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Took my new mac 19 out for the first time yesterday,I have a 40 hp and with myself and another 210 lb guy " my son' we only got 9 knots at full throttle.The boat was not planing eoth the balladt empty,Google suggests I need a different prop
Any suggestions would be appreciated.

First, welcome to the forum! We're glad to have you and looking forward to helping you.

A different prop is a possible solution but there are a lot of things that could account for your symptoms. If you were doing 9mph then you were above your theoretical hull speed of around 6.4mph so you were beginning to come up on a plane.

Which 40hp engine do you have? Which prop do you have? What was your engine RPM at wide open throttle (WOT). How clean is the bottom of the boat? What were the sea conditions during your test? Was the boat running close to level or was the nose up and stern down appreciably at 9mph?

Bill

I have a 2021 yamaha 4 stroke 40 hp ,the hull is clean low winds and calm sees,i don"t know the type of prop but I know the previous owner bought the engine brand new so it has whatever standard prop that engine comes with.has anyone ever put. trim tabs on a Mac 19?I was considering it don't if that would help.

thanks for the info Bill,I,m in no hurry on the water but my wife is,I could try to get a new wife but the boat was a lot cheaper

Rum boogie wrote: ↑Sun Jul 20, 2025 2:32 pm I have a 2021 yamaha 4 stroke 40 hp ,the hull is clean low winds and calm sees,i don"t know the type of prop but I know the previous owner bought the engine brand new so it has whatever standard prop that engine comes with.has anyone ever put. trim tabs on a Mac 19?I was considering it don't if that would help.

I'm assuming F40LA or something very similar. Clean hull and calm seas puts the problem back on the engine or prop. I'm going to assume that your engine or throttle cable is not the problem since it is relatively new and you did not mention any concerns regarding it.

You have to know your RPM at WOT to know if your prop is right and working correctly.

If you can't reach the manufacturer's target RPM (on a functioning engine) then your prop is too large, has too much pitch, or both.

If you can exceed the manufacturer's target RPM (a very bad idea) then your prop is too small, has too little pitch, has a blown hub, is slipping in the water, or any combination of the above.

It is possible to hit the target RPM at WOT and still not go as fast as you think the boat should be if the previous owner prioritized low-speed thrust and fuel economy over speed. That would just mean that the prop was right for him but not right for you (or your wife) if higher cruising speed is the priority.

A definitive answer needs RPM at WOT and the numbers stamped on the hub of your prop. If by any chance you have a 4-blade prop that would be an important bit of information as well.

Thanks for the help I talked to the previous owner and he's bringing bringing me his old prop which I believe had a lower pitch.I'm going to change props and install s tachometer.And put in at key largo this weekend,I'll keep you guys informed. Ray.

She wasrunning nose up she wouldn't level out.

Nose up with full down trim = common issue, The motors need wedges to get enough down angle to actually get the nose down.
I bought the wedges when I thought I would be using the speedboat concept more, but now really never try and plain the boat so never put them on.
https://www.amazon.com/T-H-Degre-Posit ... DER&gQT=1

I also added a "tail" to the outboard, not sure if it helps.
https://www.amazon.com/Davis-Instrumen ... 4JM?gQT=1

I have heard of people having someone run up to front of the cockpit to get some weight up there, bring the nose down to get on plane.
If you have some gear to move forward, that may help.