Prop advice '93 Glastron 175 4.3 Alpha

[Flemming]

I'm here soliciting advice on giving my '93 Glastron SSV 175 (17' weight unknown) some longer legs since its got a relatively modest 18gal tank;
motor's a 4.3 Merc I/O Alpha drive said to be 205hp and is currently in fine tune;
current prop is a solas ss ME 14 1/4 X 19 (3 blade) which I've been running for about 20yrs. great hole shot for playing on the water but not so much top end;
top end is limited by the redline ... about 45mph on the speedo. WOT would likely blow the motor ... not going there;
anyway, I'm looking to get some more range (gas mileage) with a prop switch: I'm confident I could turn a 21 and it would be nice balance but, are there any opinions on a 23? Normal load is usually 4/5 people and light gear.
Thanks.

 

[Bondo]

I'm confident I could turn a 21 and it would be nice balance but, are there any opinions on a 23? Normal load is usually 4/5 people and light gear.

Ayuh,..... If Whatever prop ya use, allows the motor to turn 'tween 4400, 'n 4800 rpms at Wot, trimmed, 'n Runnin',....

Yer Golden,.....

I'm guessin' the 21" Might get ya there,.....
I'm also thinkin' a 23" is gonna be Too Tall, 'n lug the motor into detonation which leads to self destruction,....

 

[steelespike]

Bondo is right on.Your motor needs to be able to operate within its wot rpm range. When propped right you can operate at what ever rpm you prefer.
Best cruising speed could be from just on plane up to 1000 rpm higher.
Not like a car motor; your motor is working hard like a car a car pulling a trailer up a never ending hill.As you increase pitch you are getting closer to overdrive.
Your motor will handle a slight run past the red line long enough to get an rpm and gps speed.Like running your car through the gears you approach
and hold red line a number of times.Typical car redline is usually around 5500.
Keep in mind as you increase pitch hole shot will slow, ability to stay on plane will likely be fussy.The various rpms will require more throttle.
Of course those rpms should supply more speed.
With 5 people aboard the boat should still be able to reach at least the low end of the rpm range.Occasional ventures below minimum is ok.
If your serious about fuel economy you need a flow meter. Some can do real time mpg.
I think you would need a flow meter to tell the difference of a good setup and a perfect setup.

 

[Flemming]

Thanks all for the replies. I will heed your advice; never really though of under revving the motor but it makes sense: well explained. Now, would it be possible to get the same gas mileage with a prop pitch 19 as a 21 only going slower? Seems there would be less of a load on the motor - would that make it more efficient? or is the efficiency designed around the wot within the rpm range? I find it interesting but not so much as to get a flow meter - at least not yet.
And the reason for my confidence in a 21 is that is what the boat came with; switched to the ss 19 to get the hole shot for skiing and likely would still use it for skiing days. The original 21 is aluminum and so badly pitted to be only good as a spare.

 

[steelespike]

A 19 compared with a 21.
The 19 should get out of the hole quicker a tiny saving in fuel.It will take less throttle to reach the same rpm but will likely be slightly slower
and will require more rpm to reach the same speed of the 21.The 19 may be easier to keep on plane at reduced throttle.
The 19 should do better as you add weight.I think you can see that if the 21 is at the bottom of the rpm range economy might suffer.