Water pump keeps running on Mainship 390 for no reason

You have a manifold on the forward bulkhead located in the compartment below the lower station. Shut off the lever on the inlet to the manifold. if the cycling stops the problem is a leak downstream of the manifold. If the cycling does not stop its the pump or the accumulator.
As far as your impeller is concerned, I have a single Yanmar 370 on my 2003 Mainship 390. If that is your engine its best to remove the pump and change the impeller on your workbench. You will need a ratchet wrench (as opposed to a socket wrench) to remove the pump. I'm not on the boat right now so i'm not sure of the size (12 mm comes to mind).
For oil changes, My 390 came with an oil exchange system and I expect your does also.
If it does, oil changes are simple. Your oil exchanger has a 3 position lever and a toggle switch. One position does your engine and one does your generator. If you have twins, the third position doe your second engine. If you have a Yanmar the horizontally mounted filter can be messy. Shut the engine down and wait a while for the filter to drain. Drill a small hole in the top rear of the filter to facilitate draining, then slide a trash bag as best you can between the filter and the top of the engine. If you have Cummins or Cat, I can't help.
Good luck
John

Paul Baker said:

My boat is docked behind a private residence in Lauderdale Isles. I have a bilge pump that doesn't get all the water out. We plan on going to the Bahamas in January for 6 months or so, so I'm looking for someone to show me how to change impellers, do oil changes and change filters. Only had the boat since July. Whenever i have had someone to "fix" something they spend hours without rectifying the issue.

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Paul, maybe I never answered this.....

Call, Atlantic Marine Power in Ft Lauderdale.
Have them change out the impellers, change the oil and filters, including the fuel filter.
Hang over their shoulder, ask questions and importantly, get part numbers.
Some folks keep the "old but good" impellers as back ups.
My recommendation, even if you keep the old impellers buy at least one impeller for each engine. Oil filters, buy at least one spare for each engine. I dont keep a large quantity of oil on board.
Oil can be found. Just know the W rating.
Some folks keep spare fuel pumps on board. I guess I have been fortunate.
Fuel filters? That depends upon your luck. I keep 4 of each type of fuel filters on board.
Keep a length of each diameter of hose on board and hose clamps, back up fresh water pump. Ah and a spare alternator with pulley installed. Spare belts, fuses and and. I think you get the drift. Various tools and a multi-meter, crimps and crimping tool Finally, if you are venturing into the great unknown, a membership in both tow boat organization.

Try not to overload yourself with parts.
Others will no doubt chime in.
Hope that helps.

390 leak

Before you do too many tests check the fore and aft washdown faucets. A whack on the faucet will delay the need for immediate repairs.

gotfw said:

Before you do too many tests check the fore and aft washdown faucets. A whack on the faucet will delay the need for immediate repairs.

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And the question is, How big of hammer?

390 water

Try your hand first. The O rings go bad after a year or so.

'no field workers that are worth the extortion of $100+/hr rates charged"

The Auto, RV and boat industries have the same labor problem.

Anyone skilled enough to fix all the different systems wants $40 an hour , not the $20 a yard might pay.

The yard has to pay for 35 or 40 hours a week, every! week so most of these bright folks eventually wander off to auto or truck shops that have the ability to sked the work and keep the tech busy.

Way back boatyards would build boats on spec , to keep their crews intact in the slow season.

Greetings,
Mr. FF. "... wants $40 an hour..." HAH! I'd gladly pay double+ that for competence.

RT Firefly said:

Greetings,
Mr. FF. "... wants $40 an hour..." HAH! I'd gladly pay double+ that for competence.

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Where are you folks finding this inexpensive labor?

We cut our pump on and off when we need pressure water when cruising. Lesson learned from having all the fresh water run out from someone not shutting a faucett or shower all the way off a couple of times

Group9 said:

We cut our pump on and off when we need pressure water when cruising. Lesson learned from having all the fresh water run out from someone not shutting a faucett or shower all the way off a couple of times

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Always it is either "someone" or "not me."