Opinions range from the high and mighty: "Since you are going to spend a lot of time building, you should use the best material available no matter what the cost" to the economy version "Grab some Luan underlay, build a quick cheap boat and get out there and have some fun"
Most of us would prefer to understand the options a bit better.
Marine Plywood Grades
In order to be a Marine Grade the plywood has to conform to a certain number of criteria:
- The Glue MUST be waterproof
- Voids are to be avoided
- Face and inside veneers have to be good
Any glue used for marine plywood or for that matter also used in exterior use has to conform to the a stringent test. Plywood assembled from it needs to be able to withstand boiling in water for several hours without de-laminating. This is WBP glue and stands for "Weather and Boil Proof". Glues vary in their tenacity while being boiled.
The capacity of a plywood to not de-laminate under boiling is not only dependent on the quality of the glue but also on the materials used and the care which was used in the manufacture.
My page on Marine and Exterior plywood glue
Voids in Marine grade plywood spell disaster.
Voids not only weaken the plywood but create a ready path for water infiltration. For this reason Marine Grade Ply must not have voids.
The veneers used for the manufacture of the plywood must be of high quality.
Depending on the grade, knot size and frequency is controlled. If a repair needs to be made on an inside ply there are rules for this. If a ply needs to be joined inside it can not be butted. The thickness and number of plies is also controlled.
Marine Plywood Grades
Although a British Standard, it is used by marine plywood manufacturers world wide. The plywood does not have to be of British Manufacture. It is an extensive document which details all characteristics of the plywood.
- The BS 1088 standard is a marine plywood specification that applies to plywood made from untreated tropical hardwood veneers that have a stated level of resistance to fungus growth.
- BS 1088 plywood must use a glue, which has been tested and shown to be highly resistant to weather, micro-organisms, cold and boiling water, steam and dry heat. There are standard BS tests for the glue.
- The Face Veneers must have a solid surface without open defects. Face veneers must be free of knots except "sound pin" knots, (no more than six in any one square foot,) the average of pin knots cannot exceed 2 per square foot over the whole surface of the plywood sheet. The grain in the veneers should be reasonably regular. Edge joints are limited, and end joints are not permitted.
- Core veneers requirements are similar to face veneers except that slits are allowed as long as they are small. Pin knots and edge joints are also allowed. As in face veneer, end joints are forbidden.
- Manufacturing Defects are strictly controlled. Poor bonds, overlaps and pleats, and gaps in faces are not allowed. Gaps may be filled using veneer inserts glued with appropriate adhesive.
- The Moisture content of the plywood must fall between 6% and 14% when it leaves the factory.
- Boards will be equally sanded on both sides.
- Finished size The length or width of a standard sized sheet can not be less than the stated size nor more than 6.3 mm (0.25") larger.
- Squareness -- The lengths of the diagonals of a sheet shall not vary by more than 0.25% of the length of the diagonal.
- Thickness tolerances:
4 mm +.02/-0.6 ; 6 mm +.04/-0.65 ; 9 mm +.06/-0.75 ; 12 mm +.09/-0.82
15 mm +.1/-0.9 ; 18 mm +.12/-0.98 ; 22 mm +.16/-1.08 ; 25 mm +1.8/-1.16
- Face Veneer thickness -- For any three-ply construction, each veneer sheet will not be any thinner than 1/8 of the total thickness of veneers assembled dry. Multi-Ply Construction-- This applies to boards thicker than 4.8 mm (3/16")
Each face veneer shall be a minimum of 1.3 mm and not thicker than 3.8 mm.
Each core veneer shall be no thicker than 4.8 mm
Lloyd of London Certification
According to their website
Lloyd's Register Type Approval is an impartial certification service providing independent third-party Type Approval certificates attesting to a product's conformity with specific standards or specifications, and verification of an appropriate production quality system.
In other words Lloyds ensure that the standard is met. They do not continuously inspect production lines.
Many plywood are made according to the BS 1088 marine plywood standard but they have not all gone to the trouble and expense of getting certified. On the other hand some manufacturers claim to be up to this standard but are not. As usual it boils down to you get what you pay for and it pays to go to a reputable dealer.
Lloyd Speak of their certification
This standard is very similar to the BS 1088 with somewhat reduced quality.
It is a marine plywood standard using the same glues, but has somewhat more relaxed requirements as to the quality of the veneers. The face veneers are often of slightly lower grade and thinner. There are no voids and requirements for joins are similar.
It is generally considered an excellent material if not as pretty as the higher grade. Bright finishes are not necessarily as successful as with the BS 1088. I have used both grades and have not seen any defects in either.
There are many exterior grades being manufactured. The glues are usually essentially the same as the marine grades. The outside veneers vary a great deal depending on the chosen grade. There is a possibility of voids. Boats have been successfully built using higher grade exterior plywood.
This is an Australia / New Zealand standard. Marine Plywood manufactured to AS/NZS 2272 standard, is a structural plywood intended for hulls of boats and other marine applications. It has a Type A (wbp) phenolic bond glue and is manufactured from woods based on density, bending strength, impact resistance and surface finishing characteristics.
None of the marine species are naturally durable and preservative treatment will be required. Hoop pine is often used.
The Engineered Wood Products Association of Australasia is a rich source of information and explains the standard in more detail.
Brewster is another good "down under" site to wander through.
The durability of the plywood wood is not always of paramount importance in modern boatbuilding
The marine plywood is rarely left unfinished. It very often has an epoxy coating, often with glass cloth as well. Okkume is a prime example of a widely used plywood wood which is not particularly resistant to rot and fungus.
You may not always need the absolute best grade of plywood. Often a high end exterior ply will do as well particularly in area that do not particularly stressed.
The Plywood for Boatbuilding FAQ edited by Craig O'Donnell is a good read. Various designers and builders give their thoughts on plywood. John Harris who designed the Skerry I build is part of this article.