aardvarkash10 1,065 Posted May 31 Share Posted May 31 Apropos the discussion about hoving to, drogues and chutes etc, I picked up a cheap sea anchor for a play. I very much doubt that we will ever need it since we are mostly in the gulf. But it's an interesting exercise. The sea anchor in question is a cheap PVC item probably intended for drift fishing. It's a 90cm diameter cone with a 15cm outlet. Now the math. This thread gives me a formula to work out the drag of the anchor. It looks reasonable and I couldn't find a different formula. This page gives several formulas for calculating the force from wind on a ship's hull. I used the simplest one. I want to know what the likely load is on the bridle and rode. Assume a 40kt wind, what is the load as a sea anchor holding the yacht to the wind? Wind on the beam makes the windage force formula: Force(T)=0.00052 X A X v2 Wind on the bow, the constant becomes 0.00039. A is the presenting area and v is wind speed in m/s SO is 10m at the deck, 3m beam, and I estimate averages 0.5m above the waterline at deck level. Add an average of another 0.1m in height to the top of the cabin. So A is 10 X 0.6 = 6m2 for beam, and 3 X 0.6 = 1.8m2 for front on. Solve for a 40kt wind or about 20m/s, and maximum presentation (wind on the beam) F=0.00052 X (10x0.6)X (20 X 20) = 0.00052 X 6 X 400 = 0.00052 X 2400 =1.25t So maximum load on the rode is 1250kg To allow a 2x engineering margin, I need a 2500kg tackle! Wind to bow, we get a more reasonable 0.28t and so 600kg tackle for a 2x margin. At 30 to 45 degrees to the wind, we sit at about half way between the two. How am I doing engineers? Quote Link to post Share on other sites
K4309 353 Posted May 31 Share Posted May 31 23 minutes ago, aardvarkash10 said: How am I doing engineers? Your formulas are lacking units. As an engineer, this makes my eyes hurt. 1 Quote Link to post Share on other sites
aardvarkash10 1,065 Posted May 31 Author Share Posted May 31 Just now, K4309 said: Your formulas are lacking units. As an engineer, this makes my eyes hurt. I can understand that. Also a transposition error that I've corrected. F is in metric tonnes ( I get that's not a unit of force, so shoot me), A is area in m2 and v is wind speed in m/s Quote Link to post Share on other sites
K4309 353 Posted May 31 Share Posted May 31 There are so many variables to consider for an 'accurate' answer. Noting the difference between 'accurate' is subjective, as in being about right, or being correct to 10 decimal places. There are all the obvsious technical elements like dynamic load, sea state and water force on the hull in addition to wind force, drag from hull appendages at a certain drift speed etc. But the bits I would be most interest in are the type of rope, as in stretchy or completely in-elastic, the ability for the drogue to 'bite' the water, and as you alluded to, the ability for the drogue to not rip to bits. The short story I would think is to come up with a reasonable solution I would recommend relying on imperical data rather than theoretical data. As in, what works in practice (what have others used, etc) Do you want the rope to be stronger than the drogue? Then get a big rope. Do you want the rope to protect the drogue, then get a stretchy rope. The drogue you describe sounds very much like a fishing drogue, designed to keep a fizz boat bow or stern to the waves, and to let it drift slowly in light to moderate conditions. I'd expect it would get dragged fairly quickly through the water by a quality Spencer in 40knts, best effect may be to hold the bow (or stern) into the wind, but it would largely be ineffective. From an engineering analytical point, then the drag from hull appendages may have a greater proportion of influence than the drogue, and this will skew all of your analysis. You could just go with some 14mm nylon anchor rode which is what? 6t breaking load (I'm guessing, haven't checked) and be done with it, assuming you have spare anchor rode, and wanting to use anchor rode as it is stretchy. 1 Quote Link to post Share on other sites
aardvarkash10 1,065 Posted May 31 Author Share Posted May 31 Totally agree K. I reckon the anchor would fail well before a decent 12 or 14mm line would. In terms of accuracy, I'm interested in seeing if I'm in the right order of magnitude. The variables outside the calculation are so, well, variable, that anything to the right of a decimal point would be self-delusion. I'm going to give it a shot with 50m of cheap retail 12mm anchor rode and a bit of chain to see what happens. That rode has a working load around 800kg iirc. More than enough for any weather we might be out in. Quote Link to post Share on other sites
Black Panther 1,692 Posted May 31 Share Posted May 31 Use whatever you use for your anchor. You are overthinking it. 1 Quote Link to post Share on other sites
aardvarkash10 1,065 Posted May 31 Author Share Posted May 31 Yeah, but it's Friday and I'm 2/3 into a bottle of shiraz Quote Link to post Share on other sites
K4309 353 Posted May 31 Share Posted May 31 8 minutes ago, aardvarkash10 said: Yeah, but it's Friday and I'm 2/3 into a bottle of shiraz Well if you are bored and trying to find tedious equations to work through, I'm sure I could find some insurance policies for your to assess? I reckon from a technical point of view, you probably aren't far off on the wind force calculation, but the biggest factor would be determining the slippage of the drogue. You'd need that to determine the force on the rope. The windage calc can approximate the force on one end of the rope, but you need to assess the force on the other end. Infact, the slippage / force on the drogue is probably what will define the max force on the rope. If it freely slips through the water, there will be zero force on the rope. If it is stuck solid then you could use the windage calcs to determine force, but we know it's not stuck solid, indicating the windage force isn't the one you want to put the effort into assessing. 1 Quote Link to post Share on other sites
aardvarkash10 1,065 Posted May 31 Author Share Posted May 31 Agree. You need to resolve for both, and they interact. I'm with BP. Close enough will do. Quote Link to post Share on other sites
Black Panther 1,692 Posted May 31 Share Posted May 31 In Ks example zero slippage is an anchor, any slippage would be less force. Use your anchor cable and it's enough at maximum force which the same as being anchored. Done. 1 Quote Link to post Share on other sites
Psyche 728 Posted May 31 Share Posted May 31 All this talk of heaving too in 100kts has got you excited? Are we going to see Stepping Out on the news- Lone sailor with wine bottle in hand yells instructions to coastguard during gulf storm "F off, can't you see Im testing my drogue?" 3 Quote Link to post Share on other sites
harrytom 679 Posted May 31 Share Posted May 31 used a shute in a 18ft stabi off colviille bay, 10knt easterly incoming tide ad plan was to drift fish and the shute would help us stay put nope. it was 2m diameter no exit point for water like a drogue, Set so was probaly 5m behind vessel .Surprised me at the rate of drift 1.2knt east watching gps, At the I thought now how big does a drogue have to be to hold 40ft yacht head to wind in 50+knts and seaway. 1 Quote Link to post Share on other sites
aardvarkash10 1,065 Posted May 31 Author Share Posted May 31 9 hours ago, Psyche said: All this talk of heaving too in 100kts has got you excited? Are we going to see Stepping Out on the news- Lone sailor with wine bottle in hand yells instructions to coastguard during gulf storm "F off, can't you see Im testing my drogue?" Pretty much it! Confidence build exercise. Jos is fine in big seas and wind, I still get a bit jittery. Calculations are my hail Mary. Quote Link to post Share on other sites
Island Time 1,284 Posted June 1 Share Posted June 1 I've the approx loads and formulas somewhere. But I'm on my 2nd Whiskey and can't be arsed looking for them 😆. Iirc the max holding power of my rocna 20 is around 2000-2500 kg. The anchor gear is specd to suit this. My para anchor is a Coppins storm fighter, 5m diameter, with a centre pressure relief system held closed by really heavy bungee. In the storm we were in, I don't believe the loads on the primary rode -under bow roller - (on a bridle to a primary winch as well) I don't believe that the loads were any bigger than being anchored. If anyone is interested and near Gulf Harbour, I'd be happy to go out in 40-50 kn and deploy the chute. Anyone want to film it? 1 Quote Link to post Share on other sites
Island Time 1,284 Posted June 1 Share Posted June 1 Some real life references here https://www.paraseaanchor.com/para-sea-anchor-testimonials/ Quote Link to post Share on other sites
K4309 353 Posted June 1 Share Posted June 1 3 hours ago, Island Time said: I've the approx loads and formulas somewhere. But I'm on my 2nd Whiskey and can't be arsed looking for them 😆. Iirc the max holding power of my rocna 20 is around 2000-2500 kg. The anchor gear is specd to suit this. My para anchor is a Coppins storm fighter, 5m diameter, with a centre pressure relief system held closed by really heavy bungee. In the storm we were in, I don't believe the loads on the primary rode -under bow roller - (on a bridle to a primary winch as well) I don't believe that the loads were any bigger than being anchored. If anyone is interested and near Gulf Harbour, I'd be happy to go out in 40-50 kn and deploy the chute. Anyone want to film it? More to the point, what whiskey are you drinking? I've always been a fan of the Isla's, never fully appreciated Speyside's. 1 Quote Link to post Share on other sites
Island Time 1,284 Posted June 1 Share Posted June 1 25 minutes ago, K4309 said: More to the point, what whiskey are you drinking? I've always been a fan of the Isla's, never fully appreciated Speyside's. Glenlivet 15 yr old single malt. Pretty nice! 1 Quote Link to post Share on other sites
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