Jump to content
Sign in to follow this  
MUSTANGER7

BOAT PROP PITCH AND SIZE???

Recommended Posts

MUSTANGER7

I have a Suzuki 9.9 hp 4 stroke  on a 14 ft jon boat when I run the motor with  a 9.25 in diameter with a 7 pitch I get 16 mph at 5800 rpm, I tried same motor with a 9.25 in diameter with a 9 pitch I still get 16mph but at only 5100 rpm. I have the motor set so that the trim tab on the motor is at same level as bottom of the hull, wondering it I either raised or lowered the motor an in or two if that would give me more speed. I did have a hydraulic jack plate on it and it seemed like the height its at now was the sweet spot, the jack plate which moved the motor back about 4in seemed to create a drag. This is a boat that I customized with decking and carpet along with comfortable chairs so I did add some extra weight which I know affects the speed. What I can't figure out is the reason for difference in rpm's apparently each inch is supposed to change the rpm's by about 200rpm. Any thoughts or ideas are appreciated, I mostly use the boat for fly fishing the rivers etc. Enclosed some pictures. The railing on the front is to rest a jigger pole on

IMG_3700.JPG

IMG_3697.JPG

IMG_3698.JPG

IMG_3699.JPG

IMG_3701.JPG

Share this post


Link to post
Share on other sites
C.J.L.

Not sure if they make one for your size motor but you'd get a little bit more speed with stainless steel prop.  Less flex to them.  But your gains with a motor that small are going to be slight.  Sounds like you have a good set up already.  16 mph is hoofing it for a little rig like that with all that weight.  

Share this post


Link to post
Share on other sites
Two Barrels

The engine’s cavitation plate should be level or even 1-2” higher than the bottom of your boat’s hull.  Make sure that is correct before swapping props.  Your motor has a designed RPM range for full throttle operation.  Verify that with Suzuki and select a prop accordingly.  The engine should be most efficient the closer you can prop it to work within that range.  I am betting the first prop giving you 5800 rpm is closer to the correct one.

Share this post


Link to post
Share on other sites
MUSTANGER7
4 hours ago, Two Barrels said:

The engine’s cavitation plate should be level or even 1-2” higher than the bottom of your boat’s hull.  Make sure that is correct before swapping props.  Your motor has a designed RPM range for full throttle operation.  Verify that with Suzuki and select a prop accordingly.  The engine should be most efficient the closer you can prop it to work within that range.  I am betting the first prop giving you 5800 rpm is closer to the correct one.

6000 rpm is the max so the 5800 is best, wondering if I changed the height I could be more rpm out of the 9 pitch, the cavitation plate is at the exact level of the boats bottom. They do make a stainless for $100, not sure if 1mph is worth it. Thinking of trying the slick paint approach.

 

Share this post


Link to post
Share on other sites
Coalman

Try this

http://www.suzukimarine.com/~/media/Marine/Accessories/Suzuki Propellers/propellers_PDF.ashx

 

From the looks of your boat and the mention of a jack plate you must run shallow. I would shy away from SS in shallow applications. They have no give. Your motor takes the hit.

Aluminum is forgiving and nothing a good file can't get back in shape.

 

Also take into question are you looking for speed or power. It is all in the pitch.

 

 

Share this post


Link to post
Share on other sites
NW River Mac

If you are looking for more speed I would suggest that you move the motor height/angle pin to a higher height.  Try it up a hole or two.  I think if you want more speed taking a bit of the bow of the boat out of the water would help quite a bit.  Diing this will also increase your RPMs, unless with all your added features the motor isn't powerful enough to make this happen.

 

Think if it like this: Ideally when the boat is wide open you want as little of the boat touching the water as possible.  This creates less drag and more speed.

 

good luck

Share this post


Link to post
Share on other sites
MUSTANGER7
38 minutes ago, NW River Mac said:

If you are looking for more speed I would suggest that you move the motor height/angle pin to a higher height.  Try it up a hole or two.  I think if you want more speed taking a bit of the bow of the boat out of the water would help quite a bit.  Diing this will also increase your RPMs, unless with all your added features the motor isn't powerful enough to make this happen.

 

Think if it like this: Ideally when the boat is wide open you want as little of the boat touching the water as possible.  This creates less drag and more speed.

 

good luck

this motor has power trim and tilt which is unusual on a small motor, think I might move it an inch higher and try it out. I do run real skinny water out here on the flats for redfish, snook and stainless on other boats along with jack plates some of the rivers going out are a foot deep at low tide.

Share this post


Link to post
Share on other sites
salmontogue

Outside of the very detailed explanation in the USCG Masters Manual and the very popular Chapman's, this is the best explanation of cavitation and ventilation I have seen. The plate on an outboard motor gearcase is an anti-ventilation plate not an anti-cavitation plate.  I remember this well, I failed this topic on the USCG Masters exam although I did pass.  This subject was also on the exam for the Towboat endorsement.

 

Be careful with motor height, assuring that water pickup is not interrupted.  Check your motor's manual to assure overheat protection is present.  Additionally, pressing the operating RPM limit is not advised even if your motor has over-rev protection.  Coalman is correct, shallow water operation with a stainless prop is hazardous to your gearcase and will not gain much speed if any.  I think your current speed is more than adequate for your application.  If you require more speed, you may want to consider a bigger boat, reminiscent of Richard Dreyfuss in "Jaws".

 

Is it cavitation or ventilation?

HANK JOHNSTON   September 01, 2009 at 6:33am

1 / 1

Cavitation is a little-understood concept, but it can impact propeller performance.

In a recent e-mail, a reader requested help with what he believes is a cavitation problem. He described his boat as a 1994 16-foot center console with a 1994-model 70-horsepower engine. His boat has been problem-free until recently.

"Lately I have had a problem with what seems to be cavitation — the boat gets up on plane just fine, levels off, gets to cruising speed, then it feels like the prop is losing its grip," he said. "The RPMs go up, and the boat slows down."

He thought he may have a striped hub in the propeller, so he had the hub replaced. There was no improvement with the new hub. He also checked the lower unit oil, and did not find any signs of metal debris in the oil, which would have indicated gear failure.

According to his e-mail, he has not changed anything in the setup of his boat. He did read my article on ethanol in the fuel, and was wondering if low octane could cause such a problem. He told me that he has recently developed a leak in his hull from running up on too many oyster reefs.

Cavitation is often confused with ventilation. Cavitation occurs when there is an extreme reduction in pressure on the back side of the propeller blades. Under normal conditions, water boils at 212 degrees, but if you reduce the atmospheric pressure sufficiently, water can also boil at room temperature.

As your propeller begins turning through the water at an ever-increasing rate of speed, the pressure on the back side of the blades is reduced, and if that pressure is reduced low enough, the water will begin to boil and form water vapor on the blades. This usually occurs near the outer or leading edge of the blade.

The water vapor bubbles migrate toward the center of the blade, where the pressure is higher, and the boiling stops. The vapor bubbles implode against the blade's surface. The resulting energy release is so great that it can chip away at the blade surface, leaving a cavitation burn.

There are numerous possible causes of cavitation. Nicks or damage to the leading edge of the propeller blades are one of the most common causes. If your propeller cannot cut through the water smoothly, it will cause disturbances in the water flow, and many times will result in cavitation.

A popular myth is to sharpen the leading edges of your propeller blades so they can cut through the water like a sharp knife through butter, and this will make your boat run faster. A sharpened leading edge can actually cause cavitation, which will increase the slippage of your prop, and the boat will run slower. Propeller blades that are bent or have pieces broken off the edges will also suffer from cavitation.

Probably the sneakiest propeller problem is worn blades. In our environment of sand, oyster shells and hurricane debris, we are constantly wearing down the edges of our prop blades. This process comes on slowly over a long period of time, so we don't always notice it, but when the propeller diameter gets small enough, you will experience performance problems.

Ventilation is the result of air or exhaust gases being pulled into the propeller blades. This causes the blades to lose their grip on the water, the engine RPMs go up and the forward speed of the boat is reduced.

Sometimes when the propeller RPMs increase, it can also cause a massive cavitation, which only compounds the problem. Most outboard and stern-drive engines have anti-ventilation plates made onto the lower unit housing directly above the propeller. Many people mistakenly call them cavitation plates or possibly anti-cavitation plates, but that is incorrect.

The anti-ventilation plate does not stop or help prevent cavitation. Its sole purpose is to prevent surface air from being sucked into the negative pressure side of the propeller blades.

Ventilation can also come from exhaust gases being introduced into the blades of the prop. Most propellers have some sort of ring around the trailing edge of the exhaust hub. This ring creates a high-pressure barrier that prevents exhaust gases from being sucked back into the blades.

Other sources for ventilation include the leading edge and sides of the motor's lower unit and the bottom of the boat. Inspect your lower-unit housing for any imperfections in the surface directly in front of the propeller. Knicks, gouges, scratches or barnacles can all disturb the water flow to the propeller, and cause ventilation.

The same is true for the bottom of the boat. Holes and chips in the bottom surface from such things as oyster reefs can cause disturbed water flow to the propeller. Depth-finder transducers and live well or wash-down water pick-ups or drains could disturb the water flow to your propeller.

In the case of a boat that has a little age such as the one mentioned in the e-mail, you should check the condition of the bottom. The boat bottom should be perfectly straight for at least 5 feet forward of the transom.

The boat can be checked with a long straight edge placed along the bottom from transom forward. If the bottom is concaved upward from the straight edge, you have a condition called a hook. If the bottom is convex, the condition is called a rocker.

A hook in your bottom will cause the boat to have excessive transom lift. As your speed increases the pressure of the water flowing under the hook in your bottom will cause the transom of the boat to lift out of the water and the bow to be pushed down. Ventilation occurs because the propeller is being pulled closer to the surface.

A rocker will generally cause your boat to porpoise at running speed.

Ventilation or cavitation, they are both bad for your boat. I hope that I have helped you to understand what each is and you will be able to keep either of them from interfering with your boating pleasure.

If you have any questions about your boat, motor or trailer, drop me an e-mail at
theboatdr@yahoo.com.

 

Perk

Share this post


Link to post
Share on other sites
MUSTANGER7
NW River Mac
On 6/20/2018 at 6:23 PM, salmontogue said:

 

Outside of the very detailed explanation in the USCG Masters Manual and the very popular Chapman's, this is the best explanation of cavitation and ventilation I have seen. The plate on an outboard motor gearcase is an anti-ventilation plate not an anti-cavitation plate.  I remember this well, I failed this topic on the USCG Masters exam although I did pass.  This subject was also on the exam for the Towboat endorsement.

 

Be careful with motor height, assuring that water pickup is not interrupted.  Check your motor's manual to assure overheat protection is present.  Additionally, pressing the operating RPM limit is not advised even if your motor has over-rev protection.  Coalman is correct, shallow water operation with a stainless prop is hazardous to your gearcase and will not gain much speed if any.  I think your current speed is more than adequate for your application.  If you require more speed, you may want to consider a bigger boat, reminiscent of Richard Dreyfuss in "Jaws".

 

Is it cavitation or ventilation?

HANK JOHNSTON   September 01, 2009 at 6:33am

1 / 1

Cavitation is a little-understood concept, but it can impact propeller performance.

In a recent e-mail, a reader requested help with what he believes is a cavitation problem. He described his boat as a 1994 16-foot center console with a 1994-model 70-horsepower engine. His boat has been problem-free until recently.

"Lately I have had a problem with what seems to be cavitation — the boat gets up on plane just fine, levels off, gets to cruising speed, then it feels like the prop is losing its grip," he said. "The RPMs go up, and the boat slows down."

He thought he may have a striped hub in the propeller, so he had the hub replaced. There was no improvement with the new hub. He also checked the lower unit oil, and did not find any signs of metal debris in the oil, which would have indicated gear failure.

According to his e-mail, he has not changed anything in the setup of his boat. He did read my article on ethanol in the fuel, and was wondering if low octane could cause such a problem. He told me that he has recently developed a leak in his hull from running up on too many oyster reefs.

Cavitation is often confused with ventilation. Cavitation occurs when there is an extreme reduction in pressure on the back side of the propeller blades. Under normal conditions, water boils at 212 degrees, but if you reduce the atmospheric pressure sufficiently, water can also boil at room temperature.

As your propeller begins turning through the water at an ever-increasing rate of speed, the pressure on the back side of the blades is reduced, and if that pressure is reduced low enough, the water will begin to boil and form water vapor on the blades. This usually occurs near the outer or leading edge of the blade.

 

The water vapor bubbles migrate toward the center of the blade, where the pressure is higher, and the boiling stops. The vapor bubbles implode against the blade's surface. The resulting energy release is so great that it can chip away at the blade surface, leaving a cavitation burn.

There are numerous possible causes of cavitation. Nicks or damage to the leading edge of the propeller blades are one of the most common causes. If your propeller cannot cut through the water smoothly, it will cause disturbances in the water flow, and many times will result in cavitation.

A popular myth is to sharpen the leading edges of your propeller blades so they can cut through the water like a sharp knife through butter, and this will make your boat run faster. A sharpened leading edge can actually cause cavitation, which will increase the slippage of your prop, and the boat will run slower. Propeller blades that are bent or have pieces broken off the edges will also suffer from cavitation.

Probably the sneakiest propeller problem is worn blades. In our environment of sand, oyster shells and hurricane debris, we are constantly wearing down the edges of our prop blades. This process comes on slowly over a long period of time, so we don't always notice it, but when the propeller diameter gets small enough, you will experience performance problems.

Ventilation is the result of air or exhaust gases being pulled into the propeller blades. This causes the blades to lose their grip on the water, the engine RPMs go up and the forward speed of the boat is reduced.

 

Sometimes when the propeller RPMs increase, it can also cause a massive cavitation, which only compounds the problem. Most outboard and stern-drive engines have anti-ventilation plates made onto the lower unit housing directly above the propeller. Many people mistakenly call them cavitation plates or possibly anti-cavitation plates, but that is incorrect.

The anti-ventilation plate does not stop or help prevent cavitation. Its sole purpose is to prevent surface air from being sucked into the negative pressure side of the propeller blades.

Ventilation can also come from exhaust gases being introduced into the blades of the prop. Most propellers have some sort of ring around the trailing edge of the exhaust hub. This ring creates a high-pressure barrier that prevents exhaust gases from being sucked back into the blades.

Other sources for ventilation include the leading edge and sides of the motor's lower unit and the bottom of the boat. Inspect your lower-unit housing for any imperfections in the surface directly in front of the propeller. Knicks, gouges, scratches or barnacles can all disturb the water flow to the propeller, and cause ventilation.

 

The same is true for the bottom of the boat. Holes and chips in the bottom surface from such things as oyster reefs can cause disturbed water flow to the propeller. Depth-finder transducers and live well or wash-down water pick-ups or drains could disturb the water flow to your propeller.

In the case of a boat that has a little age such as the one mentioned in the e-mail, you should check the condition of the bottom. The boat bottom should be perfectly straight for at least 5 feet forward of the transom.

The boat can be checked with a long straight edge placed along the bottom from transom forward. If the bottom is concaved upward from the straight edge, you have a condition called a hook. If the bottom is convex, the condition is called a rocker.

A hook in your bottom will cause the boat to have excessive transom lift. As your speed increases the pressure of the water flowing under the hook in your bottom will cause the transom of the boat to lift out of the water and the bow to be pushed down. Ventilation occurs because the propeller is being pulled closer to the surface.

 

A rocker will generally cause your boat to porpoise at running speed.

Ventilation or cavitation, they are both bad for your boat. I hope that I have helped you to understand what each is and you will be able to keep either of them from interfering with your boating pleasure.

If you have any questions about your boat, motor or trailer, drop me an e-mail at
theboatdr@yahoo.com.

 

Perk

 

Perk, interesting stuff.  I did know that boiling did cause cavitation but didn't know about the ventilation problem.  Very cool stuff to know.  Fortunately, I don't have either problem with the oars of my drift boat. lol

Share this post


Link to post
Share on other sites
WI Outdoor Nut

Run the prop that is giving you 5800 rpm's.  Then raise your motor one hole and retest. 

Share this post


Link to post
Share on other sites
ccavacini

Nothing to add but, I like your set up.

Share this post


Link to post
Share on other sites

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
Sign in to follow this  

×