Technical
stephenhb
stephenhb
| Sailing Performance |
| By Mike James The review here is written from the viewpoint of either cruising or racing the boat two up, or rarely with more crew, mostly Round The Island (of Wight) races. Short handed sailingWe have two children ( March 1998 and September 2000) and when we are sailing this means we have effectively 1.5 crew. One helming, the other rushing about. It is definitely possible to hoist a spinnaker with 2 crew and recover it successfully without the need for a snuffer. Short handed on Wednesday evenings racing it is often the case . General Sailing The boat is responsive to the helm, never being particularly heavy even when heeled, although there is some weather helm when beating. The rudder appears to be the same as some other 29 foot Westerly with a transom hung rudder, as it is listed in spare part lists under the same heading, at about £1400 ! So repair rather than replacement seems to be the order of the day. The helm tends to need fairly constant attention or an autopilot because the hull does not have a great deal of inherent directional stability, although it is possible to lash the helm for brief periods. Off the wind it can become more of a handful, a beam reach with spinnaker often resulting in holding the helm hard over to hold off the broach. Letting the main out or the kicker off controls this. A check on how close you are to hull speed is that the cockpit ends up at about sea level at high hull speed , with water just washing the Treadmaster if the boat is level. A tubby crew can make this worse ! I have never been out in more than a recorded 35 knots of wind, in Southampton Water in the GK24, when reefed with 2 reefs in the main and a 100% genoa. but others have had to manage 50 knots off Brighton and lived to tell the tale. The GK24 is definitely a sailing boat, because of its Quarter Ton racing ancestry. Because of this it has a relatively large rig compared with other small boats. Our fractionally rigged boat has 35 feet of mast. It is necessary to reef earlier than these other boats to turn down the power. In lighter airs, It is faster than many other cruising boats, even 30+ footers. You can literally sail in circles round Westerly Centaurs in a light breeze. When you first step aboard, the boat heels quite noticeably, but then you are over 4 feet out from the centre line. Under sail, with main and No. 1 genoa the boat heels noticeably to the breeze but can easily hold full sail up to 15-18 knots apparent. With crew ballast the limit is higher. In an extreme case, I have feathered up in 30 knots of wind without the main flogging, in a sudden gust. Genoa Only Performance As this is more manageable we quite often use this configuration for low-intensity sailing in stronger winds. We don't have to deal with the mainsail trying to blow away We hoist a 100% jib/genoa and sail slower, or motor-sail. On another occasion we were lazy and sailed in 20-25 knots of wind with only the 135% genoa up. We had started up off the wind but eventually we hardened up and we were still making to windward and overtaking other boats. As the wind gusted first there would be lee helm due to the off balance sail plan and then weather helm as the boat heeled. Once I got used to it, it was OK but it took concentration. Putting up the main would have been easier. GK24 Squall I have been in a squall which took the boat over to the point where the cockpit coamings were stopping the sea coming in. Reaction of the boat to waves The boat will point all over the place when rolled by the wash of a power boat, but if you hold the helm firmly it ends up mostly back on course, although a little worrying at the time. Because the boat is not very long, the problem with progress is rapid pitching in a choppy sea : the Solent is full of power boats and tidal races that stop you dead in light winds. In heavier winds, crew on the rail make a huge difference. Out beyond the Needles the wave period is longer and the boat begins to ride over the waves without stopping or losing much way. Out there we can overtake boats that leave us standing around the cans inside the Solent. It is easy to stall the boat, even ending up going backwards after hitting a wake , so one has to free the sheets and bear away for speed before hardening up again. If you don't, the hull just makes leeway. In this respect it is like a performance sailing dinghy. In waves, spray comes over the bows, but they usually lift over the waves (fast pitching!!), and so the occupants of the cockpit are showered rather than drenched |
stephenhb
| GK24 Scale Drawings |
| By Keith Fletcher You can download AutoCAD(.dwg) drawings of GK24 Slingshot or a low-res render in Acrobat(.pdf) The ACAD drawing is pretty much to scale, but don't order sails or masts off it!!! |
stephenhb
First off, no liability accepted, make your own mind up!
The centre 2 holes line up with the caps bulkhead brackets and the lowers deck plate (55mm centres for 8mm dia. Holes), and the outer holes were added to provide a bit of extra support with 2 extra bolts through new holes drilled in the deck.
From 6mm 304 plate (couldn’t get 316 – see how we go with corrosion – easy to replace), tight radius folded, no welds or V cuts.
Fitted with 4 No. M8 x 40 316 S.S. bolts each, with 316 plain washers inside and out, and oversized 30mm dia. washers on the inside of the new outer holes. Plain nuts backed up with nyloc nuts to provide a bit of extra thread grip. Not over tightened, to reduce the effects of the clamping load reducing the working load of the bolts.
Only slight problem was the hump in the deck from the lowers, meaning the outer holes on the lowers can’t be drawn up really tight without stressing the deck with a hard spot, so I put an extra oversized washer between the chainplate and the deck to space the gap and allow tightening.
The angles of 105 & 115 were almost right. If anything the caps could have been 102.5, and the lowers 117.5, but I suspect everyone’s boat will be slightly different due to deck flex, and the angle can be easily measured. I had to ease the clevis pin holes to allow the bottle screw to fit in a straight line (not enough clearance on the clevis pin to allow the bottle screw to articulate in the line of the load).
The height of the hole is lower than the old delta bolts, so check you have enough to open up the bottlescrews to add some more shroud length.
The whole thing now looks bullet proof, and I believe more elegant than the larger versions made that cover both shrouds with one piece, and allows proper angling of the shrouds. Went nuts with the Sikaflex, so bit of a mess to clean up though...
Downsides – Shroud base is narrowed by about 20mm each side, and over time the outer edges of the chainplates may start to dig in to the deck, and the inner side lift. I’ll just keep an eye out for flex, and maybe replace the M8 bolts every couple of years. At least I can now.
The Makefast V bolts look incredibly small for the job when you take them out….. what were Westerly thinking. I’ve now got 8 bolts per side holding the rig up, instead of 4 (any one of which failing would have toppled the stick). So have a huge confidence boost in the rigs stability – was getting paranoid about the V bolts failing, having replaced all the standing rigging last year.
Took me 2.5 hours to fit all 4, on the mooring, with a hand drill, mole grips, ratchet set and rat tail file. Took the shroud loads up on a genoa halyard tied around the middle stanchion base whilst working on each one. Old nuts came off fairly easily despite being well and truly rusty.
Cost – Bolts and washers S$32 (about 10 quid – I’m lucky that stuff is dirt cheap out here). The chainplates….. A lunch for a favoured contractor of mine (who also made me new teak rubbing strakes for the same price).
Hope this might be useful. Will send some pics of the finishes article when back from Christmas in Australia buying more bits for Slingshot...
Cheers,
Keith in Singapore
Update from Keith 07jan2008
Note I've changed the angles for the lowers to 110 degrees - I've had totake them off and get the angle closed up slightly from 115 - the bottle
screw wasn't sitting right, and I didn't want to have to add a toggle (more
to go wrong).
So these are amended from the previous PDF, but again most boats are going
to be slightly different particularly for the lowers depending on how much
the deck has humped - mine has quite a lot - so each case on its merits.
The use of 304 was through necessity due to stock, and so far after 4 weeks
in a humid and salty atmosphere no sign of corrosion.
They only cost me about 50 quid for the 4, so to replace them on 316 when
its available would be cheap and quick.
Now this out of the way to stop the stick falling off, on to the next job on
the endless list - new forehatch (or at least seals) to stop my new sails
getting wet.....
stephenhb
| Reprinted from a public guide courtesy of Hall Spars. The goal of a properly tuned rig is to have a mast that is straight athwartships, have control of sail shape, and get proper helm balance in a variety of conditions. Most importantly, the resulting mast tune should not impart any excessive strain or loads on the spar and rigging or the structure of the boat. The basic adjustments for tuning a rig are actually quite easy and straight forward and not the mystery that some people might make them out to be. To begin with, we should define the difference between fore and aft tune and transverse tune. Fore and aft tune basically refers to rake and mast bend. Transverse or lateral tune refers to setting the mast up straight sideways and setting up the uppers to minimize lean. Let’s begin with rake, which is determined by headstay length. Rake affects helm by moving the center of effort of the sails relative to the center of lateral resistance. A longer headstay gives more rake which gives you more weather helm. A starting point for arriving at the correct rake is to measure the designed rake of the sailplan. A typical 40’ boat would have about 15-18” of rake. To calculate rake, hang a plumb bob from the main halyard and measure from the aft side of the mast along the cabin top to the plumb bob. This should be done with the backstay tensioned at about 60%. The actual amount of rake you end up with may vary depending on the normal conditions you sail in and may be a compromise between what is optimum in light air vs. heavy air. The second aspect of fore and aft tune is mast bend. A certain amount of mast bend is desirable. Mast bend is determined by the relationship between the masthead position, deck partners and mast step. If we have decided on the proper rake, then the masthead position is fixed and we have the deck partners and mast butt positions to adjust to induce mast bend. By either moving the mast forward in the deck collar or moving the mast butt aft in the step, we can induce some bend into the rig. Another factor that can affect mast bend is the angle at which the butt of the mast is cut off. If the mast is resting on the forward or aft face, the resulting moment will have a major effect on bend. We normally radius the butt of the mast so that the spar will bear near the center axis of the section thereby minimizing the bending moment regardless of the angle of the spar to the step. Other factors that control mast bend are double lower shrouds, babystays, and inner forestays. Doubler lower shrouds can be tuned to increase or limit mast bend. Babystays are typically used on boats with single in-line lowers and pull the rig forward down low in the same way as forward lowers. Inner forestays with a staysail can put a large bending moment in the spar and are usually opposed with running backstays or aft intermediates. Spreader sweep is also a big factor in mast bend, but this factor is a design feature of the spar system that is not an owner variable since most spreaders are fixed rigidly to the spar. Swept aft spreaders will facilitate some mast bend and in-line spreaders will restrict mast bend. So what does all this talk of mast bend mean-why is it important? A certain amount of forward bend is healthy, making the spar more stable and less likely to pump in a breeze. Most mainsails require a certain amount of mast bend to set properly and as the breeze increases the combination of more backstay tension and more bend will flatten the main. This will keep the boat standing more upright and ease the helm. Another consideration here is headstay sag. Being able to control the amount of headstay sag with an adjustable backstay device will allow you to optimize the shape of the genoa for a range of wind strengths. With an adjustable backstay, particularly hydraulic types, it is extremely important to establish a maximum backstay load not to be exceeded and some lower reference points. A good limit is 30-40 percent of the breaking strength of the backstay wire or rod. This allows some margin of error in the system in the case of shock loading. Now we will discuss lateral tuning which is probably the most important tuning process and often the most confusing. particularly with multiple spreader rigs. Keeping the mast straight athwartships over a range of wind strengths and sea conditions is essential to keeping the rig in the boat. As well as keeping the spar straight or in column, we are also interested in having the upper shrouds tight enough to minimize how far the mast leans over the side. This will help reduce weather helm. Although the effect is small, most boats have too much weather helm in fresh air, and it is important to minimize it any way we can. The first step in lateral tuning is to center the mast in the boat. The backstay and upper shrouds should be relatively loose at this point to minimize bending the rig. Pull a 100’ steel tape up on the main halyard and measure to the chainplate or to a point on the gunwale on each side. Adjust the upper shrouds until you get the same readings port and starboard. At this point check to see that the spar is firmly secured in the partners with wood wedges or preferably hard rubber wedges. They should be very tight so the mast cannot work or move at the deck. Using Spartite is the most efficient and effective means and required for carbon spars. The next step is to tension the upper shrouds at the dock. Make sure the turnbuckle threads are lubricated to prevent damage from galling. A dry lubricant is preferred, such as Rig Lube or Bike Aid. The upper shrouds should be tightened as tight as you can get them with a 10” crescent wrench. Don’t use a larger wrench or an extender as you can damage the threads. Additional tensioning must be done under sail. The lowers and intermediates should be fairly loose, or just tight enough to keep the mast straight. If the boat has discontinuous rigging, it is important that the diagonals be very loose before tensioning the verticals. They will tension as the verticals are tensioned. Now we are ready to further tension the upper shrouds under sail. In about of 15 knots of breeze, put up the main and tighten the leeward upper shroud. Keep track of the number of turns while tacking back and forth until the uppers are snug on the leeward side with the boat heeled at 20 degrees. Don’t worry about the lowers or intermediates at this stage, the object is to just fully tension the uppers. Another method for tensioning the uppers without sailing is to heel the boat over at the dock using a halyard. The halyard must lead through a fair lead lock at the masthead and have a clear lead directly abeam. This method saves time and also allows adjustment of the lowers and intermediates but one should exercise caution tuning this way. Now that we have the target tension on the upper shrouds, we are ready to straighten the mast with the lowers and intermediates. With a single spreader rig, simply tension the lowers until the mast appears straight when sighting up the sail track or grove. This should be done under full sail in 15 knots of breeze. If the boat has double lowers, generally the forward lower will be tighter than the aft lower. The forward lower does most of the work supporting the mast laterally. The aft lower acts primarily to limit mast bend as the backstay is tightened in heavy air. At the dock, the lowers will be a good deal looser than the uppers. Under sail on the leeward side, the lowers will flop around a lot, and it is a good idea to use a shock cord lashing to take out the slack and prevent fatigue. Multiple spreader rigs are more complex, however the tuning process is essentially the same. After the uppers are secured, start with the D1 shrouds (lowers). These should be tight enough to prevent sagging to leeward at the first spreader in 18 knots of breeze. The next shroud up in a double spreader rig would be the D2 or the intermediate. The adjustment of the D2 is very important as it has a large effect on the transverse bend in the upper part of the spar. It should be set up fairly loose in the beginning of the tuning process and gradually tightened to eliminate sag at the second spreader. If the D2 is too tight, the upper spreader is pulled to windward and the masthead is relatively to leeward. Many people view that as the tip falling off, with the solution to tighten the uppers, when the correct adjustment is to ease the D2. This situation is undesirable as the angle between the upper shroud and the spar at the upper tang is reduced. The final fine tuning adjustments of the diagonal shrouds should be done at 20 – 30 degrees of heel since the adjustment is relatively insensitive at low loads. |
stephenhb
Definately 24ft - the rest we can debate!
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stephenhb
Adding internal rigging wire
A possible cure for the deck is to add an additional internal tie bar or rigging wire. I took this route and I am still evaluating whether more reinforcement is needed.
I will be writing an article about this when I am satisfied. In my case the constraint was that there should need to be no disturbance of anything already in the hull. On the port side this was easy but to starboard the removable chart table was the constraint on how low I could place the pad eye on the bulkhead on the starboard side.
For symmetry, I placed the pads in the same relative positions port and starboard.
The S/S plates were fabricated by a relative who used it as self-training for his fabricators job.
The bottlescrews came from a secondhand chandlery, but it took the purchase of 4 bottlescrews before I had a pair that had not been stretched.
I drilled a hole in the triangular plate under the deck and fitted a clevis pin through it . In the end I reversed the pin so that the split pin faced away from the cabin.
The rigging wire came from my re-rigging after 20 years , complete with swaged on ends. I used some £10 Chinese boltcroppers to cut it and they worked ... (Bolt croppers constructed to an accuracy of about 3 mm, I have never seen such wibbly wobbly nuts and bolts) The bolt croppers are therefore in my tool locker as I had an unfortunate experience (not my boat) in 2004, and I now value bolt croppers as being somewhat useful on a boat.
The other ends are Norseman screw-on terminals and by a process of shuffling in the second-hand chandlers I managed to get two with the little insert 'bullets'. These just tighten up and grip the wire without the need for a hydraulic press.
Then I just tightened up the bottle screws and waited. The tension remains in the wires and the rigging above deck is now tighter.
Pic 1. The backing pad: 13mm ply epoxied to the bulkhead.. I propose to glass this in with some tape as well.
stephenhb
Figure 1 - Existing Mast Head “U” Bolt arrangement
Fractional rig GK24 yachts utilise a different arrangement. They have an angle bar bolted to the deck, tapered up to the shroud attachment points, as shown below.
Figure 2 - Fractional Rig Arrangement
The Modified arrangement is similar to the Fractional rig detail apart from the spacing of the attachment points. This is due to the shroud position and mast rake.
Under deck securing is not affected. The forward bolts remain attached to the main transverse bulkhead brackets, and the aft bolts to the existing tension bar. This is attached onto the longitudinal frame bracket.
Figure 3 - New Mast Head chainplate arrangement
stephenhb
stephenhb
| There are two numbers stamped on the transom of most GK24's. The first has the form SOU and a series of numbers, presumably the yard or build number. This number is missing on some GK's, and it's location changes from boat to boat. The actual hull number is a bit more tricky! Look at the bottom edge of the transom, just above the trailing edge of the skeg and you should see something like Xnnn, where nnn is the hull number of your boat. It is near on impossible to see this with the boat in the water, as invariably it will be painted over with the boot top and hidden behind the front edge of the rudder. Below is a picture of where you "should" find your hull number |
GeeKay
rig chain plates together with the internal ties bars for sale
together with the rest of the GK equipment advertised on the
discussions.
On Oct 6, 1:43 pm, stephenhb <stephen.homanbe...@googlemail.com>
wrote:
> take them off and get the angle closed up slightly from 115 - the bottle
> screw wasn't sitting right, and I didn't want to have to add a toggle (more
> to go wrong).
>
> So these are amended from the previous PDF, but again most boats are going
> to be slightly different particularly for the lowers depending on how much
> the deck has humped - mine has quite a lot - so each case on its merits.
>
> The use of 304 was through necessity due to stock, and so far after 4 weeks
> in a humid and salty atmosphere no sign of corrosion.
>
> They only cost me about 50 quid for the 4, so to replace them on 316 when
> its available would be cheap and quick.
>
> Now this out of the way to stop the stick falling off, on to the next job on
> the endless list - new forehatch (or at least seals) to stop my new sails
> getting wet.....
>
> 83KViewDownload
>
> GK24mhChainplatesFinalACAD2000.dwg
> 117KViewDownload
>
> GK24mhChainplatesFinalimage.pdf
> 151KViewDownload