How thick should my cabin bulkheads be??

[mongo75]

I'm restoring a 1968 Luhrs 25' Flybridge. I can't remember how thick the old wood was that I pulled out. I plan on using 1708 to glass both sides. Do you think 1/2" is thick enough for bulkhead panels, or should I used 3/4"? I don't want this thing to be too darn top heavy, but at the same token, I don't want the whole cabin to collapse if I jump a 2' wave at 30 knots.

Here's a link to my project
http://www.bloodydecks.com/forums/boating-discussion/86033-epoxy-worth-extra-money-over-vinylester-resin.html

OK- after looking at the old pics, I recalled the sides where actually 3/4" hardwood planks held together with biscuits. I'll still take advice though!

[Roly]

David Gerr's "Elements of boat strength" is a good book on the subject.

[mongo75]

I don't have the book, does anyone out there that can maybe shed some light on this question?

[Roly]

Firstly, I am not a pro boat builder.
Imo, 12mm (1/2") glassed with 1708 would be adequate. 5/8ths would have less flex.. That is guestimating your beam and depth of hull and position and height of the bulkhead. But you will not regret paying the 20$ for the book. There are a lot of specfications in your rebuild to be calculated. Get it from a library if you don't want to own it.;)

[mongo75]

Thanks again, I'll look it up!

[mongo75]

Just bought it on ePay!

[Kaptin-Jer]

Mongo.
Since it sounds like you are building new bulkheads why not look into using composite (closed cell with glass on both sides)? It is much lighter and also stronger that wood.

[mongo75]

I wish I could afford the stuff- I have to resort to laminating ply to build my cabin.

[Kaptin-Jer]

I hear that!! but I don't think the foam stuff will be that much more than Marine grade ply. (You are using Marine grade??) Worth checking out. Try posting the question in the material forum. There are a lot more knowledgeable people that can answerer the price question better then me. I just wanted to give you an option, something to think about.

[mongo75]

Honestly, I'm using regular old 1/2" ply from Lowes. They don't sell marine ply, and I know if I encapsulate it properly, it'll do fine. It's my understanding that the difference is marine ply has no voids and it's glued together with waterproof glue? I hate "cutting corners" but I'm force to save money where I can. I pay about $30 for a sheet of 1 side sanded 1/2" and the same sheet of divyncell is $110 plus shipping. A finished piece is $225!! Call me cheap, but that stuff is expensive!!

[Roly]

As I understand it BS1088 no longer is a standard.(To Loyds anyway) Glue should be resorcinol (red -brown)or at least waterproof. Wouldn't worry too much about small voids as long as it doesn't sound hollow over a bigger area say than a match box. Big voids can delam. under edge-ways compression.
Just saturate any end grain before you encapsulate.

[Kaptin-Jer]

Gunny your preaching to the choir. I just finished awl-gripping my boat. I wanted to put on a coat of clear but I ran out of reducer and I can't wait to have enough money to buy more because it's costing me $20.00 a day on the hard, and I can't work on the boat during the week, so she is going back in the water. I'll clear coat while it's in the water. You do what you can. It will be perfect. Only you and I will know it wasn't done the "right" way and I won't tell.

[mongo75]

I hear ya- and believe me, she will be sound and reliable, just not as expensive as some other rich folks boats LOL

[Kaptin-Jer]

Lots of people forget that most of us are amateurs that just want to be in a boat in the water. It doesn't matter if we didn't fare the rudder to the exact wing calculations, it will still turn the boat, or only put one coat of bottom coat on instead of two, We'll put another on next year. --Only 5 coats of Awl grip--we'll do it again next year. It's the art of skillful compromise, between the "right way" the expense, the time and your skill in making the finished product look good and be safe. All done with out touching the mortgage money or college funds. A lot of the Pros in this forum are too use to using other peoples' money and working on other peoples' boats and take a lot of pride in doing things to perfection. I wish I could, but we have to compromise, and find alternate solutions to get the same results.

[Roly]

Well said, kapitan-jer

[AroMarine]

Mongo, for major bulkheads and gunwhales and decks 3/4" is the way to go. For cabin sides and tops 1/2" is ok with proper battening. If you seal everything up with a min of 1.5oz mat you should be ok. AC fir ply from lowes is better than pine. Pine has a lot more oil in the wood.

[mongo75]

Thanks for the info, sounds good to me. I think I'll put 3/4" on the sides as well because I'll want the extra strength due to having the big window cut out on the sides. For the roof, I'll lay some thin 8oz glass on a 1/2" sheet, and once it's glued and screwed to the supports / bettens, I'll sheet the entire cabin with 1708 (Lord knows I bought enough of the damn stuff) and that'll give some good structural integrity. My biggest fear is having the whole damn cabin collapse on my family if I get in nasty stuff, ya know?

[mikentx]

The original Bulkhead Plywood on a 1968 Luhrs is 3/4 inch thick

[mongo75]

Thanks again Mike- I guess I missed a few measurements before I tore her apart LOL

[the1much]

only 5 coats of awl?,,,man you are a cheapo hehe ;)
and mongo,,,,dont worry bout the ply,,everyone gives me crap bout it,,but i did a boat down here 6 years ago,,,its next door,,,,and i used CHEAP ply,,,and even coated it with even cheaper poly resin,,,and i still go over once in a while and jump on it,,,i havent fell through yet hehe ;)