Tune-up & WOT speed puzzle / 350 MAGS+BIIIs

[FastMarkA]

When I got my new-to-me 300 SLX last season, it had 100 hours. At WOT, it would easily reach 52-53 MPH @5000-5100 RPM.

Since the boat is >3 years and over the 100 hour mark, I decided to do a tune-up towards the end of last season: plugs, caps, rotors, distributors, fuel filters, water impellers, and oil/filter changes.

It accelerates much more smoothly than before (although it was never bad by any means), but now, I can easily hit the rev limiter at 5200-5350 RPM, but my WOT speed is 50.6 MPH. (And yes, the Smart Craft alarms go crazy...as they should.)

Before the tune-up, I would never come close to the rev limiter, so that tells me the engine is firing better with the new plugs/equipment? What puzzles me is that nothing in the underwater gear has changed, but I'm losing 2 MPH. (The boat was in a heated garage over the winter and has a clean bottom.)

So my questions are: (1) any thoughts as to why I'm losing 2 MPH and (2) maybe the added power means I should upgrade from 24P to 25/26P props?

(Of course, within all of this, I fully recognize losing 2 MPH off the top end, a speed I hit maybe 3-4x a season, is not the end of the world...it's more puzzling than anything.)

 

[scoflaw]

Sounds like your over trimming it. How fast is it when you trim down to 5100 ? How do those props look ? If your gaining rpm's and losing speed your props are slipping or at a bad angle in relation with the hull

 

[SCORPIO]

Hubs slipping maybe?

 

[paulswagelock]

Speed measured by the GPS, not speedometer, correct?
Wind and current the same as the last time? Same crew, fuel amount, water amount, waste tank amount as last time?
2 MPH can be from many small factors as it is not much difference and there are lots of variables.

 

[FastMarkA]

Good point on the trim that I foolishly omitted. Drives all the way down/bow up tabs I can sometimes hit the limiter, although speed is more like 48 MPH. When I trim up, then the limiter hits for sure.

Props are perfect. (No visible damage, and they haven't contacted anything since I've had the boat.)

Speed measured by the Vessel View, which was my benchmark before. Next time I go out I'll fire up the GPS and compare the two.

When I did the 50.6 run this year, I had empty water, empty holding, and 3/4 fuel...most of the same gear.

I'll play with the trim a bit more, including the tabs. Now that I think about it, previously when I'd hit my 52/53, it was knee-jerk to lift the drives. But now, if I'm getting a little more power, I can probably leave them down as the force should lift the boat out of the water more, even if it takes a few extra seconds.

All this being nothing more than a "mine is bigger than yours" curiosity, right?

 

[JimT]

Is there a difference in the water depth where you are running this year compared to last year? Shallower water will slow you down.

 

[FastMarkA]

Is there a difference in the water depth where you are running this year compared to last year? Shallower water will slow you down.

I have to be honest; when I read this, I thought it was akin to the car dealership line, "your blinker fluid is low." But then I Googled and voila...some new knowledge for me!

I'm running in about 18'-20' of water, so I feel that should be deep enough...but maybe I'll go a little further out and see if that makes any difference.