|Wood Fasteners, Mechanical|
Mechanical fastenings should be of material suitable for the service intended. Ferrous fastenings should be hot-dipped galvanized. Among the usual non-ferrous types brass is not acceptable in salt water applications as it will corrode from de-zincification and is inherently soft and weak.
Caution should be used in selecting fastening material because of the problem of galvanic action which can arise if dissimilar metals are used close to one another. A bronze washer used with a steel bolt will result in the eating away of the steel. Proper selection of fastening materials will significantly prevent corrosion and thereby extend their service life.
Marine applications of stainless steel alloys (chromium-nickel) are subject to a phenomenon known as contact corrosion or more commonly, crevice corrosion. Stainless steels which are in contact with each other or placed in tight joints (nuts and bolts), swage connections (standing rigging), or used to fasten wood planking below the waterline, corrode at an alarming rate. The vehicle of crevice corrosion is electrolytic cell formation. If the stainless steel is unable to naturally form a thin film of chromium oxide to shield the material from attack, corrosive liquids such as salt water are able to establish electrolytic cells with chloride ions and corrosion takes place. In short, stainless steel depends on oxygen to provide protection against crevice corrosion.
Grade 316 L (passive) stainless steel is the most accepted material for marine applications due to the introduction of molybdenum to the alloy. For example: grade 304 stainless steel has 18% chromium and 8% nickel in the alloy while grade 316 L has 18% chromium and 10% nickel and 3% molybdenum. Grade 304 is quite susceptible to crevice corrosion when employed in tight spaces and unable to generate chromium oxide. The 316 L material will last longer in the same application.
Chandlers usually stock only brass and stainless steel, both being very unsuitable for underwater fastenings. The grade of stainless is rarely mentioned and is often only Type 304.
Generally, stainless steel fasteners should not be used underwater. However, they are used quite frequently, but only if all of the following conditions are met will they be satisfactory:
- Austenitic grade at least Type 304, preferably Type 316.
- Not passing through wet wood.
- Ample sealant under the head and in between mating surfaces.
- The item to be fastened is less noble than stainless; i.e. all the copper alloys and, with some risk of hole enlargement, steel and iron.
Note: Condition (b) indicates that stainless wood screws should never be used underwater.
The choice of stainless steel fasteners below the waterline should be carefully considered based on the water salinity, grade of stainless steel fastener available, and material of other fasteners and fittings in the hull. Stainless steel may be subject to varying degrees of accelerated crevice corrosion. For more information, see Metal Corrosion in Boats, (Reference 13).
The number, size, type and spacing of fastenings for various applications are given in Lloyd's Rules and Regulations for the Classification of Yachts and Small Craft, Part 2, Chapter 4.
A general guide for use of the various types of fastenings follows:
- Lead Holes. Lead holes for wood screws should be about 90% of the root diameter of the screw for hardwoods and about 70% of the root diameter for softwoods. For large screws and for hardwoods, a shank hole of a diameter equal to the shank of the screw and of a depth equal to the shank may be used to facilitate driving. Lag screws should always have a shank hole.
The lead hole for the threaded portion of a lag screw should have a diameter of 65-85% of the shank diameter in oak and 60-75% in Douglas Fir and Southern Pine with a length equal to the length of the threaded portion. Denser woods require larger lead holes and the less dense require smaller holes. For long screws or for screws of large diameter, lead holes slightly larger than those recommended here should be used. The threaded portion of the screw should be inserted by turning and not by driving with a hammer.
Where possible, screws should be selected so that the unthreaded shank penetrates the joint for greatest strength and corrosion resistance, and to facilitate the drawing together of the members. In this case, the shank hole shall extend the full length of the shank. If conditions prevent the shank from extending through the joint, the shank hole shall extend completely through the member containing the head, to prevent threads from engaging in that member, which might prevent the joint from drawing up.
Figure A: Typical Wood Screw
Figure B: Wood Screw Properly Inserted And Countersunk
- Lubricants. Suitable lubricants such as wax, grease, or heavy paint, but never soap should be used on screws, especially in dense wood, to make insertion easier and prevent damage to the screw.
- Depth. Penetration of the threaded portion for at least a distance of 7 screw diameters for hardwoods and 10-12 in softwoods is required for maximum holding power.
- Loading. If possible, screws should be placed so that they are loaded across the screw and not in the direction of withdrawal.
The spacing, end distance and edge distances for wood screws should be such as to prevent splitting the wood. Lag screws should follow the rules for bolts. For further information concerning wood screws, see Wooden Boat, Issue 54 & 55 (Reference 17).
Hot dipped galvanized cut boat nails have traditionally and are still being used in boat building. Barbed or annular ring nails have been successful and are suitable depending upon their application (usually smaller scantling vessels). Smooth, thinly coated or plated nails, with small irregular heads and long tapered shanks such as horseshoe nails and ordinary "cut nails" (i.e. hardwood flooring nails) will not provide sufficient holding power and should not be used. In addition, wire nails are not acceptable for hull construction.
- Lead Holes. Lead holes for nailed joints may be 3/4 of the diameter of the nail without causing loss of strength.
- Types Of Load. If possible, nails should be loaded across the nail and not in the direction of withdrawal. This is especially important in end grain.
- Spacing Of Nails. The end and edge distances and spacings of the nails should be such as to prevent splitting of the wood.
Boat Spikes And Drift Bolts
- Lead Holes. Lead holes for boat spikes should be the size of the short dimension of the spike and should extend approximately 75% of the spike depth. The lead holes for drift bolts should be slightly less than the bolt diameter and of a depth equal to the bolt length.
- Type Of Load. Where possible, spikes and drift bolts should not be loaded in withdrawal. This is especially important in end grain.
- Insertion. A clinch ring or washer may be used under the head to prevent crushing of the wood. Spikes should be driven with the edge of the chisel point across the grain to avoid splitting the wood.
- Spacing of Spikes and Drift Bolts. The end distance, edge distance and spacing of the spikes should be such as to avoid splitting the wood.
- Bolts. Bolt holes should be of such diameter as to provide an easy fit without excessive clearance. A tight fit requiring forcible driving of the bolt is not recommended.
- Placement Of Bolts In Joint. The center to center distance between bolts in a row should be not less than four times the bolt diameter.
The spacing between rows of bolts should be 5 times the bolt diameter for a bolt whose length from the bottom of the head to the inner side of the nut when tightened is 6 times the bolt diameter or longer. For short bolts, this distance may be decreased but in no case should be less than 3 times the bolt diameter.
The "end distance" from the end of a bolted timber to the center of the bolt hole nearest the end should be at least 7 times the bolt diameter for softwoods and at least 5 times the bolt diameter for hardwoods. These requirements should be relaxed where necessary in the case of bolted planking butts to allow the "front row" of fastenings on each side of the butt to be bolts.
The "edge distance" from the edge of the member to the center of the nearest bolt hole should be at least 1 1/2 times the bolt diameter. For bolts whose length is over six times their diameter, use one half the distance between bolt rows and in no case below 1 1/2 times the bolt diameter.
For perpendicular to the grain loadings (joints at right angles), the edge distance toward which the load act, should be at least 4 times the bolt diameter.
In general, all groups of bolts should be symmetrical in the members. The individual fastenings should be offset slightly as necessary to avoid placing more than one on the same grain.
- Washers. The importance of washers, especially under the heads of fastenings which may be loaded in tension either because of external stresses or because of swelling stresses, cannot be overstated. The weak link in most metal-fastened wood structures is not the tensile strength of the wood or of the fastenings, nor the withdrawal resistance of threaded fastenings. The weak link is almost always the cross-grain crushing strength of the wood under the heads of the fastenings. Care should be exercised in drawing nuts down on the bolts too tight and crushing the wood.
- Wickings. A suitable wicking should be fitted in way of the faying surface of the joint at each through bolt subject to moisture.
Excerpted from Wood Hull Inspection Guidance (NVIC 7-95)