Which Prop? Old merc

[summitlt]

Got a 15' Glastron tri-hull with a 85 merc on it. Running WOT (no tach) id be guessing around 5,500 rpm it pulls 33mph on the GPS. It has a pretty damn good holeshot, enough to set ya back like it wants to go NOW!

I know this boat could do a lot better, but not sure what to use for a prop. OR how to find out what I have.

 

[mthieme]

Re: Which Prop? Old merc

There should be numbers stamped on the prop either in between a pair of blades on the hub or on around the hub where the prop nut is.
The number format should look something like 48-73142A40 with maybe a 17P afterwards.
By "perform better" I assume you mean faster. This would necessitate a prop with more pitch but you would give up some of your hole shot.
You really need to know what's on there now to move forward.
It's tough enough w/o a tach.

 

[steelespike]

Re: Which Prop? Old merc

Really can't make an effective determination without the prop size,and max rpm and speed.Perhaps you could borrow a shop tach or buy a Tinytach.
they are reasonable,easy to install and work on any similar motor.
Being over or under propped can prematurely damage your motor at worst
and deliver poor performance and economy at best.

 

[Frank Acampora]

Re: Which Prop? Old merc

That 15 foot Glastron should be going around 40+ MPH GPS with the 85 Merc and an average load. But, until you know what prop you now have on it and what RPM the engine is turning at wide open throttle, you really can't figure whether the engine is running at peak power, and what prop to put on the engine if it is running well.

 

[summitlt]

Re: Which Prop? Old merc

Took it out today. Ran a 33mph with a 770lbs load of people and gear. The prop is stamped 48-77346-A45-19P.

Afterwards went out with Just me and a buddy and ran 35mph. Its definatly not going to tach anymore as it is, gets up on plane really quick, especially with the lighter load.

I assume 19P is 19 pitch

 

[steelespike]

Re: Which Prop? Old merc

I wouldn't make any prop changes without a tach.You might check the setup.
If you don't have any ventilation problems the motor is probably too low.
The antiventilation plate(just above the prop) even with the bottom of the boat is considered a starting point.Raise the motor a hole at a time to a point just before venting becomes a problem.Be sure it pumps water.An inch could gain a couple of MPH.

 

[summitlt]

Re: Which Prop? Old merc

I'll see if I can dig up my tinytach, im sure im getting full revs out of it though. And Im sure it can handle more prop.