Less mph same rpm

[92mariner]

Took the boat down river on Saturday 4000 rpm = 30 mph, me the wife and kids on board. Yesterday take her down river 4000 rpm 24 mph, a noticible difference in speed. Same people on board. I don't know where to start looking for the problem? Prop looks okay nothing wrapped around it. Should I start by looking at the lower gear unit or could this be caused by loss of spark or compression? Any thoughts.
Thanks
Billy

 

[timmathis]

Re: Less mph same rpm

It could be the prop starting to spin.
Do you know how to mark the prop and hub to see if it is spinning?
Tim

 

[JB]

Re: Less mph same rpm

How did you have the weight distributed, 92mariner?

Do you have power T&T? Did you trim it out exactly the same amount? A difference in trim can make a huge difference.

If weight distribution and trim were the same and you were measuring water speed (not GPS speed) you may have a fouled prop that is cavitating or a slipping prop, as Tim suggested.

If you were measuring speed with GPS it could have simply been a difference in the current.

 

[bottomfeeler]

Re: Less mph same rpm

Took the boat down river on Saturday 4000 rpm = 30 mph, me the wife and kids on board. Yesterday take her down river 4000 rpm 24 mph, a noticible difference in speed. Same people on board. I don't know where to start looking for the problem? Prop looks okay nothing wrapped around it. Should I start by looking at the lower gear unit or could this be caused by loss of spark or compression? Any thoughts.
Thanks
Billy

 

[bottomfeeler]

Re: Less mph same rpm

Just wondering if it could be the obvious. Is your trim setting the same? I don't know if you have power or manual trim control. This will sure affect speed.

 

[jtexas]

Re: Less mph same rpm

This might be even more obvious: weight gain. Not your family, the boat.

Is this a trailered boat that you left at the dock Saturday night? Might have a leak you don't know about? Yet?

Seems almost certain to not have anything to do with the engine...and not the gearcase either if you didn't hear any racket coming from there. That leaves just the prop and the hull.

 

[Bob_VT]

Re: Less mph same rpm

Ambiant temperature. Was it different?

The hotter the day the worse it will perform.
The cooler the day the better.

 

[Texasmark]

Re: Less mph same rpm

All good answers guys. But I'm throwing my hat into the ring for bad trim position. Nothing else changed that could affect the boat that much. A leak might but several things we don't know concerning that and if it leaked he probably already knew it. On temp, it affects mine too, but not that drastic a change especially so soon.

Mark

 

[Motor Boater Bill]

Re: Less mph same rpm

Was everyone sitting in the same places? Distribution can make a difference. Not knowing what the boat and motor are, 4000 seems a bit low for WOT, so if the motor is not at optimum RPM, then how the load is distributed front to back could be significant. Just a thought. I was watching my buddy wallow upriver the other day, and when I suggested he have his fishing partner move from the front seat to the middle, it made a big difference.

 

[mikael.block]

Re: Less mph same rpm

hi ..

how about fuel ?..

20 gl or more can make a different...

mikael

 

[Texasmark]

Re: Less mph same rpm

Thinking some more about this, I am still learning my boat; I do a lot of cause and effect type things.

So the other day I was blasting away and deliberately stuck the nose in the water. As I recall I dropped 12mph but it had minimal effect on my rpms. I guess my slip increased with the added drag of the nose down attitude is why my rpm's stayed up. Trimmed her back out and got my mph back.

I know for a fact that my prop will slip with added drag as it's in the 10% range when "hooked up" and anything will eat away at that; especially more hull drag.

So let's stay focused on too much hull in the water and what caused it. Could be any of the above, but trim is the quickest to prove and very logical to explain.

HTH,

Mark

 

[jtexas]

Re: Less mph same rpm

Snagged a trotline?
Dinged the prop on Saturday?
Tachometer or speedometer malfunction?

But seriously, Is this a flood control river? Could have been a change in current velocity from one day to the next.

did we lose the thread originator?