7.4L Mag MPI RPM/MPH - prop slip?

[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 it was another prop on it and the previous owner told me that the speedometer was off.
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/1998csx23/mercmarineprop.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?

 

[Don S]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Moving to the Prop forum

 

[Silvertip]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Apparently so! If you add pitch you will pull RPM off and mess up your hole shot. If you decrease pitch you will rev the engine even higher although hole shot would be improved. Since a prop of a given pitch and design from manufacturer "A" will not perform the same as the same pitch but different design from manufacturer "B", your only option is to try props with different designs. If you look the prop test results at the very top of this forum you will see there is little to be gained once you optimize your existing setup. You might pick up a MPH or two, but hole shot may suffer -- or vice versa.

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I understand the impact of pitch/hole shot/top speed - I'm looking for answers as to why I've got a boat that tests out at over 60 mph, doing 51-52 at 5200 RPM.

Given the boat is loaded with fuel, and I'd expect to pick up as much as 5mph, but.. I'm probably 10+mph off where this boat should be.

Let me be more specific:
1) 5200 RPM is outside peak power / peak torque for this motor... Is it far enough outside that it indicates I need to reprop?

2) I'm running a 23P prop.. I'll call Glastron, but anyone know if this sounds about right for a 7.4L Mag? Any way the prop diameter could be wrong?

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I understand the impact of pitch/hole shot/top speed - I'm looking for answers as to why I've got a boat that tests out at over 60 mph, doing 51-52 at 5200 RPM.

Given the boat is loaded with fuel, and I'd expect to pick up as much as 5mph, but.. I'm probably 10+mph off where this boat should be.

Let me be more specific:
1) 5200 RPM is outside peak power / peak torque for this motor... Is it far enough outside that it indicates I need to reprop?

2) I'm running a 23P prop.. I'll call Glastron, but anyone know if this sounds about right for a 7.4L Mag? Any way the prop diameter could be wrong?

By my calculations, from a formula I learned yrs ago:
5200rpm divided by 1.5 (gear ratio) = 3466.6666 X 22 (prop pitch -1) = 76.26 divided by 10% slip = 68.639 mph or, 15% slip = 64.821 mph
In my opinion, your prop hub is Slipping Some. Go figure, here's the negative factors, a fairly heavy boat, big block torque, fast hard acceleration, prop age? Stress on a small, old, rubber hub!?

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

1. Tach problem mpi systems cut out below 5200, confirm your true rpm

http://www.outdrives.com/merc/sternspecs.html

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

1. Tach problem mpi systems cut out below 5200, confirm your true rpm

http://www.outdrives.com/merc/sternspecs.html

I guess in plain english what your trying to say is there's a rev limiter at 5k rpm, on the 454/385hp mpi (multi point injection) so his tach is wrong?

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I guess in plain english what your trying to say is there's a rev limiter at 5k rpm, on the 454/385hp mpi (multi point injection) so his tach is wrong?

The tach is sitting between 5000 and the next tick - 5200.
I've heard to RPM limits on this motor: 5050 and 5200.
Even if it's turning 5000, I've got huge prop slip...

ideas?

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

In my opinion, your prop hub is Slipping Some. Go figure, here's the negative factors, a fairly heavy boat, big block torque, fast hard acceleration, prop age? Stress on a small, old, rubber hub!?

That would make sense. I look at the hub and don't see any major indications of slips / movement.. Wouldn't slipping impact acceleration also? Can they slightly slip or do they just let go?

I can mark it and see if it rotates.. They should not slip at all, correct?

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Mark it good, and check it, either way, this will eliminate any more questions about it.

 

[JustJason]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

In my opinion, your prop hub is Slipping Some. Go figure, here's the negative factors, a fairly heavy boat, big block torque, fast hard acceleration, prop age? Stress on a small, old, rubber hub!?

DCG9381
if its a newer mercury prop, such as the mercury mirage you stated,
it should have a derlin (very strong plastic) hub, and it will not slip at all.

don't take this the wrong way, but i highly doubt a 23ft glastron is going to hit 65. If it is, it's going to be on a day the water is glass, and the outside air is cold and dense. It's going to be on a top speed prop, that will give horrible acceleration and take forever to get there.

as a rule of thumb, the bigger the diameter prop, with less pitch... the more torque you'll put down to the water, the more pulling power you'll have (for towsports), and it will give the best acceleration and holeshot.
The smaller diameter prop, with more pitch.... everything is the opposite, however your top speed will go up because there is less drag.
multibladed props (more than 3) will offer better acceleration, handleing, and holeshot than a convential 3 bladed prop, at any diameter or pitch, however top speed with suffer because there is more drag.

4 forces at work on a boat... pretty much the same as an airplane. thrust, lift, gravity(weight), and drag.

you should play with mercurys prop selector... it will give you better advice than anyone here.

http://appcenter.mercurymarine.com/propSelector/home.do

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

don't take this the wrong way, but i highly doubt a 23ft glastron is going to hit 65. If it is, it's going to be on a day the water is glass, and the outside air is cold and dense. It's going to be on a top speed prop, that will give horrible acceleration and take forever to get there.

Would you say the same thing if it was a 23' Baja with a Mag MPI. I think Glastron has gotten out of the performance boat market, but this one of their relatively recent (last 10 years) entries.

The boat is 3800 lbs. That's probably unloaded. Add 60 gals of fuel and two people. With 385 HP, I still think it should do more than 50.
My brother owned a PowerQuest of roughly the same size and weight, with the non-Mag motor. It would do around 62mph (ideal conditions).

Mercury's prop selector errors out "outside of recommended pitch range" when I put in current performance of the boat, probably due to the fact that I'm getting a ton of slip.

If I enter the weight & size of the boat, it's recommending something in the 21 pitch range.
Remember, I'm hitting the limiter with the 23 pitch.

I'll mark the hub, but I'd think if it was slipping, it'd be pretty chewed up...

If this is a 50mph boat, I'm fine with that - but it's just not what I was expecting. I just want to be sure it's working correctly. Cruise speeds take more RPM than I'm expecting also. (4000rpm - 33mph)

 

[JustJason]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

i just did merc's prop selector... it goes "outside of pitch range" because your bouncing it off the tach already.
i did it telling it it was a runnabout/open bow with a weight of 3800, 60gal of fuel. then people and gear i did a modest 400. i don't know your gear ratio so i went with the lowest of 1.36-1
it spits back a prop of 15.5X17, which is not even close to what you have.
if you do the match, and subtract some pitch... a 17 pitch prop should put your rpms close to normal. you should play with the prop selector some more. and definitely need to know your gear ratio.

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I guess in plain english what your trying to say is there's a rev limiter at 5k rpm, on the 454/385hp mpi (multi point injection) so his tach is wrong?

Well before jumping to all kind's of speculation and conjecture.........and prop slip calc's, i for one would check the accuracy of a old tach on a new to me boat. Prop's can be very expensive and he is also looking for top performance, taking a few moment's to verify the tach is very basic.

You might want to post your known gear ratio as well,....... Believe me there are a few people here that can and will help you along the way..


Your current number's show a 29% slip based on a 1.50 gear ratio

Prop Slip Calculator

​

Use this calculator to figure out just about anything in regard to propeller, ratio, RPM and speed. You can change one parameter and recalculate the rest. Use the CalcT button to figure the theoretical speed from the actual speed and slip. Be sure to take a look at the Prop Slip page for an explanation of how to find your prop slip. revised 8-26-24

Gear RatioPitchRPMTheoretical SpeedUse this button to calc Theo speed from Slip & Actual SpeedSlipActual Speed %​

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

 

[Esox]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Well before jumping to all kind's of speculation and conjecture.........and prop slip calc's, i for one would check the accuracy of a old tach on a new to me boat. Prop's can be very expensive and he is also looking for top performance, taking a few moment's to verify the tach is very basic.

You might want to post your known gear ratio as well,....... Believe me there are a few people here that can and will help you along the way..

I second that. Verify the tach and find out what gear ratio you have and go from there.

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

I agree.. The prop experts are saying that I need to verify my tach also.
They're telling me that the Mag MPI motor should be limited at about 100-200 RPM on top of maximum operating RPM.

I don't have an inductive tach, so I'll have to find something around here that will work... I'll verify RPM and get back with you guys.

 

[dcg9381]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

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...

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

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...

Well, sorry to hear your engine is only putting out 3900rpm, but good to hear your prop seems fine, but something is still not right, and according to my formula for speed at that rpm, with that prop it's pretty close to your net result:
3900rpm divided by 1.5 (gear ratio) = 2600rpm X 22" (prop pitch -1) = 57.20mph -11% = 50.9 mph!
I'd bet if you could get it up to 5000rpm, you'd see about 65mph. good luck, Mke

 

[Tail_Gunner]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Just a little engine refresh and it smok'n time, good luck with that boat, im in the hunt for something very similar........but up here in Orygun them old Carlson's are a rare breed.....

 

[MikDee]

Re: 7.4L Mag MPI RPM/MPH - prop slip?

Just a little engine refresh and it smok'n time, good luck with that boat, im in the hunt for something very similar........but up here in Orygun them old Carlson's are a rare breed.....

James Bond boat's! I always like their speed, and style! along with Baja's, they're just timeless design's, never go out of style.