OMC Zephyr 15HP 2 stroke cooling question

I have a 1981 15HP OMC Zephyr saildrive in my boat. The power unit is basically a 25-35HP OMC outboard top end de-tuned to 15HP for longevity and attached to a different bottom end, which protrudes through the the hull sandwiching the fibreglass with what is essentially a giant O-ring (ISO ring) that is no longer available to buy.
Replacing the impeller requires removal of the whole motor and one part that usually ends up suffering damage to the ISO ring. For this reason I'd rather not disassemble it to change impellers anymore.
The current cooling system draws raw water from an intake on the very bottom of the drive leg, pumps it through the motor and then out through a wet exhaust/water muffler/through-hull.

Finally we get to my question...
I need someone with knowledge to tell me if it'll work, because everyone I ask gives me a different answer.

I want to make a barbed/threaded fitting that will fit the cooling system drain cock hole so I can attach a hose from a 12v water pump and feed water through the motor, letting it find it's own way out...either though the exhaust or the old intake. If necessary I could attach a temp probe/relay to keep it at a pre-set operating temp. If necessary I could even regulate flow with an inline valve....it wouldn't require much pressure.
I figure I'd never have to fight with replacing the internal impeller again.

Will it work and if not is there a way to make it work??

I'm thinking the electric motor running any adequate pump would probably need much more electric power than your little engine's charging system can produce.
I'm picturing a dead battery in a few minutes.
Even if it could keep up, it's probably not designed for a high continuous current draw like that. I'm thinking 10 amps to run a big enough pump continuous. Maybe way more. I'm probably guessing conservative.

I don't know those Zephyr's but I'm picturing a typical small outboard magneto type charging system.

The engine has a 35 amp marine alternator and the amount of water moving through the engine is enough to cause only spits and spray out the exhaust...not much water movement.

I am confident I can power the pump and I can also provide a continuous duty pump that will deliver the water flow that I require. I need to know if I can pump water into the engine and let it flow where is wants to without damage and also if continuous flow would be ok or if I have to regulate flow to maintain temperature.

the thermostat that is already in the engine will regulate flow. The trick will be feeding the motor the same place as water enters now.
You just have to use a non-positive displacement pump -basically an impeller pump that is happy having it's output blocked (any marine impeller pump like the one on the motor now= Jabsco etc). An alarm would be a good idea so the exhaust doesn't start a fire in the event of a pump failure.

Spits and sprays? don't be fooled by what you're observing when the boat is up on land running with no load on it. You're seeing spits because the thermostat is 98% closed. At full rpm under load, it will open wide and there's several gallons per minute flowing through the motor. It takes that much to remove the heat under load - when the motor is making the full 15HP at the prop, it rejects about 15HP+ worth of heat, which is about 50,000BTU per hour (about the same quantity of heat that comes out of a small home furnace). -Internal combustion engines are very inefficient + the water takes some exhaust heat in the cooled exhaust too)

There's 114,000 BTU in a gallon of gas, and more than 1/3 of that energy goes out via the cooling water. Basically 1/3, 1/3, 1/3 of the fuel energy goes to the 1) prop, 2) the coolant and 3) out the exhaust of a typical internal combustion engine. So you basically need to get rid of 40,000 BTU for every gallon of gas you burn. If you burn 2 gal/hour, you need to extract 80 or 100K Btu per hour which will take about 5gpm flow with room temp water in a typical small engines.
I'm not an engineer, but those are the basic concepts engineers use to size cooling pumps. The rule of thumb for DIYers is to double the expected needs.
The thermostat only opens wide under a load.

This will be an expensive pump. If you think you're going to use a bilge pump or something, remember you need to meet about 50 or 70 psi restriction to flow. Bilge pumps quit pumping at 5 psi of head pressure.

Here's an idea- belt drive the pump. Why electric? If you have a proper alternator, then it sounds like there's a way to belt drive a pump. An engine mounted Jabsco etc. like an inboard boat typically uses.

kimcrwbr1 said:

The water pump does not supply water pressure as much as water flow.

Click to expand...

It has to do both. It has to keep supplying flow and create the pressure by pushing it's way through the water jackets and exhaust.

My guess is an RV sink pump wouldn't last more than a few hours running continuous. It's designed for intermittent use - probably would like a bladder tank even to prevent too long of continuous duty.

The more I think of it, Mechanical (belt driven pump) is the way to go - instead of converting to electric and then back to mechanical. That's a lot of power and heat lost in conversions.