Symptoms that a prop change is needed

[TheRodFather]

I am ignorant to prop selection.  My boat is an 84 Landau Magnum TR, it is a 17ft aluminum with what seems like the same or very similar hull to a tracker of that era.  It has a 70hp Merc that has a fresh 30 over rebuild on it as of last summer.

 

It has always seemed like it revs pretty quick, gets on plane quick, but top speed isn't really anything to write home about.  In fact I never really reved it above about 4.5k because there just never seemed to be any resistance and I was afraid it was going to over rev.  I realize that the timing sets the max rpm, but like I said, it just didn't seem to be working all that hard for the revs.  It will get on plane quick and then stay on plane even if I bring revs back to 3k or so, so that's what I usually did to save gas, and because of the worn upper motor mounts it had since I bought the boat.

 

I replaced the motor mounts when I did the rebuild, the old ones the rubber bushings (uppers) were broken, the top of the power head could move forward and back about an inch because the rubber connecting the inner and outer sleeves sheared.  They were captured mechanically, by design, so I ran it, but it always made me a little Leary about going fast but now that everything is tightened back up, I am curious to know if there is more to be had with the combo.  This being my first boat, and not having ridden in many, I don't really know how it "should" feel.

 

What are some things that would tell me the current prop is a good fit, or not?

 

 

[OnthePotomac]

Merc should be able to tell you the peak RPM is for that engine.  If this will help, I have a 60hp Yammie on my 17'6" tin with a 14 pitch SS  prop and it peaks out full throttle smack on the motor recommended running max of 5500 RPM, but with a full 21 gal gas tank, two adults and gear the best I can get on calm water and the right air temp is around 34.7 GPS.

[slonezp]

https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

[TheRodFather]

3 hours ago, slonezp said:

https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

 

Thank you for the link, but my boat/motor isnt anywhere close to the choices in the dropdown.

[WRB]

First things first. Before assuming the pro is a problem check you engine height, it should be 3 1/2" from the plane surface to the prop shaft with engine level.

Second is rpm's turned at wide open throttle (WOT) must bed within the mfr's range, 5,500 to 5,800 rpm's for example. You can always back off the throttle if it over rev's. 

If engine over rev's you need a prop with more pitch or the hub clutch is slipping.

Tom

[Bankbeater]

I would check your motor first as @WRB suggested.  The only time I've ever had to replace a prop was when it was damaged.

[Way2slow]

Before messing with the prop, do as suggested.  Make sure you setup is right, engine height and trim angle.  If it takes a lot of trim up to get it to quit barging through the water, might want to look at increasing setback. 

 

Then make sure the motors is doing what it should be.  I know you recently rebuilt it, so may be time to do a compression and timing check to make sure after break-in nothing changed.  Also make sure the butterflies and carbs or throttle body are going fully open.

 

After all this, then you might want to look at the prop.  If it looks like a beaver had part of the blades for lunch, then it's time for rebuild or replacement.

The proper pitch should be able to turn within a couple hundred of recommend max with your normal load.   If more than 300 below max, then consider going down on pitch.

If it's more than 100 over max with normal load, might want to look at going up one pitch.

Don't use a light, empty boat unless that's how you run most of the time.  Two people, gear, gas, live well full etc, is how you want to check it if that's how you normally fish.

[Elkins45]

My boat is maxing out at 4400 if my tach is to be believed. It’s mounted as low as it will go. Before I go out and spend $300 on a new prop I’m going to raise it up a couple of holes and see what happens. All it will cost is a little time and some sealer for the bolt holes.

[Way2slow]

One word of caution when raising the engine height.  Don't get the anti-cav plate much more than in inch above the transom/hull if you don't have a water pressure gauge.   You never want your water pressure dropping below 16psi when on plan or making hard turns.

[WRB]

Before raising or lowering the engine height (not trim angle) measure where it is now.

The engine needs to be trimmed flat and parallel to the plane surface in the center of the boat bottom for standard hulls. You can use a 2x4 board (4 is actually close to 3 1/2") to eye ball the engines propeller shaft centerline, close enough for proper engine height. 

If the prop is damaged it needs to be repaired or replaced. The cavitation plate on the engine is about 1/2" above the prop blade. Most outboard engine props are 13 1/2" dia., putting the cavitation plate surface around 7" above the prop drive shaft centerline. The water pump intake holes are below the plane surface with the shaft at 3 1/2" below the plane surface so the engine doesn't suck in air.

Tom

 

 

[TheRodFather]

Thank you guys, I will check those things and report back when I get a chance