60hp hits 6000

[fishfeatures]

Hi,

Boat in my avatar is a 13 ft fletcher ski boat. Powered by a 1990 johnson 60hp. It over went a complete rebuild over the winter (boat and motor). I'm currently running a 13 1/4 x 17 pitch and speedo says 38mph ( I've no gps). RPM is hitting 6000 -6100 at wot , I have no idea of boat weight but with me and fuel and motor I guess 700lbs.

I think the engine should be propped to 5500 or so, but to do that I would have to go up to a 21. The only thing is that we have no prop shop over here that lends props so I'd have to buy the correct prop if i get it wrong then I'll waste good money. Now I have only noticed this as I never had a tach before the overhaul. i have 2 props of the same size. Can i cup one to see if it helps ? Can I cup it myself and if so how? No proper prop shop near me to do a good job.


EDIT: props are actually 13 3/4 x 15.. good thing I doubted myelf and went out and checked them.

 

[steelespike]

Re: 60hp hits 6000

The 60 is rated for 5000 to 6000 rpm.Your actual speed is probably about 37.
I would say you're propped perfectly. The boat should have a strong hole shot probably stay on plane easily at reduced throttle.
If your more comfortable at about 5600 a 19 should put about there.
It may slow your hole shot but may hold speed or give you a little more.

 

[fishfeatures]

Re: 60hp hits 6000

thanks SSpike . If going at 6000 rpm is ok for the engine then I'll stick with what I have. Thanks.

 

[steelespike]

Re: 60hp hits 6000

As with any motor keeping a little below the maximum rating in normal operation is easier on the motor.
It is easier on the motor to be able to rev freely within its rpm range.

 

[Frank Acampora]

Re: 60hp hits 6000

If the current props are aluminum, yes, you can cup them yourself. Cupping will make the prop "grab" the water better and will lower RPM by about 200-400. The downside is that it will take a good bit of time and will not be as accurate or pretty as a machine cupping. The upside is that you can change the amount of cup at will to suit your application. Look at some cupped props to see the size of the bend in the blade. Then, using a 2 X 4 set on end or an accurately curved block of steel in a vise, place the props trailing edge on it. Using a smallish hammer--16 oz --tap the trailing edge to bend it. Do this gradually and work all three blades.

Aluminum props are cast. Try to bend them too much at once and they will break at the bend. Working them gradually will "cold forge" them and the aluminum will be a little more malleable. Done carefully, you can cup and remove cup from the blades. Do it too many times and the aluminun fatigues and again will crack.

 

[Texasmark]

Re: 60hp hits 6000

If the current props are aluminum, yes, you can cup them yourself. Cupping will make the prop "grab" the water better and will lower RPM by about 200-400. The downside is that it will take a good bit of time and will not be as accurate or pretty as a machine cupping. The upside is that you can change the amount of cup at will to suit your application. Look at some cupped props to see the size of the bend in the blade. Then, using a 2 X 4 set on end or an accurately curved block of steel in a vise, place the props trailing edge on it. Using a smallish hammer--16 oz --tap the trailing edge to bend it. Do this gradually and work all three blades.

Aluminum props are cast. Try to bend them too much at once and they will break at the bend. Working them gradually will "cold forge" them and the aluminum will be a little more malleable. Done carefully, you can cup and remove cup from the blades. Do it too many times and the aluminun fatigues and again will crack.

Frank, stay with me on this:

fish..., If you don't balance your effort in doing your own cupping job, you will set up vibrations in the engine which you absolutely don't want. A prop shop can ensure a uniform cup on all blades. As you said, once you find out what you want in the way of cupping, go and buy it or take your "model" to them and let them balance it out.

Cupping could add as much as an effective 2" of pitch increase at top end giving you lower rpm's and better speed in addition to holding better. Hole shot is relatively unaffected so you get a good hole and good top end.... best of both worlds.

Mark