Prop diameters
 

Hi!

I have bought two Merc 2.5 EFI engines (SVS and PCU) with 15 inch midsections. The engines will be mounted 18,5-19 inches senter to senter.

I have been reccommended by some to mount the engines with the propshaft 1 inch below the bottom, and by others to mount them even - 1 inch above the bottom. Merc Hi Performance reccommends 0,25 below the bottom with the Sportmaster lower unit at 90 mph.

With the 15 inch midsection the propshaft will be approx. 0,5 inch below the bottom with the engines mounted all way down.

If it proves to be a little high (expecially at cruise), is it possible to compensate with larger diameter props?

I plan to use Bravo 1 props 28 inch for cruise and Chopper II 28 inch props for wot. I will also try Mercury ET props for wot.

Boat rigging starts in 4 weeks! Can't believe it soon spring again.

Enclosed a picture of a twin Merc 2.5 rigged Phantom 25 with 6 inch setback. I will only use 1 inch setback due to overall balance.

Cheeers, Toffen G :drink:

Re: Prop diameters
 

The only way to know what the best motor height is will be to test. Diameter may compensate for increased slip but only to a point. By increasing diameter it's possible to increase slip by increasing the stern lift. Likewise it's possible to to reduce slip by reducing diameter if the slip is being caused by excessive prop diameter.

There are no clear cut rules, just guidelines to follow. The most important thing is to provide the proper information to the person working on your props.

Matt

Re: Prop diameters
 

Hi!

Maybe Trottle Up or BBlades can help me here:

- How high pitch can Mirage props be delivered in counter rotation?

- How high pitch can Tempest props be delivered in counter rotation? If low pitches now, will the pitch be higher later?

- How high pitch can Raker props be delivered in counter rotation?

Thanks!

Cheeers, Toffen G

Re: Prop diameters
 

Hello Toffen,

First I would echo Matt's thoughts. Get a base line and we go from there. I would make sure you can go down at least to 1" below however, just in case. (ultimately, go as high as you can without too much slip.)

Second, left hand Mirage Plus to 29P. Left hand Tempest to 23P.

Take care.