Help with selecting/understanding a prop

[jethro]

I have a kicker motor that I want to try and get a slower prop for. I know it's uncommon, but I'd like to run the motor a little higher in the RPMs while going slower. I can't figure out what my motor came with stock, there is nothing written on it anywhere unless I just don't know where to look. My motor is a 2004 Mercury 6hp 4 stroke. As I understand one concern with prop selection is making sure it doesn't allow too high an RPM... is this a concern with a 4 stroke though? I have to imagine even this small motor has a timing based rev limiter? What number makes a slower prop... say I have a 9 pitch, is 6 pitch a slower prop?

 

[lugoismad]

Each degree of pitch is 6% of change in the pitch of the prop blades.

so a 12 pitch prop is 6% more aggressive than an 11 pitch prop.

a 12 pitch prop would (without any loss due to slippage) go through 12 inches of water in one full revolution.
An 11 pitch, 11 inches. And so forth.

It sounds to me like you want a lower pitch prop.

This would allow you to run higher RPM because your prop is grabbing less water.

Unfortunately, I don't know enough to answer the rest, sorry.

 

[DaleH]

Usually 'pusher' props have big/wide 'elephant ear' shaped blades and typically 4 blades at that. Found this on-line, Mercury Black Max ... maybe they make the pusher 4-blade in other pitches.

I entered your OB info in this Mercury Prop Selector = https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

... and they recommended the Black Max props too, but I'd go to a dealer. Or, try out that Turning Point prop calculator and select the option to have their Techs check the data for you. A good thing with Turning Point props is that you can exchange them for the prop YOU need ... so you aren't stuck buying something that didn't work out for you!

Turning Point Prop Calculator = https://www.forum.tinboats.net/viewtopic.php?f=4&t=37433

 

[jethro]

When I put in all my info about my motor it says nothing is available from Turning Point. You sure you are putting in a 6 horse 4 stroke Dale?

 

[DaleH]

When I put in all my info about my motor it says nothing is available from Turning Point. You sure you are putting in a 6 horse 4 stroke Dale?

I did not try your OB in the T-Point selector, only the Merc one. Regardless, you want a much lower pitch (I'd say at least 2" lower) with big fat blades, 4 if you can do it.

Once when I blew up a big V6 OB a friend gave me a 90hp OB motor to use until the new motor arrived - hey it was toona season! - since it could use the same rigging, fuel and control system. Off with the old dead one and on with the new 'old' one in just a few hours, very NASCAR pit crew-like operation. I found a 4-blade, big-bladed low pitch "houseboat or barge boat" prop and threw it on there. Son of a gun, but that old 90hp V4 would just get the boat up to slow planing speed around 14mph ... and I still made it offshore ... slowest boat in the fleet!

 

[jethro]

and I still made it offshore ... slowest boat in the fleet!

Slow and steady wins the race!