The Offshore Voyaging Reference Site

Five Ways That Bad Boats Happen

Wander through any marina or row through any mooring field (or popular anchorage) with an experienced eye and you will see a lot of scary things:

  • Hulls that will pound horribly going up wind, both motor boats and sailboats.
  • Motor boats that burn two or even three times more fuel than they should to move a given tonnage through the water at a given speed.
  • Sailboats that need way more sail area than they should to sail well.
  • Sailboats that are just plain slow.
  • Sailboats that need huge rudders, and often two of them, to be even remotely controllable.

That’s just a few of the things I see. The list of naval architecture sins could fill pages but, enough, you get the idea. A person could be forgiven for thinking that boat hull design must be some kind of poorly understood black art where coming up with a good hull is a matter of pure luck. After all, what other factor can explain the number of truly terrible hull designs that assault the senses at every turn?

But, in fact, the fundamentals of good hull design have been well known for years and the majority of naval architects know these fundamentals. So how do all the bad boats we see come to be?

I thought it would be useful to this book to look at that on the theory that if we can understand how bad hull designs happen, our readers will be better equipped to avoid bad hulls when boat shopping.

So, let’s dive in to the dark and terrifying world where bad boats are spawned.


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More Articles From Online Book: How To Buy a Cruising Boat:

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  2. Is It a Need or a Want?
  3. Buying a Boat—A Different Way To Think About Price
  4. Buying a Cruising Boat—Five Tips for The Half-Assed Option
  5. Are Refits Worth It?
  6. Buying a Boat—Never Say Never
  7. Selecting The Right Hull Form
  8. Five Ways That Bad Boats Happen
  9. How Weight Affects Boat Performance and Motion Comfort
  10. Easily Driven Boats Are Better
  11. 12 Tips To Avoid Ruining Our Easily Driven Sailboat
  12. Learn From The Designers
  13. You May Need a Bigger Boat Than You Think
  14. Sail Area: Overlap, Multihulls, And Racing Rules
  15. 8 Tips For a Great Cruising Boat Interior Arrangement
  16. Of Cockpits, Wheelhouses And Engine Rooms
  17. Offshore Sailboat Keel Types
  18. Cockpits—Part 1, Safe and Seamanlike
  19. Cockpits—Part 2, Visibility and Ergonomics
  20. Offshore Sailboat Winches, Selection and Positioning
  21. Choosing a Cruising Boat—Shelter
  22. Choosing A Cruising Boat—Shade and Ventilation
  23. Pitfalls to Avoid When Buying a New Voyaging Boat
  24. Cyclical Loading: Why Offshore Sailing Is So Hard On A Boat
  25. Cycle Loading—8 Tips for Boat and Gear Purchases
  26. Characteristics of Boat Building Materials
  27. Impact Resistance—How Hull Materials Respond to Impacts
  28. Impact Resistance—Two Collision Scenarios
  29. Hull Materials, Which Is Best?
  30. The Five Things We Need to Check When Buying a Boat
  31. Six Warnings About Buying Fibreglass Boats
  32. Buying a Fibreglass Boat—Hiring a Surveyor and Managing the Survey
  33. What We Need to Know About Moisture Meters and Wet Fibreglass Laminate
  34. US$30,000 Starter Cruiser—Part 1, How We Shopped For Our First Cruising Sailboat
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  36. US$30,000 Starter Cruiser—How It’s Working Out
  37. Q&A, What’s the Maximum Sailboat Size For a Couple?
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  39. A Motorsailer For Offshore Voyaging?
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Matt

Two further thoughts on hull form coefficients (prismatic and the many others):

– The optimal range of many of the important coefficients for a given cruising speed is really quite narrow. Differences that you can’t even see when the boats are side-by-side on land can make a 10% difference in efficiency. The hull shape has to be determined by the performance requirements, period.

– Historically, form parameters (prismatic, block, etc) are calculated for the entire boat. This is fine when the boat’s pretty close to fore/aft symmetrical, but with many modern designs you need three sets of these coefficients – forebody, afterbody and whole hull – to accurately understand what they’ll mean in reality. Fore/aft balance – meaning that the weight distribution, the prismatic coefficients, etc. are comparable in the fore and after bodies – is one of the key traits that make Dashew boats perform so well in rough conditions.

RDE (Richard Elder)

I’m afraid I have to differ with the way John has defined the design criteria for the hull form of the A 40 on a number of occasions. If I might paraphrase: ‘No compromise from the optimal hull form will be permitted for the sake of interior design (or for anything else for that matter). ‘

In point of fact cruising sailboats do have interiors and those interiors have to be habitable for human beings. Thus interior design is one of the trade-offs that must enter into hull design.

Let me elaborate from one of the examples presented in this post. If the prismatic relationships and fore & aft balance of the Dashew designs represent the optimum hull form for long distance cruising sailboat, an A-40 designed according to those principles would have a 40 ft. waterline on 40′ overall length, and a beam of only 9’.

If uncompromisingly designed to the Dashew standard of optimum hull form the A40 would have an interior layout unlike any 40′ sailboat built since 1940 and one that would find no buyers.

It follows that:
1- The Dashew hull form is not sail performance-optimal for a 40′ boat because of scale factors. Perhaps so, but I’d have to see this hypothesis proven.

Or:
2- There are other hull forms with enough beam to permit an acceptable interior layout that deliver acceptable performance characteristics. If you accept this premise then you have taken interior design into consideration in designing the hull form. As you should.

I suggest that if you could weigh, quantify and graph all the desired performance characteristics and desirable interior design requirements and overlay them, the curve would be somewhat rounded on top with a fairly broad range before it fell off the cliff at either end. Within that curve of acceptability you might find a beam of between 10.5′ to 12′, but it definitely will not be a Dashew hull form or an Open 40. For example if increasing the beam from 10.5 ‘ to 12’ yields a 100% improvement in interior liveability while sacrificing 2% in desired sailing characteristics and adding 3% to cost that would be an example where interior design should drive hull design rather than the other way around.

Matt Marsh

It should be noted that one cannot simply scale a boat equally in all directions and expect the result to be usable. Stability, displacement and sail area do not follow the same scaling rules. Directly scaling a Sundeer 60 to 40 feet would yield a very tender boat, unable to fly the necessary sail area.

To (very approximately) scale a hull shape while keeping the result in the same general performance family, one should scale the length as L^1, the beam and draught as L^0.7, the displacement as L^2.4, and the sail area as L^1.7. The form coefficients (Cp, Cb, etc.) stay the same. A 40-foot version of the Sundeer 60 would have a 10’3″ beam, 6.3 tonne displacement and 523 square feet of sail.

In a voyaging design, the hull shape and proportions are set according to performance and seakeeping criteria, and the size is set according to the required load-carrying capacity and interior volume.

This is quite distinct from designs intended for marina dwelling and short inshore cruises, in which the length and cost are set according to marketing requirements, and then all the other parameters are adjusted to increase volume and load-carrying capacity until performance is too severely compromised. This is an equally valid design process, but it’s intended for a different kind of boat with a different mission.

RDE (Richard Elder)

Hi Matt & Eric & John,
Both your responses are excellent, both in scope and detail. Exactly the kind of discussion I was hoping to provoke!

Erik de Jong

Hi Richard,

I like the way you think!
There is however a significant difference between, let’s say, a Prismatic coefficient (Cp) and a length to width ratio.

If a hull shape is designed with a non optimal CP, there is nothing else in the design that can compensate for that. You will end up with a hull that makes more waves and requires more power to push forward compared to a better optimized hull shape.

The width of the vessel has surprisingly little influence on the waves that a boat is generating, assuming that both the length and the weight of the vessel are kept the same as well as the Cp.

In other words, where the Adventure 40 has a very different length to width ratio, she can still be optimized for the same hull shape to decrease wave generating in the same way as the Dashew’s have done with their designs.

Now you are probably thinking that a wider boat does not have the same weight as a narrow boat, and you are correct there, but optimal Cp is only determined by the relative speed of the vessel, and not by the length-width ratio or any other ratio for that matter. In order to push a hull forward, a certain power is required, the closer you get to hull speed, the more power it will require.
Where the Dashew designs are long, they have very high hull speeds compared to the Adventure 40. In both cases, the designer asked himself what an acceptable cruising speed is. For the Adventure 40, I have pushed that to 90-95% of hull speed, while for Steve and Linda Dashew, the acceptable speed might have been around 75 or 80% of hull speed (I just throw a number in here, I do not have the different parameters for a Dashew designed boat at hand).
Relatively speaking, the Adventure 40 has a lot higher power requirement than a Dashew boat since the relative speed is a lot higher. In order to be able to do that under sail, the Adventure 40 needs a lot more sail than what a Dashew boat would carry, again, relative to the size of the boat. The only way to make that possible is by increasing the sail carrying capabilities of the boat. Te simplest way to do that is by increasing the width (but be sure not to change the Cp while you do that!), the wider the boat, the more sail it can carry. The other way is by lowering the center of gravity, but this usually increases the weight of the boat since ballast needs to be added, or draft needs to be increased. Widening the boat does add some weight, because more material is required to build one, but there is no way around that.

Now back to your first statement, the Adventure 40 will be optimized exactly as Steve and Linda Dashew optimized their boats. Optimization is mainly in creating a hull that goes easy through the water (ie, generates no significant waves), and the width to length ratio decreases when increasing the relative speed of the vessel because of the increased demand for sail carrying capability.

The Adventure 40 interior is actually significantly compromised by the hull shape, where a “modern” 40 footer has three full size cabins and two washrooms, the A40 has only a cabin and a half without getting much more living space back for that compromise. When comparing the empty volume of the hull, I would say that the A40 is about 30% smaller than most other 40 footers that one can buy these days.

Now compare it t an open 40. Well, that is just impossible. This because both the A40 and the designs of Steve and Linda Dashew are displacement hulls, while the Open 40’s are planing vessels. A vessel that goes up on a plane generates much less waves as a displacement vessel, so the Cp of an open 40 is much less a dominant factor in the design. What is dominant is the requirement for speed. In orde to increase the speed, the sail carrying capability of such a racer needs to be as big as you can get it without increasing the weight. There are two ways to do that: increase the beam of the vessel as much as you can, and bring the weight to windward by adding water ballast, canting keels, move sails that are not in use to windward etc.

Best Regards,
Erik
s/y “Bagheera”

Erik de Jong

I now see that Matt and John type a lot faster than I do, but we are all three basically sailing the same.

Niels

On the basis that “skinny and pointy” are highly desirable efficiency attributes, I have often wondered why the MacGregor 65 has had so much derisory comment over the years. Perhaps it’s their execution rather than their concept which is flawed.?

Matt

I suspect the MacGregor 65 has been tainted in some eyes by association with the MacGregor 26, an inexpensive trailerable motor-sailer which shares a name but little else with the 65. Although the 26 is darn near ideal for its intended purpose (introducing new boaters to a little bit of everything without breaking the bank), it is pretty cheap in both price and quality and tends to be operated by people who are still learning the ropes. So the name draws a bit of derision in some circles.

Simon Wirth

Hei Niels
i had the fortune to sail on a MacGregor 65 for two weeks. As it was a modiefied version, I can only comment on my experience on this boat. But from what I read only and what I have seen, I suspect it is a combination of the following points. The MacGregor 65 doesn’t look like mutch in the harbor beside some other boat of here size. The freeboard is low compared to what you mostly see this days. Because of this, she has a wetter bow then many smaller boats. I have been told that, because of her long and fine lines you see the boat flex quiet clearly heading into the waves. An last, the standart interior looks downright cheap compared to what you see at boatshows this days.
Just my two cents
Regards
Simon

Ted Tripp

I often marvel when the shrimpers in their 60′ boats pass my slip and barely leave any kind of wake. But when the recreational fishermen in their white plastic 25 footers pass at the same speed, I have to grab on to something! I always think that it must take a lot of wasted fuel to move the water that way.
What are the hull qualities at work that creates this enormous difference and what are the recreational fisher designers after?

Matt Marsh

Having the ability to do 40+ knots severely compromises the boat’s ability to operate under 15 to 20 knots. The appropriate hull shapes and balance properties for these speed ranges are just too different.

In somewhat more technical terms, the ideal hull shapes for moderate to high planing speeds tend to be flat-faced prisms, with all stations aft of midships having nearly the same shape and the beam at the transom being roughly equal to the beam at midships. The centre of gravity of a high-speed boat must also be relatively far aft to ensure correct longitudinal stability, and the planing surfaces should have an angle of attack on the order of 5 degrees when the boat is at rest. This means the afterbody prismatic coefficient is greater than 1, guaranteeing that the wave-making resistance below planing speeds will be enormous.

Erik de Jong

In addition to what Matt writes above here, the small fishing boat is probably sailing on 100% of hull speed, which is the top of it’s capabilities if it is a displacement hull. While the 60′ boat sails maybe at half of the displacement hull speed it is capable of doing.

Just for the sake of experiment, motor away with your own boat at 100% of it’s capability, and look at the waves you will be generating. If you slow down to half that speed, you will practically have no wake left behind your boat.

Generating waves like that takes a lot of power, be it diesel, be if wind causing the rigging to move forward. So in fact, if you want to sail efficient, a 25′ boat should not sail faster than 4 knots, while a 60 footer could still sail fairly efficient at 8 knots.

When motoring, stay away from hull speed. Practically, one should avoid 70% of hull speed and faster under those circumstances.
While sailing, go for it! the sky is the limit!