SailNet Community - View Single Post - rpms quiz
Thread: rpms quiz
View Single Post
post #6 of Old 09-11-2010
Senior Member
RichH's Avatar
Join Date: Jul 2000
Location: Pennsylvania
Posts: 4,463
Thanks: 42
Thanked 249 Times in 235 Posts
Rep Power: 18
If the hose to tube connection from the tank was partly loose, the following 'may' have been acting:
The fuel delivery line is operating under vacuum, a VERY small amount of air (cc/HOUR, etc.) is drawn in, an air bubble slowly builds up in a 'high' spot in the line (top/head of fuel filter, etc.) until it partly takes up the majority of space in the diameter of the line, the 'bubble' purges downstream (called an 'air slug') which passes all the way through the system. Yanmars being 'easy to bleed', especially can handle small 'slugs' ... and you only get noticeable drop in rpm.
The design problem is that fuel delivery systems on small marine engines are 1. vacuum motive (larger marine engine are usually pressure motive), 2. small marine engines use very poor 'connectors' that can easily 'suck air'. The standard 'compression fittings' that are used are in actuality 'one time tighten only' fittings; if you open them you should remove the 'ferrule' and 'trim back' the copper so that the ferrule fits over 'uncompressed' copper. This vacuum motive configuration allows a boat builder to quickly and cheaply install the supply lines TO the engine - 'quick and dirty'.

The best in marine design is to have a fuel pump AT or IN the tank, the delivery line PRESSURIZED, the connections of the delivery lines 'double flared' or 'ring seal' connectors. The downside is that with a pressure system if there develops a leak one will fill the bilge, etc. with oil - and if the oil is pumped overboard you can encounter heavy fines, etc.
So, for a bombproof fuel delivery system on a small boat ... change out all the copper lines to stainless steel using *double flared connectors*; and, put a 12vdc pump just after the tank.
BTW - such a system will allow the filters to be more efficient in total capacity of 'crud' removed before the filters become plugged.
RichH is offline  
Quote Share with Facebook
For the best viewing experience please update your browser to Google Chrome