Just to clarify something, and I am sorry if I am being a bit thick, but I am working on the assumption that you start afresh with a totally new raw water pump, which is going to bit a bit painful if you have to buy it new, though I've seen good ones secondhand from time to time at the local chandlery
. Think au$500 plus new, I know this as I have a spare on my shopping
list for when we go cruising, but not before.
I cannot see how you could possibly reuse the existing pump, aside from it being gear driven, I would also assume the bearing cannot take any side load so a belt drive, even if you could fit a pulley, would not work. If you are thinking of remaking the pump to handle the side load I guess that could be done but I suspect it would be a lot of work with an uncertain outcome. I am usually keen to give it go with this sort of thing but this one would scare me, that's for sure.
As for RPM, it's not too hard. Our jabsco
is rated to a maximum of 3000 rpm, which is ok given we are propped for a maximum engine rpm of 2000 rpm, and I am guessing the Ford Lehman
would be similar? Anyway, minimum rpm they recommend is 500 to give enough flow for our setup, idle speed of 650 rpm and you can see we could get away with a slight reduction in ratio, however as it is we currently have a ratio of about 1:1.4 which gives us good flow across the rev range, particularly at our preferred cruising revs of 1200 rpm. FWIW our engine is theoretically 96 HP but with the current
prop we are limited to 80 which I think is smart and certainly enough power. Either way, it means we are producing about the same amount of waste heat as you will be at WOT.
Finally, you may have considered this already so apologies if I am stating the obvious, but with a conversion to something similar to our setup you can easily tap in an electric
bilge pump as an emergency cooling
pump. Probably possible with your current setup but a heck of a lot easier when all the hoses are at eye level.
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