best hull for short steep chop

[maarty]

Hi Pierre,

You say with your log example: "If you want to beat all the sailboats for comfort ratio and capsize ratio" that your log cannot be beat. And then you say:
"I will guarantee you it will capsize". So which is it?

Capsize ratio has a lot to do with displacement, beam, depth of ballast and ballast/displacement ratio and hence the righting arm.

Carl's Sail Calculator doesn't show ballast to displacement ratio (easy enough to figure out by dividing ballast/displacement), nor does it show directional stability or how close winded a boat a boat can sail.

Motion comfort is definitely an important factor in choosing a boat. Good capsize and motion comfort numbers coexisting are a plus. Carl's site is not perfect, but it is a rich source of data on many boats, and a good starting point.

I suggest that generally speaking, high capsize, and low motion comfort numbers together can indicate a less close winded boat that is less comfortable when sailing against choppy waves.

 

[SamSam]

I forgot this is essentially science...numbers. I define short chop as roughly 1 m tall and about 2 m long. I am curious for determing hull design for a future sailboat. Your opinions are much appreciated. Thank you.

It might help also to give an idea of the use and load. Like day use for two wild and crazy guys or overnight trips for a family of four. No matter what, it looks like you're going to get wet!

 

[Tim B]

A simple ratio to determine the effect of pitching is to look at the wavelength vs the (static) waterline length. A ratio of 0.5 generally represents a maximum pitch response (often close to 100% of the wave slope). Ratios above 3 generally suggest fairly minor pitching, and as ratios approach 0, pitching also reduces significantly. However, the wave slope is also important, and is defined as atan(A*L/pi) for a pure sine wave. In this case, 32 degrees. This provides an indication of what the bow will encounter. Essentially, the steeper the wave, the finer bow required. Full motions simulation is required to really describe what will happen, but there are a limited number of people who have the time and money to do this.

So for this sort of sea-way, the longitudinal weight distribution is not too critical, but the bow shape will be of considerable importance. A lightweight boat with plenty of sail area and a fine bow will be a good bet in these conditions.

In your case, a 1m wave with 5m or 10m wavelength will be more dangerous.

I have massively simplified the dynamics calcs here, as there is a fully-coupled 2nd order 6DOF system to consider in the full set, but there is enough to get a rough idea of the 1st-pass figures. To progress further, you need hull and mass characteristics.

Cheers,

Tim B.

 

[BATAAN]

The Polperro fishing cutters of the UK had to beat out of port to go to work. This developed a type that had the power to smash to windward. Doesn't mean it was dry or comfortable.

 

[Mr Efficiency]

I forgot this is essentially science...numbers. I define short chop as roughly 1 m tall and about 2 m long. I am curious for determing hull design for a future sailboat. Your opinions are much appreciated. Thank you.

2 metre wavelength ? And 1 metre height ? That is shorter-than-short, and I don't know what sort of circumstances could produce it, I'd suggest 10 metres with a one metre height would be pretty testing for most boats, and qualify as a very nasty short chop. That 2 m stuff would be advancing at about 3 knots, almost a standing wave !

 

[asathor]

Perfect Chopper

I think you are asking for a Rafiki 37 or the even more rare True North 34. Stan Huntingford drew them based on the NW cost fishing boats that would work alongside larger processing boats or carry the catch in themselves, in any weather including nasty chops in the narrow straits. They are very stable in a chop but if you found yourself exceeding hullspeed you are probably riding a tidal wave, however they are said to run like a steamtrain with a good suit of sails as they carry enough canvas. I tried to buy a Rafiki 37, but the foam core had deteriorated too much and both deck and hull were wet, also the leaking iron oil tank was boxed in by the floor and galley. The Owner of the True North I looked at loved it too much for the current market, he was probably right. Some True North were sold as bare hull kit. I ended up buying a very different boat, a Tartan 37 c/b, it is much lighter but to my surprise the ride is amazingly smooth, with no hobby horsing, along with everything else that you hear about them being pretty much right on. I will be adding an inner forestay so I can de-power and go to windward in a blow. The headsail is very large on the T37 but it balances superbly stock so on inner storm jib would probably end up matching a double reefed main. I sailed the Tartan in the infamous Whitefish Bay on Lake superior in a nasty chop and 25mph wind. To sail into something like that you will need a pilothouse on anything under 50 feet to stay dry. In a smaller boat you would have to tack at 120-130 degrees true not to be too miserable.

 

[jim lee]

You want to beat to weather in chop? Take a look at a J/35 and copy whatever they did. If it gets uncomfortable, ride outside on deck. The bone jarring isn't so noticeable on deck.

-jim lee

 

[Tim B]

That would be a big rig and a fine entry then!

Tim B.

 

[BATAAN]

2 metre wavelength ? And 1 metre height ? That is shorter-than-short, and I don't know what sort of circumstances could produce it, I'd suggest 10 metres with a one metre height would be pretty testing for most boats, and qualify as a very nasty short chop. That 2 m stuff would be advancing at about 3 knots, almost a standing wave !

Sounds like the Fraser River entrance on a good ebb. We came across there one day and watched a 40 foot heavy ketch get totally out of control from the extreme pitching. He threw the wheel from one side to the other but couldn't get her to stop trying to round up so gave it up and fled to hide behind the breakwater till the tide turned. BERTIE, our modified SPRAY, continued on her way and tied up in Vancouver. Funny how old traditional types have a certain sea sense about them that sometimes get lost in yachts.

 

[mikel2me]

LOL!!! I grew up boating Western Lake Eire and it can get ugly in a minute...

 

[gilberj]

Those shallow places that are blessed with a good fetch are pretty particular. Western Lake Eire and Lake St Clair and Saginaw Bay in Lake Huron, mostly do not get deeper than about 25 feet except in the ship channel. When the wind gets up a little the waves begin interacting with the bottom and they become steep and short. I have seen similar conditions in a few localized spots here on the BC coast, but the water is deep enough the inshore waves do not generally reach the bottom. Interaction with tide is the big thing here and can be just as nasty.
I think any boat needs to have power to carry sail and probably a fine entry, relatively slender hull, or a good run...I have watched another Spray 'Joshua' and was impressed with the way she moved.

 

[bntii]

I sail on the Chesapeake & like my old Rhodes for the chop we can see at times.

She is narrow and moderately heavy:

Under sail she tears along nicely given a bit of breeze & has the weight to cut through the small short stuff.
Motoring is a different story as the hull can hobby horse to a stop if not fitted with enough balls in the diesel.

 

[sharpii2]

In descending importance, I would use these criteria for looking for such a boat:

1.) a long sharp bow, for obvious reasons,

2.) moderate to heavy displacement. A D/L of 250 to 300. Keep in mind a lot of so called light boats can approach this range when loaded up with cruising gear. As a general rule the heavier boat does better than the light boat when under canvassed. The opposite is true when the boat is over canvased. The heavier boat tends to plow through it and the light boat, (without a lot of rail meat) ends up slamming.

3.) a fractional sloop rig so it can sail decently with the main alone. A 3/4 rig is my favorite. And there are a lot of those around. When able to sail with the main alone, there is, at most, just one trip to the foredeck to strike the jib when it starts getting rough. I use either a roller furling system or a down haul for the jib, so I don't have to go up front at all.

I'm not putting 4.) in the list, because it is true with any boat you can get. And that is thorough knowledge of the boat. The more you know your boat, the more successful it will be. Even in circumstances it wasn't primarily designed for. I figure it takes at least two seasons to get to know a boat. And then you have to put it through its paces.

It is then that you find out if it can sail reasonably well with the jib struck and make upwind progress with the centerboard retracted. It is then you find out if has a problem getting stuck in stays (Can't come about), and if it does, what you can do to work around the problem. With a former boat of mine, I had to back the jib in order to swing the bow through the wind. When the jib was furled, I had to back the main. Knowing that, I never again got stuck in stays. I was able to make this light, beamy ceterboarder sail up a very narrow channel dead up wind in total confidence.

 

[BATAAN]

The seas of the English channel are well-known for their vicious chop and a certain type of vessel was developed to cope with it. Here is an excellent yacht, based on traditional designs, by Bevil Warington Smyth from 1924 that was fast, seaworthy and comfortable (for a 29' boat). Unlike modern racing equivalents like the Farr 30, MARIE MICHON can carry quite a bit of food and water and has an easy motion. The original made a cruise to Finland with a crew of 3 without engine and was well-known for her speed and seaworthiness.

 

[sharpii2]

One thing I developed over the years is my Heft factor (Hf)

It is supposed to give a hint on how well a boat will handle a chop.

It goes like this:

20*displacement volume/(Beam squared*Length)

Both Beam and Length are hull Length and Hull Beam.

The lower the Hf, the more sail area a boat can carry for it's given weight, ballast ratio, and ballast placement.

The higher the Hf, the less the boat can be bounced around by lumpy seas.

I consider a value of 0.75 to 1.00 to be a good compromise, 1.00 to 1.25, a little on the heavy side, 1.25 to 1.50 to be heavy, and 1.50 up to be very heavy.

A boat does not have to be heavy for its Length to have a high Hf. A 15ft long, 2 ft wide kayak with a 200lb paddler, and perhaps 20 lbs of gear will displace approx. 4 cf.

4*20=80. 80/(2*2*15)=1.33

Of course, hull shape has a lot to do with it too. A reasonably sharp bow and a moderate to high Hf make a good team.

A low Hf boat, like the Siren 17 I used to own is great down wind and even on a reach. When beating into short head seas, it was a different matter. The poor thing would slam to a stop on every third wave. It was well endowed with sail area and it needed every bit of it to make windward headway.

The Siren weighed about 850 lbs, including me the skipper. It was about 6.33 feet wide and about 17 ft long.

850/64=13.28. 13.28*20=266. 266/(6.33*6.33*17)=0.39. So it had a Hf of 0.39, which is pretty damn light.

I'm not knocking the boat. I'm just using it to illustrate my point.

I imagine most work boats in the age of sail had high Hf's.

This was so they could carry a reasonable load and be short enough to fit into small harbors.