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tshino 09-19-2007 06:35 AM

plumb
 
I see that most new go fast designs have plumb stems or bows.
What are the advantages for speed from a plumb stem and what might be the drawbacks? And for cruising?
thomas

alan white 09-19-2007 09:57 AM

The chief advantage of a plumb stem (or stern) is an increase in waterline length, which translates to a higher displacement speed potential.
This matters more in heavy wind areas than in light wind areas.
The reason is that with very little power, skin friction slows the hull far more than frontal area. The opposite is true at higher speeds.
Genoa jibs and various other large sails set far easier today than in the past due to light weight and roller-furling and reefing. The ability to easily pile on sail in light air allows a longer waterline without the penalty of light air inefficiency.
For what it's worth, a longer waterline allows more displacement per foot of overall length--- useful in saving dock fees charged (as they are), by the foot.
Much has been written about the ultimate sea-keeping qualities of short (or no) vs. long overhanging bows. The St. Pierre dory is legendary for its sea-keeping manners in extreme conditions and it has high and long overhangs. For an open boat, long and high ends always contribute to dryness and therefore safety. Such designs also are more resistent to plunging, tending to gain bouyancy the deeper they are driven.
Enclosed boats will also tend to plunge more with plumb bows, but without the same adverse effects in terms of safety except in more extreme conditions that would trip or broach the hull. However, in such circumstances, many other design factors also come into play, like beam, deadrise, displacement, rig, etc.. Many of the most famous sea boats of all time had plumb stems (Bristol Channel boats, e.g.). Lighter boats known for seaworthiness generally had longer overhangs.
So many factors contribute to the net effect of a design, one can't really analyze any one feature without considering all of the others.
Much depends on local conditions, too. Areas with mild winds can still have rough seas and sometimes the reverse is true. All of these things tend to contribute to boat design.

Alan

jehardiman 09-19-2007 11:57 AM

Quote:

Originally Posted by alan white (Post 161322)
The chief advantage of a plumb stem (or stern) is an increase in waterline length, which translates to a higher displacement speed potential.

Alan

While longer waterline length generally translates into more speed, the correct answer for the reason for plumb bows is the rules.

Most rules use LOA as a factor. Having a plumb bow maximizes LWL for a given LOA. In general, and for many reasons given by Alan White above, a different type of bow would be better. Plumb bows are favored by the rules the boats are built to, not any inherent weight or hydrodynamic advantage.

Edit to add: And berthing fees and regulatory laws are also part of "the rules".

alan white 09-19-2007 01:27 PM

It's difficult to say that what is considered by every marine architect to be a means to go faster is really caused by "the rules".
Perhaps the rules favor faster hulls on purpose, and not entirely by accident.
In any case, the question was about what is the speed advantage of a plumb stem, not why so many are seen.
Respectfully, if you reread the question, you will see that "the rules" is not the correct answer.

Alan

SamSam 09-19-2007 02:44 PM

Quote:

Originally Posted by tshino (Post 161296)
I see that most new go fast designs have plumb stems or bows.
What are the advantages for speed from a plumb stem and what might be the drawbacks? And for cruising?
thomas

Normally a go-fast boat is a powered, cigarette type boat. They don't have plumb bows. You must be talking about fast sailing craft.?.?

jehardiman 09-19-2007 04:14 PM

Quote:

Originally Posted by alan white (Post 161339)
It's difficult to say that what is considered by every marine architect to be a means to go faster is really caused by "the rules".
Alan

(Italics my emphasis)

A plumb bow does not make a faster boat in all situations...Period... Indeed, sailing waterline length increases with overhang and therefore would tend to drive to long overhangs that were seen up until the recent (70's and 80's) changes in some measurement rules (which also drove open reverse sterns BTW) and the introduction of new classes based on LOA only.

What makes a faster sailing boat is higher SA/D and SA/S,lower D/LWL, high aspect rigs and large righting moments. None of these is driven by the selection of a plumb bow.

There are other reasons for having a older or modern plumb bow. Working channel cutters had plumb bows because of tonnage laws, not because of speed or seakeeping requirements (indeed, there were speedy in spite of their plumb bows and horrible D/LWL, but never could contend with racing yachts of the period). Modern plumb bows reduce weight and pitching moments, two things that are required by light vessels with high aspect rigs and keels, but the bow doesn't make the boat fast, the tall rig and righting moment do.

Bows are selected to fit into the design requirements. There are some reasons a plumb bow is a good fit, and other reasons it is not. Unless I was length constrained for whatever cause, or was going for a marketing look, there are really no compelling reasons to give a sailing vessel a plumb bow.

Mychael 09-19-2007 05:44 PM

So in simple terms with all other things being equal, how does the ride of a boat with plumb bows compare to a simliar size/shaped boat but with long bow?

Mychael

alan white 09-19-2007 05:45 PM

Quote:

Originally Posted by jehardiman (Post 161366)
(Italics my emphasis)

A plumb bow does not make a faster boat in all situations...Period... Indeed, sailing waterline length increases with overhang and therefore would tend to drive to long overhangs that were seen up until the recent (70's and 80's) changes in some measurement rules (which also drove open reverse sterns BTW) and the introduction of new classes based on LOA only.

What makes a faster sailing boat is higher SA/D and SA/S,lower D/LWL, high aspect rigs and large righting moments. None of these is driven by the selection of a plumb bow.

There are other reasons for having a older or modern plumb bow. Working channel cutters had plumb bows because of tonnage laws, not because of speed or seakeeping requirements (indeed, there were speedy in spite of their plumb bows and horrible D/LWL, but never could contend with racing yachts of the period). Modern plumb bows reduce weight and pitching moments, two things that are required by light vessels with high aspect rigs and keels, but the bow doesn't make the boat fast, the tall rig and righting moment do.

Bows are selected to fit into the design requirements. There are some reasons a plumb bow is a good fit, and other reasons it is not. Unless I was length constrained for whatever cause, or was going for a marketing look, there are really no compelling reasons to give a sailing vessel a plumb bow.

A plumb bow doesn't increase speed, no. Hull speed in displacement boats is governed by waterline length, however, and if the overall length (for whatever reason) has been established, any efforts to get maximum speed potential will involve lengthening the waterline, which results eventually in a plumb stem.
As I said, "So many factors contribute to the net effect of a design, one can't really analyze any one feature without considering all of the others."
We are in agreement then, because my meaning was that many other factors would effect whether a plumb stem was a good or a bad idea.
Perhaps I shouldn't have used the Bristol Channel Cutter type to illustrate the viability of a plumb stem. I was only trying to show that perfectly seaworthy types have had plumb stems.

tshino 09-19-2007 10:34 PM

In my characterization of "go fast" boats I had in mind the open60's, mini 650,
570 etc. I didn't intend to conjure up power boats in a sail forum. I am only beginning to grapple with the complexities of boat design so isolating various design elements for analysis is natural, if ultimately a little futile...one must begin by breaking apart the whole...and as Alan points out, always with the awareness that all the parts influence each other. Having looked exclusively and perhaps not very thoroughly at only traditional and neo traditional designs until very recently, I am also intrigued by these new racing designs' use of chines, exceptional beam, and the horizontal cut at the top of the sail. Would anyone care to recommend design texts they feel are particularly useful?

thomas

alan white 09-19-2007 11:28 PM

I must make this same recommendation once a week. Dave Gerr's "The Nature of Boats", a very down to Earth book about all those interrelated factors and how they effect each other.
Offshore boats have come a long way in recent years. Much of the changes are technological materials advances, answers to questions like, "How do we make a racing sail that doesn't need to have a mast six foot higher than the last bit of sail that actually creates more drive than drag?"
Chines are a new one on me----- I haven't been reading magazines of late, I'm afraid. Of course they assist tracking and good flow lines, which Phil Bolger's been saying for years, but someone who knows more than me would be the better one to answer that one.

A.

Paul Scott 09-19-2007 11:50 PM

Quote:

Originally Posted by Mychael (Post 161370)
So in simple terms with all other things being equal, how does the ride of a boat with plumb bows compare to a simliar size/shaped boat but with long bow?

Mychael

Mycheal, I think it depends on the stern, weight, and flare of the hull, but our boat Amati (you can look it up on Schooner Creek's website) has an almost plumb bow (within a couple of degrees), and is 40' by 10.5'. We sailed from Vancouver BC to Schooner cove recently in 25-30K straight downwind in breaking 8'-10'ish seas (backside), and it was fine, even sailed by the lee about half of the time. Never went down the mine and stopped, just steered around & sliced through. She's pretty light (9600 lbs), and a Bob Perry design. It has to be about balance. I've sailed plumb bows that were plain dangerous (like an Int 110 in & 7' chop, or a Sol 18 in 4' chop), and others like ours, and our U20 (Jim Antrim) which are great in the big stuff. Actually as good as a 30 sq m I sailed quite a bit in big stuff. Which was really good, but WET. Amati and the U20 are dry- the U20 even in 45K, and 15' crud. Both have good flare though. That and Bob Perry is really good at seaworthy boats. Antrim too.

The ride of all three was similar, the 30 sq m being a bit softer (but it was more v'd than the other two), Amati and the U20 more manouverable, but they have modern underbodies. Bit of a wrestling match with the 30 sq m on the tiller downwind. U20 pounded more, but it's a fat little sucker. None of them stuck a bow in the backside of a wave and stopped. The 110 and Sol 18 would go down the mine and keep going down the mine. Stuffing it is the phrase that comes to mind.

I don't think it's simple to compare a plumb bow to a long overhang bow. Both can be good. It's about balance. What would all things being equal be, if you think about it? Same WL, but different LOA? Or same LOA, but different WL? Same D/L? Different D/L? SA/D? B/L? Keel configuration?

Got to love sailboat design.

Paul

Faster 09-30-2007 03:36 PM

Greetings all
 
New to this forum, here on the Left Coast.

The design rationales above address the whys and whats of plumb bows.. but I think it also comes down to what's fashionable at the time - and now plumb bows seem to be fashionable. Maximizing LWL for a given LOA is likely the main justification, as mentioned above.

But practically speaking as these "fashionable" ideas migrate to more cruising oriented boats, the plumb bow creates a problem. It is very difficult, esp with chain rode, to get an anchor aboard these boats without scarring that proud, vertical stem. On my early 80's raked stem design I really have to work at it to get the anchor to swing into the boat as I retrieve it. Not so with these new plumb bows.

We have friends cruising the Caribbean on a relatively plumb bowed Bene 36.7 - and in the trades they really had to work out a technique to keep their ground tackle from attacking the boat. Some makers have added extended rollers to get some clearance, but there goes the alleged LOA advantage as the marina happily measures the prod for your moorage rate.

You are already starting to see some elaborate SS protection plates mounted on such boats.

marshmat 09-30-2007 03:49 PM

A lot of good points have been made, all of which seem to me to be correct in their context.

An example that I think summarizes a few of the things that have been said:
Consider two fictitious racing rules, Rule A and Rule B. They are identical except for how length is measured. Rule A measures by LOA and Rule B measures by LWL.
To maximize the speed under Rule A, a designer will stretch the waterline length to the longest he possibly can. Thus there are no overhangs, the bow is plumb and the hull leaves the water right at the stern. With LOA of 20 m, this boat might have LWL of 19.8 m.
Under Rule B, the LWL at rest is fixed. The LOA, though, can be anything (within reason). So you get long overhangs at bow and stern, shaped to increase the effective LWL as the boat heels. You also might see a big bowsprit and other tweaks so as to increase the sail plan area. The rest LWL might be fixed at 20 m, but the LOA might be more like 28 m.
Now we go to the cruising market. Some builders like to follow the trends of the fastest, cutting-edge racers and use this as their marketing approach. So when LOA-limited classes are popular in the races, this builder will produce broad-sterned, plumb-bow boats loosely modelled on the popular racing classes.
Other builders don't follow the racing trends so much and prefer to focus on what they think is best for cruising. Now the more traditional raked bow forms come into favour, with their generally more tame seakeeping, drier ride and easier anchor handling.
So in the end, it boils down to what the designer feels the boat should be doing, and how it should look and feel while doing it.

Paul Scott 09-30-2007 10:08 PM

Bye the bye, we anchor from the stern. So does Rave. In waves it's a bit noisy, but plumb bowed boats tend to have the mast forward, so a lot less hunting on the hook.

Paul


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