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Old 09 October 2010, 04:22   #11
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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
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Old 09 October 2010, 05:54   #12
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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.
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Old 09 October 2010, 09:00   #13
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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.
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Old 09 October 2010, 13:26   #14
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Originally Posted by jwalker View Post
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?
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Old 09 October 2010, 14:51   #15
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Old 09 October 2010, 15:03   #16
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Is this what you mean by a deep forefoot?
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Old 10 October 2010, 21:38   #17
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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
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