I do not really like wood as a structural material. Inconsistant properties requiring larger safety factors ( cost and weight ) and unequal strength in various directions. The primary structure uses marine grade ockoume and these are the structural strength values I have selected: tensile/compressive 10,500 psi. Modulus 1,245,000. Shear parallel to grain 980 psi.
For plywood the percentage of wood in any direction depends on the construction of the ply but for stuctural purposes I have assumed 50% each way for tensile and compressive. So for the ply figures are: tensile/compressive 5250 psi. Modulus ( 75% of timber ) 933,750. Shear parallel to grain ( 65% of timber ) 637 psi.
No extra strength is considered for the epoxy and glass ... so it is just bonus safety factor. For the honeycomb sandwich panels the honeycomb strengths are from the manufacturs tables.
The actual formulas for calculations for strength and stiffness are available from structural engineering texts and I will not go into them here. What I hope to provide are some insights into the methods used for estimating what the loads on the structure are and the decisions re factors of safety. Safety factor is the ratio of the breaking strength of the material to the design point of the stucture. If the design load on the plywood was 1000 psi the safety factor would be 5.2 . Generally with a high confidence in the material and the design estimate safety factors of 1.5-3 are typical. When those are fuzzy up to 6 may be desirable.
First for the hulls. Panel strength. This is the strength required to resist hydrostatic and dynamic pressure. Basically to keep the sides bottoms and top skins of the hulls from breaking due to being under waves. The basic process is to estimate how deep in the water a part of the hull will go then figure out the pressure at that depth and add in a dynamic pressure component for the load caused by the speed of the boat. With that figure out how thick the hull skin needs to be to support the load at the spacing between stringers.
For 10 feet aft from the bow where the panel is 14" wide the design depth is 10' under water with a velocity of 20mph. The design pressure is 5 psi. For the structure it is assumed the panel has fixed edges. Safety factor at design is 2.3 . The 10 foot immersion would be in storm conditions.
At midships the panel is 20" wide with a design depth of 5.5' ( the bottom of the cabin would be in contact with the water at this immersion ). Safety factor here is 1.95 . The plywood is the same thickness throughout the hull.
At the stern panel width is 17.5" and design depth is 7' underwater. Safety factor 2.1 .
Panel deflection was also checked to see if there would be excessive panting of the panels. Maximum deflection at load is about 1/8" so in normal operation it would be negligable.
A second consideration for the hulls is their strength as a beam. For example when a wave lifts the bow it bends the hull upward. Similarly waves impacting on the side flex the hulls. Both strength and stiffness are concerns here, so that the hull does not break nor wiggle around causing drag. Two design point were considered. The first was that the entire weight of the boat was support on the bow and stern tips of one hull. You could get to about 1/2 this load in a boatyard by blocking on the tips of the hulls. This was a sanity check. The expected design was also worked out based on 3/4 of the weight of the boat at 12' aft from the bow and 5 ' forward from the stern. This approximates the maximum bending load that could be produced as waves support the hulls.
For the sanity check the safety factor was 1.1 vertically and 1.0 laterally. For the design case the safety factor was 2.6 vertically and 2.4 laterally. Deflection at the design case was about 3/4" which would not produce noticable drag increases.
The cabin holds the hulls together. It is subject to large torsional loads from the hulls. The design point for this is also the support 1 hull 12 aft of bow and the other hull 5' forward of stern condition. This is the maximum likely torsional load. The torsion produces shear loads in the cabin with them maximum at the joint of the cabin top and bottom. The low shear strength of the plywood makes this interesting. The safety factor here is 2.3 . There are of course lots of bolts holding this all together :)
On the bolt issue a calculation was made for the bolts that hold the struts to the hulls. The design loads for the front strut were 150% of boat weight in tension. 50% of boat weight in shear. Safety factors 3.3 in each case. For the aft struts 50% fo boat weight in tension and 25% of boat weight in shear with safety factors of 4.9 and 3.3. In total 60 3/8" silicon bronze bolts hold the hulls to the struts.
Selecting design loads for the honeycomb panels in the cabin and elsewhere was an issue as well. For the aft deck which is 4' x 11' and concievably might be pooped at 5.5' above the water a rough guess was required. I used the novel estimate of 8 guys standing in the middle... or 1500 lbs. This produced a safety factor of about 3.3 in tension and compression of the thin 4mm ply skins with a 3" honeycomb core. The shear loads in the core have a safety factor of 4.4 .
For the cabin in addition to the torsion loads there are panel loads most notably people standing on the cabin and the bending loads from the strut attachment points. For the people loads a theoretical panel 5' wide and 11' long ( the width of the cabin approx ) was used assuming it was supported on the sides and a 200 lb load placed there. Safety factor was about 20 . Deflection was also checked and that was about .2" The actual cabin has support from the engine room bulkheads but in the forward part of the cabin top this load is likely. For the cabin top 4mm ply was sufficient to meet these design points. For the cabin floor 6mm was used due to the increase point loading from things that might be dropped in the cabin and for the stut bending loads and to support the engine.
The forward stut is both a beam and subject to internal fuel hydrostatic loads. The shear loads in a thick beam such as this are very high. Two design point were evaluated for the front beam. The first was a static condition as if at the dock with .375 of the weight of the boat acting on the lever arm of the beam. Safety factor tension/compression was 52 and in shear 33. The second design point was a dynamic loading condition assuming 75% of the boat weight + a 200% factor for dynamic shock loading. Safety factor in tension/compression was 4.3 and in shear 2.7. This design point was conservative. It is important to keep the basic structure together. If the tip of the hull breaks off it is pretty inconvient but with all the water tight bulkheads not a panic. If the hulls falls off you don't get back on your own.
Copyright © 2003 Tony Bigras.