RickK's 170 rebuild

[gman 82 aquasport]

By looking at your work one would guess that you have done this before Those stringers look great..

[gran398]

DB...great idea by your friend on the "mother of invention" tool.

Nice work Fearless Leader

[RickK]

By looking at your work one would guess that you have done this before Those stringers look great..

Nope - nothing of this scale.  "White knuckling it" plus getting some good tips from the Master Rebuilders here and the WWW.

[gman 82 aquasport]

Yeah the www can do wonders   I took a lot of ideas from boatbuildercentral...they had some good tutorials still it looks great

[RickK]

I have a post over there for ideas right now.  Good guys there.

So anyway, I'm getting the pieces of the puzzle together and now need to figure out the exact stringer layout. I had 3 stringers as seen in one of the pics above.  but I don't think I can use 3 anymore due to the tank.
The following drawing is what the thought is


I know that I need continuous longetudinal stringers for strength and I think I can fit 2 in.  
The stringers I made are 5.5" wide at the top and about 9" wide at the bottom and are roughly 11-12 inches deep. I would like to raise the floor an inch or two to facilitate self bailing - as she sits right now (see pic posted a few ago) the scuppers are under water at rest. Plus I'll need that height to accomodate the new tank.
The drawing shows that the chine width at the transom is 63" and it starts narrowing and at the front of the tank is 55".
If I move the stringers in from the outside 14" (outside stringer top measurement) I think I can can get about 135" of stringer before they disappear into the hull.  This unfortunately runs through the tank so here is my question.
At the top of the drawing you can see the profile of the stringers and the profile of the tank. I need to notch the tank a couple inches into each stringer to allow this to work - will that still give me good strength?  Thinking I will glass the cutouts back in, inversed as a ledge.
I will use the 90 degree stringers as bulkheads for and aft of the tank.
I think I supplied enough pics to give you a good idea of how she sits in the water, the old stringers and drawings to help you visualize the plan.  
The console back edge was 64" from the transom so that is where I started the back of the tank - might move it forward 2 more inches to allow the pie cover for the fuel feed and guage to be inside the console flooring.

Any thoughts?

[seabob4]

Rick,
You'll be fine on notching the stringers to accept the tank width, as, given the size of your stringers, you aren't removing that much material.

Most definitely glass back over the notches, just make sure that you cut the notches big enough so that when you re-glass, the tank will still fit...

You wouldn't believe the amount of "notching" we used to have to do at Stamas.  Bell housings, fuel coolers, starter motors, dip stick tubes, you name it...

[Capt. Bob]

I need to notch the tank a couple inches into each stringer to allow this to work - will that still give me good strength?  Thinking I will glass the cutouts back in, inversed as a ledge.

I see no structural reason not to do it that way. The stringers also support the tank in many boat designs so why is yours different. If you support the tank on the sides and bottom, it (stringer support in that area) should be just as strong, maybe stronger. Using a poly tank, you can foam that sucker in and it becomes part of the hull support system. It's a 170, not Miss Budweiser.
http://www.youtube.com/watch?v=XDOHp2VX-xU

SB beat me to it

[seabob4]

Must be the Maker's, CB...  

[gran398]

Maker's is good.

Per the diagram...the stringers will be notched nearly  in half.

Let's all sleep on this.

And put our head's together manana.

[Capt. Bob]

That Maker's 47 is a bit stronger than the usual one and compliments my ever aging reflexes SB. If I was a gunfighter, I'd be on Boot Hill by now.

As far as the stringer section goes, I'm no boat builder but I do remember a few (very few) concepts of forces acting upon objects.
Visualize the 170 hull as completely flat like a sheet of plywood or a piece of paper. Using a sheet of paper, place it on a table and observe the forces acting on it. The big one is gravity pushing it down on the table and the opposing force is the medium(wood) that supports the paper. Now slide the paper toward the edge of the table so that a section cantilevers over the edge. The composition of the paper (material, thickness) will allow the paper to resist a turning (bending) moment up to a point. As you continue to slide the paper over the edge, the downward force of gravity (which remains constant) will over come the upward force of the medium (no longer the density of wood but of air) and the paper will succumb to that bending moment.

Now, fold the sheet in half, long ways, so as to create a crease (keel) down the middle and open the paper. Now slide it over the edge with the "hull" perpendicular to the edge of the desk and notice how much further the sheet extends until it can no longer resist that bending moment. Continue to add creases to the paper on each side of and parallel to, the original "keel" crease and try again. These additional creases along with the original "keel" crease redirect/distribute the forces that act on the paper exactly like the stringers and keel in your hull. The size of the stringer(s) as it runs parallel to the keel can be wider to reduce the number needed or smaller if one preferred to distribute the forces acting on the hull more evenly for some engineered reason. As you(Rick) stated, the stringers stiffen the hull helping to resist bending moments and as SB stated, they (stringers) can also have multiple purposes such as bulkhead, engine, fuel tank, generator mounting surfaces and even wiring support . Even with all that, the thing they do the best in your small hull is support the deck, which when connected to the stringers adds even greater resistance to these bending moments we often refer to as hull flex.

As SB stated, you need you concentrate on providing support and clearance for the tank itself.  
After all, your not trying to act like Hugo Vihlen are you?

Good luck.

[RickK]

Yup I agree and the decking will help stiffen it up.  I need to make sure to adequately support the decking with cross members so there will not be much flex in the floor. Distance between tops of the stringers appears to be 23" going down the center and 14 on sides.
Look at the notches in the stringers on this Seacraft - can definitely see the platform for the deck though.

[gran398]

The good news is there will only be a small amount of material removed. Something to consider is the expansion of the poly tank. Maybe allow 3/8th's per side where it fits the notches?

[seabob4]

Scott, my experience with roto-molded tanks is that they expand in the "unsupported" areas, i.e. the tops, bottoms, and sides, basically in the center of those areas.  The corners, which are basically the dimensions of the tank, remain the same.  This expansion is not the same as foam expansion, which can actually break stringers and bulkheads.

Typically we like to leave 1/2" on all sides of the tank to allow for expansion.  On the same token, by CFR requirements, a tank must not be able to move more than 1/4" in any direction in regards to the the mounting surface.

What one doesn't want to have is a situation where the tank, regardless of the material, is force fit into the tank compartment...

[gman 82 aquasport]

I'll toss my 2 pennies in here :) I assume that you are going to put some type of floor under the tank that will let water or stuff pass under it, that being said, if you are going to add a front and rear bulkhead where the tank is and it looks like your stringers will be cut less than half for the tank notch,and it looks like the depth of the notches will be 4 inches or less, my outlook would be to double the glass on the notches put in a floor for the tank bottom to sit on(rubber strips), bulkheads(1/2 inch glass covered ac fir) to keep everything in place and add structure, then foam under the 2 outside angle bottoms..that should support the tank well and your stringers should still be plenty strong does this sound feasable... I sometimes look at stuff to simply and might miss something Thought of this after I posted it, your stringers will still be the full width on the bottom so I can see no reason to think there would be a loss of structural support for the hull, as hull, stringer and floor will all become one

[gran398]

You asked for thoughts, so...

To follow Bob and Gary's line of thinking....the only place you'd need to be concerned with tank expansion is on the "fat" parts of the tank. Supporting the tank from the bottom as Gary said, with bulkheads on the ends...and set it in place with foam per each corner, as Chris did on mine. As Bob pointed out, the corners won't expand. Then chock it on top.

If you were to foam below the tank...and here's the empirical question...per the schematic, is it possible the expanding "fat" portion of the bottom of the tank could impact (potentially damage) the base of the stringers via the foam?