Will a DEEP spade rudder prevent chine tripping ?

The angle of the rudder shaft will determine the force vector when the rudder turns. A vertical shaft has only a side force. A forward inclined shaft will lift the bow when turning. A aft inclined shaft will lift the stern when turning.

 

So with what has been written go back to your origanal thoughts about a deep spade rudder and what have you learned what will it do for chine tripping ????

 

The simple statement in the text, to which you refer, can not be regarded as a general rule. "Tunnels" has put it in the right perspective. The initial heeling may be influenced by rudder angle of attack and the depth of the resulting force vector. But in a high speed situation, the asymmetrical pressure distribution over the two bottom sides, and the resulting heeling moment is far more powerful than the rudder forceonce the boat is into a turn.

It is all about balance; position of VCG, bottom loading, is the outer chine wetted, depth of keel strake a.s.o. Hence my somewhat laconic answer.

The risk with the "rudder stabilization" mentioned, is that if the boat is, say skidding from a wave slighly yawed (Skidding sideways), a neutral rudder position will cause the boat to "heel the wrong way" into the next wave, digging the chine and causing a serious broaching. In a boat with a high CG, a "non-tripping" double chine as Tunnels has shown is a far better and safer way to get in balance. But its angle is not trivial; the dynamic lift vector from it should pass above the VCG, otherwise it is reinforcing the heeling. So the simple answer to your original question is still NO!

 

Simple way is anti trip chines , the dead rise needs to be 12 degress to 16degress ,no keel at all just a nice sharp natural vee ,after of the bow fore foot where a skeg or dual fins would be to alow a point to pivot on so the back of the hull will slide and swing controlled by the rudder , The dead rise needs a camber so is almost semi flat in the centre where the keel would be !at hi speed this will act to make the hull rise slightly and give less wetted area The high point of introduced rocker should be close to but just aft of where the pivot point of the hull is , this also make the hull rise to have less wetted area. The rudder needs to have hydralic steering to be very positive and slow to turn . It speed the fin forward can and will airate and loss some of its bite and at the point it is possible to turn the wheel quite sharp and the whole bottom will side side ways with the outside chine well above the water so will not trip .the hull noticably will drop the bow . ! How do i know ?? comes from the first boat i designed and made when i was still at highschool and built after i started work .
I poured over pictures and scketchs and drawings and built models till i understood what all these things were and what they did ! . My first pay packet i bought a sheet of plywood and scavenged round the neighbourhood under houses and any where for nice clean straight timber till i had Enough to start ! I did everything myself cutting the timber to size and hand planing smooth and made everything myself as well . The boat was only 14 feet long but was all mine ! with my ideas !and when it went in the water i was so proud .It was a good boat completely predictable and i never had to change anything all the time i owned it ,, My father was a good self taught engineer and he showed me how to weld and shape steel and so i made all my own parts for the boat .
I was only 17 years old when all this took place ! 50 years ago! I have never ever lost my love of boats . Manythings have changed but they are still the same ! :!:

 

Gonzo

My I/O boat has the exact opposite actions you state when I trim the drive ( rudder assembly ) in & out. I trim in to force the bow down in choppy water water. Both straight & turns. Excessive trim in can cause prop cavitation & or bow hooking in a evasive sudden turn. Only did that 2 X . Gets the hair up on my neck. Triming out raises the bow and just causes sluggish wide turns.

 

A rudder and an outdrive are two completely different things. One is a foil and the other a propulsion unit.

 

I thought the shaft housing & the skeg of the motor are the rudder also.

I was playing with high speed steering if the motor stopped. When the motor was turned off at 30 mph, I still had reasonable steering .

 

When you turn with an outdrive, the thrust of the propeller is what produces most of the turning force. A rudder is a passive steering device.

 

True gonzo.

But I am in the process of building a 21' constant 23 degree deadrise runabout with a seperate prop & rudder setup.

So I am trying find out what are the requirements for the rudder, to cause heeling into turns. Typical 305 CI V8 205 hp motor. I will have a power steering system from a Chevy car or another marine engine to handle the expected steering loads.

 

I have a feeling the constant deadrise hull will require a lot larger area rudder, to push enough water sideways to allow good turning.

 

what kind of speeds are you talking about when you want to do all this turning ??

 

Nonsense! You have not understood what makes your boat deviate from a straight line. In order to follow a curved path, there must be a force on the HULL (not the rudder) pointing to the center of curvature. This is the centripetal force. The rudder is actually introducing an OUTWARD force on the rear of the hull, which is giving the hull an angle of attack ("yaw") in relation to the original speed direction. The resulting transverse force on the hull is what is making your boat to follow a curve, the rudder is only introducing the initial deflection (in fact a force counteracting the wanted centripetal force) and controlling the following angle of attack.

The rudder force necessary to introduce this can be very small, depending on the balance between the hull´s lateral dynamic center of pressure (note also possible fin location as discussed by Tunnels) and its center of mass (the "dart effect"). The lateral COP varies with trim on a V-bottom hull, and the trim may be influenced by the vertical inclination of the rudder shaft, as Gonzo has said. All outdrives legs have their rudder shaft inclining forward, in order to create a slight lateral "rudder balance".

When you trim your drive leg, the vertical force from the propeller thrust line plus the lift from the anti-ventilation plate is something quite different from what Gonzo is mentioning.

With a flat bottom, all of the centripetal force is acting horizontally on the hull side, causing an outward heeling couple together with the center of mass trying to go straight forward. In addition to this, there is a transverse flow from the chine and across the flat bottom panel. This is causing a low-pressure zone near the chine, increasing the outwards heeling tendency.

Increase the vee or introduce a double chine in order to get a positive pressure near the (outer) chine and the tripping is reduced, but don´t ask the rudder to correct a bad hull design, that is asking for trouble.

 

If a 21' runabout does not need to have a rudder. How would you get the boat to turn ?

" There is a force on the hull " .............. " ( NOT THE RUDDER ) ".....your logic.

Rudder companies are wasting our time ??

 

Well, have you ever been onboard a boat? Suppose your boat is making head on a straight course and you want to make a starboard turn. How do you make that happen with the help of an aft rudder? It goes like this:

First you turn the rudder shaft anti-clockwise, so that the starboard side of the rudder blade becomes the "high-pressure side" (and vice versa), forcing the rudder to deviate to port, i.e. AWAY from the center of the turn, which then causes the nose of the vessel to point slightly "offtrack" to starboard.

In this yawing position, the rudder is only balancing the (often small) forces that try to restore the hull to a straight course; the rudder as such is NOT contributing to the centripetal force that is acting on the mass to make it follow a curve to starboard. There are numerous examples of hulls that become longitudinally instable while turning, tending to run into an inward spiral. In those cases you have to carefully apply contrarudder in order to keep a steady turn, but still, the side forces that makes your boat to cirkulate around a turning center comes from the hull itself. The rudder is only setting up the angle of attack needed.

Actually, to prove my point, take a look at someone on a surf board, see any rudder there? No? In that case, the pressure asymmetry needed to make the hull deviate from a straight course is created by the "helmsman" moving his weight.

Earlier you wrote something like "..determining the rudder size needed to set up the necessary heeling....". The rudder is not the major heeling factor. What makes the hull heel in a turn is the asymmetrical bottom pressure resulting from transverse flow when the hull is moving yawed.

 

The bigger & deeper the rudder. The faster & more the boat turns in a direction at any speed.

Do not need any therotical flapping of your lips. I do help to make race boats handle better & more safely.

What do you do in the marine world ?