strange speed/prop issue

[94OMC]

Hi,
I have a '94 Four Winns with 5.0L OMC. According to the paperwork that came with the boat, the max speed should be 45-48 mph @ 4600 rpm. The boat still has the stock 21" SS prop, but I'm seeing speeds of 42 mph @ 5,000 rpm.

How is this possible? The speed was verified with GPS, but I suppose the tach could be off...

Any ideas?

thanks

 

[stevens]

Re: strange speed/prop issue

The top speed indicated in the advertising is very often only attainable under ideal circumstances - nearly empty tanks, no passengers, no extra weight or gear, clean & slick hull, flat water, engine in top tune, the right prop, the right use of engine trim & trim tabs, etc...

You're within 5-10% of the advertised speed, which is probably just normal. Alternatively, if you really want to look into this, you should first verify your tachometer, then pick a fast prop that gives you the optimal max RPM. However, if you prop for maximum speed, you will most likely also lose a bit in accelleration / hole shot.

 

[Bondo]

Re: strange speed/prop issue

Ayuh,.....

Check the Tach.... If it's Right, Get a Bigger Prop....

That 5.0 should be Topped Out at about 4600rpms,.. Maybe 4800.....

 

[94OMC]

Re: strange speed/prop issue

Thanks for the replies.

Some more info:

-nearly full tank (28 gals)
-2 people and dog on board
-clean hull
-little wind, just a little bit of chop

I'll find a way to verify the tach, because I don't want to be over-revving the engine if it really is getting to 5k rpm. Hopefully the tach is just off. Like I mentioned before, this is the stock prop so I would imagine it came propped correctly from the factory.

Thanks

 

[Dandelion]

Re: strange speed/prop issue

This one is simple: Try a 23" prop. The higher pitch will reduce your rpm's by a few hundred, and increase your top speed by a few mph, which is exactly what you're looking for.

 

[94OMC]

Re: strange speed/prop issue

This one is simple: Try a 23" prop. The higher pitch will reduce your rpm's by a few hundred, and increase your top speed by a few mph, which is exactly what you're looking for.

Not so easy it seems...

According to a prop slip calculator, the engine should have only been turning 4,125 rpm for 42mph @ 15% slip (tach said 5,000). Also, at 4,600 rpm the boat should be going 46.8mph - right in line with the 'advertised' top speed.

Now I need to find out why my tach is off, and why I'm not hitting the WOT rpm. The boat probably had some more in her as I wasn't pushing it 100%. I know the boat has a different control box than what it came with from the factory - I'll check to see if it's allowing the throttle plate to open all the way.