Raw Water Pump or Stern Drive Pump

[kwoolard]

I own a 22' SportCraft with a 1974 888 Mercruiser that is driven by a Ford 302. My father-in-law previously owned this boat and had some overheating problems....not sure exactly what was causing it. However, along with having a water pump in the outdrive he installed a raw water pump that is belt driven. The raw water purmp picks up water from the hull (seacock and strainer). The water line from the outdrive and the water line from the hull or seacock "T" together and enter the raw water pump. From there the raw water pump feeds the circulating pump. So basically what I have is two water lines feeding the raw water pump...one pressurized from the stern drive pump and the other from the hull or seacock.<br /><br />Should I keep this set up or should I remove the water pump from the stern drive and just use the raw water pump or should I remove the raw water pump and just use the stern drive pump? <br /><br />My worry is that if I leave both pumps on, one might be working against one another and I will not get the proper water flow.<br /><br />I would think that two water supply lines would be more than enough to supply the cooling system for the motor.

 

[Drowned Rat]

Re: Raw Water Pump or Stern Drive Pump

Interesting setup. The only problem I see is if your "engine" raw water pump goes out, your L/U will then be pumping all it's water out through the seacock unless there is a check valve in the system, then you'll probably blow a hose or seal. Your engine will get nothing. As long as both pumps are in good order, it shouldn't be a problem. If you take one of them out, I think it has to be the engine mounted one, otherwise how would your L/U get cooling water? The engine can only intake so much water, seems like overkill.

 

[airman]

Re: Raw Water Pump or Stern Drive Pump

I'll add to the confusion by disagreeing with Drowned Rat and suggest that the belt driven pump is a superior pump. I work on five sterndrive boats, all cooled with belt (or gear) driven raw water pumps. The water through the legs has all been bypassed and we draw water thru a seacock and strainer, like yours. The water in the leg provides no appreciable cooling. There will still be water flowing through with the exhaust as well. The strainer provides protection from clogging the engine/heat exchanger with junk in the water and the impeller can be changed with the boat in the water if you use the belt drive pump. I don't like the dual pumps either, unless you install check valves.

 

[airman]

Re: Raw Water Pump or Stern Drive Pump

To clarify - The water in the leg provides no appreciable cooling to the leg. All of our legs have the exhaust bypassed as well, with no problems.

 

[Drowned Rat]

Re: Raw Water Pump or Stern Drive Pump

I stand corrected.

 

[kwoolard]

Re: Raw Water Pump or Stern Drive Pump

I didn't think about what would happen if the raw water pump failed...thanks for the clarification. Since the engine doesn't have a problem with cooling anymore I may just leave the setup like it is and install a check valve after the seacock. Any ideas on where to find one for my application?<br /><br />Thanks for all the help!!

 

[rodbolt]

Re: Raw Water Pump or Stern Drive Pump

hello<br /> mercruiser uses the water from the outdrive pump to assist in cooling the upper drive on alpha and some pre-alpha drives. on most repowers that use a belt driven or shaft mounted pump I usually take the impeller out of the lower unit housing and put the pump back together and just use the engine pump.<br /> however some folks just cap the seawater in line off at the transom. I would not like this cause if it leaks it will fill the boat up rather rapidly. dont use 2 pumps. a malfuntion in one will cause a malfuntion in the other at best or sink you at worst.<br /> the last few I did I ran the water from the transom out the hull with a brass thru-hull and double braided 3/4 oftwall hose just double clamp everything and use the engine mounted pump to cool the engine. just remember the upper unit is water cooled and there is a limit on how much water the engine needs to stay cool unless a riser or manifold is plugged.<br /> good luck and keep posting

 

[kwoolard]

Re: Raw Water Pump or Stern Drive Pump

This is the serial number off of my outdrive (4013860). Is this a model that needs some cooling for the upper unit? Assuming that the upper unit is water cooled, it would not be a good idea to remove the water pump in the outdrive....right? <br /><br />Is it possible to remove the water pump from the outdrive yet still use the raw water pump to pull water through the outdrive using the same water line from the outdrive? Basically, using the raw water pump to do the same job as the water pump in the outdrive, but just in a different location.<br /><br />Do you guys have any recommendations?

 

[airman]

Re: Raw Water Pump or Stern Drive Pump

All of our drives are Volvo's ar Sterpowr's so I don't want to stick my neck out too far on the leg cooling issue other than to say that it isn't a problem to bypass the water on a Volvo leg. The method Rodbolt suggests implies that he has bypassed the Merc legs with no problems either, but maybe he can confirm that. I think your idea is sound, and an improvement on the leg pump because you can use the strainer. Also, the belt driven pump is easier to inspect and service than the leg's pump.

 

[rodbolt]

Re: Raw Water Pump or Stern Drive Pump

hello <br />yes yours should have the water pocket.<br /> maybe I was less than clear. I apologize. the method I personally use to do this is to take the seawater inlet pipe at the transom. dissconnect the 3/4 inch inlet hose from it. adda 3/4 BRASS not plastic through hull well above the ater line. hook the tube from the seawater inlet at the transom to the thruhull fitting I just added. now while the engine is running I have a killer stream of water sprying out. use a large clam shell to deflect it down or you will fill your neghbors boat when docking.<br /><br /> method 2 which I have never done but have seen done is to remove the impeller from the pump and reassemble the pump witout it.<br /><br /> method 3 which again I have not tried and dont reccommend is to simply cap the nipple off inside the transom. <br /><br />if you use method 1 then be aware that when flushing you MUST repeat MUST supply water to both pumps. if the outdrive pump melts it will cause a leak in the drive oil.<br /> good luck and I hope this clears up any problems.

 

[Fishermark]

Re: Raw Water Pump or Stern Drive Pump

When I repowered my 24' Sportcraft with a Mercruiser 5.7 liter engine I was told by the dealer that I needed to install a "thru hull" 1.25" pickup and strainer and bypass the cooling hose from the leg (Alpha 1). The engine I bought already had the raw water pump installed on it. I found quite a bit of disagreement between local mechanics as to whether or not the water still needed to be pumped through the lower unit for cooling. Just for the record, Mercruiser says it does.<br /><br />What I wound up doing was removing the rubber hose that comes from the gimbel housing and connects to the transom. I removed the plastic pipe from the transom assembly and blocked off the transom there. Now the water still goes through the outdrive and splashes against the transom. (It doesn't cause a problem).<br /><br />I had considered doing something like what Rodbolt recommends but decided not to at this time. In fact, I had thought about adding a tee in the line with a shut off valve - it would make a great raw water wash down for the boat.<br /><br />For the raw water pickup I used a 1.25" pickup and strainer, then added a seacock, next comes a strainer assembly (see picture):<br />

<br /><br />In order to flush the engine, (I boat exclusively in salt water), I close the seacock, remove the lid from the strainer and put a hose in the strainer basket. Add "ear muffs" to the lower unit and flush away! It sounds complicated, but is actually very easy. I just have to make sure I remember to open the seacock back up before running the engine in the water.

 

[kwoolard]

Re: Raw Water Pump or Stern Drive Pump

Fishermark, I like the idea of using the leg water as a washdown for the boat. I guess you can find many uses for that water after it has done its duty for the sterndrive. <br /><br />Where would you recommend getting that type of strainer and what brand?<br /><br />I have another question on this cooling topic and that is my thermostat. I removed my thermostat cover this weekend and did not see a thermostat located in the housing. According to my diagram for this type of cooling you should be able to see the thermostat once you remove the thermostat cover, is this true? If not, where is the thermostat located if it is not under the thermostat housing cover? This is my first project with an i/o if you haven't figured that out yet!<br /><br />In my opinion, he may have removed the thermostat since he had a cooling problem. However, I would think that this would pose problems for running the engine in the winter, and would also not let the engine get up to its recommended operating temperature.<br /><br />I guess you never know what you have from a hand me down until you start investigating. Unfortunately he is not here to answer all my questions.....that is where pros like you guys come in.

 

[Fishermark]

Re: Raw Water Pump or Stern Drive Pump

I would not be the least bit surprised if someone removed the thermostat in order to try and solve a cooling problem. (Not a good idea of course, but that doesn't keep people from doing it ). You can get the strainer at most any marine store - I know Boater's World has them: Strainer <br />(That's the same brand I have on my boat).<br /><br />Good luck!

 

[Fishermark]

Re: Raw Water Pump or Stern Drive Pump

Sorry, I accidently double posted a response - you can ignore this one!

 

[recess]

Re: Raw Water Pump or Stern Drive Pump

Using a raw water pick-up instead of the stern drive pump has some interesting possibilities. I am running a 20' Grady with the old OMC stringer, electric shift. (I know, make a boat anchor out of it) Apparently water leakage is the main culprit of the deterioation of these units. Can the impellar be removed, block off the water intake, use a raw water engine mounted pump, and then dischage the coolant and exhaust through the transom, not the stern drive. Wouldn't this eliminate the water in the drive as well as the heat build up in the lower unit? Actually, couldn't the old water passages be filled and capped with lower unit oil to aid in the cooling?<br />The hose on the intermediate unit could be used to fill the old water passages with oil and the old water pick-up tube could be modified as a drain. Thanks for any input you might have.

 

[recess]

Re: Raw Water Pump or Stern Drive Pump

Using a raw water pick-up instead of the stern drive pump has some interesting possibilities. I am running a 20' Grady with the old OMC stringer, electric shift. (I know, make a boat anchor out of it) Apparently water leakage is the main culprit of the deterioation of these units. Can the impellar be removed, block off the water intake, use a raw water engine mounted pump, and then dischage the coolant and exhaust through the transom, not the stern drive. Wouldn't this eliminate the water in the drive as well as the heat build up in the lower unit? Actually, couldn't the old water passages be filled and capped with lower unit oil to aid in the cooling?<br />The hose on the intermediate unit could be used to fill the old water passages with oil and the old water pick-up tube could be modified as a drain. Thanks for any input you might have.

 

[kwoolard]

Re: Raw Water Pump or Stern Drive Pump

On the thermostat issue....What is the recommened thermostat for my engine??<br /><br />888 Merc - Ford 302<br />1974 - Standard Cooling<br /><br /> <br /><br />I know that marine stats are lower than automotive; apprx 140-160 is the standard for marine. But does it matter which thermostat I use as long as it is in that range?

 

[airman]

Re: Raw Water Pump or Stern Drive Pump

All of our sterndrives (again, Volvo's, Sternpowrs and a Konrad) have the water and exhaust stuff bypassed. Water is drawn thru a seacock and exhaust (with cooling water) goes straight thru the transom. No problems and 9000+ hours on one of the boats now.

 

[flashback]

Re: Raw Water Pump or Stern Drive Pump

Kwoolard, I have a raw water cooled chevy used in fresh water and run a 140 degree Tstat... if you are in salt water I would suggest the 140 over the 160. If the engine gets too hot you run the risk of salt deposits... By the way Fishermark, the washdown trick is a great idea. Now where to put the hose bib???

 

[rattana]

Re: Raw Water Pump or Stern Drive Pump

I agree with Flashback, if you are running a raw water cooled engine you should use a 140 degree thermostat. If you have a closed cooling system you could run 140-160 degree thermostat.