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To let dry or to leave the anti-freeze... that is the question to be answered.

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When I started doing the process of winterizing, I started looking and multiple forums of boaters and there are mixed feelings about what to exactly do when winterizing (specifically related to the water draining). The main procedures can be boiled down to the following:

  • Flush the boat and then drain the plugs, hoses, etc. and let air dry.
  • Flush, drain, then reconnect all, add or run antifreeze through everything and leave antifreeze inside.
  • Flush, drain, then reconnect all, add or run antifreeze through everything and drain once more the antifreeze and let air dry.

These are the working theories... Benefits of the anti-freeze are that it has got corrosion inhibitors. But it has also been extensively published that gycols can be more severe to metals than not having them present so the corrosion inhibitor will protect from the glycol, but who knows if its better or worse than regular water or air. In addition, the corrosion inhibitors need to be present at a certain concentration to protect and usually they are added in too low of concentrations.

We can all agree that steel needs both water (moisture) and air (oxygen) to corrode. So the best thing to do is to run some quick experiments to see what really is the truth. Keep in mind that what I did you may need to do with whatever anti-freeze you use as this may only be representative to what I have been using. I use a "splash" branded antifreeze from Menards that essentially is the same as the Dowfrost and uses corrosion inhibitors.

Here is a link to information on the Dowfrost: 


and the splash -75 that I use:



The test conducted was simple. Purchased a thin steel rod and cut it into several sections. Each section was exposed to different conditions:

  1. BLOCK GLYCOL/WATER SOLUTION: Extracted some glycol/water solution from the block. Placed the rod inside a bottle, mixed it so that all the rod got wet and let the rod partway in the air and partway in the solution. Left the bottle cap untight so that it is slightly exposed to the ambient (as the block would be). Here the engine was run first with tap water, taken to temp, drained and re-ran quickly with 5 gallons of glycol. The block solution was obviously diluted but had some color to it from the glycol. I even tested the block water in my freezer and even through it was diluted it would not freeze. It would form crystals just like the -75 concentrated but not harden.
  2. BOTTOM OF ROD SOAK/TOP DRY: Here another rod was placed in a beaker filled with the same block solution as 1. The bottom of the rod was allowed to soak in the solution for a couple of minuted and the top was left dry. The solution was then poured out of the beaker (drained) and slightly covered  with cling wrap to leave some humidity inside. The humidity of my insulated garage with snow covered cars that melt inside will also affect it.  
  3. SPLASH -75 AS PURCHASED CONCENTRATE: Same procedure as 1 of placing a rod in the bottle and mixing to fully saturate it and then let it sit half way in, but the solution was straight from the purchased bottle.
  4. TAP WATER: Same procedure as 3 but with tap water and no glycol.


The test has been going for about two weeks and there are already some very interesting results.

  • The tap water (4) as expected is the worse. So adding the anti-freeze with corrosion inhibitors is a good thing.
  • The fully concentrated antifreeze (3) does a decent job but some corrosion is already observed towards the bottom submerged area. The air (top) side is pretty good.
  • The dilution (1) results in less corrosion protection and there is more noticeable corrosion that the concentrated (3). The air side though is doing better than the submerged which indicates that the liquid solution is more susceptible to corrosion than the oxygen exposure of the initially wet rod (top/air side).
  • The rod that was halfway soaked (2) and then all the solution drained is not bad on either side. Just a bit of corrosion where the fluid is touching the bottom but still better than (1).

Not bad to already be seeing this in two weeks. But the boat is stored for about 7 months of the year where I'm located. SO I will update on this again in a couple of months. For now, my thought is that the best option appears to be to drain water as best as possible to get the concentrated antifreeze in there to coat the surfaces and prevent freezing. Then drain and let dry. AGAIN THE DISCLAIMER is that this may be unique to the antifreeze brand being used and your winterizing procedures. Plus my tap water is pretty hard and so it seems to be bad stuff for the steel. Anyway, its a very easy test if you decide to give it a go yourself. 

The attached microsoft onedrive link shows images of the testing:


I was going to try to imbed some images but I'm not an expert in this an was not able to get it working... sorry about that.

Edited by Javi
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This is an interesting experiment.  Thanks for doing it.

I will note that your block is cast iron, not steel, but the results are probably similar.  It might be worth repeating on cast iron to make sure.

The takeaway for me is that raw water inside an iron block is always bad, and that only worrying about it over the winter doesn't make sense if you don't worry about it through the summer.  I do not regret my decision to add half-closed cooling.  In my case, the decision was easier since I boat almost exclusively in salt water, but after reading all of these threads about it I'm sure I would do it even in fresh water.  It's one investment in my boat that I'm sure will hold its value. 

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That is a good point. Your situation will apply well because we all know that salt water is the worse but as you can see, tap water is not good by any means. So while flushing does help, it is still working on it. Comparing tap water to the typical lake I run in would be interesting, but I think tap may be worse with the chlorine. Would need to test to confirm though.  

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Cast Iron seems like it will do slightly better than steel in terms of long-term strength performance, but both seem they will corrode at similar rates on newly exposed surfaces.



http://www.uobabylon.edu.iq/uobColeges/fileshare/articles/Corrosion of cast iron.pdf

"Cast iron and steel corrode; however, because of the free graphite content of cast iron (3% - 4% by weight or about 10% by volume), an insoluble graphitic layer of corrosion products is left behind in the process of corrosion. These corrosion products are very dense, adherent, have considerable strength, and form a barrier against further corrosion. Because of the absence of free graphite in steel, the corrosion products have little or no strength or adherence and flake off as they are formed, thus presenting fresh surfaces for further corrosion."

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@Javi:  Nice work, thanks for posting.  Question, is your tap water run through a water softener, if yes is there some salt content?  It would be also interesting to run the test with the lake water that the boat uses, that water will also have a variety of impurities that may alter the results (probably not in a good way).

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Agreed, since its such a simple test and I do have some left over steel rod, I may try pulling the plug and pouring some block lake water into a vial and doing the same test during the summer months with lake water just to see how comparable it is. Most lakes around here are similar I would think, since they are all connected (Yahara Lakes-Madison).

Overall, I bet that what we will learn is that probably corrosion will happen anyway, possibly even more during the summer months. For me, I added pencil anodes to the block which get eaten up very quick so I know they are working hard. In the end, it probably doesn't matter that much what method is followed because the cast iron can handle a lot. But since some of us just like to pamper our boats, why not try the best option especially when its not that much harder.

The tap water I am testing with is not softened. The spigots to the outside of the yard as well as the kitchen are not hooked up to the softener in my house. I flush with the outside spigot.

Edited by Javi
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If that's what you're getting with tap water after only two weeks, then I think it's safe to stay that a boat sitting unused all week in the summer time (and probably longer in many cases) is WAY worse than any winter lay-up solution.

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Did you degrease the rod before you started the experiment?  Most of the rod that I buy has a good amount of mill lubricant left on it.  A chemical degreaser or just a nice scrub with hot soapy water should do.  Regular dish soap is my tool of choice.

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Are your samples in light? Light is adding another difference?  But as said Lake / Sea water is not treated so is that another affect.  And all of our waters differ. The one thing I see over years. The boats I worked on that use antifreeze have less rust and corrosion. and our impellers are lasting much longer. But not a controlled experiment. All I will say is after running 30 plus years of experiments one finds difference in outcome with very small changes from the person doing the set up.  


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All good points:

To have a control, the rod was wiped with lab grade acetone (concentrated). That gets rid of everything in terms of oils. So again that is going to expose the surface. 

The bottles are not covered in the dark, but they are in a corner of the garage where no direct light hits them. Light will generally induce bacteria growth much quicker which can also contribute to corrosion.

All bottles are exposed to the same (minor) indirect ambient light from the garage door small windows during the day. Garage is cold at around 46 F and between 20-60% RH depending on if I drive into it with a snowy car or not.

I agree that not draining may be better for the impeller vs. letting the system dry out. If you are letting dry out, you should then follow the manual instructions (as stated in my 06' manual at least) which indicate to remove the impeller, especially if you don't plan on replacing it every year, because it will dry and may stick when starting next season. A little pure silicone spray to the impeller before storing may help keep it in good shape.

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