Following Sea D, SD and Planing

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Gludy

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I think it would be fair to say that for example SD boats are considered to have more problems in a following sea than planing boats. The same for displacement boats of course.

Whilst accepting the wisdom of this claimed fact I have to admit it confuses me.

Why should a planing boat cope better with a following sea than an SD boat? After all the SD boat will usually have a keel that, if anything would resist the sea turning your back end so you broach.

Has this 'wisdom' come about because of the speed of D and SD boats - them not being able to outrun the following sea so that there comes a point when there is no flow over the rudder and hence no steerage?

I really do not know the answer to this, so will someone please explain why the fact of following sea performance of D and SD boats has arisen.
 
Unsure, but even with a planing hull, things can become difficult to manage beyond a certain point, esp with an outdrive boat where you start "surfing" down the wave and losing steerage: it all gets a bit exciting in the wrong kind of way, and I chicken out and drop back to displacement speeds.

dv.
 
You said it in the pennultimate sentence the SD has not the speed to "outrun" the following sea. My Broom Crown being SD and only having 150 engines felt awfull in such as the steerage would almost disappear with the water going the wrong way over the rudders, the bow would lift alarmingly and the feeling of imminent broaching would often be there.

You make a point about a keel whilst it does aid stability under power by keeping the hull straight, imagine that the following seas have forced the stern round toward broaching, at that point of course the keel will start to aid the sea in completing the broach as it becomes "side or beam on" to the sea. For the same reason one lifts tabs in a folowing sea on a planning boat.
 
Surely as good a reason for all boats to be built as per old ships life boats ie.double ended.Flat sterns ok for space but not sea kindly?
 
Suncoast has said it all. As soon as the keel gets a bit side on to the waves the boat is pushed sideways and if the bow ploughs into the face of a wave in front and the boat has little or no steerage, the boat effectively pivots about the bow = broaching. Even worse than a following sea is a quartering sea in which the keel is continuously side on to the waves
Having said this, some of the best seaboats in the world have keels eg Nelsons, RNLI lifeboats so it cannot be so simple as keelboats = bad, planing boats = good in a following sea. I think it also depends on the design of the keel, the hull and the rudders. Some keels are cut away at the stern possibly to reduce the effect of a following/quartering sea and some hulls have more rounded stern sections possibly to ease flow of water. Also, the bigger the rudders, the better they'll work with little flow over them. Where is David Marsh when you need him?
 
Thanks for the point on the rudder and of course you are right.

However if you are right about the problem with SD being the lack of speed, as per my original point, it really means that SD's are not more prone to following sea problems - just that slower boats are more prone to following sea problems. An SD boat that can do 25 knots is no more prone to the problem than a planing boat that can do 25 knots?

This is the very point I am trying to resolve - is there another reason than just speed because if there is not then its a myth that SD hulls are more prone to broaching in following seas.

Further - say a following sea has a current of 5 knots - the whole surface layer of the sea is moving in your direction at 5 knots.
Then is this situation as long as you have any positive speed at all you are moving fater than the water - you would have to be standing still to have no steerage. So its not the current that causes lack of steerage.

Then on top of that 5 knots current you have the wave action which as we all know moves in an elliptical fashion - so the top of the wave is always travelling to some extent in the direction your going - this water moving within the wave must add to the total speed of the water and it must be this water that causes the water past your boat to exceed your relative boat speed and cause loss of steerage. Hence near the top of the wave or just coming off the wave on the way down you lose steerage.
 
Yes, exactly Mike.
I can understand that once you have lost steerage the keel possibly helping the broach but as you say there must be more than that to it - what are considered the best sea boats including offshore lifeboats are SD hulls and they must face pretty bad following seas.

I really do not know the answer to this one.
 
looking at a few nelsons etc vs less sea-kindly hulls - I have the impression that the Nelsons & other "proper" sea boats have a keel going well forwards, almost to the bow, vs something starting 1/2 way back or more.

Presumably the full length keel helps reduce the pivoting when an overtaking wave pushes the back of the keel around.

re. the lack of steerage - you'd still have water from the prop flowing past the rudder, so some steerage, esp. with big rudder.

Some books advocate reducing power if necessary to keep a safe speed, then applying more at the time when extra steerage needed.


Our displ hull has a reasonably deep keel at the back, tapering forward to nothing well before the bow - and a large rudder. Following/quarter sea swings the boat around hugely, requiring a lot of correction & even with that we make a zig zag course which confuses oncoming boats a bit, I imagine.

It also allows turns w/o steering if a channel bends , but I'd much rather it was easier at sea.
 
Rudders ? I was talking outdrives: there is some rudder effect, but steering is mostly vectored thrust from turning the drive unit. This has only happened to me a couple of times, but it wasn't pleasant, because I ended up in a dilemma: applying more throttle down the wave maintains steerage (up to a point), but means that you smack into the back of the next wave. Ease off as you go over the top but keep planing, and the steering doesn't do much as you surf down, which is scary if you start to go sideways. Slow right down to semi-displacement speeds (10knots -ish) and it all comes back under some sort of control.

dv.
 
Interesting but if you could speed up surely that would be much easier to control in the following sea? I know, because it is a D hull you cannot get more speed but if it could be cured by speedm then the issue would come down to speed not the hull.

Also - an I am only guessing - keel towards the front would help straightem the boat not braoch her whilst keel towards the back would help broach her. Front and back being fore and aft the natural pivot point of the boat.
 
I'm really not sure why this is, but it definitely felt more under control at 10knots.

Im guessing, but I think it might be to do with the fact that it takes a large amount of energy to get a planing boat planing (to get over the hump). So if a planing hull is well below planing speeds, being downhill for a few seconds won't be enough to get it planing: it won't speed up as much, and you get to keep the vectored thrust.

dv.
 
As mentioned in recent previous thread, its not about a particular boat as such, whether D, SD or planning it is about how well the boat is designed, screwed together and a well engineered propulsion system.

Drove several old Watsons and Solent class lifeboats and believe me just because they were lifeboats did'nt mean they were great in all sea conditions, their main criteria was to stay afloat no matter what and self right. Down a serious sea the Watson in particular was a complete pig, once you started sliding down the back of a big un you were going faster than that which you could achieve with the engines, once that double ended stern got quartered you were in the do do. At least you knew when you broached and rolled she would just pop up again and on you went.

In many ways a smaller planing boat works really well in serious weather (talking big RIBS and commercial stuff) as you have the ability to drive yourself up and over and even power yourself 45 degrees off the back of a wave (in similar fashion to a surfer only on the other side of the wave), travel down to the trough at the bottom and turn quickly to square up again to the face of the next and drive up to the crest. Works kind of like tacking for a sailing boat so you avoid pounding into the back of the waves all the time.

The main thing to consider in any hull form in real bad stuff is power up the face but just enough the maintain speed over the ground, knock it back once you crest, let the crest run under you and as the stern clears the crest power on again, in this way you give yourself as much steerage as possible through either the drives or by thrust over the rudders, if you are already going quickly over the top you will have nothing left and probably will stuff yourself up the back of the next. In other words what ever hull you drive you have got to 'drive' it and work the boat to make things happen.

Perhaps the issue is not SD, D or planing but why are some boats better in weather than others be they one or the other hull form.

You can take the same position with cars, as an example, there are some very butch and chunky looking 4X4's out there but many are no more than pose machines, stick em in a mucky field and its all wheel spin and no traction, just because it looks like a roughty toughty boat does'nt mean it is!!.
 
Nautical
Thanks for that good contribution.

Accepting that there are good and bad designs of all types of boats. D, SD or planing - are you saying then that in general the hull type does not relate to how well they handle in following seas - rather thae whole design?

You also seem to be saying that power - the power to power up the wave is important - so an Sd baot with say a top speed of 25 knots hence ample power should be , in general, at no more advanatge or disadvantage than a planing boat?

I am trying to draw some conculsions out of this from you experience, hence the questions.
 
In a ideal world, one could build the perfect all weather boat, however boats are just like everything else in life no one product can do everything perfectly. My view would be that if you wanted the best possible hull form for travelling down the weather a SD hull with plenty of beam and a full bow section(not narrow hull like early Nelson's) a fairly shallow keel that finished before the props and really big grippy props with large surface area rudders. Downside of course as she would'nt handle as well up the weather where you would like a narrower beam and finer for'd sections.

Then of course with SD you are burning lots of fuel, compromising on interior space, quite rolly polly, very expensive to build, so planning hulls become much more attractive and some are pretty darn good in bad weather as well as long as you drive them according to the conditions and as mentioned in some conditions work better than either SD or D, all depends what you want to do with a boat, go down the weather, up the weather, accross the weather, work on the boat, hang on for grim death, rescue something or just survive the biggest storm ever. Every boat / hull is a compromise there is'nt one that is the best at everything.
 
"Perhaps the issue is not SD, D or planing but why are some boats better in weather than others be they one or the other hull form."

that is exactly what I am beginning to think - the issue is what characteristics make a boat peform poorly in a following sea?

So far:-
The lack of power to drive the boats through waves.

Then only when in a siuation of no steerage:-

1. Large keel aft of the centre pivot point.
2. Large rudder.
3. Trim tabs badly set down - instead of up
4. ??????
 
Well to be precise, SD if you want to go down the back of a wave, a modified one to go up the face of one, a planing one to go across, a D to survive a hurricane, ...........but joking aside, all things being equal if you want to have the best in a following sea then a beamy modified SD as described previous would be my choice. Going up the face of a big un probably nothing to choose between a good SD and a good Planing, the SD might give you a slight advantage if you were going at speed but if its that big you won't be anyway.

Hope that helps.
 
Thanks.
So its a myth that an Sd boat is poor in a following sea then. It maybe poor but nor because it is an SD but for other reasons.

I still do not have it clear why you would choose a beamy SD for a following sea, even after reading the thread... could you please expand? Keep it simple because I am really very simple at heart. :-)
 
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