constants across boats sizes

OK I think this is a fairly logical question. I have read and studied enough to know I am too old and too inexperienced to ever become a boat designer so if and when I start my project I will buy plans from a proven design professional. I like Ted Brewer's style and the fact that like me he is also an avid model railroader. Now back to my question.

all else being equal will the physics and handling of a boat be the same as a larger/smaller boat if all proportions are the same. Let's say I find a boat I love at 25 feet but that's out of my league financially so using trusty computer I recalculate all dimensions to 18 feet. (forget interior for now).

Will the boat have the same performance and handling at 18 feet that it originally did at 25 feet? I can see where a few things don't change such as water weight and momentum. so assume the water is calm and the winds constant. I know a 10 foot boat won't handle a 20 foot wave the same as a 100 foot boat.

 

Hi paul. Size matters. The only drawback to increased size is moorage, usually charged by foot.
There are economies of scale. Two boats, one 18', one 25', both require an engine, running gear (shaft, strut, bearing, stuffing box, prop), electronics, head, holding tank, navigation lights, anchor, compass, the list continues. You may as well get more boat and in the process, all things being equal, you are likely to get more speed, as well (your designer will explain). This applies, within the constraints of design, right up to the point when someone says "hey, we've got room for a reefer (trash compactor, hot tub, etc.) and starts weighing it down - Then you get a bigger engine, for that a bigger shaft...and so on.
IMO, there are no easy formulae for deciding what size, what type (displacement, displacement cat, planing, whatever). You will need to meld your personal tastes with the very real factors of trailerability, moorage (does a 25'er overall cost the same as an 18' overall, do they use the documentation to charge for moorage and it states a length between perpendiculars, is it a different class of slips for vessels over 28' and the charges double, are cats too wide for the marina)... you can see how important a little research becomes and then, meld what you believe with what a designer can do within those constraints. Also, trailerability is good but if you are going to spend real time on this thing and keep your wife happy, bigger is always better. Do not be intimidated by size alone. A bigger boat is always easier to handle (if it weighs more), everything else being equal, than a smaller boat because it moves slower and is less abruptly affected by a gust. The only problem arises in that when you screw up, the bigger boat tends to cause more damage. Just remember not to try to power your way out of marina trouble - if you screw up, put it in neutral and get out a fender. never yell at your wife or the girl working the fuel dock. Let's get this straight right now - it will be your fault when you hit something. Learn to say "sorry, I got a little out of shape there" and go on with a pleasant day. Talking off the cuff here, may have forgot something or mis-worded something but am hoping others will chime in as I head out to play cards (Saturday night)...

 

I see where you're headed, Mark twice the boat does not necessarily equal twice the cost. there are some pretty big boats on ebay for a song while the small catboats seem to bring a huge price for their size. not sure why. maybe that's a question for the catboat forums.

My size limitation is that of trailerabilty as I want to build here now in my home state of Indiana. Not known as a cruising state, but we do have a pretty nice 11000 acre man made lake nearby with some interesting shoreline. actually a great place for a novice sailer to learn. and later move to Washington where my family is. and a trailerable boat can double as a pretty nice camper provided you don't try to do the jeep trails over the continental divide in Colorado.

I have looked at many of the formulas for hull speed motion comfort etc. I don't quite understand how length alone determines speed and I am sure that is just a rule of thumb but the one concrete formula I do understand is the one to reconcile my checkbook. and I am sure most would agree that is the most constant perameter in boating. good luck at cards.

 

Paul, what you're thinking is a scaling factor or device, which doesn't work in size changes more then say 10% to 15% of the original. Look up the laws of mechanical similitude and relativity.

For example, lets say you want to half the size of your boat, all around. You've found the perfect 30' boat, but want a 15'. Well the law of similitude says you'll decrease it's surface area by 4x (not 2), it's hull volume drops by 8x (not 2) and it's stability dives at a rate of 16x that of the 30' perfect boat. In a nut shell you've made the boat half it's original size, but it weighs 8 times less and is 16 times less stable. Not such a good idea.

You can make very slight proportional changes (a few percent at most) or you can use other "tricks" but you don't want to venture very far out onto this design guess work limb, because it will bite you in the transom.

So, your 25' down to a 18' conversion is a 28% reduction from the original, which is well outside the "reasonable" grounds, I mentioned.

Yes, displacement directly translates into project cost. This too is subject to the same laws of similitude and relativity. This is why a boat that is 20' long can be quite affordable (lets say 20K for example), but a boat only half as much longer (30') can be several times that (120K) of the 20' boat. Developing scantlings for a boat's structure is also subject to these same physical laws. They are a primary reason most designers and architects are prematurely bald.

As far as you not understanding how length alone governs hull speed, well it's more then just length, but it's also just a lack of conceptual understanding. In this regard, given certain hull form shapes, you can conclude specific S/L ratios with a specific amount of power. Once certain shapes reach their "theoretical" limit (commonly called hull speed and represented at S/L of 1.35) the wave train they are generating as the pass through the water, generates sufficient drag that over coming this shape and length induced limitation is imposable with the other variables unchanged. Even if you double the available power, some shapes (which are generating huge amounts of drag at these S/L ratios) just can't climb (literally) over their own, self generated wave train. Just because you don't understand this doesn't mean it's incorrect or invalid, it just means you don't understand. Is it "hull speed law", well for some shapes yes, but other shapes no, not even close. It's the conceptual understanding stuff that lets you know which shapes.

 

Clear enough, PAR kind of like doubling the size of a digital photograph quadruples the file size because you have not only 2X the width but you have 2X the length as well. I guess if it is the asthetics of the boat you have fallen in love with, the answer would be to search out a proven plan for that configuration in the look you want. and I have noticed the more prolific architects often offer similar look designs in several sizes adjusted for the physics. I just don't have the knowledge to mess with dimensions I guess my creativity (or lack of it) should be applied to the paint scheme. Thanks for the explaination.

 

Yep, now you got it, though not only does it double in width and height, but also in depth (all three dimensions).

For example a box 12" square is a cubic foot. Double this and its not 2 cubic feet, not 4 cubic feet, but actually 8 cubic feet (24" long in each dimension).

I offer styling clues across several sizes yachts as well. You sort of stick with something that works or offer a "family" resemblance.

It's not hard to pick up styling clues from a design you like and incorporate them into a different design, be that smaller or larger.

In fact, it's the aesthetic portions of the yacht that will make it "your" boat and the areas I recommend most back yard builders fool with the most. Changing the shape of a bulwark or rub rail, maybe cabin trim or possible making a rounded coming instead of the square one shown in the plans. There's lots of stuff you can do, that will not affect the dynamics of the design, but will possibly dramatically affect the look. This is where a home builder can really shine. No pro can put the love and attention into a project a guy in his back yard can.

I guess what I'm saying is don't give up, keep looking, you'll find something close, or that can be made to be close.

 

I like your style, PAR.

I can easily see that boat design is a little more artistic and philisophical than designing an aftermarket carbureter for a Chevy. After all you are creating a personality that will be called "She" not "it" Would we dare refer to our wives as "Things"?

I am sure when choosing an architect, how you work with him(her) can be just as important as the more mechanical aspects of design.

There is just something about those Catboats, I can't get out of my mind. "Simple Elegance" comes to mind.

 

Um. I'd like to clarify the above comment about an aftermarket carburetor for a Chevy to any of the readers who are also gearheads. having been to more than a few custom car shows in my life, I know there are those who can see art in the design of an engine as well as that of a boat. and yer right.

there was a vintage 1928 Evinrude 4hp on ebay a few weeks ago that looked like once cleaned up and restored would look as good bolted to your fireplace mantle as the back of a wooden rowboat. I was only referring to the utilitarian designs found at discount auto suppies.

 

If you compare the Demon line of replacement 600 to 650 CFM carburetors to the Holley, Carter or Rochester OEM's, you'll see the difference in form following function.

The function has to be there, but the form is subject to debate. Bolger bore this home in many of his designs, which usually produced children only a mother could love, though they sailed well. I believe you can have your cake while you're eating it, if you're a clever designer and the mark of a truly good one, is the most functional designs also happen to look great too.

 

Hi Paul,

Don't give up so easily. Go to the trouble to shop around some, there are boats and boats and boats and some more boats.

If you have no boating experience then buy a crate of beer and give boat owners a beer or two for a ride on theirs. Note the conditions, space and whatever the boat has to offer. You'll begin to develop a feel for what you really are after.

If you plan to rough it out there alone or with a budy, then you can buy a small boat. If you plan to have the wife along, think bigger like Mark indicated, bigger is better. Wiemen are after safety warmth and comfort, only bigger can offer that.

As for the budget, who knows what you may come across. Guy has a bad experience on the water and gives the boat away, guy moves away from water, will take what he can get... just get involved in some way and it will come to you.

Designing and building can be as expensive as buying a boat.

8m is a nice size boat, much better than anything smaller.

 

Your point is well taken, Fanie.

Succesful boat ownership is as much about resourcefulness and craftsmanship as it is about a big checkbook.

And will the boat I am happy with as a beginner still make me happy after a few years or even months of experience? One should probably consider his experience now and add a few years. lest he become bored right away. But don't get too far ahead of yourself either and become discouraged.

I hear there was bargains galore following the devistation by hurricane Katrina and while those boats might be structural totals there was probably thousands of dollars in radios hardware sails fittings etc to be had from those haul away treasures.

I see it's all a juggling act. and for me personally it's about building something as much as sailing it.

I am a long way from South Africa but if you are ever sailing through Indiana I would be happy to buy you some beers for a ride on your boat.

as was stated at the end of the travel channel "Megayacht" show,
"The only thing better than having a megayacht is having a good friend who owns a megayacht"

Thinking a sailing club might be my answer at least for awhile.

 

Hi Tom,

It is a mind set really. I can tell you now if you think you can then you are right. If you think you won't make it, then you are are also right.

Most of my friends think you have to swim in money to get a boat other than the odd bass boat. I, thinking I live only once, I have to make the most of it, decided to go for what I want, and that's what I'm doing. One day at a time.

I do research every day and do something towards it every day. It is lonely, very lonely. IF you find someone that will actively help you, you are very lucky. Most will come by once in a while and offer idiotic remarks, so...

If you are too old for it, then I guess you are around 105 or there abouts. Many people die before they finish their life long dream. So what, you got to follow you heart and did something about it other than watching TV and be a couch potato.

There is nothing you cannot do if you put your mind to it, but it really is up to you.

 

Fanie,

I don't know much about boats. Like you, I am researching and learning everyday in preparation to tackle that project. But I do know artists and I consider boat builders to be artists.

There is a certain personality associated with boat builders as rugged individualists, perfectionists, in meditation with their goal.

Unfortunately, that also brands them as untouchable and reclusive and while many people would love to help, they feel somewhat alienated or awkward.

If boating is making you lonely, I would suggest getting involved in some club activities and set your sites to the goals of the club. They are just folks trying to have some fun and while they might not have the perfectionism and talents of a mastercraftsman, if they are creating a safe sturdy project to enjoy together then a goal has been realized. Let the kids paint that old rowboat behind the garage and if it looks a mess and them too so be it. If it floats and they are smiling you have created a masterpiece.
I must remember this too.

I am thinking toward joining our park sailing club for learning and socializing while doing my own "secret" project in the garage at home.

Those boat club fishfrys go a long way in the creation of your own floating Picasso.

We need to think positive but to a realistic goal. and for God's sake, have fun.

 

This is a refreshing thread. Usually, most that ask the sort of question Paul No Boat started with already have some ideas and biases locked up and don't respond all that well to suggestions. The advice and thoughts given are valuable and costs much less than finding it out by yourself. Here is one added thought. Even if you want to build a boat yourself, it would be far better to learn how to sail and sail several different boats before getting serious.

Buying a boat similar to what you think you might need is a good idea. I bought a 22' sailboat on ebay from a charity a couple years ago. It was a Hunter 22 and not a real high end boat but has turned out to be a pretty nice one and a good sailer for my use. The big incentive for doing this is cost. I paid only $720, including transfer fees. I had to drive about 700 miles round trip to pick it up and that adds some to the cost. It was very neglected and had an atrocious amount of barnacles on the bottom but cleaned up pretty nice. Stuffed into a corner was a spinnaker, practically unused that would cost more than the boat did. I have less than $1K in it. If I give it away, I'm ahead.

One recommendation might be to look for a centerboard or swing keel boat so you can trailer and launch at ramps. This will save much cash that you can use other things. Two that are often seen are a Hunter like mine with a centerboard and the Catalina 22 with a swing keel. There are others like the San Juan 23 and Spirit 23 and also a Venture that I rate a bit below these but still adequate for lake sailing.

I have built quite a few boats, both sail and power, but would not build if a suitable one can be had for a small fraction of what it costs to build, not even including the labor.

Advice is good but not a substitute for experience, especially experience gained without too high a cost. Good luck!

 

Thanks Tom,

I am enjoying the input on this and several other threads and I have found that many experianced sailers, like yourself, are not too judgemental of us newbies.

I am not a boat person yet, but I am fairly mechanically and artistically inclined and I think I could turn out acceptable results, given the right guidence.

I do kind of have something in mind that seems to satisfy my desires, that being Ted Brewer's Cape Cod Bay. an 18 foot catboat as it would be fun to sail on 11000 acre Lake Monroe here in Indiana and could handle the challenges of some bigger water later when I have gained some experiance.
and not too complex in it's systems, either.

I plan to join the Lake sailing club run by the park district, but I am not sure how much catboating could be learned there so I might change my mind on the design to something more common in this area.

I just missed a really nice 18 foot sloop on ebay a couple weeks ago for 2500 bucks looked great and basic.

http://cgi.ebay.com/ebaymotors/ws/e...0410585377&viewitem=&sspagename=STRK:MEWAX:IT

BASIC being the keyword for me not only in boating but for anything. I would imagine there are a lot of people who spend half their retirement on a boat way too much for them not only in size but in complexity of electronics, navigation, maintainence, cruising lifestyle, etc. Sailing is a CULTURE. not learned in an afternoon.

As I have browsed plans I notice that the smaller plans always have pictures of completed boats and as you look larger they switch to artist's renditions of what a completed boat would look like, so I figure the larger the boat the greater the odds it will never be completed. There must be a formula for that theory somewhere. LOL

Thanks for your patience and encouragement.