Are we making any real design improvements??

Double-enders are usually good-looking and very well-balanced boats, but they don't win races. There are a number of reasons for this - they tend to bury the stern when under spinnaker and they don't surf.
The reverse-angle transoms are good for cruisers because of the added deckarea, but for racing that deckarea is only added weight where you don't want it!

Best regards,
Søren

 

As the LCB is moved back, it becomes more and more difficult to get the necessary volume behind it in a double-ender. It's an idea left over from boats of substantial displacement with the LCB in the middle of the boat.

In the US, the term "reverse transom" would mean the top edge forward of the bottom (i.e. less deck area).

 

Ahh - I thought it was the other way round! Then the answer to SuperPipers Question should be: Nothing happened to the reverse-angle transoms - they're still here, just a little more vertical.

Oddly, the angle of the transom has always been dictated by the rules. In the days of the IOR, the transoms were extremely flat - under IMS it's quite the opposite.

The old IOR-transoms were often very elegant, The IMS-transoms are very ugly. Have a look at Siemens Mobile, designed by Frers - probably the ugliest boat afloat!

Best regards,
Søren

 

Another favourite topic of mine: Tumblehome.

The new breed of Open ocean racers seem to have very rounded topsides. This feature was illegal in the last Volvo Ocean Race. If a VOR60 looks like a canoe, the Open60 looks like a kayak. If tumblehome was illegal on a crewed boat but not illegal on a solo boat then the rule was probably not a safety item. What are the advantages of tumblehome? Has the tumblehome philosophy changed in the past 2 decades? Are the slab-sided IMS boats more or less efficient because of the apparent lack of rounded topsides?

 

I've noticed that some of the slab-sided IMS boats are floating with the stem knuckle well above the water when upright. When they heel they "sink" to make the waterline longer. "Rule thinking" again, I believe.
I believe that is more or less the case with ACC boats too. I read somewhere that the waterline is increased about ½ meter this way.
I would also like to know if there are any advantages of tumblehome - in my eyes it is ugly and impractical.

Best regards,
Søren

 

Thank you for your kind words. Wing masts make sense on beamier boats where staying can be more simply accomplished. For a normal monohull, the added weight, windage and complexity does not seem to pay for itself. Obviously, reducing weight aloft and drag are good things performance wise as the allow a boat to carry more sail area longer and with a flatter incident angle.

Double enders offer some advantages in very slow speed vessels but as they approach hull speed there is a tendancy to squat in the stern which creates a lot of drag. The steeply reversed transoms were intended to cheat the IOR rule and was seen as lowering the center of gravity near the stern. The truncated transoms come from trying to move the center of bouyancy and gravity futher aft, and the open transoms lower the vertical center of gravity and provides a way to quickly drain the wide open cockpits popular on race boats (unless your race boat happens to be from New Zealand).

Regards,
Jeff

 

Iain Murray's challenge for the first AC using IACC class boats had a couple of Etchells, radically lightened and re-keeled and re-foiled, as trial horses. From watching them and talking to the guys who sailed them, I don't think they were consistently on a Melges' pace. The problem was that the reduction in waterline stuffed the shape. The same problem was found when IOr boats were converted to IMS.

Re fine bows. This is a trend that has been going on for many decades - but like so many, I think it's technology driven. Older spars and sails weighed so much more that the resulting pitching moment had to be fought with extra buoyancy up forward. Stick a few bricks at the hounds of a modern dinghy to compensate for the heavy old rigs, and you'll find the boat nosedives and pitches excessivley pretty quickly.

An example of how old the trend is can be seen by looking at Uffa Fox's 14s from the '30s - they show a fining of the bow and a broadening of the stern.

One area I can see as an advance is the shift to U shaped sections, from Vee shapes. The advantages are, of course, lower wetted surface and more dynamic lift. But why did it take so long for designers to go to "U"s and "planing flats" along the keel? In hard-chine skiffs, the use of sheet ply restricted the amount of U, but not in other classes. When many designers tried to increase volume (to reduce beam or float more weight), they simply increased the deadrise and stuffed up the planing qualities. It went on for years, until boats like March Hare came along. Even then Mike Jackson wrote how scared he was to go against conventional wisdom and go from Vee to U.

Yachts stayed Vee for even longer, until Carter etc started flattening hulls and then (IIRC) Peterson and Farr brought in very U-shaped bows - more effective, but ironically Farr at least did it to increase the rated displacement 'cause the "shoulders" of his U were on the fwd depth points. Ironic that IOr cops abuse yet it was the reason that Farr moved to a faster shape!

Some dinghy designers see more immersion of the stem as an advance. Maybe topsides flare should also be seen as an advance, although sometimes (IMS and IACC) it's restricted by peculiarities in the rules or boats.

IOR sterns leaned forward, btw, because the aft girth measurement was taken from the forward meeting of sheerline and transom. Moving the transom forward shortened the rated length, which was taken from one girth to another. However, moving trhe transom forward also put the measurement on a fatter part of the boat, increasing the rated width aft and therefore pushing the rating back up. So placing the corner of the transom was a game depending on the exact shape of the hull underneath IIRC.

Have you guys seen Francis Herreshoff's designs for canters and inclined solid wingsail cats from the 1940s (IIRC)? Scary. Jim Young (famous NZ designer, who emplpyed a kid called Bruce) built one of Herreshoff's canters. I wonder where it is?

 

When Olin Stephens' Columbia beat Septre, there was a huge fuss in the press about how Columbia's v-eed sections got through the waves better than Septre's U sections, complete with aerial photos showing differences in the splash. The theme of Stephens' genius with bow sections ran through several AC challenges. So, I suppose it took the lesser mortals a while to discover that different, i.e. much lighter, boats could use a different shape.

 

Fiery Cross (built in 1953ish) is still in NZ. Was banned from racing unless the keel was fixed. Jim Young used to cant the keel mailnly for reaching and leave it in the centre for the upwind stuff as it had no other foils apart from the rudder. It's a 45ft by 7ft double ender.

 

Improvements

In ten years of sailing and racing I have become somewhat confused!
I'm on this guy's catalina 22 and look at my GPS
and it reads 5.5 knots he said "this boat won't go that fast" (water line).OK my GPS is lying.
Im trimming another guy's catalina 22 on the inland waterway, 18 to 20 knots of wind, I hate
these boats but I got to admit, it is boring a hole thru the water, we catch up to and pass a benataue
36 and burn it up.
What I am getting at is, that even my stinky McGregor 25 has gone 12 K for extended periods W/ spinnaker, and commonly goes faster than hull speed even up wind.
Apparently there is some force at work that is not quite planing but close.
Thanx again!

 

That either be your engine or what is loosely termed "semi-planing", which basically translates to "can't quite get itself up and over the hump, but has enough power to try"

Steve

 

semi planing

I'll call you steve now!
Perhaps I just don't know the actual water line length to do the calculation, therefore I and others just think we are going faster than the theroetical max when actually our WL's are longer than we think or the published data is wrong.
Is there an accepted procedure for measuring a boat to find this ever so elusive number or is it a closely guarded secret of designers much like magicians who don't give away their secrets either.
Thanx kindly,
STEW

 

First of all hullspeed is typically quoted as 1.34 times the square root of the waterline length and that formula is derived from the height and speed of waves. That formula described the theortical speed at which the bow wave moved aft combining with the quarter wave and altering longitudinal trim there by creating an inclined plane that requires a large increase in energy to climb. Planning is defined as the point at which the boat has climbed up on its own bow and stern wave.

There are a number of ways that a boat can exceed hull speed without actually planning. The most common in displacement monohulls is surfing where the slope of the wave face creates a downhill inclined plane and gravity imparts the necessary energy to exceed hull speed.

There is also a state called semi-planning in which a boat can exceed its theoretical hull speed by producing a smaller bow wave and by moving the center of buoyancy aft so that the stern does not squat. Semi-planning hulls are usually very narrow or will often have very fine bows to trick the water into thinking that the boat is very narrow. Few, if any, multihulls actually plane. They get their high speeds from thier narrow beam and minimal wave production. Boats like the Transpac sleds get their high speeds the same way. Modern IMS typeform racing sailboats pretty frequently exceed thier theoretical hull speeds by reducing the size of the bow wave with a very fine bow and a center of buoyancy that is located pretty far aft reducing the amount of squatting at speed.

With regards to your anecdotal evidence, a Catalina 22 has a waterline that is roughly 19.33' and so would have a theoretical hull speed of 5.89 knots. A Catalina 22 would have a bear of a time hitting that speed very often because of its hull form, minimal stability, and inefficient sail plan and foils. Often when a GPS shows a boat like a Catalina 22 or Macgregor 25 seeming to be moving upwind at speeds approaching or exceding hull speed, there is a favorable current adding to the speed over the ground. For a Magregor 25 to sustain 12 knots it would need to be in big enough waves to be surfing and to use the momentary energy of the wave to accellerate it up onto a plane. It is generally cited that a boat designed to plane with any regularity needs to have roughly 400 square feet of sail area per 1000 lbs of displacement. A Magregor is no where near that.

Respectfully,
Jeff

 

Bringing old designs up to date

Bringing old designs up to date

 

bringing old designs up to date

I agree the D/L ratio is always a good start to any design,and depending on witch ratio is used ,it will set the requirements as to both speed ,power requirements and performance.
I am currently working on building a Redwing 23',which is based on an old design of Howard Chappelle,brought up to date by Karl Stanbough.