Dimensioning of plywood cockpit floor

Hello everyone,

I'm currently working on the design of a cockpit floor and need some advice on the structural requirements. Sure, the floor is supported by a system of frames and stringers, but I'm unsure about the critical load cases and appropriate design assumptions for dimensioning the plate, frames, and stringers.

My primary concern revolves around localized, concentrated loads from the crew. I've identified three scenarios — do these seem appropriate and are there others I should consider?

1. Distributed Load: A person is standing generally on a supported area of the floor.
   
   
2. Concentrated Load 1 (Stepping): A person stands with one foot (full weight) in the center of an unsupported panel.
   
   
3. Concentrated Load 2 (Extreme): A person kneels (focusing weight onto a smaller area) in the center of an unsupported panel.
   

If these load cases are applied, what is the maximum acceptable deformation or deflection, so that the floor still feels nice and rigid? What are the common industry standards or rules-of-thumb (e.g., deflection-to-span ratio) used in boat design to ensure adequate stiffness in the cockpit floor?


Many thanks for your advice.

 

I calculate the panels, in general, so that their maximum deflection is less than half their thickness.
Consider a load of 4 people per square meter, although the weight of the water in the cockpit will likely be much greater. Whatever weight you consider, you should take into account the vertical acceleration in that area.

 

Thanks TANSL for the reply.

Are there any other opinions on that topic?
In my case I am talking about a 8 m plywood sailboat with an open cockpit. So there is no possibility that greater amounts of water accumulate.

According to ISO 12215-5 the panel thickness should be not less than something around 4-5 mm. But ISO doesn't encounter any local forces from crew, etc.

I am trying to get the structure done for a 8 mm thick plywood cockpit panel.
@TANSL are you now referring to a maximum deflection of 4 mm for the whole structure or for a local load, according my mentionned load cases, on an individual panel of the cockpit floor?

My current cockpit area looks like this. The individual unsupported panels measure 650 x 266 mm.

Later the frames and the two green longitudinal parts will get some openings. The web depth depends on the outcome of these consideratios.

 

8mm ply with no fiberglassing, just paint?
No dimensions given in your graphic but it looks reasonable.
I concur with Tansl's comments and reasoning.

 

Even without water accumulation, the ISO standard always provides a design pressure for each panel, depending on its location. Water doesn't need to accumulate for a load to be placed on the panel. Many of the loads on the panels are very short-lived, but the panel must withstand them.
Regarding the cockpit, the ISO standard provides a design pressure, distinguishing between walking and non-walking areas, i.e., the bottom and sides of the cockpit. I assume this takes into account the crew's weight (I don't know the exact amount) in the walking areas. However, if you want to be more cautious, you can add any N/mm² you deem necessary to the ISO design pressure to account for the potential accumulation of people on a panel.
As for deflection, if all the panels are 8 mm thick, I would consider a deflection of 4 mm for each panel. Keep in mind that this criterion for maximum permissible deflection is usually more stringent than the ISO criteria.
When an opening is created in a panel, the perimeter of the opening is usually reinforced with reinforcements similar to those already present in that area, and all connected together. It is not necessary to recalculate the perforated panels.

 

Hi ..

Wouldn't it make more sense to relate the maximum deflection to the aspect ratio of the panel, or to its size in general? If a 600 x 300 mm panel deflects by 4 mm, that's quite different from a 400 x 200 mm panel deflecting by 4 mm. Hence my question: what is generally accepted as the maximum deflection for a panel or surface to feel “rigid”? When you step on it, you shouldn't feel the panel giving noticeably under your feet. And for this, the deflection per length should be used.

Regarding ISO... I have worked my way through most of the standard and written my own calculation tool for my specific application (amateur build plywood multi-chine with open cockpit).
Below, I have posted the main data for the project, information about one of the panels in the cockpit, and my calculation results.

As you can see from the results, the design pressure (P_DS) is specified as 5.585 kN/m². Based on a panel measuring 650 x 266 mm, this would be an approximate weight of 98.5 kg – evenly distributed over the entire surface of the panel. Even a 5 mm thick panel would feel far too thin for this application from a practical point of view.

Main data:
{
"Type_of_craft": "recreational",
"Type_of_hull": "Monohull",
"Type_of_boat": "Sail",
"Design_category": "B",
"L_H": 8.0,
"L_WL": 7.9,
"B_H": 2.53,
"B_WL": 1.915,
"B_C": 1.158,
"D_max": 1.376,
"T_C": 0.268,
"Draught_at_mLDC": 0.268,
"m_LDC": 1860.0,
"m_light": 1530.0,
"rho": 1025,
"A_S": 42.0,
"Maximum_Speed_at_mLDC": 16.0,
"Beta_0d4": 22.5,
"maxGZ_60": 0.638
}

Panel data:
{
"panel_id": "COCKPIT_BOTTOM_0.96",
"panel_type": "deck",
"access_type": "accessible",
"x_position_m": 0.967,
"Z_Q_m": 0.891,
"Z_SDA_m": 0.891,
"panel_length_l_mm": 650,
"panel_breadth_b_mm": 266,
"curvature_cb_mm": 0.0,
"curvature_cl_mm": 0.0,
"material_name": "Plywood 8",
"sigma_uf_NPmm2": 30.0,
"tau_u_NPmm2": 1.0
}

Results:
-------------------------------------------------------
| PANEL: COCKPIT_BOTTOM_0.96 (deck)
| Material: Plywood 8
| x = 0.967 m
| l = 650.00 mm
| b = 266.00 mm
| k_CH = 1.000
| c_l = 0.00 mm
| c_b = 0.00 mm
-------------------------------------------------------
Design Pressure: 5.585 kN/m²
k_DC: 0.800
k_DYN: 3.000
k_L: 0.522
k_AR: 0.744
k_SUP: 1.000
k_SLS: 1.223
P_BS base: 51.340 kN/m²
P_BS: 15.932 kN/m²
P_DS base: 17.997 kN/m²
P_DS: 5.585 kN/m²
k_c: 1.209
σ_d (Design stress): 13.500 N/mm²
σ_uf (Ultimate flexual strength): 30.0 N/mm²
tau_uf (Shear strength): 1.0 N/mm²
MINIMUM THICKNESS t_p: 4.63 mm
Strength test using the moment of resistance:
-> SM_req: 2.95 mm³/mm, SM_actual: 3.57 mm³/mm. Test passed: True
Shear stress test (inlay shear test):
-> Shear test: Required thickness: 2.08 mm. Minimum thickness: 4.63 mm. Test passed: True

 

It's a bit late to review your calculations, so I won't do it until tomorrow. Regarding deflection, I don't think I explained myself well. I don't want all the panels to deform by 4 mm. What I'm calculating is the maximum deflection of each panel, which occurs at its center and is completely different for each panel because it depends, among other things, and logically, on its aspect ratio. If you'd like to discuss this privately, so as not to bore the rest of the group, send me a private message.
Cheers

 

Hello @hashtag_laeuft , here are the results I have for your boat.

 

Hi .. thanks for your calculation results.
There are only slight deviations in our calculation results. I implemented the simplyfied method, which might use some more safety margins.
You used some other material properties than me, resulting in different minimum thickness.

But I doubt that a 8 mm Douglas Fir plywood panel of 650 x 266 mm, only supported on the edges, deflects less than 0.4 mm when an evenly distributed pressure of 5.575 kPa is applied. With that area this would correspond to a mass of 98 kg. I can't imagine that this is real.

ISO 12215-5 doesn't include any deflection calculations.
Might you please share the formulas you use to calculate the maximum panel deflection? That would be great.

 

However, unfortunately nobody replied to my core question, yet.

Are there any rules or guidelines for the maximum acceptable deflection per panel size, so that the floor still feels nice and rigid?

 

The ISO standard considers that the panels have all four edges fixed, not supported. Based on this assumption, I use the formulas from thin plate theory (Timoshenko), with fixed edges and a uniformly distributed load across the entire surface. I take the maximum allowable deflection to be equal to half the thickness because it seems reasonable to me; I can't give you a technical explanation.

 

Local panel point-load capability should be around 10,000 psi for an 8 meter sailboat. This lets you put a 1/4" bolt backed by a common washer anywhere you might want to put one. And for larger hardware sizes, you can use 1/8" backing plates that extend 1/8" beyond the edge of the common washer. Laminating epoxies start around this value, so if you use epoxy annuluses for hardware, The epoxy bond will be stable for a long time. Fir plywood needs some help getting to these values. You'll want to laminate the panel's bottom surface before installing the grid. I'd assemble the floor and grid entirely and then put it in the boat.

Some simple tests you can do to get a feel for for a panel's fastener holding ability and ding resistance is to put a panel on a press over a 4" cutout and apply 20,000 pounds to a 3" metal disk. That shouldn't leave a mark. A 2" disk will leave a mark but not be badly damaged. Very good marine ply can manage this with no coatings or reinforcement. Although it might need to be a bit thicker for the exact test described above. If you try a few tests with mahogany, meranti, okoume, and fir with a few different glass skins, say 1, 2, and 3 layers of 10 oz weave, and you press a few different bolts out through the panels, you will get a feel for how to best construct them. You might also want to find an outboard motor and plonk the toe down on the panel a few times. 8 meters seems to be about where global strength issues and point load issues combine in a way that good designs can leverage weight and cost to a high degree. It just depends on how fussy you want the build to be and whether you want it to last 50 years.

Purely from a weight standpoint, a plywood that is stiffer than fir and with a better surface finish and better ding resistance would allow a thinner skin for the same fastener holding power. But global stiffness, sound deadening, thermal insulation, cost, and the ease of forming eye-pleasing curves may push you towards a thicker, lower density core.

 

I have attached a document in which I explain how I calculate the deflection of the stiffeners.

 

Hi together ..
many thanks for your replies and your time.
Thanks @TANSL for the calculations.
Thanks @philSweet for your explanations. Not exactly to the topic, but still very valuable.

 

Is your concern about deflection or ultimate strength?