ps. Don't know if you've ever actually drained / sucked the oil up through the dipstick hole with a standard suction pump. It is a royal, slow PITA. Takes 5 minutes? Ha, try about an hour of continuous pumping as it comes out in literal drips and drabs. I wish the oil pan drain plug was actually accessible on these engines, cause it would be a heck of a lot easier to drain the oil out.
absolutely have sucked oil up thru the dipstick hole... i use a mityvac fluid evacuator.
on some of the equipment I service it is designed that way, because that is the ONLY way it can be done. there is no hose attached to the pan
because its prone to failure, causing engine damage, as stated by the manufacture...
boats arent the only objects that have been built
around the engine, rather than the engine being installed after the object was built
many oil services I do dont allow for the possibility of an oil drip... its considered careless and shoddy work in some enviroments... intel is about the nit-pickiest place you've ever seen. so we suck the oil out and pump it in...
when the oil is hot, it will suck like water (almost).... a gallon of oil takes about 70-90seconds to be sucked thru the dipstick tube, at the engine operating temp of 180degrees.
and i would suppose it would depend on the design of the oil pan more than anything else, as to whether enough oil can be removed to "clean the oil" when changing it... and if the suck tube reaches to the bottom of the pan.
some pans have a sump for the drain plug, and some have a sump for sucking from the dipstick tube...most have neither one.
truthfully, i dont know if an oilpan used on a marine engine is designed differently than any other, but there are many random and thoughtlessly designed oil pans in existence.
there will always be a bit of old oil left in the engine oil pump and galleries that will remain, and even a pan designed with a drain plug sump will continue to drip oil for a couple of hours if it was left it open that long.... but normally we drop the oil, wait a 3-4 minutes and then replace the plug... while it is still dripping/accumulating in the pan....
i also dont know the shape of the pan on my westerbeke 30, but ive seen the insides of oil pans on all types of engines, and many of them either have a ridge/boss built up around the drain plug for strength, or the plug is mounted on the side of the pan..... both which leaves a fair amount of oil in it when draining from the plug....
I dont believe that 10% of the old oil remaining in the pan is any worse than going over the oil change schedule 20-30 hours. if the filter is doing its job and if the engine is being serviced timely, the oil in the pan wont be in very bad condition, and the dilution ratio will be low enough that you can be assured of good service life from the oil, even if you do run over a bit on the scheduled hours til the next service...
some people change the oil every year no matter the amount of hours they put on it, and others opt for changing it at a certain number of hours, no matter how much time has gone by.
there are a lot of variables and a lot of leeway when deciding how and when to change the oil, and what method is best.
as for the drain hose that is attached to the oil plug hole for draining... I know there can be many exterior pressure lines on an engine that could fail at any time during its operation, but never have I seen any applications where there is an external line attached to the pan below oil level, that is intentionally left without a shutoff at the pan...
but because some of these engines are installed in such a way that the pan cannot be touched, let alone reaching the oil plug, this could be one of those things that is a bit specific to the boat industry for the convenience of changing the oil if one chooses to drain it rather than suck it out... and thats OK if thats the only way it can be done.
I have seen a large number of engines that were installed on flat skid-plate units that have no "below" to drain the oil into, and they have a long hose connected to the pan in place of the drain plug.... this is intended to be ran to the side of the pan and over the edge so the oil can drain into a bucket... or sucked out. some of these hoses even have to run uphill a couple of inches to get to the edge, leaving some of the old oil in the engine, and this is an OEM arrangement. the hoses frequently fail near the connection and drip due to the vibration of the engine.....
but there is always a shutoff valve in the oil pan drain hole that the hose connects to....
as normal as it is
in a boat to have the hose connected to the drain hole without a stopper valve, its still a bad idea... but I will concede that with proper maintenance and inspections, the risks can be very low... but then, if you could get to the drain on the oil pan to replace the old hose with a new one, you could probably operate a quarter turn valve there also
