Joe, The 3.8" 1450 with reduced diameter. 50 mm? Are you sure it is not 46 or 47 mm? After we clean up a stock 1450 they are normally closer to 49 mm.
Did you ever run this on your Roadrunner? If so, how did it work?
Normally a stock 1450 will have more lift than a cupped 1400 prop, but this depends on how the cupping is done.
If the cupping is formed to only increase the overall pitch of the prop this cupping will ALWAYS add lift. In effect this prop has a greater pitch/diameter ratio. Meaning the 1400 prop may now be a 1500, 1600, 1700 in terms of P/D ratio and the lift will increase with the P/D increase considering the Rake remains the same.
Note: when I reference 1400, 1500 I am not indicating that the prop will have the same lift characteristics as an Octura 1600, 1700 ect. Progression, rake have an effect on the lift and some series of Octura props that have higher P/D ratios actually have less lift than some of the Lower P/D props that Octura makes.
Octura 1200, 1400 series props would be considered low P/D ratio props. 1400 indicates that the pitch is 1.4 times greater than the diameter. Octura 1450- 1.4 X 50 mm = 70 mm = 2.75". Octura 2150 - 2.1 X 50 mm = 105 mm = 4.13".
Do to the differerences in progression and rake between these two props, the lift of each prop is not much different. On the other hand the 1450 and the X450 have the same P/D ratio of 1.4, but the lift characteristics of each prop is very different as most boaters have expericenced. Again, this is due to different progression and rake between the two props.
Now while it is possible to maintain mininmal lift while increasing P/D, it is not so easy to control prop walk as the P/D increases. This is the one of the reasons you need to change rudder trim with the different props Joe. There are two other things that will cause a need for extra rudder trim becuase of prop lift. 1. The higher ride hight if the prop will reduce the amount of rudder in the water, increasing the amount of propwalk. 2. The increased ride hight will change the attitude of the hull and turnfin causing it to pull more down force on the right front sponson. Both of these will depend on the sensitivtity of the hull. Some hulls (normally older less responsive designs), will never be effected in this way.
If the desired efffect is to have increased Pitch while maintaining the same or less lift when working with a prop, the rake and progression must be changed accordingly.
Note: Backcutting a prop to make it visually appear to have more rake does not actually change the rake at all. For some props backcutting will reduce lift simply because the highest pitch area of the blade is being removed. This is effectively reducing the P/D ratio which is really what reduces the lift.
To more simply address Joe's problem, the H-4, H-7 and cut & cupped 1450 all have a much higher P/D ratio than the stock 1450. If the 1450 (Ron's) is cut to 46 mm and cupped to 3.8" the P/D ratio of this prop is about 1.8. This is much more than the original 1.4. Again this higher P/D is causing extra lift that is blowing the the tramsom out of the water.
Now, when Joe runs the stock 1450 the transom stays planted, but speed is not what he wants, so he cups prop to increase pitch. The boat runs faster and the transom is still hooked up. So, we are asking why did this cupped 1450 with increased P/D ratio stay hooked up??? The answer is simple... The prop is heavier so it holds the transom in the water......just kidding LOL!
Because the prop is full bladed the engine can't pull it as easily as the other three smaller bladed props. Therefore, the cup that Joe was able to add was less than what are in the other three props. This means the prop still has a relativly low P/D. Also when Joe cupped the prop, he may have added the cup in such a way that he also added rake and effectively reduced lift while increasing P/D.
All three of the other props can be made to have the same lift as Joe's 1450.
They simply need to be cupped in a way that will maintain the desired pitch and reduce lift at the same time.
Not all hulls of the same type (rigger, tunnel, Vee, Cat) require the exact same prop lift.
When someone calls me for a rigger prop it is important for me to know who's rigger design. If it's a design that I have not previously cupped a prop for I may not hit it right the first time. This is one of the reasons I offer one FREE recupping.
If it comes from me I want it to be right. When I have a customer with a unique set up that I'm not sure about and the prop is totally the wrong one, I exchange the prop at no additional charge. The only bad thing about this is that I stay so busy, as most of you know, it takes a looooong time to get a prop from me.
Hope this helps.
Good Boating,
Andy
Did you ever run this on your Roadrunner? If so, how did it work?
Normally a stock 1450 will have more lift than a cupped 1400 prop, but this depends on how the cupping is done.
If the cupping is formed to only increase the overall pitch of the prop this cupping will ALWAYS add lift. In effect this prop has a greater pitch/diameter ratio. Meaning the 1400 prop may now be a 1500, 1600, 1700 in terms of P/D ratio and the lift will increase with the P/D increase considering the Rake remains the same.
Note: when I reference 1400, 1500 I am not indicating that the prop will have the same lift characteristics as an Octura 1600, 1700 ect. Progression, rake have an effect on the lift and some series of Octura props that have higher P/D ratios actually have less lift than some of the Lower P/D props that Octura makes.
Octura 1200, 1400 series props would be considered low P/D ratio props. 1400 indicates that the pitch is 1.4 times greater than the diameter. Octura 1450- 1.4 X 50 mm = 70 mm = 2.75". Octura 2150 - 2.1 X 50 mm = 105 mm = 4.13".
Do to the differerences in progression and rake between these two props, the lift of each prop is not much different. On the other hand the 1450 and the X450 have the same P/D ratio of 1.4, but the lift characteristics of each prop is very different as most boaters have expericenced. Again, this is due to different progression and rake between the two props.
Now while it is possible to maintain mininmal lift while increasing P/D, it is not so easy to control prop walk as the P/D increases. This is the one of the reasons you need to change rudder trim with the different props Joe. There are two other things that will cause a need for extra rudder trim becuase of prop lift. 1. The higher ride hight if the prop will reduce the amount of rudder in the water, increasing the amount of propwalk. 2. The increased ride hight will change the attitude of the hull and turnfin causing it to pull more down force on the right front sponson. Both of these will depend on the sensitivtity of the hull. Some hulls (normally older less responsive designs), will never be effected in this way.
If the desired efffect is to have increased Pitch while maintaining the same or less lift when working with a prop, the rake and progression must be changed accordingly.
Note: Backcutting a prop to make it visually appear to have more rake does not actually change the rake at all. For some props backcutting will reduce lift simply because the highest pitch area of the blade is being removed. This is effectively reducing the P/D ratio which is really what reduces the lift.
To more simply address Joe's problem, the H-4, H-7 and cut & cupped 1450 all have a much higher P/D ratio than the stock 1450. If the 1450 (Ron's) is cut to 46 mm and cupped to 3.8" the P/D ratio of this prop is about 1.8. This is much more than the original 1.4. Again this higher P/D is causing extra lift that is blowing the the tramsom out of the water.
Now, when Joe runs the stock 1450 the transom stays planted, but speed is not what he wants, so he cups prop to increase pitch. The boat runs faster and the transom is still hooked up. So, we are asking why did this cupped 1450 with increased P/D ratio stay hooked up??? The answer is simple... The prop is heavier so it holds the transom in the water......just kidding LOL!
Because the prop is full bladed the engine can't pull it as easily as the other three smaller bladed props. Therefore, the cup that Joe was able to add was less than what are in the other three props. This means the prop still has a relativly low P/D. Also when Joe cupped the prop, he may have added the cup in such a way that he also added rake and effectively reduced lift while increasing P/D.
All three of the other props can be made to have the same lift as Joe's 1450.
They simply need to be cupped in a way that will maintain the desired pitch and reduce lift at the same time.
Not all hulls of the same type (rigger, tunnel, Vee, Cat) require the exact same prop lift.
When someone calls me for a rigger prop it is important for me to know who's rigger design. If it's a design that I have not previously cupped a prop for I may not hit it right the first time. This is one of the reasons I offer one FREE recupping.
If it comes from me I want it to be right. When I have a customer with a unique set up that I'm not sure about and the prop is totally the wrong one, I exchange the prop at no additional charge. The only bad thing about this is that I stay so busy, as most of you know, it takes a looooong time to get a prop from me.
Hope this helps.
Good Boating,
Andy
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Great post Andy, I'm sure this sheds light on things for alot of people! B)
