Engine Stringers - Design and Compostion

[Herman]

Looking good, well done!

[ChrisN67]

Thank you Herman.

[GG]

Originally Posted by Herman

I guess we are mixing things up, me as well:

2 different things:

The SIZING on the glass fiber can be volane or silane, with (in the past) volane for epoxy, and silane for polyester. This has seen a huge amount of development, and silane now being perfectly compatible with both epoxy and polyester.

The BINDER keeping the glassmat together. This still only works for styrene resins, so polyester and VE. Using these mats with epoxy indeed is not a good idea.

I have no idea of what multiaxials look like in your place, but the stuff I sell is free if binders. Even the CSM which can be applied on the multiaxial is free of binders, and held together with the stitching only. (makes for a slightly more "fluffy" appearance, but who cares?).

Still, using CSM (powder bound, emulsion bound or stich bound) with epoxy does not really make sense to me, except for perhaps some specific reasons.

its all good .

[ChrisN67]

Originally Posted by marshmat

Chris,

We understand that you're an individual repairing a personal boat, and don't want a full engineering study. And if it were an 18' bowrider, people probably wouldn't be questioning the "No time to read, just build it" attitude.

What you've described here, though, is a 33-foot boat with, if I read correctly, a pair of 440-horse diesels. You're spending thousands of dollars on materials, and putting in thousands of dollars worth of your time.

In this context, spending perhaps three hours with a comfy chair, a $35 book and a pocket calculator is the absolute minimum of due diligence that one ought to do.

(If you do, you'll find that several of the things you've mentioned- new stringers being only in a two-metre section, motor mounts being installed into the tops of the stringers- are not ideal ways to do it. The first one leads to a hard spot at the end of the stringer, ie. a stress concentration in the hull skin, and can be alleviated by making the stringers much longer; the latter can let water into the stringer, motor mounts on L-angles through-bolted laterally to the stringers is better.)

You mention that the hull is currently propped up by the chines- have you measured how much the bottom has warped as a result? A distortion of perhaps five millimetres from the original shape could significantly affect the boat's performance at high speed. Before you start installing new stringers, you're going to have to spend some quality time under the hull with a straightedge and measuring tools- if you install new stringers into a hull that has warped or distorted, you'll lock in the flawed shape and all your effort and expenditures will be for nothing.

The repair you're undertaking is certainly doable, but you need to be honest with yourself about how much work it is going to be, how much it will cost, and when you need to put down the grinder and pick up a book.

As an aside, I heard about a Mayday call a few years ago involving a boat not too unlike yours. Nobody knew exactly what happened, because the thing sank in under a minute- but the crew, upon being rescued, thought that the inboard engines had punched right through the hull when the boat came down hard off a wave at high speed. I've also seen reports of such boats from well-known builders whose hulls broke up after spinning out in turns. Uber-beefy in one spot is meaningless- it's the sharing of loads between appropriate structural members that makes a boat strong.

Point well taken, I agree that just because it looks ok and passes some engineering "rule of thumb" learned long ago, that does not imply it was an ocean-bound thumb.

I actually have more recpect for proper planning and engineering than I let on. I do not think one or ten passes of any book will suffice (there will always be something missed in the absence of a fully comprehensive understanding) I brought a boatbuiulder to Kuwait to do the initial structural work (core and transom) He left with a stringer lamination schedule on the dry erase board.

It was designed for a "100 Knt hull". Not sure what that means But I can tell you that after all the stagger laminations om the inboard stringers were complete it is bullet proof. For the engines to " punch through" They have to go through a set of epoxy stringers with 14 total lams, a 6' x 3" x 3/4" tapping plate then the hull bottom, which at the keel, is about 16 lams of alternating +-45 and 0/90 then Airex R63 then 8 layers +-45 and 0/90 then the original skin.

The hull bottom was checked for geometry and faired. I do not want to use antifouling so I opted for 8 coats of AWL grip over 6 coats of Interprotect and 545 primer.

Although technically Awlgrip is not designed for below the waterline due the to the effect of UV inhibitors in the crosslinking of epoxy (and other additives), if enough layers are applied and overcoat times are respected, it will give enough protection for 30 days soires. (Hopefully)

In the end; I enjoy working and it and learning. I am lucky enough to have the resources.

Thank you all for the input. Gonzo / Marsh, your points are well taken.

[marshmat]

Glad to hear you've got it figured out, Chris Happy boating!