Boat Design Questions

[AlistairM]

HI

Now this is all theoretical but after some reading matter I was pondering a couple of areas;

Some assumptions - A fully displacement boat will sit "in" the water and shove it out of the way as it progesses, whilst a lighter semi displacement sits on top and moves through the waves. - This may or may not be right

Question 1; > Is it possible to design a semi displacement boat that will offer an "upright" and comfortable ride, or is there always going to be a trade off between speed and comfort.

Question 2.> Are the lenght of the boat and the amount of freeboard directly proportional. I.e. is it safe to say the longer the boat the more freeboard there will be and therefore the more space below is available. Or is freeboard calculated differently.

Apologies if these are simplistic and they are just musings as I am interested in the design processes, rest a sure I will never likely put these in to practice.

Thanks

Alistair

 

[TimBennet]

Firstly, you have to decide what it is about 'the ride' that you find uncomfortable. There's a lot of research but roll interval and accelerations are the usual culprits. So it is possible to identify the hull forms that would give you 'comfort'.

But to go fast in a sailing boat you have to have features that will allow the power from the sails to be developed and exploited efficiently. When you list them, the overlap with the comfort factors is not encouraging!

So you're as with all things in boats, you're into the world of compromise.

Power boat design is actually much simpler except . . . buyers often want living space and low purchase price. I live in hope that my ideal prospective customer will walk through the door one day - a very rich, very slim, very short person with no family, no great desire to wash frequently and the constitution of an ox. But failing that, everyone else has to prioritise their choices.

 

[sighmoon]

HI

Now this is all theoretical but after some reading matter I was pondering a couple of areas;

Some assumptions - A fully displacement boat will sit "in" the water and shove it out of the way as it progesses, whilst a lighter semi displacement sits on top and moves through the waves. - This may or may not be right

Question 1; > Is it possible to design a semi displacement boat that will offer an "upright" and comfortable ride, or is there always going to be a trade off between speed and comfort.

Question 2.> Are the lenght of the boat and the amount of freeboard directly proportional. I.e. is it safe to say the longer the boat the more freeboard there will be and therefore the more space below is available. Or is freeboard calculated differently.

Apologies if these are simplistic and they are just musings as I am interested in the design processes, rest a sure I will never likely put these in to practice.

Thanks

Alistair

1: When you say upright, do you mean you don't want it to heel over? Catamaran? Lightweight (if that's what you mean buy semi displacement) and wide boats tend to heel less than heavy, narrow boats, but damp the motion more, which can make it feel a little jerky, as Tim said.

2: The amount of freeboard depends what you're trying to achieve. Low freeboard in absolute terms (rather than relative to boat length) means less windage (i.e. faster) when sailing upwind, but can mean a wetter ride on a narrow boat and cramped accommodation - both stop becoming issues on a big boat. The proportion of freeboard to length is relevant structurally rather than hydrodynamically - forestay tension could bend or break the boat if it's too 'sleak'.

 

[TimBennet]

Freeboard though as a function length overall is actually an inverse relationship. Small boats have proportionally higher freeboards (as a function of their lengths) because even their owners want to stand up.

Internal volume is certainly influenced by freeboard, but more so by beam and the distribution of beam with length.

Longitudinal strength is normally OK in yachts of normal form but interesting was a concern on the Jubilee rowing barge Gloriana. Although an 'open boat' she had to have structural members running the length at rowing bench height to take the logitudinal loads, and she doesn't have any rigging!

 

[AntarcticPilot]

Unless the hull is a planing hull, then it is a displacement hull. Light displacement is still displacement, and subject to the same limitations as any other displacement hull.

Basically, a displacement hull always displaces the weight of the boat to give buoyancy. A planing hull also displaces its own weight when stationary or at low speeds. However, a planing hull, by its shape, can generate lift when moving above some speed, so the boat has less drag, allowing higher speeds. Paradoxically, a planing boat may use more power at low speeds than it does once planing.

Few cruising yachts plane; the requirement for a keel to prevent leeway tends to be incompatible with planing.

 

[Davegriff]

Alistair,

Reckon a trip to the lbrary is needed at the least to even begin understanding boat design.

A displacement hull will have a rounded bilge and narrow entry/exit profiles. A semi-displacement hull will generally have rounded bilges with narrow entry, wide exit profile, and a planing hull narrow entry, wide exit profile with hard chines. All give different performance, stability, uses etc.

 

[sighmoon]

Longitudinal strength is normally OK in yachts of normal form but interesting was a concern on the Jubilee rowing barge Gloriana. Although an 'open boat' she had to have structural members running the length at rowing bench height to take the logitudinal loads, and she doesn't have any rigging!

I was thinking of the extremes - i.e. if low freeboard is fast, why don't we see racing yachts with very minimal freeboard. Along those lines, Uffa Fox reported a Skerry Cruiser snaking over the waves. Or more recently, the Americas cup explored the limit:
http://www.youtube.com/watch?v=PrlX6wvY2m8

 

[AlistairM]

Thanks for all the responses. I guess I was barking up the wrong tree with the freeboard idea, I will need to revisit my literature (Tom Brewer). I guessed there would probably be a tradeoff between comfort and speed, be interesting to know if there are popular designs or certain types different people go for.

Particulary greatful for the responses given that these are only theoretical questions and hold no other purpose than to increase my own understanding.

Thanks

 

[TimBennet]

I was thinking of the extremes - i.e. if low freeboard is fast, why don't we see racing yachts with very minimal freeboard.

I'm not sure the speed gains in reducing freeboard are particularly significant. There will be a weight saving and reduction in CoG, but there is some suspicion that narrow, low freeboard yachts might 'trip' on their deckedge when thrown sideways and thereby roll easier. Also high freeboard (and coachhouse volumes) are significant factors in improving the AVS. So theres not really any real 'pressure' to go with low freeboard.

But the biggest problems with low freeboards are they make headroom difficult. Owners know that racing boats must have a life after their pure racing days are over if they are to be economically viable for regular people. That's why club racing rules like iRC encourages a reasonable habitable space.

But classes outside that, which aren't of 'normal form' and are pushing the structural envelope will be much closer to the edge. The skerry cruisers were probably as close to the structural edge for wood construction in their day as the AC yachts were ten years ago. I once saw a classic old S&S 62 footer built by Aberking and Rasmaussen that wobbled all the top of every wave. When it blew hard and the forestays were loaded, none of the cabin doors would fit their holes properly. I dug around and found all her bronze strapping was gone! She was built so lightly to be competitive in the Med in the 1950s that there were no margins for her structure to 'age'.

I will need to revisit my literature (Tom Brewer)

His brother Ted is generally regarded as the more knowledgable! His 'Understanding Boat Design' is a good primer. In that he does give his formula for calculating a boat's 'Comfort Factor'. It's only a light hearted idea, but does give some idea to the interplay of factors, rather than trying to use one feature of a boat to praise or condemn a design.

After that Dave Gerr's 'The Nature of Boats' is also very readable.

 

[mjcoon]

1: When you say upright, do you mean you don't want it to heel over? Catamaran?

Though there has also been a design which you might call a gimballed hull (or rig). The mast and keel are connected and balance forces in the usual way. But the hull remains upright and does not contribute "form stability". But of course you still have the problem of pitching in waves (while wondering about the integrity of all the mechanics!).

Mike.

 

[ianat182]

For a pretty good handling of most of the design questions there is a small book 'How to Design a Boat' author John Teale, that explains just about everything needed to build power or sailing craft, from dinghy to large high speed cruisers and ferries,with many useful tables and formulae, lines drawings and full explanations. The book is probably still on Amazon UK and was originally priced at £8.99,published by Adlard Coles Nautical.


ianat182

 

[Bodach na mara]

For a pretty good handling of most of the design questions there is a small book 'How to Design a Boat' author John Teale, that explains just about everything needed to build power or sailing craft, from dinghy to large high speed cruisers and ferries,with many useful tables and formulae, lines drawings and full explanations. The book is probably still on Amazon UK and was originally priced at £8.99,published by Adlard Coles Nautical.


ianat182

I would add to that "Fast Boats" by the same author, for some rather eccentric thoughts. For traditional sailing yachts, "Cruising Yachts" by Harrison Butler covers all of the basics in understandable form and throws in the flaky "metacentric shelf" theory for good measure. It is more likely to be found in a museum than a library however.

There are lots of us (maybe most) that have studied yacht design but never come near to designing a boat. Well maybe a kayak or a tender. It is all part of he fun and is something you can do at this time of year when sailing is not really attractive.

 

[davidej]

My meagre offering to this debate is that high freeboards can cause parking problems.

New(ish) to mid-size AWB with high freeboars and no bowthruster means that you have to be very careful to avoid the bow 'blowing away' when manoevering in a marina.

 

[wotayottie]

Some assumptions - A fully displacement boat will sit "in" the water and shove it out of the way as it progesses, whilst a lighter semi displacement sits on top and moves through the waves. - This may or may not be right

Wrong. Any and every boat sits in the water displacing a weight of water equal to its own weight - Archimedes. Displacement / semi -d /or planing relate to the shape of the hull . A planing hull will reduce the effective weight of the hull when the boat is moving by generating an upward force in a similar way to your hand generating a force if you put it out of the car window.

Question 1; > Is it possible to design a semi displacement boat that will offer an "upright" and comfortable ride, or is there always going to be a trade off between speed and comfort.

There is no measureable qualtity called comfort. You cant put a number to it because its entirely subjective. But yes a semi-d hull going in a straight line will be upright assuming it isnt loaded to one side and the motion going faster will have far fewer low frequency movements in it. You might find faster = more comfort. You might not.

Question 2.> Are the lenght of the boat and the amount of freeboard directly proportional. I.e. is it safe to say the longer the boat the more freeboard there will be and therefore the more space below is available. Or is freeboard calculated differently.

No, they arent related. Freeboard is a function of average density of the boat. A submarine blow the surface has no freeboard. Pump out the tanks and make the sub a lot lighter and all of a sudden it has freeboard. With yachts feeboard if often also a result of human height - you need to accommodate people of 6'3" on board so you need much the same deck height in a 30 footer as in a 70 footer so tha latter almost always looks sleaker.

 

[PeterWright]

Freeboard is entirely a matter of choice and totally unrelated to length.

Sailing Barges, when thet were trading were obliged not to less than 3" of side, despite their length of around 80ft. It just so happened that most barges had a protective iron band around the edge of the covering board, to protect the hull from minor knocks when coming alongside. This band was almost invariably 3" deep, and it's top edge was level with the top of the covering board, so it provided the ideal loading guage - just keep on filling the hold until the water reaches the bottom edge of the iron band!

Of course, a natural consequence of this was that the slightest angle of heel caused the sea to rush in through the lee scuppers leaving the lee deck awash up to the hatch coaming. Add a seaway to this and waves would be breaking over the hatch tops. Woe btide the mate who failed to get the hatch cloths water tight after covering up!

Peter

 

[JimC]

You might like to read "How to Design a Boat" by John Teale. Very interesting and instructive. Reduces prismatic coefficients, conic sections etc. etc. to layman's language and contains worked examples of designing various types of boat.

http://www.amazon.co.uk/How-Design-Boat-Power-Sailmate/dp/1408152053/ref=sr_1_1?ie=UTF8&qid=1359403517&sr=8-1

 

[ianat182]

Bit late there JimC!!


ianat182

 

[JimC]

Bit late there JimC !

OK, sorry, but I did supply a link so it wasn't a total waste of time