PDA

View Full Version : Engine & Prop Info Needed!!!!!



Stan
03-18-2000, 02:12 AM
Decision Time!!!


RE: 1990 18 2+3


I just had installed an almost new Mercruiser 350 w/ Alpha I,Gen II, 250 HP with the current 2 barrel carb, all stock set-up. Thru hull exhaust.

I took it out for a test ride the other day and the results were worse then I expected;
With the 15 / 18 pitch prop ( wrong prop, I know ) the speed was a dismal 43 mph. In all my excitement during the very short test run I didn't look at the rpm's !!!

I would like to see 60 mph.. if possible from this boat, but don't want a temperamental beast on my hands.

My options as I see them are as follows:

New 4 barrel carb ( definitely )
High rise intake manifold ( probably )
New exhaust manifolds ( possibly )
New cam ( don't really want to do this )

I think I might be able to get a new Edelbrock carb, manifold and possiblly a marine exhaust at a discount price from a friend ( we all work every angle when we can ). What do you feel the installation should cost me?

Should I re-prop the boat with the current setup and see where it gets me, then do the engine work….or….just bite the bullet,,do the engine work, THEN re-prop ?

I will have the boat out again next week so I can get some more info on RPM"s

What props do you guys suggest for this boat as it sits…and after the modifications ?

Thanks

Stan........

kevin
03-18-2000, 06:51 AM
stan you realy need to know your rpms but you know that now. as far as your intake i am getting ready to make a change to alum also, but watch out on the high rise part. you need to find out the torque curve of your engine im sure some one out there knows this, and pick the right intake for that application, high rise intakes are more for higher rpm and it seems what i have found is most small blocks need to wide open throttle at about 4500-4800 this would require a low rise dual plane intake like the edelbrock performe KEEP in mind this is just based off of the research i have done for my own application and there are a lot of true motor heads out there that can probably prove me wrong good luck

RickR,GroveCity
03-18-2000, 06:53 AM
You might be hitting the rev-limiter.
Borrow and try a couple of props and get a baseline before you do engine mods.
Is your Merc a Magnum?

------------------
RickR mailto:riggerb@aol.comriggerb@aol.com</A>

[This message has been edited by RickR,GroveCity (edited 03-18-2000).]

Francis O. Saffell
03-18-2000, 01:03 PM
Handy Dandy Theoretical Top Speed Formula
(modified 0 times)
Owen

Here's a formula I'd like some people to try and compare to actual in the field data so we can get some idea of the drag characteristics of
various donzi models and engine/prop combinations. To use the formula you need to know:

The top rpm range of your engine ERPM
The Gear Ratio for Prop Shaft RPM GR
Prop Pitch Used PP

Here's the formula

(((( ERPM / GR ) X PP ) / 63360 ) X 60) = Theoretical Top Speed in MPH.

63360 is inches to one mile.
60 converts miles per minute to miles per hour.

Example:

Mercrusier 502/410 hp
ERPM 5000
GR 1.36 (Bravo 1)
PP 24

((((5000/1.36)x24)/63360)x60)= 83.56 TMPH

This should at least give some people an idea of what they can realisticly expect from
their setup and boat and point out areas for improvement.

Mar 01, 2000 02:58:59 A.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
Scott B

Profile | Email

Props n gears do lie tho. As I recall, a 12 - 18% "slip" factor needs to be calculated in to account for the slippage of the propellor in the
water. Keep in mind, (stretching brain here,gawd its early still) that a 23 pitch prop,will screw its self 23" thru a block of wood in one
revolution, but H2o isnt wood. Slip varies with the effficiency of the hull,drive,and prop design. Considering such, a 23 cleaver, a bow
raising prop, might carry the boat better than say a 4 blade of the same pitch, but the 4 blade,by lifting the stern,might be faster by
reducing the wetted surface of the boat,hence reducing drag.
When I ran calcs on my SeaCraft,theoretical vs radar gun, it calced out to a 14% slip factor,which is actually pretty low.I have not run
Donzi #'s,but would expect them to fairly low also. FWIW,YMMV!

The other Scott
Just north of Cuba

Mar 01, 2000 05:25:49 A.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
CDMA

In four years I will be a naval architect. I will get back to you. For me the formula is as follows

( tree on boat x boat on trailer)/4 ( months till boat is finished) = 0 mph

Chris

Mar 01, 2000 06:06:40 A.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
PaulO

Look at this one:
http://member.aol.com/ejoejr/PropAnalyzerAppletG.html

Mar 01, 2000 08:07:16 A.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
RickS

18 Classic
4500 RPM
1.49 Drive Ratio, or is it 1.48? Can't remember. I used 1.49
19 Pitch Quicksilver

Calculated MPH = 54 MPH
Speedo Observed MPH = 52 MPH

4% Difference, but then I'm using a speedo reference and not radar or GPS.

Mar 01, 2000 09:29:03 A.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
PaulO

18 classic 350 Chevy Volvo 280
approx. 270 HP
1.61:1 ratio
4950 RPM
GPS speed 58 MPH
23" pitch volvo ultra

Mar 01, 2000 11:33:50 A.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
Owen

Yes I understand about the slippage factor - thats sort of what we're looking or.
PaulO, your TMPH = 67. To get any meaningful data I guess we need to include engine type (wieght) and hull type. You can simplify the
equation to.

((ERPM/GR)xPP)/1056=TMPH

I plugged this in on my non-donzi and was surprised how close I was.

3.7 Merc
4700 ERPM
1.84 GR
21 PP

Observed 43 MPH
TMPH 52 MPH

Mar 01, 2000 03:06:22 P.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
Owen

http://member.aol.com/ejoejr/PropAnalyzerAppletG.html

PaulO, Thanks for the site.

Mar 01, 2000 03:21:14 P.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
Danhouck

1996 16' DOnzi
305 chevy to alpha
ERPM=4600
GR=1.5
PP=21

Speedo says ~60 mph
Equation says 60.985

That's pretty sweet.

Mar 01, 2000 06:15:44 P.M.

RE:Handy Dandy Theoretical Top Speed
Formula (modified 0 times)
GEOO

Profile | Email

Bam Marine has a few calculators. Click on; Tech Stuff Prop and speed calculators! http://www.go-fast.com/index.htm?/first_page.htm
GEOO

Mar 01, 2000 07:14:13 P.M.


Stan According to the prop calcs you would need a 24 Pitch prop to run at 60 with 10% Slip.

Looped
03-18-2000, 01:16 PM
Owen,
That's a great memory that you have.

On the same subject of the OMC years of Donzi, Does anyone know what products that were cut back in quality in the late 80's to early 90 18' Classics? I am about to look at one, but more info on them would be helpful.

Thanks,
Looped

GEOO
03-18-2000, 06:43 PM
Stan, 250hp with an Alpha should go 55 to 60 mph. I would play with props and check your rpm's. Find out what's not right before you add engine parts. How did you check your speed? Maybe your speedo is clogged?? GEOO

PaulO
03-18-2000, 10:38 PM
Stan,
Just finished the install of the new cam and edelbrock performer RPM mainold on my formerly 260 mercruiser. Unfortunately, I have a ton of other work to do before I get this thing in the water and test it. I can tell you this though, the edelbrock performer is basically a stock replacement. If you want to move the HP/Torque curve up the rpm range a little, the performer RPM is probably your choice. This is still a dual plane manifold. The good news is that it fits under my engine hatch. By the way, there is a nice little spot in the water passage to install a pencil zinc. I also coated the insides of the water passages with paint.
Paul

RickR,GroveCity
03-19-2000, 07:13 AM
Looped
My 90 18 2+3 is first rate. The hull lay up is the same as earlier models and the deck is balsa cored.
The only differences I see are

Faria Guages instead of VDO

A black plastic dash insert that looks great but develops cracks after a while.

The cut out for the dash takes some of the structural integrity out of that area of the deck.

The air scoups are not the high dollar castings of the earlier models.

The late model OMC King Cobra with the Cone Clutch (like a Bravo) is a grea drive rated at 650HP

I'll be at the show with Genuine Risk is you want to look her over.

When calculating speed add 1" for cupped props and subtract 10% to 15% for prop slip.
A well tuned prop can have &lt; 10% slip on a well setup boat.
Mine has &lt;9%
5150RPM,1.43Gear Ratio, (22 cupped=23)=
73.4 GPS MPH



------------------
RickR mailto:riggerb@aol.comriggerb@aol.com</A>

[This message has been edited by RickR,GroveCity (edited 03-19-2000).]

Forrest
03-19-2000, 08:17 AM
PaulO . . . danger on the pencil zinc, dude. I tried this trick in an attempt to get more life out of my Weiand Stealth intakes (BTW, great intakes) on my Magnum. After a few months of salt water use, I decided to check the zincs . . . gone, not there. I figured that they must have been doing there job and had expired. Anyway, I installed a couple of new zince and after a few more times out, I noticed the the temp was rising on one engine. I went through all of the "traditional" trouble shooting procedures and came up with nothing. Just for grins, I decided to pull the circulation pump off of the front of the engine in question and guess what I found stuck in the pump's passage way. Yep, you guessed it, the missing pencil zinc.

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

AVickers
03-19-2000, 01:14 PM
This might be a stupid question, but is the speedo accurate? The pitch on the original question seemed way too little -- I run a cupped 15" X 24 at about 4600 and the boat is fairly quick...

The first two things I'd work through would be the prop and the carb (probably in that order) and I'd find out how fast the engine was turning to get your mid-forties speed with the low-pitch prop.

Stan
03-19-2000, 11:41 PM
Thanks for all the info guys......I think my next step is to pay attention to my RPM's on my next rip out. Before I drop a ton of $ into this engine I think I need to play around with some props.

Thanks again....I will keep everyone informed

Stan....

Looped
03-20-2000, 01:30 PM
RickR,

Thanks for the info. Now I have a few things to look for when I go up to see it.

Looped

Terrell
03-24-2000, 01:29 PM
Does any one know how this formula works when you are running a DP setup? It would seem to me that you slip factor would be less, but I dont know the true pitch on my props, since they just have a Volvo number on them. Any thought?

Forrest
03-24-2000, 05:33 PM
Scott, that was Holley Marine Products that had cast-bronze water passages in their aluminum intakes. Aparently they are so new that they are not shown in the Holley catalog yet.

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