Drive train is a term that I borrowed from the automotive industry because no marine term exists to describe the complete drive system in powerboats.
While the term "running gear" is often used to describe the shaft, propeller and strut, the components of the complete drive train consist of the engines, struts, engine mounts, transmission, shaft, stuffing boxes, struts and, of course, the propellers.
Before we can properly survey a drive system, we first have to have a solid understand of the basic mechanics and the stresses to which the system is subjected. In this chapter, we'll cover the basics and deal with some of these issues.
Perhaps one of the primary reasons that we find so many problems occurring with drive systems results from failure of designers and builders to think in terms of the entire drive system.
The drive trains in inboard powered boats, unlike those of outboards or stern drives, can span considerable distances.
The span of the system runs from the front of the engine to the tip of the propeller shaft.
In large yachts such as those built by Broward Marine, where engine rooms are actually forward of amidships, the length of the drive train is often over forty feet long.
A rule of thumb is that the alignment of the drive system becomes increasingly critical in direct proportion to its overall length.
Unusually short drive trains can tolerate substantial degrees of misalignment while unusually long drive trains can tolerate very little.
The reason for this should be obvious: the degree angle of misalignment off center is magnified in direct proportion to the length.
A general rule of good system design is that the drive train should never be made totally rigid unless the hull is also totally rigid.
Most production built, fiberglass hulls are not rigid and are subject to considerable flexing.
While one rarely notices this, and it normally doesn't cause any obvious damage, many hulls will flex as much as 1" out of central axis alignment.(1)
It you have trouble believing this, try stringing a guy wire for a long distance anywhere between two points on the interior and watch how it alternately slackens and tightens while underway.
Only steel and aluminum yachts have a substantial degree of rigidity. However, while fiberglass hulls tend to be flexible, the foundation for the drive train must be quite rigid.
Since engines are mounted on stringers, this means that the stringer system must have sufficient strength to maintain stability from engine to propeller.
If it doesn't, bending and twisting of the hull as it travels at speed over rough water will throw the system out of alignment and cause accelerated wear and possible damage to system components.
(Additional spaces are added for easy screen reading.)
Table of Contents: Chapter 11
11. Drive Train 241
Engine Mounts 243
Strut Bolts 249
- 2nd Edition
by David H. Pascoe
Publisher: D. H. Pascoe & Co., Inc.
David Pascoe - Biography
David Pascoe is a second generation marine surveyor in his family who began his surveying career at age 16 as an apprentice in 1965 as the era of wooden boats was drawing to a close.
Certified by the National Association of Marine Surveyors in 1972, he has conducted over 5,000 pre purchase surveys in addition to having conducted hundreds of boating accident investigations, including fires, sinkings, hull failures and machinery failure analysis.
Over forty years of knowledge and experience are brought to bear in following books. David Pascoe is the author of:
- "Mid Size Power Boats" (2003)
- "Buyers’ Guide to Outboard Boats" (2002)
- "Surveying Fiberglass Power Boats" (2001, 2nd Edition - 2005)
- "Marine Investigations" (2004).
In addition to readers in the United States, boaters and boat industry professionals worldwide from over 70 countries have purchased David Pascoe's books, since introduction of his first book in 2001.
In 2012, David Pascoe has retired from marine surveying business at age 65.
On November 23rd, 2018, David Pascoe has passed away at age 71.