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boardjnky4    8

I have a 15.5 now with 5/8" clearance.

I went from a 15x13x.105 to 15.5x13x.105 and I lost 2mph on top speed. Same RPM when surfing and wakeboarding.

A bit curious as to how I lost top end and didn't drop RPM. Do you think that cavitation or slippage would due to clearance would cause that? Jim at Acme seemed to think it might be he couldn't put his finger on it.

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volzalum    280

Yes, it could be cavitation. Technically with everything equal (pitch and cup), the larger prop should reduce rpm at same speed so top speed should increase. Also note though that the larger prop gives more rear lift so try putting a little weight in the rear and see if the top speed increases (i.e., weight will put the rear of the boat down some similar to the 15").

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boardjnky4    8
17 hours ago, volzalum said:

Yes, it could be cavitation. Technically with everything equal (pitch and cup), the larger prop should reduce rpm at same speed so top speed should increase. Also note though that the larger prop gives more rear lift so try putting a little weight in the rear and see if the top speed increases (i.e., weight will put the rear of the boat down some similar to the 15").

right right. So I might need to shim my strut just 1/8 or 1/4 inch to get rid of that cavitation.

I do run some lead in the bow that might be causing more cavitation than needed at empty ballast, top speed. However, I'm seeing same RPM while wakeboarding and wakesurfing. Also seeing no difference in fuel consumption (DIACOM confirmed). As such, doubt that weight in the rear will help.

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volzalum    280
6 hours ago, boardjnky4 said:

right right. So I might need to shim my strut just 1/8 or 1/4 inch to get rid of that cavitation.

I do run some lead in the bow that might be causing more cavitation than needed at empty ballast, top speed. However, I'm seeing same RPM while wakeboarding and wakesurfing. Also seeing no difference in fuel consumption (DIACOM confirmed). As such, doubt that weight in the rear will help.

When you are wakeboarding and surfing, you are weighted enough to probably not have cavitation or slipping. The ½ inch prop size change at most would impact the rpm 50 rpm, so it may be so close that you are not seeing the difference. When empty at WOT (how often do you actually do that?), the rear is probably getting air to the prop. 

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volzalum    280

I finished tightening the prop strut bolts and aligning the engine this afternoon. 3 of my motor mounts were only finger tight to start, so that will now be a periodic check for me now. Surprisingly, I have better clearance on the through hull log now than I did from the factory (it’s dead center now). I got the alignment to 0.0025 all the way around.

Final measurements, I went from ½ inch clearance to 1-⅜ inch clearance on the 14 inch prop (still have 1-¾ clearance on trailer) so subtract ¾ inch for 1.5 inch prop diameter change to 15.5 inch prop and will have ⅝ clearance. New 15.5 inch prop gets here Tuesday. Going with the Acme 2847 which is 15.5 x 15 x 0.075 cup. Going to see how the change impacted the performance of the 14 inch prop this weekend.

Here are a few pictures. 
 

FC4D8F8E-298A-4D1D-836F-F5A0894F52AE.jpeg

67A3D084-C2A3-497B-BB25-74907D2AB905.jpeg

E1EF9CC4-48B7-4E76-80F8-2E9A25C31BA5.jpeg

A1924D44-0C40-4165-ADEC-0939FABA403C.jpeg

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Jlagos    23

Nicely finished. So will the plate beneath the strut change the angle of the thrust produced by the propeller. If so, will it then try to lift the back as you were describing to boardjnky4. How about turning at idle?

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volzalum    280

I know the bigger prop will give more lift, and I expect the 1.5 degrees to give a little more as well. When I went from the 15 inch prop to the 14 previously, I had to put 250 pounds of lead forward of the cockpit to compensate for the smaller prop. I expect to be moving most of it back to the rear when the 15.5 goes on. As for idle steering, I do not expect any difference. The angle change does not impact the prop efficiency significantly (about 1.5% more thrust loss compared to before the mod) I will find out this weekend when I run the 14 inch prop after the adjustments. 

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boardjnky4    8

So I was eye'ing up the 15.5x15x.075 as well. That's just a really strange cup for a 15 pitch prop.

What engine is in your boat? A PCM 6.0?

Please let us know what your top speed and rpm end up being.

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boardjnky4    8
18 hours ago, volzalum said:

When you are wakeboarding and surfing, you are weighted enough to probably not have cavitation or slipping. The ½ inch prop size change at most would impact the rpm 50 rpm, so it may be so close that you are not seeing the difference. When empty at WOT (how often do you actually do that?), the rear is probably getting air to the prop. 

Almost never, it's just a data point to benchmark from.

I was led to believe the RPM decrease would be significant. With the speed control constantly changing things, it would be impossible to measure a 50 rpm difference. Heck the difference in chop/wind at any given moment can change the rpm more than that.

I guess I was just expecting too much.

Edited by boardjnky4

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volzalum    280
55 minutes ago, boardjnky4 said:

What engine is in your boat? A PCM 6.0?

I have the XR550 in my boat with the 1.48:1 V-drive.

54 minutes ago, boardjnky4 said:

Almost never, it's just a data point to benchmark from.

I was led to believe the RPM decrease would be significant. With the speed control constantly changing things, it would be impossible to measure a 50 rpm difference. Heck the difference in chop/wind at any given moment can change the rpm more than that.

I guess I was just expecting too much.

ACME said I would lose too much cruise speed if I dropped below the 15 pitch prop. With the original 15x15.75 I was 42 mph at WOT and with the 14x16, I am at 40 MPH WOT. The 14x16 also increased the RPM for holding surf speed. Since my engine will hit the 5400 RPM with either prop, and then go into control I am not sure what the final change is. I saw between 100-150 rpm difference for the 1 inch diameter and .25 degree pitch change. I also tried a 14x17.75 before the 14x16 and it had similar rpms to the 15 but could not hold surf speeds for anything.

I'll update after this weekend and the comparison to the 15.5 inch prop next weekend.

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boardjnky4    8
13 minutes ago, volzalum said:

I have the XR550 in my boat with the 1.48:1 V-drive.

ACME said I would lose too much cruise speed if I dropped below the 15 pitch prop. With the original 15x15.75 I was 42 mph at WOT and with the 14x16, I am at 40 MPH WOT. The 14x16 also increased the RPM for holding surf speed. Since my engine will hit the 5400 RPM with either prop, and then go into control I am not sure what the final change is. I saw between 100-150 rpm difference for the 1 inch diameter and .25 degree pitch change. I also tried a 14x17.75 before the 14x16 and it had similar rpms to the 15 but could not hold surf speeds for anything.

I'll update after this weekend and the comparison to the 15.5 inch prop next weekend.

OK that's a big difference from my 409/1.72:1 setup

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volzalum    280

Yes, I was actually disappointed in the boat when Centurion put the 1.48 in it instead of the 1.72. They essentially defeated the benefit of the more powerful engine by cutting it off at the v-drive ratio.

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Troy    234

Fantastic! Can't wait to hear the outcome of all your hard work. 

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boardjnky4    8

Since I'm stuck with the prop I have, I'm going to have it re-pitched to a 15.5x14x.105 at Nettles.

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volzalum    280

So my new prop (15.5x15x0.075) came in and is installed. I dinged my 14 inch (14x16x0.105) on Sunday, so when I send it off for Repair I will get it pitched to a 14x14. I can say the prop angle change was noticeable on the 14 inch prop. It was barely able to get to surf speed going upstream (though there was a 2.5 mph current), and I lost 1 mph on the WOT (dropped from 40.5 to 39.5). At my empty cruise speed of 24 mph, the rpm changed just under 100 rpm. I will get results with the 15.5 inch one this weekend. Pictures attached. 

65C361EB-B6A1-4573-A7CC-189427E871BE.jpeg

68EA0760-6CC7-49A7-8E0B-702A4F0B5C3F.jpeg

50F088A7-95F8-43A7-8FF6-773941A6D0DE.jpeg

DFA1B5AC-EFA4-4ECA-9BC0-974363D1E896.jpeg

12AF1AF0-7A88-46AA-B636-7B9D6214AE03.jpeg

F4287BA7-43D2-405B-BFF1-A223C391C51C.jpeg

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volzalum    280

So I am very happy with the mod. With the 15.5 inch prop, I gained 2 mph at top end. Dropped 400 rpm at surf speed and 24 mph cruise speed. Does 28 at old 24 mph rpms. Holds all ballast with no issues now for surfing. I was able to accelerate to 15 mph (upstream) with all ballast full with testing quicksurf (with the 14 inch prop I could not get above 9). At normal lean I was at 3100 rpm for surfing setup.

So if you have enough adjustments in your engine mounts (room to go up in front and down in back) and enough clearance in the through hull, then this mod is 100% worth doing. I am very happy.

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Troy    234

That is awesome news!!! Wave back to normal?

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volzalum    280
1 hour ago, Troy said:

That is awesome news!!! Wave back to normal?

Yes. There was no impact on the wave. It was fun seeing its limits. In goofy surf configuration, a gps speed of 11.9 (upstream in 2.5 mph current) was the fastest I could get it with the 14 inch prop. There was a 4.5 mph current on Saturday and I took it up to 15 mph upstream (so 19.5 mph water speed). But at my rider profile settings, the wave was still clean and similar size.

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