smugglers blues

I have a 1984 Scarab III. Rebuilt motors, started with stock Merc 330s but they are bored .60 over, cams with just a tad more lift, had the quadrajets tweaked a little and hi perf exhaust manifolds. On the first run out I was running 50 mph at 3000 rpm WOT after it was trimmed out. I know this isn't right? I posted this earlier this week in just general boating and they advised me to post in Tech Q&A. Tachs are right and motors seem to be running good. Props are mirage 23p. Any advice? I know that it should be running at least low 4000s for rpm. To get the RPMs there I would have to be running 17 or 18p props and I know that doesn't seem right either? Motor sounds loaded down at 50 mph at 3k.

wjb21ndtown

Smuggler, something is goofy with your numbers. Click this:

http://www.rbbi.com/folders/prop/propcalc.htm

It's a prop/speed calculator. Your outdrives should be 1.50 gear ratio. At 5000rpms with 23ps you would only be going around 35mph with average prop slip conditions. I factored 15% to use a plain jane # and came up with 37mph.

Your first step is going to have to be to borrow another set of wheels, and go from there. It sounds like your props are more 28-30p props.

smugglers blues

Hey thanks for the reply. Great link. I just tried that link and entered in the same info except 1.1 ratio (I think thats whats on my Borg Warner Trannys) and it tells me 50mph. I just don't feel right feeling like the motors are totally loaded down at 3k rpm. So now what do I do? I feel like a 17 or 18p won't get my boat out of the water?

flyinlow

The 1.1 ratio in your trans is correct but the reduction is in the drives
most 330 trs drives have about a 1.5 ratio which would give you a final drive ratio of 1.5

blue thunder

How did you validate your tachs were right?

wjb21ndtown

Flyin' low is right, that 1.1 # is incorrect. There is definitely a gear reduction in your outdrive, so your prop #s are off.


Borrow a set of props and take it out again. Where are you located? You may find that members on here would be willing to lend you a set of props to test. I know I have sh!tloads of wheels laying around. I'm in SE MI. I'd give you a hand if you're in the Lk St Clair area.

Inspector

iTrader: (1)

I had a 1983 Scarab III with TRS drives, BW 72 transmissions and @ 500 hp engines. The boat ran 68.6 MPH on GPS at 5450 RPM with 23" Mirage props. From what I remember the slip was a little over 13%. The common gear ratio for that vintage TRS drive was 1.5 to 1 and the BW Trans in performance boats was usually 1 to 1. When I first purchased the boat it had 420 HP Mercs and would run 61.5 MPH on GPS at 5100 RPM and the same props. With the information provided I believe your engines are probably @ 370 HP as the 330s had Small Oval Port Heads so the top speed should be 55-58 MPH with the correct props. I think the 23" props are too big for your application and that the speedometer in the boat may be off (reading too high) this giving strange slip numbers. I would try 21" props and a GPS speedometer to see where the slip ends up. Good luck and have fun.

smugglers blues

Hey Everyone

I appreciate all your input. The 50mph I got was from a GPS. I did verify the props are 23p. I would love to borrow some props from someone to test. I live in SoCal. Is it true that each number I go down on props only adds 200rpm?

firehawkcat

When i propped mine it was about 300rpm per pitch

Rookie

iTrader: (4)

You do understand that it is theoretically impossible for a 23 pitch prop to run 50mph @ 3000RPM. One of your #'s is off. If 50 was GPS and you know that the prop is definitely a 23, then you tach is off. TRS's are 1.5 ratio

http://go-fast.com/Prop_Slip_Calculator.htm