Drogues, Following Seas & Hulls

[Gludy]

Having recently read up a fair bit on boat handling in rough seas I am finding some confusing advice and I have a number of questions:-

According to Dag Pike in his book Power Boats in Rough Seas, planing boats rely on their speed for stability when slowed down because of rough weather are reasonably unstable and in a following sea the stern can be caught, the boat turn on its bow and broach.

According to a number of sources the problem with the other extreme – displacement boats, whilst more stable at displacement speeds, is that, they are slow and so can easily be caught by a following sea and guess what – the same problem, the stern can lift, rotate about the hull and broach!

Based on this advice I am not sure what the case is should you have the power with an SD hull to travel at high speeds and hence avoid the sea? The advice seems to be that such a boat in a head sea is very good but the reverse is true in a following sea ….. but is this based on the assumption that the speed is not there.? I do not know.

A planning hull with a following sea normally has enough speed to avoid the sea passing it but this ceases to be the case as the seas build up and the boat is forced off the plane. So I guess that if it is not planning then the D hull would be better and maybe the SD hull would still be able to maintain a higher speed through the waves. Again I do not know.

I am not repeating other threads here, I am stating what others say because some of it clashes and is therefore confusing.... in particular I am leading to my questions on drogues.......

Enter the drogue (not sea anchor or parachute) – this is towed behind a boat under power and helps prevent the stern turning and so helps prevent a possible broaching situation. Apparently it will only shave a knot or two off your speed. How many of you out there have a drogue? How many have ever used one? What is your experience of them? It seems that they can be fairly small 20 inches for a 20 foot boat, 30 inches for a 50 foot boat and about 35 inches for a 60 foot boat .. so they are not expensive. They could, it seems (and sorry for this) save your bacon if you are caught below 'escape speed' in a following sea. Yet I do not know a single person with one.

 

[rich]

You could get a Forcast before, you go out!

 

[Gludy]

Re: Drogues, Following Seas & Hulls

Yes, I study the weather a lot but there can be occasions when you can get caught out. The cost of the drogue is very low for the safety function it can offer.

In addition when you have an engine failure, hit a lobster pot and stop both engines etc, you may wish or need to slow the drift of the boat down and so in that cae employ a sea anchor. Even a smaller drogue would help slow the boat down and may give you time before you hit thr rocks.

I believe in levels of safety at sea, many lines of defence and in this case, you are right in putting the weather forecasting at the top of the list but as I see it a drogue can come in very handy.

 

[omega2]

My best advice is to pick a following wave and try to stay with it, you will fall off it eventually but a bit of gas will prevent the next one overtaking you, then you can ride that one. As far as a drogue is concerned, with wind over tide and at anchor we stream a fish box from the transom, this keeps us fairly straight and prevents us doing 360's around the anchor warp.

 

[Gludy]

Re: Drogues, Following Seas & Hulls

If you are keeping up with a following wave then is not there very little water over the rudders and hence the steering ineffective?

I have had experiences in my previous boat where it was almost as if the steering had no effect and I had to speed up to get control.

As I see it, without knowing, it would appear best to always have enough speed to plough through and go faster than the waves but its when this is prevented that the problem occurs.

Your comments about a drogue at anchor are interesting .. thanks.

 

[omega2]

Re: Drogues, Following Seas & Hulls

With the size of your boat you should experience any trouble in staying with a following wave, I find no trouble with loss of helm even if I lose one, just speed up a bit to prevent being pooped

 

[omega2]

Re: Drogues, Following Seas & Hulls

And to finish my thesis, if it gets too snotty then put the hook down and get the rods out, and wait for moderation.

 

[squidge]

Re: Drogues, Following Seas & Hulls

Just a thought , If you are being overtaken by a following sea then trailing anything including a drogue have an adverse effect ? If you are not going fast enough to out run the seas then you would be slowed by the drogue , the wave would catch you carring the drogue and in the worst case get tangled aroung you prop and or rudder? Surely the best course of action is to set a course towards the seas and then deploy the drogue, sea anchor or parachute from the bow to prevent it being blown off and sit it out?

 

[omega2]

Re: Drogues, Following Seas & Hulls

HERE HERE squidge, I was trying to get that very message across diplomatically but you tell it how it is

 

[hlb]

Re: Drogues, Following Seas & Hulls

It going round the prop is the big danger. These things are used on sailing boats, dont think they make much sence on power boats.

Anyway I always thought boating was about fun. It's getting far to complicated on here of late. If it's rough I go and tootle up the river, or go to the pub. Maybe I should be out there with my parashoot thingy out, dragging the legs down from the roof, taking the next apropriate wave up the nearest beach!!

 

[Gludy]

Re: Drogues, Following Seas & Hulls

No- I do not think so.

What matters is that you relative soeed to the drogue - as long as you are moving forward under power then you will be going faster than the drogue.

Imagine for example the sea is moving at 10 mph and you are moving forward at 8mph - you have a following sea passing you of 2 mph but you are still 8 mph faster than the drogue!!!

The books say that its important to use a drogue in these conditions on a power boat - lifeboats carry them.

Its all about preventing broaching and the danger is in a power boat.

 

[Talbot]

Re: Drogues, Following Seas & Hulls

Gludy,
I know that the others on this forum seem to be trashing your views, but there is a place for a drogue on a powerboat being used in the circumstances you describe. Provided the cable to the drogue is long enough there should be no problems with it being caught around the props/rudder until the time comes to recover, when a lot of care will be needed to get it right. For all power boats and even yachts, there comes a time when running fast enough to stay ahead of the wave is no longer feasible, you are then vulnerable to the rudder stalling - this is particularly true of a mobo due to the much smaller rudder area. A drogue towed astern will maintain directional stability in these circumstances, and that is why the rnli carry them. You could not stream this sort of drogue from the bow, cause you would then be makeing considerable sternway, and risk a high probability of rudder damage. A parachute anchor comes into its own in hese circumstances cause it will minimise drift, but of course is much bigger and needs very strong deck fixings . Even if you are consideringa drogue from the stern, I would strongly recommend getting advice on the strength of the fixing on the boat, cause it would be a ery bad time for the fixing to carry away!

 

[squidge]

Re: Drogues, Following Seas & Hulls

Gludy you have found me out , I am as you know a raggie(although i do also own a rib and have a lot of fun with it /forums/images/graemlins/smile.gif). Having a displacement hull and only a little power i have no choice but to take the seas on the chin if i am unfortunate enough to be caught out (we are talking gales + ?) ,not happened as to have to deploy anything yet thank goodness. So please excuse my lack of understanding of the handling of power boats in adverse conditions. I just think that hypothetically a displacement hull or a hull off the plane will act in a similar way and the safest action is to lie facing the sea .
"
In addition when you have an engine failure, hit a lobster pot and stop both engines etc, you may wish or need to slow the drift of the boat down and so in that cae employ a sea anchor. Even a smaller drogue would help slow the boat down and may give you time before you hit thr rocks."

I agree but bow towards the seas so that a following wave cannot not swamp you.

 

[webcraft]

I don't come here often, but found this post interesting.

Most of you can just get the hell out and into sheltered water / alongside when you hear a bad forecast or conditions start to deteriorate - that's the advantage of a fast boat. In addition, most of you with planing or SD hulls probably don't undertake very long passages in terms of passage time from a safe refuge.

If however you are going to be making passages that leave you hours from a safe landfall then that is a different kettle of poissons.

When raggies consider using a drogue it is to stop a boat surfing and broaching when she is probably already running under bare poles or a scrap of storm sail. Let's suppose the seas are too big to power your way home at high speed and you have to drop back to displacement speeds. Most mobos will have more windage than eg a sailing vessel under storm jib, so the wind is now likely to be accelerating them to dangerous speeds (ie severe danger of broaching). In addition most mobo hulls are not directionally stable unless under power, so now you are in big trouble.

So - in these survival-type conditions you might want to consider a drogue . . . it would certainly be the weapon of choice for most raggies in this situation.

However, most power boats, as well as being directionally unstable at low speeds in these sort of survival conditions, also have a hull form aft that I would have thought is unsuitable for presenting to large steep or breaking following seas. MoBos tend to have high, powerful bows though, so I would have thought that if you got caught out in conditions where you might consider a drogue then you would be better off riding to a sea anchor or parachute deployed from the bow.

Having watched a 35-40ft SD cruiser overtake us coming into Kilmore Quay last year in a F7 gusting 8 I have to say that nothing would prompt me to go out in anything other than a heavy displacement MoBo in the open sea in anything over F6. The skipper obviously knew what he was doing, but it looked very marginal. He was using his speed top maintain control of the vessel, but in another hour with a full gale and bigger seas this would not have been possible, and the boat would have had to drop into displacement mode. At that point I think he would have been in serious trouble.

All very much IMHO of course, as all the boats I have sailed have rarely exceeded 8 knots! If the post was helpful all well and good otherwise feel free to tell me to b***er off back to Scuttlebutt.

- Nick

 

[kindredspirit]

Re: Drogues, Following Seas & Hulls

Hi Gludy,

Is this problem overated with SD hulls? i.e. problems with a massive following sea.

I've been in situations with huge following seas e.g. the Blasket Sound in 1999 and didn't seem to have a problem.

The most recent was after rounding Slyne Head which is notorious, last year. We were doing 12 knots and the waves coming from behind us would lift us high, scoot us along and then deposit us in the trough. The GPS readings went from 22/23 knots to 6 knots, then back up to 22 knots, then back to 6 knots continuously for about 25 minutes. I'd say the swell was only about 12 to 15 feet but the power of the water was tremendous. The boat didn't seem to have a problem (it tracked true) although naturally we were on "high alert".

The real problem would be if the engine stopped. I wouldn't like to be in that situation then.

 

[trev]

Surely the trick is not to be caught out in such conditions in the first place, and if it looks like deteriorating to such an extent then you should run for shelter.
It's normal in any boat with a heavy following sea to keep one hand on the wheel and the other on the throttles to give and take as required.
Most boats whether planing or SD will be able to cope with a fair sized chop - and take shelter if it gets too bad (passage planning and all that).
I don't know of anyone with experience of this drogue thingy, I would assume it's probably more as an emergency measure on a long exposed open sea passage. I don't think many of us do more than five or six hours steaming at a time, and watch the weather very carefully.

 

[hlb]

I'd like to know how you could let the thing out and retrieve it in these humongus seas, whithout the aid of some massive trawler gear!

 

[Sammo]

Re: Drogues, Following Seas & Hulls

Paul

I too can give you a first hand instance here.
About ten years ago we were in Rothesey and had to make the passage up the Clyde. The wind was prevailing from the south west F7 gusting 8/9, going straight up with steep breaking waves and white stuff everywhere, I was skipper.
The boat was a Semi-Displacement Broom European 35ft with twin 80hp fords, top speed about 10 knots.

After we set out I soon realised that as the waves were overtaking us from behind we were in for a roller coaster ride. As the boat had an aft cabin there was no fear of being pooped but as I was in displacement mode the chances of a broach where very real.
As Kevin says the boat seems to stop dead then surge forward as it starts to surf with the water foaming about 4” below deck level at the bow.
With us the base of the trough was the tricky bit as the boat did have a tendency to broach, as we were nearly flat out I found the best way to swing the head round was to go astern on one engine with the occasional cross wave but most of the time steering like a demented train driver.
When we reached Holy Loch I realised that we were on a collision course with the chain ferry that was coming across, then to my relief I realised that he had stopped and was letting me take a straight course across his bow. He could obviously see the dangers involved in me trying to go round his stern.
Past Holy Loch the white stuff died away and we carried on to our mooring.
The drogue Idea might have helped to stay straight but the fact is lines and propellers don’t mix, and as you can imagine the last thing I needed was something to tend.

Later on that evening on the moorings the wind increased to a full gale and a worried crew asked if we should set watches to tend the ropes.
No say`s I we`ll do it another way, and in the finest tradition of MoBo`ers I took them up the road, bed and breakfast.


……………………..
make haste slowly





to be fair I nicked the picture so here`s a link to the broker
above boats for sale

 

[whisper]

Re: Drogues, Following Seas & Hulls

Don't understand these figures. Surely if you are towing a drogue you are both travelling at the same speed or are we talking very,very stretchy elastic here /forums/images/graemlins/grin.gif

Like many others here I follow the system - or atleast try to - of getting myself on the back of the wave, as near to the top as possible. Then, by playing the throttles you can usually manage to stay there for a while. If you fall over the front of it, try to keep that as gentle as possible but then quickly open up again so you keep ahead of this wave and then sit on the back of the next one and so on. Some SD boats may not have the power or hull form to be able to do this, though last Summer I saw a Channel Island 22 doing it brilliantly coming into Salcombe with a 2.5 to 3 m. following sea.

 

[Bergman]

This high speed bit sounds awfully risky. A big motor boat at high speed has an awful lot of energy which must go somewhere if it stops quickly.

Apart from that, how do you manage in the dark. Running away from waves that you can't see sounds very difficult.

As a raggie I know very little about motor boats but what the fishing boys do in really nasty stuff is "dodging". That is point the boat at the waves with just enough power to maintain steerage way and sit it out. That takes the most punishment on the strongest part of the boat, the bows. You are watching where the weather is coming from, not trying to look over your shoulder and provided the engine keeps going you are unlikely to broach.

I think the lifeboats only use their drogues where they are forced to run down weather. I know Whitby Lifeboat used to sometimes use theirs to get back into harbour in bad conditions.

So the raggy view is use the speed to get to shelter before the nastiness starts and if you can't dodge it that way stick the pointy end into it and dodge it that way.