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Old 02-08-2013, 13:03   #1
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Displacement and LWL in Seas

After a two week trip along the coast of UK on our vivacity 650 it got me thinking about the boat displacement, it's LWL and their ratio - and how it affects boat behavior in the seas.

We've got 21 feet twin/bilge keel Vivacity 650. It's documented displacement is 1150 kg (2535 pounds).

Displacement/LWL ratio - 150. (classed as ultra-light in one of the articles about ratios)

On day 1 we left with a very marginal weathercast for our experience level - it said bf6, NE winds (onshore), and we had to cross Thames Estuary with it's shallows. Waves forecasted were 1.5m.

We were motorsailing, wind was building, so were the seas. Due to the shallows we had to follow waypoints exactly, and this sometimes meant taking waves on the beam.

It was not so bad initially, till we arrived at one location where the waves really started to build -swell coming from the sea plus additional waves due to wind against tide combination made quite a messy combo.

Some of the waves started really hitting us on the beam on the verge of broaching - I could no longer hold the course with our tiller. This really scared us, and we decided to change our plans and run for shelter. This shelter was still 20 miles from our position.

Running with those seas in our little boat was also quite scary - each wave lifting our stern and trying to turn the boat around. It wouldn't just simply surf straight down the wave - it was quite tense trying to keep the heading. I couldn't imagine leaving the boat under autopilot in this..

So long story short - is it just our inexperience, or our boat is simply not suitable for that sort of thing, and we should stick with her coastal inshore waters?

is 150 ratio too small? I believe it is, but I would love to hear your opinions

Would longer waterline make things smoother?

Thanks
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Old 02-08-2013, 13:24   #2
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Re: Displacement and LWL in seas

Not sure if it's a ratio thing or not. One of my early boats was a 21 foot retractable keel... in the conditions you describe, it would have been (and was) scary also.
Not sure why the waves were trying to turn you around... what was your sail setup at that point? almost sounds like it was "tripping" on the keels.... maybe a a bilge keeled boat does that...?
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Old 02-08-2013, 14:46   #3
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Re: Displacement and LWL in seas

More waterline would reduce the displacement/lwl ratio even further.

This is not a problem of any ratios. This is a problem of the absolute size of the boat. My boat is also categorized as "cruiser/racer" on that ratio -- about 200. But with 25 (English) tons of displacement, she is steady as she can be in tough conditions.

To reduce the tendency to broach when sailing downwind, put away the main and sail on headsail alone. If you have enough wind to maintain a reasonable speed, that's the magic bullet for that problem.

Kudos to you for tackling such a serious passage in such a small boat. Those are conditions many sailors will never see. Don't expect a one-ton-ish boat to be easy to handle in a F6 with a sea running, tide running, and shallow water, and did I understand correctly, wind against tide? . Any boat will be a certain challenge to handle in that. Sailing down wave faces in any boat presents a big helming challenge, and forget the autopilot. If you can manage such conditions more or less in such a boat, you will soon be ready for anything. That's real sailing. Cheers. In future, you might want to avoid the wind against tide where possible.
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Old 02-08-2013, 15:06   #4
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Re: Displacement and LWL in seas

Quote:
Originally Posted by Dockhead View Post
More waterline would reduce the displacement/lwl ratio even further.

This is not a problem of any ratios. This is a problem of the absolute size of the boat. My boat is also categorized as "cruiser/racer" on that ratio -- about 200. But with 25 (English) tons of displacement, she is steady as she can be in tough conditions.

To reduce the tendency to broach when sailing downwind, put away the main and sail on headsail alone. If you have enough wind to maintain a reasonable speed, that's the magic bullet for that problem.

Kudos to you for tackling such a serious passage in such a small boat. Those are conditions many sailors will never see. Don't expect a one-ton-ish boat to be easy to handle in a F6 with a sea running, tide running, and shallow water, and did I understand correctly, wind against tide? . Any boat will be a certain challenge to handle in that. Sailing down wave faces in any boat presents a big helming challenge, and forget the autopilot. If you can manage such conditions more or less in such a boat, you will soon be ready for anything. That's real sailing. Cheers. In future, you might want to avoid the wind against tide where possible.


Nicely put, Dockhead. Excellent post.

Moving up to a longer boat sure made a difference to us, especially to one having a hull shape that discourages hobby-horsing.

Ann
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Old 02-08-2013, 15:09   #5
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pirate Re: Displacement and LWL in seas

The Thames Estuary can be a bitch... strong tides and shifting sand bars that swirl the current round... to be honest with 1.5 metre forecast waves you were chancing it in a Vivacity... the keels are not exactly ideal for those conditions... the sea's would have been short and steep..
Its not so much the LWL that counts more the design of the boat itself... if you yet to tackle more trips like that and travel further afield yet keep the costs down try switching to a fin keel Corribee 21... or better yet.. a Hurley 22... have taken both across the Biscay to Portugal... the Hurley in December when even big boat owners don't fancy the trip... its a proper little yacht with a bigger boat feel..
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Old 03-08-2013, 01:54   #6
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Re: Displacement and LWL in seas

Dockhead: Thanks for the advice of genoa only .. I will defo try it next time. Altough became very picky about the weather - hopefully temporarily

Boatman61: Thanks for the suggestions, our initial purchase idea of this boat was to learn sailing basics - we bought it blindly from ebay, no surveys, no seeing in real life either.. Now that we basically rebuilt the whole boat from scratch I find out it's not really the boat for that sort of voyages


I know it's a long shot, but could it have been due to barnicles on the hull ? The whole trip we were scratching our heads why our outboard could not achieve anything over 4 knots. On the last day of our trip we decided to beach it and scrub the hull. It was my first scrub ever, so I don't know what counts as "a lot of barnicles", however after we were finished, our speed with outboard only at max throttle went from 4 knots to 5.5 knots - 38 % difference!

Now that Cheechako mentioned this "tripping the keels" thing..

One thing kept going in my head the whole trip, especially while waiting out the weather in Ramsgate with 30 knots blowing and seeing bigger boats leave - "I want a damn bigger boat, so I can sail more!"

But hey that's life and that's why I am now looking to crew on bigger boats before I go ahead and buy a bigger one - hopefully not from ebay this time! :
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Old 03-08-2013, 06:11   #7
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Re: Displacement and LWL in seas

Also, if the boat has appendages that create much drag: do not try to sail her too fast. Boats with much drag and bottoms not optimized for planning become LESS hydrodynamically stable when sailed very fast. Sailing too fast often comes from having too much sail up which additionally makes the boat aerodynamically LESS stable.

Go fast enough to keep good steerage though.

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Old 03-08-2013, 07:20   #8
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Re: Displacement and LWL in Seas

Quote:
Originally Posted by parito View Post
very marginal weathercast for our experience level

shallows. Waves forecasted were 1.5m.

shallows we had to follow waypoints exactly, meant taking waves on the beam.

waves really started to build -swell coming from the sea plus additional waves due to wind against tide combination made quite a messy combo.

Hi,

As you have noted in your post that LWL and displacement have an impact. The Queen Mary could have weathered your seas.

Wind against current/tide and wind or swell over shallows makes a very short steep sea where 1.5 meters looks and feels like 5 meters or more.

I never sail in marginal weather. I love my boat and my life. When I am at sea I have to cop what I get (though I always sail in the correct season). I can stay off shore and decide when to approach the coast.
Day sailing and coast hopping you can't do that so as you only sail the tricky bits you need to only sail in good weather.

Good work for having an alternate port! They come in very handy


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Old 03-08-2013, 07:25   #9
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Re: Displacement and LWL in seas

Quote:
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could it have been due to barnicles on the hull ? went from 4 knots to 5.5 knots - 38 % difference!
Yep.

My dinghy is in the water the whole time and every second week she slows down from 15 knots on the plane to about 4 knots and not planing. So I scrub her bum of weed... not even barnacles.
So growth really does affect speed and handling.
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Old 03-08-2013, 07:51   #10
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Re: Displacement and LWL in Seas

Good experiences. Have you ever read Maurice Griffiths?
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Old 03-08-2013, 08:04   #11
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Re: Displacement and LWL in Seas

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Good experiences. Have you ever read Maurice Griffiths?
Negative.. but now you got me interested
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