Is it possible? Stumped on this one.

Wildboating

So we finally had a chance between life plans, break downs and so on to take out boat for a small trip last night.  I had a load of half fuel, full water, 6 people and normal overnight gear on board.  We pulled out of the marina and took off east running between 3800-4000 rpm and cruising at 32mph for about 30minutes when the speed started dropping off mph by mph until I reached 17mph.  At this point the RPMs had not changed, the engine had no alarms there were no odd sounds and so forth.  I throttled back and brought the boat to a stop, opened hatch, all looked, smelled and sounded good.  All gauges were also at normal ranges.  There was no wind and hardly in current or waves in the lake.  After about 20 minutes of checking around, I throttled back up and got back to full 32 mph before pulling into a local marina for dinner.  The trip home at times seem like it was also trying to fall of at times however a storm started heading in, waves and wind were way up so the drive was completely different.  So back to my original question?  Is it possible for something to have happened that would allow rpm's to stay the same yet the boat speed cut in half??

aero3113

It almost sounds like your prop hub is going bad but that would give you a rise in RPM's.

Wildboating

Boat has Bravo 3's , they don't have hubs I don't believe.  Thanks for the suggestion though.

Dream_Inn

Hmmm, I'm betting it was nothing.  I say that because I've seen at times I'll slow down just due to a change in current and wind and just have to bump it up a bit.  Then at times it goes the other way, when things get real smooth and no wind, it'll speed up.  If all was normal getting back on plane, that's my opinion.

Michael T

I'm with DI. Any mechanical feature I  can think of would alter the rpms.  Of course the other thing I can think of was some debris was entering your pitot tube and you "thought" you were slowing gown but your mph were just dropping. That might explain the dropping mph with a steady rpm.That has happened to me a number of times over the years. Do you know for sure your speed had actually gone down or did it just "feel" like it?

bella-vita

When u say boat dropped to 17 mph where u still on plane because on my 342 if I go slower than 25mph I start to lose plane 

Weber

When you dropped to 17 I'm sure you weren't planing anymore, was your throttle at full and what rpm?

LaRea

The '03 and later 342 models will stay on plane down to about 15 mph or even 13 depending on load.  (Rinker changed the hull shape from '02 to '03.)  But both engines have to be healthy, and you need to adjust the trim tabs all the way down and the drives all the way down. 

If you just reduce power, without adjusting tabs and drive position, yes it would come off plane at 17 mph.

Black_Diamond

The transom change was in '04. My '03 has the deeper set outdrives. Seeing the difference I can believe the lower planing speed of the newer 342's. Let alone the added engine compartment room. 

Wildboating

The drop in speed was observed via the gps unit, the factory gps, lowerance 2400 i believe, if you hit exit button twice real quick the display switches to a digital speedometer.  

We definitely dropped to 17, we were starting to fall off plane as the rear was settling in.   I am taking a 60 mi one way trip this weekend, I will closely observe and try to capture any other information if it happens again.  The only thing i noticed was my gps was reading high 11V low 12's when this happened.  Could this have been the result of something going awry with my voltage?  We have been noticing lower than normal operating voltages.  Any ideas where to start diagnosing this?  Now I think of it, I believe we ran the generator the rest of the trip home.

Black_Diamond

Low voltage will cause all kinds of problems with electronics, including engine performance. Start at your batteries: connections, ground and load test the batteries too. 

Wildboating

I am working on this today Alswagg,  Thanks for the help with it.

ruwallydog

Wow...those lower planing speeds would be great. My lowest is approx 25-28 mph depending on conditions.