Wayward slow speed 'bow-steer' on deep vee hulls?

Rocksteadee said:
Try pushing a pencil in a straight line from the stern. The slightest lack of concentration and it will turn
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IMHO you have it right. This is not a deep V issue but a sterndrive issue. Nobody could accuse Sealines of being deep V hulls but both of my Sealine sterndrive boats steered like a drunken chicken at slow speeds. It is all about the point at which thrust is applied to the hull. Sterndrives push the boat right from the transom whereas shaftdrives push the boat from a point about 1/3rd of the way forward of the transom. That plus the fact that sterndrive boats have their engines further back and hence the CoG further back and that makes the bow very light and easier to deflect
 
Deleted User said:
IMHO you have it right. This is not a deep V issue but a sterndrive issue. Nobody could accuse Sealines of being deep V hulls but both of my Sealine sterndrive boats steered like a drunken chicken at slow speeds. It is all about the point at which thrust is applied to the hull. Sterndrives push the boat right from the transom whereas shaftdrives push the boat from a point about 1/3rd of the way forward of the transom. That plus the fact that sterndrive boats have their engines further back and hence the CoG further back and that makes the bow very light and easier to deflect
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This sounds like it makes perfect sense until you look at a Botnia stern drive patrol boat: steers straight as a die at slow speeds, and even planes at ultra slow speed which sounds like a contradiction.

The props of a shaft drive push from the transom, or very close to it.
 
This sounds like it makes perfect sense until you look at a Botnia stern drive patrol boat: steers straight as a die at slow speeds, and even planes at ultra slow speed which sounds like a contradiction.
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To be honest I think we need to hear from an owner rather than hearing what it looks like! And if it planes at slow speed, that generally means the hull is beamy and rather flat at the back to create the lift. Not a bad thing but it aint a deep V

The props of a shaft drive push from the transom, or very close to it
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No they dont. They push from the point at which the gearboxes are mounted which is usually well forward of the transom
 
Deleted User said:
To be honest I think we need to hear from an owner rather than hearing what it looks like! And if it planes at slow speed, that generally means the hull is beamy and rather flat at the back to create the lift. Not a bad thing but it aint a deep V.

The props of a shaft drive push from the transom, or very close to it.
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No they dont. They push from the point at which the gearboxes are mounted which is usually well forward of the transom
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Interesting!

A good analogy is the comparison between bicycle and a tricycle. A bike leans over easily and starts to turn, a trike is supported horizontially and needs the steering to be rotated to initiate a turn.

A deep vee is like the bike with no dynamic stability, the low vee is supported by the flat sections at the stern and is like a trike.
 
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My Levi Corsair has 26deg deadrise, it tracks fine at low speed although you can steer it a little by loading one side or the other. It is twin engined on shafts, both same rotation. I have a wandry rib though that may have something to do with the steering play!
 
Bouba said:
) a pair of counter rotating props on the same drive should make her track true like a torpedo
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Not sure about that .

My single engine s23 on outdrive used to wander and was very easily affected by wind but correcting this becomes second nature . Avoiding over correction is the trick. Sometimes doing nothing would result in the boat sorting itself out.
My present boat has twins on outdrives and is far better but even so it does need some steering correction.
 
gordmac said:
My Levi Corsair has 26deg deadrise, it tracks fine at low speed although you can steer it a little by loading one side or the other. It is twin engined on shafts, both same rotation. I have a wandry rib though that may have something to do with the steering play!
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26° is a serious amount of deadrise. Wow!

It's beginning to look like it might be a weight influenced issue: heavy hulls have the required inertia to track track straight?
 
You could ( designer ) have a bell shaped bottom as opposed to a sharp v edge , where both sides of the hull form a keel .

These act a lifting pads as well as minimise the alleged rudder effect of a knife V off balanced by waves .
Also have extra wide chine flats .These reduce the rolling tendency both at D and P speeds .
At P the faster you go the stiffer in terms of roll resistance it gets .

Deep V is the aggregate of the dead rise angle it’s the tiny little details that count .In no particular order of importance———

1-Chine width

2- Lifting pad ( bell shape bottom not knife )

3- Additional lifting strips that run the L of the hull to the stern

4- And as said any centrifugal effect ( if any applicable? ) running in the long horizontal axis .

Loads of the above will resist turning -at any speed obviously more so at speed like my boat - as said to effect a turn I have throttle back and slow right down , to reduce the actions of the above four so the actual rudders can effect a turn .

Not much of the above 4 components it will wander about all over the pace like a small rib with a OB
 
Portofino said:
You could ( designer ) have a bell shaped bottom as opposed to a sharp v edge , where both sides of the hull form a keel .

These act a lifting pads as well as minimise the alleged rudder effect of a knife V off balanced by waves .
Also have extra wide chine flats .These reduce the rolling tendency both at D and P speeds .
At P the faster you go the stiffer in terms of roll resistance it gets .

Deep V is the aggregate of the dead rise angle it’s the tiny little details that count .In no particular order of importance———

1-Chine width

2- Lifting pad ( bell shape bottom not knife )

3- Additional lifting strips that run the L of the hull to the stern

4- And as said any centrifugal effect ( if any applicable? ) running in the long horizontal axis .

Loads of the above will resist turning -at any speed obviously more so at speed like my boat - as said to effect a turn I have throttle back and slow right down , to reduce the actions of the above four so the actual rudders can effect a turn .

Not much of the above 4 components it will wander about all over the pace like a small rib with a OB
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Am I correct in saying that larger planing hulls accelerate through the transition zone and straight into the plane with very little trim change, unlike small light boats which fall into a hole left by the bow wave and often rear up at alarming angles of trim?

So perhaps the dynamics of hull design are significantly different depending on length/weight.
 
The steering mechanism has a lot to do with it as well

Our first boat, 22 ft or so, 120 engine hours with a 5.7 V8 280hp engine and dual prop drive very little play in the steering and little or no wander.

Our second boat, same manufacturer, same hull shape but 24 ft. 5.7 V8 " 320" hp, I suspect less, with the same dual prop drive, A new boat. Same steering mechanism, but very sloppy steering, it wandered all over the place.

Fast forward 10 years or so to a "new" boat with a D4 and DHP drive/ hydraulic steering. I was expecting it to be like the hydraulic steering equipped RIB I did my PW2 training on in 2001, Wrong, it's worse than either of the previous boats with cable steering ! The slop at the steering wheel is incredible; I expected a hydraulic system to be almost slop free like on the the numerous industrial machines I have been involved with over the years.. No problems with the helm pump etc, it's "just the way it is " apparently
 
GrahamHR said:
The steering mechanism has a lot to do with it as well

Our first boat, 22 ft or so, 120 engine hours with a 5.7 V8 280hp engine and dual prop drive very little play in the steering and little or no wander.

Our second boat, same manufacturer, same hull shape but 24 ft. 5.7 V8 " 320" hp, I suspect less, with the same dual prop drive, A new boat. Same steering mechanism, but very sloppy steering, it wandered all over the place.

Fast forward 10 years or so to a "new" boat with a D4 and DHP drive/ hydraulic steering. I was expecting it to be like the hydraulic steering equipped RIB I did my PW2 training on in 2001, Wrong, it's worse than either of the previous boats with cable steering ! The slop at the steering wheel is incredible; I expected a hydraulic system to be almost slop free like on the the numerous industrial machines I have been involved with over the years.. No problems with the helm pump etc, it's "just the way it is " apparently
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That reminds me of my first real car, a Cortina MK2 with steering box rather than the rack & pinion in the later Escort: it was an old farm car and there was 2" of loose play in the straight ahead position. You had to guess when to add steering input to correct a severe deviation off the road.

You could have brought up a very valid point.
 
rustybarge said:
Am I correct in saying that larger planing hulls accelerate through the transition zone and straight into the plane with very little trim change, unlike small light boats which fall into a hole left by the bow wave and often rear up at alarming angles of trim?

So perhaps the dynamics of hull design are significantly different depending on length/weight.
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Yes I think you are “ correct “

Planing

A planing hull is simply one so shaped that a degree of dynamic lift is added to its natural buoyancy during the time when its speed of advance exceeds that rate at which solid water can close in abaft of it .


I think we all first need to agree that there is no one binary point of planing. It is not either on or off. It is a transitional regime, and therefore, any attempt to define a point of planing is somewhat nonsensical.


What actually defines planing using the most widely accepted definition is —-

The fact that if the speed increases the trim angle will decrease.

If you aren't planing, increasing speed in the sub planing regime results in an increase in trim angle, for a planing hull. Again, for a semi-planing hull that might not happen at all, but for a true planing hull, the speed where it attains planing status is the point where the trim angle decreases as the speed increases.

Look guys this is pretty simple.
Think about an airplane wing. To supply a given amount of lift at a given speed you need a specific angle of attack. If the speed increases you have to decrease the angle of attack or you will get an increase in lift. As you approach planing speed the trim angle doesn't decrease and indeed, the lift is increasing since you are supporting an ever increasing portion of the hull with hydrodynamic lift.
But once you are on the plane, even though the hull may be heavy and still have a significant portion of displacement lift, if &the trim angle drops as speed increases, it has attained planing status.
This is why using arbitrary measures like % of weight or amount of lift, are not appropriate, a heavy hull might have a different lifted height when compared to the same hull when lightly loaded. But in both cases, when the trim angle decreases, planing has been realized.

How much of the displacement volumn must be above the surface to be "on plane"? all of it, most of it, half of it, any of it? What % if any ?
Since planing begins to occur when hydraulic forces lift the hull, how much until it is actualy "planing"? Any lift replaces displacement forces with dynamic ones, so some say the begining of planing occures when there is any lift not associated with displacement.

Therefore to some they see no reason for any of the qualifiers at all. many shapes can plane to one degree or another, no reason to put those conditions on it. I also do not see why 50 percent is the magic number either, if any portion of the weight is lifted out of the water by hydrodynamic lift you will reduce the drag and increse speed.

Some say that planing occures if any of the weight is suppored by the dynamic forces on the hull from the water. A little or a lot of the weight being supported is illrelevant, the fact that the hull is not fully in displacement mode means that lift forces on the hull are partially supporting the weight of the boat.

I think the real problem is the word "planing" itself is obsolete and based on archaic ideas about fluid mechanics. The origin of the word assumes it is even possible to be above or "on" the plane of the surface. We know a lot more about the process and forces involved but are stuck with these obsolete terms.


Getting back to the topic some suggest taking into account the “subjective feeling “ of the persons on board, I believe that anyone who has been on board a planing vessel can witness that it's movements became "stiffer" and more jerky when encountering waves at high speeds than it was during low-speed navigation. A mathematical explanation for this behavior is that the perturbation of hydrostatic lift component is in linear relationship to the vertical speed of the incoming wave disturbance, while the dynamic lift component increases as a square function of the vertical speed of the perturbation. Hence, the vertical accelerations become much more severe in high-speed regime.
It makes me think that perhaps it can lead us to a completely different criterion for the definition (by convention) of the planing, based on the vertical acceleration response of the vessel to incoming waves, or to a single standardized perturbation which could be reproduced in towing tanks. The planing regime would then be indicated during sea trials by the on-board accelerometers, rather than through the GPS readout .

Just some food for your creative minds.


I think that at the end, it is a matter of semantics and not physics or engineering. It would be better to say, for example, "this vessel at a speed of 54 knots has 93% dynamic lift to displacement ratio", rather than argue whether it is planing or not

I think we can all agree that, when looking trim angle it becomes pretty clear where planing starts.

The more you think about it one soon realises this becomes different for every hull, and for every load condition, every thrust angle, and for every offset of the thrust vector from the line of the planing surface, as well as things like prop rake. It's possible to significantly change the onset of planing by modifying those variables.

When all things are considered, it is far more complex than a simple % of lift. For lightly loaded short, wide hulls, the change in trim angle happens much more quickly with increasing speed. For much larger hulls with heavier bottom loading, the curve will likely be more of a gentle hump, but the reality is, when you are past the peak of the trim angle curve the hull is now in the planing regime. Since there a any number of variables that can push the actual speed at which the trim angle starts to decrease - that's not a bad thing to use for a rule of thumb .
*
Maybe planing is like time: we all know what it is, until we think about it
 
As an experiment this weekend I tried the leaning effect to create a turn and can confirm it does indeed effect a turn on both tender and mobo without trim tabs. I can confirm also that trim tabs deployed unevenly to counteract any lean has a pronounced effect on steering wheel centre point when on the plane. Intuitive I suppose but nice to confirm
 
We have a 31 footer with twin V6 Mercs and sterndrives.....at first you tend to overcorrect the wander but as you bevcome more relaxed its pretty easy to keep her in a straight line at slow speeds...just as well as we wre right at the top of the Hamble and its a long way below 6 knots. I heve definitely notived an improvement as well since we replaced both dives and transom shield assemnlies a couple of years ago...theres always some slack in sterndrive steering (mercs anyway after a while).

Because she is narrow with twins (at time of build the widest towable beam US regs) she did tend to lean into the wind a lot and the slightest of crew movement would involve a lot of fiddling around with the tabs so there was always a lot to keep me busy. Found a gadget by Mente Marine,(swedish I think) which replaces the 2 Bennett rocker switches and is connected up to the same wiring...really easy. Went out and set it to the desired angles for me trim wise at about 20 knots, pressed and held the set button...job done...now when we go out, it comes on automatically when we are moving down river, senses when we are getting on the plane and operates the trim tabs to level out the boat to the desired attitude......touch of up trim on the drives and just steer it...beautiful....tabs are automatically lifted again and only operate in a side wind or if anything changes....put the boat into a turn and the system disengages till you straighten up again....I have nothing to gain from explaining all this but the website is easy enough to find..and a couple of friends have also fitted this system with similar results....
 
BruceK said:
As an experiment this weekend I tried the leaning effect to create a turn and can confirm it does indeed effect a turn on both tender and mobo without trim tabs. I can confirm also that trim tabs deployed unevenly to counteract any lean has a pronounced effect on steering wheel centre point when on the plane. Intuitive I suppose but nice to confirm
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That sort of fits in with my bicycle analogy that I made earlier in the post: deep vee leans over easier and initiates turn....:)
 
Offshore57 said:
We have a 31 footer with twin V6 Mercs and sterndrives.....at first you tend to overcorrect the wander but as you bevcome more relaxed its pretty easy to keep her in a straight line at slow speeds...just as well as we wre right at the top of the Hamble and its a long way below 6 knots. I heve definitely notived an improvement as well since we replaced both dives and transom shield assemnlies a couple of years ago...theres always some slack in sterndrive steering (mercs anyway after a while).

Because she is narrow with twins (at time of build the widest towable beam US regs) she did tend to lean into the wind a lot and the slightest of crew movement would involve a lot of fiddling around with the tabs so there was always a lot to keep me busy. Found a gadget by Mente Marine,(swedish I think) which replaces the 2 Bennett rocker switches and is connected up to the same wiring...really easy. Went out and set it to the desired angles for me trim wise at about 20 knots, pressed and held the set button...job done...now when we go out, it comes on automatically when we are moving down river, senses when we are getting on the plane and operates the trim tabs to level out the boat to the desired attitude......touch of up trim on the drives and just steer it...beautiful....tabs are automatically lifted again and only operate in a side wind or if anything changes....put the boat into a turn and the system disengages till you straighten up again....I have nothing to gain from explaining all this but the website is easy enough to find..and a couple of friends have also fitted this system with similar results....
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I will Google that to have a look. :)
 
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