Hollow water lines?

Maynard Bray has stated that he is a" relentless Proponent of hollow water lines" Why would this be so important? What might be the advantages/disadvantage? looking forward to your comments

 

Mostly, they make the bow look really, really sweet. Under a counter stern, they add a lot to the visual impact. They may also (I really am ignorant here) have good seakeeping qualities.

 

Generally, hollow waterlines happen naturally at the bow when you combine a fairly plumb stem with flared forward sections.

To say that forward hollow waterlines make a boat fast would be wrong as it depends on so many variables as to be way beyond the scope of a post here. However, it is certainly a feature that has been shared by a great number of successful and fast sailboats.

 

Is some one using tank testing and then talk about?

 

Hello

Fish move in the water, boats on the water. There is a difference.

Hollow waterlines seems to shorten the effective length of waterline.

 

That is true but only in extreme cases.

Looked at another way hollow waterlines are the forefoot area stretched down and forward giving more length for a given 'rest of the boat' shape.

 

If (like Tcubed said) a boat with a spoon bow and all convex waterlines forward, has the bow extended to form a more plumb shape, hollow waterlines will be the result. Is this faster or slower?

The length will increase and therefore the Hull speed will increase. The wetted surface will increase and the speed will decrease. The rate of lateral acceleration of water in the bow wave will change but that is a bit complicated and its effects are variable.

In a convex waterline, the lateral acceleration is highest at the bow and decreases thereafter. In a hollow waterline, the acceleration is least at the bow, may increase in the hollow and decreases thereafter as the waterlines, of necessity, become convex. Whether the acceleration increases or not in the hollow depends on the curvature there. If you make a plumb bow and convex waterlines on the same boat, it becomes a different boat with way more forward buoyancy and a comparison to hollow waterlines makes little sense.

The effects of all these components are then affected by the sea state in some unknown way.

I have no real clue what the final result will be. Mr Bray may know but I think it is just his guess and will not be true for all situations. My latest build of a 15' 8" lapstrake sailboat has some hollow in the waterlines forward.

 

Actually the displacement curve (or curve of areas) is much more relevant than just waterline shape to determine fluid accelerations due to the passage of the boat.

It works out that with a fairly plumb stem you usually need to put in hollow to get a nice curve of areas. It is not too surprising therefore that when this is worked out well the hollow looks 'right'.

Myself, i am not a big fan of overhangs, particularly excessive overhangs- If you have a boat x long, might as well make the whole thing x long instead of just the deck and get rid of pounding, better handling (there are other factors involved as well of course) and speed. I can't think of any work boats with huge overhangs, for example.

 

All true Tcubed. I assumed we were talking about waterlines plural which also defines the curve of areas, and thus volume distribution, at all points. Of course, what the eye sees can be quite different from what the water sees.

 

Other notable designers have advocated hollow bow waterlines. Among them are/were the Hereshoffs and Colin Archer too. Archers waterlines were the result of, not the primary intent of, the well known curve of areas that he advocated.

A sailing canoe is a different breed of cat from the boats that Bray may have in mind. A sailing canoe design must surely give some attention to forward bouyancy. Skinny boat and pitching moment generated by the sail suggest that fuller bow sections are appropriate. Deliberately designed hollow bows, then, are not what the doctor ordered. A paddle propelled canoe is a different deal in that the bows can have hollows without serious penalty unless the boat is to be used in rough water.

 

At certain S/L ratios, a hollow entry is beneficial. It can suck down the leeward surge and these WL's aren't necessarily a requirement of upright stems. In pure displacement hulls, where the S/L will be limited to 1.4 or less, most often considerably less, then hollow WL's are helpful. It also helps move displacement toward midship, which is also beneficial. At these lower S/L speeds this feature can keep flow attached longer.

If you incorporate the same concave WL's into a hull capable of higher S/L figures, then it will detract from performance, possibly limiting it a fair amount. In these hulls you want seperation and let her power, drive through it so the flow can release cleanly, off a crisp transom or chine edge.

This of course is a very generalized view as there are many other elements to consider, such as entry half angle, how quickly the displacement comes on in the forward (and aft) portions of the hull, forefoot shape and depth, design brief requirements, etc.

In craft limited to S/L's governed by WL length only, then a fine, hollow entry and exit, within the limitations of displacement, will part the on coming rush and leave the flow behind with minimal fuss and disturbance, which is the whole point.

 

I have a 10 ft rotomolded kayak with rather exagerated hollows at bow and stern. It is OK for just lazing around, but when paddled hard it throws up white water well back from the stem. Trying to go faster is like trying to push a hole through a brick wall.

My 13 ft kayak is supposed to come from a racing background, also has hollows at bow only. At max effort it is the fastest of my three boats.

The easiest boat by far for cruising is my 12 ft home made wood canoe which is a pack boat or lost pond style. It has the finest entry and exit of the three boats, and long, gentle waterlines at bow and stern, just barely hollow.

Strangely, the shortest (and fattest) boat has the longest glide, just seems to go on forever. Small boat behavior passeth all understanding.

 

Very interesting - thanks everyone. My personal interest was rowing hulls. Modern open water sculls look to be essentially wave piercing and certainly don't have any hollow in the water lines. On the question of overhangs and flare - These would seam to provide reserve buoyancy with out increasing surface area/drag. And How come in this age of omnipotent computers we don't Know how much surface area a person can drag around?

 

"Small boat behavior passeth all understanding." Truly a gem of keen observation, AK. From this distance, I would expect that the best glider has the finest finish on the wetted surface. However, I have seen but few rotomolded boats with a really slick finish. One of the exceptions was an Old Town 13 footer that I had one time. It was slick and did glide pleasingly. That is, it did until it accumulated a rash of battle scars. Oyster shell and other hazards along with a paddler who was easily distracted, or maybe just plain dumb, did it in. After collecting some scars it did not glide as well and the wake pattern became slightly different with evidence of eddy making.

Another thread somewhere in the forum, examined the concept of smooth surfaces. Some one with a 420 sailboat as I recall. Marchaj did some serious work in regards to surface quality. Smoother is better. Smoother has some qualifiers. An attractive shiny surface is part of the recipe but there is also the absence of, or at least minimization of undulations to be dealt with.

I built a 16 foot narrow chesapeake style sharpie that Chapelle might well have prescribed. It's flat bottom and elevated forefoot sure enough does not have the slightest suggestion of hollow waterlines. Being a glutton for punishment and maybe a bit of a show off, I put a concours like finish where it counts. It is very lightweight at 310 pounds including me, oars, and all the gear. It will easily out glide one of those lovely Tern kayaks. The subject Tern is a 14 footer and well finished. It has a moderate amount of hollow in the fwd WLs. Go figure! Weight and inertia could have something to do with the difference perhaps as in F=Ma). The gliding performance is always at fairly low speeds of course. SO I am inclined to think that sophisticated design influence is less a factor at very low velocities.

 

Hollow waterlines in a sailing hull, in the form of a clipper bow ,in a boat under 55 feet, tend to let the bow drop into the trough of a wave before building up any real buoyancy. This stops the boat dead when it runs into a headsea. One can clearly see this while riding a BC ferry from Vancouver to the Island on a long weekend summer day. The clipper bowed boats tend to plunge deeply and stop dead when they hit the ferry wake. Spoon bowed boats barely slow down. My first boat had deep V, hollowed waterline foreward. In a head sea , it often dropped off a wave and slamed her foreward topside into the front of the next one, with a teeth jarring slam, when well heeled, going to windward. My current boat has a slight outward curve to her bow lines, and that has led to a huge improvement in windward peformance and comfort in a head sea.
Brent