Reply by Guy Fredrickson on January 21, 2012 at 7:26am

This is the second major design iteration.

The centerline length is 16 ft.

The widest beam is located deadcenter along the centerline.  And this corresponds to the lowest point of the rocker.

At deadcenter, the elevation of sheer is 24 in., the width of the sheer is 76 in., and the width of the bottom is  56 in.

The elevation of chine/transom point is 8.6 in. and the elevation of chine/stem point is 11.8 in.

This picture is the "bend curve" used to bend the plywood side panels before they are rotated into position.

This picture shows the traditional plan and profiles views.

This picture shows the placement of the frames.  Notice that the center frame is located deadcenter at the widest and deepest location of the boat.

And here are the 3D-rendered views of the boat.

 

 

 

 

 

There is no economy of plywood.  Each side panel requires two 10-ft sheets or three 8-ft sheets.  That's simply the consequence of the higher sides.  The shape of the boat is completely dependent on two parameters: i) the "bend curve" and ii) the as-cut dimensions of the plywood side panels.  One of the most suprising things to me was how small changes in either can have significant impacts on the boat's 3D shape.

 

Permalink Reply by Guy Fredrickson on January 21, 2012 at 8:41am

Here is something that became apparent to me.  It is nothing new and you see it in some of the drift boat designs, particularly the fiberglass and aluminum drift boats, but also some of the wooden variety (e.g., the Classic Rouge River Dory and the Colorado River Dory in Fletcher's book).

 

In some designs the profile of the continuous rocker bottom may start to become too exagerated.  A way to limit this, at least in my design approach, is to provide a "flat section" on the "bend curve".  In this picture the "flat section" extends 2 ft. either side of dead center.  This would give the boat a flat bottom 2 ft. either side of dead center.

 

 

I'm not interested in this feature on my boat, but I did think it was interesting and I now see why it is done on some designs.  Many of the Hyde boats have a flat section that is about 5 ft. long.  They do this so that the boats draft a few inches less water than an equivalent continuous rocker.

Permalink Reply by Guy Fredrickson on January 21, 2012 at 7:39pm

Well... I built a poster-board model.

And it's a DAMN GOOD thing that I did.

It was all coming together nicely until the transom.  My calculated dimensions of the transom did not agree with the physical model.

I went back to the spreadsheet and, sure enough, found an error in one of the calculations.

NOW I've got it working!!!

And NOW I will build a new model just to make sure!!!

Permalink Reply by Guy Fredrickson on January 21, 2012 at 8:06pm

Here's the failed model.  You can see the obvious mismatch between the transom panel and the side panels.

I'll do it again tomorrow.

 

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