Skin thickness

[bearwen]

Hello All,
Well I have been playing with the vectorlam spread sheet for sandwihich cores and well I have a few questions. First thing I am building a 30' offshore speed/fishing boat (Baja, checkmate type but less fancy for fishing and High speed fun) and well I am using 1/2" balsa core 9 1/2 lbs density (was going to use foam but core shear was low and didnt feel good about it) anyways I am looking at 1 layer gelcoat 1 layer 17oz biax w/ mat balsa core then 1 layer 17oz biax w/mat the progran says that this is good to go based on DNV rules but it seems thin to me. I have no experience with sandwhich cored boats only delt with and seen solid glass boats so looking for insight. Also I am considering building the whole boat sandwhich core is this a waste? Should I just do the bottom of the hull the transom and the decks sandwhich? If you dont sandwhich the sides of the hull how thick should they be how many layers? The boat is 30' LOA 8'6" beam with a 24* deadrise @ transom the stringers are spaced @1' for now.
Thanks
Bearwen

[Karsten]

Usually you wold use solid laminate in the bottom and sandwich everywhere else. Reason is that the bottom is dynamically loaded (slamming) and you have to make sure that the sandwich really works. There have been problems with delamination and core shear failure. Since the bottom is dynamically loaded the static shear strength of the core is not very helpful. You want high toughness and balsa is the worst core in that respect. Use linear PVC or Corecell instead. You should be o.k. with a sandwich bottom if you don't use female moulds. If you use female moulds there is always the core bond issue. Use perforated core and a good vacuum. It's impossible to see if the core is bonded to the skin in every location. If you use perforated core you have to check if glue is comming through each hole.

The stringer spacing looks like it's designed for single skin. 1' is pretty small. A proper sandwich should deal with about twice that spacing. If core shear is a problem you can use higher density or thicker cores. My guess is 25 - 30mm for the bottom and about 20mm for the topsides and deck. Superstructure can be 15mm. Just a rough estimate. You will find that the core is quite expansive. If it's a custom build boat I would go for epoxy and get rid of all the mat except against the gelcoat. It's not structural and just soaks up expansive resin. Using epoxy allows you to use better material properties, you get rid of osmosis and the boat is much tougher. If it's engineered well it will last forever. The DNV minimum fibre weight requirements are a good guide. Although they are a bit heavy for a boat this size. I wouldn't get too worried if you use a bit less.

Cheers,
Karsten

[bearwen]

Karsten,
Here is what I got I am using epoxy resin the boat is 30' with a 8.5' beam and a 24 degree deaedrise. The main question I have is the laminate schedule what I am looking at is 1 layer 20 mil gelcoat 1 layer 17oz biax w 3/4oz mat then 3/4" balsa core then 1 layer 17oz biax w 3/4" mat this will be built over a male mold and vacuum bagged or infused. The big question I have is are these skins thick enough they seem thin to me but according to dnv they are more than enough. I have thought about foam core but the sheer strength was low and the cost is high plus I am using epoxy so osmosis is next to nill. The next problem I have is should I core the rest of the boat ie gunwalls, decks, etc... and if I core them how thick of a core and what laminate schedule? I thought for the main hull and transom 17oz biax 3/4 balsa 17oz biax but use 1/2 or 3/8 balsa in the exterior gunwalls and on transom use 2 layers in and out of the biax. Then for the decks and hatches and inner gunwalls, sundeck etc... use 1 maybe 2 layers 18oz woven roving then 1/2 or 3/8 balsa core then 1 or 2 layers 18 oz woven roving. How does this sound to you guys? The inner decks and walls I am sure are good regardless of what core and how many layers the big question is the skin thickness on the hull bottom sides and transom.

Just a thought but what about foam core on the bottom and balsa everywhere else????
Thanks for the input.
Bearwen

[Blue Heron]

Bearwen,
I'm no expert, but that laminate thickness seems way too thin. one layer of 17oz biax and a layer of .75 oz mat is only going to be about 1/8" or so thick.

I've been reading Dave Gerr's "Elements of Boat Strength" and it seems like it would be a good resource for what you're trying to do. He uses a formula based on length, beam, and hull depth at midships to calculate a Scantling number that you use as a constant in other formulas you find throughout the book. You can calculate laminate thickness for hull bottom, topsides, decks, stringers, etc. He also gives formulas for calculating core thickness and layup schedules for cored laminates. I started on a spreadsheet using his formulas and I've peen playing with it a little.

If you're considering homebuilding a 30' go-fast boat, the $30 or $40 the book costs would probably be money well spent, even if you just use it as a frame of reference to cross check other sources.

Dave Williams

[Karsten]

The DnV High Speed Light Craft minimum fibre weight requirements are 2400 g/m2 in the hull bottom outer skin and 1600g/m2 in the topsides outer skin and complete hull inner skin. The weights are scaled with the boat length but scaling stops below 20m. This is for sandwich construction obviously. Since your boat is only 9.15m long I wouldn't be surprised if you can meet the strength requirements with about 3/4 of the above fibre weights.
For an accurate analysis you would have to know the frame and stringer spacing, boat displacement and speed plus the area it's operated in. Are you going to use it on a lake or jump waves on the ocean and does it meet the requirements to be "classed" as a high speed and light craft?
Foam in the hull bottom and balsa everywhere else is a option.

[Ssor]

My only comment here is that nothing too strong ever broke, and being twenty miles off shore is not a good place to learn that you didn't do your figures right.

[bearwen]

Ok,
Well I have been playing around with the spread sheet and according to the spread sheet the single layer biax 1708 inside and out is strong enough I added 1.5 oz mat to bulk it up to meet the DNV minumum skin thicknesses of the spread sheet (I have also added to the biax 1 layer on the outside that is all good but I want to know if I can do it safely with only 1 layer inside and out lighter is faster). Ok Karsten here is the boat offshore wave jumping, poker runs and skiing with my kids frame spaceing is 3' stringer spacing is 1.5' dispalcment well I cant find the number but its in the 6000 to 8000 lbs range speed well not sure I am looking for 65 mph minumum (there are so many options with power plants I havent decided).

You can down load the spreadsheet that I am using from the company vectorply there web address is http://www.vectorply.com/lamdesign/vectorlam_sw.htm

Thanks for all the input so far.

[Karsten]

I'm happy to give general advise but if you ask me to engineer your boat without having seen a drawing I have to decline. It can lead to all sorts of problems for me. Therefore I'm not going to do that. If you want your boat engineered for a good price I can refer you to somebody. They will do a professional job (which might end up on my desk in the end) especially if you also use their materials to build your boat. Just sent me a private email.

Cheers,
Karsten

[marshmat]

A word about VLAM. It's a very powerful and useful tool, but you must know how to interpret its results. Much like FEA, you must treat output numbers with caution. The numbers you get from VLAM depend only on the materials you told it to analyze- not on the conditions you're using the boat in. It cannot possibly take into account your hull geometry, running conditions, dynamic and shock loading, etc. It is your responsibility to build in substantial safety factors adequate for the conditions you intend to run in.

[roob76]

just a suggestion if you don't allready have an exact design.http://www.boatplans-online.com/stud...CS25_study.htm

[War Whoop]

30 foot boat in the 65 mph + speed range better check again outside skin that would be gelcoat skin out then 1 0-90 18 Oz and 1 -45+45 17 Oz with a 5/8 or 3/4 " Core inside could be a 0-90 oz single skin but Why ?? be safe and balance it..

By all means core the bottom..Thats where the load is at!

[roob76]

the 65 mph + speed range better check again

oops i guess i missed that part!

[zerogara]

65+ for fishing? Is that "fly" fishing?
How fast do drug running boats go these days? DEA?

Wow! Spreadsheets, some CSM, bi-axE, core, and a 1 ton motors.
Are you sure you understand the consequences of something going wrong with hull integrity at 65+?

Why don't you buy a Cigarette and think why you can not convert it to a sports fisherman and still maintain its speed!
http://yachtworld.com/listing/pl_boa...11&slim=quick&