Monohull vs Cat????

General boating discussion
longranger
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Monohull vs Cat????

#1

Post by longranger »

I have heard this hashed out a whole lot of times with NON ALLOY but thought it might still be usefull asking this same question for alloy examples. My bias is towards a Cat because of thier superior ability to handle wind chop (that hurts my back), lateral stability during drifts and fuel economy in no particular order. I still prefer the looks of a nice deep V, particularly with a flybridge. Please let the opinions fly.

Thanks,

Mike B
peterbo3
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#2

Post by peterbo3 »

G'Day Mike,

In my very humble opinion, the smaller alloy cats (under 20') do not cut it. They are generally pressed alloy & are not designed for heavy use. They are pressed because of the design & build restrictions related to their size. Glass is definitely the better option in this size range due to the builder being able to achieve correct hull shapes via a mould.
But once you get up around 23- 25' & over, alloy cats come into their own. You can get the heavy plate & internal framing that gives alloy boats their strength. This size allows the boat to be built in a similar manner to mono hulls.
The wider hulls also give you the option of I/B or O/B power.
Seeing that you are talking about flybridges, I guess you are looking at a larger, non trailerable boat. If so, the alloy cat can be custom built by any reputable alloy builder. All that is needed are plans & there are a number of naval architects who are experts in this field. The glass cat needs moulds & you are limited in what you can do unless you willing to pay for a "one off".
Bullshipper has done a fair amount of research on alloy cats. Perhaps you could shoot him a PM.
Down this part of the world, a lot of water police boats are now alloy cats, where as before, glass cats were the vessel of choice. They seem to ride as well as the glass boats because the larger size allows the hulls & tunnel to be built in the correct ratio to achieve the "air cushion" effect.

Even though I own a plate mono, there is no way it will outperform a similar size cat in most situations except fuel usage & initial cost.
Regards,

Pete in Brisbane
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Bullshipper
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#3

Post by Bullshipper »

I think that aluminum has a lot of advantages, and would agree with Pete that 24' is almost a minimum length for a cat as the sloping tunnel takes up X amount of length(foreteck+bowcover) no mater how you slice it.

The advantages to aluminum might include some of the following:
Weight-fuel milage and trailerability
Overall height- tall fibergalss cats are hard to pull from molds, have shallower tunnels for the same reason. Shallower tunnels mean less fuel economy, more sneezing and tunnel slap, lower gunnels. The bottom of alloy sponsons can have flat planing surfaces instead of rounded bottom sponsons, that are easier to pull from molds, but cost you more in fuel, overall speed, loading capability and have a more tempermental ride trying to make a displacement shaped sponson plane with excessive horsepower.
Features-aluminum can be a one off effort not requiring molds, more expensive foam panel fillers or wood that will wick water and blister.
Strength- alloy boats are stronger, and have not had the tunnel failures/stress cracks found on most brands of NON ALLOY cats
You start with a single layered material that has no defects and only requires proper design and welding. There are no mutiple lamination schedules to build a thickness up with resigns that have varying amounts of hardner, saturation, cure and application variation, affected by temps, humidity, age of product, cleanliness and surface prep- in other words less variables.
But cats have more welding than monos, are noiser, tend to bob, are much more expensive to build, and do not lend themselves to extended stays in warm salt marinas, IMO.
longranger
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#4

Post by longranger »

Thanks Peterbo3 and Bullshipper. Just the kind of knoledgable feedback I need. This is going to be a tough but joyous decision process.

I am planning a vacation from my home in Southern Cal to the pacific northwest this spring to view and seatrial a few monos and cats all in the 25 to 30' range. Want 9.5' beam or wider. Can't justify or afford anything bigger. For cats I will look at Maxweld, Armstrong and Nichols. For monos still narrowing the field but Norvelle is definately at the front of the line. Have called most of the likely candidates and have estimates from several.

I will go pilothouse with comfortable overnight capability, full marine head and flybridge with controlls if possible. Meant for serious fishing offshore but comfortable enough for family cruising. Would love diesel but probably can't afford it. Assume it will be quality 4 stroke outboards. Good but not irrationally expensive electronics including a state of the art autopilot. If 9.5' beam will probably get a trailer but will be keeping her in the water. The marina is only minutes from my home and work so I use a boat more if it is ready and waiting--300 hrs on motors last year with a lot a drift fishing.

The trip for seatrials is hopefully going to allow me to put these beasts though thier paces in at least moderate slop. First marine trip where I will be happy to see small craft advisories :). Testing in calm seas will tell me very little. From other peoples reports most of what I will seatrial will have excellent seakeeping abilities. Just need to find out which feels best for me. This is going to be fun. I will keep this board posted when I take my trip.

One final thing. I am looking at buying new but would welcome the savings of a comparable used boat. In our current economy I have seen quite a few bargains with everything I need except an alloy hull. If any of you see or hear of an alloy bargain that sounds like my kind of boat please PM me.

Mike B
Ironwoodtuna
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MY HYPER CAT

#5

Post by Ironwoodtuna »

Hey Bullshipper:

Here are some early autocads of my Hyper cat 34' x 14' with stepped inner tunnel, 1st third of the hull has vee shaped hull with wide flare, the 2nd third has curved displacement rounded outer and stepped inner tunnel and last 1/3 has a hard chine. It will have a Hysu foil, 435hp Hamilton Jetdrives and 40-knot crusie at 18-20 gallons per hour. top end of 45-50knots, 300-gallons per motor for a near 1,000NM range. What do you think??? Marty
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skypoke
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#6

Post by skypoke »

Longranger,

Here is my experience with an alloy cat. We built one from plans by a New Zealand NA. We had the parts plasma cut and put her together in our spare time. It's an 8 meter hull plus extended stern platform. Power is 2 X 150 Hondas. This is a full planing hull design which yields excellent fuel mileage at reasonable speeds. We average 2.5-3 nmpg at operational speeds. Running in the Gulf of Mexico steep chop offshore Texas, typical seas are 3-5 feet, short period straight up and down stuff. Ride is far superior to any monos I've been on of comparable size. I'd say it's not as smooth riding as a comparable size Glacier bay into head seas but will run off and leave it in following seas. Our pilothouse style hull with cabin is very light in comparison to a similar glass hull.

We've had zero issues with corrosion, we wet slip the boat. We did not paint the hull, but coated it with Nyalic, a clear coat which after 3 years is now at the end of its useful life. I like the neo technical, military style look of the bright hull but will say the first question most people ask is if we intend to paint it. We basically hit her with a water hose after an offshore trip. I'm not into cosmetic maintenance at all, it's hard enough to find time to fish, much less hand buffing and polishing. She's way rugged, we've been known to fetch up against a piling and pivot in reverse to make the slip during high wind conditions. She's bounced against a few offshore rigs during tie ups in rough conditions. Not a mark on her, with full length pipe rails and square spray rail aft, you'd have to really work to ding her.

As far as ride, I'd give her high marks. We've been in some serious snot and never an uneasy moment. She does not pound while running or trolling. On the drift the side to side roll you'd experience with a mono does not happen, while the roll is far less, almost unnoticeable really, there is a quick movement as the wave passes under first one then the other hulls.

You're looking at some pretty pricey boats there. I'd suggest some sea trials on various hulls, and also consider the appearance factor. Your alloy rigs are not going to be babe magnets, by and large. It all comes down to priorities.

Chuck
Texian, born and bred.
longranger
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#7

Post by longranger »

Thanks Skypoke,

Great info. that confirms my experience with Mono vs Cat in NON ALLOY. All things being equal, particularly cost, I would go cat in a heartbeat. Unfortunately the cats cost more for a variety of reasons. For all the reasons people on this forum understand I am now only shopping alloy. There are very few alloy cats coming up on the resale market, always my prefered method for buying a boat or car. The few nice ones have been well taken care of and have prices pretty close to new.

Not a midlife crisis thing but I am 51 and really want this. It will stretch but not break my finances if I go new carefully choosing just the right vessle with few comprimises. Still it is hard to ignore the relative abundance of used alloy monohull bargains in this economy.

My plans are to take a trip to test a few of the most highly rated models in the northwest next month. Am also strongly considering getting a set of plans and having a custom cat like you constructed. Unfortunately I don't have the time or facility to do the build myself and might kill myself trying to aquire the skills. Would love to see pics of your Cat. Also PM me contact info for the NA you used if you think it is appropriate.
Thanks all
Mike B
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Re: MY HYPER CAT

#8

Post by Bullshipper »

[quote="Ironwoodtuna"]Hey Bullshipper:

Here are some early autocads of my Hyper cat 34' x 14' with stepped inner tunnel, 1st third of the hull has vee shaped hull with wide flare, the 2nd third has curved displacement rounded outer and stepped inner tunnel and last 1/3 has a hard chine. It will have a Hysu foil, 435hp Hamilton Jetdrives and 40-knot crusie at 18-20 gallons per hour. top end of 45-50knots, 300-gallons per motor for a near 1,000NM range. What do you think??? Marty
[/quote]

It looks like an interesting design project Marty.

I am not sure if I understand the logic in designing the sponson this way, but am sure you have your reasons. It looks like you want to do a variation on a malcom tennent design, but his hulls where set up with about 25% of the power you are planing to install as he had knifier bow tips to gain economy at typical trolling speeds.

I am also unclear why you want to step the tunnel as a lack of turbulence will help your fuel economy according to what I know.
Bob5292
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#9

Post by Bob5292 »

Can someone explain what a Hysu foil is and how it is different from what other Hydrofoil assisted cats are using?

Thanks
Bob
skypoke
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#10

Post by skypoke »

My understanding is the Hyusa foil comes from the design board of the South African fellow who has put a lot of study and design effort into optimizing foils for cats. They will design and mfg a foil for new or existing hulls. They are pricey. Last I checked, around $12K.

There are others who basically take an airfoil shape and come up with a foil design. I suspect there's lots more to it than bolting on a foil and go. I believe Morelli and Melvin design their own foils. Melvin was a Boeing engineer for years so I'd be pretty comfortable with his work.

Placement, balance with main and stern foils, all takes initial fitting and adjustment. I've always been intrigued with the concept. Some day we may mill one out and install on our cat though it's not optimal for this. Most foil borne cats have asymmetrical hulls, ours doesn't. That Bladerunner cat (the large one) linked to appears to have conventional hulls and appears to be running nice and high on its foil.

Chuck
Texian, born and bred.
Ironwoodtuna
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Re: MY HYPER CAT

#11

Post by Ironwoodtuna »

[quote=It looks like an interesting design project Marty.

I am not sure if I understand the logic in designing the sponson this way, but am sure you have your reasons. It looks like you want to do a variation on a malcom tennent design, but his hulls where set up with about 25% of the power you are planing to install as he had knifier bow tips to gain economy at typical trolling speeds.

I am also unclear why you want to step the tunnel as a lack of turbulence will help your fuel economy according to what I know.[/quote]

Like Bullshipper, I have owned a NON ALLOY cat and always liked the cat's ability to run faster in bigger seas wit a softer ride and better fuel economy. THen I got into aluminum and live the idea even more of building a specialized fast cat that will handle the snottiest of seas while giving me greater, speed, range, fuel economy and security far offshore.

In my research between my own cat experiences and online research of multiple cat builders, both glass and alloy my design I beleives covers most issues well, but after reading a thread reply from Bullshipper, he makes an interesting observation on the Hysufoil system which if true would ake me away from using the Hysufoil on my design.

Here is my design thoughts based on self experiences and overall cat owners experiences.

THe displacement hull of Glacier Bay delivers a extermely smooth ride in a short wind chop up to 3-4' in height, when a mono hull would get pounded to death. But the fuel economy is lower and the hull's inability to lift the boat because the knife hulls that so nicely slices through the short chop wave does the same in larger but taller waves of oncoming. The knife blade displacement hull slices/sinks into the oncomingwave. In coming off a tall wave, like running up the rips of Montauk in the 22' model I owned it was extremely dangerous. So IMO is still an issue in a larger cat even 30' that I take the issue serious in my design.

Whereas, the World Cat's narrow sponsoned vee hull will run smoother and get better fuel economy because it will semi plane and cut swells very good, but its narrowness make the exageration of the bob-effect is danerous to the passengers.

Now as you go to the wider Austrailan built aluminum and NON ALLOY cats they will generally ride better in quartering sea and following seas, but not have as smooth a ride in taller and shorter duration waves/rip waves which is still an issue to me. So the manufacturers with the taller wider tunnels run better in the taller shorter waves.

Now the english Bobkat hull configuration (with or without the Hysufoil) is very interesting and makes a lot of sense. When reading and researching all the different articles about the comparisons of each of the different CAT hull designs in books, magazines and on-line, it maybe self-serving, but I think their design is valid, but I also beleive it be improved upon even more.

My design is based on a 14' x 36' design.

Here is what I taken into serious consideration when designing my cat:

1st, I like bow flare that my Ironwood monohull provides above the hard chine line and beleive that if built into a cat will add an additional benefit. It will give the cat hull the necesary LIFT when the hull is being run into shot tall building head seas or rip waves. More on this later...

2nd, I think the deadrise area of the front of the hull below the hard chine line from where the bow flare ends, is important to have a Vee to slice through the waves, what degree of angle for the vee is where a lot of thought must be considered and implimented. This section of the hull's deadrise/bottom would be made from 5/16" plate with a 1/2" keel and sitting on three Tee stringers that are 2-1/2" x 1-1/2, gusseted and welded to the CNC router cut frames of which one would be a water tight bulkhead. From the hard chine up, the plate matieral would be lighter.

Here is copy of a section from Bobkat's website write up on the comparison in design thought:
Comparison Hulls
A comparison of Power Catamaran Hull Types
(Reprinted from the Spring 2000 Issue of Power Multihulls Magazine By Prof.Eng. Bob van Niekerk)

The registered trademark BOBKATTM covers a range of power catamarans from 2.5m to 33m that have similar looking hull shapes, but with detailed changes for different sizes and speeds. The shape took 23 years to develop after which it underwent a series of refinement for the next 10 years. It is still not perfect, but when compromises were forced on the design, safety and comfort always took precedence over the outright speed.

The almost symmetrical bows prevent wandering when quartering seas and provide plenty of buoyancy when needed. It is almost unknown to take green water over the bows even in the Cape of Storms' worst seas. Stepping the bows in the front section of the tunnel allows asymmetrical sponsons for the rest of the way to give that all important banking to the inside, which is considered a prerequisite for safe sea keeping - and keeping your passengers on board.

The convex shape incorporates the equally important non-trip below the wide chines to further improve safety in beam seas and quartering swells in a large following sea. The rounded section does not slam and gives a comfortable ride in rough water, even when jumping the large swell at high speed. Spray rails are used at the bows to keep spray down and to provide a dry ride, even with a Southeaster off the bows.

The patented tunnel chines running fore and aft behind the bow-step are there to mix this solid water with air to form a two phase medium that is compressible and therefore reduces the tunnel slamming experienced by other tunnel shapes. The tunnel is also the highest of the hull shapes listed and when the foils are fitted on the Type 'J' BOBKATTM the effective tunnel height allows high-speed travel in severe sea states. The 22m patrol boat, for instance, can take 3m high head seas at 26 knots without discomfort.

At other headings there is almost no limit to speed accept in following seas when the boat starts flying off the crest and loses propulsion while airborne. The tunnel chines lift up in a flattened S - shape near the stern as does the tunnel roof, to provide an increased tunnel area for waves to enter when travelling at speeds below 20-knots in following seas. This not only lifts the stern but prevents that sudden forward lurch you get with other transoms. For the sports fisherman this feature also allows for backing down at speed when fighting a large fish without any danger of swamping.

end of copy:

3rd, So, I beleive that its important to have a sharp Vee bow shape for entry. The sharp entry cuts and parts through the on-coming wave but then having by letting the shape of the hull change you gain additional benifits. That second 1/3 of the hulls tunnel being narrower makes sense for fuel economy. As in the Fountain and Go Fast stepped hull boats. In my design there would be a full 4-6" step between the forward vee and the displacement next portion of the hull. This step allows more air to mix with the water and in turn making the hull easier to move forward at speed (but no difference at slower & trolling speeds).

4th, Here is where I think differently:
The 2nd 1/3 of the hull having a stepped inside of the tunnel makes sense. While under speed, and when running across wave tops the hull makes the displaced warter (splashing) water mix with the water to soften the ride. I think of it as a bean bag effect, since you can't compress water, these water beads as they are created, meet and mix with air there is a softening effect (just not sure how much). But, here is where a think a smart alloy builder can make the tunnel inner steps work better. Using a plate roller, and the right grade aluminum, the builder would roll a radius into the tunnel plates to create the steps. Instead of all the long seam welding where each inner corner is made into a hard 90-Degree right angle step, I think that by making a wide radius rolled rounded step (an upside down "J" shape step). This will eleminate one welded seam/corner, while making the aluminum in the verticle tunnel plate a) stronger and stiffer in the longitunal sections. b) while the rolled radius corner will displace the water and provide a softer landing as the water in the wave will flow into the center of the tunnel for a better. Thus giving a for a softer ride when hull is displaced deeper than normal while entering hard into a larger wave.

The last 2/3rd of the boat's tunnel would be a continuation made from one long rolled plates that are welded together.

5th, having the second 1/3 of the hulls outside shape be a displacement style will make the ride much softer, especially when the hull comes off one wave and lands into the next wave. But I beleive that here again a small lifting strake is IMO required to help give the hull in this area the necessary planing lift to keep fuel hull more efficent.

6th, THen the last 1/3 of the hull should have a hard chine bottom, and again would step where the hull shape changes from the ouside hull from displacement to had chine shape. This step again would be 3-6". The hard chine bottom would be angled 8-12 degrees from the inner tunnel edge. This last section of the hull would give greater planning ability for better speed and fuel effencency.

Here again, I think this shape will also help on the bob-roll effect while drifting beam to in seas.

The two side of the vertical tunnel plates that are rolled to make the inner tunnel steps, I would make a 4-foot x 5" long, angled keel that would stick down below the bottom plate. THis keel would provide a GO STRAIGHT effect when running in following seas or following quartering seas. Remember the veel shaped bow keeps the front of the hull from jerking the hull to one side or the other and this would keep the BACK SIDE end going straight.

My sides would be generally taller than most cats, the deep sponsons/tunnel would give me a flushed deck ove the engines. Then by having 30-32" gunnels from the deck to the gunnels it will a good height and security for fisherman to lean against without having railings in the cockpit area. Then toe rails for them to tuck their feet under while fishting larger fish.

The outer bow sides which is where my near 45-degree outside flare will come from will be quite tall (bulwark looking) so the vessel has the abilitiy to ride up (lift itself) and if the wave is very large and breaking will be much less likely to break over the bow.

With my transom design where the sides of the hull creates a platform over that stick out and over the jetdrives allowing the angler to stay within the cockpit and walk a large hooked deep diving tuna around
rear of the boat without having to navigate the line around an outdrive or jetdrive that would normally be sticking out beyond the transom.


Got to go to dinner, more later.
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Bullshipper
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#12

Post by Bullshipper »

There are lots of theories, but if this is you first effort builting an alloy cat, I would try to copy a design that you know rides well and tweak a one or two variables to get what you want.

Roger Hill in New Zealand has a lot of testing and tweaking experience in finding the right combination of sponson width/length holding transom deadrise fairly constant at 13 degrees on alloy cats ranging in size from 24 to 80 feet in length.

They have excellent fuel economy, are fairly simple to build, seem to hold to the same inner superstructure design parameters for different sizes, and have a fine reputation of holding together in rough seas without overbuilding. The NZ code is strenuous, but successfull.

Cats are sensitive to weight, and are not as forgiving to overbuilding and extra weight like a mono, so staying within the plate width parameters of 3, 4, and 5 mm plate will save you money and make the hulls efficient.

So, I think its best to let their experience with 100's of new designs be my guide, and just tweak 1 or 2 things on the planning hulls like:

Load the boat lighter by increasing sponson lengths to get less load per foot of length

Move the motors inboard on the sponson to provide space on the outer chines for trim tabs, and not disturb the water going to the props

Balance the boat to make it sit and ride a bit more bow happy so that
slap in not a problem in head seas at slow speeds, and improve her ride in a following sea and beam sea

Employ trim tabs to push the bow down at higher speeds to improve the head sea ride.

Slope the motor wells down to raise the tunnel.

Go to a minumum of a 10' beam, and preferably a 11'11" for a 29' length.

I have al ready tested some of these ideas on my od 94 seacat, so I know that the alloy will be a lot better with less weight, more beam and length.

Changing more than that on on already good design, is enough for one effort. More steps can follow on following hulls after sea trial evaluations.

Now, why do you want to maintain diesels inside of narrow sponsons?
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#13

Post by NIGHTHAWK »

Bullshipper wrote: Now, why do you want to maintain diesels inside of narrow sponsons?
I had the same thoughts. Why?
28' MAXWELD MAXCAT (Twin 225 Hondas)


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