Dashboard Tachometer...how accurate??

[paulgp6022]

On my boat (see signature) , how accurate is the factory tach? Should I have a different tach for seeing my true RPM's for prop selection? I currently am getting 4900 RPM's on my 2003 Mercury 75hp. Manual says 4750-5250 rpms is the range. Does this sound like I need a lower pitch prop? I currently have a 12x21 on it as reccommended by the dealer. I am thinking I could go to a 12x20. What do you guys think? Thanks for any info.

 

[Spidybot]

Re: Dashboard Tachometer...how accurate??

Tachs are generally accurate.<br /><br />Think you're pretty close to the ideal pitch. One inch down will put you at 5100 rpm with the same top speed but better hole shot, roughly.<br /><br />Adjusting x-dimension and trim may help but the planned use of the boat should be considered too.

 

[paulgp6022]

Re: Dashboard Tachometer...how accurate??

Thanks for the reply. I am new to boating, what is "x-dimension"? Thanks again.

 

[Spidybot]

Re: Dashboard Tachometer...how accurate??

X-dimension is an expression for the motor mount height on the transom. Higher mount means less mass in the water and less drag. <br /><br />Read this post for more

 

[paulgp6022]

Re: Dashboard Tachometer...how accurate??

I see now. My motor is mounted on the top holes, so there is room to go higher.

 

[Spidybot]

Re: Dashboard Tachometer...how accurate??

You could work on these details. As stated I think you're pretty close to the ideal for a general purpose setup and although you might gain something, there is a limit to it. <br /><br />Getting serious about it also calls for some of the other devices mentioned in the other post.

 

[paulgp6022]

Re: Dashboard Tachometer...how accurate??

I guess I am concerned about "lugging" my new motor. I don't want to mess it up.

 

[paulgp6022]

Re: Dashboard Tachometer...how accurate??

ok

 

[Spidybot]

Re: Dashboard Tachometer...how accurate??

At 4900 rpm wot you're not lugging. <br /><br />Again, it sounds OK to all recommendations.

 

[paulgp6022]

Re: Dashboard Tachometer...how accurate??

Thanks for the information!