View Full Version : Propellers.....Help.....!!!

04-17-2000, 07:30 PM
RE: 1990 18 2+3
Merc 350 260 HP

Propeller performance so far:

Mercury Black Max propeller

14 x 21 4000 rpmís = 47 mph
14 x 19 4400 rpmís = 45 mph

I canít believe my speed went DOWN !!

Any suggestions??

A friend is suggesting I try his Land & Sea variable pitch propeller 14 x 12-22.
Any thoughts ?

Will a propeller off an OMC King Cobra fit my Alpha ?

I feel I should be able to achieve between 55 to 60 mph with my setup. Isnít that very reasonable !!


04-18-2000, 02:04 PM
Stan, I ran your numbers; assuming a 1:50 gear ratio; on the computer http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html The number add up. Approx. 12 to 13% slip. I think an 18 with an Alpha and 260hp should run 55 to 60mph!!. Are you trimming the outdrive up, to loosen the hull?

04-19-2000, 05:09 PM
Yes, I probably need to learn to work my trim better. The engine is rated 4200-4600 rpm. I think the Thunderbolt V ignition system has a rev limiter built in. Can that be changed ? Will that be bad for my engine ?

More Questions:

Do I need more rpmís to achieve the speed I want ?

Do I need a different prop ? ( Obviously ) What prop ?

I am about to try a Torque-Shifter prop and thoughts ?

Will a 4-barrel carb help the situation?

The list goes on and onÖÖ


04-19-2000, 10:17 PM

I tried Land and Sea Torque shifting prop. Great for water skiing did not gain top speed.
The props you have seam to be working correctly; Speed vs slip vs rpm's.
Donzi's run slow unless you trim the bow up alot at full thottle. Is your engine fresh?? You should be able to hit 55mph GEOO

04-20-2000, 12:41 PM
Here is my 2 cents about the Torque-Shift prop.
They tell you when you buy the prop that "THIS IS NOT A SURFACE PIERCING PROP" they aren't kidding. while testing his brand new prop my freind gets his boat on plane,(pulls great!!!) then feels the shift (just like you're supposed to) reaches 40...50...trims it out a little, 55...60..wow this thing looks like it's going to work great!!! 65...trim it little more and BA-BLAM oops... what was that?!!!
we stop the boat, raise the motor,and inspect the $600 investment "well at least we still have 1 blade left". It's a good thing that we were going up river.
Don't get me wrong I'm sure it's a great product. Just be carefull.

04-20-2000, 01:10 PM
I have a 96 18 2+3, 350 2-barrel, 210 or 215 hp, 14X19 Quicksilver prop. At 3700 ft elevation at WOT, drive trimed down I'm close to the speed and RPM you list for the 19 pitch, maybe a little more speed though. With drive trimed all the way up though, RPM's peak at 4600 and speed is 54 MPH (speedo).

04-20-2000, 03:43 PM

Your HP should be 260 with the 350 2- barrel