hollow entry?

[rickc]

Most hard chine Vee bottom hulls I look at seem to have a reverse curve at the bow where they meet the waterline. Doesn't this create drag since the water is forced abruptly to the side rather than flow in a continous arc around the hull?

[gonzo]

Are these planing or displacement hulls?

[Willallison]

As you suggest Rick, the enemy of planing hulls (well any hull actually, but I'll assume you're talking about planing hulls) is drag. And whilst I don't quite get the gist of your question, the best way to stop drag is the ensure that the water parts company with the hull as soon as possible. That's one of the main functions of chines - they push the water out sideways, away from the hull, so that it can't curl up the hull sides.

[rickc]

In particular I'm thinking of displacement hulls. I was aware that the sharp chines helped to separate the hull from the water on a planing hull but I didn't know how the hollow entry plays into this,The hollow seems to act like a plow.

Is this plow effect an advantage to a displacement hull? or is it there just because this is what results when you twist a plywood bottom to meet the stem ?

What brings this up is I am considering a new boat to build. While looking at several hard chined designs I noticed that most had a hollow in the entry. I remembered that Chapelle said in one of his books that the chines should be either all the way in the water or all the way out. and something to the effect that water doesn't like to be surprised as it flows around a hull.
Last week while I was helping a friend haul his fiberglass sailboat(McGregor I think) that has spent much more time being pushed around by a 6hp outboard than by wind . it occured to me that this heavy boat has gone thousands of miles on very small engine. I studied the bottom of his boat to me it is shaped very much like a modified sharpie except with large radiuses in the chines (and a big centerboard). So with that idea in mind I sketched out a few ideas using a modified sharpie bottom. The flow looks good along and under the hull and I like the fact I wouldn't have to torture any plywood to build it I am just wondering what I am overlooking.

Rick






http://home.nycap.rr.com/richboat/BUBBE1.jpg

[yipster]

just an obvious thought since most hard chine Vee bottom hulls seem to have them: maybe guest is refering to the speedrails? they run from bow down al the way to the stern, form a bit of tunnel and keep the spray down.

[yipster]

gee, did i take 3 minutes to write above? you beat me on my posting guest and speedrails only apply to planning hulls ofcourse.. now i dont know..

[Doug Carlson]

Rickc,

Ted Brewer in his book, Understanding Boat Design, has a section in which he talks about flam (convexity) and flair (concavity) in the hull above the waterline. If I understand correctly, its a function of material and construction method but is also a design choice.

The functional effects of flam and flair in the bows seem to be:

flam- more reserve bouyancy
less effective spray control requiring spray rails or the like

flair- finer entry
no secondary spray delection needed if correctly designed

Its my impression that most if not all of Brewer's boats are designed with flam while most if not all of Phillip Bolger's boats have flair, so your not the first person to ponder this question.

Doug Carlson

PS: I think parts of Brewers book are available at his website.

[gonzo]

Many hard chined plywood boats have awful water entries. It is usually the consequence of trying to modify a planked hard chine design. A good example of what can be done with plywood is the line of kayaks by Cheasapeake Light Craft.

[Willallison]

I'm not sure that we've yet got exactly what you're talking about Rick - or at least I'm not. But it occurs to me that you may be talking about the shape of the 'waterlines' as opposed to the shape when seen in section....?

[rickc]

My apologies for not better explaining the question, Lets try a couple of drawings

The drawing below depicts what I always thought was meant by "hollow entry", the water represented by the blue lines instead of fllowing around the convex shape of the boat as you might expect actually goes into a "hollow" created by the twist in the plywood bottom necessary to meet the stem The flow becomes "S" shaped
http://home.nycap.rr.com/richboat/chine1.jpg

In the second picture the chine and keel converge at the stem at or below the waterline ( this could be a flat bottom or a modified sharpie) The water flows around the hull in a continous arc as show below:

http://home.nycap.rr.com/richboat/chine22.jpg

My question is: Which is more desirable in a displacement hull?

[Guest]

Rick, in your sketch, there is no hollow in the lines of the boat which is just a wedge moving to the left. The water is experiencing an acceleration normal to (at right angles) to the surface of the hull and is thus pushed out and then flows back in toward the stern. Of course your sketch is only two dimensional and does not look at the vertical acceleration of the water resulting from the displacement of the boat.

It has been established long ago by imperical testing of otherwise identical hulls that a round hulled boat is more efficient at displacement speed than a hard chined hull. Any sharp edges requiring the water to change direction rapidly will increase drag. In many cases, the difference is not enough to offset the extra complexity of building the round hull.

Tom Lathrop

[Guest]

Rick, The first two sketches did not show up when I first looked at your post so my answer is skewed. The hollow you show will increase drag based on the same principle mentioned for a chined hull. That is, it requires the water to change direction (momentum) more rapidly than the non-hollowed shape. One falacy though is that plywood will not naturally take a hollow shape even though from some perspective, it appears to. If you look at the actual waterlines of a plywood bottom like a modified sharpie, there will be some convexity to the lines since plywood can bend in only one direction in a single plane.

Tom Lathrop

[gonzo]

A hollow forward may make a boat drier. Also, you may design a fine entry with reserve flotation. It is not directly related to your question, but something to think about.

[yipster]

its gettin a interesting conversation! i think the question is basicly simpler meant, the first drawing -with deeper V- has less sharp corners and thus WSA, when aproaching hullspeed it also gives some upward lift. a sharpie as i recall has a flat bottom to make construction easyer. rite? just my 2 cents :-)

[shu]

I think by looking at this from the standpoint of a water molocule being approached by the bow of a boat, we can see that the finer the angle of entry, the less that water molocule will be accelerated laterally away from the centerline. A small amount of hollow will decrease the angle of entry, thereby gradually accelerating the water molucule. But the more hollow you have, the greater the secondary angle of attack (for lack of a better term) of the hull sides; all other aspects of hull geometry being the same.

The less you accelerate the water molocules, the less bow wave you will generate, resulting in less drag, but perhaps more importantly, in the hull-speed regime you will not have as big of a "hill" to climb. So, for hull-speed and slightly higher, I think a small amount of hollow can be advantageous, given moderate beam. Really narrow hulls (multi's) seem to be a different matter.

[gonzo]

Chines at displacement speed create more drag. That is because a fluid flowing along a curve has a maximum speed before it separates and creates eddies. The smaller the radius the slower the speed the separation starts. On the other hand, a sharp chine at the water exit can help. Well designed, a sharp edge aft will give a clean water exit above a certain speed.