low rpms at full throttle

[turbocorrado]

1997 wellcraft eclipse. 20 ft bowrider with a 4.3 mercury. pretty sure its a 190hp with an alpha 1 gen 2. current prop is a 14.5x19, the engine is supposed to rev to 4400-4800, i took it out for the first time and at full throttle am only hitting 4000 rpms, playing with trim i was able to get my top speed to 37mph. im not sure if the prop on there is the correct prop or not. the PO had no idea what he was doing. should i try a 14.5x17?

 

[ihearth2o]

If you were running solo when you took your wot rpm reading, I would see if they make a 15p prop too.

 

[turbocorrado]

i had 4 people total in the boat when i did it. according to the factory brochure online the v8 came with a 13.75 x 21

 

[ihearth2o]

If that's your normal load, then I would reprop based on the rpms with that load. rule of thumb is 200 rpm change per 1" of pitch difference. Go down in pitch to increase rpm and vice versa. If you're looking to pull skiers, especially slalom, you'll want to stay at the upper end of your boat's recommended rpm range.

 

[QC]

I agree ^^^^ however... I would also verify the tach is right. Was speed by GPS? Top speed seems a little low as well, so I am wondering about performance period. Although a prop change may indeed be in order, everything, including the data, needs to be confirmed as correct or you may be masking something. Did she plane out OK? If so, then I always check tach. Last tuneup? Other stuff on board? Soaked foam? Prop condition? Stock gear ratio? All of this needs to be considered.

 

[turbocorrado]

I agree ^^^^ however... I would also verify the tach is right. Was speed by GPS? Top speed seems a little low as well, so I am wondering about performance period. Although a prop change may indeed be in order, everything, including the data, needs to be confirmed as correct or you may be masking something. Did she plane out OK? If so, then I always check tach. Last tuneup? Other stuff on board? Soaked foam? Prop condition? Stock gear ratio? All of this needs to be considered.

just bought the boat and did a TON of work to get it water ready, was not seaworthy when i got it. tach is correct, its brand new just installed, speed was gps confirmed. it was originally 32mph untill i trimmed it up a bit to reach 36-37mph. new cap and rotor, i still need to do plugs and wires. im assuming stock gear ratio, unsure of soaked foam, boat seems to float perfect and floor is solid, also didnt have issue getting on plane. i had to give it a good bit of throttle to get on plane but it did it quickly.

its my first real boat thats not a tiller, and was my first time operating a real boat as well, so ive got a huge learning curve to get over

 

[turbocorrado]

heres a pic of the prop, i want to replace it and the hub but i want to make sure i order the correct prop if i need to change pitch

 

[QC]

Sounds pretty good so far. I would do a compression check as well. The fact that you planed easily continues my tach concern. Does the tach have a switch for OB, 4, 6, 8 cyl? If so, and that is set right, then we'll drop the tach question. Checking on the gear ratio and then the results based on your numbers. Stay tuned.

 

[QC]

Based on my calcs, I am seeing 6% slip which is too low. Either the pitch, speed, gear ratio or RPM is wrong I think. I used a 1.81 gear ratio. If I use the 1.62 that's in the 205s the numbers are more believable. that's a 16% slip, and that prop is pretty beat up, so I kind of believe that. One way to chek the gear ratio is to pull the plugs, rotate the engine in gear and count how many times the prop turns. Also need to do for compression check. Two birds with one stone?

 

[Rick Stephens]

A 17in. is what I run on a 3.0 liter. I would also suspect your tach. I would think a 19in. would be about the perfect starting place for a wheel on a 4.3liter. Rick

 

[steelespike]

I too suspect the tach using a 1.81 gear ratio i calculate at 4000 rpm 37 mph = 7% slip.
​ Possible but highly unlikely I too wonder if we are using the right ratio though my info clearly indicates 1.81.
I think we need to prove the ratio.
I also would like to see a lightly loaded wot rpm and gps speed.It's my feeling 4 adults would run about mid 40s.
Lightly loaded pressing 50.

 

[500dollar744ti]

Have you done a WOT run with just you? That prop looks like it has spent some time in the mud, 19p is what I believe should be on there.

 

[turbocorrado]

im pretty sure the tach is correct, its set on 6cyl and is a brand new faria that i just installed. i will look into the ratio, is there any way to tell without turning the motor over, a serial number anywhere? i havent had a chance to run it with just me in it. i will attempt to do that, is it possible that 14.5 is to big in diameter and thats part of the issue?

 

[ihearth2o]

Check for a decal on the port side of the upper gear housing. First set of numbers will be the gear ratio followed by the serial #. I'm getting 8% slip using 1.81 ratio at 4000rpm/37mph. That would be a pretty high performing set up though. With that said, IMO your tach is not at issue. With 4 passengers at WOT, it is not off the recommended range by that much. If you were solo, I would suspect your tach would read within range. How much fuel were you on? What were wind/current conditions? Have you washed the hull?

 

[QC]

I'm confused ^^^^^ I agree that slip will come down with fewer people, but how would that validate a 6 - 8% slip with more people? Slip should go up with more load...

 

[ihearth2o]

I wasn't trying to establish a relationship between slip and boat load at all. In fact, as I understand it, load could affect rpm and top speed, two things needed to determine slip, so any slip change would depend on those deltas.

 

[turbocorrado]

Check for a decal on the port side of the upper gear housing. First set of numbers will be the gear ratio followed by the serial #. I'm getting 8% slip using 1.81 ratio at 4000rpm/37mph. That would be a pretty high performing set up though. With that said, IMO your tach is not at issue. With 4 passengers at WOT, it is not off the recommended range by that much. If you were solo, I would suspect your tach would read within range. How much fuel were you on? What were wind/current conditions? Have you washed the hull?

actually this reminded me the boat has a grippy bottom paint on it, thats probably not helping things. i had about 25-30 gallons of fuel, i believe its a 50 gal tank

 

[jestor68]

I don't know which slip calculator you guys are using; but the BAM calculator shows 10% with his numbers using an actual 36 mph.

The prop selector estimates the 19 pitch should do about 4600 rpm @ 42 mph using his boat's weight and as stated load, considering a like new motor making like new power. You semi tired motor can't do it anymore.

I suspect what we're dealing with here is a 17 year old motor that's been run hard and is now showing the result of that abuse.

Tune it up the best you can, then prop it to attain the proper rpm range.

 

[turbocorrado]

I don't know which slip calculator you guys are using; but the BAM calculator shows 10% with his numbers using an actual 36 mph.

The prop selector estimates the 19 pitch should do about 4600 rpm @ 42 mph using his boat's weight and as stated load, considering a like new motor making like new power. You semi tired motor can't do it anymore.

I suspect what we're dealing with here is a 17 year old motor that's been run hard and is now showing the result of that abuse.

Tune it up the best you can, then prop it to attain the proper rpm range.

motor was replaced in 2006, its possible it is worn out again. this fall il run some tests on it

 

[QC]

I don't know which slip calculator you guys are using; but the BAM calculator shows 10% with his numbers using an actual 36 mph.

Pretty simple actually. Use 37 MPH and you get 7%, and use 36 you get almost 10%. 1 MPH makes a big difference down in the 30s. You make a god point though. A tiny change to any of the variables and the results change.