thinking of changing prop what should I consider

alan ber · Feb 9, 2013

I have a 1984 mercruiser
model 488
4 cylinder
Alpha 1
present prop # 4214 aluminum
boat is a 21 ft walk around cuddy sea ray

I don't push boat, used for intracoastal fishing , just thinking about
changing hoping too get a little more power.

No trim tabs, infact tabs were never installed when boat was new

I realize trim tabs will help and planning on doing at some point but in mean while considering changing prop.

Presently throttle handle all the way down, at full speed, engine rpm never red lines and cruises around 4000.
engine makes no unusual sounds at full throttle., or sounds of slipping , runs good.

Not sure what prop mfg suggests on this engine

what should I consider too get more speed,? larger or smaller then mfg reccomends
Stainless?
5 blade?

Boat stays on trailer , lower unit and bottom of boat clean like new.

thanks for your help

achris · Feb 9, 2013

Re: thinking of changing prop what should I consider

The less blades are more efficient, so stay with a 3 blade. And stainless is better than aluminium.

The suggested revs for a 488 is 4400 to 4800, so you are WAY over-propped. Drop down 3" to 4". (I have no idea what a '4214' propeller is!) If you continue to run like you are, you are going to kill that engine....

The engine manufacturer only recommends a rev range for the engine at WOT, not (and NEVER) a prop size. The 'recommended' prop is the one that allows the engine to reach the recommended revs at WOT....

Chris......

Silvertip · Feb 9, 2013

Re: thinking of changing prop what should I consider

To add a bit to the last sentence in the above post -- RPM at WOT should be with what you consider an average load. In other words don't make WOT runs to determine rpm with a light load when you typically run heavy (or vice versa). "Doesn't red line" means nothing as there is no red line on a marine tach. Give us an RPM number at WOT with an average load.

steelespike · Feb 9, 2013

Re: thinking of changing prop what should I consider

Check under the prop nut for numbers,
You did pretty good getting us the performance numbers but a little vague about wot rpm and speed.
Your gear ratio may be stamped on the out drive.
Actually a 3 blade may be slightly faster than 4 blade but a 4 blade will usually have a better hole shot and stay on plane easier at reduced throttle.
4 blade will usually handle better at speed and around the dock.

achris · Feb 11, 2013

Re: thinking of changing prop what should I consider

steelespike said:
... Actually a 3 blade may be slightly faster than 4 blade but a 4 blade will usually have a better hole shot and stay on plane easier at reduced throttle.
4 blade will usually handle better at speed and around the dock.

SS is absolutely spot on with these statements. 4 blade will also shed weed better, due to the higher rack angle and smaller individual blade size. The big 'BUT' is that most 4 blade props (and large diameter 3 blade) increase bow-lift. On most boats that's ok, but on a well designed deep Vee ocean-going hull, that seriously BAD news..... The prop tries to hold the bow up over a crest for as long as it can. When it can't hold the boat any longer, it lets go, and the hull slams. It's bone-shattering and very disconcerting. I had a 3 blade Mirage on many years ago, then changed to a 4 blade "Offshore" series (now called "Vensura"). The Mirage was terrible (on my boat) and the Offshore was better, but the best prop on my boat has been a bog stock, out of the box, SS "Vengance"....

The best way to find the best prop for you..... try them out....

Chris......

jestor68 · Feb 11, 2013

Re: thinking of changing prop what should I consider

alan ber said:
I have a 1984 mercruiser
model 488
4 cylinder
Alpha 1
present prop # 4214 aluminum
boat is a 21 ft walk around cuddy sea ray

Presently throttle handle all the way down, at full speed, engine rpm never red lines and cruises around 4000.

Are you saying that the most rpm the motor turns at wide open throttle is 4000 rpm?

As a place to start; the Merc prop selector recommends a 19" pitch prop.

steelespike · Feb 12, 2013

Re: thinking of changing prop what should I consider

Not trying to be argumentative but ;it's my understanding the 4 blade provides stern lift thus the suggestion it improves hole shot
and the ability to stay on plane better at reduced throttle.
The 4 blade is often suggested for a boat that has trouble with porpoising.

achris · Feb 13, 2013

Re: thinking of changing prop what should I consider

As a Merc dealer back in the 90s, I had access to all the different props, and was encouraged by Mercury to take them all out and try them on as many different boats as possible, including my own.

Here's what Merc say about the 4 blade Vensura (used to be called the 'Offshore' series)...

VenSura

The VenSura is a tough, durable saltwater propeller for single or twin outboard applications, 135hp and above. The VenSura’s four-blade design delivers better acceleration and holding than a three-blade propeller. The VenSura’s geometry provides strong bow lift – even in rough seas.

Available in 17- to 23-pitch
Right-hand and left-hand rotation

VenSura | Propellers | Mercury Marine

And from personal experience I can attest to that fact....

jestor68 · Feb 13, 2013

Re: thinking of changing prop what should I consider

achris said:
As a Merc dealer back in the 90s, I had access to all the different props, and was encouraged by Mercury to take them all out and try them on as many different boats as possible, including my own.

Here's what Merc say about the 4 blade Vensura (used to be called the 'Offshore' series)...

Depending on the design, the 4 blade prop is considered a "bow lifter". It provides more stern lift on take off, and more bow lift at speed.

steelespike · Feb 13, 2013

Re: thinking of changing prop what should I consider

So are we saying most 4 blades provide bow lift or just the Vensura and those like the Vensura?
If 4 blades are said to stay on plane better at reduced throttle they must provide bow lift at speeds closer to wot.
I guess we could say that a 4 blade intended for a pontoon isn't intended to provide much bow lift.

jestor68 · Feb 13, 2013

Re: thinking of changing prop what should I consider

steelespike said:
So are we saying most 4 blades provide bow lift or just the Vensura and those like the Vensura?
If 4 blades are said to stay on plane better at reduced throttle they must provide bow lift at speeds closer to wot.
I guess we could say that a 4 blade intended for a pontoon isn't intended to provide much bow lift.

Do pontoon boats have a bow?

jimbo_jwc · Feb 13, 2013

Re: thinking of changing prop what should I consider

My 87 196 CC came with this 48-78118-a40-17p prop its a 3.0 L Mercruiser Alpha One outdrive . I have just bought a 4 blade same pitch for grip climbing waves of Lake Erie

achris said:
As a Merc dealer back in the 90s, I had access to all the different props, and was encouraged by Mercury to take them all out and try them on as many different boats as possible, including my own.

Here's what Merc say about the 4 blade Vensura (used to be called the 'Offshore' series)...

VenSura | Propellers | Mercury Marine

And from personal experience I can attest to that fact....

achris · Feb 13, 2013

Re: thinking of changing prop what should I consider

jestor68 said:
Depending on the design, the 4 blade prop is considered a "bow lifter". It provides more stern lift on take off, and more bow lift at speed.

Where in the literature does it say a 4 blade gives 'stern lift' and 'bow lift'?

steelespike said:
So are we saying most 4 blades provide bow lift or just the Vensura and those like the Vensura?
If 4 blades are said to stay on plane better at reduced throttle they must provide bow lift at speeds closer to wot.
I guess we could say that a 4 blade intended for a pontoon isn't intended to provide much bow lift.

Well, what I would do is an internet search on manufacturers of 4 blade props and have a look see what they say about them....

jestor68 said:
Do pontoon boats have a bow?

I assume that question is rhetorical...

steelespike · Feb 14, 2013

Re: thinking of changing prop what should I consider

jestor68 said:
Do pontoon boats have a bow?

It's the pointy end.lol

Tail_Gunner · Feb 14, 2013

Re: thinking of changing prop what should I consider

Four blades generally provide "Over All Lift". While merc's line may provide bow lift and i have little experience with them at the end of the day the stern will ride higher with a 4blade..hence the term stern lifter...Now that lift does come from the stern..but the general attiude of the boat would would have the bow pointed upwards

NewbieCVX17 · Mar 29, 2013

Re: thinking of changing prop what should I consider

Hi I'm new here, but last yr I bought a hub and replaceable blades from canadapropeller.com. It's a very easy system, if you break a prop, jump in the water and change it within 10 minutes. I gave them the necessary info, and they gave me the proper pitch and # props for me. Justa thought!

achris · Mar 29, 2013

Re: thinking of changing prop what should I consider

NewbieCVX17 said:
Hi I'm new here, but last yr I bought a hub and replaceable blades from canadapropeller.com. It's a very easy system, if you break a prop, jump in the water and change it within 10 minutes. I gave them the necessary info, and they gave me the proper pitch and # props for me. Justa thought!

They're fine for smaller engines, but high horsepower engines (anything over about 60hp) should stick to a single casting prop...

thinking of changing prop what should I consider

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