How from the keel do I mount my Johnson 50?

[jtrom]

It's a longshaft going on a lightweight alum boat. I have heard the cavitation plate should be even with the keel or an inch above it. Which is correct?. My factory manual doesnt have any info on it....

 

[nwcove]

Re: How from the keel do I mount my Johnson 50?

its "even" with, or an inch "below". which is gunna be a bit of trouble with a long shaft.

 

[boobie]

Re: How from the keel do I mount my Johnson 50?

Whats the measurement of your transom? 15 " or 20 "? What's the HP rating on the boat?? And the cav plate can be an inch above the bottom of the boat for less mtr drag.

 

[iwombat]

Re: How from the keel do I mount my Johnson 50?

It should be as high as you can get it w/o cavitation. That usually ends up being about an inch or two above the keel.

 

[1946Zephyr]

Re: How from the keel do I mount my Johnson 50?

A 50hp Johnson on a lightweight aluminum boat?? Wow......gotta be a wild ride

 

[Haffiman]

Re: How from the keel do I mount my Johnson 50?

The installation height depends on several factors.
The deeper the V-bottom, the higher you may lift. (flat bottom, just above flush).
Propeller type. (cupping and SST props normally allows more lift).
Flat water or open ocean?
Priority of top speed, hull-shot, low planning speed?
There are general rules as mentioned above, but at the multiple tests should be done to get what you want out of it.

 

[jtrom]

Re: How from the keel do I mount my Johnson 50?

The boat is a '59 Starcraft 15. Ft (actually measures 14'2") It seems awfully light for a 50 horse but according to the factory brochure that is what its rated for, up to 50.p

 

[jtrom]

Re: How from the keel do I mount my Johnson 50?

The boat is a '59 Starcraft 15 ft. (actually measures 14'2") It seems awfully light for a 50 hp but according to the factory brochure it is rated up to 50.

 

[wilde1j]

Re: How from the keel do I mount my Johnson 50?

Motor height should be as high as it needs to go to make the top of the AV plate just run dry on top @ WOT. There's no way to exactly measure or predict where this will be, as it's an empirical process. The motor also should not cavitate excessively in hard turns in flat water when height is right.

 

[dc095]

Re: How from the keel do I mount my Johnson 50?

empirical ?

 

[nwcove]

Re: How from the keel do I mount my Johnson 50?

It should be as high as you can get it w/o cavitation. That usually ends up being about an inch or two above the keel.

didnt know that standard boat props were meant to be semi-surfacing, but it is good info!

 

[5150abf]

Re: How from the keel do I mount my Johnson 50?

Put it on as high as it will go then run it, if it cavitates drop it down and run it till you find a spot where it quits and that is where it needs to be.

Every boat is a bit different so what works on one may not on another, I have a 6'' setback and run my almost 2'' above the bottom of the boat.

Take your hat off before you whack the throttle, that thing is gonna fly.

 

[wilde1j]

Re: How from the keel do I mount my Johnson 50?

didnt know that standard boat props were meant to be semi-surfacing, but it is good info!

They aren't. The prop is always fully under water (or should be). Surface piercing props are completely different and are almost never on recreational boats.

This is a picture of the first Arneson surface drive unit: