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View Full Version : Best prop for 96 18' Classic



Ranman
07-05-2000, 01:43 PM
As some of you may know, I recently purchased a 96 18' Classic. It has an all factory Merc 350 (250HP) backed by an alpha One with thru-prop exhaust. Not exactly sure which prop is on it now (I have not looked yet), but it is an "elephant ear" SS Prop. The boat runs an honest 58-59 mph on the GPS w/ 2 people and a full 40 gallons of gas. A fellow at the local boat store told me I could gain 4+ MPH with a 4 blade cleaver, however, I would have to feather it up onto plane. I am not super interested in bottom end punch as I don't plan to pull anything and slight feathering out of the hole doesn't bother me too much. Does anyone know what the best prop for top end would be given this setup? Stability is also important as I want *safe* (chuckle) speed. Any info is appreciated.

P.S. I'm working on getting my ride in the registry. Thanks.

Barry Phillips
07-06-2000, 07:48 AM
Ranman: I have 98 Classic 18 with a
350 /250HP. I am running a 23 pitch
Merc Vengeance SS prop, my speedo
indicates 62mph with 4 aboard, I have
been clocked on radar at 61.5 . Alone
with 20 gallons of fuel and ideal
conditions I have seen 67 mph. I am
running a Brovo 1 Performance drive.
The Vengeance is a standard round ear
design. I'm not sure about a 4 blade
cleaver, 4 bladed props work better
with jack plates used on outboard
applications or Land & Seas stern
jack for IOs. I would look at a 3 blade
raker design like a Stilletto. If your motor
is a 250 like mine than it's a 2 barrel carb
think about a 4 barrel carb and a new
intake before you change your prop.
This is my plan.
Good Luck
SO-SLO

Stan
07-07-2000, 12:42 AM
RE: my 1990 18 2+3

I bought the boat with out any power. I installed a 1996 Merc 350 w/ Alpha 1 Gen 2
350/250 hp....2 barrel carb...same as you guys. VERY Low hours on the engine. I don't seem to be able to get near the speed you guys are getting. So far I have been playing with an alumn prop...14 x 19
it gets me 4100rpm = 49mph @ 1500 ft elevation.

I think I need to go to a 14 x 21 stainless steel to see some more speed. What do you feel is holding me back at this point ? Prop, elevation, 2 barrel carb ( I've been ichin' to put on a 4 barrel and highrise intake.

By your numbers I should also be in the high 50's to low 60’s !

What rpm’s are you turning at the high speeds ?

I NEED MORE SPEED !!!!!

Stan

Barry Phillips
07-07-2000, 06:48 AM
Stan: Everything, 19 pitch is not enough
for 18 with 250hp. I tried a 21 Mirage
last year, down from my 23 pitch
Vengeance and lost 4mph. 61mph down
from 65 the boat had better midrange
rolloff but lost it in the top end. Look
for a good SS 23 pitch prop for your
set up. Check out www.propswap.com (http://www.propswap.com)
try a Mirage or Mirage Plus, Laser,
Vengeance and again a Stilletto these
props are a great value, I have read
good things about them. Your 250
is a good plateform for a new intake
and a 4 barrel. A turner suggested
I think about a cam, tuned exhust such
as IMCO, EMI , Gil etc. I have Gil tips
already. He also suggested bumping
up the rev limiter to 5200rpm this is what
the MPI Mag runs which have the same
bottom end. Right now I'm trying to pull
the money together and convince my
wife why another few MPH is a good
thing, 65 is fast enough for her.
Good-Luck
SO-SLO

PaulO
07-07-2000, 08:47 AM
Stan,
Something is wrong there. Selecting the correct prop involves several things but, the most basic is getting your max RPM to coincide with the max power of your engine. 4100 RPMs is too low. Your max power is probably around 5000. Now the problem; that 14x19 prop should allow that motor to spin to 5200-5500 if everything is OK. Unless you somehow got one of the few big-block Alpha drives with the 1.36:1 ratio, you have got 1 of four things I can think of off the top of my head:

1. Your engine is not running correctly (possibly mis-adjusted throttle linkage also).
2. Your aluminum prop is really in bad shape.
3. You have a serious hook in your hull.
4. Your tach and/or speed measuring device ( hope you are not going by a speedo) are way off.

Start with the easy stuff first. See if the drive ratio is stamped on the port side of the upper gearcase. Check that the carb is opening all the way. Check that the flame arrestor is not clogged. Engine running on all 8? Borrow a prop before you buy one.

Hope some of this rambling helps.
PaulO

RickSE
07-07-2000, 10:16 AM
Stan, are you the one in Southern California? I'm in Flagstaff, if you are then one of these days we should get together at Mead or Havasu, maybe we could figure out your problem.

Well you probably know my story but I'll say it again. I have a 96 18 w/350 2-barrel & Alpha 1 w/1.47 gear. At Powell, 3700 ft., and with a 19 pitch Quicksilver prop the boat would run 53-54 MPH at 4600 RPM, trimmed up. As you may remember I decided to remove the 2-barrel and install a 4-barrel Holley 650 and Edelbrock Performer manifold. This combination ran 4900 RPM with the old prop but I never really checked the speed, I was to excited about trying my new 4-blade Revolution 4 props. 21P Revolution 4 ran 4200 RPM @ 56 MPH & 19P Revolution 4 ran 4600 RPM @ 56 MPH.

After these runs I sent the 4-blades back and decided the 2-barrel cam was not taking advantage of the carb. So I ordered a new cam, Crane Cam 272/284 degs. duration & 454"/480" lift, This cam is specified for marine use, see http://www.cranecams.com/master/apps/chevy13.htm. You also have to change the springs if you use this cam. At this point the engine is out of the boat and will go back in this Sunday. I'm not expecting big numbers out of this but 15-20 more HP and maybe a few more MPH would be nice. We calculated 232 HP at 3700ft. with the 2-barrel set-up and 250 HP with the 4-barrel. My goal is to be able to hit 60 MPH at Powell. I will order a new 3-blade 21P, Mirage or Vengeance, soon and go up in the next couple of weeks and give it a try.

[This message has been edited by RickS (edited 07-07-2000).]

[This message has been edited by RickS (edited 07-07-2000).]

BERTRAM BOY
07-07-2000, 11:08 AM
Boy, am I glad I'm at sea level.
BERTRAM BOY

Forrest
07-07-2000, 12:37 PM
RickS, that's the same cam kit that I used in my 383 X-18 project. Since I have not started the engine yet, let me know how it runs. I'm close to getting it in the water. HLSM.com has been slow (almost 6 weeks now and five phone calls) getting my parts to me. I used them about four times in the past and until the last order, they were fast. I'll try them one more time before putting them down on the "do no use" list.

------------------
Forrest

PaulO
07-07-2000, 01:17 PM
Ricks,
I am expecting my 383 motor in about a week and will then have a 21P Mirage plus available. Beautiful condition. Let me know if you are interested in it.
PaulO

RickSE
07-07-2000, 03:06 PM
PaulO, I might be interested in the prop, it depends on cost. I can get new ones at dealer price and have the advantage of returning them if they don't work. Dealer cost on a new Mirage is probably around $350. Does yours have the exhaust vents? Not sure when they started using the vents.

Stan
07-09-2000, 12:01 PM
My drive is stamped 1:47 I sent my serial #'s to mercruiser and they confirmed 350/250 hp. I gonna start with basics...new spark plugs regapped to 35 degrees ( the old ones were at 42 degrees). Timing is next

Any other suggestions ?

Any one have a prop they could let me borrow to do some testing ?

Stan

PaulO
07-10-2000, 08:46 AM
RickS,
Yes, it is a new one with the vent holes. Will be looking for $200.00 as long as it remains is its current condition until sold. Feel free to try it first and return it if it doesn't work.
PaulO