Top speed too low?

[Chuckles88]

I have a 1989 18' Glastron futura 185. It has a 4.3 in it with a 14x17p prop. At 4700rpm's I'm at 37mph gps checked. After reading through other threads it seems my top speed is low. Can anyone chime in? Thanks

 

[achris]

Hello Chuckles88, and :welcome: to iboats....

Those figures look perfect...



If you think it should be going faster, then check the health of the engine, compressions, timing, and that the carb is opening up fully... Is this a 2 bbl carb, or a 4bbl?

Chris.......

 

[Scott Danforth]

Is that your WOT RPM? Will it go higher?

What is your gear ratio?

If your ratio is 1.84, your at about 10% slip

 

[Chuckles88]

It's a 2bbl. I was getting 4100 rpms but then changed plugs and ended up with 4700. They aren't marine but I don't believe it matters? I was worried it was water logged due to age and the top speed.

 

[Chuckles88]

I can't seem to find the gear ratio stamped anywhere. I have had it to 4800 but that was 2 adults, 2 kids and a full tank

 

[achris]

Did you check compressions, check the timing, check that the carb is opening fully? Those are the easiest and quickest things to check, before worrying about water-logging...

 

[Chuckles88]

I haven't done that yet. I'm sure it's in my near future. That wouldn't change the 4700 sweet spot or the mph More rpms?

 

[achris]

If your engine isn't healthy, then the max revs will drop, obviously. Picking up 600rpm on just a plug change tells me something else is also going on....

Chris........

 

[Chuckles88]

I'm not sure if the plugs were ever changed. They were extremely carbon fouled and one snapped when removing it.

 

[Chuckles88]

Should the boat go over the recommended 4800 with the throttle pressed all the way?

 

[achris]

Should the boat go over the recommended 4800 with the throttle pressed all the way?

If the prop has a small enough pitch, yes. You select the 'correct' prop pitch so that at full throttle, the engine is spinning at the manufacturers recommended RPM, in your case, between 4400 and 4800... I prefer to 'prop' an engine at the upper end of the range...

You put a prop on and take the boat for a run. Open the throttle full (WOT) and see what revs you get. If it's too high, you select a prop with a LARGER pitch. Each 1" of pitch will change the revs about 200... If the WOT revs are too low, you go DOWN in pitch....

This all assumes the engine is in good condition and running properly. Changing pitch to mask a problem doesn't help. So before doing prop runs, make sure the engine is healthy. Do a compression check, timing check and ensure the carb is opening up fully when you push the control lever all the way.... You also need to measure the revs with a calibrated tacho. The dash tacho isn't near enough accurate.(I've seen dash tachos 500rpm out)

Chris..........

 

[Chuckles88]

So if the guy before me had everything set right and run it at 4700 rpm(if the tach is on of course) and get 37 mph at wot, maybe that's what the boat does? I will check all of the items you listed so I know. If I did have water retention, more weight equals more prop slip at wot? Or once I get to top speed, it wouldn't make a big difference? Thanks Chris

 

[achris]

If the boat was set up correctly with the current prop, and it's now water-logged, you'd see a substantial drop in top speed.

Check the things I suggested, and if they are all in spec, then 32 knots is your lot...

Chris.........

 

[nola mike]

Seems slow to me. Don't know what the weight of your boat is, but mine is listed at 1700#, has similar HP, and still hits upper 40's at 4500 rpm with a 23p prop and 1.84 ratio drive. It could almost touch 50 when I got it 10 years ago.

 

[Chuckles88]

Have to add I'm at 4200' elevation. Could my current setup be cavitating too much with a 14x17p prop without me knowing?

 

[nola mike]

No, but you might need to rejet your carb to compensate for the elevation. Don't know at what point the elevation becomes relevant, both in terms of carb tuning and effect on engine performance overall...

 

[Rick Stephens]

I run often at 5000 ft. I also run at 700 ft altitude. If heading to 5k I just rotate the choke so it is non-op. Yes, my boat runs a little rich at 5k, but it does just fine for the most part. I would doubt that he needs to do much adjusting of mains to run at 4200. Easy enough to check plugs. If they show really rich, then drop the mainjets a little.

 

[achris]

First 'step' in adjusting for altitude is at 5000'.

Merc changed the 'adjustment procedure' for altitude a few years back. It used to be all about rejetting, now they just recommend changing the prop to bring the revs back to spec.

Chris...

 

[Rick Stephens]

First 'step' in adjusting for altitude is at 5000'.

Merc changed the 'adjustment procedure' for altitude a few years back. It used to be all about rejetting, now they just recommend changing the prop to bring the revs back to spec.

Chris...

That makes a great deal of sense.

 

[Chuckles88]

Checked timing, checked carb to see if it opened all the way and replaced distributor and rotor. 4800 rpm at 39 mph with a 14 x 17p. Plains very quick. My estimations have me thinking I could go to a 14.5 x 19 to get me right in the middle of the 4400-4800 and bring my top end up a bit. Does this sound correct? Thanks!