Figuring out Gearbox ratios is giving me a constant headache, need help

[L&LLandServices]

14ftx8ft Alumitech, 5 seater, rear double single steer with triple bucket front, all ss rigging, 50 gal fuel tank, sea deck on front and wallboards. Have a brand new 555 big block 724hp 686 trq at 4800 rpms. Question is, I’m looking at running a 82” 3 blade R series. What gear box is going to produce the most thrust with me only turning the motor 5000-5200 rpms tops ?

Please help my head hurts

 

[OneBFC]

Well, first thing you should probably consider is that you don't have enough prop. Unless you want to run 100+ mph I'd look at adding another blade. A 3 blade @ 82 just isn't enough for the engine specs you list. My little tiny 122 cubic inch engine runs a 3 Blade 80 inch with wider blades than those R's on a 2.38 ratio. You make WAY more power than I do.

Just a comparison point....

That all said, add a fourth blade and run a 2.55. That puts you right at 5100 RPM with prop at 2000 RPM. Those R blades are pretty good in that range.

Now, if you want to run the glades at warp speed....then sure, that 3 blade setup with it pitched way past the 3 mark might work out ok. Gear ratio should probably stay the same @ 2.55.

Good luck with it!

 

[SWAMPHUNTER45]

Table 3 with Sensenich suggests optimum blade rpm between 2,000 - 2,400

Going by your data you sort of fall between a 2.37 - 2.55 in my opinion.

That power would really be a better fit for the JX series of blade.

As OneBFC told you 3 blades are not gonna get the job done if your data is accurate. Hence the saying the proof is in the prop. With that kind of power 4 "S" blades or 4 JX will be needed. The "S" and JX blades work well with a slower rpm. If ride quality was a consideration and not racing I would be going with them. Hard to beat that power spec mated to a 2.38 or 2.55 with 4 wide blades.

 

[L&LLandServices]

Thanks for the input fellas, and yes power Numbers are accurate. I have the dyno sheet in my hand.Looks like I’m calling to get a Four blade this week. :cheers: :thumbleft: