does tis seem right 90 johnson

energizedsbs

Recruit
Joined
Jan 2, 2005
Messages
5
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
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: does tis seem right 90 johnson

Im no expert but would start with a compression check.Be certain it is running on all cylinders.Who rebuilt the motor.Check the vertical height. Anti vent plate should be about even with the bottom of the boat.Did you adjust the trim? What size prop are you running.What size on the 70 and what rpm did it run.Is your speed measured with a gps.<br /> Even though your power increase is around 30% your speed increase would most likely be less depending on the designed speed performance<br />caracteristics.By the way what is the max horsepower rating for the boat.
 

energizedsbs

Recruit
Joined
Jan 2, 2005
Messages
5
Re: does tis seem right 90 johnson

compression is all good.... its definitly running as good as it can....a johnson dealer rebuilt the motor....yes i ajusted the trim as far as it would go till i started losing speed....prop is a 13 3\8 dia and 17 pitch....gps yes...the max for the boat is a 90.... i just thought a little 16 ft with a 90 would be faster.... arent the 115 the same in weight... i should have just got one of those.... im looking for at least over 50mph...41 doesnt cut for me...lol thanks for the help<br /><br /><br />sam
 

G DANE

Commander
Joined
Nov 24, 2001
Messages
2,476
Re: does tis seem right 90 johnson

In order to see how you are doing, try to calculate the speed formula, earlier posted by JB. Usually quite accurate. If your boat isnt too heavy your should be turning at least a 19" on that boat. Formula is:<br /><br /> posted June 21, 2002 09:50 AM <br />________________________________________<br />There is another formula for predicting top speed if ideally propped and trimmed.<br /><br />Prop HP=A<br />Boat gross wght.=B<br />Bottom factor=C: 180 for V bottom, 200 for nearly flat, 225 for Flat.<br /><br />Take the square root of A/B and multiply by C.<br /><br />Example: My Montauk has 70hp, weighs about 1900lb. with max load. 70/1900 =.037. Sq. rt of .037 =.19,<br />Nearly flat bottom factor, 200 X .19 = 38.4mph.<br /><br />GPS WOT reads 38.5mph <br /><br />--------------------<br />Red sky at night. . .<br /><br />Are you sure your tank and fuel connections are good to supply the fuel the V4 needs ? Tank vented, lines tight ? How deep is the motors antiventilation plate compared to lowest point of keel. At 5600 RPM with a 17" pitch your theoretically speed is 45.5 MPH which leads to a slip percentage of around 10%, not uncommon. Sound like the motor isnt performing its best.Do the speed calculaon and compare.
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: does tis seem right 90 johnson

Youre right at 10% slip. Not bad for that prop. But the rpm should be around 5800-6000 with an average load. <br /><br />A 17" seems a bit small for a V4 on that boat but if I'm not mistaken the Malibu has a lot of hook and is made for skiing -- or not?
 

energizedsbs

Recruit
Joined
Jan 2, 2005
Messages
5
Re: does tis seem right 90 johnson

no not the malibu i have.... i know what your talking about though.....me and two friends picked up the boat with no motor on when i stripped it... i took everything out of it exept a cooler seat and a small little counsole...its a small light boat... i just dont know why its only running around 41... doesnt seem right...i know the motor is runnig good so is it the prop you think??
 
Top