Bulletproof water pump?

[Squeakit]

I have a 5hp British Seagull motor which uses a hard plastic 4bladed impeller (called a "rotor") that doesn't touch the sides of the housing and basically never wears out. Starting the motor dry won't hurt it. Motor can sit for 20 years and the impeller doesn't get hard and brittle and fly apart and plug up the cooling system. It is basically bulletproof unless you suck up a whole bunch of sand or gravel and it pumps water like crazy. Why don't modern motors use this simple and trouble-free design instead of the offset rubber impeller that seems to cause so much grief? Am I missing something here? Would really appreciate some comments from the many knowledgeable experts on the board. Thanks.

 

[roscoe]

Re: Bulletproof water pump?

Very good question.<br /><br />It's kind of like this fishing tackle question.<br /><br />Why are spinner rigs made with a nylon leader, instead of coated steel leader? Answer: Because the fish can't bite off the steel ones and the lure manufacturer would go out of business.<br /><br />At least I can make my own lures.

 

[merc16]

Re: Bulletproof water pump?

i know i agree with you squeakit i guess if all motors used impellers like that the would never have anyone in to get a new one put on so they wouldnt make as much money i guess its the same way as some cars today

 

[Squeakit]

Re: Bulletproof water pump?

Roscoe & Merc: Unfortunately, you guys are probably right on about this. I was hoping there was a better reason than planned obsolescence, especially because impeller failure can be catastrophic. Some smart engineer could make a bundle by producing a retrofit replacement for the rubber impeller. Don't think one would work in the offset housing, though. So you'd have to replace the entire pump and every engine has a different configuration making this potentially great idea economically impractical. I guess the manufacturers have got us all by the short hairs.

 

[Walker]

Re: Bulletproof water pump?

Well for one thing, if the impellor blades don't touch the sides of the housing then its a centrifugal pump and I doubt it would pump enought water at low speed to cool a bigger motor. In fact it probably wouldn't even prime the entire cooling system until you reached 2000 rpm.

 

[Squeakit]

Re: Bulletproof water pump?

Hi Walker: What you said makes sense. The seagull is a small block with one cylinder and a very straightforward cooling system (no T-stats) and at idle speeds the water volume is much lower. I guess the bigger engines need the displacement pump action of the impeller to force enough water thru the block. But for little motors with simple design like the Seagull it sure works well. Thanks for your input!

 

[FredM]

Re: Bulletproof water pump?

I think Walker is on the right track. You need a displacement type pump in an outboard that is likely to be on a planing boat. If you think about it, when the boat is on the plane only the lower unit is in the water, the water pump is generally on top of the L/U and above water level. It is only when the boat is at rest that the pump is submerged and easily primed. With the 5 hp seagull you would expect the water pump to be submerged and primed at all time and not be in a planing situation. In this scenario the centrifugal pump would work.<br /><br />Just my thoughts

 

[Squeakit]

Re: Bulletproof water pump?

Thanks Fred. It begins to make more sense now. The Seagull manual states that the lower unit must be submerged to a certain level and that the pump would not operate on an "air-water mixture." In other words, it was not self priming. The Seagull is designed for slow speed and high torque and there is no way it is going to plane a boat, although my 5hp does come real close to planing my little 12ft aluminum pram. I guess a lot of R and D went into the modern water pump, but they sure seem to cause a lot of grief.

 

[FredM]

Re: Bulletproof water pump?

Hi squeakit, I think what tends to happen is that people neglect and/or abuse the water pump. Too often I see people give their motor a "quick start" on the trailer at the ramp before backing it in. It takes less than a minute to damage an impeller running it dry. <br /><br />The impeller is a cheap item and critical to the longevity of the motor. When I bought my first motor the owner hadn't changed the impeller for five years. It was still peeing ok but when I replaced it I found the fibre washer that was supposed to be under the impeller had either disintegrated or been left out sometime and allowed the key to grind away on the pump housing face plate. Lucky for me the key was damaged and not the housing. So a new key, impeller and washers fixed it up. <br /><br />The important point here is that just because the tell-tale is peeing does'nt mean all is well with the water pump. I check my'ne every year<br /><br />Fred

 

[Squeakit]

Re: Bulletproof water pump?

Excellent info Fred, and thank you. Moral is take care of your water pump if you want to keep your engine. Know what you mean about "dry starting" - some young guy at the launch ramp the other day started a beautiful older Merc 6cyl and sat there revving it for about 10-20sec before he backed the boat into the water. I don't butt into other people's business, but I sure felt like telling him he was going to smoke that wonderful old motor if he kept this up. Anyway, this is sure a great forum with so much useful information for those of us that love (or hate) outboards!

 

[merc16]

Re: Bulletproof water pump?

squeakit,<br /><br />just five seconds running out of the water will damage an impellor yes ive also seen this happen befor with a guy on our lake was crossing thorugh a shallow channel with a johnson outboard and he lifted the motor out of the water and coasted down it while it was idleing!!!!!!! and you know what happend then? his engine overheated three quarters of the way down and it just shut off and i had to tow him back for he could not start his engine again! so to anybody never start or run your engine out of the water!!!