1996 Johnny 90 hp prop question

[Osprey 175]

I have a 1996 Johnson 90 VRO[removed!].. At WOT I am turning 4250 rpm. The motor should be @ 4500-5500,so it may be lugging a bit. Performance is not bad. My 1996 Aquasport Osprey 175 does about 33 mph@WOT and cruises nicely @28 mph. Another guy I know has the SAME rig and his tops out @ 35 mph,with a similar load on board. My boat has a thick, lumpy, crappy, bottom paint job,and 100 pounds of kicker and bracket[6 hp merc]on the transom. I feel this is the reason he is a tad faster on top end. My question is this. Should I replace the prop with one that will get me in the rpm range of 4500-5500@WOT? I am a few hundred rpm under spec[250] but will this effect peformance/fuel consumption enough to bother with? I was told my WOT rpm is far too low and should be close to the max[5500]@WOT. Will I get any increase in top speed or improvement in fuel mileage?. The boat is fairly heavy for 17 footer,well over 2200 lbs with kicker, gear, and fuel[not including 3 grown men]. Another guy told me his Osprey 175 would top out @42-43 mph with a 90 hp Yamaha. Sounds a bit optimistic to me!Anyway, just wondering if I will get a performance boost with the engine spinning faster than 4250. any thoughts???/ thanks for the help!!! bob

 

[Solittle]

Re: 1996 Johnny 90 hp prop question

Not only should you get a performance boost but more importantly your motor will last longer. What you are doing is like trying to climb a hill pulling a trailer in 4th or 5th gear - very hard on a motor.

 

[ob]

Re: 1996 Johnny 90 hp prop question

What prop are you currently using.Dia,pitch,stainless or aluminum?You can also look into raising the height of your engine on the transom.Good starting point is to have the lower unit anti-ventilation plate roughly even with bottom of boat hull when trimmed level.As a rule you would like to have the engine mounted as high as possible without experiencing excessive prop ventilation at 2/3 throttle while executing a tight turn.Optimal height placement will afford less water drag from lower unit and allow the prop to develop higher rpms.Only a water test will deterimine the final placement.

 

[Jack Shellac]

Re: 1996 Johnny 90 hp prop question

You definitely need to get those RPM's up or you will eventually ruin your motor. It should be turning at least 5250 and preferably 5500. Here's some benchmarks: My 17' Cape Horn and 90 Johnson [15 HP Kicker]is similar to your rig. With a 12 3/4 x 15 SS prop, it turns 5550 RPM and 36 MPH. With a 12 3/4 x 17 SS prop, it turns 5100 and 39 MPH. I run the 15" P prop unless I have some special reason to run the other. This should give you an idea of which way you need to go.

 

[JB]

Re: 1996 Johnny 90 hp prop question

Hi, Bob.<br /><br />Let's not forget our sponsor. <br /><br /> http://www.boat-propellers.net/