HTM Mishap
02-05-2002, 04:58 PM
#41
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I would want to know the facts of what happened as it effects as all. The speeds now a days is getting out of hand. I am a firm believer of displacement. 150 miles an hour for a 30 foot boat(I would feel a little more safer in a boat with some more footage 38-46 foot) that isn't even a true tunnel like a Skater. I am not saying anything bad but you have to wonder about the materials being used in todays boats. Most of us can't afford a brand new Race Style Skater or MTI that is built with a higher grade fiberglass that can take the pressure at that speed.Most boat builders are trying to break the envelope of speed but at a budget, trying to beat the top names with speed but with 1/4 of the budget. Even the big names barrel roll or tumblesault but for the most part the boat stays in tact drives, engines , cockpit etc.
02-05-2002, 07:41 PM
#43
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I don't know if this is the place to continue the discussion on spining props in or out on twin engine setups, but i hate to see information put out and everyone guided accordingly, perhaps wrongly.
There is no question that, especially with surface piercing props, the downward rotating blade develops more thrust (forget torque) than the upward traveling blade when the propshaft is parallel to the water surface. This is because the water is slightly denser a foot or so deeper and there is less "blowout" potential as the upward traveling prop blade gets closer to the surface. Knowing that, it would be simple, spin them out because the moment arm of the higher thrust (upward force)is on the outsides or larger than if it were on the insides of the props by as much as a foot on each side.
But! A slight difference in the propshaft angle to the water makes a big difference in the effective angle of the prop to the oncoming water. If the propshaft is angeling up toward the boat bottom (trimmed in), the upward traveling blade suffers a pitch reduction (angle of attack reduction) and the downward traveling blade gets an even better bite. (The upward traveling blade is actually receding from the oncoming water to some degree) This is P factor of a CLMBING aircraft, ie it drifts to the left...requires right rudder. (The opposite happens in a power dive)
Now the outward spinning prop has even a greater advantage in the "righting" tendency.
Most people don't trim in however, they trim neutral to out. If trimmed out the opposite effect occurs and "physics" or intuition can be misleading.
We know all hulls are different but on average it is very likely that a trimmed out drive more than compensates for the rotation effects by the better bite the upward traveling prop gets when spun inward. This is why some folks notice an increase in speed, both blade planes are sharing the thrust load better and negates the stern lift that spinning in EITHER direction provides if the propshaft is horizontal. That gets defined as bow lift, rightly or wrongly because that is what it looks like.
People who spin their props inward are not irresponsible crazies who will sacrifice anything for a mph (though i might!). They are a lot, maybe a majority, of the racing cats, who have tested to tears and are doing what works for them. More than likely, though we all say we trim neutral, the inward spinning guys are trimmed out a bit.
Sorry this is long, but i know some will say, "what about torgue?" You tell me what the forces are with a moment arm of a few inches and i'll tell you what the reactive force the buoying is at a few feet. The difference is quite dramatic. We don't mind that the engines are spinning in the same direction...the forces are simply not that great. Neither are they on the props.
Ted
There is no question that, especially with surface piercing props, the downward rotating blade develops more thrust (forget torque) than the upward traveling blade when the propshaft is parallel to the water surface. This is because the water is slightly denser a foot or so deeper and there is less "blowout" potential as the upward traveling prop blade gets closer to the surface. Knowing that, it would be simple, spin them out because the moment arm of the higher thrust (upward force)is on the outsides or larger than if it were on the insides of the props by as much as a foot on each side.
But! A slight difference in the propshaft angle to the water makes a big difference in the effective angle of the prop to the oncoming water. If the propshaft is angeling up toward the boat bottom (trimmed in), the upward traveling blade suffers a pitch reduction (angle of attack reduction) and the downward traveling blade gets an even better bite. (The upward traveling blade is actually receding from the oncoming water to some degree) This is P factor of a CLMBING aircraft, ie it drifts to the left...requires right rudder. (The opposite happens in a power dive)
Now the outward spinning prop has even a greater advantage in the "righting" tendency.
Most people don't trim in however, they trim neutral to out. If trimmed out the opposite effect occurs and "physics" or intuition can be misleading.
We know all hulls are different but on average it is very likely that a trimmed out drive more than compensates for the rotation effects by the better bite the upward traveling prop gets when spun inward. This is why some folks notice an increase in speed, both blade planes are sharing the thrust load better and negates the stern lift that spinning in EITHER direction provides if the propshaft is horizontal. That gets defined as bow lift, rightly or wrongly because that is what it looks like.
People who spin their props inward are not irresponsible crazies who will sacrifice anything for a mph (though i might!). They are a lot, maybe a majority, of the racing cats, who have tested to tears and are doing what works for them. More than likely, though we all say we trim neutral, the inward spinning guys are trimmed out a bit.
Sorry this is long, but i know some will say, "what about torgue?" You tell me what the forces are with a moment arm of a few inches and i'll tell you what the reactive force the buoying is at a few feet. The difference is quite dramatic. We don't mind that the engines are spinning in the same direction...the forces are simply not that great. Neither are they on the props.
Ted
02-05-2002, 08:25 PM
#44
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Ted, so are saying that during a turn with the inside prop deeper in the water that an inward spinning prop won't tend to walk the stern out?
I would agree that straight line runs won't make a big difference unless the hull doesn't like the added lift. But in a turn it just makes sense.
I would agree that straight line runs won't make a big difference unless the hull doesn't like the added lift. But in a turn it just makes sense.
02-05-2002, 08:39 PM
#45
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I don't think spining props inward is necessarily "better" it just should not be rejected out of hand.
If in fact a prop exerts enough force at speed to cause "walking" then the effect would be there definitely. What i don't know is if that is a significant force. The only thing for sure is that thrust is very large. Because the prop is "beating" the water to some degree there is lift also...much, much smaller. Then there are factors that make a difference but are even smaller. I don't want to attribute handling problems to the wrong things that's all.
Hydrodynamics is witchcraft not engineering...a "science" that is empirical and a body of knowlege that has come about from testing and observation. The most accurate statement is probably that nothing is true for all boats all of the time.
Ted
If in fact a prop exerts enough force at speed to cause "walking" then the effect would be there definitely. What i don't know is if that is a significant force. The only thing for sure is that thrust is very large. Because the prop is "beating" the water to some degree there is lift also...much, much smaller. Then there are factors that make a difference but are even smaller. I don't want to attribute handling problems to the wrong things that's all.
Hydrodynamics is witchcraft not engineering...a "science" that is empirical and a body of knowlege that has come about from testing and observation. The most accurate statement is probably that nothing is true for all boats all of the time.
Ted
02-06-2002, 09:44 AM
#46
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Hi Gregg,
How are you doing? Are you going to be at the Tradewinds on the 16th? We will be there, I hope to see everyone there. My cell# is 973.214.9686 Call when you are in North Jersey.
Philip
How are you doing? Are you going to be at the Tradewinds on the 16th? We will be there, I hope to see everyone there. My cell# is 973.214.9686 Call when you are in North Jersey.
Philip
02-06-2002, 11:09 AM
#47
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Hi Ted, I think you are absolutely right, all we are concerned about here is Torque, and not thrust. Thrust is a linear force, and torque is a rotational force.
I am sure every boat exhibits different handling tendencies spinning “in” vs “out” , but I have found in both cases, 28’ and 32’ Skater that the boat is much more stable and handles better spinning out, I have not seen any speed gains spinning “in”, only a deterioration in stability and handling. I have been thinking about why? And IMO I think there are many “forces” and variables at work here. In an outboard boat these forces might not be the same as an inboard boat seeing there is no “in line” torque produced by the rotating engine mass. As we all know for every action there is an opposite and equal reaction. Torque produced by a right hand rotation, right side prop spinning out, would tend to force the left sponson down when the left side prop is out of the water and not producing torque, the effect would be reversed when the right side prop is out of the water and the left side prop is hooked up and producing torque. I would rather have the high side sponson being forced down by prop torque rather then being lifted up by torque. I also think that the advancing blade would enter the water sooner spinning out due to the angle of the water coming off the bottom of the sponson or dihedral. I also have noticed that spinning “in” tends to lift the stern of the boat which in an outboard boat I find not very desierable due to the reduced weight of the outboards over inboards. I am wondering in your outboard cat, what have you found to work best.
I am sure every boat exhibits different handling tendencies spinning “in” vs “out” , but I have found in both cases, 28’ and 32’ Skater that the boat is much more stable and handles better spinning out, I have not seen any speed gains spinning “in”, only a deterioration in stability and handling. I have been thinking about why? And IMO I think there are many “forces” and variables at work here. In an outboard boat these forces might not be the same as an inboard boat seeing there is no “in line” torque produced by the rotating engine mass. As we all know for every action there is an opposite and equal reaction. Torque produced by a right hand rotation, right side prop spinning out, would tend to force the left sponson down when the left side prop is out of the water and not producing torque, the effect would be reversed when the right side prop is out of the water and the left side prop is hooked up and producing torque. I would rather have the high side sponson being forced down by prop torque rather then being lifted up by torque. I also think that the advancing blade would enter the water sooner spinning out due to the angle of the water coming off the bottom of the sponson or dihedral. I also have noticed that spinning “in” tends to lift the stern of the boat which in an outboard boat I find not very desierable due to the reduced weight of the outboards over inboards. I am wondering in your outboard cat, what have you found to work best.
02-06-2002, 02:52 PM
#48
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We spin in. We tried it out and we got bad handling...never raced with it though, it was that bad. Skater recommended we spin in and we feel they were right. I get the feeling that in right turns we do get some climbing (two rights and a left) but it could be our imagination, it is so hard to tell.
I really was trying to deemphasize the consideration of engine torque in my blurb above completely, and talk about torque forces on the props as being quite small. They are there though and when there is not a lot of boat in the water, they may contribute, i just don't know how much...we need more GOOD data on that subject.
Ted
I really was trying to deemphasize the consideration of engine torque in my blurb above completely, and talk about torque forces on the props as being quite small. They are there though and when there is not a lot of boat in the water, they may contribute, i just don't know how much...we need more GOOD data on that subject.
Ted
