Whats your guess???

2940 Views 30 Replies 7 Participants Last post by 1boatnut, Jul 30, 2002

OldMagicfloat

Discussion Starter · Mar 20, 2002

I have a 27 Ft Baja with a 502. The only mods to the motor are Gil offshore exhaust. I can get about 5500rpm from the motor running 23p Bravo lab finished prop. I tried a buddies 25p and it put me right in the RPM range it should be,but I did not like the handeling if a lot of trim was used. I am now going to install a 800 carb,Dart intake manifold and change the cam to the same cam as in a HP 500.

What pitch prop do you think I will have to run ? (or start with)
Anyone else running this type of set-up ?

1 - 20 of 31 Posts

laster

· #2 · Mar 20, 2002

What kind of prop was your buddies 25? What did you not like about the handling? With the cam and intake changes I would think you would be looking for around 5400 RPM. A Stock 24 Bravo I should be a good place to start. Should run around 74 MPH if you have the horsepower to push it.

OldMagicfloat

Discussion Starter · #3 · Mar 20, 2002

The boat seemed to be squirrly in the rear.My buddies prop was also a Mirage plus

laster

· #4 · Mar 20, 2002

The four blade should reduce that squirrely feeling. I will stick with my original guess.

26scarab

· #5 · Mar 20, 2002

I think Laster is right on the prop, 24 B1 but I my guess is it will run 68mph.

Think about it a non-stepped 27 Fountain with an HP500 is going to run around 70-71 and a stepped one might hit 73-74.
I'm judging my guess on my friends 27 Fountain which is ALOT smaller.

Your Baja is probably similar to my Scarab, it will take alot of power to get it to run fast. I'm currently making 670hp and the boat is running 80.1 on GPS. I'm spinning a labbed 26 B1 5800rpm's. What our boats need is and extention box to help carry the bow. If you add a box , raising the "X" it will be alot easier to run fast.

OldMagicfloat

Discussion Starter · #6 · Mar 20, 2002

26scarab,I certainly agre about the box. Baja is not very agressive on the x dimension to start with.
Only 1 question though,If I can spin a 24p now,wouldnt you think with the extra ponies I will have to go bigger?

26scarab

· #7 · Mar 20, 2002

I thought you said you where spinning a 23p 3 blade 5500. Adding the extra blade (24 Bravo 1) will probably lower your existing set up 200-300rpms. Which will also lower your top speed , usually around 1-2 but it is worth it for the extra stability. Plus you will probably pick up 3-4 in the midrange cruising rpm's.

Have you been to BAM marines web site? they have a cool prop slip/speed calculator. you can input the numbers you already have and then play with pitch and rpms to find your speeds

OldMagicfloat

Discussion Starter · #8 · Mar 20, 2002

your right I am running a 23 3blade. I missed the fact that the 24 you were speaking of is a four blade. Makes sense. Thanks for the heads up on BAM Marine. I will give it a shot.

GottaHaveIt!

· #9 · Mar 20, 2002 (Edited)

Here ya go:

Prop
Slip Calculator

The above prop slip calculator is much
easier to use than the Bam version.
I use 10% for the slip, it's a good
overall number for a deep v.

I would go bigger than 24 pitch B1. Try
a 26 B1 and work towards making
enough HP to turn it 5,200 RPM.

I have found from testing various props
that the 4 bladers tend to lift the stern
of the boat and upset the overall
handling of the boat at top speed (makes
it "squirrely"). The Mirage Plus is a good
prop for a stock power setup but to take
advantage of 600-plus HP a four-blade
prop is almost a must-have.

*edit*
With the mods you mentioned in the
first post of this thread, you should
have no problem turning a 26 B1 to
5200 RPM
*end edit*

-Chris

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OldMagicfloat

Discussion Starter · #10 · Mar 20, 2002

Chris,thanks. Cool calculator
I would have to agree .I figure if I can turn a 25 now between 5100-5200 rpms,with the extra hp i will need bigger. Too bad,alot of 25p props for sale now-cheap. Oh well

26scarab

· #11 · Mar 20, 2002

For that boat to pull a 26 bravo 1 to 5200 it's going to have to make at least 585hp. Adding what was described is not going to get it to that.
I think the 26B1 will just load the motor down too much, you want to spin it around 5400-5500rpm.

I figure that boat with around 50 gallons of gas is going to weigh around 5400-5500 lbs.

When my boat was stock (502mpi) I was running 64GPS at 5200 with a 23 mirage. Well I had the rev limiter bumped up and put a 22 Quad 4 Hydro motive on and the boat ran the same but at 5500rpm's, well it handled 100% better , way more stabile and it accellerated (sp) alot faster. That prop added just enough stern lift to make the boat more stable but not enough to make it squarrly.
I tried a Hydro motive prop with the new motor and it had WAAAY to much stern lift, it was like driving rear wheel drive car on ice !
I think the 24 B1 is the way to go, your mods will probably get you close to 500hp. I would suggest maybe milling the heads to bump the CR up a little, just don't forget to do the correction cut on the intake.

We should have a "guess your top speed contest"
how soon will you have the boat in the water?

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OldMagicfloat

Discussion Starter · #12 · Mar 20, 2002

April 1st. But I dont know if the mods will be done by then. Still waiting on some stuff.but if not no problem,I will run the boat as is ,get gps readings,and re-check after the modifications are complete.

GottaHaveIt!

· #13 · Mar 20, 2002 (Edited)

Yup, it's gonna be interesting to see
what speed you get out of it. I agree
the 26 B1 is overkill and that it will take
over 550 HP to turn it to 5200 RPM; I was
figuring the boat weight at 4,700 Lbs;
my 25 Outlaw is 4,750 w/ full fuel.
I may be repeating myself but my 502
Mag EFI w/ CMI's and Vortec billet flame
arrestor turns the NON labbed (stock)
26 B1 to 4,700 and GPS'd 69.7 the other
day.

I'd love to turn 5,500 RPM but the general
concensus is for long engine life keep the
RPM's below that, so I figured I'd stick to
the stock ECM's 5,150 and shoot for
making enough HP to turn the 26 to 5,100 which will be around 74 MPH.
--
There will always be someone faster,
but I wanna be the fastest normally
aspirated 25 Outlaw! And turn my
own wrenches as well!

*edit*
Well, now that I think about it, 5,500
is not that high if the right parts are
used. The Mag engines have the right
parts. Mercruiser probably runs them
at lower RPM and lower compression
to save on unnecessary warranty
expense.
I agee, more compression and more
RPM!

*end edit*

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26scarab

· #14 · Mar 20, 2002

Gottahaveit,
Those Baja's are that light ? My boat weighs 4800-4900 dry , and has a 80 gallon tank so there is another 480 lbs if I have a full tank.
I know a 27 Fountain is 4700 dry. I just though the Baja's were heavier than that.
I think I will still stick to my 68mph.

It just seems what my boat would do on paper and what it does in the water is not quite the same.

This should be an interesting saga !

Keep us informed !

Speedwake

· #15 · Mar 20, 2002

hey guys, I'm waiting to Cobra Marty to discover this place. He's a local guy and I bought a used Mayfair Dual Ram steering system off him last year for my 288. He's an ER Doc at a suburban Philly hospital and he's a calculation nut. I think Marty has software for every known boating related formula and calculation.

maybe somone could casually drop him a line somewhere else since I'm unable to.....

OldMagicfloat

Discussion Starter · #16 · Mar 20, 2002

I would be happy to. Is he where I think he is?

GottaHaveIt!

· #17 · Mar 21, 2002

OK I'm off by 600 Lbs..

Originally posted by 26scarab Gottahaveit,
Those Baja's are that light ? My boat weighs 4800-4900 dry , and has a 80 gallon tank so there is another 480 lbs if I have a full tank.

OOps, the Baja website specs the 25
Outlaw weight as 4,750 Lbs. So with
100 gallons of fuel, 5,350 Lbs; plus pax
and misc safety equipment.

-Chris

26scarab

· #18 · Mar 27, 2002

Just though I would bring this back to the top

26scarab

· #19 · Apr 26, 2002

did you ever run the boat ?
What speed and prop did you end up with?

OldMagicfloat

Discussion Starter · #20 · Apr 26, 2002

26Scarab, I have not even started the boat yet. The guy that was selling me the 800 carb and Dart manifold ended up stiffing me. I purchased a Holley 830 and Merlin manifold.I bought it from a guy in California, and it got damaged in shipping.I am able to repair the stuff,now just waiting to hear from the guy if he wants me to repair it or if he is going to claim it under the post office insurance.I can buy the parts for about 75.00-100.00.<but then he needs to discount that from the price I paid.

With all this going on,I lost my job Monday. I plan on keeping the parts if possible,but most likely not installing them until I can get a positive cash flow. I will post the info as soon as I find out.I might install the carb and manifold without a cam change,but I need to do more investigating before I do that. I might get some gain just from that up-grade.Any input??

Thanks for the interest

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