can i increase my wot

[basspirate]

i have a 2000 johnson 130 on a 20 ft robalo cc with a 15x17 prop. my engine will get up to to 4500 rpms and my speed is 20 mph. i'm looking to get a little more out of my engine. i've checked and there is no room left on the throttle arm to go further. my carbs at full throttle are wide open but can go further past wide open to starting to close in the opposite direction. i know my boat is a little underpowered but only 20mph seems really slow.

 

[ezeke]

Re: can i increase my wot

WOT is when the throttle plates are horizontal. Any further and they will begin to close in the wrong direction, reducing speed.

You may reach higher RPM by raising the motor higher on the transom but you must be careful that the motor is able to have adequate water for cooling.

At 20 MPH your current setup calculates 38% slip at 4500 RPM.

You should be propped to reach 5800 RPM. I would recommend that you try a prop with 13 pitch rather than your current 17 or, at most, no higher than 15 pitch. .

 

[kenmyfam]

Re: can i increase my wot

4,500 RPM at WOT seems a little low to me.
Set up is huge as stated already. If set up is at the optimum then dropping the prop pitch a couple of inches may well help out.

 

[jay_merrill]

Re: can i increase my wot

Yes, you can change it. As previously mentioned, you have too much pitch on your current prop.



???

 

[basspirate]

Re: can i increase my wot

i cant raise the engine any more. i'm on the bottom hole of the engine. my cavitation plate lines up with the bottom of my hull. does anyone have any idea of how much less prop slip a 15 pitch would give

 

[Bob_VT]

Re: can i increase my wot

How old is this condition? How long have you had the motor and been running it?

Before you spend any money on props you need to find out if it is a problem with the motor.

 

[basspirate]

Re: can i increase my wot

i got the boat at the end of last season. i've only used it a few times so far. the engine runs fine. it has 125 psi on all cylinders. it's firing on all 4 and i can see gas going through the carbs on all 4 cylinders. since this appears to be a prop question i'll go annoy the guys in the prop forum for a while.

 

[jay_merrill]

Re: can i increase my wot

As a general rule of thumb, you will gain 200 rpm for every inch of decrease in prop pitch. Conversely, you will lose 200 rpm for every inch that you increase pitch.

Since this is subject to a lot of variables, treat the general rule as just that - general. You will probably experience some variation of the 200rpm v. 1 inch, relationship.



???

 

[countvlad]

Re: can i increase my wot

i have a 2000 johnson 130 on a 20 ft robalo cc with a 15x17 prop. my engine will get up to to 4500 rpms and my speed is 20 mph. i'm looking to get a little more out of my engine. i've checked and there is no room left on the throttle arm to go further. my carbs at full throttle are wide open but can go further past wide open to starting to close in the opposite direction. i know my boat is a little underpowered but only 20mph seems really slow.

i believe that your boat weights a lot... thats probably one of the reasons why its so slow....

now, after you reached WOT with the motor down, did you try to trim the motor out to see if your RPMS would increase? if not... try that...
my boat is a 1976 renken cc that weights ith full gear,motor etc around 1900lbs with an 120 1989Johnson i do 40mph@4800 rpms motor down.... once i start trimming the motor out my rpms and speed increases bringing the motor in the recommended manuf rpm range... and btw i have a 13*17 pitch prop .. i believe your prop might be too big for that motor....