Jack plate vs tilt n trim

[jimmbo]

Your engine has a WOT range of 4500 - 5500, and if it has the Turtleback(bubble) Exhaust Cover, it makes its best power close to the 5500 mark. If it has the Flat Exhaust Cover, it will make its best power around 5200 rpm. What exactly are you putting it on, and your intended usage. If the boat is a 16 or 17 footer a 17" is usually a good choice, a 15 ft can use a 19. Tops speed difference is usually very slight(within 1 mph), if both the props are with the WOT rpm range. Outside of it, either too high or too low, will show up a speed loss. A tach is really the only way to know. Depending on the rigging, you may be able to use a higher performance prop. These can run at higher transom heights, like those offered by a JP. For boat speeds below 45mph, the best engine height is having the Anti-ventilation Plate flush with the hull bottom. At speeds above 50mph, a small gain will occur by raising the engine higher(about 1 mph per inch higher), but unless a higher performance prop is used, you might experience a loss of speed from excessive slip.

 

[tramsdell]

Its an aluminum 18ft starcraft super fisher. 3 batts. 30ish gal fuel tank. Im thinking 17 would be a good place to start.

 

[ahicks]

I don't thing you're way off there. Agree that prop or a 19 will likely be the prop of choice.

 

[jimmbo]

A 17 would be a good starting point.
Regarding engine mounting height. Being an Aluminium boat, it probably has a keel running down the Centerline of the hull. Does it extend all the way back to the transom or, does it taper off prior to the Transom? If it does run all the way back to the Transom, it will increase the turbulence in front of the prop and will require the AV plate to be located below the bottom of the hull.

 

[tramsdell]

If i remember correctly the keel stops stortly after the bow bend. Theres two longitudinal ribs that run the rest of the length to stern.