prop change vs. trim tab installation

[OceanVU]

New to me 19'Grady w/1985 175hp black max merc. Currently has a 3-blade michigan wheel 13.75X21 prop. With normal load, boat goes about 50mph with only 4600rpm indicated on tach. Boat is a little too fast at minimum planing speed, and I would like to improve rough water ride with family on-board (boat seems over-propped for family type use). <br /><br />My questions: Will trim tabs or prop change give most benefit? If I install trim tabs, how will it effect prop selection? Specific prop selection advice for family type use?

 

[walleyehed]

Re: prop change vs. trim tab installation

Trim tabs (smart-tabs) would be the best "addition" you could make, however, with a 21P at 4600, the max speed would be around 43-44 at best.<br />Depending on how you are measuring speed, I would disregard that for the time being and verify the tach..that's the gauge that tells if we're running the right prop and where we need to go.<br />The speedo or tach-one of the 2, is way off, and I would highly recommend we find out the tach accuracy first.

 

[Texasmark]

Re: prop change vs. trim tab installation

After you answer Kenny's question my comments here are that your problem of stability and planing out at a lower speed will be solved by tabs. <br /><br />The planing speed of the boat is determined by load, distribution, hull design, and propulsion thrust angle. (don't think I missed anything).<br /><br />Going to a lower pitched prop would just help you to maintain a certain speed at the planing point under varying conditions.<br /><br />Like a high pitch (prop) would load up and the boat would fall back off plane, and you would advance the throttle to get back on top and then she would runaway, then you cut back cause you are going too fast and the cycle repeats it'self.<br /><br />Example: Like on a heavy assed deep V you might have to get to 20 to get the butt outta da watta.<br /><br />But on a John boat (nice flat bottom, light boat) with a light load, it could plane out at 15.<br /><br />So the tabs would help to offer more surface area of boat bottom and being they are so far aft, provide a lot of lift for a relatively small area.<br /><br />My 2c<br /><br />Mark