Prop Guys
12-25-2002 | 09:29 PM
#1
Guest
Posts: n/a
Prop Guys
For you prop experts; In reworking a prop for less blade area most shops want to cut diameter. Why would this be better then reduceing blade area [size]? Is a 4bl prop slower because of increased blade area? would cuting the blades to have a total area approximately equal to the 3bl work to increase speed and still keep the benefits of the 4bl? When changeing fron a 3bl to a 4bl cleaver use same pitch or drop down one size? All this is given that drive height is set.
12-26-2002 | 10:43 PM
#3
Charter Member #1093
Joined: Feb 2001
Posts: 660
Likes: 0
From: Lighthouse Point & Ocean Reef FL
mjuwalters,
You ask a good question, and I think the reason you're not getting much response, is because the answer is not black and white.
I've worked on my race props since I started in the late 60's. Usually started with a good foundation such as Merc or Hydromotive, or Hi-J or Menkens in the old days. The current crop are more efficient then earlier models, although top end on the surface piercing side, isn't much higher then it was in the days of 2 blade props, ET has dropped dramatically however.
Each hull responds differently to each propeller. There are some general givens: such as a 4 blade will create more stern lift than a 3 blade and a round ear will plane easier than a cleaver style blade in a non surfacepiercing environment. beyond that, it's a matter of test and see.
One of those givens is that a 15.25 (pitch) 4 blade will turn less rpm than a 15.25 3 blade, assuming the same pitch. Further, a 15.25 diameter 4 blade (or 5 blade) will create more stern lift than a 3 blade of equal diameter because there is more blade surface creating lift (down swing of the blade) with each revolution. By reducing diameter in the 4 blade prop you increase RPM but not necessarily forward speed.
It is very difficult to reduce pitch, other than removing cup. But removing cup may bring unwanted issues into play, such as blow out or trouble getting on plane with an aggressive X dimension.
Removing diameter is easier but merely seeing an RPM increase isn't a move forward. An alternative to removing diameter is removing leading edge and leaving diameter alone. In this scenario you reduce the size of the blade from the hub out towards the tip. This may not impact top end, but it will generally increase acceleration, such as thru a turn or from mid point of a turn up to top end.
All this to say, there isn't a forthright answer to your question
Find a prop company that will allow you to try a prop and continue to return them until you find one(s) that make you happy. Both Hydromotive and Merc Hi-perf have fair try and return policies with only a restocking charge.
If it's any consolation, I went thru 9 different sets of props dialing in my Black Thunder (triple engines). After one year of aggressive testing, I ended up selecting props that were 2 MPH slower on the top end than the best WOT props, but gave the best performance in terms of on plane, lowest RPM to stay on plane and best mid range cruise.
Many of the poeple on this Board have good recommendations for props shops. For my 2 cents, I highly recommend Dennis at Merc for his wheels. If budget is no concern then Bob Hering. Bob built and tuned the wheels that we set our Unlimited Hydro lap and overall race records with. He also has a very fair try and return policy, but we're talking something better than $1,500 per prop..... Another guy who has done great work for me, is Jeff Titus in Lake Placid, Florida. He does business as Performance Propellers. Most know him as an outboard prop guy, truth be known his props have set as many inboard records as outboard.
If you need any phone numbers for these folks, e mail me and I'll be happy to get them for you.
Good luck
You ask a good question, and I think the reason you're not getting much response, is because the answer is not black and white.
I've worked on my race props since I started in the late 60's. Usually started with a good foundation such as Merc or Hydromotive, or Hi-J or Menkens in the old days. The current crop are more efficient then earlier models, although top end on the surface piercing side, isn't much higher then it was in the days of 2 blade props, ET has dropped dramatically however.
Each hull responds differently to each propeller. There are some general givens: such as a 4 blade will create more stern lift than a 3 blade and a round ear will plane easier than a cleaver style blade in a non surfacepiercing environment. beyond that, it's a matter of test and see.
One of those givens is that a 15.25 (pitch) 4 blade will turn less rpm than a 15.25 3 blade, assuming the same pitch. Further, a 15.25 diameter 4 blade (or 5 blade) will create more stern lift than a 3 blade of equal diameter because there is more blade surface creating lift (down swing of the blade) with each revolution. By reducing diameter in the 4 blade prop you increase RPM but not necessarily forward speed.
It is very difficult to reduce pitch, other than removing cup. But removing cup may bring unwanted issues into play, such as blow out or trouble getting on plane with an aggressive X dimension.
Removing diameter is easier but merely seeing an RPM increase isn't a move forward. An alternative to removing diameter is removing leading edge and leaving diameter alone. In this scenario you reduce the size of the blade from the hub out towards the tip. This may not impact top end, but it will generally increase acceleration, such as thru a turn or from mid point of a turn up to top end.
All this to say, there isn't a forthright answer to your question
Find a prop company that will allow you to try a prop and continue to return them until you find one(s) that make you happy. Both Hydromotive and Merc Hi-perf have fair try and return policies with only a restocking charge.
If it's any consolation, I went thru 9 different sets of props dialing in my Black Thunder (triple engines). After one year of aggressive testing, I ended up selecting props that were 2 MPH slower on the top end than the best WOT props, but gave the best performance in terms of on plane, lowest RPM to stay on plane and best mid range cruise.
Many of the poeple on this Board have good recommendations for props shops. For my 2 cents, I highly recommend Dennis at Merc for his wheels. If budget is no concern then Bob Hering. Bob built and tuned the wheels that we set our Unlimited Hydro lap and overall race records with. He also has a very fair try and return policy, but we're talking something better than $1,500 per prop..... Another guy who has done great work for me, is Jeff Titus in Lake Placid, Florida. He does business as Performance Propellers. Most know him as an outboard prop guy, truth be known his props have set as many inboard records as outboard.
If you need any phone numbers for these folks, e mail me and I'll be happy to get them for you.
Good luck
12-27-2002 | 06:56 PM
#4
Registered
Joined: Dec 2002
Posts: 959
Likes: 0
From: chesapeake bay md
Steve thanks for your reply. My insterest is more in how or why to modify props in general. Althrough I do have several sets of props to test. Just wondering what to expect. If it was not so dam cold here, would be testing now.Your comment on acceleration may apply to my setup.Your idea about removing blade area is what I was thinking But I have no experence to back it up.But I do know once you take it off you can not put it back. May have to kill one set to get it right.[ go to far then back up a little]. Any way thanks for your insight and have a good holiday.
Michael
Michael
12-31-2002 | 08:33 AM
#6
Registered
Joined: Mar 2002
Posts: 5,143
Likes: 1
From: Cajun Country
mmwalters,
SteveDavid couldn't have said it better!!!!
The whole prop thing is trial and error. Especially when you are trying to fine tune a prop to a a paticular boat/motor combination. I have spent many days at the boat ramps with race teams trying different sets of props, modifying props on boats or bringing them back to the shop, working them and testing again the next day. The more succesful teams usually spend a lot of time testing props and working them until they are just right for a paticular race condition.
There is nothing wrong with removing blade surface area along the leading egde and not removing diameter. It can be time intensive doing that because the blade needs to be thined so there is an edge along the leading edge.
If yu were to reduce the blade surface area of a 4 blade prop to that of a comparable 3 blade there is still one more leading edge cuting through the water, of course that is going to cause resistance.
Of course the whole process can get very expensive working and reworking a prop and it is possible to go too far with something and then you are left with a paper weight. We try to make sure that the customer understands this before we get involved with a project that modifys the prop beyond it's original design.
Our main objective is to try and understand what the customer wants from a a set of props and work toward that goal. Just because a certain prop works on a paticular boat/motor combination doesn't mean it is going to satisfy a different customer with the same set up.
I would make sure you find a shop that will work with you and understand what your goal is. We are always open to new ideas that customers may have and will incorporate them into a prop that may, or may not, work for an application.
Matt
Throttle Up Propellers
[email protected]
www.throttle-up.com
SteveDavid couldn't have said it better!!!!
The whole prop thing is trial and error. Especially when you are trying to fine tune a prop to a a paticular boat/motor combination. I have spent many days at the boat ramps with race teams trying different sets of props, modifying props on boats or bringing them back to the shop, working them and testing again the next day. The more succesful teams usually spend a lot of time testing props and working them until they are just right for a paticular race condition.
There is nothing wrong with removing blade surface area along the leading egde and not removing diameter. It can be time intensive doing that because the blade needs to be thined so there is an edge along the leading edge.
If yu were to reduce the blade surface area of a 4 blade prop to that of a comparable 3 blade there is still one more leading edge cuting through the water, of course that is going to cause resistance.
Of course the whole process can get very expensive working and reworking a prop and it is possible to go too far with something and then you are left with a paper weight. We try to make sure that the customer understands this before we get involved with a project that modifys the prop beyond it's original design.
Our main objective is to try and understand what the customer wants from a a set of props and work toward that goal. Just because a certain prop works on a paticular boat/motor combination doesn't mean it is going to satisfy a different customer with the same set up.
I would make sure you find a shop that will work with you and understand what your goal is. We are always open to new ideas that customers may have and will incorporate them into a prop that may, or may not, work for an application.
Matt
Throttle Up Propellers
[email protected]
www.throttle-up.com
12-31-2002 | 11:56 PM
#9
Registered
Joined: Jul 2002
Posts: 1,387
Likes: 0
From: Houston,texas
CHRISTMAS BREAK
WE HAVE BEEN GONE ON CHRISTMAS VACATION AND SORRY WE COULD NOT GET BACK TO YOU SOON ENOUGH,althouh we are always willing to work towards the customers goal and meet those expectations,if you cant achieve your goal with someone else ,we are here for you now.good luck/Darin
01-01-2003 | 01:15 AM
#10
Guest
Posts: n/a
cutting the diameter down usually results in more low to midrange slip making it difficult to get on plane and increasing cruising rpms,most often in a pleasure boat it is best to take a little off the the backside(not the leading edge edge) 4 blades are usually a little slower top end but you gain better hole shot and midrange cruising,depending on your X diamention a 3blade might not work well for you espically when cruising in following seas
