Why do some boats lean into turns?

[JizzaDaMan]

angle of limiting stability (I think that is the term).

actually known as the angle of vanishing stability (AVS), the angle at which the righting moment becomes 0 and the boat will capsize.

 

[K^2]

Yes, of course. And with higher CoM, that angle is decreased, so low CoM is still preferable. No question about that.

 

[Johnca]

Note: My mother tongue being French, I apologize for the quality of my english

I have aluminium boat with almost a flat bottom hull (very small ‘’V’’ shape.At speed over 28 mph the boat turn flat and slide. So the radius of the turn is very long. At speed under 28 mph the boat lean and turn shaply. To solve this sliding reaction I am wondering if a fin installed center of the boat could help to turn shaply? Yes if the boat height is to high the boat could flip over. For ex: the water ski specialize boats have fin…Correctcraft boats, mastercraft boats.

What do you think of this solution?

 

[sophiecentaur]

Good idea. I think a (small) skeg about half way along could help you a lot. I think that, above 28mph, the hull starts planing so high that the only contact with the water must be right at the back, so there is very little turning moment (steering effect) of the prop and the hull. The skeg will maintain a 'pivot' well in front of the outboard.
btw, if you have a heavy load at the front (keeping the nose down), does the boat turn better at speed? Certainly, a towed water skier would tend to lift the bow out of the water unless the tow is attached very low to the transom.

 

[Johnca]

Thank you for the info.

Now I am going to visit dealerships with boat equip with small skeg and mesure the hight, length and the position (middle, forward or backward). Than with different boat builders there will be a ''tipic'' place for the skeg.

I think personnaly that the skeg should be in the middle of the 19 footer boat?? Just in front of the pylon were the tow is attach now. I will make sure that the tow is lower as possible just to clear the outboard motor.

For your info the boat nose stay low during skiing. I am not worried about boat flip over the width is 96 inches.

For me it is interesting to use physics to solve daily problem and experiment.

 

[sophiecentaur]

Thank you for the info.

Now I am going to visit dealerships with boat equip with small skeg and mesure the hight, length and the position (middle, forward or backward). Than with different boat builders there will be a ''tipic'' place for the skeg.

I think personnaly that the skeg should be in the middle of the 19 footer boat?? Just in front of the pylon were the tow is attach now. I will make sure that the tow is lower as possible just to clear the outboard motor.

For your info the boat nose stay low during skiing. I am not worried about boat flip over the width is 96 inches.

For me it is interesting to use physics to solve daily problem and experiment.

I haven't spent enough time above about 6kts to have had the opportunity to study high speed boating personallly. But I think the basics of Physics must apply to much of this question (except the magic of hydrodynamics / aerodynamics which are usually totally counter-intuitive!)
If you are in a position to make changes to your boat (re-position the skeg after trials) then you have a good chance of getting a good final solution. I might suggest that a position behind the centre may be better - but look at existing designs first. Too far forward may compromise low speed turning.
It sounds like an interesting project.

 

[Johnca]

I haven't spent enough time above about 6kts to have had the opportunity to study high speed boating personallly. But I think the basics of Physics must apply to much of this question (except the magic of hydrodynamics / aerodynamics which are usually totally counter-intuitive!)
If you are in a position to make changes to your boat (re-position the skeg after trials) then you have a good chance of getting a good final solution. I might suggest that a position behind the centre may be better - but look at existing designs first. Too far forward may compromise low speed turning.
It sounds like an interesting project.

I will keep you inform !


John from Canada

 

[Pkruse]

Johnca: most boats like yours put the skeg at the rear of the boat. Keeps things stable, like an arrow in flight.

But moving it forward makes the boat more responsive. So it is a trade off.

Think of canards on an airplane. They make the plane so unstable that nobody thought of using them before the development of computer controlled flight. A human can't respond fast enough to keep it flying straight.

 

[d3mm]

I recommend people trying to modify boats go to a specialist forum eg boatdesign.net

There is a great amount of misinformation on this thread (eg jybing is uncommon, centre of gravity vs centre of bouyancy) about a very specialised and safety critical issue, that is also governed by national and international standards and regulations.

Your country's marine and inland waterway regulator almost certainly has the legal authority to sieze boats deemed not compliant with safety standards and prosecute the owners. You are also much more likely to have an accident.

 

[Pkruse]

Here in the USA, commercial boat design and construction is heavily regulated for boats carrying freight or more than six passengers. Privately built and operated boats have none of that. I've built, repaired, or modified hundreds of them. I have to get them inspected, but that is just to make sure they are homemade and not stolen, and that I've paid taxes on all of my building materials. Something as simple as installing a skeg or modifying one on a small skiff is a quick afternoon project that requires no government interference. It is not uncommon to tweak a keel or skeg design to suit the desires of the owner.

 

[sophiecentaur]

I recommend people trying to modify boats go to a specialist forum eg boatdesign.net

There is a great amount of misinformation on this thread (eg jybing is uncommon, centre of gravity vs centre of bouyancy) about a very specialised and safety critical issue, that is also governed by national and international standards and regulations.

Your country's marine and inland waterway regulator almost certainly has the legal authority to sieze boats deemed not compliant with safety standards and prosecute the owners. You are also much more likely to have an accident.

Afaik, in the UK, there are no legal regulations at all for private boating. Commercial requirements are very tough, however, and you will not get insurance for a boat that is not seaworthy. You also need no qualifications at all to take any boat (privately) out to sea.
Of course, you'd be mad to act irresponsibly / negligently because you could always be sued if you cause damage or injury (and you could die).
The RYA encourage good behaviour and offer courses at all levels for boat users. Also the RNLI offer advice and help (for free!).

Unintentional gybing should be uncommon in any but the lightest winds.

 

[Pkruse]

When I got my engineering degree, it was nothing more that than a license to continue learning, which I've been doing for nearly four decades. The single most important thing I learned in my early education was how ignorant I was about so many things, and that I had better correct that ignorance in any particular topic before doing work that requires that knowledge. This thread is an example of that. Certainly if you don't have an extensive knowledge base concerning boats, go correct that problem before building, modifying, or using them. I've easily survived many situations that could have easily killed me or someone else only because I mastered the knowledge base before attempting the task.

The best way to learn about boats is to spend time with people who have mastered them. It is best to do that in person aboard their boat or in their work shop. It is good to take boats apart and study their design. Read all you can. Network with the masters online. Do whatever helps you to master the art. Then go enjoy yourself in the safety that comes with understanding.

 

[Pkruse]

On the subject of jibing, and using the US spelling: the first time I jibed was the only time it ever caught me by surprise and the first time I capsized a sailboat. It was an 8 foot pram that was the first boat I built myself. My mother made the sail and my dad made the spars. I was in light wind near shore in 3 ft of water. I was 11 years old. The point here is that another way to learn is to start with a very small boat and use it in very safe conditions.

Since then, I've jibed thousands of times, even at sea in 60 knot winds and 20 foot breaking seas. It was safe because I knew how to do it safely. I was flying a single small storm sail that was reefed down to its minimum size, and I had full control of the sheet line and other rigging required for safe jibing under conditions that made coming about far more dangerous.

 

[d3mm]

I think it was Russ Watters, in a long paste, who said jybing was uncommon, which I regard as an error. As you know, this only applies to small boats and not in racing. The discussion on center of gravity and center of bouyancy was crazy.