Am I under proped?

[GeoRXT]

The boat is an '88 Angler 19' with a '96 Mariner 125hp. Boat is about 1,600 pounds. Engine is in great condition between 143-150psi compression across the board.

Took it out yesterday for the first time and I saw that the max rpm was way over the limiter. Limiter should be around 5250 and was at 7000+. Of course I did not run it at this. But, I needed some numbers so I ran it at 6,000rpm and it yielded 25mph. Now, it was 2 guys and 2 girls on the boat and it had 3/4 tank of fuel which should be around 27 gallons.

The prop is a Solas D3X13X19R (M) If that helps.

 

[Dhadley]

Re: Am I under proped?

The numbers aren't adding up right. Are you sure you were only going 25 mph?

 

[GeoRXT]

Re: Am I under proped?

The numbers aren't adding up right. Are you sure you were only going 25 mph?

Well the gps read between 21.8-22.2 knots...

 

[coolguy147]

Re: Am I under proped?

dont knwo hwo to ready knots in mph wise but i think 20 knots is fast isnt it?

 

[GeoRXT]

Re: Am I under proped?

I just used on of those online conversion calculators!

http://www.onlineconversion.com/speed_common.htm

And compared to my near 80mph jet ski, it's turtle slow...

 

[coolguy147]

Re: Am I under proped?

25mph at 7k rpm lol? somethign aint adden up really

 

[Dhadley]

Re: Am I under proped?

Yep, that's about 25 1/2 mph. Are you sure the tach is right?

 

[Campylobacter]

Re: Am I under proped?

Yep, that's about 25 1/2 mph. Are you sure the tach is right?

seconded, check the tach. Something isn't right.

 

[mthieme]

Re: Am I under proped?

Seems to me with what you have you should be going twice as fast.
TO get the rpms down and the speed up from you're at, you'd need like at least a 30P prop which of course isn't right.
Got barnacles?

 

[daman4469]

Re: Am I under proped?

Geo, How fast are you going when you max out the engine? If you are still going slow (under 40 MPH i would say) at 7 grand, then you may be under propped. Is this a new boat to you, or did this just start? It also may be a trim issue.

 

[GeoRXT]

Re: Am I under proped?

Geo, How fast are you going when you max out the engine? If you are still going slow (under 40 MPH i would say) at 7 grand, then you may be under propped. Is this a new boat to you, or did this just start? It also may be a trim issue.

Well, at 7k it was doing around 27.8 knots. Definately less than 40mph. I just bought the boat 2 weeks ago...The motor was trimmed all the way down because it is not working at the moment.

http://forums.iboats.com/showthread.php?t=294135

 

[daman4469]

Re: Am I under proped?

It sounds like it could be an under prop issue. But it could be many things. However, a 125 on a 16 1/2 footer should really push you. Does it cavitate (spin without pushing) when you accelerate/top out? If so, then you are under propped. I suggest taking your prop off and measuring it for its pitch, unless its stamped on your prop. The last number you said there tells me its a 19" pitch. On an engine like that, you should be running a 22 or so.

http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurement

This is a great resource for measuring your prop. Its not as tricky as it looks. I suggest trying this. If you are running a 14 prop or something like that, then its definitely your issue. That is, of course, unless it is stamped on the prop.

 

[GeoRXT]

Re: Am I under proped?

It sounds like it could be an under prop issue. But it could be many things. However, a 125 on a 16 1/2 footer should really push you. Does it cavitate (spin without pushing) when you accelerate/top out? If so, then you are under propped. I suggest taking your prop off and measuring it for its pitch. Sounds like you should be running a 21 or so.

http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurement

This is a great resource for measuring your prop. Its not as tricky as it looks. I suggest trying this. If you are running a 14 prop or something like that, then its definitely your issue.

It's a 19 footer actually...

And the prop is a Solas D3X13X19R

It does cavitate a bit on take off...

 

[daman4469]

Re: Am I under proped?

Is that number stamped on the prop? Youre sure thats what it is? If its cavitating on take off, check the height of your cavitation plate. The plate, at full trim (all the way down) should be a couple inches above the beam on the back. Of course, its going to go at an angle, but estimate. Is it like 3 or 4 inches? or 5 inches below or above? as long as its a little above the beam youre alright....

Props are tricky....too high of a pitch and the prop will try to push the boat too far each turn, and cavitate....
Too low and you will have too much "torque" and it will spin.

If it only cavitates a little on takeoff and runs fine when it planes out, I doubt its an underprop issue. Mine will cavitate a little too on takeoff. I tihnk any boat will if you gun it too hard on the start.

 

[GeoRXT]

Re: Am I under proped?

Well I dont really gun it off the start because the engine mounts need replacing.
The number IS stamped on the prop.
As for the cavitation plate I would have to check. What beam on the back though? I dont get that? The bottom of the boat?

And I know about tricky props! On my jet ski I would have to either have the prop bent or replaced after every little mod I did to it. Even on the same setup I gained 2+mph with a different prop and can get another 1.5-2.5 mph with fine tunning...

 

[daman4469]

Re: Am I under proped?

Well I dont really gun it off the start because the engine mounts need replacing.
The number IS stamped on the prop.
As for the cavitation plate I would have to check. What beam on the back though? I dont get that? The bottom of the boat?

And I know about tricky props! On my jet ski I would have to either have the prop bent or replaced after every little mod I did to it. Even on the same setup I gained 2+mph with a different prop and can get another 1.5-2.5 mph with fine tunning...

Yeah, the lowest point on the back of the boat. Usually near where your drain plug is. Comes to a point. Thats the bow...I'd check that. When I bought my boat it cavitated big time because the plate was 4 inches below the beam. Definitely can be a big deal...

 

[GeoRXT]

Re: Am I under proped?

I will check tomorrow and post back. The boat is a flats boat too, I dont know if that matters as far as speed. Water was fairly calm at about 2'.

But going back to the rpm issue, what should be my next step?

 

[daman4469]

Re: Am I under proped?

I will check tomorrow and post back. The boat is a flats boat too, I dont know if that matters as far as speed. Water was fairly calm at about 2'.

But going back to the rpm issue, what should be my next step?

Well, assuming everything checks out ok, that would bring us to the fact that you are running higher RPMs than you want to. Double check your guage. If it is consistent, then I suggest this (Be sure everything is what you are saying, cause props are lots of $$$ !!)

To drop your RPMs, raise your prop #. Just an fyi....a 19" pitch means that, theoretically, for every turn of the prop, your boat will move 19" forward. Raising that number to say, a 21" would make the prop spin a lower number of times to get you the same distance. In short, 1" increase in prop pitch will lower your RPMs about 200 or 300 (usually a bit more). So if you want to be running around 5500 RPMs, you would probably want around a 23" prop, based on what I'm told. And go stainless. Aluminum props are cheap and replaceable, but you will def suffer some performance. I'd check with a local boat shop first; they know motors in and out and may bring up something I am missing. Plus they can recommend props and such.

And although your boat is a flat bottom, I'm sure there is a "point" in the back. Double check that cavitation plate height. And make sure your tach is right. Let your engine idle and see what it reads. It should probably read in the 1000 or 1500 range. If its any higher, your guage is wrong. Or youre idling high

 

[GeoRXT]

Re: Am I under proped?

Well, assuming everything checks out ok, that would bring us to the fact that you are running higher RPMs than you want to. Double check your guage. If it is consistent, then I suggest this (Be sure everything is what you are saying, cause props are lots of $$$ !!)

To drop your RPMs, raise your prop #. Just an fyi....a 19" pitch means that, theoretically, for every turn of the prop, your boat will move 19" forward. Raising that number to say, a 21" would make the prop spin a lower number of times to get you the same distance. In short, 1" increase in prop pitch will lower your RPMs about 200 or 300 (usually a bit more). So if you want to be running around 5500 RPMs, you would probably want around a 23" prop, based on what I'm told. And go stainless. Aluminum props are cheap and replaceable, but you will def suffer some performance. I'd check with a local boat shop first; they know motors in and out and may bring up something I am missing. Plus they can recommend props and such.

And although your boat is a flat bottom, I'm sure there is a "point" in the back. Double check that cavitation plate height. And make sure your tach is right. Let your engine idle and see what it reads. It should probably read in the 1000 or 1500 range. If its any higher, your guage is wrong. Or youre idling high

Oh, forgot that fact. It idles at 4K...I surely thought that was WEIRD! That's on neutral...

 

[daman4469]

Re: Am I under proped?

Oh, forgot that fact. It idles at 4K...I surely thought that was WEIRD! That's on neutral...

Does it sound like its idling at 4 grand? or can you tell the guage is wrong? if it isnt idling that high, then you arent running as high of rpms as you think. Youre running about 3000 high. So that 7500 is really 4500. THAT makes sense. Is that 7500 maxed out? Or do you have more throttle? Im guessing you have more throttle....