A prop question (enertia or other versus older props)

[nnl1987]

Currently have a a 4.3l v6 omc fresh rebuild. I've experimented a bit. The prop it came with, old aluminum prop 14.5 x 21 wot at 4300rpms full tank two adults. I got a dirt cheap stainless steel prop same pitch and diameter (14.5 x 21) 3900 rpms same scenario with weight factors (once again this is a very old prop same age as probably the omc engine.

My max wide open throttle is 4300-4600. My question is what kind of results would I get with something newer in design (say a mercury enertia or equivalent other) I've got a chance to get a mercy enertia 15x15 stainless steel. But don't really know what to expect with it. The price is definitely right on it but don't want to jump on it until I get some further input. I don't know if it will end up overreving the engine. Thoughts?

 

[dingbat]

Currently have a a 4.3l v6 omc fresh rebuild. I've experimented a bit. The prop it came with, old aluminum prop 14.5 x 21 wot at 4300rpms full tank two adults. I got a dirt cheap stainless steel prop same pitch and diameter (14.5 x 21) 3900 rpms same scenario with weight factors (once again this is a very old prop same age as probably the omc engine.

My max wide open throttle is 4300-4600. My question is what kind of results would I get with something newer in design (say a mercury enertia or equivalent other) I've got a chance to get a mercy enertia 15x15 stainless steel. But don't really know what to expect with it. The price is definitely right on it but don't want to jump on it until I get some further input. I don't know if it will end up overreving the engine. Thoughts?

A drop from a 21 to a 15 pitch prop will increase rpm roughly 1000 rpm.

 

[nnl1987]

By math that is correct as well. Just didn't know about the diameter change and the over all design of a old prop versus something newer? Not even sure if they would be all that different in there designs that would impact the rpms.

 

[dingbat]

By math that is correct as well. Just didn't know about the diameter change and the over all design of an old prop versus something newer? Not even sure if they would be all that different in there designs that would impact the rpms.

You can play with blade design and the cup to change bow and or stern lift, but that’s about it.

Diameter has little effect unless the lower units doesn’t have the clearance for the larger prop.

Pitch is in effect the gear ratio. You can’t go from a 21:1 to a 15:1 ratio w/o increasing the rpm to maintain the same output speed.

By chance, im running a 15” x 15” Enertia on my boat. Pushes my 22 foot, # 5,200 offshore hull with 225 HP outboard at 5,850 rpm.

 

[Pmt133]

If you're going to start playing around with testing props... I'd confirm the tachometer is accurate. You'd be amazed how off and inconsistent they can be. Once you know what you've got, get some GPS speeds of the vessel at various RPM then compare props. I try to test on the same day or at least same atmospheric conditions and temp.

 

[Scott06]

Currently have a a 4.3l v6 omc fresh rebuild. I've experimented a bit. The prop it came with, old aluminum prop 14.5 x 21 wot at 4300rpms full tank two adults. I got a dirt cheap stainless steel prop same pitch and diameter (14.5 x 21) 3900 rpms same scenario with weight factors (once again this is a very old prop same age as probably the omc engine.

My max wide open throttle is 4300-4600. My question is what kind of results would I get with something newer in design (say a mercury enertia or equivalent other) I've got a chance to get a mercy enertia 15x15 stainless steel. But don't really know what to expect with it. The price is definitely right on it but don't want to jump on it until I get some further input. I don't know if it will end up overreving the engine. Thoughts?

Of your tach is accurate you should be looking at a 19” enertia is a great prop if you can afford it or find one used

 

[Chris1956]

Props with more rake tend to be higher performance. However, a lot depends on the hull and power-to-weight ratio. Some hulls can only go so fast, regardless of power. Usually, you need to be able to trim the motor, lifting the bow to gain speed. Some boats trim up more than others. My old winner was a straight deep vee, bow to stern. It would not trim up, but the hull rose up in the water as you went faster. At high speed, she would ventilate in the corners. Just the nature of the hull design.

Gee, what are the max recommended RPMs for your engine? isn't 4600RPM about max for that engine? If so, and you drop pitch 2", you will add about 400 RPM to your WOT RPM. You run that RPM for any length of time, and you will no longer need to worry about getting the correct prop, as you will be under tow to the repair shop.

 

[nnl1987]

If you're going to start playing around with testing props... I'd confirm the tachometer is accurate. You'd be amazed how off and inconsistent they can be. Once you know what you've got, get some GPS speeds of the vessel at various RPM then compare props. I try to test on the same day or at least same atmospheric conditions and temp.

Tachometer is correct. I've been testing on the same day a few hours a part from each other. Compared the tach on the dash with a digital timing light.
My boat as you would expect with a stern drive is a fiberglass deep v hull. It's heavy haven't weight it yet (donzi ragazza 19'). It's got a set of trim tabs that I've been playing with those help with being heavy in the ass.

The max wot is 4300 to 4600. So the aluminum prop that it came with is about right for the it (4300rpms with it). l've just been on the hunt for a decent stainless steel prop that will suit the boat. I will be looking at a 19inch stainless steel in the future. Not sure if I could get something 18inch they seem to be in incriments on the odd numbers.

 

[Scott06]

Tachometer is correct. I've been testing on the same day a few hours a part from each other. Compared the tach on the dash with a digital timing light.
My boat as you would expect with a stern drive is a fiberglass deep v hull. It's heavy haven't weight it yet (donzi ragazza 19'). It's got a set of trim tabs that I've been playing with those help with being heavy in the ass.

The max wot is 4300 to 4600. So the aluminum prop that it came with is about right for the it (4300rpms with it). l've just been on the hunt for a decent stainless steel prop that will suit the boat. I will be looking at a 19inch stainless steel in the future. Not sure if I could get something 18inch they seem to be in incriments on the odd numbers.

Historically yes only available in odd pitches, some 4 blades were only available in even pitches (Michigan Votex 4 blade for example), so you would take a pitch one inch lower than the three blade and keep rpms up with the extra blade.

Many of the high end SS props are now available in inch increments as some of the newer engines have tighter power bands. Enertias are avilable in inch increments. As you saw lower RPM with the SS prop one that bites like enertia or a revolution 4 you wont get as much clip and thus lower WOT rpm. IF you got a 19" version of the SS prop you ran you would probably be right where you want to be rpm wise

 

[Pmt133]

Tachometer is correct. I've been testing on the same day a few hours a part from each other. Compared the tach on the dash with a digital timing light.
My boat as you would expect with a stern drive is a fiberglass deep v hull. It's heavy haven't weight it yet (donzi ragazza 19'). It's got a set of trim tabs that I've been playing with those help with being heavy in the ass.

The max wot is 4300 to 4600. So the aluminum prop that it came with is about right for the it (4300rpms with it). l've just been on the hunt for a decent stainless steel prop that will suit the boat. I will be looking at a 19inch stainless steel in the future. Not sure if I could get something 18inch they seem to be in incriments on the odd numbers.

Sorry. Didn't mean to offend if you took it that way by the way. People just tend to believe without verifying what they see. You've already taken a major step in the right direction and sounds like you are going about it properly.

I'd want to be on the higher end of those RPMs that way if you have a load of people it doesn't drop out of range but that's more a personal preference.

 

[nnl1987]

Sorry. Didn't mean to offend if you took it that way by the way. People just tend to believe without verifying what they see. You've already taken a major step in the right direction and sounds like you are going about it properly.

I'd want to be on the higher end of those RPMs that way if you have a load of people it doesn't drop out of range but that's more a personal preference.

No offense was taken. The 18" inch pitch should bring it closer to the higher end of the rpms. Why I was wondering about only ever seeing them on odd numbers. I will be looking at either 18 or 19.

 

[jimmbo]

I have been running an Enertia since 2006. It is a very good Prop, and it has a Tenacious Grip on the Water. Your Boat will likely ride Higher out of the Water, the entire Boat, not just the Bow. However a 15X15 is probably not enough Pitch. Your old AL Prop, was it cupped? The SS you put on, was it an OMC SS Prop? If so it was cupped, and that could account for some of the Rpm drop, and a better grip on the Water(less slippage) could account for the rest of the Rpm drop.

 

[nnl1987]

I have been running an Enertia since 2006. It is a very good Prop, and it has a Tenacious Grip on the Water. Your Boat will likely ride Higher out of the Water, the entire Boat, not just the Bow. However a 15X15 is probably not enough Pitch. Your old AL Prop, was it cupped? The SS you put on, was it an OMC SS Prop? If so it was cupped, and that could account for some of the Rpm drop, and a better grip on the Water(less slippage) could account for the rest of the Rpm drop.

Not sure how to tell if it's cupped. The subtle between props is unknown to me at the moment. First time I've owned a bigger motor besides 25hp 2stroker. I could take a picture of it along with pulling up the old model numbers. The stainless steel one I put on was indeed a omc prop.

I never did put the aluminum and stainless steel side by side to look at them closely. I will start there along with a few pictures.

 

[QBhoy]

Hi. Couple of things from me.
So that enertia prop in particular, at that size..isn’t the normal enertia prop. That’s almost certainly the enertia eco prop. They have a huge diameter in comparison. So much so, that should you have a trim anode fin or similar on the drive, it will contact it. Not usually a prop that’s generally considered ideal for your application perhaps. The enertia prop usually commands such a high price that it’s not probably wise to spend loads on one, until you know exactly what size you require. Modern props can sometimes play outside the general rules of expectation in terms of what rpm to expect compared to another general type prop previously. The tempest plus for example, can commonly show cruising rpm and top end rpm similar to a prop of an inch or two more in pitch.
If I was you, I’d be looking for a more steadfast and predictable prop like a laser 2 or similar. The 21” laser 2 for example will have a diameter under 14” and may bring you just into the targeted range.
To complicate things further, props like this can also end up giving more rpm than a previously tried ally prop of the same pitch (despite the rules suggesting that a stainless will drop rpm compared to the ally of same pitch). Just because it can lift the hull more, allowing more rpm and speed to be achieved from the engine…a hull pushed through air, is easier to do, than a hull pushed through water..allowing the engine to produce its best