high speed sailing in non-planing quarter tonner - mechanism ?

[mikejames]

Once or twice on near the limit sails especially on flat water , Forthought (Westerly GK24 - heavy quarter ton design) has done a strange thing - the bows go down, there's a sheet of water spraying both sides of the bow and the rudder goes light. The boat speed increases well over hull speed -
On the GK24. the bow section is quite raked, marina unfriendly and makes effectively a deep-V hull if pressed into the waves

Typically this is in the lee of the Isle of Wight on the Round the Island race when the wind gusts just east of Cowes on the way back, with spinnaker, genoa and main up.

All the crew are sitting as far back as possible asking if they can take the genoa down and I am suggesting that they stay put, despite it only being half way through completing the spinnaker hoist

One of the crew mentioned it had also happened on a half-tonner that he had been on in Portsmouth Harbour

Symptoms

Flat Sea
Bows down , stern up
overcanvassed - gust
light helm but responsive and balanced
fast ! (about 1.5 times hull speed)


Whats happening ? so I can repeat this

And its nothing to do with my tame dolphins , alcohol or fatigue ..

 

[sailorman]

possibly too little bouyancy in the bows causing the rudder to loose grip

 

[graham]

Probably planing on her own wave,as the wave moves aft the bow appears to go down. Best example Ive reached on my 22 footer was a steady 14 knots on the log for 5 or 10 minutes.Very sphincter clenching stuff but as you say perfectly controllable.

 

[Adamastor]

I think graham's got the right idea. They messed around with the sterns and especially the bits between the foils of IOR boats- bustles and other unmeasurable nasty shapes- to give them less OMG downwind. If they had been allowed to draw flat runs and IMS asses, ton-boats would plane like a dream. A mate of mine used to see high teens and low twenties on his Petersen 33 with the same OMG values you're talking about- curtains of spray from the first stanchions aft, and the same feeling you get going "over the falls" on a surfboard.
I guess the boat's just designed to be that smidgen short of dynamic lift from the hull that lets her sit back and plane on her buttocks, so she tears rather than skims along, falling off her quarter-wave and trying to push her fat midriff not so much up out of the water as through the water.
I don't care what they say: performance isn't absolute- it's completely relative. Making little boats fly beyond their limits is where it's at, not making already fast designs just do what they're intended!

 

[gardenshed]

no disagreement with what's already written.
GK24 so, moderate displacement, not much flat surface to promote planing and if you imagine the simple force of the rig pushing the boat forward, this force is also trying to tip the bow down.
The combination of the wave pattern that is developed at high hull speed and the force of the rig isn't reacted by hull lift so she digs in and sends spray all over the place. On the back of a large wave, you may get surfing (where the wave helps push you through the the transition into semidisplacement/planing speed) but in flat water the boat tends to feel as if it is tripping over.
I was once on board a small boat (IOR mini-tonner) going from Loch Riddon to Rothsay when we had exactly the circumstances that you mention, i.e. flat water and very strong gusts. Full main and kite and we eventually had a gust that pushed the bow under water. This brought the run to a dead stop tipping the rear of the boat up, bringing the rudder out of the water and almost driving the wee thing completely under. Quite scary and a bit of damage done.
The designer was on board so we immediately had a discussion about the trade offs/compromises related to volume distribution along the length of the boat, hull shapes that planed more easily , the need for upwind performance as well as downwind and the racing rule of the day (IOR). Needless to say the next boat was less extreme and had different vices...........
With your GK24, there is little that you can do except keep the weight back and enjoy the ride. Keep the hatch shut and make sure that it can't slide forward if things do come to an abrupt halt with the bow pushing itself under water.

 

[Racecruiser]

I don't know the hydrodynamics involved but I'm sure you do the right thing by keeping the weight aft and the boat under the rig!

I think it's a 'displacement plane' - just invented that term! The rudder going light is perhaps due to some aeration at speed.

Do you get a bit of humming from the boat? That and the wake flattening off is usually a sign that you're beyond hull speed.

Bearing off I guess will help to keep her upright as the apparant wind goes forward with extra speed. 'Yeehah' I think is usually shouted at that point!

 

[wotayottie]

heading for pitchpoling? /forums/images/graemlins/grin.gif

 

[boatmike]

Almost certainly! The boat in question has insufficient reserve bouyancy in the bows to resist the forward pressure on the sails. When in this perilous state it would only take a good gust to dig the bows in on a wave and have the wonderful experience of flying in a boat......
May not actually pitchpole but would certainly broach. You might find it fun but listen to your crew. They seem to have a better developed instinct of self preservation than you do!

 

[bbg]

If you are expecting these types of conditions (and you're racing) it might be prudent to take the anchor and chain off the bow and store it as low as possible in the center of the boat. It will reduce the tendency of the bow to drop, and improve performance generally.

 

[KenMcCulloch]

[ QUOTE ]
.......
I was once on board a small boat (IOR mini-tonner) going from Loch Riddon to Rothsay when we had exactly the circumstances that you mention, i.e. flat water and very strong gusts. Full main and kite and we eventually had a gust that pushed the bow under water. This brought the run to a dead stop tipping the rear of the boat up, bringing the rudder out of the water and almost driving the wee thing completely under. Quite scary and a bit of damage done.


[/ QUOTE ]
I once had a Yeoman (half-decked keelboat) and they were notorious for doing exactly that. Several were lost due to over enthusiastic downwind sailing leading to the bow burying itself in a wave and the cockpit flooding. LJs were worn at all times!

 

[afp]

I can remember the Trapper 30 we used to own doing much the same thing on a race from Poole to Hamble. We were using our oversize spinnaker at the time.

It wasn't obvious from the cockpit until water started to squirt up from the feet of the chap who was flying the spinnaker from the shrouds.

We dumped the spinnaker, much to the delight of the rest of the fleet behind us, who had smaller spinnakers, until a gust of 35+ knots came through as we got up to Calshot, and flattened a number of them.

 

[Daydream believer]

We used to tell the owner of the boat that we raced in to get the spinnaker up to create some lift.
I am not sure if he believed us - we did not really- but he was paying the bills & it was fun.

 

[footsoldier]

I don't think this wins a record for resurrecting a long-dead thread, but the attempt surely deserves a 'highly commended'.

 

[jamie N]

I don't think this wins a record for resurrecting a long-dead thread, but the attempt surely deserves a 'highly commended'.

Perhaps, but it's certainly relevant to me at the moment, and (again) thanks to DB for it.

 

[Minerva]

We used to tell the owner of the boat that we raced in to get the spinnaker up to create some lift.
I am not sure if he believed us - we did not really- but he was paying the bills & it was fun.

Well that anecdote was worth waiting 16 years for!

 

[Daydream believer]

Well that anecdote was worth waiting 16 years for!

47 years since we did it
So probably just as bad

 

[Vimmy]

Would an adjustable length pole actually create adjustable bow lift?

 

[DownWest]

In a similar situation we planed clean through the fleet at Burnham. The rudder was vibrating enough to loosen fillings. We were so far ahead crossing the line, that they didn't give us a gun, assuming we had joined the many retirements.
Ah, just read the title.... This was a 27ft hard chine fin and bulb, but I think it was the only time it realy planed.