Propellor too small: first next size ?

[Vronsky]

I'm hitting the 'rev limiter' each time I go WOT (DF115A)
Motor has the standard issue aluminium 3x13.75x17 size
I read elsewhere that 14" x 19 is more common for the boat & engine I have.

What would be the next size up for aluminium, and also in steel as I understand a steel prop brings out the best in this engine ??

THANKS,
V.

[Moonlighter]

Props need to match the engine to the boat and load, so there is no such thing as a standard issue.

Having said that, the 17” pitch prop that you currently have is NOT the normal starting point for a Suzuki 4 stroke. 17” is a common starting point formany 2 stroke outboards like Yamahas, but Suzukis are geared totally differently and trpically run larger diameter and pitch than that.

On my DF115, I run a Suzuki stainless steel 14 x 20” 3 blade prop and it is perfect on my boat, I can still hit just over 6100rpm at WOT. The 115 is the max rated hp for my hull.

This prop is a very popular choice for may poeple running Suzuki 115’s and 140’s that are well matched to their particular boats.

My boat is 5,5m long cuddy cab and the boat/motor/trailer as I currently have it set up, including fuel and gear, weighs in at around 1200kgs.

[Vronsky]

My boat appears very similar, same max power. My understanding is that the aluminium 3x13.75x17 was on the motor when it came from the distributor. Technician did a -very- short sea trial (not actually at sea but some canal nearby the dealer) and declared it was okay.
Are the steel prop sizes a bit different from the alu range ?
Could the (for this model popular) 14x19 I mentioned also be an aluminium, and it's corresponding size in steel the 14x20 you refer to ?

Thanks
V.

[Moonlighter]

It is rare nowadays for a motor to be supplied to a dealer with any propellor - the dealer will usually specify a prop to be supplied based on their knowledge and experience of fitting that motor to a particular hull. The purchaser will usually be asked - do you want a stainess or alloy prop? Then the price of the selected kind of prop then is included in the deal.

When setting the motor up, they should water test the boat and confirm that the prop is suitable, and if not, change it for one that is.

To determine the optimum size prop, it is NECESSARY to open the engine right up to full throttle and ensure that the engine will rev to, in the case of the Suzuki, about 6200rpm at WOT. But to not hit the rev limiter. This will ensure that the engine is then operating in its optimum rev range at cruise speeds and will have good hole shot acceleration.

This is where dealer experience comes in - due to Suzuki’s engine running in requirements, it is not acceptable to rev a brand new engine to WOT before it’s properly run in.

So an experienced dealer will select a prop based on their vast experience with putting that motor on similar boats. As I said, the Suzuki 14 x 20 is often the go-to choice for boats like mine where 115 is the max hp and the motor and hull are a good conbination.

So thats what my dealer supplied to me, and it was a great choice. He said to me to run the motor in, then do a WOT test, and let him know the max revs the engine reached at WOT when trimmed right out.

If that test shows the prop is wrong, the dealer should be contacted and requested to replace it at their expense. They usually have a stock of props for this purpose. If yours is a new motor fitted up by your dealer, take it back and tell them its over-revving and you want the prop replaced with a suitable Suzuki prop that is correctly matched to your boat.

Yes, the sizing is slighly different between the Suzuki ally and stainless props. Particularly diameter is often a 1/4” different.

A Suzuki stainless steel prop is superior to the alloy one. It will:

- Grip the water better, meaning there will be less tendency to lose traction in turns and in rough or aerated water and it will accelerate faster
- be more efficient, because the blades are thinner and flex less
- be more resistant to dings and scrapes.
but it will cost considerably more than an alloy one.

The downside of any stainless prop is that if you hit any hard objects there is a greater chance of the engine’s lower unit being damaged as the shock of the strike is transmitted more directly to the gearbox than an alloy prop - which would bend or break and thus absorb the shock. So if you are regularly boating in shallow rocky or log filled areas where bumping the bottom and hard objects is a common thing, an alloy prop is a better choice, yes it will get dinged up, but it is far cheaper to replace a dinged alloy prop than a gearbox!

But if you predominantly boat in open waters where prop strike is unlikely, a stainless prop will perform far better overall.

Finally, as a rough guide, every 1”” change in prop pitch (assuming the same model and material) changes revs by about 150-200rpm. So if you go from a 17” alloy to a 19” alloy, max revs will be reduced by approx 300-400rpm. An engine that is easily hitting the rev limiter will need at least an extra 2” of pitch to get it into the proper rev range. Maybe more. It can be a trial and error job to an extent.

But I do think the choice of a 17” prop to start with was a very poor one.

Best plan - talk to your dealer, work with them to get the optimum prop for your boat. If you want to upgrade to stainless, you will expect to pay extra. Good dealers always work with owners constructively to get the right prop sorted out.

[D'oh999]

My Dealer started with a 14X19, Alum. Which is close to my previous Yamaha on a similar weight boat (shorter but wider and deeper Dead Rise.)

Hit the rev limiter, then a 14x21, again hit the limiter, 14X23 Alum. maxed out at 6100 RPM. The 130 Yamaha would never have turn that much pitch 13 1/4X19 was the most prop and still 500 RPM below max rated.

I had to wait for a Stainless, and suspected a 200 RPM drop, but it didn't drop at all, still 6100 RPM, lightly loaded. But man, the SS sure made a difference in Acceleration and cornering. A lot more like the 2 stroke I remember.