hydrodynamic fairing?

[black_sails]

[EDIT tryed to fix formatting it all turned into a solid block somehow]

Something not entirely dissimilar was posted by me almost two years ago... (to a few ribs and jabs admittedly), to bring people back up to speed without having to either remember me or go look up other posts of mine i'll just resummarize the relevant part: I'm a bit of a hobbyist and I want to play around with boat design for fun as much as seriousness. My actual boatbuilding is still years off (between work and school I wont be near the lakes for multiple years) so this is more like a way of staying in touch with my hobby interests and dreams in the meanwhile.

Previously i'd contemplated whether a variable length cat hull was possible (though I hadn't considered demountable ones at the time), however one thing i'm wondering now is might it be possible to have something to improve hydrodynamics in front of the floating parts of the hull? Like at low speed (ie 7 knots) apparently shape doesn't matter apparently so lets literally assume it's just a long rectangular box. (to also maximize floatation within the length/not even tapering)

Then to improve the hydrodynamics at speed I basically stick a wedge on the front and rear of these hulls of whatever length so it's more efficient and faster, but since this isn't additional floatation (just a fairing) it shouldn't affect the legal length or tonnage either. (or create legal registration nightmares when I try to explain how the boat isn't one length) I'm sure this has been done before (everything has been done before) but I was wondering if others could comments, offer references I might not find on my own and similar regarding this topic.

[2big2small]

There are multi-part tenders and dinghies on the market if you need inspiration on how to do this.

The legal length will depend on how the jurisdiction you register the boat in defines length. They may have a view that anything that can be bolted on and off, such as for example a pullpit and bowroller, are part or not of the length.

[boatpoker]

If you're fairing has volume it will certainly affect "tonnage volume" (gross and perhaps net). It would most likely affect TML (tonnage measurement length) but depending on how it's done that would have to be considered a grey area for now.

PS. I am a Transport Canada Appointed Tonnage Measurer with recognition in UK, BVI, Honduras, Costa Rica, Bahamas and Antigua.

[black_sails]

Originally Posted by 2big2small

There are multi-part tenders and dinghies on the market if you need inspiration on how to do this.

The legal length will depend on how the jurisdiction you register the boat in defines length. They may have a view that anything that can be bolted on and off, such as for example a pullpit and bowroller, are part or not of the length.

Have any examples off hand or is there a specific search term to find that? (just like I didn't know the term "demountable" for a catamaran hull before)

This would definately be a US vessel, registered under the simplified tonnage measurement. I'm a little confused how something that could bolt on and off should change the length, because by definition I could have a 2 foot one, 8 foot one, and 12 foot one I didn't tell anyone about... or I could leave all of them at home and just stick with the 'legal length'.

Although the vessel I first try it on wouldn't technically need it, i'm wanting to test it with the idea of scaling it up. Such as in the future if the early retirement windfall hits, i'm building something at 78 feet just at the limit of simplified tonnage - with maximum floatation volume inside the squared up cat hulls. Yet what I add to the front and back (possibly only in international waters even) though technically being extra length is to give a more streamlined shape for when I don't want to move at trawler speeds. The extra piece does not have to have any floatation, it might be it doesn't even have to be enclosed. (just like modern pickup beds can maintain laminar flow behind the cab despite being completely open)

Originally Posted by boatpoker

If you're fairing has volume it will certainly affect "tonnage volume" (gross and perhaps net). It would most likely affect TML (tonnage measurement length) but depending on how it's done that would have to be considered a grey area for now.

PS. I am a Transport Canada Appointed Tonnage Measurer with recognition in UK, BVI, Honduras, Costa Rica, Bahamas and Antigua.

This would be a US vessel under simplified tonnage, my idea is to have the fairings open to the air while maintaining laminar flow to the otherwise blunt-ended hulls on the cat.

[boatpoker]

Originally Posted by black_sails

This would be a US vessel under simplified tonnage, my idea is to have the fairings open to the air while maintaining laminar flow to the otherwise blunt-ended hulls on the cat.

I'm not sure of US rules but in the countries I've worked with the "simplified" rules would not be accepted for any multihull with accessible volume.

[black_sails]

Originally Posted by boatpoker

I'm not sure of US rules but in the countries I've worked with the "simplified" rules would not be accepted for any multihull with accessible volume.

My bad, I should have said open to the WATER and not actually providing any floatation or storage at all. Since i'm not a hydrodynamic expert (and if the idea can't be "legal" it doesnt matter anyways) it might be shaped like a cone with an open back, or a wave piercing vertical wedge, or a bulbous dome separate from the front of the cat hull and held off at whatever distance is required. Like I know there are some legal exceptions to how length is counted, certain types of swim platforms off the rear, certain kind of... decorative I think things off the front, the whole point being that neither is usable as either displacement or storage.

With the further change that the fairing would be detachable once no longer in international waters for instance, since i'm pretty sure if I steam into port with ten feet hanging off the front of something it'll be legally treated as ten feet longer. I suppose you could say i'm basing this off assuming the laws change in international waters? (because one reason for 'flags of convenience' is taking advantage of these laws anyways, it matters once beyond the coast)

[Thalas]

I think you are adding an incredible amount of complexity to accomplish a relatively undefined goal.

It sounds like hypothetically, you want more speed on a cruising catamaran and theorize that adding length and a more piercing shape will accomplish this. Is that right?

It's true that more waterline on equal tonnage does equate to more speed, but I think you'll find the "piercing" front end of a hull is a very small factor in resistance. At the end of the day, wide hulls simply have to move more water out of the way than narrower hulls.

Friction on the hull is the other big factor. I've researched if there were shark skin like surfaces (unfortunately don't exist), hydrophobic coatings (not ready for prime time) and even silly solutions like a small, solar powered air pump pumping bubbles at the bottom of the hull traveling up the sides at speed (this would actually decrease drag, but how much and at what level of benefit/maintenance.

I thought I wanted an Outremer until I sailed on an FP. Now I want to day sail on an Outremer in great conditions for a fun day on the water and cruise the Caribbean and Med on an FP. If you wanna go fast, the safest bet is to buy a boat built for speed. Otherwise, enjoy the ride and 10kts ain't nothing to sniff at.

[hellosailor]

Maybe I'm missing something here, but unless your goal is something like to keep the OAL under sixteen feet so it can be unregistered...WTF is the point? What do you expect to accomplish by fixing or changing the "legal" length?

You go into a marina, and you'll pay by the foot, based on their measuring tape, not the name of your boat. You call it a 38, they know the bowsprit and davits go longer, out comes the tape and no one cares what the "legal" papers say.

Then there's the matter of the attachment being non-structural to the hull, flooded, but somehow being solidly affixed enough to remain in place while in motion? Hey, if it is flooded, that seawater has a mass of about 63# per cubic foot, it is going to need more than a couple of 1/4x20 bolts to hold it on.

I'm not seeing it.

[thinwater]

The fairing will be full of water and will be moving up and down with the waves, often quickly. This means the fairing will need to be stronger than the hull, since it will be lifted full of water (it wont drain that fast). The boat will also pitch like a boat with bow and stern crash tanks flooded.


Thus, it will cost more to build and sail worse than with fixed bow and stern sections. I know you don't want negative input, but your not bringing anything we can understand. Build a small model. Something.

[black_sails]

Originally Posted by boatpoker

I'm not sure of US rules but in the countries I've worked with the "simplified" rules would not be accepted for any multihull with accessible volume.

My understanding (for what that is) is US law allows it', it's mostly based off GRT and ultimate length. As well as intended usage - an 80 foot fishing vessel requires full international tonnage standards of whatever that is, with all the engineering analysis and calculations and such. Whereas a private vessel only used privately would not, but nor would a 76 foot fishing vessel, homebuilt, under a certain tonnage.

I don't have all the docs handy but I think there are three main types of tonnage a US vessel can be registered under.

Originally Posted by Thalas

I think you are adding an incredible amount of complexity to accomplish a relatively undefined goal.

It sounds like hypothetically, you want more speed on a cruising catamaran and theorize that adding length and a more piercing shape will accomplish this. Is that right?

Bingo! But it's more "efficiency at speed" as well, like the ability to make a given passage at say 12 knots instead of 8 knots with a blunter shape, when you start adding currents that might be 3-4 knots in the wrong way that's effectively a doubling of speed.

I don't know of it's incredible complexity - i'm not a marine engineer, i'm more like a hobbyist who wants to play with boats, play with armchair engineering, and is slowly learning whats involved in actual real marine engineering.

I find myself wondering "is it possible to..." and wanting to build a small boat to try it, on the lakes, and wondering whether an idea would scale up or not. I fully accept most people would call this a waste of time - but that's what it is - a hobby - to answer questions that I have.

That said if I KNOW something would be a 100% failure of intent entirely then I don't want to build it. I just don't want to be dissuading from trying something (since this is half for fun, and lots of past 'inventor' types doing things for fun actually came up with something useful after multiple iterations of exploration) just because other people aren't doing it, or it's not a common design.

I'm not convinced it would add incredible complexity - but yes it's probably more than i'm thinking right now. Until I modeled it out in CAD I wouldn't know what forces are working on what lever arms, and thus what structural modifications are required, and added weight, over a simpler system. I'm more wondering at what level something becomes counterproductive to it's original intent and is abandoned wholesale.

I mean at the simplest level it might just be a few feet added of the right shape the same way that people at Ecomodder add a 'whale tail' rear to their car to dramatically improve the aerodynamics. Or it lets me experiment with different front of hull shapes like a bulbous bow without having it that way 'permanently'.

Originally Posted by Thalas

It's true that more waterline on equal tonnage does equate to more speed, but I think you'll find the "piercing" front end of a hull is a very small factor in resistance. At the end of the day, wide hulls simply have to move more water out of the way than narrower hulls.

Friction on the hull is the other big factor. [snip]

If you wanna go fast, the safest bet is to buy a boat built for speed. Otherwise, enjoy the ride and 10kts ain't nothing to sniff at.

Well it's more I want a wave piercing design to shove the waves to the side, instead of under the cat, under the hope it will be a nicer ride. This also means that I need a high bridgedeck because the idea is to slice through the waves instead of pounding over them.

My biggest reason for wanting "more length than I registered with" is that my philosophy of rougher seas isn't that the seas are too big, it's that you didn't show up with a big enough boat for the seas. If seastate 4 is rough in your 40 footer it's probably no big deal to a 120 footer - any increase in length should make a nicer smoother passage, IMHO, so i'm wanting to maximize the length for the available money and for the weight (and cabin volume) instead of the normal tetrising and cramming that occurs to shove the most boat into a given length. Which is then short and fat and squat and pounds over the waves. I'm not even seeking a cabin that is as long as the cat hulls for instance, it would only take up about half of that. (or more accurately, i'd be taking the cabin you could fit on 40 foot cat hulls and putting that on 80 foot cats, so they can be half the width for the same approx floatation)

I'm talking some bigger lengths here but thats all long term idealistic thinking/i'd like to build a small boat to test ideas that I could haul on a trailer, long before I ever scale up to the same idea on a larger scale.


I'm aware of friction but that's really an issue for speed, everything i'm talking about even powerboat mode isn't going to be planing hull speeds. I'm more interested in long distance efficient cruise at speeds above what a monohull can do, which as near as I can tell from reading older articles on things on length to width ratios of the hulls happens north of something like 10:1. So it's about how far can you push an idea without it being counterproductive. I dont want to plane water with 2000hp but rather run 12-18 knots without extreme fuel costs, and at a waterline length shorter than normal hullspeed. (which apparently works fine with really long narrow hulls according to some old study I read many years ago)

Originally Posted by hellosailor

Maybe I'm missing something here, but unless your goal is something like to keep the OAL under sixteen feet so it can be unregistered...WTF is the point?

You go into a marina, and you'll pay by the foot, ,,. You call it a 38, they know the bowsprit and davits go longer, out comes the tape and no one cares what the "legal" papers say.

Imagine that scenario except I really do break it down to park in the marina. Say I design a boat for 40 feet with long thin hulls, but the front and back 6 feet are removable because that goes beyond the limits of the cabin which is shorter than the length of the boat. So now i'm paying for 28 feet in the marina, but I have the hydrodynamic efficiency of the 40 foot hull on the open ocean. Same issue if I have to pay to go through expensive locks and similar.

Will this save enough money to be worth the hassle or complexity? I dont know. It's just an idea I thought was fun to play with. If you dont find it fun and instead are frustrated at me you can either or ignore me, but I cant help myself just having thoughts like this... wondering if I could do something a certain way.

Originally Posted by hellosailor

Then there's the matter of the attachment being non-structural to the hull, flooded, but somehow being solidly affixed enough to remain in place while in motion? Hey, if it is flooded, that seawater has a mass of about 63# per cubic foot, it is going to need more than a couple of 1/4x20 bolts to hold it on.

...thats perhaps the best counter so far. Though for what it's worth this is just the latest iteration of an idea of trying to get maximum length when in international waters, and minimum length (and width) when in port or going through locks or maybe through a tight river. I posted once before about transformable length... and although I can think of ways to possibly do it, like something telescoping out of the hull for instance, that's got it's own issues. Including legal registration issues at some point if the effective tonnage changes because the shape changing hull is now displacing more.

It's for an experimental boat, what else do you want me to say? Some people tinker with cars too trying ideas unlikely to work as well as they think.

Originally Posted by thinwater

The fairing will be full of water and will be moving up and down with the waves,

I know you don't want negative input, but your not bringing anything we can understand. Build a small model. Something.

Actually I was hoping the wave piercing hull would push through the waves instead of riding up and down and slamming all the time. The extra weight might even stabilize things... although I admit handling it's weight could be an engineering challenge. (this could be an idea that "works better in carbon fiber and not in wood")

I don't mind negative input, I only ask that it be constructive. To either point out what other problems I have to solve, and to ask whether that really is worth all the hassle.

I'm trying to explore some ideas i've had in my head about boats my entire life and since in a few years i'll finally have a chance to start building them as a hobbyist and doing things instead of just riding in them i'm wondering whether ANY of my ideas have merit, or if i'm just fooling myself.

[boatpoker]

I just looked it up ....

US Tonnage Guide

If you measure via the simplified measurement and the NET Tonnage comes out to less than 5 tons, your boat will not qualify for documentation.

[CaptTom]

People have been sailing the oceans for thousands of years. Hull designs have evolved to their current state for a reason. It's not like an automobile, where adding a whale's tail will slightly improve gas mileage. We're talking water, not air. The forces are significantly different, as are the structures required to resist them. And cars are designed to maximize looks, comfort and carrying capacity, not efficiency. So there's a lot of room to play around with things like aerodynamics.

It's true that "there's no replacement for displacement." If you want to go faster, in heavier weather, you need a bigger, heavier, stronger hull.

I don't think you can get those things by bolting on or hinging down an appendage while underway. The complexity and materials required would be costly, take up a lot of space, decrease reliability and increase maintenance effort and cost. And in the end, the whole thing would be a lot more fragile than a normal hull.

Simply put, it's cheaper, easier and safer to just buy and operate a boat which will give you the performance you want.

[boatpoker]

Originally Posted by CaptTom

People have been sailing the oceans for thousands of years. Hull designs have evolved to their current state for a reason. It's not like an automobile, where adding a whale's tail will slightly improve gas mileage. We're talking water, not air. The forces are significantly different, as are the structures required to resist them. And cars are designed to maximize looks, comfort and carrying capacity, not efficiency. So there's a lot of room to play around with things like aerodynamics.

It's true that "there's no replacement for displacement." If you want to go faster, in heavier weather, you need a bigger, heavier, stronger hull.

I don't think you can get those things by bolting on or hinging down an appendage while underway. The complexity and materials required would be costly, take up a lot of space, decrease reliability and increase maintenance effort and cost. And in the end, the whole thing would be a lot more fragile than a normal hull.

Simply put, it's cheaper, easier and safer to just buy and operate a boat which will give you the performance you want.

So no room for innovation and everything that can be invented has already been invented ?

[black_sails]

Originally Posted by CaptTom

People have been sailing the oceans for thousands of years. Hull designs have evolved to their current state for a reason.

It's true that "there's no replacement for displacement." If you want to go faster, in heavier weather, you need a bigger, heavier, stronger hull.

I don't think you can get those things by bolting on or hinging down an appendage while underway.

Simply put, it's cheaper, easier and safer to just buy and operate a boat which will give you the performance you want.

Things like the Sea SLICE and SWATH hulls didn't exist thousands of years ago yet are seaworthy, there are always new ideas. Most dont work. Sometimes one does. Those two hulls probably would be exceptionally difficult to make out of wood but are more suited for something like steel as well - new materials can change things.

I don't mind criticisms that tell me what i'd need to overcome to make something work but I feel a little shot down if the answers sound like generations of naysayers. Even if an engineer tells me that, there was a time when flight was just a fantasy too. It's more like a question of first is it possible, second how far can it be taken, and third is it worth the time effort and energy? I already said i'm a boat hobbyist and I like to tinker, whats wrong with letting me tinker? Or even helping me tinker by clueing me in to what challenges i'd face trying to make it work better then let me decide whether it's worth my time for the benefits vs detriments?

I'd already called someone asking about how experimental hulls might be classified and they couldn't give me a firm answer without seeing engineering drawings but it was suggested it would have to be legally flagged for the maximum possible displacement, length, and width - if i've got some kind of shape changing hull. It can squish into something smaller, but expanding beyond legal length even in international waters creates legal ambiguities - it'd be like refitting at sea which legally makes it a different boat, but since the legal requirements are primarily insuring that someone is enough captain to pilot boats "up to" a size, refitting into a smaller size doesn't create the same problem and could likely get a waiver of some sort if the main goal was smaller size in the marina or going through locks.

Fairings are just one example of the real goal - a shape changing hull - I was just hoping to explore the idea a bit more before throwing it out completely. The US Navy works on shape changing hull designs as experiments seeking some advantage, should we tell them that no progress is possible hullforms of the past and they should be satisfied with I dunno, windjammers or something too?


So if this is the hangup, lets say it's not something for at sea, lets say it's a completely different reason... does that change whether I can get advice from a wider group of people? Maybe my reason for wanting bolt on fairings is I made a 60 foot long boat that I can remove 10 feet on the front and rear, so that it can be towed on a 40 foot flatbed. Call them break down demountable catamarans. Is it really so harmful to let me mentally explore the idea or let me touch minds with others who find the idea possibly interesting? :-/

[boatpoker]

I don't understand what legal ambiguities you are talking about and I know of no legal requirement for such a vessel to be tonnage registered or classified by any of the classification societies..... I find that a little curious since tonnage measurement and registry are a big part of my business.