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| 5.0 gxi 270hp bogs real bad on take off |
| Author |
Message |
    
brian stacy
New member
Username: donkey413
Post Number: 1
Registered: 06-2009
| | Posted on Wednesday, June 24, 2009 - 03:37 pm: | 
|
I have an 2004 crownline 216 ls open bow boat that i just bought. it has a duo prop volvo outdrive. it runs about 46 mph top end with a full tank and 2 people. however the more people in boat it doesnt seem to affect the top speed. it only turns about 4500 rpm and seems to run perfrct until you go to take off. it hits 2500 rpm and just sticks at that rpm until boat levels out. the thing that gets me is at half throttle it runs up to 2500 and stays there like a rev limiter or something. and if you go ahead and give it full throttle it doesnt make any difference it seems like it has a limiter on it. the rpm just stay the same until it levels out. any input would be great i just feel like it something stupid causing this. thanks for looking |
Robert A. Fierro
Advanced Member
Username: sandkicker
Post Number: 626
Registered: 06-2008
| | Posted on Thursday, June 25, 2009 - 11:02 am: |
|
I am assuming that once it levels out the RPMs climb to "normal"???
IFSO... This is normal. A boat is very different from a car. The HP that it takes to move a boat hull depends on LOTS of things, among them how much of the hull is out of the water and the "attitude" of the boat...i.e. level or "nose up".
The RPMS that an engine achieves is dependant on the load on the prop and the torque curve of the engine. As the hull climbs over its own bow wave (and then levels out), less of the boat is "in the water" and less oomph is required to push the boat and the load on the prop will drop and the RPMs will increase. |
brian stacy
New member
Username: donkey413
Post Number: 2
Registered: 06-2009
| | Posted on Thursday, June 25, 2009 - 11:48 am: |
|
it does level out but it just seems funny.you would think from half throttle to wide open it shoul at least very 2 or 300 rpms and it doesnt. thanks for you your input though i might just be wigging out on the deal! |
Robert A. Fierro
Advanced Member
Username: sandkicker
Post Number: 632
Registered: 06-2008
| | Posted on Thursday, June 25, 2009 - 04:15 pm: |
|
There was a period of time when I had the wrong prop on my boat... it was overpropped. I reached MAX RPMS at about 75% full throttle (even on a plane) and no more RPMS from there to wide open.
What most people don't understand is the physics behind an gas engine's "throttle". The word "throttle" literally means to "Choke"... if you "throttle" someone, you grab them by the neck and squeeze so they can't breath.
This is exactly what the throttle on an engine does, it restricts the engine's ability to breath and THEREFORE, it's POWER....not it's RPM.
This is why when an engine is in neutral you can get max RPMS with only a very small amount of throttle opening...in neutral, an engine does not need much "power" to run at max rpms...
Now... put the engine in gear with a load. That lever in your hand is a power control lever, not an RPM control lever. It takes torque to spin a prop (BTW, HP is proportional to torque X RPMs)
and the curve of torque vs RPM for a gas engine is not a straight line... it is a curve with a peak. Sooooo when you open the throttle all the way, the engine will only put out as much torque as it can, and if the load on prop is such (due to the angle of the hull and how much is in the water) that the engine will only turn 2500 RPMs at maximum torque, then that's all the RPMS you are going to get. As the drag on the hull reduces, the load on the prop drops and with maximum power output, the RPMS out of the engine will rise.
How fast would you run if you had a monkey on your back who was squeezing your throat? and you were running up hill?... wouldn't you go faster if the hill was less steep? and/or the monkey stopped squeezing so hard?
What I forgot to ask and this is important... with a "full load" on board, how long does it take to get to max RPMs??? |
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