wooden boat building/hull questions

I am building my first flat bottom power dorry 20' long 5' bottom beam and top beam 7.5'. Question i am looking to do a main frame bottom boards 2x6 and side board 2x4. I want to do a main frame every 4' with a half frame every 2' in between. To maintain strength in the floor but lighten it up on the sides, and ease the construction is this feasible?

 

I assume you mean 1 1/2" X ...
Sounds over built even at that but an actual 2" X ... Unthinkable.
Sounds more like a surf skiff than a dory.
Pics or dwgs?
But your omitting every other side frame upright sound's good.

 

That is way overbuilt. How did you calculate the scantlings?

 

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I thinking of the main frames being 1.5x5.5 bottom with 1.5x3.5 sides sandwiched between two bottom boards with a filler board in between the the two bottom boards. The frame in between would be only a single 1.5 x 5.5 board. My idea is to eliminate notches in the floor and provide an ample joint surface to the outter skin. I also plan to run box stringers from transom forward. Notched into the bottom of frams and boxed in between.

 

I also thought of using 1/2. Or 3/4 plywood to sandwich the bottom 1.5 x 5.5 and side 1.5 x 3.5 . I want more than a lap joint on the sides.

 

Massive overkill.
Find some plans.
How big of a motor are you planning on?

Have you taken your board sizing and calculated the weight? don't forget to add seats, etc.
Are you going to fiberglass this boat or just paint?
What will you use to join the wood?

 

Really way over the top in regard to scantlings, as everyone else has suggested. Your planking is also ridiculously thick. Is this an ice breaking dory?

Typical frames for a boat of those dimensions would be 1x4's with 1/2" planking on 16" - 24" centers. This would still be heavy, but at least reasonable, considering I don't know much else about the design, like target speed range, weight estimates, etc.

Where is the center of gravity on this puppy and how do you plan on powering it?

 

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http://www.spirainternational.com/hp_sitk.php
This is kind of the idea I leaning towards. Just not as big and I don't plan to curve the whole front up just the bottom. I am also leaning towards doing the pod style outboard mount instead of a splash well. These plans look like they run 2x4 lumber and I have seen them with 2x6 bottom.

 

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I was leaning towards using heavy duty construction adhesive PL for all lap joints or sandwiches of panels with screws in all lap joints . Than I plan to cover the whole outside in fiberglass for protection from water and impacts with docks or underwater obstacles. Then coat all of the frames in resin with probably at least two coats of resin. Than put the floor down and coat with resin and fiberglass to the sides to prevent water getting under the floor.

 

Tell us how it goes.
I understand why people don't listen to well meaning people.
But there can be a penalty.

Do a weight study, then pick out your motor and update us.

Did you see the thread on a guy who added length, structure, and equipment to a boat, adding 5tons and then was surprised when the boat did not go as fast as he wanted?

 

If you don't design the shape of the boat and its volume distribution, appropriately for the anticipated scantlings, how will you know if it'll float the way you want, such as right side up or at the LWL you envision? Spira's dories are designed around a specific set of weights and their respective locations. As a powerboat, they're less than desireable in terms of efficiency, comfort underway and build materials, but yeah, you're the designer/builder. It's just as easy to build a boat that will not meet any of your expectations, as it is one that does. You can guess and hope or take a more refined course, it's only money and time, right.

If you're going to use PL as the adhesive, you'll spend more than using epoxy, ounce per ounce. Epoxy is superior in every regard, particularly dynamic loading, which boats are highly subjected to. PL is s static load adhesive with a very poor modulus of elongation attribute. PL works great to keep paneling on the walls of your family room or to keep plywood from squeaking over the floor joists, but ask it to accept dynamic loading and it'll fail, because of this elongation modulus weakness.

Lastly, there no sense in using epoxy, unless you're going to encapsulate all the wood, not just places you think you should. The only advantage to its use as a coating, is to stabilize moisture content. Unless the pieces (all of them) are completely coated, including screw holes, cutouts, etc. you just slathering on really costly goo that's not going to prevent moisture gain.

Given the costs of the Spria plans and others in this general size range, what's the point of the "hunt and peck" approach to the design? You'll spend more on a single coat of paint on this puppy, compared to the plans, so is the budget this tight, you need to peel off a hundred bucks? If this is the case, you're going to cry, when the cost of lots of other stuff hits the fan. The last place you want to cut a corner is in the plans.

 

Very good points.
You will help yourself if you figure out the weight and cost. Both mean while you are designing, you need to keep a running list of the parts, and the equipment.
The hull is going to be relatively cheap compared to motors, etc.

 

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I will do some changing up my plans for the weight materials. I been doing some reading on the glues . What do you recommend than for the joints and covering the outside of the hull . Sounds like epoxy is best for joints but not for covering because of the cost. Am I going to lose strength if I use polyester resin in my joints and to cover the outside? Or will I be better off doing all the joints in epoxy and than only use polyester resin to glass the outside? I know polyester doesn't bond to epoxy but epoxy will bond to polyester. Since you said coating the inner parts with resin will not have any benefit as far as protection from water. What do you recommend to cover the inner parts with? Would a oil base paint or stain be the best approach for that ?

 

Epoxy is best for gluing joints and coating the hull.

Polyester was shown long ago to not work well with wood, it doesn't stick well and doesn't protect from water as well as epoxy.
No question that it is cheaper.

Why do you want to coat the outside with epoxy/ glass or polyester?
Strength, abrasion, water resistance???

Here is a reference: http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook 061205.pdf

The statement that coating the inside does not help with protection from water is an opinion, not shared by the guys who did the most to popularize epoxy / wood construction - the Gougeons.

 

As mentioned there's no advantage to using polyester for the adhesive or for the sheathing. Considering the total cost of build materials, cutting the adhesive or sheathing corner seems silly, if you'd expect some durability. It's sort of like putting the cheapest set of tires you can find on a Corvette and expecting it to still handle well afterward.