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my new prop...

OOPS, I just hit 904fps.
Thanks for the figures leo, my previous prop was at 879 fps but I got 50 extra lbs of push by going 2 inches longer hub. Hmmmmm.

Bob
 
This is what I get out of this forum is that if you want low end brute force along with lower cruising rpm's which equal better fuel economy you want either a larger prop diameter with less pitch or increase your current prop pitch for greater bite.

If you want snap and top end go with less pitch in your current prop to achieve higher rpm's (but don't surpass your recomended prop rpm limitations.)

So if I change my reduction from 2:1 to 2.38:1 I will be able to achieve even greater torque numbers but maintain optimum rpm's which in turn would give my boat much more thrust at same rpm's; right?

I guess I worded that right, let me know if that sounds right.
 
Leo,
I'm running a large prop now on my ZZ4 350/355. I run the 3 blade "H" series wide (12") Sensenich set on the #1 setting. When I change to a 2.38:1 I should be able to run it up to 2- 2 1/4 pitch setting. I think this would make a big difference in my thrust numbers. Am I thinking correctly?
 
I have been saying this for years run a higher ratio gearbox and swing more prop I have been running 2.68 for 8 years now and you will always swing more prop with more ratio. I have talked at least 20 people into converting from a 2to1 or a 2.38to1 to running a 2.68 and every single time EACH person has said the 2.68 is way better. That's a fact. It's starting to catch on now but in the beginning everybody especially the airboat manufactures hated 2.68 because they thought you had to turn more RPM's but in reality you can turn the same or less with a 2.68. As for pitch I'm sure nobody here feels anybody else's answer is correct. I feel there is no one answer and here is why. First I think pitch and prop settings for EACH boat will be different. A 3000lb boat will like different pitch and RPM then a 1200lb boat. Just like the same 700HP motor in two cars likes a different tune or gear when it's in a 1,500lb car to a 4,000lb car. Even if you use the same exact motor to power two different vehicles why would two different car's or boats act differently with a gear or prop change. I'm sorry guy's but a 600ft pound of torque motor with a 2.68 gear box and a 80inch Sensenich will need different pitch settings from a 1,200LB 12 ft boat then it will in a 16ft 3,000lb boat. It's not just max thrust that makes the boat run right, you can set a prop to where it reads the most thrust and then run it and then change the pitch and you will like how the boat runs better. Peak numbers never perform the best. You ALWAYS give something up when you go for a PEAK numbers it is simple physics 101.
 
Yep thunder... I agree life is always about compromise ... you always give something to get something ... no free lunches. You just gotta know what you want and what you are willing to give up to get it.

I couldn't agree more about the boat differences ... my new 200,000 pound boat takes more to move it than my old one did for sure. once it gets going it still dances like a waterbug on the surface which is what we both like ... gotta be free ... I cant stand a boat that plows on the nose or its butt. Everyone else please take note of the fact that this type of running is not for everyone. They put turn chines on most boats for a reason ... to help you turn. Thunder and I like the sprint car slide a free running hull provides .. but bad things can happen quickly if you are not paying attention ... remember the richochet rabbit cartoon ... yep been there done that and will do it again I am sure. While testing my new boat I was in a serious crosswind situation and having some rudder issues which I am fixing, and if I was not paying attention I would have wrecked in to stan who was sitting still. I had to apply throttle when my butt was already biting a hole in the seat cushion, skinny water, hard bottom, big bump at the shoreline coming up, was already going faster than I should have been, and now we need more throttle to complete the turn, aww man. but we did fine slight bump but the passenger stayed in the seat and I missed stan.

That brings to mind the old addage that when you are out of throttle you are out of control on an airboat so always keep a little back for emergencies because they happen when you least expect them ... thats why they are called accidents otherwise they would be called stunts.
 
Leo: I forgot to add that I run a 3.12 box on a 500+hp smallblock swingin a 82" sens superwide set past the last mark, the protractor reads 20+ degrees at the tip my max engine rpm is so far 4900 which is a prop rpm of 1570 which I will be slowing down once I get the engine broke in. It is still gaining rpms which my last one did also. when it peaks out I will pitch it more. I just keep havin trouble with those darn docks I can't slow down maybe I am gonna need a sea anchor to drag while docking.
 
Leo,


Isn't it possible to design a prop to perform better with more pitch at less RPM's that less pitch at a slightly higher RPM? (Talking Static Thrust numbers only) Because on my setup (a 0540 straight valve) I run a 78" power shift signature series prop and I have proven on a scale with less pitch turning 2750 RPM’s I get less trust than with more pitch turning 2550 RPM's.

All this talk of less pitch more trust is deceiving; prop design has yet to be taken into consideration. We are talking about boats not planes.


Kevin
 
kb, i think what leo was really saying; for example.....if you added another blade and lessened your pitch you could still get more thrust. not so much as less pitch is more thrust but more prop with less pitch at optimal rpm/peak power could possibly create more thrust....
 
leo_081103 said:
In terms of pitch, the flatter your pitch, the greater your thrust! Go flat or bite more until to a point when your engine is at it's peak. In your case you gone flat thus the extra thrust all through out the prop envelope... good thing that the prop characteristics matched your boat!

quote]

I understand that duckluv, but some people may not.

Again there are to many other variables in prop design to say less pitch makes more thrust. Like the prop I run, after a certain RPM it cavitates generating less thrust, thats why I have to run more pitch with less RPM's.


Kevin
Click to expand...
 
Well as always there is a point when something looses efficiency and the most efficient prop is a two blade the more blades you add the less efficient a prop becomes. Hey country try putting a clutch on your gearbox I did that about 4 years ago and let me tell you that is the funnest thing I ever drove. I loved pulling up to a dock or boat push in the clutch and sit still people new something was up and after 5 minutes they realized my prop wasn't moving! Makes for a killer hole shot on the hill too bring her up to 5,000RPM and drop the clutch that was soooo cool. I ran a 3.21 box also, I tested one 5 years ago but it didn't go as I expected.
 
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