rpm issue

[jfa473]

I have a 1996 Astro 18.5" with a mercury 150 efi that i just bought. The boat runs real well . Old tach was bad installed new quick silver tach. The problem i'm having is the boat only runs at 4900 rpm and gps at 56.9 mph. The boat is fast enough for me but the engine redline I belive is 5400 rpm. It doesn't matter if I have 2 people in the boat or 1 with a full tank of fuel or:% almost empty same mph and rpm. Motor trimmed up all the way watching rpm and gps to see if i'm losing mph but i'm not. And the boat diggs pretty deep on the hole shot and doesn't get on plane to fast. It has a trophy 25p prop

 

[Silvertip]

Re: rpm issue

Check engine mounting height and verify that the tach is accurate. If the engine is in proper tune and there is no marine growth on the hull, you might consider a prop with less pitch. Work with a local dealer. They often have a loaner program which allows you to try various types before buying. The numbers you provide show 8 to 11% slip which is pretty good. However my data shows your engine has a 1.87:1 gear ratio and a 5000 to 6000 RPM wide open throttle operating range. A 23P prop would put you at or near the middle of that range. You would likely pick up a little top end but more important would be a better hole shot. Food for thought but you might want to post this in the Prop forum. Walleyehed is the prop guru and would give you the best recommendation.