Trailer cruiser revisited... as a trimaran

[Fanie]

Imo the hull is shaped only so water can flow to the prop. If the narrowing down towards the stern then the whole area aft above the draft would be shaped like that.

That shape increase the draft, it reduces space in the hulls and I think it's going to increase the wetted area as well so create more drag on a displacement hull. I don't see any positive property in that shape for using with outboards on a smallish 8.5m trailable cat.

[Willallison]

Displacement cats work on the principle of high length:beam ratio's. To a point, the higher the ratio, the more efficient the hull. Yes, increasing the fairbody draft can increase the wetted surface, but that is usually more than offset by the improvement due to slenderness. If that were not the case, then displacement cats would rarely be more efficient than planing cats at the sorts of SL ratio's that we are talking about.
Many displacement cats are (or at least were) a development of sailboat cat hulls. The CS hull, as Tennant called it, is a development that reduces that tendency without resulting in a completely immersed tansom as would the case be with a more conventional displacement hull.
There was an article kicking around that graphed the efficiency of the CS hull vs 2 others - I think a traditional displacement hull and an equivalent monohull. The CS was better on almost all fronts. Unfortunately a quick google failed to turn it up.
If anything, this shape would reduce overall draft - though probably not in the o/b powered form that Matt is looking at. A displacement boat, by its very nature remains immersed at virtually the DWL. A conventionall shaped boat, if it is to resist squatting in any way, will have fairly stright and level buttock lines running aft. This mean that fairbody draft remains essentiall constant thoughout. Shafts protruding out the bottom of the boat will therefore increase that draft. They will also, as you say Fanie, be exposed to damage.
There are other attributes of the shape that are worth bearing in mind to. The boats can dry out without fear of damage to appendages for the reasons above. They also tend to exhibit lower wash - I suspect mainly due to the lower trim angles.

Sorry, but based on my research and experience, I would have to be presented with a very godd reason for using anything other than this shape for a cat hull

[Willallison]

Still can't find that graph. Tennant refers to it in this piece off his website, though the grapgh itself isn't there...

Frequently in magazines, and on web sites, you will find largely unsubstantiated claims of “greater speeds”, “lower fuel consumption”, “longer range” etc for a particular brand, or type, of boat. However, when the results of fuel consumption tests are published it allows everybody to look beyond the advertising and estimated performance figures and get down to what a boat is really achieving. We pride ourselves at producing very fuel-efficient powerboat designs that give high speeds on displacement hulls with as low as possible horsepower requirements. We also claim that our efficient “CS” hulls allow for longer ranges and higher speeds with a given amount of fuel. To make sure we can substantiate these claims, when possible, we compare our fuel consumption data with other similar boats. This serves as a continual validation of our design ethos.



We recently had a 13.6m [44.6’] “Escape” displacement power cat launched here in New Zealand and an independent fuel test completed on her. We managed to find published, professionally conducted, controlled fuel tests for three other types of vessels of a similar size with comparable parameters. These were all professionally built and had been designed by reputable naval architects. These were used for a comparison.



The fuel consumption curves are for: a round bottomed, with an aft chine, “semi-displacement” powercat, a hard chine planing powercat and a “fast” cruising monohull. Our vessel, the “Escape” has the full displacement “CS” hull form that was developed by this office. The fuel consumption was matched to the displacements of the vessels so that the slightly heavier planing cat and monohulled vessel would not be penalized. ie: calculated on the basis of fuel consumption at various speeds per tonne of displacement.



The results are shown in the accompanying graph. Right across the speed range the “Escape” was using less fuel than any of the other vessels. Her closest rival below 19knots was the semi-displacement powercat. However, as can be seen from the graph above 10 knots the “Escape” is using, on average only 65% of the fuel per tonne of the semi-displacement vessel! Even below 10knots the “Escape” is only burning 60% of the semi’s fuel. This means more than 40% more range for the “Escape” at this speed.



At all speeds compared, the planing cat was using more fuel than the “Escape”. At the planing cat’s drag hump at around 11 knots the Escape was using only 43% of the horsepower of the other boat! Above this speed the planning boat gets onto the plane and her fuel consumption begins to drop until 18 knots where it begins to rise once more. At the top measured speed of 23 knots that the Escape reached with her 200Hp motors she is still burning only 90% of the fuel of the planning cat. The planing cat can go faster than the displacement cat ie: it has a higher top speed. But it uses an additional 480HP (total) to get another 6.5 knots of speed! This clearly shows the high price exacted by higher speeds. So unless very high speeds are required in a boat of this length the displacement cat is clearly superior. However having said that I should point out that we have produced displacement power catamaran designs that easily achieve more than 30 knots from very modest horsepower. But of course they have to be longer.



This graph also shows the second advantage of the displacement cat. If the speed can be reduced to a slower cruise speed then the result is a greatly increased range. If we look at 15 knots for example the Escape is only using 50% of the fuel of the planing vessel. For a given amount of fuel this equates to twice the range at that speed! To increase her range the planning cat could increase her speed to 18knots, at this point she would still only have 55% the range of the Escape. The other option for the planing cat would be to reduce her speed to 8 knots to achieve the same range as the Escape achieves at 15 knots. At 15 knots the Escape has a range of 616 nm (with 10 % reserve) and she would cover this distance in 41 hours. The planing cat would take 77hours to complete this trip. This means she would arrive 1.5 days later! On a return trip cruise you would lose three days out of your holiday just to passagemaking!

[Willallison]

Nearly.... but this one doesn't have the conventional displacement cat hull on it, which is frustrating as it's the the one I'm hoping to show the comparison against

[Fanie]

Displacement cats work on the principle of high length:beam ratio's. To a point, the higher the ratio, the more efficient the hull.

Where did you get this ?? Because as far as I know this is not true.

[Ad Hoc]

Fanie

Look at a typical Series hull form that is long and slender. These are have low displacement length ratios, and most high speed ferries, in one form or another, are basically like these too. That is, long slender high-speed displacement catamarans. Some more efficient than others, but that is down to their L/B and hence its displacement length ratio.

[marshmat]

Keep the good discussion coming, everyone

I'd like to address a few of the points raised that directly relate to my particular project:

Originally Posted by Ad Hoc

Boats of this size are even more about the functions and its practicality. All these are the "design drivers". No need to focus on hull forms and other aspects for now, since your SOR is quiet varied and may possibly change.

Therefore just keep focusing on the the "issues" you really want from your boat. Then once that has gone around the houses several times, then see what type of hull form and layout will best provide you with the 'design intent' that satisfyies all and allowing for "extras" later on too.

I agree completely. Perhaps I neglected to mention earlier in the thread: the design brief for the trimaran I'm working on now has been two years in the making. My original solution was a planing monohull that, after much analysis and some model testing, was deemed feasible and likely a great boat- but not efficient enough, and too rough in choppy weather, for my planned operating conditions. The tri evolved as a more efficient, likely more comfortable design that, although somewhat more complex, would maintain the necessary people-carrying space and cargo-carrying capacity.
To summarize:
- Inland lake, canal, and some Great Lakes use in conditions up to and including small-craft warning
- Overnight for 4 or one week for 2, comfort level a step up from tent camping but no luxuries
- Carry at least a tonne of lumber, building materials, etc. which may be in awkward sizes
- Minimal wake, minimal fuel consumption
- Trailerable with a normal minivan or light pickup
- Would be nice to be able to carry a small sail rig, but is primarily a power design
- Quick to launch, recover, or prepare for lockage

Originally Posted by Willallison

Displacement cats work on the principle of high length:beam ratio's. To a point, the higher the ratio, the more efficient the hull.... (etc)

Agreed. In my proposed tri design, the vaka has L/B = 11.2, a value decided on (along with the form coefficients, sectional area curve, etc.) as the result of a long string of Michlet/Godzilla simulations. The form I've drawn works best at around 16 knots, and I believe it to be capable of about 20. There may be trim issues, of course- I am planning to try some proper CFD simulations and eventually a model test to make sure that the boat will trim properly.

I like Tennant's stern form, and would probably use something similar if I were doing an inboard-engine powercat. It shares a lot of similarities with the Sea Bright skiff (the traditional one, not the Atkins tunnel-stern thing) and is undoubtedly a clever design. For various reasons, I don't think it's the way to go for this boat (outboard instead of inboard power, somewhat slower operation than that hull is intended for, would like to be efficient under sail with the engine tipped up, and would like a fair bit of rocker for quick turning in tight canals and when under sail).

The upward angle of the buttock lines in my current hull is about four degrees, which seems for the moment to be a good compromise between low drag and ability to maintain proper trim. Note that the amas are very far aft, and could be equipped with trim adjusting foils.

Keep the discussion coming, friends.... I'll let you know how I'm progressing soon.

[Fanie]

Aaarrrgghh... I apologise. I completely misunderstood what was said there... yes correct. A 1:1 ratio is a planing hull and a 12:1 ratio is a good displacement hull. I'm like the English say here but not all there

[Ad Hoc]

marshmat

The point I'm making is that, in your original post, you ahve stated having some 9 iterations of your 'design' to date. This proves my point coupled with your comment above.

You need to sit down and draw up a list of 'wants'....what you want your boat to do. Think it through a few times then FIX IT.

Until you have fixed your design intent or SOR, you shall be constantly changing/evolving your SOR because each iteration does not converge on one solution that satisfies your SOR.

Your "wants" may well have to be compromised in order to converge on a solution, only later on once the vessel has been designed to meet your SOR will you know what, if any; ie cost, size etc. But until you have a fixed and finite list of your wants, for your vessel, you shall be chasing your own tail until the cows come home with "design solutions".

[marshmat]

Hi Ad Hoc,

I agree completely. What you say above is absolutely true. And it is essentially what I have been doing. (See post #52.)

Perhaps you and I simply use slightly different design methodologies or terminology. The approach I've been taking here is the same approach I use for most design problems:
- Define the initial requirements and use profile
- Consider all feasible design directions, and decide on one or two general concepts that are likely to result in something that meets the design goals
- Rough out that concept to form the basis of a feasible design
- Check back against the design goals and see if they can be met with this design
- Refine the design goals in more detail
- Build on the previous iteration of the design to better approach the design goals
- Repeat from point 4 until satisfied

I don't believe in firm, fixed goals that never change. As a concept progresses towards a completed design, one gains a better understanding of the design requirements, and ought to evolve them accordingly. Note that my use profile and the core design requirements have not changed substantially since I started this project ( Playing around with a 10 m trailer cruiser ) although some tweaking has been done (I've cut back on the need for luxury and thus weight/size, and relaxed my desire for high speed, among other things).

My design "iterations" are not complete re-workings; they are incremental evolutions of the concept, with a few bugs being worked out each time. Most yacht designers would likely include eight or ten of my 'iterations' in each loop around the design spiral. As a matter of personal preference, I like to break this out into smaller bits, evaluating the design against the goals at each stage.

And, AdHoc, I very much appreciate your comments. You're forcing me to analyze and explain the rationale for my own design process, which I normally do not focus on in this way. It is proving to be a very useful and enlightening exercise.

[Ad Hoc]

marshmat

Only too happy to assist in anyway, having someone prodding you mentally (with real questions not just smart arse one liners) is always a good thing, whether we like the questions or not. As you modestly put, forcing you to reanalyse, as that is surely the purpose of critiquing a design and its SOR to ensure everything has been thought of before money is spent! My comments in general are not always welcomed by many of the "arm chair" designers that seem to exits here, as they don't 'understand' what design really is! (Which don't bother me one bit, i dont feel like i need pats on the back or have anything to prove). My questions are exactly the same i ask myself as does my colleague to me and me to him. Not everyone can do a "self-risk" independent assessment...

The reason why is say fix the SOR, is that it helps to focus the mind and converge on one solution at a time. It does not mean it is the only solution. Having too many variables, and constantly change ones at that, will not allow for a solution nor establish where compromises shall need to be made to meet the SOR (if not met), certainly not in a timely manner. (Hence my reference to your previous comments and 9 other iterations....see my point?)

My design spiral is not that different to yours. However, I have just 2 issues, first, I always start with the weight estimate and then the sketch based upon the weight estaimte (from previous data), then rechecking the weights based upon the sketch, before going into any kind of 'design' iteration. If the SOR is not met, i start again, just adding or slightly modifying the first 'design'.

FYI, i design in the commercial field, not leisure, so my 'design spiral' may be different to yours? not knowing your field.

[marshmat]

My field is actually engineering physics, although I have done some work in high-efficiency building design as well as a lot of time with solar cars. Boats are more of a hobby- I love doing this kind of work and am well-versed in the theory involved, but am not doing it professionally or to sell plans- just to gain experience and have fun.

I do see your point regarding a clear, definite SOR. I am at that point now with this boat. Locking down the performance and other clear, numerical requirements is fairly straightforward- the difficulty with a boat like this (as with most projects I've been on) is that, once a design begins to emerge, you get things like "well, if that's how it's likely to evolve, it would be SO cool to also have......." And so the SOR and the resulting design gradually adapt, on an ever-tightening spiral, until at some point they are close enough that only minor details are being tweaked. I like to do this in ever-finer steps: start by getting the weight, speed and seaworthiness dealt with in a preliminary design, then progressively refine the result to account for comfort, living space, build cost and time, etc.



At present, I am happy with the hull and the simulations I've done on it so far, although there is still more analysis to come that may result in small changes to the hull. There is still a sail rig to come- for a sailboat I would have started the rig early on, at the same stage as the hull, but this is primarily a powerboat. The motorsailing ability here is more of an add-on, something to play with when conditions are appropriate- for about 60-70% of the proposed use profile for this project, sails and masts are not practical (low bridges, narrow canals, inland lakes with no consistent wind).

[Ad Hoc]

marshmat

"....well, if that's how it's likely to evolve, it would be SO cool to also have...."

and there in lies the problem. Every design and i mean every design i have ever worked on (in the past 20 years) have an element of this. You can't change the SOR once fixed. Minor changes like, oh, i would like blue seats and not red, or tinted windows not clear etc are easy and not really worth worrying about. The way to think of is this, if a modification or change is wanted/needed it falls into one of 2 categories:

1)cost
2)performance.

More often than not both need to be addressed. But when it is just a cost issue, that is for the client (you in this case) to justify. But these 'costs' shouldn't affect the design concept, just the overall price.

Whereas hmmm...an extra genny for air/con or, id like a few more 24DC batteries just in case i'm out at sea too long etc etc, all add weight which affects performance (and cost). When a change like this affects your performance, the original SOR is no longer valid. So you either have to do some serious compromises, or start again.

In the commercial fields I am in, starting again is not acceptable, for both, designer and client, so a compromise is found.

This is what you need to do. Fix your SOR and any "hmmm...that would be nice"..revue against the cost and performancee goals, (not to mention delivery time). Reviewing the "cost" of your implications will answer the question of whether you really want/need it.

In 'designing vessels' this comes from the Specification and General Arrangement, drawn in complement with the Spec. These two documents are the bible. Change them at your peril. Since this is the 'fixed' SOR.

[marshmat]

Ad Hoc: I agree. The specifications and requirements cannot continually change- that causes an endless circle of redesign, which is definitely not good.

The thing is, when I started this thread, I had a pretty good SOR- but it wasn't perfect. There were gaps. I had a start on a design that would meet the requirements I had. But since I don't yet have experience with this type of boat, I decided to seek feedback from my friends on here rather early in the process. In the last few weeks, the gaps and contradictions in the SOR have been fixed up, thanks in large part to the advice and commentary of everyone on here. Simultaneously, I've been manipulating the preliminary design to see how it might all fit together.

To summarize the peformance requirements, as they stand now, which is pretty much final:
- Inland lake, canal, and some Great Lakes use in conditions up to and including small-craft warning
- Overnight for 4 or one week for 2, comfort level a step up from tent camping but no luxuries
- Carry at least 1000 kg of lumber, building materials, etc. which may be in awkward sizes
- Easy access for swimmers, or to low docks / shore
- Minimal wake, minimal fuel consumption, quiet under power
- Trailerable with a normal minivan or light pickup
- Would be nice to be able to carry a small sail rig, but is primarily a power design for cruising at 20 knots or less
- Quick to launch, recover, or prepare for lockage- should not take significantly longer than my current boat (~5min to prep and launch from trailer)
- Weight target of 1.5 t or less in trailering condition (achievable, based on structural estimates to date)
- Simple, relatively inexpensive build from commonly available materials (developable ply/epoxy)

What I have now could best be described as an outline for a design that meets all of these requirements. And from now on, the performance requirements are not going to change substantially. Styling is still open, of course. So is the interior layout (I have a pretty good idea how everything will fit, but there may yet be a more efficient way).

Just to give everyone an idea of the level of fitout we're thinking of for this one: Electronics will consist of engine instrumentation, bilge pumps, a basic GPS, a class D VHF, running lights, a few cabin LEDs, and a couple of charging ports for things like an iPod. The stove, if any, would be a 1-burner propane unit. The head would be either a portapotty or a simple pumpout tank. Water for the sink and head will be pumped by hand. Drinking water will be carried in refillable bottles. Refrigeration will consist of a portable cooler and ice block (or Peltier module). There will be a single berth in the bow and another in the stern, one of which will share a compartment with the head. The cockpit settee/dinette will fold down to a double bed. There will be a fixed windshield, but the top will be fold-down canvas like on an 18' bowrider.

Oh, and there will be no less than six bilge pumps (at least two in each hull), a full set of lifejackets, flares and man-overboard gear, two beefy anchors, the usual complement of sound-signalling gear and portable spotlights, etc. And my fiancee is insistent that we have a proper wooden ship's wheel at the helm

My current focus, as far as this design is concerned, is now on the interior arrangement, the styling, and the structure. More to come in a day or two, unless it's too sunny around here to stay indoors

[Fanie]

Carry at least 1000 kg of lumber, building materials, etc. which may be in awkward sizes

Does the wife know about this

What on earth for ? If it's firewood then I'm sure youll be surprised to see how long a bottle or two with gas lasts...