www.iFourWinns.com

I was able to hit 52 - 53 mph GPS indicated last season. This season, the most I have seen is 44 mph. What gives? Motor still spins to the same RPM, props are the same.

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2011 SL242 5.7 VP GXi - 320 HP Dual Prop

That is basically impossible.
If you have the same prop (traveling the same distance per revolution).
If you have the same amount of rpms (same amount of revolutions of prop).
Then the speed has to be the same.

If your boat was heavier - you would run less rpms.
If your engine was not producing the correct power - you would run less rpms.
If your prop was smaller - you would run more rpms.

Now - if your prop was smaller and your engine is not producing the correct power - then you could get the same rpms, smaller prop, lower speed.
If your boat was heavier and your prop was smaller - then same rpms, smaller prop, lower speed.

Bottom line, something has to change. It is just a simple gear calculation.
You can even computer your theoretical speed using the formula here:
http://www.csgnetwork.com/marinepropcalc.html
Prop slip - use 0.1 for a 10% efficient prop which is about what a duoprop will provide.
Gear ratio - 2:1 I assume.

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The Dance II
2007 278 Vista
6.2L Mercruiser

Not sure if you leave your boat in the water for the season, but if you do, any chance that your boat's bottom is fouled up more this year as compared to last year? That algae crap does not take long to grow once it starts going and can quite negatively effect boat speed.

One other thought, I'd check to make sure your power trim on your outdrive is working since you'll really need to trim that drive out once on plane to get your maximum speed.

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Current Boat
- 2001 Four Winns 328 Vista "Hour Decision"
- 2010 310 Mercury Inflatable

Previous Boats
- 30' Doral Prestancia
- 25' Doral Citation

Slim chance as it isn't over revving, but may have prop bushing.....I am thinking i remember your boat is inside or in a raised slip on the new dock you built at the new house.

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http://i1339.photobucket.com/albums/o71 ... wqm355.jpg


08' H210SS
VP 5.7Gi

Slipping hub?

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Mike
2005 Four Winns 230/240
VP 5.7GXi/DP
1998 F-150 XLT
Boat Pic

A slipping hub / prop bushing would cause an increase in rpms at wide open throttle.
Trim down too far and you would see a decrease in rpms at wide open throttle.

He stated no change in rpms or prop size. Yet slower speed. Again, that is basically impossible.
One or the other has to change to change speed.

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The Dance II
2007 278 Vista
6.2L Mercruiser

The boat has sat in the water for 8 weeks, something I have never done before, so maybe its algae. Its coming out on Saturday, so I'll update the thread after

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2011 SL242 5.7 VP GXi - 320 HP Dual Prop

I'll still bet algae is very much a significant contributing factor in the speed reduction based on your latest posting above.

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Current Boat
- 2001 Four Winns 328 Vista "Hour Decision"
- 2010 310 Mercury Inflatable

Previous Boats
- 30' Doral Prestancia
- 25' Doral Citation

In theory it's possible for a slipping hub to yield "normal" engine RPM's with lower prop RPM's. Unlikely perhaps, but I'm not ready to say his claims are impossible without considering the possibility.

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Mike
2005 Four Winns 230/240
VP 5.7GXi/DP
1998 F-150 XLT
Boat Pic

I agree that in theory you could have a slipping hub and have the same rpms. If the hub is only slipping a little.
8-9 mph difference means the hub would be spinning like crazy.

Also Algae can cause an issue but 8-9 mph difference when trimmed up would be more like barnacles hanging off the hull and again, you would see a reduction in rpms at that much speed difference.

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The Dance II
2007 278 Vista
6.2L Mercruiser

It is also theoretically possible that a bent prop could yield lower speed at the same RPM... if the bent prop resulted in both a lower pitch and a higher drag due to the bends.

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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings

So I pulled the boat this weekend, so much growth on the bottom. Some of it wiped off easily, some of it not so easily. Stuff had even started growing on the props , even though the boat was used every weekend for tubing and skiing

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2011 SL242 5.7 VP GXi - 320 HP Dual Prop