long shaft motor vs a short

[brian3127]

i have a long shaft motor on my boat and i think i could go with a short. the prop hangs way below the transom. would that make a difference. in how it performs in the water. maybe less stress on the transom. and if a shorter shaft motor would be better how hard its it to take the extention off of it.

 

[ezmobee]

Re: long shaft motor vs a short

It would make a huge difference. The horizontal plate above the prop should be even to about an inch above the boat's keel.

 

[brian3127]

Re: long shaft motor vs a short

my motor is a 1975 40 hp johnson. it has an extention on the motor. how hard is it to take the extention off. what parts would i need to do this. and you said it would make a big difference. in what way

 

[Silvertip]

Re: long shaft motor vs a short

Think of the position of the prop. Being way below the keel means it is akin to having a long lever strapped to the back of the boat. As the prop tries to push forward, it is acting on that long lever thus trying to raise the bow. Having the prop up higher decreases that tendency and provides better push. In turns the long lever also comes into play wherein it tries to flip the boat by squirting the stern out from under you. This is why they make 15, 20, 25, and 30 inch legs. If it didn't make any difference the manufacturers would only make long ones.

 

[brian3127]

Re: long shaft motor vs a short

Think of the position of the prop. Being way below the keel means it is akin to having a long lever strapped to the back of the boat. As the prop tries to push forward, it is acting on that long lever thus trying to raise the bow. Having the prop up higher decreases that tendency and provides better push. In turns the long lever also comes into play wherein it tries to flip the boat by squirting the stern out from under you. This is why they make 15, 20, 25, and 30 inch legs. If it didn't make any difference the manufacturers would only make long ones.

thanks

 

[ezmobee]

Re: long shaft motor vs a short

Check out this thread http://forums.iboats.com/showthread.php?t=302093

 

[scottrn]

Re: long shaft motor vs a short

It would make a huge difference. The horizontal plate above the prop should be even to about an inch above the boat's keel.

Would 2 inches be too far up? My motor is a 20 but the new boat has 22 inches from transom to keel..... to far?
Thanks

 

[ezmobee]

Re: long shaft motor vs a short

Maybe it is, maybe it isn't. Only to know is to test. If you're ventilating all the time then it's too high.

 

[Solittle]

Re: long shaft motor vs a short

Maybe not the best comparison but I run a 75 9.9 longshaft Johnny on a 15" transom of my 13" tinny and it works just fine. Sure you wold be better of with a short shaft but you may want to try it as is before you go through the hastle. You may be surprised.