Props and propheight for twin outboard set up - Phantom 25?
 

Hi!

I have a 1985 model Phantom 25 offshore performance boat. It is currently rigged with a Promax 300 SVS. The boat runs 76-78 mph spinning a stock A45 Tempest 25 at 6.300 rpms. I use a CMC powerlift and 7,5 inches setback. Propheight is approx. 2 inches below at WOT.

The Phantom 25 model was normally produced as a race hull for the European offshore circuit. The manufacturer and designer – Steve Baker – retired some few years back. When raced the boat was rigged with twin Mercury XR2 and Mercury 2.4/2.5 performance engines.

Steve Baker rarely check his emails, but I have been told from him that my particular boat is built extra strong .. and heavy. The hull weights approx 2.500 pounds / 1.150 kilos incl. the Promax 300 SVS and driver. The boat is narrow – approx. 5,5 feet wide. But is extremely stable at speed. No chine walk at all.

Next year I will mount two Merc 2.5 EFI on my Phantom. The hull is still very strong, and both my dealer and I are confident that it will handle the power. I have been told by Steve Baker that the engines should be mounted 23 inches senterline to senterline. Then the engines can be turned all over to both sides without interferring. I will use Latham hydraulic steering with a tie bar.

The engines will be mounted directly on to the transom not using powerlifts / brackets due to the balance of the setup. This requires a correct mounting height of the engines to be able to get best performance (read best speed in this case). I know it will be a question between high speed performance - and cruise speed. Too high mounting height – the slip will affect the cruising speed.

Mercury Hi Performance reccommends engine mounting height is 8 (28 inches) inches if running a Sportmaster lower unit and in excess of 90 mph. Estimated top speed will be 90 -100 mph. This means a propheight approx. even with the bottom. It seems a little high for me. What do you think will be the best motorheight / propheight with this setup?

I plan to rotate the propellers inward to create bowlift. What do you think?

I know I have to test and test propellers. Would a Bravo 1 propeller in 28 or 30 pitch be a good starting point? Or maybe a 3 blade Tempest 29 (if it can be bought in left rotation). Or other suggestions.

Kind regards,

Toffen Gunnufsen
Lillesand
Norway :D