motor height for 1950's Johnson

Not sure if this is a boat question or a motor question, I'll try it here.<br /><br />I've seen alot of people here say that the Ventilation plate should be even with the bottom of the boat. I'm currently building a new transom, my Ventilation plate is a whole 3 inches low. <br /><br />My question - does this apply to the old Johnyrudes also? I ask because they have a plate that is just above the Ventilation plate, which covers the water intake passage for the water pump. This plate has about 4 3/32 or so holes in it. Should I still have this old of a motor up this high? Will these holes be sucking air into the water pump if they are above the flow of water? This picture is small, but it shows the cover plate, just above the Ventilation plate.<br /><br />

Re: motor height for 1950's Johnson

These old engines can't be run very high on the transom as they will ingest air quite easily. Just keep the wate over those holes on the cover plate.

Re: motor height for 1950's Johnson

I've found my '58 18 has to sit a little lower than the normal recommendation, with the plate an inch or inch and a half below the bottom, if it's any higher the water pump doesn't always prime.

Re: motor height for 1950's Johnson

Great info, thanks! I will adjust my "in-progress" transom. I think I'll go with the vent holes at the bottom of the boat, instead of the plate.

Re: motor height for 1950's Johnson

I did the same as you, read these post about raising O/B. I had no problem with picking up water, what I had was alot of cavitation problems when turning, and these wern't sharp turns either. Lowered motor back to original position. BTW, mine is a 56 Johnson 30.

Re: motor height for 1950's Johnson

Hi 55 Crestliner,<br /> Actually, you can run those motors quite high on the right set-up. I run a '56 30hp Johnson on an oldutility racer with the motor at about 19-inches - cavitation plate way ABOVE the bottom. I also run a 15 1/2-inch 2-blade prop with a good cup in it and it still pumps plenty of water at speed - the blades throw water into the intake screen in the leading edge of the exhaust outlet. Your average 3-blade aluminum prop would have lost its bite several inches below where I run mine. Better to build the transom a little low and then raise the motor with wooden spacers than have to cut the transom down after you've gotten it all finished.<br />- Scott

Re: motor height for 1950's Johnson

Chinewalker, (or anyone else), You mentioned your 2 blade prop - Do you think a 13p prop (2 blade cupped) is too much for an 18 horse? I tried one the other day, but it felt slow in the top end, and acceleraton was very much slower. Does this prop need a bigger motor, or does the motor have something wrong with it? <br /><br />Much faster with a 9x9.<br /><br />I know I need a tach, and no, I have not done a compression test - didn't seem to need one.

Re: motor height for 1950's Johnson

Hi 55Crestliner,<br /> You basically answered your own question with the 13-pitch prop. Unless you're running the motor on a VERY light performance hull, .ie a hydroplane or the like, the 18hp will never swing the 13, and even then you might have to jack it up high just to get it to plane off (prop will partially ventilate allowing motor RPM to increase, allowing the boat to plane off).<br /> So, to answer your question - no the prop does not need a bigger motor, and no, there's nothing wrong with the motor - you just need a smaller boat.<br /> PS - When I say smaller, I mean a hull that weighs in at less than 150 lbs....<br />- Scott

Re: motor height for 1950's Johnson

"there's nothing wrong with the motor"<br />Thanks, that's reasuring.

Re: motor height for 1950's Johnson

Originally posted by cajun555:<br /> I did the same as you, read these post about raising O/B. I had no problem with picking up water, what I had was alot of cavitation problems when turning, and these wern't sharp turns either. Lowered motor back to original position. BTW, mine is a 56 Johnson 30.

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I now know what ventilation is! Looks like I'll be cutting that transom down a bit more. It's reving up when going into a mild turn. I think I've just figured out the biggest reason - my boat is so flat bottom in the rear, whenever you turn, it's basically lifting the keel, and the motor. On a V hull, it seems it's just rotating at the keel.