Yamaha 9.9 4Stroke what prop on 5ton catamaran? Fixing spun prop?

[Franziska]

Hi,



we cought a plastic bag on one prop and unfortunately the rubber bushing of the prop separated from the aluminum housing so that we now have a "spun prop" essentially the prop did not deliver power to the water anymore as it slipped.



We fixed it temporarily in McGiver style by two axial set screws between aluminium prop and bronze shafts but I guess I need a replacement prop.

Or can this be repaired with a new bushing? Unfortunately cheap shear pins seem to be a fix of the past.



I'm wondering if it's not a good moment to improve performance now if I replace the standart prop now.

The damaged prop came as standard with the engine and did not get matched to the boat. It's a 11 3/4 x 7R.



The boat is a 35ft cruising catamaran with deep vee, Wharram style hulls. Liveaboard displacement about 4.5-5tons.
We have two Yamaha 9.9 4strokes from 2014 as main engines. Max speed so far with both engines is about 6.5kn in calm water.



Would reducing pitch possibly improve our top speed?

I found two possible versions, as I will replace both engine props I'm leaning towards the cheaper one:


This, rather expensive one:
https://www.buitenboord.com/propelle...fdgfjjv9hmjjq7


And this cheaper one:
https://www.svb24.com/en/aluminium-p...-6-9-9-hp.html

[rslifkin]

Do you have any kind of tachometer on the engines to see what RPM you're reaching at full throttle? And are these the high thrust Yamahas? I'm guessing they are based on the quoted prop size.

[Franziska]

Unfortunately no rev meter.


Yes, these are the typical Yamaha 9.9 high thrust 4stroke motors in long shaft version.


See attached.

[rslifkin]

The stock prop on the high thrust 9.9s is probably at least close to what you need. In my opinion it would be worth picking up a pair of cheap battery powered inductive tach/hour meters to put on the engines and see if you're reaching rated RPM at WOT. If you are and you're not hitting the rev limiter, the current props are probably about the right size. If you're not reaching rated RPM, then I'd go for lower pitch replacements.

As far as the spun hub, a prop shop may be able to fix it, but not all outboard props have hubs that can be replaced. For some, a spun hub means it's time for a new prop.

[MicHughV]

You can have speed or power, but not both.

Those engines typically can run up to 5,000 rpm, but a good operating range is around 3,500 rpm for good fuel economy.

Adding pitch is the way to add speed, but only of the engine is capable of turning it without lugging it down, which is unlikely, and which may require a reduction in prop diameter, so little to be gained there.

Either way, little benefit is to be gained by changing the prop.

Yamaha is well known for matching their props right for their engines, and I'd stick with what was on there.

The difference in the above props is the thickness of the blades, and whether or not that will make a difference for you would be difficult to ascertain.

In the event you nick a prop for any reason, it would be good to carry a spare, and if it were me, I'd opt for the less expensive one as being the more ulitarian.

[Munoz]

That SVB prop is not designed for the high thrust motors.

I would either stick with the stock prop or drop down to the 11³/4 x 7 prop (this is what I run on a 35ft mono, gives 5 knots at 4000 rpm, 6 knots at 4900 rpm (full throttle).

I use a cheap rev-counter like this:

https://www.amazon.co.uk/Timorn-Repl.../dp/B09DL5TYLT

[Franziska]

Hi guys,

ok, the SVB are out. Shame, they had a good pricetag.

Looking at non OEM props I found these and they would even deliver to us in the Marina in Tunisia:

https://www.aliexpress.com/item/1005...yAdapt=glo2deu

Could this work?

[Munoz]

Originally Posted by Franziska

Hi guys,

ok, the SVB are out. Shame, they had a good pricetag.

Looking at non OEM props I found these and they would even deliver to us in the Marina in Tunisia:

https://www.aliexpress.com/item/1005...yAdapt=glo2deu

Could this work?

Not very well, again that prop is for a standard gearbox (2x reduction), not the high thrust (3x réduction). You really need to make sure the diameter (11¾ inches) is correct

[Franziska]

Originally Posted by Munoz

Not very well, again that prop is for a standard gearbox (2x reduction), not the high thrust (3x réduction). You really need to make sure the diameter (11¾ inches) is correct

Thanks, will heed that comment.

[Franziska]

Anyone see something low budget available?
I have hard time finding something less than 80€..

[Jammer]

Originally Posted by Franziska

Would reducing pitch possibly improve our top speed?

No. Stay with the stock prop.


I had the Yamaha 8 HP high-thrust on a previous sailboat. The 8hp and 9.9 are exactly the same engine except that the 8 HP is derated by using a different carburetor.


The stock prop technically leaves you overpropped, with too much slip and too low RPMs. But I actually switched to a lower-pitched prop, same blade count, same diameter, and it had zero effect on speed with RPMs being nearly the same (very slightly higher).


I believe that the real problem is that the prop area (blade area times number of blades) is too small to provide the necessary thrust regardless of pitch. Since the outboard cannot accept a larger diameter prop, there's not any practical way to improve things.

[Tedd]

Originally Posted by rslifkin

In my opinion it would be worth picking up a pair of cheap battery powered inductive tach/hour meters to put on the engines and see if you're reaching rated RPM at WOT.

I second that. I bought a RunLeader tach on Amazon for about 25 bucks that does the job quite nicely. Well worth it.