bow steer

gtflash · 06 October 2010, 17:22

why does bow steering hapen more in a following sea?? Ive never worked out the science

Hightower · 06 October 2010, 22:21

Sounds interesting! Describe this "Bow steering" I've never encountered this. Perhaps this is what some people call a "breach"?

BogMonster · 06 October 2010, 23:04

Sort of an explanation here http://www.ifish.net/board/archive/index.php/t-743.html

"Code Brown"

I never noticed it on the Humber.

m chappelow · 06 October 2010, 23:51

In a following sea ,the stern lifts ,,picks up ,,on a wave and as the boat is pushed forwards ,,surfs ,,the bow then being lower starts to dig in deep which can then act like a rudder and slows ,any sideways movement caused by the wave on the stern then makes the bow sheer direction ,unless its corrected by steering or slowing down ,, the stern will then in effect piviot on the bow, swing around and broach to and with a possible beam on capsize .

depending how deep a forefoot there is on the bow ,,with fast boats they dont have them ,,but boats such as the north east fishing coble are notorious for broching to in a following sea as the bow is much deeper than the stern,

one reason why in a following sea ,some boats will tow a drouge or a long rope behind the boat to help stop it from broaching to , i used to tow a few crab pots behind my old fishing boat

pic, of a northeast coble with a deep bow or forefoot ,,fantastic sea boat but in a following sea they can be a bit tricky ,

gtflash · 07 October 2010, 17:32

Quote:

Originally Posted by m chappelow

In a following sea ,the stern lifts ,,picks up ,,on a wave and as the boat is pushed forwards ,,surfs ,,the bow then being lower starts to dig in deep which can then act like a rudder and slows ,any sideways movement caused by the wave on the stern then makes the bow sheer direction ,unless its corrected by steering or slowing down ,, the stern will then in effect piviot on the bow, swing around and broach to and with a possible beam on capsize .

depending how deep a forefoot there is on the bow ,,with fast boats they dont have them ,,but boats such as the north east fishing coble are notorious for broching to in a following sea as the bow is much deeper than the stern,

one reason why in a following sea ,some boats will tow a drouge or a long rope behind the boat to help stop it from broaching to , i used to tow a few crab pots behind my old fishing boat

pic, of a northeast coble with a deep bow or forefoot ,,fantastic sea boat but in a following sea they can be a bit tricky ,

sounds feasable, but if your fast, the wave shouldnt have much force on stern will it??

BogMonster · 07 October 2010, 20:24

Quote:

Originally Posted by gtflash

sounds feasable, but if your fast, the wave shouldnt have much force on stern will it??

I'm guessing that something named after a cow (Daisy ...

) is probably not all that fast when being chased by a big sea

willk · 07 October 2010, 20:45

Quote:

Originally Posted by gtflash

sounds feasable, but if your fast, the wave shouldnt have much force on stern will it??

You'd think that, but if your bow digs in and the stern lifts, the force needed to pivot the hull might not have to be that strong.

alystra · 08 October 2010, 16:14

I've been looking recently at the ground handling characteristics of tailwheel aircraft as opposed to tricycle undercarriage planes. (Don't ask) Apparently, tailwheel aircraft never want to go in a straight line and will swerve off course unless prevented by instant application of lots of rudder. Apparently this is because the centre of gravity is behind the main wheels.

I wonder if it follows that, when a fast planing boat digs it's bow in, the centre of drag (or the centre of bouyancy) moves forward of the centre of gravity of the boat, making it directionally unstable.

Thinking about it, I think most fast planing boats are not directionally stable at slow hull speed, needing constant helm input. Could this be because the centre of bouyancy/drag is forward of the C of G? When planing the C o B/drag obviously moves aft as half the boat is out of the water. With C oG ahead of C o B the boat becomes directionally stable.

Above based on limited understanding of elementary physics.

Brambles · 08 October 2010, 21:49

Quote:

Originally Posted by gtflash

why does bow steering hapen more in a following sea?? Ive never worked out the science

How was your engine trim?

Hightower · 09 October 2010, 07:22

I was out in some moderate Sea's yesterday and deliberately tried to induce some Bow steer (broach/breach) by travelling in a following Sea. I experimented (unsuccessfully) with different speeds and trim angles. Perhaps the conditions weren't bad enough!

gtflash · 09 October 2010, 08:22

I understand a boat like mine with a sharper bow will bow steer more than a constant v, however I struggle to understand how a boat travelling into the back of a wave in a following sea is more likely to bow steer than crashing into a large head sea, surely in both instances the front has massive grip and the aft very little??? If your travelling at speed, the boat will always be faster than the forces on the stern so in theory, there would be little or no force???? Im confusing myself

We had a small hook at lymington, but were on a turn buoy in a following sea, and the wave probably broke at our stern, on starboard while turning starboard, i probably trimmed in too much also. Seems likely, and understandable, however it got me thinking about the whole bow steering in general

Pikey Dave · 09 October 2010, 09:54

As I undertsand it (& I could be well off the mark) in "normal" conditions, the engines are pushing the boat & steering is controlled by changing the direction of the engine (thrust). So you actually contol the direction of the boat by moving the stern in the opposite direction that you want to go. In a following sea, the sea is pushing the boat from behind & depending on the relative speed of boat & sea, the sea could be pushing the boat faster than the engines are. At that point you have virtually lost control of the boats direction. The boat gathers speed due to being pushed from behind AND gravity as you slide down the face of the following wave & bonzai! you hit a small tree

As the bow digs in, it's pot luck which way the boat turns, depending on the angle of the waves & hull characteristics. It's a bit like trailer wag when going too fast downhill, the inexperienced brake too hard & the trailer passes you & you end up broadside on, on your side.
In a head sea, the opposite happens, it's the old immovable object & irresistible force scenario. The engines are pushing against the sea & the relative thrust hence steerage is higher. like dragging a trailer UP hill, much more stable & controlled. Planes have exactly the same issues with headwind & tail wind. Planes always take off & land into the wind if at all possible as the relative speeds & thus control, increases.

jwalker · 09 October 2010, 13:00

Bow steer caused by a deeper than ideal forefoot can be very strong and frighteningly uncontrollable. As a general rule for a rib hull, the angle of contact of the bow sheer to the water needs to be approx 45°, less is ok but leads to a flattening of the hull form as in some Northcraft boats. I've owned a bow steering boat... for a very short time, because it was awful in a following sea.
There are boats out there which I'm wary of and I would need to sea trial fully in following seas if considering a purchase.

Hightower · 09 October 2010, 17:26

Quote:

Originally Posted by jwalker

There are boats out there which I'm wary of and I would need to sea trial fully in following seas if considering a purchase.

RIBs? or Hard boats?

jwalker · 09 October 2010, 18:51

Ribs

willk · 09 October 2010, 19:03

Is this what you mean by a deep forefoot?

steve292 · 11 October 2010, 01:38

All (ALL) planing powerboats suffer from a lack of balance between bow and stern volumes because you need the flat planing surface at the stern to give you lift & a fine entry at the front to cut the swell.
Boats have very little dynamic stability when running the crest of a wave and at that moment, a small change in trim caused by the bow digging in can result in a serious torque to turn the boat sideways (broach) because of the difference in immersed volume between the bow and stern . The bow digs in and the fat stern gets thrown around. Which is why powerboats designed for offshore use taper back slightly at the transom, to offset some of this difference in volumes

We can reduce (but not eliminate) the risk of broaching by having as much reserve buoyancy in the bow as possible but here is the compromise....a fine deep entry will contribute to broaching but does not pound. A full bow, shallow will run faster, resist broaching but will pound.. .

To prevent broaching you need to run faster than the waves, if you are in a following sea.

hope this helps,
Steve