# Ring Frames, especially High Modulus

Discussion in 'Fiberglass and Composite Boat Building' started by BlockHead, Jun 30, 2019.

1. Joined: May 2013
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Location: Croning NY

I'm using The Elements of Boat Strength and SketchUp to design a 50' cruising cat.
LOA/LWL 50', BOA 25', Hull Beam 6.24', and the Depth of Hull 10.85' (excluding the height of the bridgedeck).
I'd appreciate any thoughts on the following questions.

A1) Can the scantlings of a cat be calculated or extrapolated from the book?
A2) If so, is the Scantling number (Sn) calculated with the BOA or with the Beam (hull)?
A3) If it's calculated with Beam (hull) then is the volume doubled (for two hulls) before being divided by 1000?

Assuming a Sn=(50'*6.24'*10.85')*2/1000=6.77, my 8 cored ring frames are intrusive (5.89" sided and 6.28" molded).
B1) Do ring frames really cross the ceiling at full molded dimension?
I don't think I've ever noticed seeing that.

I know aramid and carbon are used strategically even in some mass production.
C1) Is anyone making aluminum or carbon ring frames to save space?
C2) How big approximately, would equivalent solid uni-directional, carbon, fiber ring frames be?
C3) How much approximately, would equivalent solid uni-directional, carbon, fiber ring frames cost?

Thank you.

Last edited: Jun 30, 2019
2. Joined: Sep 2011
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### TANSLSenior Member

I would not risk calculating a catamaran of 50', in FRP, with the book The Elements of Boat Strength. There are more modern tools that will allow you to better optimize the structure of your boat.
In general, extrapolation is not good practice, it can be a serious mistake, unless you are dealing with properties that vary linearly, which is not, at all, what happens with the scantlings of a ship, and even less of a multihull.
Mix FRP and aluminum in the structure, if that's what you're thinking, it's not advisable.

jehardiman likes this.
3. Joined: May 2013
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Agreed, I'd never trust myself and/or The Elements of Boat Strength and SketchUp to build a million dollar boat that lives would depend on.

I'm mainly interested in designing the accommodations. I want to know the hull thickness(es) so that I know what space I have to work and what internal structures I have to work around.

Are any more modern tools relatively inexpensive?

Thank you.

Last edited: Jun 30, 2019
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### TANSLSenior Member

There are no cheap tools if you just want to calculate one boat. Any tool is very cheap when it comes to designing several boats, especially thinking about how they allow you to optimize your structure, save material and labor. Maybe it's cheaper for you to hire an expert on that subject.
I have developed an application that allows calculating the scantlings in accordance with the ISO 12215 standard, which is the one applied for vessels of less than 24 meters in length. My software, SCT, is not free but allows you to use it during a 30-day trial period, which should be enough to calculate your boat.

5. Joined: May 2013
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Ya, didn't think so :/

A trial of SCT sounds good though.
Does it work for cats?

Thank you.

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### TANSLSenior Member

Although ISO 12215-7 (which should regulate catamarans) has not yet come into force, SCT performs what is called "direct calculation" for the wet deck, inner bow shell and transverse beams. So, yes, you could use SCT for your catamaran. Do not have any problem in asking me your questions, if you need it, by means of a pm or email.

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### jehardimanSenior Member

I concur with TANSL here, The Elements of Boat Strength is rather simplistic with respect to multihulls. While ring frames could have some usefulness with the actual skin of the hulls, it is really the bridge deck connections that deserve the most analytical work. While loads on monohulls increase with dimension^3, loads in the bridge deck of multihulls increase with dimension^4. Make the joint too stiff and the hull fails, too light and the hull separates, too heavy and the boat performs poorly because you can't make weight. While ring or space frames, especially ones integrated into the bridge deck, have some usefulness it is important to match strength, stiffness, and weight in the composite shell to bridge deck connection to prevent making unknown load concentrators. The actual analysis of the hull to bridge deck connection should be rigorously detailed.

8. Joined: May 2013
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Location: Croning NY

Would the 30 days start when I downloaded it or installed it.

Thank you.

9. Joined: May 2013
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Location: Croning NY

Interesting thoughts.
Thanks.

10. Joined: Sep 2011
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### TANSLSenior Member

The 30-day trial period starts counting the first time you start the application.

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