Re your post in P235 forum about blocked cooling water system on outboards, I suspect that far more outboards overheat because of scale build-up in the cooling water channels than because of impeller failure.

It's not very common for outboard engine impellers to break up, and, if they do, they'll probably lose one or more complete blades, resulting in ongoing rather than just temporary engine overheating, ultimately leading to head gasket failure.

All outboard impellers are "soft" or Very Low Pressure impellers with broad tolerances, whereas most inboard/sterndrive engine impellers are Medium Pressure or High pressure with a larger number of shorter, stiffer blades with close tolerances; these are subject to higher stress at the roots, and can also be affected by cavitation, and are as likely to suffer from small pieces breaking off as complete blades breaking off, dependent entirely on the cause.

All engine manufacturers encourage you to change your impeller every year, not only because they make extremely high profit margins on spare impellers, but also because it is impossible to predict impeller life in engine running hours because there are too many variables such as usage, water quality, climate etc etc. That said, all Japanese engine manufacturers specify well in excess of 1000 hours for their impellers, whilst very few leisure users will exceed 100hrs/yr.

The principal "enemies" of outboard impellers are no water [hence NEVER start your engine, even briefly, without immersing the water intake - a bucket will do...] so keep an eye out for plastic bags around the lower unit; regular running in oily or diesel contaminated water [avoid the Gulf of Mexico Javascript:insertsmilie('[:D]') ]; regular running in silt/sand laden water [so don't drive it up the beach!].

The result is that annual replacement is overkill for the leisure user. I have owned a variety of outboards up to 90hp, and run an impeller for at least three seasons, whereas I either have, or carry out, a basic service every year, especially changing the gearcase oil.

On the other hand, EVERY outboard used in salt water will suffer from scale build up in the cooling water channels over the years, hence the oft repeated advice to run your engine in fresh water, or run fresh water through it with a hose and intake muff, after every usage. Since this isn't always practical for me, every two or three years, I will run my engines for about 1/2 hour in a dust bin to which I have added a full packet of washing machine/dishwasher descaler powder!

Geoff Sheddick

Thanks for your comments on overheating problems. I can certainly concur that outboards eventually suffer from restrictions because of "scale" in the cooling channels or pump as I have had several small engines die as a result. I didn't find that running in fresh water did much good (perhaps the cleaning barrel out our club was contaminated?) but my solution was to buy a Honda air cooled 2hp and I've not had any troubles (famous last words...). However, I'm interested in your use of washing machine/diswasher descaler but am a little confused about the science of the whole thing - excuse me if I'm being a bit nieve In dishwashers salt is used to soften the water to prevent scale so why if a motor is used in salt water does it get scaled? Also, aren't descaling solutions acidic and isn't acid likely to corrode the waterways. I suspect that we are not dealing with normal hard water type scaling but what actually is happening here?

Incidentally, our 6hp Tohatsu that you responded about in the 235 topic is has had less than 3 seasons use so I am hoping that the will not be any cooling channel corrosion yet...

Chris Cobb

235/48 "Tarakihi"

I confess that I don't claim to know the scientific answer either.

However, I believe that scale builds up in engine cooling channels because mineral deposits are left behind as water evaporates when a hot engine stops, just as I observe that scale can builds up in my dishwasher's pipework even though we add dishwater salt to the actual washing mix.

Perhaps another member has the scientific explanation?

Oh, and I wouldn't dream of leaving my engine sitting in even a mild acidic solution for very long, and I should have added that I always follow any use of descaler by a thorough fresh water rinse both internally and externally. And lastly, I freely admit that I have never stripped an engine down before/after so I have no idea how much difference it actually makes!

Geoff Sheddick

Thanks for your comments, they were most useful.

When my impeller was last changed (after about 18 months) it only showed signs of a small crack on one of the rubber blades so it probably would have lasted a few more seasons.

Brian Banham 235/59

You were right to change the impeller when you did, because once any impeller has any damage visible to the naked eye, it should never be refitted except as a short term emergency measure.

If you go to the last page of the attached link, you will find the very useful Jabsco troubleshooting guide for INBOARD engine impellers - they don't manufacture or sell outboard impellers, and I don't know of an equivalent guide featuring outboard impellers, but the principles are very similar.

http://www.jabsco.com/files/artdoc4851.pdf 

If you compare any outboard impeller with the inboard engine ones shown, you'll notice how the blades of the outboard impeller are much longer, are often tapered, usually fewer in number, and, if you feel them, moulded from a much softer compound, all of which relates to their low efficiency, lower pressure design.

By the way, just because your inboard or outboard impeller comes out of the pump housing with its blades all bent, doesn't necessarily mean that it is worn out! - as is often implied by those wishing to sell you a new one. After cleaning it& inspecting it, just pop it in some hot water for a while, and, unless it is age hardened or heat damaged [when it will remain stiff and unyielding & should be thrown away], the blades will gradually relax back. And which way round you put it back in doesn't matter either.

Geoff