Anyone have direct or second hand knowledge regarding the ability of HDPE boats in larger sizes (i.e., 30’ + LWL) to avoid excessive flexure or deformation with constant use? Any more or fewer difficulties in this regard with flat-bottomed or V hulls? I’m looking for feedback regarding monohulls rather than pontoons supporting a platform. Thank you.
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Flex/Rigidity in HDPE Boat Hulls 30’+
- Thread starter Scow
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- hdpe hdpe boats
I can't address boats that size but having had both whitewater and touring hdpe kayaks, I can tell you the material doesnt have much rigidity on its own.
The short kayaks were okay without reinforcement but the 16 foot touring boat needed a full length aluminum tube to stop it flexing in use.
I had a strap come loose and wrap around the car wheel on one trip. The front of the boat bent past 90⁰, breaking the al tube. I replaced the tube and the boat was as good as new, no discernable damage to the plastic body at all.
Engineers will give you the hard data, but I suspect you'll need some serious stiffening on a 30+ foot hull.
The short kayaks were okay without reinforcement but the 16 foot touring boat needed a full length aluminum tube to stop it flexing in use.
I had a strap come loose and wrap around the car wheel on one trip. The front of the boat bent past 90⁰, breaking the al tube. I replaced the tube and the boat was as good as new, no discernable damage to the plastic body at all.
Engineers will give you the hard data, but I suspect you'll need some serious stiffening on a 30+ foot hull.
Thanks. I’m glad you mentioned it. I’ve looked at their site before but was thinking the boats were too small to be indicative for what I’m “needing.” However, as there are standard models listed up to 3.60 m BOA, I think it suggests at least a possibility of going up to BOA of 5.0 m. I’m looking for something around 15 m x 5 m LOA x BOA and they have boats at almost 12 m x 4 m.
The maximum deformation, at its center, of a thin plate with all four edges fixed, subjected to a uniform load perpendicular to its surface, is proportional to:
The Young's modulus of HDPE can be up to ten times lower than that of a fiberglass laminate. This means that, for the same load, an HDPE panel will deform much more than an FRP panel of the same dimensions. If deformation is an index of the fatigue to which the material will be subjected, it is reasonable to assume that HDPE may have a considerably shorter lifespan than FRP.
By the way, I posed some technical questions to Bruno, and he either doesn't know the answer or hasn't wanted to answer me.
- a coefficient that depends on the plate`s aspect ratio
- the load to which it is subjected
- the smallest dimension of the panel raised to the fourth power
- Young's modulus
- the thickness of the panel raised to the cube.
The Young's modulus of HDPE can be up to ten times lower than that of a fiberglass laminate. This means that, for the same load, an HDPE panel will deform much more than an FRP panel of the same dimensions. If deformation is an index of the fatigue to which the material will be subjected, it is reasonable to assume that HDPE may have a considerably shorter lifespan than FRP.
By the way, I posed some technical questions to Bruno, and he either doesn't know the answer or hasn't wanted to answer me.
Bruno Tideman.
You should study the longitudinal strength of the ship-beam, rather than the deflection of a panel. To achieve adequate behavior in this respect, you will need to significantly increase the number and scantlings of the longitudinal elements. That is, girders with a high-modulus strength and thick panels.
In any case, my impression (I have no practical experience with HDPE) is that this material must be considerably inferior to FRP and even aluminum if we are looking for sufficient strength with minimal weight.
I beg to differ here. It is correct that a material with lower modulus of elasticity (E) will deflect more than a stiffer material (like HDPE compared to FRP or aluminium compared to steel) when loaded to its elastic strength limit, but that is not to say it is inferior as such. If we check the dimensioning rules for local strength (as differing from global hull strength), it is completely doable to come up with a HDPE hull weight comparable to alu. It will have similar (or better) safety margin regarding ultimate stress, BUT it will deflect more. You design for deflection limits, not for stress limits!
This is just the same design dilemma we saw many years ago, when uninformed yards copied steel drawings, added some thickness and applied those for aluminium; skin cracks all around. The point is: the more elastic the material, the more sensitive for load concentrations ("point loads") it is, and the constructor must be aware of the differences.
We have an analog situation with long-distance piping for water distribution etc, where the pe-quality PE80 (~MDPE) is accepted for 50 years with operating pressure 6 N/mm2, and where pressure peaks from pressure pulses ("water hammer") may occur. One aspect of polyethene that so far is not included in the scantling rules is the strong time dependence of E for all variants. For instance, the PE80 shows an E=600 to 800 MPa for a 3 minute load of 3 MPa, while it is E=200 MPa for 1 year at 3 MPa. For shorter pulses, E is even higher.
For a boat design this means that the material has higher stiffness for short impact slamming conditions than for long-term global loading. The scantling rules so far do not consider this effect. In addition, the elasticity per se will reduce the slamming peaks due to the reduced pressure-wave speed, compared to stiffer materials. This is the explanation for the impression of a "softer ride" in a GRP runabout than the corresponding alu-version; the GRP is more elastic, since they both are normally designed to some accepted yield stress level, following Hooke' law.
@baeckmo, I think we're on the same page.
Speaking of local loads, as you rightly point out, comparing panels of the same dimensions and thickness, an HDPE panel will flex much more than an RFP or a metal one. And that's certainly true if we're working below the elastic limit. So, I think we're both saying the same thing; we don't disagree at all.
The ISO 12215-5 standard, as far as I know, doesn't take into account the deformation or deflection of the panel at all. It only requires that a certain stress, direct or tangential, not be exceeded at any point along the panel's thickness. It omits deflection, although for me, that's the critical value that must be considered. In my calculations, I always ensure that the panel's deflection doesn't exceed 50% of its thickness. That's a much more demanding requirement than the ISO standard. Aside from the percentage that each of us considers appropriate, I think we agree on that as well.
Regarding the pipework, I have no experience with it and therefore won't venture to comment.
I'm not entirely sure what you mean by a "more elastic" material. But it's clear to me that a material that deforms more than another, without reaching its elastic limit, will give a smoother ride than a more rigid material (by rigid I mean a material that deforms less under the same load). So I think we agree on this as well.
One issue that concerns me, which I asked Bruno about but he didn't seem kind enough to answer, is the safety factor applied when designing with HDPE. Specifically, what design stresses should be used, what percentage of the yield point should be used? Do you have an opinion on this?
Thank you.
Speaking of local loads, as you rightly point out, comparing panels of the same dimensions and thickness, an HDPE panel will flex much more than an RFP or a metal one. And that's certainly true if we're working below the elastic limit. So, I think we're both saying the same thing; we don't disagree at all.
The ISO 12215-5 standard, as far as I know, doesn't take into account the deformation or deflection of the panel at all. It only requires that a certain stress, direct or tangential, not be exceeded at any point along the panel's thickness. It omits deflection, although for me, that's the critical value that must be considered. In my calculations, I always ensure that the panel's deflection doesn't exceed 50% of its thickness. That's a much more demanding requirement than the ISO standard. Aside from the percentage that each of us considers appropriate, I think we agree on that as well.
Regarding the pipework, I have no experience with it and therefore won't venture to comment.
I'm not entirely sure what you mean by a "more elastic" material. But it's clear to me that a material that deforms more than another, without reaching its elastic limit, will give a smoother ride than a more rigid material (by rigid I mean a material that deforms less under the same load). So I think we agree on this as well.
One issue that concerns me, which I asked Bruno about but he didn't seem kind enough to answer, is the safety factor applied when designing with HDPE. Specifically, what design stresses should be used, what percentage of the yield point should be used? Do you have an opinion on this?
Thank you.
Exactly!!
For any low modulus material, this is the criteria to satisfy, the amount of deflection, not stress.
It is what is always overlooked when someone wants to change a design from a metal to a composite, or from steel to aluminium.
Everything discussed refers to a thin panel with its four sides embedded, which may or may not be welded to a boat (that's irrelevant).
I'm pleased, and surprised, to see that everyone thinks the important, though not the only, variable to consider is the panel's deflection, and I'm surprised because no one had mentioned it until now. So I'm glad to see that I agree with the great designers on this forum.
I haven't seen (though I'm not saying they don't exist) any references or restrictions on panel deflection in either the regulations of the Classification Societies or ISO 12215. Therefore, I'd like to know, regarding deflection, what criteria and maximum values the experts on this forum use, and what those criteria are based on. Thank you.
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