Ok lets say you have a 72 inch prop pitched at i dont know say 20 degrees now you go to a 74 inch prop same prop just from 72 to 74. Now with the 74 you are going to take some pitch out of the prop because you went bigger witch is only an inch at either end. Now how much different is the 74 going to be for push I mean you have to take pitch away to keep it turning like the 72 so arn't you just maybe getting just a liitle bit dealing with this sanareo.
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Prop Thrust Question
- Thread starter gat2013
- Start date
A
Anonymous
Guest
With an engine of the same power, turning the same RPM via pitch adjustment, the larger prop with less pitch will 9 outta 10 times make more low-speed thrust, at the expence of more torque roll, slower engine acceleration and theoretically....boat speed. (Although in my opinion at typical airboat speeds is very arguable)
A lighter boat more concerned with being quick and agile will like the smaller prop and a heavier boat or one that is more concerned with low-speed push (especially dry running) will like the bigger prop. JMO
Felber
A lighter boat more concerned with being quick and agile will like the smaller prop and a heavier boat or one that is more concerned with low-speed push (especially dry running) will like the bigger prop. JMO
Felber
Waterthunder
Well-known member
The rule I go bye is run the largest diameter prop you possibly can until boat roll and throttle response become unacceptable! Once again this will be different on every boat. You do not run the same diameter prop on a palm beach as you do a Thurman! This is why I laugh when people state what prop and what RPM you should turn. There is no way to correctly answer this type of question without knowing the hull or the drivers terrain!
A
Anonymous
Guest
I agree you can't answer what the perfect prop is without knowing more. After all, any prop is a compromise. But the question was about push, and that question in my eyes is very easy to answer.
Felber
Felber
plumcrazy
Well-known member
paul pettit used to hand build some of the best wood props ive ever ran, back before composite came along,he said something to me years ago that made since ; " just emagine this prop turnin in a pool of jello its nothing but a screw , may sound simple but every time i study a prop design thats new i still picture it that way, you got to know a lot about a airboats to pick a good pakage, hull type , power an props there so many combanation that work and more that dont
When there were no carbon props. just wood, The HP of an engine was determined
by the size prop the engine would turn at about 2900 RPM. A 472 and 500 Caddy
would turn a 72 38 wide blade about 2900 RPM on the average Caddy Engine. A
72 38 wide blade is a 260 HP prop. Stick this same prop on a Straight Valve 0-540,
? 260 HP. turns about 2900.
The Book may state that the Caddy engine was 300 or 325 HP, but that was taken
at a higher RPM. you want get it without a gear or belt. The HP was stated more than
your to get. On the other hand, the cont GPU was stated as 175 HP at 2400 RPM,
but the airboater wants to turn it 3000 RPM. He can turn it about 2900 on the
average GPU with a 72 32 Wide Blade Prop. The 72 32 Wide Blade prop is a 220
HP prop.
Now, grab a 72 inch Water Walker Signature Series Prop. Put it on your boat. Set
the pitch in the blades to where the engine will only turn the same RPM's it turned
with the wood prop. Let's say 2900 RPM. Now level the engine. Level the blades,
put a protractor on the tip of a blade and check the degrees. We call this true pitch.
If you wright this down every time you install a prop on an engine, you will have a
record of where to set the prop on comparable engines.
Take the same Signature Series prop, bolt on a short hub and your prop will be a
70 inch Prop. Now set the pitch to where the engine will only turn 2900 RPM. Then check the degrees at the tip of the blade. This would be known as 2 degrees more
on a wood prop. Hinch, 70 34. Now why would your GPU run the ground better with
with the 70 inch Sig than the 72 inch?
When the blade was designed, It was designed in the neutral position, on the 2 mark,
not the 1 or 3 mark. The # 2 mark on the blade should be the most efficient point
to set the blade. The 2 blade signature was designed for 200 to 300 HP. So, 200 to
about 235 HP use the 70 inch. About 235 to 270 HP, use the 72 inch. 270 to 300
HP, use the 74 inch. Use the length of the prop with the style of your blade to get as
close to the neutral position as possible. This works good for me and my customers.
Chuck
by the size prop the engine would turn at about 2900 RPM. A 472 and 500 Caddy
would turn a 72 38 wide blade about 2900 RPM on the average Caddy Engine. A
72 38 wide blade is a 260 HP prop. Stick this same prop on a Straight Valve 0-540,
? 260 HP. turns about 2900.
The Book may state that the Caddy engine was 300 or 325 HP, but that was taken
at a higher RPM. you want get it without a gear or belt. The HP was stated more than
your to get. On the other hand, the cont GPU was stated as 175 HP at 2400 RPM,
but the airboater wants to turn it 3000 RPM. He can turn it about 2900 on the
average GPU with a 72 32 Wide Blade Prop. The 72 32 Wide Blade prop is a 220
HP prop.
Now, grab a 72 inch Water Walker Signature Series Prop. Put it on your boat. Set
the pitch in the blades to where the engine will only turn the same RPM's it turned
with the wood prop. Let's say 2900 RPM. Now level the engine. Level the blades,
put a protractor on the tip of a blade and check the degrees. We call this true pitch.
If you wright this down every time you install a prop on an engine, you will have a
record of where to set the prop on comparable engines.
Take the same Signature Series prop, bolt on a short hub and your prop will be a
70 inch Prop. Now set the pitch to where the engine will only turn 2900 RPM. Then check the degrees at the tip of the blade. This would be known as 2 degrees more
on a wood prop. Hinch, 70 34. Now why would your GPU run the ground better with
with the 70 inch Sig than the 72 inch?
When the blade was designed, It was designed in the neutral position, on the 2 mark,
not the 1 or 3 mark. The # 2 mark on the blade should be the most efficient point
to set the blade. The 2 blade signature was designed for 200 to 300 HP. So, 200 to
about 235 HP use the 70 inch. About 235 to 270 HP, use the 72 inch. 270 to 300
HP, use the 74 inch. Use the length of the prop with the style of your blade to get as
close to the neutral position as possible. This works good for me and my customers.
Chuck
Night Ranger
Well-known member
How much horsepower does it take to turn a 78 X 60 wood prop with a 2 to 1 gear reduction?
Waterthunder
Well-known member
Pitching them at Zero or Power Shifts mark or in the middle of the Sensenich's 3 marks will give you the most efficient thrust per ft pound of torque at peak blade speed! I have realized that and also Mr Wine told me a this a few times as did the guy's at Sensenich. They designed their props and I'm sure they know best however for me I run wide open less then 1% of the time so I'm more concerned at making more thrust at the RPM's I run my boat at 99% of the time. This is where many people miss the many advantages of a 2.68.
Yeah I posted it about push the reason was because you know how when you see your friends on the lake and they ask you what size is your prop and you reply 72 there always is one of them that says oh you should have got a 74 but seeing how you would have to turn the 74 down in pitch so it would swing the same RPM as the 72 I cant see how it would push NIGHT AND DAY DIFFERENT"S at full throtel or the top end of the RPM. I have always heard guys say this about the two props so I figured I would ask and see what some answers where.
plumcrazy
Well-known member
some times its just the drivers preferance, we have all seen the guy in the marsh , well i should say newbie who cant run dry on a boat you know is capable ,some guys love that snap some love that low end push, thats why this subject brings out a lot of passion, like the song say we did it our way 
A
Anonymous
Guest
Well Plum you hit the nail on the head just like some others...It's a friggin SCREW. No matter how you slice it, there are ways to walk outside of certain parameters to increase efficiency and tailor it for the customer's needs better, but it's still a screw that gets work done over time. Work done over time is HP. Torque is force...
Before I get myself all wound up again and talk too much.....I think some people should pick up a machinist's handbook and look at feed rates and clamping force values of common SAE threads.
Do you want a ACME thread, or do you want a fine pitch? A small diameter ACME will have a great feed rate with very little force applied. A large diameter fine pitch will have a much slower feed rate with WAY more force. The negative of the large diameter or excessively high pitch? You better make some real power in either case, or choose the right size for the torque applied....I know I sound retareded...But do your homework and weed out the BS cuz this ain't theory..It's FACT!! And I mean that in the nicest way guys.
Felber
Before I get myself all wound up again and talk too much.....I think some people should pick up a machinist's handbook and look at feed rates and clamping force values of common SAE threads.
Do you want a ACME thread, or do you want a fine pitch? A small diameter ACME will have a great feed rate with very little force applied. A large diameter fine pitch will have a much slower feed rate with WAY more force. The negative of the large diameter or excessively high pitch? You better make some real power in either case, or choose the right size for the torque applied....I know I sound retareded...But do your homework and weed out the BS cuz this ain't theory..It's FACT!! And I mean that in the nicest way guys.
Felber
You are right Felber, I try to explain how things work using HP to pick your prop so
most everyone can understand. Most people buy their engine by the HP rating. Many
don,t know that the 220 GPU develops 405 FT LBS Torque.
The misconception of how an airboat runs is in the design of the airboat, not the
engine or the prop. The book is written on engines and props but not airboats.
Example: If someone told me to build them a ground running boat and they had a 14
X 7 hull and they wanted to run a SBC 400 direct drive and wanted a carbon prop.
I would not use a 2 blade 74 or 76 or 78. I would use a 68 inch 3 blade. I would
build the engine stand lower and put about 3 inches tip below top of transom. When
the center of thrust is closer to the ground, your boat will run the ground better. The
higher the engine is mounted, the more the boat wants to push over on it's nose.
We went from 78 inch wood prop on a customers caddy boat to a 68 inch 3 blade cypress
and turned his junk boat into a rocket ship. You still have to match the style prop
with RPM your going to turn it. Chuck
most everyone can understand. Most people buy their engine by the HP rating. Many
don,t know that the 220 GPU develops 405 FT LBS Torque.
The misconception of how an airboat runs is in the design of the airboat, not the
engine or the prop. The book is written on engines and props but not airboats.
Example: If someone told me to build them a ground running boat and they had a 14
X 7 hull and they wanted to run a SBC 400 direct drive and wanted a carbon prop.
I would not use a 2 blade 74 or 76 or 78. I would use a 68 inch 3 blade. I would
build the engine stand lower and put about 3 inches tip below top of transom. When
the center of thrust is closer to the ground, your boat will run the ground better. The
higher the engine is mounted, the more the boat wants to push over on it's nose.
We went from 78 inch wood prop on a customers caddy boat to a 68 inch 3 blade cypress
and turned his junk boat into a rocket ship. You still have to match the style prop
with RPM your going to turn it. Chuck
Waterthunder
Well-known member
It will cruise better but will run the hill worse! Your tip speed is low now and adding more pitch you will turn it even slower if you take out pitch it will probably run the hill better but your cruising RPM's will go up it depends on what is more important to you!
Crabby, It has to be the right 3 blade. A 3blade Water Walker Cypress 68 inch or a
3 blade sensenich short narrow blade 68 or 70. Chuck
3 blade sensenich short narrow blade 68 or 70. Chuck
DAGWOOD0520
Well-known member
Crabby let us know what you feel is different between the Q and the three blade when you try them I'm sure it will be intresting to hear. Likes and dislikes at least.