1972 johnson how far down should the prop set from the bottom of the boat?

[hooter85742]

Okay I have a 1972 johnson 40hp. I have 2 different lower gear boxes. one is 6 inches longer than the other. The shorter one puts the top of the prop right at the bottom line of the boat. The longer one has it 6 inches below the bottom line of the boat. Oh the boat is a 1972 arrow glass 14 foot 7 inch v bottom hull.

I also have more than one prop I have a 10x10 10 1/2x10 and a 10x12. I have used the 10x10 and it will not get the boat on plane. the 10x12 gets the boat on plane with the lower gear box and a tail andded to the motor. The tilt is at the last hole the last spot away from the boat. But put it to the hole at the closest to the boat and it pushs the front of the boat down into the water at full throtlle. That should give an Idea about what I have now. Speed wise its not fast.

 

[kfa4303]

Re: 1972 johnson how far down should the prop set from the bottom of the boat?

HI hooter. The general rule of thumb is that you want the cavitation plate (plate just above the prop) to be even, or 1"-2" higher than the keel (bottom) of the boat. If it sits much below the keel it will cause drag and reduce performance. That 40 hp should definitely push a 14' boat along nicely. I have a 20 hp on my 14' aluminum, v-hull and it pushes it along at about 20-25 mph at full throttle and is very stable. As you've realized there are many variables at work when trying to get you boat to perform it's best. You have to find a balance between prop size and pitch, the weight distribution within the boat itself and the tilt/trim of the motor, which is why so many modern motors have infinitely adjustable tilt/trim, rather than the old pin system. There's really no magic formula, you just have to experiment and see what works best for your applications (i.e. fishing vs. skiing vs. cruising). No one boat can do everything equally well and any gains in one area of performance often mean loss in others. Aside from following the advice regarding the cavitation plate, you might be best served by spending a day on the water under whatever you expect to be the most typical conditions for your boat and systematically adjust each variable on the boat until the best combination is found. You'll be doing lots of starting and stopping and changing of props, but you'll learn lots and it'll be worth it. If you're generally boat alone set it up as such, if you're usually with 1-2 other folks, bring them with you and/or get an equivalent amount of ballast. However, while you can put ballast almost anywhere, people have to sit in seats so there weight isn't quite as adjustable, so bear that in mind. no substitute for good 'ol fashioned trial and error.

 

[hooter85742]

Re: 1972 johnson how far down should the prop set from the bottom of the boat?

Well knowing that the placement of the cavitation plate helped alot alot. I know that I had a few problems with the motor. I have them fixed and now as you said to to just run it and find the right prop and pitch then it will be just right. I think I am close it ran get the last time out.