does tis seem right 90 johnson

[energizedsbs]

i have a little 16 foot malibu.... its a really light boat.... i redid everything and got a rebuilt 93 90 hp johnson.... my ? is that it at 5600 rpm im only running 41 mph... my old johnson 70 hp went the same.. if need anymore info i'll post more.... thanks alot folks

 

[ob]

Re: does tis seem right 90 johnson

Need to let us know what diameter and pitch prop you're running.The engine rpms are in direct relation to the engine setup,and prop selection on a givin hull.Otherwise,I'd guess you're running a 13 3/8 X 17 prop(if its OMC),in which case 41 mph should be ballpark.Bare in mind that the 90 is a measurably heavier engine that the 3 cylinder looper.

 

[energizedsbs]

Re: does tis seem right 90 johnson

13.375 dia and 17 pitch ss prop....i just thought the boat would go faster...

 

[OBJ]

Re: does tis seem right 90 johnson

Hows the engine set up on the boat sbs....meaning, where is the anti-cavitation plate in regards to the keel of the boat? High, low, same....? <br /><br />Edit: Are you using the TNT?

 

[Silvertip]

Re: does tis seem right 90 johnson

Look at it this way. Assuming both a 70 and 90 HP engine turn 5500 RPM at wide open throttle and both use the same diameter and pitch prop, and have the same gear ratio in the lower unit, then the speed will be identical. It's simple math. Everything is turning at the same speed so there would be no advantage. However, the 90 has the advantage in that the amount of weight it will move at 5500 RPM is greater because it has more power. It is also possible to increase the prop pitch on the 90 which will provide higher speed (theoretically) while still turning at 5500 RPM. The 90 should also get a better hole shot. So you see, there are advantages to a bigger engine, you just need to set it up with right prop, use the trim properly, and have the motor height set properly.

 

[Dhadley]

Re: does tis seem right 90 johnson

The 70 and 90 have totally different gear ratios.<br /><br />In your other thread we were looking at the hull design. It really doesnt matter how light the boat is. In fact if the boat is really as light as you described in the other thread, you would be running a much larger pitch prop. Or the rpms would be off the scale with a 17.<br /><br />A couple of things to look at -- <br /><br />First look at the tach and make sure its an OMC/BRP tach set on position 6. Not 4.<br /><br />If thats OK look at the hull and see how much hook is in it.<br /><br />If the hook is normal (about 1/16"-1/8" right at the back) then verify the motor is running on all 4 and the sync & link is correct (including timing).