composite material - weight critical application

[Ulrich]

Hello All
I am planning to add a strong but light weight superstructure on the sundeck of my vessel (trawler yacht 62'). Composite seem to be the best choice but I could not find any advice about the skin thickness using Divinycell H80 foam. Offers received indicate a weight per m2 of 17 KG which is way above my expectations. Divinycell H80 is just about 2.8 KG / m2 so do I really need 14 KG of resin, fiber, filler & paint per m2 ? Can somebody give me a proposed laminate schedule for this sort of application?
Help, please!

A bit more information would help to find a serious answer. length, width, load etc.
You otherwise get opinions and estimations if your happy.

Regards
Richard

[Ulrich]

Thanks, Richard. Roof dimension is 6m*4m. Coamings 18m2 (windshield excluded), bulkhead/partitions 11m2. Structure to be mounted on sundeck (aluminum) and to withstand wind load of 65 knots continuously or 85 knots gust (I am living in a tropical area). Would be great to get your advice.
Regards, Ulrich

Hmm,

thats quite a bit more than just a Flybridge, so to say.

Are you aware of the much increased windage, in addition to the weight?
Did you ask your NA about this plan?

Not to chicken out here, but there has to be a substantial check if that makes sense on your vessel without compromising its seaworthiness.

Before that is not on the safe side I fear to contribute to any of such plan.

The composite layup is your minor problem I assume. (the weight estimation was not too far from reality when the roof is unsupported and low quality materials are used, btw)

Regards
Richard

[Ulrich]

The vessel is 78t empty (steel hull/alu superstructure) and 95t fully loaded (15t fuel). The existing superstructure subject to be removed is 850 KG. The last NA I had involved in this project took copies of the plans, 60,000 Thai Baht down payment and vanished. The design has been made by an Australian Architect and fellow cruiser, but he is not a NA.
The mission is to keep things as much weight neutral as possible not to tamper with seaworthiness.
I fully understand that such a specific advice I am asking for is inappropriate because of all the variables. But perhaps you are able to answer the following questions:
1. the current roof of about 3*4 meters is composite build in 2000. Foam is 1/2" overall thickness including non-skid outside is 17mm i.e. the skin does not exceed 2mm each side. A rough weight estimation ends up with about 120KG considering foam 2 KG/m2 and resin, fiber, filler, paint 4KG / m2 each side. Now this was in the year 2000 and the roof is still fine after 10 years and 35,000 NM including one Pacific crossing, storm force winds and green water. Nowadays we have better foam, resin and better reinforcements such like Aramid, Kevlar, sort of 3D cloth etc. etc. The new roof is twice the surface but has a lot more support than the current one.
Do I still need to calculate > 10KG /m2 for a superstructure?

2. solid 4mm aluminum weights 11 KG/m2. Everybody says that composite easily outperforms alu by weight to strenght. YouTube has got a nice video about that! But they don't say which core they used and the laminate. Can you answer the question what is the proposed laminate schedule on 30mm Divinycell H80 foam to achieve a comparable performance as 4mm solid aluminum plate (performance means here mainly wind load and not a sudden impact)?
Thanks for thinking about that!

Originally Posted by Ulrich

The vessel is 78t empty (steel hull/alu superstructure) and 95t fully loaded (15t fuel). The existing superstructure subject to be removed is 850 KG.

That is a severe amount of weight!


The mission is to keep things as much weight neutral as possible not to tamper with seaworthiness.

Understood.

I fully understand that such a specific advice I am asking for is inappropriate because of all the variables. But perhaps you are able to answer the following questions:.......
. The new roof is twice the surface but has a lot more support than the current one.
Do I still need to calculate > 10KG /m2 for a superstructure?

yes.
If there is a supporting structure (can be built into the new roof), a layup of about three times 350 Gsm omnidirectional and 2 separating (in fact connecting) layers of about 80 Gsm on both sides of a medium density foam (60kg m³) should be about on the safe side.

2. solid 4mm aluminum weights 11 KG/m2. Everybody says that composite easily outperforms alu by weight to strenght.

The layup I estimated above is 3x350 + 2x 80 = 1230 gsm, plus resin in handlayup = 1,5 kg = 2,730 kg. say 3kg, lets assume the same for the core, you´ll have 6kg m².

Stiffeners almost always add 30% to the weight, then we have say 10kg. No matter built in by another layer of roving or a stiffening profile, or frames in Alu.

A Al structure would have 11kg plus 30% = 14,3 kg, but a severe advantage when it comes to weathering, water intrusion, fatigue resistance.

All the figures above are nothing but estimations.

No, not because I have not enough info. Just because I do not know how to calculate them right!
I am neither a NA, nor a graduated engineer, just a merchant and boatbuilder. And I estimate and interpolate by experience, rather than rules. So, I might be wrong (on the safe side)

I would like to have Alik´s thoughts on this. (being familiar with the Thai market, and composites alike)

Regards
Richard

[Ulrich]

Thanks a lot, Richard. Very helpful, indeed. I have addressed the questions also to the foam manufacturer (DIAB) asking them to benchmark their product and laminate recommendation(s) versus 4 mm aluminum plating under weight / strength assessment.
For my understanding: 2.730 KG laminate is one side, right? So we are about 9 KG / m2 plus stiffeners?
The sundeck (or the entire deck house) is aluminum. Would you favor an aluminum solution of my project?
Thanks again, experience often out weights theoretical assessments and desktop calculations!
Regards, Ulrich

Yes one side only was calculated. (one shall not do that late at night)

Yes again, I would prefer Aluminium on profiles.

Viel Erfolg!

Regards
Richard

[Herman]

Check the www.alcanairex.com website, there must be a XLS sheet somewhere, which will allow you to to some calculations.

[AndrewK]

Your 6x4m area is comparable to a lot of 12 - 13m catamaran coach house tops.
For these catamarans a light build is 600gsm biaxial or DB glass over 20mm 80kg foam, for hand lamination this equates to ~4.7kg/m2 total 115kg + support structure.
A more conservative laminate will be 600g biax/400gDB either side of same core, ~6.3kg/m2.
In a catamaran at least half of this area is supported by the front and side windshields as well as transverse cabin entry bulkhead.
Not sure if yours is just a roof top with only post support which will have greater uplift loads to be considered.

As Richard said you need to talk to a composite engineer or NA, if Alik can not help I can recommend one.

[Ulrich]

Thanks a lot everybody! The manufacturer of the foam (DIAB) came up with a laminate schedule which would allow 10.1 KG / m2 (4.52mm laminate on 30mm foam). Diab had the project drawings.
Anyway, I appreciate all comments and the comparison to a catamaran top sounds very close to my application. The 25 m2 roof top has quite some support already and a few more stiffeners may allow 20mm foam and the laminate as proposed. This would eventually bring the weight down another 30% to some 7 KG / m2. I will discuss this with the Diab engineers.

You know that the 30 mm foam makes the roof stiffer than a 20 mm would? So, I would stay with the diab proposal. (Which was surprisingly close to my estimation)

Still my favourite would be a Al roof.

Regards
Richard

[Herman]

Also, the extra stiffeners create extra weight, not only calculated weight, but also "unforeseen" weight, due to more or less sloppy work.