Slamming

[Ubergeekian]

OK folks, I'm going to show my ignorance, but hey, if you don't do that, you never learn. So...

I see a lot of references to slamming. What does it mean?

 

[jellyellie]

In what context?

 

[sailorman]

OK folks, I'm going to show my ignorance, but hey, if you don't do that, you never learn. So...

I see a lot of references to slamming. What does it mean?

modern flat bottomed boats leaping off of a wave into a hole. they slam as they hit the bottom of the hole.
my MAB flat bottomed boat doesnt

 

[planteater]

modern flat bottomed boats leaping off of a wave into a hole. they slam as they hit the bottom of the hole.
my MAB flat bottomed boat doesnt

So what causes it if it isn't the shape of the hull?

 

[Madhatter]

its a song title by Bob Marley

 

[Garold]

OK folks, I'm going to show my ignorance, but hey, if you don't do that, you never learn. So...

I see a lot of references to slamming. What does it mean?

To a non boaty person ................it's when a boat does a belly flop. I had a catamaran, and they are really good at it. Slap a flat hand in a sink full of water and you'll get the idea.

Maybe ignore my last idea. One too many glasses of wine this evening I believe.

Cheers

 

[rr_123]

I see a lot of references to slamming. What does it mean?

It's a way of drinking tequila. With salt and lemon.

An acquired taste, but easily acquired!

 

[Crabman]

So what causes it if it isn't the shape of the hull?

Its normally a **** helm if you are talking yachts going upwind.

 

[Trident 24]

Tonic water, shurely?

 

[elton]

To me it means the boat slamming down on air momentarily trapped between bilge keels.

The first time it happened I asked my instructor if we were hitting the sea bed (half a mile out)

 

[LadyInBed]

So what causes it if it isn't the shape of the hull?

Sailing too fast for the wave conditions.
Weight balance of the boat - probably not enough nose weight.
Maybe sailing at less than 45 degrees to the waves.

 

[V1701]

I thought it was due to or exacerbated by having too much weight forward. I think it can be cured, at least partially, by thinking about and altering weight distribution. For example moving your anchor and chain into the bilges midships or by getting several fat blokes to sit in the cockpit...

 

[TradewindSailor]

True slamming is caused by large area of hull hitting the water surface all at once.

Consider a boat like the topper dinghy : It has a flatish bow section. If it comes off a wave, the bows will fall and hit the surface and will slam.

If however you have a deep V shaped bow, the bow enters the waves gradually and doesn't slam, making the ride smoother.

This is the best I can do at the moment ....

 

[johnphilip]

More heel less slam

Perhaps this is obvious to many but I did not realise it until we raced the boat for the first time in a bit of breeze.
I had been disappointed that our Elan slammed into the seas, so I would reef to reduce it with little effect. However if you keep plenty of sail when going to windward and the boat somewhat on her ear, no slamming at all. Uncomfortable to live with for long periods but soooo.. much better than crashing onto the waves.

 

[Bosun Higgs]

Slamming is what a Bav starts to do when heeled in a F3. The boat bangs and shakes. When you get up to F6 its so bad that (allegedly) the keel falls off!

To be serious ( there's nothing wrong with Bavs but you can wind up the owners like an old watch ), modern boats tend to have flattish bottoms to give form stability which are a great help with downwind performance and reduce rolling. But the price is that they slap on the water which can and does shake the boat. As the Elan owner points out, heeling the boat considerably presents a sloping bottom to the waves and reduces / stops the slamming.

Your Victoria has a shape which is much more "wine glass" and remniniscent of the old wooden boats. It wont slam much if at all but will roll much more going down wind in a sea. I've noticed the same thing in my Starlight. It does slam but not a lot compared with the Moody that preceeded it, but it roills a lot more instead.

 

[Robin]

The main cause is flat sections in the forward part of the hull, farther back has much less slamming effect. Old boats tended to have a sharpish bow that stayed that way from deck level down to underwater and even back to the leading edge of the keel. Later designs had more buoyancy forward and more 'U' rather than 'V' shaped sections and if these hit the water flat palm down style you get a slam.

Bilge keelers slamming is mostly from trapped air between the keels and can be really alarming as I found on a windy upwind ride on a friend's Konsort one night, sleeping was not easy!

Next you need to look at the dynamics as well as the static design shape. A boat only 'slams' when it hits the water so slamming is a mostly upwind phenomenon as the boat encounters the waves. The more directly the boat encounters the waves the more it can slam and maximum slamming is when motoring straight into the waves with a headwind and headsea.

Now consider how different designs go upwind. Older more traditional designs are not very closewinded nor are they fast upwind, so encounter the waves at more of an angle and at a slower speed. Newer designs are not only faster upwind but sail closer to the wind and therefore hit the waves more head on as well as with greater speed.

Look at the extreme example which is of going to windward under engine. Traditional designs will pitch more and even hobbyhorse up and down in bad cases going nowhere fast but not slamming much. Newer designs will not pitch as much as they have more buoyancy forward and probably have more horsepower per ton displacement as well so will make more progress albeit noisily as they slam more.

How to avoid? Well as speed and angle are both contributory factors, try to sail at more of an angle to the waves or reduce speed (reef more) or a combination of both. A skilled helmsman will weave a way round the waves too, keeping up both speed and pointing as best as achievable. Motor sailing is a good way of increasing slamming as it encourages pointing too close into the waves.

 

[snowleopard]

True slamming is caused by large area of hull hitting the water surface all at once.

That's a pretty good definition - it takes a flattish surface meeting the water square on. There are lots of parts of a boat that can do it and it depends on the hull shape. Cats are particularly bad in a beam sea when the crests of the waves passing under the hulls hit the flat surface of the bottom of the bridgedeck. To stop slamming you can alter course or angle of heel so that the offending part of the boat is no longer hitting the waves square on.

 

[Ubergeekian]

Your Victoria has a shape which is much more "wine glass" and remniniscent of the old wooden boats. It wont slam much if at all but will roll much more going down wind in a sea. I've noticed the same thing in my Starlight. It does slam but not a lot compared with the Moody that preceeded it, but it roills a lot more instead.

Many thanks for that, and for all the other useful (or entertaining) definitions.

As an aside, a day spent antifouling has given me a far better knowledge of Jumblie's underwater shape - she's basically semicircular with a sharp keel join: much more one of these

than the more traditional boats in the yard, which are more like one of these

All hail Chuck Paine!

 

[lenseman]

Slamming is what a Bav starts to do when heeled in a F3. The boat bangs and shakes. When you get up to F6 its so bad that (allegedly) the keel falls off!

I was under the impression that a Bavaria were only called a 'Bav' AFTER their keel had fallen off.

 

[fireball]

They don't fall off - they just wobble slightly ... geeze some people!!