dcg9381

I've got a 1998 Glastron CSX-23. New boat to me.
It's got a 385hp 7.4L (454) Mag MPI motor, which seems to run great.
I believe this boat should hit 65-68, properly propped.
I bought the boat with two props, the other on the boat a 21 pitch prop used for getting the boat on top of the water quickly, previous owner indicated that the speedo was off, as he had "radar'd" it.. :-)

I swapped the "speed" prop - which I believe is original (not 100% sure) - 23pitch Mercury Mirage prop, stainless, 14-5/8"x23:

http://lakebox.dnsalias.com/photos/1...marineprop.jpg


I'm hitting 5200 RPM, which I believe is the end of the operating range for this motor.
Speedo reads 50-52mph.
I double checked it with GPS and it's spot on.


Is a 14-5/8x23p prop correct for this application?
Any other suggestions?

Payton

5200 is just past the optimal operating range for your engine.
Using Bam Marine's Prop slip calculator
http://www.go-fast.com/Prop_Slip_Calculator.htm
Your slip is at 32% ( I did assume you have a 1.5 gear ratio). That is way to high. Your guess of 65-68 is right on.

Check your hull for a hook and make sure the prop is in good condition.

Pat McPherson

At 5200rpm you are on the rev limiter.
Try a bigger prop (M+ 25p) and see what you get.

dcg9381

Any way that this prop could slip? Forgive my lack of the proper terminolgy - but the composite piece that sits in the center of the prop? I imagine I'd notice if it was slipping as it would be pretty chewed up?

Pat McPherson

If the hub broke loose, I doubt you'd get on plane.

jayhawk261

When they talk about "prop slip", they are not talking about the prop physically slipping on the hub. Prop slip is the loss of bite in the water. No prop is 100% efficient, so therefore it will "slip" in the water not grabbing all the water that it can theoretically push. My prop slip is calculated at around 7% which is extremely efficient. I think 10-12% is probably pretty normal. Someone may correct me on that though.

My boat is a 24' and prior to going through the engine, I had a 454MAG MPI as well. My boat has a 24 degree dead rise and a step bottom hull which makes it more efficient than the non-step shallower v that I'm assuming the Glastron has. My boat, with a worn out engine would see 72 mph on gps. I think low 60s to 65 is probably realistic for your hull. I'd go with Pat's suggestion and try a 25P Mirage plus. I may have a BBlades labbed one for sale here soon as I am most likely going to need a different prop with the new engine.

dcg9381

I understand that prop slip is one measure of prop efficiency.. 30% is high. A boat like mine shouldn't be more than 15-20%.

My understanding is that earlier prop designs could actually slip on the rubber hub, but that probably isn't happening in my case. I can mark the hub location on the prop to be sure.

Ping me when you're wanting to sell the prop. My email is:
[email protected]

I bought an inductive timing light today that also gives inductive RPM, so I'll go back out and verify RPM as well as GPS speed (again).

There's some conjecture on the RPM limit on the MAG MPI motor. I don't hear this motor hiting a limiter, at least not like an automotive limiter.. We'll see..

dcg9381

Well, my prop is fine.
I went and bought an inductive tach, and my boat tach is way off.

23P Merage Plus
31mph 2600rpm
43mph 3460rpm indicated 4600rpm
50mph 3900rpm indicated 5200rpm

At 50, I've got prop slip of 11% - which is well within range.

The tach I can replace, no problem - although I'll have to figure out how it's hooked up on the MPI motor (just for grins). I'll also check to make sure it wasn't tampered with - I believe setting it on 6 cylinder would have it read higher, if that's possible with these tachs.

The bad news is I'm getting to 3900 rpm and 50mph.
This boat tested out in 1998 at slightly over 68mph via radar.
I'll compression test it tonight...

One of the "quick and quiet" exhaust baffles - the stainless part is clattering pretty bad, I assume that's all internal valving though...

Pat McPherson

If your quick and quiet is not open. You will not have as much power. I would think 200-300rpm down, not 1000 though.

the duke

tach sender on EFI is the gray wire. just do a direct switch, wire for wire. The gauges are marked where which wire goes.

On your Dist module, there are 2 plugs. one goes to the coil. That one has the tach pick up in it.