sailboat bows

The way I see it, since you're paying rent for the whole length, you should optimize your rent/speed ratio.

Yoke.

 

Thanks for the chuckle, Yokebutt.

 

Rent/speed ratio

Good one, Yoke!

 

Agree. Hateka.

 

If you'll look at more traditional designs, I think you'll find that a plumb stem was standard fare. Before the days of lightweight powerful engines, lightweight hulls and 6-1 A/R's, designers had to maximize hulls to get the maximum performance that was available to an LOA.

If I were designing/building a 60'+ yacht, I could probably also hire a hydrodynamnasist(sic) to to check the lines and create the slipperyist hull possible. But, as it is, I have no hydrodynamic expert, so I'll have to rely on my own judgement in regard to hull shape AND put as much waterline on the sun-of-a-gun as LOA, form and esthetics will allow.

Losing 5' of WL on a 60'er equates to roughly a 5% loss in hull speed where the same loss ao a 25'er is about a 10% loss of hull speed. Even the long overhangs of Herreshoff designs were not put there for reserve bouyancy, but were a means of beating the ratings rules. When the hulls were heeled, they would "sink" and thus, present more waterline. So, in this case, the overhangs, while adding to reserve bouyancy, are intended to create more waterline and the hull shape presented to the water depends more on section shape than on overhangs for it's reserve bouyance in many of it's missions.

So then a question arises. Two boats of equal LOA. One, for the sake of arguement, has a waterline equal to the LOA and the other has a waterline at 75% LOA. Can we draw any conclusions in regard to performance, comfort, seakeeping, accomodation, etc. from just this bit of information? Probably not.

 

When I wrote bowsprit and a gaff rig a few days ago, it was not only a joke. I just got a poster of the Volvo-Open 70 competitor ABN-AMRO-2, which has both. I assume the bowsprit to reduce weight at the forward end, and the 'gaff' to reduce mastheight. Hateka.

 

This is the QUESTION that matters, at least for me, or from a cruising point of view. (Of course there are always the rent problems referred by Yokebutt )

Please give some answers!

 

Ok, so anyone have any comment on 5' of freeboard at the bow of a 33' sailboat. The rig would be lug yawl with the main stepped similarly to a cat boat in the fore and aft position..

 

No easy answers Vega. Too many variables. I would venture to say that long overhangs will also be associated with a leaner hull form. Meaning less interior volume (accomodation) for similar LOAs.

From a cruising standpoint, I would think think interior volume jumps to the top of the list as long as it doesn't detract from an efficient hullform.

An easily driven hullform, one that reaches hull speed easily, is important also. A lean entry with a gentle exit are going to be easier create with a longer lwl.

How do you plan to generate sail carrying power? Lots of ballast down low or a combination of ballast and hull form? Hullform stability will call for greater beam and firmer bilges than low slung ballast. A wider beam will dictate a longer waterline to avoid a bluff hull.

But, a hull that derives sail carrying power from form and not ballast is going to have a lower lower ballast ratio. This equates to a hull that is subject to quick accelerations, corky.

And so on, and so on....

 

Hi

I haven't read all the posts here but would like to comment on some I have.
If for displacement hulls, we assume a simple bow shape so we are discounting things like bulbs, then we consider only simple vertical, raked or rounded shapes.

I think it is a mistake to consider a shallow raked bow as pushing the water under it. Water has a lot of mass, 1000 kg per cubic metre, more if it's salty.
So in order for the hull to push water down or sideways, that water has to push the water next to it and so on. Water is incompressible so this means a very large mass would have to be moved, many thousands of times the hulls weight. So that can't be what happens. The only place the displaced water can most easily go is up to the surface where it becomes part of the bow wave. So the water is moveing not along the hull but up from the keel to the surface at right angles to the boats movement. At least I believe that is the case wherever the hulls X section is increasing along it's length. Now we could design the hull so the lengthwise displacement distribution displaces a single outward sin wave between the bow and quater waves at max displacement speed. This gives a very long shallow (min height) displacement wave. Considering this sin wave form, the steepest part of the wave represents the maximum rate of displacement at the bow entry and the crest shows no increase or decrease in displacement which, depending on keel shape is at or near the beem. We can see from this that the water flow at the bow is the shortest distance from keel to surface at 90 degrees to the direction travelled. The least acceleration of this displaced water is the shortest distance from keel to water line, a straight line across the hull.

At the beem, there is no displacement change so the flow is along the hull there. At the stern, folw is across the hull from WL the keel in the trough of the displacement wave. The quater wave crest will then be around a 1/2 wave aft of the stern.

Now I must point out that such a hull shape while dynamically good offers a lot of surface for its displacement so a compromise between skin and dynamic drag is much more the norm but this may serve to point out the water flow pattern is much the same regardless of the bow elevation shape.

I must also say there is another flow dynamic that becomes stronger below the surface and that is where the mass of water above a moveing object is high, the water can only flow around the object by speeding up, much like through a constriction in a hose. The deeper we go, the stronger this flow pattern becomes.

Consideration of bow elevation must then ballance max displacement speed for length restriction (vertical bow) against reduced surface area for minimum skin friction at lower speeds. Then there are sea keeping and manouverability issues to consider as well. It's all compromise.

OK so here I admit, this is all what I have personally figured out, what do I know. Some you may well like to correct me, please feel free to do so.

Regards,
Ken

 

You figured it. It is all compromise. Hateka

 

Bow Aesthetics

I've not seen much discussion of the aesthetics of the situation so here I offer some.

PAR, I agree with you. The two most distinctive lines on a boat are the sheer line and the bow line/profile. A year or so ago I saw an article in Soundings magazine by Ted Danforth speaking of Ray Hunt designs. I wrote him in response to his article, “And most importantly, you included the oft forgotten element, the sheer line. What a terribly important factor! Romantically stated, but oh so true, it is simply her sheer… sheer beauty that is. She enters the harbor like a beautiful woman entering a room. Her sheer is the line we try to get right when we doodle boats.”

I consider both the sheer and the bow line/profiles as the two most important lines on a vessel. And I am convinced that a computer cannot match the human, hand-drawn line for these two. The computer renderings can be useful for viewing the design from many various perspectives, but don’ t ask it to draw the original line.

When I first became interested in sailboat design there were still quit a number of wooden boats, and many of these had long graceful overhangs that contributed to longer waterlines upon heeling. Not only were they practical, but they were beautiful. From my letter to Danforth , “Your article brought some of that feeling back to me…some of that feeling that first inspired me to want to learn of sailing yachts and their design ….that had me (virtually a non-sailor at the time) putting together a scrapbook of designs, both good and poor, for future reference. You captured not only the factual history, but more importantly, some of the essence of being involved with yachts and yacht design itself….. ‘the measure of total understanding of the nature of a boat’.”All thru the late 60’s and 70’s I experienced the dawn of the fiberglass boat, and the need for the manufacturers to cram ever-more accommodations into a certain length vessel. I saw the disappearance of overhangs. I saw the boats become more and more like just so like many “floating Clorox bottles”, I termed it. No distinctive bow shapes and straight, non-descript sheers.

Now it so happens that I got quite heavily involved with multihull craft because here is where free thought and innovation abounded, and here it is even tougher to create a pretty sheer line. Look at the number of vessels on the market that look like their sheer lines were drawn with a straight-edge ruler. And this current fad of plum bows just doesn’t do anything but add to the already boxy look of multihulls. One of our major problems in the early days of selling multihulls was not only overcoming the ‘capsize issue’, but also the boxy looks of our craft. Lots of folks aren’t interested in being associated with a ‘floating box’. Boats need to be practical, but they also need style.

Now I know I will take some heat for this next remark, but here it is anyway. I was once looking for some aero kits for a Mercedes auto to jazz it up a little. I was so disappointed in what I found in Germany designed by the Germans. My thought was they can let the Germans build it, but let the Italians design it. The Germans are all practicality, but the Italians add some flair. What’s sexier looking a Ferrari or a Mercedes?

I look thru the production multihull market right now and see some practicality, some innovation, some experimentation, but I don’t see very many pretty designs. I’m going to attach a number of illustrations to this posting, both good and poor (in my opinion). Feel free add your own illustrations and/or comments to those I presented. Oh, and I’ve just included sailboats here. The power cats are a further disaster, most are out and out ugly.

I will also address plum bows in a separate posting, as well as bulbous bows.

How about this Lock Crowther design “Investigator II”. She is a beautiful proportioned 55’ vessel with a handsome and proud bow. Or Peter Wormwood’s “Indigo” 60’ design, unique in how it disguises its freeboard into a much slicker sheer line. How about this variation on a plum/bulbous bow by Lock on the vessel “Wahoo”, pretty good. I don’t think Lock ever drew his lines with a computer. His son Brett does, and I’m not impressed with his designs aesthetically. Nor am I impressed with Tennant’s or Hughe’s, too many ruler-straight lines. I’m not even happy with mine. My 65’ is OK, but that 42’ looks like a stacked wedding cake because I tried to cram too much into its length, and add a plum bow. (Some profile illustrations of mine can be found on this page)

Like I said, the multihull vessel form is even tougher to make look good.

 

...couple of more bow examples...

Which of these examples would you choose??

 

I'm glad to find I am not alone in finding modern CAD designs uninspiring.
You guys need to retrace back to the beauty of the J's and Fifes.

 

Now, HM, if your definition of beauty finished with the J boats, you have been missing out.
That's like saying that the last beautiful woman was Mae West.
Steve