Chuckitt,

What do you think? think it will work. Figure I need to turn the prop 2000 rpms for maximum thrust and be quiet. The motor will be close to 80HP at 5000 rpms. Max torque will be in the 4000 rpm range. It will be on a 12X7 Panther with slick bottom and 1/2" rigging along with 3' rudders. Figured a 2 to 1 reduction

If it works spread the word, make it a ground runner. I'll spend the extra $$$ if the engine will take the abuse.

I don't know drake, I have never used a power fin prop. I do know i would never use a 74 inch prop on an 80 HP boat. The longest i would use would be a 66 inch with 80 HP and a drive unit. I prefer to use a shorter prop with more pitch than a longer prop with less pitch. I ask Circle S to make some 4 blade bow-tie plates for the warp drive blades to develope a good drag race prop for the 60 to 100 HP motors. If the 80 HP motor is an Extreme V Twins motor, you could send it to me to put on Scorpy. :) We would have the fastest mini in the world.
Thanks, Chuck

The engine is by EVT and it can take the abuse. I may start off with a 70" and if I can not pull it then I can start cutting it down. Powerfin sad that prop would push great at 2000 rpms on the prop. Everything I keep reading is saying bigger and slower is better.

Many times it has been proven that bigger is not better. If someone is using info from airplanes, it's not the same. Airplanes fly in light air so a longer prop can be used. Airboats always operate at ground level air riding on water or ground. In 1998, I ran a 60 inch 6 blade Warp Drive on a 220 GPU at 91 MPH in 400 ft and over 100 MPH flat out. I will give up 2 inches length anyday to gain 2 inches pitch. Most prop manufactures don't build airboats, They just know what people buy. I just hate to see someone buy the wrong part for their boat and it not perform good. Then it's back to the drawing board. I wish you luck on your build. Thanks, Chuck

So 66" you say?

Hey Drake, That's where i would start. I haven't used the Power Fin prop but if they work like the Warp Drive of IVO prop it should be good.
Thanks, Chuck

Hi Drake, Let us know how the prop works out!

Chuck is right about the performance differences between boats and planes. Airfoil performance changes greatly with temperature and humidity (barometric pressure) and is also affected by every 1000’ of altitude. A propeller is truly an airfoil or moving wing. Stunt planes have short aggressive wings and endurance planes (Rutan Voyager) have very long almost fragile wings.

I was tinkering with some clutch ideas and that got me thinking about response time to thrust and that “kind of” applies to the prop length discussion.

I too would gladly trade length for pitch within reasonable envelopes for propellers and it has a lot to do with inertia. Both of motion and rest.

Take a tennis ball on a 6’ string representing long prop and small pitch. Compare that to a tetherball on a 3’ string representing short prop large pitch. (basic, basic example)

Using yourself as the hub start rotating either of the two comparisons. With the tennis ball you may have to take a few running steps to get it in motion to even begin to get it to rotate around you. With the tetherball you could probably just start rotating to bring it up to rotation speed. Either way a lot more energy, and time, will be needed to spin the tennis ball. The power plant (you) remains constant.

When both are in rotation they will deliver about equal energy if they impact something. Tennis ball lighter but traveling faster, and tetherball heavier but traveling slower.

Now imagine stopping the rotation(s) while trying to keep the line relatively straight. The tetherball will wind-down about as quickly as you can but the tennis ball is going to take a few more rotations and time. There are other implications as well including reverse torque forces from the inertia at motion. Longer prop, more mass at distance, greater inertia to counter.

This is a simple example but I think it is a good visual.

Just my double nickels, (two cents adjusted for inflation).

So would you agree to start at 66" with more pitch or even smaller with more pitch. I am trying to keep her quiet as well. From what I read you have to be at .5 mach or lower to keep the prop sound down.

Hey Drake1, The shorter the prop, the less the tip speed will be if the props are turning the same rpm. The less tip speed, the less noise. We are running a 60 inch 3 blade IVO on the 54 HP Briggs and turning the engine 3850 with a 1.69 to 1 drive. the prop is turning 2278 and the boat ran 39 mph. I think starting with 66 inch will be good for 70 to 80 HP and you can always cut the blades shorter if need be.
I just got your phone message and will call you in the morning.
Thanks, Chuck

Hi Drake,

Honestly, for selection, I myself would defer to Chuck and the other builders/guru's here for modern prop selection for a certain power plant. From my boat experience I have read enough of their posts to trust their thoughts and integrity.

I was just passing on some things to think about.

Here is one for example: Prop performance will change dramatically with temperature and humidity and it has to do with air density or pressure. High pressure will give positive measurable improved performance over low-pressure atmosphere. Or… cold dry air performs better than hot humid air because the cold dry air has greater density than the hot humid air. (at ground or sea level)

I know…I know…your probably thinking I over spun my prop and the rivets came loose.

The difference is so great and measurable that a boat sitting on the sand will not budge to even fish tail at 95+ degrees and the same boat the next morning at 70 degrees will run a block dry no problem. (multiple first hand experiences with the boats, and the science comes from private pilot introduction to atmospheric performance) If the river droped we had to wait till the next morning to run them out to the channel.

A boat will run much better in the cool morning than in the hot afternoon. I am guessing this is one of the reasons for the posts that have confusing speed and/or RPM performance results from the same boat.

Good luck with the new boat!

Hi Drake,

Honestly, for selection, I myself would defer to Chuck and the other builders/guru's here for modern prop selection for a certain power plant. From my boat experience I have read enough of their posts to trust their thoughts and integrity.

I was just passing on some things to think about.

Here is one for example: Prop performance will change dramatically with temperature and humidity and it has to do with air density or pressure. High pressure will give positive measurable improved performance over low-pressure atmosphere. Or… cold dry air performs better than hot humid air because the cold dry air has greater density than the hot humid air. (at ground or sea level)

I know…I know…your probably thinking I over spun my prop and the rivets came loose.

The difference is so great and measurable that a boat sitting on the sand will not budge to even fish tail at 95+ degrees and the same boat the next morning at 70 degrees will run a block dry no problem. (multiple first hand experiences with the boats, and the science comes from private pilot introduction to atmospheric performance) If the river droped we had to wait till the next morning to run them out to the channel.

A boat will run much better in the cool morning than in the hot afternoon. I am guessing this is one of the reasons for the posts that have confusing speed and/or RPM performance results from the same boat.

Good luck with the new boat!
I am Papee and I approve of this message. :lol:
With all of us trying to squeeze everything we can out of these smaller power plants, Things like temperature change are very noticeable in performance. Here in PA when I leave the boat ramp in the morning during fishing season it's a cool 50-60 degrees. The boat seems to fly, then later on in the day when them temps are between 80-90 it will be noticeably slower. Now that I have my GPS I'll do some comparsions this season. I try to do all my prop adjustment testing at the same time each day. At one point in testing motor combination on my boat this difference in temp made the difference between getting the boat on plane and not.