Need for Speed - Prop Question

[Bruce]

Hi there,I just took the boat for the first ride of the season with the new jackplate installed. Pulse 170 with a 1993 Evinrude 120

I wasn't "really" expecting an increase in speed due to the fact that before I installed the plate I was running approximately 6000-6100 rpm according to my tach with a top gps speed of around 53.

The current setup is a Raker 13.5 x 22
with the prop center approx 4.5 inches below the pad. While "fooling' around with different heights on the plate, it really didn't affect the rpm's or water pressure or top speed even with the plate pretty well at the top.

So, that brings me to this, according to the Seloc manual the "wot" rpm is 6000 and I am achieving that, give or take.

I'm thinking of trying to find a 13.5 x 24 Raker which will drop my rpm's roughly 200 but may afford me a couple of more mph due to the increase of pitch.

What do you think, 13.5 x 24 or perhaps try a 14.5 x 24 and perhaps lose approx 400 rpm.

I don't know if the 14.5 would even fit onto the shaft. I'm not sure that the 13.5 and 14.5 Raker's are compatible in shaft size. (I'll scout that out in a minute)

For what it's worth though, the extra 6 inches of setback made a difference in "hole shot" which was really noticeble, however, I'm in search of some more top end speed.


Thanks in Advance for any suggestions/comments


Bruce

[Bruce]

Hi there,I just took the boat for the first ride of the season with the new jackplate installed. Pulse 170 with a 1993 Evinrude 120

I wasn't "really" expecting an increase in speed due to the fact that before I installed the plate I was running approximately 6000-6100 rpm according to my tach with a top gps speed of around 53.

The current setup is a Raker 13.5 x 22
with the prop center approx 4.5 inches below the pad. While "fooling' around with different heights on the plate, it really didn't affect the rpm's or water pressure or top speed even with the plate pretty well at the top.

So, that brings me to this, according to the Seloc manual the "wot" rpm is 6000 and I am achieving that, give or take.

I'm thinking of trying to find a 13.5 x 24 Raker which will drop my rpm's roughly 200 but may afford me a couple of more mph due to the increase of pitch.

What do you think, 13.5 x 24 or perhaps try a 14.5 x 24 and perhaps lose approx 400 rpm.

I don't know if the 14.5 would even fit onto the shaft. I'm not sure that the 13.5 and 14.5 Raker's are compatible in shaft size. (I'll scout that out in a minute)

For what it's worth though, the extra 6 inches of setback made a difference in "hole shot" which was really noticeble, however, I'm in search of some more top end speed.


Thanks in Advance for any suggestions/comments


Bruce

[Bruce]

I just answered my question about whether or not a 14.5 would fit. Irregardless if the shaft size is the same, I only have 1/2 inch clearance under the cavitation plate as it stands right now.

That makes it easy, the hunt is on for 13.5 x 24 Raker.

Bruce

[jkeiper]

Bruce;

I'm not that familiar with OMC's gear ratios. Do you know what yours is? By the RPM your turning that 22 you should be runing faster than that. Seems like a lot of slip to me, but I don't know what gears you have.

Jim

[Bruce]

Hi Jim, the gear ratio is 2.0, when I calculate the slip, it comes in around 15/16% give or take.

Maybe I need to "dial it in" more,


Bruce

[daren34]

how does it do with more trim and all the way up?
the diplomat that i had liked lots of trim
can you raise the motor up any higher on the plate or the transom?

[Mark]

The 170 is a lot of boat for a 120HP. At over 1300# 53mph is not bad. Still something isn't right. The Raker has a lot of grip and is likely to slow RPM's before it slips by 18%. First I would borrow a different prop just to make sure the hub isn't slipping. If nothing changes then I would check my tach to ensure it is working accurately. If you can turn a 22p Raker to 6000RPM you should be seeing 60 MPH.

[jkeiper]

Bruce:

That is what I thought you'd have for gears and I came up with the same slip #s. With some wokr you can get that down. I'm not at all familiar with OMC props, but you need something with a lot of cup in it. If you can get the water pressure start at 3" below the pad. Get a prop with a lot of cup. (i.e. Laser II or Tempest Plus. OMC equivlent) It will bring your RPM's down, hook up better, hold on better and you won't need much trim. You don't need the 24p. Work on getting rid of the slip instead.

Good Luck!
Jim

[Bruce]

Thanks Guys, I can definetly go higher with the plate, another 2 inches for sure. That will set the prop shaft at around 2.5 inches below the pad.

Probably get back out next week, I'll report back in.

Bruce