Classic C-Hawk 18 Rebuild

[tech_support]

moving the weight of the engine back will trim the boat to the stern, so that will help a little.

increasing the sole slope is much easier when you are replacing the stringers, you can cut the new angle into the shape.

I have had customers who built a "wedge" into the bottom side of their sole before bedding it down to the stringer tops, but that was on a boat with very wide fiberglass trapezoid stringers. You could make the "wedge" from steps of plywood, gradually working up to the bow. Unless drainage was a real problem, then I would not bother with all the trouble.

[Willc]

I'm just before tracing the first stringer from my template, so now is the time. I was figuring 1/4" per foot, but that puts me 3 1/2" higher at the bow and I'm already raising the sole an inch. So now I think I will just use 1/8" per foot and let the motor and jackplate trim the rest.

Does that sound reasonable?

Thanks!!
Will

[tech_support]

Does that sound reasonable?

sure.

3" higher is not excessive, but you might not need that much depending on the motor trim effect.

[Willc]

I'm starting on the stringers in earnest now. I've got the first one expoxied together and I'm waiting for it to cure. I've got a few questions about the best stringer layout. Pre-repair, the boat had a central stringer that was butchered for a gas tank. It was about 14" tall and ran from bow to about 12" shy of the transom. A five foot section was cut out of the middle for the tank. Only 2 or 3 inches of stringer was left in this section. I've owned this boat for over 10 years and it has done fine with this questionable modification. See the pic below:




I'm going to put the tank back in the same location. How much structural support am I really getting from the central stringer? Can I just run a 3 or 4" tall one down the middle?

The outboard stringers seem to be more for sole support than structural. Can they be replaced with cleats on the side of the hull?

Here is what I was thinking:



All stringers and bulkheads would 1 1/2" meranti. I would put ample glue cleats on stringers and hull. The sole will be 3/8" meranti with 1' stiffeners across the long runs. I would use the full size central stringer in the bow section for extra crash resistance. The dashed line would be the shortened central stringer (3-4" tall.)

I realize that there are many factors in boat design and serious engineering behind it. I am just looking for an educated opinion. What would you do if it was your boat?

BTW, I'll have to order more 3/4" plywood if I do much more than this.

Thanks,

Will

[tech_support]

All stringers and bulkheads would 1 1/2" meranti. I would put ample glue cleats on stringers and hull.

thats pretty thick, not needed. You could use 1" or even 3/4" total thickness.

Your boat had tighter stringer spacing than what you show in the drawing (smaller bottom panel size). Thicker stringers will not help at all in reproducing the same stiffness in the hull. You will either need to have the same spacing, or increase teh thickness of the hull in order to have the same stiffness you had before.

Also, you would have a wider span between supports for the sole/deck. This will then require a much thicker deck (at least 1/2").

Can I just run a 3 or 4" tall one down the middle?

Yes. In fact, with 3" you get almost all the stiffness you need. When you do the calculations on the stiffness of 3/4" laminated wood laid on edge; 3" is extremely strong. The real reason to make the stringer full height is so you can support the sole. Having the hull and sole bonded together to creates "boxes" which really make the boat solid.

Many production fiberglass boats were built with stringers only going up 3 or 4 inches, then the deck had stiffeners (stringers upside down) glassed to the underside, This way the deck/liner and the hull could be made separately, then they injected foam.

[Willc]

thats pretty thick, not needed. You could use 1" or even 3/4" total thickness.

Man, I wish I had posted last night, before I laminated two 3/4" pieces of plywood together. Oh, well!

Just to make sure I've got it right: Same stringer spacing, but I can reduce the height of that center stringer. And use lots of stiffener on the sole going over the tank, because 3/8" is kinda thin going across the 22" span.

Thanks for the quick reply!

Will

[Willc]

The stringer that I laminated together is not hard cured yet. Should I try to pull them apart. It sure would save a lot of weight.

[tech_support]

The stringer that I laminated together is not hard cured yet. Should I try to pull them apart. It sure would save a lot of weight.

that is up to you.

Same stringer spacing, but I can reduce the height of that center stringer.

Yes, and when you cut it down in the middle, round the inside corners off (not 90 degree cuts)

And use lots of stiffener on the sole going over the tank, because 3/8" is kinda thin going across the 22" span.

Or just use 1/2" on the sole. You should really only need one of two stiffeners under there.

[Willc]

I couldn't wait on the reply. The epoxy was just a little gummy. I had about a minute to spare, literally.



This saves me an entire sheet of meranti. It will be worth the grinder work to clean them up. I will round over the top for the 1708 to make the bend.

I already have 3 sheets of 3/8", which is why I didn't hop all over using 1/2" for the sole.

First real mistake and it was salvageable. I'll take it!!

[Willc]

Getting ready to tab the first stringer. Do I need 2 layers of 12 oz biax (offset) or 3? Also, once it's tabbed, how much 1708?

Thanks!

Will