Prop Diameter-who needs it?
07-14-2003, 12:10 PM
#1
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Prop Diameter-who needs it?
I'm trying some different props and I need to know how the diameter of a prop can affect different applications. With my cat a 32" Bravo one got me 89mph but with 15% slip. Trying to find cleaver/semi-cleaver style props with a 15"+ diameter is pretty tough. There are only a couple of makers that offer props with really big diameters. The Hoss hyperdrive series (a true cleaver) comes in 14 1/2", 15", 16 1/2", and 17" diameters. The Hydromotive Q4+ can be had in a 16" diameter. I'm working on a deal for a Hydromotive Q-4 (15.5"). I'm also looking at a Yamaha drag (3 blade) and a couple of true cleavers (3 blade), but their diameters are small (14.5"). Is it even worth my time to try these props because of the diameter? Do I need to compensate for the lack of blade area by adding pitch? I've had an offer to custom build a Yamaha drag with 5 blades for a extreamely affordable price. Is the addition of blades acceptable compensation for the lack of diameter? Do I even need a large diameter prop for my boat?
26' twin tunnel catamaran
540 w/600hp @ 5400
Bravo 1 drive set level with the bottom
26' twin tunnel catamaran
540 w/600hp @ 5400
Bravo 1 drive set level with the bottom
Last edited by Cord; 07-14-2003 at 12:23 PM.
07-14-2003, 02:41 PM
#2
Registered
You are doing the right thing in trying as many props as possible. My fastest prop slips about 15% and my second fastest slips about 7%. The 15% prop is a small diameter Spinelli 4 blade and is the most fun to drive. It accelerates very hard and has the top speed to back up the snappy mid range. The 7% prop is a blueprinted 15.5" dia Hydromotive and it also performs very well. It slips more coming out of the hole than the Spinelli, but has a much faster cruising speed. The acceleration is not as good and the top speed is about 1 MPH off. I tried many different Bravo props and they have a strong hole shot with very little slip rolling over onto plane. The cruising speed on the Bravo props were better than the Hydromotive and the acceleration was about the same. The top speed was off a couple of MPH compared to the Hydro. I don't know if any of this will help you since your application is quite a bit different from mine. It seems like with extremely high X dimensions Bravo props do the best in getting the boat on plane, but they may not be the fastest wheel. Good luck and keep us posted!!!
07-14-2003, 03:46 PM
#3
First and foremost. What ever your propeller says it is for pitch is wrong!
Second, your slippage numbers are incorrect due to the fact that the propeller pitch you were led to believe is correct is in fact wrong.
Point: Without an electronic pitch reader, who the hell knows what pitch you accurately have.
Everyone states that a 32" Mercury 4 blade Bravo 1 propeller is the equivalent to a 30" hydromotive 4 blade propellers which is equivalent to someone else’s bla bla bla, hence who's lying?
If there is a conversion factor then there is a B.S. factor as well.
Diameter!
It will help you get on plane faster, help slow speed cruise, aid in propeller slippage efficiencies to a point.
However, it will also kill top speed, reduce rpm's, create more propeller walk, and create more stern lift and a host of other things.
Number of blades:
You can run a smaller diameter propeller with the same pitch if you have more blades to do the work and be as efficient at the same time.
More blades will also get you on plane faster, cruise better, be more efficient at certain speeds ranges over other # of blade propellers and are (if balanced correctly) far more smoother than that of a lesser bladed propeller.
Cleavers:
These will actually plane a heavier boat faster than a non cleaver propeller, but… They do not back up as well due to the fact that they are not a symmetrical design like a round ear propeller design is. The blunt rear edge would rather push the water away than to pull it through it. The more blades a cleaver has the more noticeable the backing problem becomes.
They do not produce more stern lift, rather they do not have cup in the tips to give the lifting leverage that a round ear propeller does. What this means is that if you need a lot of bow lift from the drive or are relying on bow lift coming from the cup in the propeller, you will have less of it coming from these propellers. This is noticeable with a very high X dimension boat as you can no longer trim it out to get the bow lift without blow out and the propeller will not raise the bow as there is no cup in the tips.
More over, if your boat has enough lift designed into its hull that you do not need to rely upon bow lift from positive trim of an outdrive and or need a lot of cup in the tips of the propeller to give bow lift, then a cleaver will work fine on your application.
Try the Mercury outboard cleavers. They will fit the Bravo’s and they work well with a high X dimension. Not to mention they are not expensive and they come in pitches up to 36” and diameters of 15.5 in pitch of 34” and higher. 15” if less than 34”.
Second, your slippage numbers are incorrect due to the fact that the propeller pitch you were led to believe is correct is in fact wrong.
Point: Without an electronic pitch reader, who the hell knows what pitch you accurately have.
Everyone states that a 32" Mercury 4 blade Bravo 1 propeller is the equivalent to a 30" hydromotive 4 blade propellers which is equivalent to someone else’s bla bla bla, hence who's lying?
If there is a conversion factor then there is a B.S. factor as well.
Diameter!
It will help you get on plane faster, help slow speed cruise, aid in propeller slippage efficiencies to a point.
However, it will also kill top speed, reduce rpm's, create more propeller walk, and create more stern lift and a host of other things.
Number of blades:
You can run a smaller diameter propeller with the same pitch if you have more blades to do the work and be as efficient at the same time.
More blades will also get you on plane faster, cruise better, be more efficient at certain speeds ranges over other # of blade propellers and are (if balanced correctly) far more smoother than that of a lesser bladed propeller.
Cleavers:
These will actually plane a heavier boat faster than a non cleaver propeller, but… They do not back up as well due to the fact that they are not a symmetrical design like a round ear propeller design is. The blunt rear edge would rather push the water away than to pull it through it. The more blades a cleaver has the more noticeable the backing problem becomes.
They do not produce more stern lift, rather they do not have cup in the tips to give the lifting leverage that a round ear propeller does. What this means is that if you need a lot of bow lift from the drive or are relying on bow lift coming from the cup in the propeller, you will have less of it coming from these propellers. This is noticeable with a very high X dimension boat as you can no longer trim it out to get the bow lift without blow out and the propeller will not raise the bow as there is no cup in the tips.
More over, if your boat has enough lift designed into its hull that you do not need to rely upon bow lift from positive trim of an outdrive and or need a lot of cup in the tips of the propeller to give bow lift, then a cleaver will work fine on your application.
Try the Mercury outboard cleavers. They will fit the Bravo’s and they work well with a high X dimension. Not to mention they are not expensive and they come in pitches up to 36” and diameters of 15.5 in pitch of 34” and higher. 15” if less than 34”.
07-15-2003, 06:55 AM
#5
Registered
BOATLESS pretty much covered things, just a couple things to consider;
Does your boat need a bow lift or stern lift propeller?
This is especially important with cats.
Use slip number as a tool, don't get too wrapped up in them as one needs to know the actual pitch of the prop to arrive at the actual slip number. As a general rule stock Bravo 1's are 1-1.5" less pitch than they are stamped. Lab finished even less. Hydromotives are at, or slightly above the marked pitch. Mirage Plus' are very close to marked pitch or slightly below.
In your case because of the power to weight ratio you will have to be careful about too much stern lift. You will need a higher pitch prop. The higher the pitch the greater the stern lift, the greater the diameter the greater the stern lift.
Your prop shaft height is about normal for a cat. You really shouldn't need a prop with an excessively large diameter. Try to stick with props from major manufacturers. They have consistant quality and if you buy one you can usually exchange it if it isn't right for your boat.
The final decision in chosing a prop is what type of boating do you plan to do and where. One prop does not fit all. The same boat with different owners may desire completely different qualities from a prop. It's important to get a prop that works for you! it's your boat and you are the one who has to drive it.
Does your boat need a bow lift or stern lift propeller?
This is especially important with cats.
Use slip number as a tool, don't get too wrapped up in them as one needs to know the actual pitch of the prop to arrive at the actual slip number. As a general rule stock Bravo 1's are 1-1.5" less pitch than they are stamped. Lab finished even less. Hydromotives are at, or slightly above the marked pitch. Mirage Plus' are very close to marked pitch or slightly below.
In your case because of the power to weight ratio you will have to be careful about too much stern lift. You will need a higher pitch prop. The higher the pitch the greater the stern lift, the greater the diameter the greater the stern lift.
Your prop shaft height is about normal for a cat. You really shouldn't need a prop with an excessively large diameter. Try to stick with props from major manufacturers. They have consistant quality and if you buy one you can usually exchange it if it isn't right for your boat.
The final decision in chosing a prop is what type of boating do you plan to do and where. One prop does not fit all. The same boat with different owners may desire completely different qualities from a prop. It's important to get a prop that works for you! it's your boat and you are the one who has to drive it.
07-15-2003, 07:18 AM
#6
Registered
BOATLESS pretty much covered things, just a couple things to consider;
Does your boat need a bow lift or stern lift propeller?
This is especially important with cats.
Use slip number as a tool, don't get too wrapped up in them as one needs to know the actual pitch of the prop to arrive at the actual slip number. As a general rule stock Bravo 1's are 1-1.5" less pitch than they are stamped. Lab finished even less. Hydromotives are at, or slightly above the marked pitch. Mirage Plus' are very close to marked pitch or slightly below.
In your case because of the power to weight ratio you will have to be careful about too much stern lift. You will need a higher pitch prop. The higher the pitch the greater the stern lift, the greater the diameter the greater the stern lift.
Your prop shaft height is about normal for a cat. You really shouldn't need a prop with an excessively large diameter. Try to stick with props from major manufacturers. They have consistant quality and if you buy one you can usually exchange it if it isn't right for your boat.
The final decision in chosing a prop is what type of boating do you plan to do and where. One prop does not fit all. The same boat with different owners may desire completely different qualities from a prop. It's important to get a prop that works for you! it's your boat and you are the one who has to drive it.
Does your boat need a bow lift or stern lift propeller?
This is especially important with cats.
Use slip number as a tool, don't get too wrapped up in them as one needs to know the actual pitch of the prop to arrive at the actual slip number. As a general rule stock Bravo 1's are 1-1.5" less pitch than they are stamped. Lab finished even less. Hydromotives are at, or slightly above the marked pitch. Mirage Plus' are very close to marked pitch or slightly below.
In your case because of the power to weight ratio you will have to be careful about too much stern lift. You will need a higher pitch prop. The higher the pitch the greater the stern lift, the greater the diameter the greater the stern lift.
Your prop shaft height is about normal for a cat. You really shouldn't need a prop with an excessively large diameter. Try to stick with props from major manufacturers. They have consistant quality and if you buy one you can usually exchange it if it isn't right for your boat.
The final decision in chosing a prop is what type of boating do you plan to do and where. One prop does not fit all. The same boat with different owners may desire completely different qualities from a prop. It's important to get a prop that works for you! it's your boat and you are the one who has to drive it.
