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Old 11-19-2018, 07:06 PM   #1
CuzzA
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How to tap an outboard engines water cooling system for on board hot water?

I'm most interested in how to do this with a Honda/Tohatsu, but for posterity I think it would help others on how to do it with other brands.

I know you could machine tap the block, seems sketchy and you'd certainly lose your warranty.

I suppose you could tap into your telltale, but there's not a lot of pressure there and today I was a little shocked how cool the water was coming from a Honda 250 sitting idle, but just after a 20 mile haul. Hardly warm at all, it's as if they set this cooling system up to mix cool water before discharging as some kind of safety feature to prevent accidentally scalding someone. I thought maybe we could get water from the hose flushing port, but there's a check valve there. Water goes in, not out.

So, how can this be accomplished? I'm not interest in other options. I want the hot water from the engine. How do you do it?
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Old 11-19-2018, 07:13 PM   #2
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Re: How to tap an outboard engines water cooling system for on board hot water?

Replace the water pressure gauge fitting with one with a "T" and take it off of there. Just put a ball cock valve or suitable substitute on the end of the line. There shouldn't be any warranty issues with that.
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Old 11-19-2018, 07:21 PM   #3
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Re: How to tap an outboard engines water cooling system for on board hot water?

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Replace the water pressure gauge fitting with one with a "T" and take it off of there. Just put a ball cock valve or suitable substitute on the end of the line. There shouldn't be any warranty issues with that.
A quick internet search doesn't return any results for that other than an actual dash gauge.

How about "cooling temp sensor?"
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Old 11-19-2018, 07:30 PM   #4
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Re: How to tap an outboard engines water cooling system for on board hot water?

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A quick internet search doesn't return any results for that other than an actual dash gauge.
Your dash gauge is sensing water pressure from the engine port. You could always add a in line T fitting on the hose leading to the gauge as well. That would definitely not void any warranty. I'd make sure it's a very good connection and place it somewhere you'd notice any leaking...even though leaking shoudn't cause any concern given the the pump's capability. It's just that it'd probably harder to tap into given the bundling nature of hose/wire routing.
IMO, you'd be hard pressed to find any internet search hits on such.
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Old 11-19-2018, 07:58 PM   #5
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Re: How to tap an outboard engines water cooling system for on board hot water?

Thinking about this more, I guess it's very possible that they've done away with the legacy water pressure port and are now just electrically monitoring temp? If so, logically, there should still be a troubleshooting port on top of the block that could be utilized?
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Old 11-20-2018, 07:29 AM   #6
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Re: How to tap an outboard engines water cooling system for on board hot water?

Pee holes are feed from the pressure sent up from the pump before it hits the block. Either remove the check valve from the flush fitting or if the motor has serviceable block corrosion anodes you can drill and tap the cover on one to get hot water. Don't know about Honda but Suzuki has them and they are easy to access.

I'm kind of wondering why you aren't going with the Zuke 200s. Those motors are animals and much lighter than the Hondas. Plus they come with fly by wire controls that are way easier to rig and after having them on my boat I am only pissed I didn't have them sooner.
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Old 11-20-2018, 08:57 AM   #7
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Re: How to tap an outboard engines water cooling system for on board hot water?

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Pee holes are feed from the pressure sent up from the pump before it hits the block. Either remove the check valve from the flush fitting or if the motor has serviceable block corrosion anodes you can drill and tap the cover on one to get hot water. Don't know about Honda but Suzuki has them and they are easy to access.

I'm kind of wondering why you aren't going with the Zuke 200s. Those motors are animals and much lighter than the Hondas. Plus they come with fly by wire controls that are way easier to rig and after having them on my boat I am only pissed I didn't have them sooner.
Thanks Moose. Regarding the Zukes, one reason. Cost. Though I haven't priced the Zukes, I'd bet it would cost $10k more vs. the Honhatsu.

Also, now Honda has fly by wire... https://marine.honda.com/outboards/ist
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Old 11-20-2018, 01:34 PM   #8
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Re: How to tap an outboard engines water cooling system for on board hot water?

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Thanks Moose. Regarding the Zukes, one reason. Cost. Though I haven't priced the Zukes, I'd bet it would cost $10k more vs. the Honhatsu.

Also, now Honda has fly by wire... https://marine.honda.com/outboards/ist
My first internet search showed a single 200 being $12,240 and I believe you'll get a better deal with twins.
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Old 11-20-2018, 07:21 PM   #9
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Re: How to tap an outboard engines water cooling system for on board hot water?

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My first internet search showed a single 200 being $12,240 and I believe you'll get a better deal with twins.
That's on the 4 banger, right? Does that include rigging, prop, warranty, install? Do you think I'd be putting more stress on the 4 cylinders with a full load? Would I have the torque to get on plane efficiently?
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Old 11-20-2018, 08:43 PM   #10
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Re: How to tap an outboard engines water cooling system for on board hot water?

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That's on the 4 banger, right? Does that include rigging, prop, warranty, install? Do you think I'd be putting more stress on the 4 cylinders with a full load? Would I have the torque to get on plane efficiently?
Yep, high tech, lean burn 4 cylinders that push bigger boats than the one your building. I think they are hands down the best option without question power and money wise. Weight and fuel economy would be a big improvement over the Honda’s and they would have a smaller footprint leaving you more room. Give outboard specialties or international marine a call for a quote rigged. I think you will be shocked how affordable they are. Hell, for the same price you could go with a single DF 350 with the duoprop. Either way would meet all your needs and expectations.
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