Low RPM's

[spotshot]

I just bought a 98 Hyda sports 20' with a 150 hp johnson ocean runner with a 15 x 17 aluminum prop. The motor only ran 5200 RPM's @ 44 mph and cruised @ 34mph @ 4000 RPM good hole shot jumps right up on plane.I was looking to get a stainless prop and would like to ret my RPMs up a little.I really haven't look to see whats available.if i i went with say 15 x 15 I should get my rpms up some but what would this do to my fuel economy, hole shot cruise speed and top speed.Any help or suggestions would be helpful .

 

[Texasmark]

Re: Low RPM's

Without other interruptions your moving to a 15P SS normally will help with prop efficiency and the lower pitch will allow the engine to run up higher on the rpm curve where 2 strokes like to run. Now, just what happens to your torque curve at that point will determine whether you gain or loose in mph. If it's flat then it is basically the ratio between the 15/17 x the new rpm vs old.

17/15 = a 13% pitch decrease. Assuming your 2" pitch got you 400 more rpm, then you have 5600/5200 = about a 7% rpm increase.

So just for a number the pitch changed the most and since it went down your speed might go down 13-7 = 6% drop but the SS efficiency could add say 2% so it might only drop 4% or 44 x .96 = 42 mph.

But that's just numbers and you don't know till you run it.

HTH,
Mark

 

[jestor68]

Re: Low RPM's

I'm not sure why you think you need to get your rpm up. You are turning in the upper third of your WOT range(4500-5500) now, which is good, since that motor develops 150 hp at 5000 rpm.

Now you are operating at a 2% slip; near perfect.

A 15" will raise your rpm to about 5600, but lower your theoretical top speed to 43.

You'll be going slower and burning more fuel.

 

[spotshot]

Re: Low RPM's

if i left the 15 x 17 prop and went with a ss prop what would this do?

 

[jestor68]

Re: Low RPM's

if i left the 15 x 17 prop and went with a ss prop what would this do?

It is typically recommended to drop an inch of pitch when switching to a SS prop.

In your case that shouldn't be necessary, because most of the 17" SS props are of a smaller diameter(example: 14.25 X 17). The 3/4" decrease in diameter will offset the need to drop pitch, resulting in nearly the same WOT rpm, and improved all around performance. The debate involves whether the increase in performance is worth the triple price tag.

The use of the stainless prop with it's additional cupping should allow you to raise the motor higher on the transom(reducing drag) to gain even more speed (and rpm). Usually you can raise the motor at least one hole higher.
So; while usually gaining 1-2 mph, by raising the motor you could see 3-5 mph. Results vary depending on the boat.

Looking at it in terms of efficiency, by putting the SS prop on and gaining 2 mph; that translates to an increase in efficiency and better gas mileage, as well as better acceleration, cruise, top speed; everything.

 

[Texasmark]

Re: Low RPM's

One of the reason for the decrease in diameter of some SS props for the occasion is due to "rake". Rake is what lifts the bow of the boat out of the water once on plane, not to be confused with the bow in your face coming "outta da hole" (getting on plane) with some boats....not in your face, but over the water. Props designed for bass boats usually have a lot of rake...blades bent back to get the bow up because most of them run on a pad and the bow lift helps in the mph. If you never ran one, as once I didn't you don't know the experience. But if you have, you know what I am talking about.

On 2% slip that is totally unreasonable. 0% slip means that your prop is revolving through a solid. 2% means that your prop is basically revolving through a solid. Water is not a solid........ Guess again.

Keep in mind that the info you receive on here is from the www...that is the World Wide Webb. You have no idea as to who is blowing off and how accurate the information is....proceed with caution including what you hear from me!

Mark