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Old 10 August 2019, 02:46   #1
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Driving issue or weight distribution into steep waves, force 7

Hi all,

I was having some fun out in yesterday’s weather with my 6.5m XS. I was off St Anthony Head outside Falmouth heading up into some steep seas and a likely force 7 wind.

I wasn’t trying any heroics, just increasing my familiarity with the way the boat handles in those conditions while I was able to be alone on board (so as to be better prepared if forced to face it on a trip with family/friends).

When heading into the seas at 12-15 knots, plenty of throttle and steering adjustment was needed to avoid slamming, but then I tackled a steeper face of prob 2-2.5m and the boat came up well and usually there is a soft landing starting with a contact point around 3/4 of the hull length along and the V falls into the waves for a secure cushioned re-entry. But this time the boat rose further and fell back onto the water prop-first with the transom actually causing a big enough splash to wet me at the helm, from behind!

So, my questions are: did the wind really get under the bow and continue to lift it? Did I tackle the steep face a little too fast (even at 12 knots and throttling back) and the ‘jump’ action became too close to a ‘flip’?

Is it a weight distribution issue? I was running alone, without extra load and circa 100 litres of fuel (of a 180l tank).

It was fine - didn’t feel particularly unsafe but was a surprising shift in behaviour which I’d like to understand better because with passengers it might have been less secure.

I’ve been out in it in much bigger - approaching Cat B max faces of 4m - but with less direct headwind and am experienced in handling that therefore the request for advice!
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Old 10 August 2019, 03:21   #2
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It does sound as if you were pushed back, could be wind, could be a following wave. Could be a bit of wind and rear weight as underloaded? Difficult to say but at least you noticed it and only allowed it to happen once? What did you do differently on the next wave to stop it happening again?


When the wind is coming in at me at the same point of the wave crest i'd usually turn the nose slightly and almost slide down the back rather then power off or down.
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Old 10 August 2019, 03:48   #3
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In later waves I avoided the sharper faces with an oblique angle (or steered to less steep areas) but that meant I didn't test the conditions in the same way again!
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Old 10 August 2019, 07:40   #4
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I don't move gear around the boat but work just off the trim, with trim down pushing into big seas. When seas get to big I take them on at an angle rather than head on and rarely drop bellow 20knots. Seas of 2.5m are pretty big, last week I was out in 2m swells (according to data from wave measuring data).

2m swell.
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Old 10 August 2019, 08:03   #5
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I suspect that the steeper face had an even deeper trough on the other side, which is near impossible to see before you’re falling and too late to slow further then too.
I have a tendency to get into false sense of security thinking that the wave pattern is predictable, until it’s not anymore.

There is a very good description of an experienced powerboater trundling along in reasonably big but manageable seas only to find themselves swamped when they fell off a “rogue” wave in one of the powerboat books but the name escapes me.
The oblique angle is a good idea, lengthens the effect wavelength and gives you a better view ahead.

If you have a standard setup XS650 they are well balanced and v capable.
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Old 10 August 2019, 10:03   #6
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Ah, interesting re the trough. I hadn't thought of that. That might be a factor.

It's a stock XS650 with Merc 150hp, 180L tank and 4 jockey seats (2 singles, 2 doubles).

Really does handle well - went from Falmouth to the Scillies last summer, Falmouth to Salcombe a couple of weeks ago and regularly cruise to Fowey, around the Manacles to Coverack etc in all weathers.

She's particularly adept in a following sea.
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Old 10 August 2019, 10:24   #7
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Yesterday/today have definitely been challenging conditions down here, well done for even trying to go out! We're sheltering in our sailing yacht in Falmouth Haven for a couple of days whilst these storms blow through, and certainly have no intention of going sailing until it calms down.


Just walked around Pendennis Head and it was looking very lumpy and uncomfortable out to sea, only one or two small craft out there that I could see.
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Old 10 August 2019, 13:16   #8
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It's impossible for us to tell what happened but often it's not the waves that get you but the holes. From your description it sounds as though you tackled a wave that was actually two waves merged into one. When two wave trains are present, it will be the case that occasionally you'll find yourself on a wave that it's formed by a wave from each wave train passing through each other. These waves combine to form a much higher wave but as they move on they reform into their original sizes and leave a hole where the crest used to be. If you're on top of the crest as it separates back into its two waves the boat will fall into the hole between them. The hole forms quickly because the waves are traveling away from each other at their combined wave speed.
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Old 10 August 2019, 15:09   #9
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Steep wave boat angle very high not enough momentum going forward so she lands on her arse where the heavey bit is so at 12-15 knots too fast IMO
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Old 10 August 2019, 16:15   #10
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Thanks all - you are all right in that when I changed the angle of attack from head-on to slightly oblique and further reduced speed I avoided repeating the issue even in the same sea conditions.

Thanks for the comments - good to learn.
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Old 13 August 2019, 13:10   #11
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Quote:
Originally Posted by willbank View Post
Thanks all - you are all right in that when I changed the angle of attack from head-on to slightly oblique and further reduced speed I avoided repeating the issue even in the same sea conditions.

Thanks for the comments - good to learn.
try a 25kg bag of sand in the bow locker
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Old 15 August 2019, 09:11   #12
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I had the same sort of thing happen once in my Ribcraft 545 ( a very Beamy Boat for its length!). ..While in the Bristol Channel in a Force 8 with Chris from Osprey (we were the only Boats of any kind out that day!) ...If the wind is strong enough...i.e approaching ...or ...Gale force...the Wave is Big enough ...AND Steep enough...AND the Wind direction is right on the nose! The Greatest Care must be taken.
If you breach the Crest of the Wave at an acute angle head on the wind can undoubtably get under the Hull AND the Tubes!.. And if so has the strength to literally stop you dead!...
The wave keeps moving under the Stern of the Boat (Holding you up even more) in the event forcing you to slide backwards down the steep face of the still moving wave ..Not a nice feeling ..OR if you're REALLY unlucky ...sideways! Which can easily lead to capsize.

You'll soon learn to alter you're angle of attack and look for the gaps in the wave pattern ...this often entails a Zig Zag course,but is the only way to make headway in these conditions in (relative) safety
A heavier ...Bigger Rib is less prone but still care must be taken.
I commend the OP for getting out and getting Helming practice in adverse conditions...how else are you going to learn?..But it's always a good idea to have company!
I'm assuming anyone who pushes the limits from time to time has a reliable well founded capable and well balanced Rig.
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Old 15 August 2019, 20:10   #13
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Sorry if this is a little to far off topic track but just thought this may help a little in regards to rough water handling. I love the big swell conditions we get here and yes I know there's very little sea chop among the swell in this clip and it's more about wind chop we are talking about.

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Old 16 August 2019, 01:45   #14
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https://youtu.be/SnqvrCrvmDs

I’ve been in the same predicament off of st Catherine’s point. Multiple lands on the aft section. This video isn’t descriptive or me but shows some lovely throttle control to keep the boat from riding too far out of the water. Ribs don’t always have to go fast. There is a careful balance of too fast and too slow but ribs do go at displacement speed too.

Sometimes the waves are just too big. I hit a nasty wash in the Solent last year and landed aft first in a 8m boat. We were loaded a little to the rear and unfortunately my dads back took the impact. Why the 71 year old wasn't sat in one of my suspension seats still annoys me to this day.
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Old 22 August 2019, 04:46   #15
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I was riding the same storm in inland water, though. No swell, but same wave height, and was experience the same sensation; landing on her arse.
Very uncomfortable and very scary! Didn't know what to do. Thought it had to do with headwinds.
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Old 22 August 2019, 11:57   #16
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Fun facts:

Waves don't follow any predictable pattern but fall on a frequency spectrum that follows a Rayleigh distribution. The significant wave height is the median height of the largest 1/3 of the waves you will encounter. Forecasting services generally quote significant wave heights. Significant wave height is 1/2 of max possible wave height.

Most waves will be smaller the significant wave height.

Roughly 1 in 10 waves on average will be significant wave height or larger.

Roughly 1 in 1000 waves will be twice significant wave height. Does not take long to hit 1000 waves doing 25kts.

If you go out long enough in 2m seas you will at some point encounter a 4m wave. Could be the 2nd or the 998th wave you encounter outside the harbour.

Think you might have found the big one!
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Old 22 August 2019, 14:48   #17
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https://youtu.be/SnqvrCrvmDs

This video isn’t descriptive or me.....
That's a seriously competent performance from both the rib and the skipper. Good post GT!
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