Diameter props

Toffen · 26-12-2005, 04:43 PM

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.

Cheeers, Toffen G

sterling · 26-12-2005, 09:11 PM

Hi Toffen

Increasing your diameter will give you generally more overall lift.
The bigger props will bite better and therefore give you more bow lift and overall lift.
Cleavers will do the same on a cat ,the bigger diameter makes more stern lift,and pushes the hull higher out of the water(at the stern)and sometimes also overall lift.depending if your turn them inward or outward.
I turn them inward on my cat,wich is uncommon,most cats turn outward.
On my Bernico 21 V bottem with 2x 2,5 EFI,s I tuned them outward,because inward was so difficult to drive.(chine walking)
If your boat is not quite heavy,it don,t need so much bow lift with all that weight in the back.
I think you should try a set cleavers too,because they don,t lift the bow too much,and just pushes the boat forward,and your boat feels easyer to drive ,and it is far more neutral than a chopper.
But as always try before you buy.

cheers sterling

Maxim Gibson · 29-12-2005, 06:00 PM

Hi, still testing the new boat and wondering about props... the previous owner said it ran best on a Laser 23, we've tried one and still had no problems at all getting the revs out of it, would it be wise to try a Laser 25?

scottyboy · 29-12-2005, 07:46 PM

Quote:

Originally posted by Maxim Gibson
Hi, still testing the new boat and wondering about props... the previous owner said it ran best on a Laser 23, we've tried one and still had no problems at all getting the revs out of it, would it be wise to try a Laser 25?

Is this on the backdraft? Not sure if its like the manarfelt's but they are best with cleavers! Tryed one of them yet?

Maxim Gibson · 29-12-2005, 07:48 PM

No not yet, what kinda pitch?

scottyboy · 29-12-2005, 07:54 PM

Think mine are 26" through hub cleavers ! Not sure on diameter but think there about 14"? Are you running a jack plate? Any set back?

Maxim Gibson · 29-12-2005, 07:55 PM

Cheers, not running any kind of set back at the moment but did post a thread on here asking about whether it's best to use a stand-off bracket or a lifter, had a varied response, are you using any?

scottyboy · 29-12-2005, 08:03 PM

I am using a lifter yeah and ive gotta about 10" of set back BUT mine is a mannarfelt bat boat and i believe they require a completely different set up? Carly baby on here is the best one on here to ask he runs a 2 litre backdraft! Dont think he runs any setback or a lifter from memory? I have heard that the backdrafts are abit of a handful in 2litre form but im sure he used to run a cleaver of some sort! Drop him a pm dude!

Maxim Gibson · 29-12-2005, 08:04 PM

Ok, cheers for all the help m8..