Aluminium vs Stainless propeller

[jointventureII]

On a Bravo 2.

I get 26 or so knots from the 320hp Mercruiser diesel pushing my Bertram 25. This feels a bit slow and, with a 2.2 ratio gearset, there would be room to play. It currently spins a 17.75 inch dia prop at 21 inch pitch.
I tried a 17.25 dia prop / 25 pitch and it didn't make any difference - not to the top speed or the revs. I have read a fair bit about this and just recently finished the 2 year IIMS Surveying diploma, which contained a fair bit about props, prop selection, pitch, diameter etc etc.

Therefore I'm toying with the idea of switching to a stainless prop however I have some doubts about:
-The additional weight of the prop
-The forgiveness it may offer compared to alu (when hitting something in the water)
-Galvanic issues, of which I suffer none at the moment.

What I'm after is anyone with actual experience of having made the switch....be it outboard or sterndrive....

 

[simonfraser]

Ali prop on my previous outdrive was easy to damage, SS one on my outboard is not suffering any damage, same cruising area / speed

 

[Rappey]

-Galvanic issues, of which I suffer none at the moment.

Your aluminium prop is on a stainless shaft, surely that's worse than stainless on stainless.
The advantage of a stainless prop is better durability and performance. Thinner and more rigid blades offer less flex, higher top end speed due to less drag from the thinner blades.

 

[julians]

I've had stainless props onvarious out drives and out boards for years now with no galvanic or durability issues.

But then I've never knowingly hit anything with them either.

 

[Mr Googler]

Hopefully @BruceK will see this. He has 1st hand experience of many many props…….not that he wants to have

 

[Rappey]

I would suggest there is no advantage at all to aluminium props other than the price. Stainless is also far more repairable

 

[rogerthebodger]

I have a fabricated stainless steel propeller. I machined the boss and lazer cut the blades and tig welded then to the boss at the required angle

 

[PowerYachtBlog]

With Volvo easy answer. Given the choice I would always choose aluminium, for a simple reason, if you hit something at speed you throw the prop away, and minimal damage is done to the stern drive.
With stainless/steel if you hit say bye-bye to stern drive.
I am saying the above by experience. Some years ago I got a five inch thick plastic rope, with A series props, props destroyed, nil damage to stern drive.

I had A series prop (three and four), which tend to loose speed over the years, but the B series (four-four) on duo-props did not lose a point of a knot of cruising.

 

[Keith-i]

Is the damage risk really much greater with stainless steel? All props of that size would have a rubber bush inside to give protection. If you can avoid hitting hard things often then stainless is the way to go for performance.

 

[jointventureII]

Thanks all.

The main reason behind this, is that (as I'm seeing no change in performance or engine RPM with a considerably higher pitched albeit lower dia prop) I think the blades may be flexing too much and power being lost.

I can actually change the prop with it in the water so if I can get hold something to test then I'll give it a go.

 

[spannerman]

With Volvo easy answer. Given the choice I would always choose aluminium, for a simple reason, if you hit something at speed you throw the prop away, and minimal damage is done to the stern drive.
With stainless/steel if you hit say bye-bye to stern drive.
I am saying the above by experience. Some years ago I got a five inch thick plastic rope, with A series props, props destroyed, nil damage to stern drive.

I had A series prop (three and four), which tend to loose speed over the years, but the B series (four-four) on duo-props did not lose a point of a knot of cruising.

Its not true that if you hit something with a stainless prop you say goodbye to the drive. I rebuild at least 12-15 Volvo and Mercruiser drives plus several dozen outboard lower gearhousings each year because of grounding incidents. There are a lot of hidden reefs off the coast of Norway. We weld on new original fins and repaint the housing, I replace bent shafts and bearings and seals as required and a new prop and they are as good as new. Also sterndrives have a breakpoint coupling between the upper and lower sections so the shock loads don’t affect the really expensive parts in the upper housing. These repairs are nearly all insurance jobs.
Also once you have tried a stainless prop I doubt anyone would go back to aluminium because of previously mentioned advantages.

 

[rogerthebodger]

The way to prevent damage to the drive system is to include a shear pin into the propeller boss as some small outboards

 

[Mr Googler]

Advantages of Aluminum Props​

To start, aluminum props are very economical, often costing a third of what a comparable, stainless steel propeller will run. This is one of the top reasons why they’re selected so commonly for boats and are used by manufacturers, due to the lower cost of outfitting a boat.

Given their low cost of acquisition, it’s pretty common to see them used as a spare propeller as well because, for a low price, you gain a little insurance in the off chance you run into issues with your boat, such as a prop blade shearing off due to an impact while you’re out and about. With a few simple tools onboard, you can make repairs to get back to shore, where you can get a repair or replacement for your existing prop.

An aluminum propeller can also be easy to repair, especially when compared to a stainless steel propeller.

Disadvantages of Aluminum Props​

Though there are some notable advantages, there are also some disadvantages to aluminum props. To start, aluminum is a strong but flexible material, which is among the reasons why it’s used in many types of boats and aircraft. However, this flexibility can cause the pitch on your propeller blade to change, which can reduce performance, especially on higher horsepower boats at high speeds. If performance is vital to your boat’s purpose, it may be worth looking at the more rigid stainless steel prop options.

If you’re expecting to pull all the performance you can from your outboard engine, another potential disadvantage is the thickness of an aluminum prop blade. Because aluminum blades are thicker than stainless propeller blades, they create more drag in the water, which lowers your top speed. If higher speeds are important to your operation, a stainless steel prop will make it easier to reach a higher top speed due to reduced drag.

The third disadvantage of aluminum propellers is that they are more likely to be damaged. If you’re only running in sandy conditions, this may not be an issue, but if there’s a lot of driftwood, rocks, or underwater obstacles, you’ll find yourself digging out a spare prop much faster with an aluminum prop on your outboard motor that you’ll see on a stainless steel prop.

Stainless Steel Propellers​

Though you may initially balk at the higher cost of an average stainless steel propeller at your local prop shop, that cost also provides you with a range of options that will boost your boat’s performance and provide superior durability.

Advantages of Stainless Steel Props​

To start, a stainless steel propeller will deliver better performance in an average 3-blade prop. At the same time, you won’t lose the better handling and superior hole shot that you’d expect from a lower-pitch propeller. This makes this type of propeller more efficient in the long run, making up some of the expense of purchase.

A stainless steel prop will also see superior durability and longevity. The same blow that may snap an aluminum propeller blade will often simply glance off of a stainless steel propeller, due to the higher level of durability as a simple aspect of using stainless steel.

The high performance that these propellers produce is caused by thinner blades that reduce drag and a stiffer, stronger profile that is able to cut through the water more efficiently. This loss of drag makes it easier for your motor to reach its true top speed, with superior efficiency.

Because stainless steel props operate more efficiently, you’ll experience better performance that leads to exceptional fuel efficiency in the motor, allowing you to get more boating in for less expense. These aspects can often defray much of the cost of upgrading to a quality stainless steel prop.

Disadvantages of Stainless Steel Props​

For all these advantages, there are disadvantages as well. The main disadvantage of a stainless steel propeller is its cost. As a more expensive option, these propellers will cost more at the onset, but this cost is often defrayed through lower overall operational expenses, such as superior fuel economy, better performance, longer lifespan, and better durability.

 

[jointventureII]

Thanks again all

In terms of making the change - if changing like for like, then I would assume the stainless prop would give a slightly lower RPM, being a bit heavier and stiffer

 

[Rappey]

then I would assume the stainless prop would give a slightly lower RPM

I would have thought higher rpm with exactly the same stainless as its easier to turn, less drag. The weight of the prop is pretty irrelevant