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Prop for Yamaha 8hp 2 stroke


Ed

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I've got an 8hp Yamaha 2 stroke with the regular prop on it, but really could use a bigger prop for moving the boat (900kg 8.5 multihull) in and out of the berth, reverse is painfully poor with the exhaust coming through the prop

 

Yamaha website just made me more confused, which prop should I be looking at?

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Do you know the size of your current prop? It should have a number x another number, being diameter and pitch.

 

Assuming you can't really change the diameter of your prop (cause its the largest you can fit on the outboard) you can change the pitch to suit what you want to do.

 

A bigger pitch will go faster at full speed. If you want more power at low speeds, you can go for a smaller pitch. This will give a slower top speed but will be more efficient at slower speeds.

 

For slow speed maneuvering, you want the biggest prop you can get. You will need about 10% or 15% of the prop diameter as clearance under the ventilation plate. I'd be surprised if you don't already have the biggest diameter that will fit.

 

The prop size for the Yamaha should be stamped on one blade of the prop, near the base.

 

The pitch is how far the prop will travel forward in the water, if it didn't slip, in one full turn. This can be visualised as the distance a screw goes into a piece of wood on one full turn.

 

Your prop may be sized for a fast dinghy (say a 12 ft tinnie) and designed to plane, so really big pitch. If you only want to move at slow speeds, a smaller pitch is required, and is far more efficient. It may be that if the pitch is too great, the flow is actually stalling over the blades and its acting more like a paddle than a screw.

 

Edit - the other multi guys will probably know, but you may just need to ask for a 'high thrust' prop. I.e. a prop designed for slow speeds / work boats etc, as opposed to a fast prop, designed for planning boats. Most multi guys with an 8 hp outboard would use the 'high thrust' prop. (I wouldn't know, I've got a lead mine)

 

Diameter is the first number, pitch the second. If pitch is bigger than diameter, its a 'fast' prop. If pitch is smaller than diameter its a 'slow' prop, but will perform better at slow speeds, i.e. maneuvering.

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http://www.propellers.co.nz/yamaha-propellers/

 

Google Solas Propelors. Nz made. The rubber clutch inside thier props are harder rubber and wont slip like the Yamaha ones do. And is cheaper.

 

The genuine Yamaha prop keeps slipping when the prop comes out of the water going through waves. We nearly ended up on the break water in Okahu Bay trying to motor back in westerley front when the Yamaha prop was slipping. 

 

Looks like they are making a high thrust, four bladed, low pitch prop soon. Try one of those.

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i seem to remember yami call their prop for sailboats a dual thrust prop

 

 

https://www.youtube.com/watch?v=TcDApinY5kw

 

dual meaning both fwds and reverse, ribs + tinnies don't need to reverse much

 

as well as reduced prop pitch they are easily identifiable by the black ring on the hub that helps guide the exhaust gases, emitted by the hub, further back

 

so they are less likely to create cavitation pockets for the blades slip into.

 

have a spare anti-ventilation plate for props that tend to suck down ventilation air if anyone interested

and also a dual-thrust yami 2-stroke 8hp motor

 

​other companies like solaras? also make 4 blade props that offer more thrust

 

but the yami ones should be in plentiful supply 

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We bought one of the dual thrust props for our yammy 8. It was marvelous. Did the job in reverse, which is what we wanted. Lost a bit of top end and economy on go forward but we really wanted some stopping power for docking. Mission accomplished.

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