if its a new outboard, the dealer should be ensuring it is supplied with the correct one due to warranty?

Thanks Rob. No it's used, 220hrs came off a cat with twins.

 I am sure there is a site that lets you put in your vessel details and motor combo and it suggests the correct size prop. What is the reason for too much prop, not getting WOT? 

before swapping props, you should ALWAYS check all the following first.
-throttle cable is actually giving you full throttle (run with cowl off and give it full throttle at the engine?)
-blocked filters, wrong size fuel lines etc are not restricting maximum power.
-make sure there is no other reason such as old fuel, tank vacuum, dirty hull, damaged prop etc which may cause it
-Load vessel to maximum weight, or always slightly under prop engine.

Who installed the outboard, dealer or yourself?

Yamaha should fit as well.
Just propellers had a good selection of used props last time I was looking.

http://www.propellers.com.au/other-products/2nd-hand-props.html

Luke

The Robalo 235 is pretty similar to my BW Conquest 235, a heavy 23 footer.

The 19P will be WAY too much, but such a large prop is typical on dual installations running high top speeds.

I have a spare 15P Mirage that you can borrow that will bolt straight onto your 300. This is the standard prop on the Conquest 235's with a 250hp Verado, and it is reasonably suited for this style of boat.

The Verado 300 has a 1.75 gear ratio vs the Verado 250 1.85, so the 15P will be about right I think as it will be the equivalent to a 17P with a 1.85 ratio box. My 235 runs 37 knots flat knacker with a 1.85 RATIO AND 15p running up in the top third of the recommended rev range which is where you want to be. DO NOT run too big a prop and under rev the engine, it is the worst thing you can do to a boat motor. Assuming your 300 can push a 15P to the tope third with a 1.75 ratio, you will get 38-40 knots. PERFECT!

I changed my 15P Mirage to a 15P Enertia prop. Similar looking prop but I gained about 300 rpm and 3 knots and 20% better fuel economy as the Enertia is designed for use with heavy boats. You may be OK with the Mirage as you have a few more horsepower to push it, but I would be pretty sure the Enertia would be better for your style of boat but the Mirage will be a perfect test of the right pitch.

Drop me a PM if interested. I am North of river.

Cheers

John

Hi Mecury have their own prop selector app,

https://www.mercurymarine.com/en/au/propellers/selector/#/step-one

Ian

 Thanks Rob, a dealer fitted the engine and did suggest a prop change but to try it first. As for the net, yes there are prop selector apps but I'm not about to spend $500 or more on a prop because the Internet said it will work.

John, cheers for the info and will pm you.

 as a guide only.

 Email Ken at Propgods. Hes in Florida but very helpful and knowledgeable on the American boats. He may even have different performance numbers on your R235 & Verado 300 with different props. He did for me when I was looking...

www.propgods.com/Content2/HOME/tabid/69/Default.aspx

 Thanks Stevo, I'll do that.