Sunsetter LXI Speed/RPM Question

[Rednucleus]

Question on a friend's boat - '03 SS LXI with Monsoon 330. Today pulled a bare footer, appeared to hit rev limiter at about 4500 rpm, maxed out at 39 mph. At 30 mph rpm's 32; at 32 34-35rpm; at 36 38-39rpm. Don't most dd's speed & rpm mirror each other (like 32mph @3200)?  Hitting rev limiter mean time for different prop?  Other owners experience for top end speed? Thanks for you comments -Red

Edited August 22, 2023 by Rednucleus

[Bozboat]

52 minutes ago, Rednucleus said:

Question on a friend's boat - '03 SS LXI with Monsoon 330. Today pulled a bare footer, appeared to hit rev limiter at about 4500 rpm, maxed out at 39 mph. At 30 mph rpm's 32; at 32 34-35rpm; at 36 38-39rpm. Don't most dd's speed & rpm mirror each other (like 32mph @3200)?  Hitting rev limiter mean time for different prop?  Other owners experience for top end speed? Thanks for you comments -Red

30 = 3000 rpm’s with an acme 515, above 30 the wind resistance of the Bimini, tower, wakeboards etc start to eat some rpms, so 4900 might get you 45-46 mph.   The acme 515 is a 12” pitch prop. In theory, one rotation of the prop should get you 12” forward movement. Maybe some math wizard can finish this thought, but I think you can get close by calculating speed off prop pitch and rpms 🤷🏻‍♂️

The 525 prop has a 11.5” pitch which would mean 11.5 inches forward per revolution. So more power and less speed. 
 

with the acme 515, and monsoon II 320, and a large bimini, I can hit 4900 rpms and 45-46 on smooth water with no head wind. 

Edited August 22, 2023 by Bozboat

[roeboat]

I think that engine might need a tune up. The rev limiter should be around 5200 rpm. 4500 rpm is a bit low. We have a few sunsetters on the lake and they all push through to at least 43mph and about 5000rpm. They have the acme 515 on them.  There is a possibility that the gauges are not reading correctly too. I'd try to verify the speed with a gps speedometer app on your phone.

[roeboat]

58 minutes ago, Bozboat said:

Maybe some math wizard can finish this thought, but I think you can get close by calculating speed off prop pitch and rpms

I think its more of a happy accident that it works out so well. I actually played around with that in an excel sheet once. Its all about calculating the slip of the prop. The extremely difficult way is to use drag coefficients and such. The easy way is to use known data rpm vs actual speed and theoretical speed. It won't be perfect and the farther you get from the original prop the less accurate it'll be.  

This is what I did for a 2005 Response LXI with a Monsoon 340 and acme 515. The graph on the right is the original and the others are interpolated from the data. Its interesting seeing the efficiency dropped at low end as the boat gets on plane.

 

[Rednucleus]

@roeboat thanks for the info - speedo is PP stargazer so assume gps is accurate. 

[Steve B.]

Almost identical driveline, but not sure what prop I have. GPS speedo. At 2850 rpms, it will go 30, but right after that the rpms/speed swap. Rev limits at 4950 rpms and speed is 48.5.

[Woodski]

@Rednucleus  A few other items I would look at based on your data - trans fluid level and condition assuming prop is in good shape (what is it?), I would also throw the boat on a scale, 39 mph @ 4500 RPM is pretty far off with what should be a stock prop on it (Acme 515) and even a 525.  Was there something under the boat messing water flow across the prop?  Prop can also be a cause, how smooth is the boat?  Prop efficiency does not really fall off a lot until your pushing 5k RPM (assuming everything is in good shape)

[footndale]

00 SSLXI, 525 Acme, pulls 42mph with a footer and 3 others. Can't do a 515, loses speed.   Plus this boat has a barefoot plate on it too.

[justgary]

13 hours ago, roeboat said:

I think that engine might need a tune up. The rev limiter should be around 5200 rpm. 4500 rpm is a bit low.

I agree.  It either has the wrong prop or the engine isn't making the torque it used to, or both.

[Rednucleus]

Quick update - our group is here at Lake Curlew for the week, boat runs well for skiing.  Found dirty cap & rotor, cleaned those. Also two plug wires a bit loose. Now runs smooth full throttle maxes out at 5k, top speed 39mph. Has a tower & bimini.  Don't know what prop in use yet.

[Woodski]

@Rednucleus - Are you saying the boat has a max speed of 39 mph no matter what the engine speed is?  (Post 1 - 39 @ 4500, post above 39 @ 5000).  If that is the case, there is an additional issue either in the drivetrain or the hull.  Does the boat sound and feel like it only goes 39 mph regardless of engine RPM?   From previous post, I would check trans fluid, if the trans is slipping it will only get worse.  If that is fine, time to look at the prop and verify it's  1. correct (Acme 525 or similar)  2. not damaged  3. key is not sheared

[JL5]

Yes that is odd. I have a new this year to me, 00 SS LXI and although I really haven't run it WOT, we easily hit 44-45 this AM with 3 people bimini top on with new 525 on it.

[Rednucleus]

Quick update - pulled boat out this weekend, all 4 blades nicely dinged - amazed we did not perceive any vibration. Not sure when prop will be fixed/replaced but will report back on new top end/rpms when done. Thanks all for your input - Red

[Bozboat]

1 hour ago, Rednucleus said:

Quick update - pulled boat out this weekend, all 4 blades nicely dinged - amazed we did not perceive any vibration. Not sure when prop will be fixed/replaced but will report back on new top end/rpms when done. Thanks all for your input - Red

4 blade?  What prop is on it?

[Rednucleus]

Knew you were going to ask that - was not able to read number he hasn't pulled prop yet - stay tuned for further updates!

[Bozboat]

9 minutes ago, Rednucleus said:

Knew you were going to ask that - was not able to read number he hasn't pulled prop yet - stay tuned for further updates!

I’m thinking the SSLXI favors three blade props, so that could be the whole enchilada. 

[Steve B.]

I think my prop is the 535? Anyway, 3 blade. Haven't WOT'd since a GPS speedo upgrade recently, but 4350 rpms pulled a barefooter 41 on my '02. I can usually rev up to 4950 ish, just haven't yet this summer.

Steve B.

[Rednucleus]

Good tune up & put on spare 4 blade - acme 232 13x12.5. Now 4500 and 42 mph

They recently acquired a Sportster and will try that boat's 3 blade to compare.

Edited September 7, 2023 by Rednucleus
added info

[UWSkier]

On 9/6/2023 at 6:29 PM, Rednucleus said:

Good tune up & put on spare 4 blade - acme 232 13x12.5. Now 4500 and 42 mph

They recently acquired a Sportster and will try that boat's 3 blade to compare.

I'd really like to try a 4 blade Acme on my boat to see if it improves the ski wake.  I know there are very few 13x12 or 13x11.5 options that have more than 3 blades.

[Woodski]

@UWSkier - agree, would be a fun experiment.  The Acme 3 blades do have a lot of blade area, so may not see the difference one would with the pre CNC stuff.

[footndale]

Maybe contact OJ, they deal with people really well, so I've been told.