John B

I installed sportmaster lowers on an XZ upper on 36 Skater with stock 500 EFI's. Since the X was already +2 inches, the standard length units were used. We cut 2 inches off the long skegs before we tested. Well, we lost about 3 mph with the same loads, and lost some water pressure.

After some research, we found the stock Bravo lower was much more tapered and flow efficient. So we made the skeg on the sportmaster just like the stock unit and gained 2- 3 mph. The water pickup slots were reshaped so the water could get into the drive and went from 10 psi to 20 plus at high speeds.

Seems as though Mercury has two types of water slots in these sportmasters, long and short. The flush units they sell only work on the short slots. Mine were the long ones which did not work worth a s.....

Has anyone tried this? Does anyone know about why they have these different pickup slots?

After some research, we

mmwalters

John B OK I am heading out to my boat with a saw right now




I know you must have some before and after pictures. So let's see them

KAAMA

iTrader: (1)

Wow---that's interesting---nice going on the test! I know this may be a stretch, but would you happen to know what kind of water slots are on the XR drive and/or do they vary with those as well? I take it that you are speaking of the low water pick-up design that is in/at the tip/front of the bullet/casting---right? Thanks.

sterndrivespec

i think that you would have seen a gain in speed if you had gone with the 2" shorter gearcases.sportmasters were ment to run at the surface,and might slow you down if they are two deep in the water.we are going to do a test on my 37 v-bottom.i will post the results,chris

BenPerfected

John,
Any idea how much of the + 2-3 MPH is due to the change to the water pick-up slots vs. the reshaping the skeg? Are you back to the MPH you had with the stock (no nose cone?) Bravo lower case? It would be interesting if you could try the IMCO lower case for a comparison...it's just money. You might jump to 116 MPH!
Sprague

John B

Sterndrive,
The prop shafts were and are now 2 inches above the running bottom surface of the hull. Going up 4 inches ( another 2") is not possible. It's interesting that the hydrodynamics of the sportmaster are not good.
KAAMA and Sprague,
Both the slot and smoothing were done at the same time. I don't think the slot work had much to do with the speed change unless they caused some turbulence. The slip factor on the props was about 1% higher after the changes. We may have too much water pressure now (one leg had 30 psi), some may be overflowing the holes causing turbulence and prop slip. Obviously the original stock units had just the HD type low p/u slots, they were not the XR lowers.

I will attach some pictures later.

Sonny Spectre 30

I put 2" shorter sportmasters on my 30' Spectre Cat and picked up 10 MPH over the ZX Lowers with nose cones. I liked the idea that I still had the same amount or a little more sked in the water and feel comfortable about the boats control at all speeds and water conditions.

I would be curious to know how much thinning the leading edge of the skeg some would improve top speed. I am getting about 15-18 lbs of water pressure at high speeds. With the stock units I was running 35+ pounds. I also notice a difference in my stock vs lab props in water pressure. My lab props are set up for more stearn lift and result in less water pressure.

More comments on shaping the skeg, please!

John B

Picture side view after reworking

John B

sportmaster Front view reworked

Note leading edge compared to stock. Slots were deepened to increase water pressure from 8-12 lbs, it may to high, max now is near 30 at 110 mph.

John B

Picture of the boat, with cut down cavitation plate. Gives a rooster tail like a 6 drive.