Engine height and Mercury High Performance
 

Hi!

According to Merc Hi Performance they reccommend the following engine height with a Sportmaster lower unit and twin engines:

70 mph speed = 27,3 inch (7,3 inch increase) height
80 mph speed = 27,7 inch (7,7 inch increase) height
90 mph speed = 28 inch (8 inch increase) height

With the Torquemaster lower unit and twin engines, they reccommend:

70 mph speed = 25,4 inch (5,4 inch increase) height
80 mph speed = 25,7 inch (5,7 inch increase) height
90 mph speed = 26 inch (6 inch increase) height

I know that the Torquemaster does not have the low water pick up, and is not made for 80 mph plus speeds. Sportmaster lower unit is made for surfacing props and speeds in excess of 85 mph.

But why is there a difference in 2 inch height at the same speeds (70 and 80 mph)? Is this due to the lower unit design?

I know the height depend on the boat design and props used.

The reason for asking; I am now rigging a Phantom 25 with twim Merc 2.5 EFI SS engines capable of 90 mph + speed. So I have to use a Sportmaster lower unit due to the speed.

Cheeeers, Toffen G

Re: Engine height and Mercury High Performance
 

I think this is really a riggers questions.

Try posting this in the General Tech sections.

Julie

Re: Engine height and Mercury High Performance
 

The dynamics of the cases are completely different.

When I was still at Mercury, I had numerous TM cases run over 90 mph. Point being, TM gear cases may or may not blow out in excess of 85 mph.

The SM case is really designed to run at ultra elevated heights. That is the reason for the differential in #'s. The case is longer and can generate higher speeds if it isn't creating additional drag from its size.

You do not have to run SM because you are running 90mph. The TM will actually be faster than the SM if the boat can't fly the bow on its own. The question is how far can you jack up the motors while maintaining water pressure and boat attitude.

Re: Engine height and Mercury High Performance
 

Hi!

Thanks for answer!

My current propheight with my TM case is 2 inches below the bottom with my PM 300 and 7,5 inches setback. I have blocked the two upper waterinlets on each side, that allow me to run higher. Any higher than 2 inches, and I loose waterpressure.

I am not sure how fast the boat will run with twin Merc 2.5 EFI's. With one PM 300, the top speed is 76-77 mph. Other similar boats with twin 2.5's in UK run in excess of 90 mph using SM lowers.

The propheight using 15 inch mids will be approx. 1,5 - 2 inches below the bottom. The engines are delivered with SM lowers, so I will check out how they fit the setup - and take it from there. The boat runs flat and require bowlift with one engine. I presume two engines will change this somewhat, making it more sensitive to trim.

Cheeeers!

Toffen G :drink:

Re: Engine height and Mercury High Performance
 

Agreed,

Lift them to the moon and see how the bow carries. The higher you can go with the SM cases the better!