ProBoat contest - collaborative design thread - power catamaran

[kengrome]

I created this thread (upon the recommendation of Captain Blight) to discuss and collaborate on a power catamaran design, either theoretically or actually, to be entered into the Professional Boatbuilder's design contest the details of which are posted on their web page:

http://www.proboat.com/designchallenge.php

Previously we were discussing the particular features of a power catamaran hull so let's continue here ...

At that slenderness ratio and with the very high prismatic that the narrow hulls allow while still retaining a narrow entry and exit angle, Canters multiplier suggests that the resistance curve should not be rising too steeply at an S/L of 4 which is about our target speed. My quick arithmetic suggests that the numbers seem to work ok and that the vertical accellerations in waves will be markedly lower than a low waterplane loading monohull so the boat would, in rough seas be able to maintain a much higher speed for a given comfort value and that the power requirements of such a vessel would be well within the parameters of the contest.

If I'm not mistaken, this conclusion presumes that the lower vertical accelerations you're talking about are still 'high enough' to keep the bows from diving into waves. If they are not, and if the bows punch into the waves the catamaran will slow down dramatically -- perhaps more than a monohull with higher (but less comfortable) vertical accelerations?

The catamarans hulls need to be deep enough to allow the increase and decrease in displacement required for the volumetric variation needed to cause the boat to rise and fall through waves, thats no issue as the speed requirement is not high.

Since speed is not an issue very narrow hulls without a lot of depth can be used and they will still rise easily above the wave crests if the bows are loaded lightly enough. Shallow, wide flaring bows serve the same purpose of course, even at higher speeds, but they also increase the vertical accelerations. What I'm getting at here is that we may find it more productive to talk about a specific hull shape rather than talking in such general terms. In this regard ...

Here's the main hull for a trimaran I designed with a 15 knot top speed which is the same as this contest. Just click the image for a larger view:



I've already scaled this hull to match the contest's 18.5 overall length limit. As shown here it is only 2.5' wide which seems more than adequate given the fact that two side-by-side hulls are needed with space in between them.

Three people can sit in tandem in this hull, which means 6 could sit in two of them when used in a catamaran configuration, and that's more than the contest requires.

Then again, there's no need for people to sit in the hulls. We could just as easily put a flat deck between or over the hulls and have everyone sit up high with their feet on deck. So don't be confused by the current picture of this hull, it will probably change dramatically by the time we are through here.

Getting back to the hull characteristics ...

How might we modify this hull to 'improve' it or to make it work effectively as a catamaran hull in our contest entry boat?

By the way, I'm a visual person and pictures always clarify things for me ... so I'll make the proposed changes and post new images as we agree to them. Then we can all 'see' the changes we've been talking about rather than constantly trying to imagine what they may look like.

[Captain Blight]

I'd like to say, you do really nice renderings!

Three pieces of plywood per hull... what kind of freeboard are you looking at in the 'midship area? If a platform (cockpit, or whatever) were built connecting the hulls, would wave slap get to be an issue?

[kengrome]

I'd like to say, you do really nice renderings!

Thanks, but that's the software's fault not mine. I just tell it what to draw and it *usually* cooperates by making my input look nicer than I could ever hope to draw by hand.

What kind of freeboard are you looking at in the 'midship area?

Sorry, I forgot to post the hydrostatics, here they are, don't forget to click the image for a detailed view:




To summarize this info, I chose 1000 pounds as the design displacement, which makes this one hull a 500 pound displacement hull at 8 inches draft (that's the 0.666 ft draft line in the chart). Waterlines in the pictures are 6, 9 and 12 inches (0.5, 0.75 and 1.0 feet). Oops, sorry the other pix don't have any waterlines, here's one that does:




Bow height is only 2.0 feet so *if* the boat were driven too fast in a 3 foot chop it would pierce the tops of some of those waves. Fortunately the peaked foredeck will help the boat slice upwards through these pierced waves, thus lowering the boat's resistance to emerging from beneath them, and dampening the vertical accelerations that cause humans such discomfort.

Running the boat at slower speeds will simply allow the bows to ride up and over the same waves they might have pierced at higher speeds. Yes, speed definitely makes a big difference in terms of the boat's performance in such conditions. The slower you can go the easier it is to deal with problems like wave slap and such, because at slow speeds many issues like these don't even exist.

If a platform (cockpit, or whatever) were built connecting the hulls, would wave slap get to be an issue?

Sure if the waves are big (I should say 'steep') enough and/or if the 'wing deck' (as I refer to it) were built too low to avoid it. That's why we need to discuss and decide upon issues like this:

Do we want two or three beams connecting the hulls or a full deck covering the space in between the hulls? If beams only, do we want them arched up to avoid higher waves, or straight across because it's cheaper?

If straight across, do we want the hulls built taller which makes the boat heavier and gives it more freeboard and less 'wave slap' at the same speeds? Or do we just make the coamings taller and attach the beams (or deck) to the coaming top? Or do we keep the hulls and coamings low-profile and deal with wave slap issue some other way ... or just ignore it because it's not really a problem anyways?

[kenjamin]

Do we want two or three beams connecting the hulls or a full deck covering the space in between the hulls? If beams only, do we want them arched up to avoid higher waves, or straight across because it's cheaper?

If straight across, do we want the hulls built taller which makes the boat heavier and gives it more freeboard and less 'wave slap' at the same speeds? Or do we just make the coamings taller and attach the beams (or deck) to the coaming top? Or do we keep the hulls and coamings low-profile and deal with wave slap issue some other way ... or just ignore it because it's not really a problem anyways?

Ken, my solution is to take two flat iron skiffs with relatively high sides and such narrow beams that they wouldn't work as monohulls but would work fine if they were strapped together.
This idea is a take off on my Down and Dirty Double Dory catamaran (shown here) which was designed for displacement cruising and sailing with a 4HP long shaft motor:



In order to support the weight of a larger motor and to make sure the hulls plane efficiently, I've taken the dory hulls and replaced them with flat iron skiffs. Here's the overhead schematic of what I'm talking about:



The bottoms shown in this overhead view will not be totally flat but have slight rocker in the forward third and maybe a little turn down at the release edge of the hulls like permanent trim tabs. Because my skin burns quickly in the Florida sun, I've incorporated a rigid (but lightweight) sunshade as part of the boat's structure. I have a theory that this sunshade could play a part in stabilizing the boat when it's bouncing across the wave tops in addition to giving me a place to sip lemonade. The sails shown are all capable of being stepped at sea and their masts all store out of the way along the hulls' gunwales. To those who say it won't sail well, please remember there's a 20HP Honda four stroke attached to the central deck so how good does it need to sail to have a little fun with a fresh breeze??? The hulls have no transoms and are completely and rapidly self-bailing with a false bottom 6" to 10" above the true bottom of the boat. All beams that tie the boat together are straight but laminated with many layers of solid wood and lashed to the hulls for flexible strength. What do you think?

[Cedric Rhyn]

Pretty picture Ken but that outer chine line is way too steep, it will force the boat to run bow down which makes it tend to try to pivot around the forefoot . And 2 1/2 ft beam on each hull of a small cat? Way too wide, why dont you read John Welsfords postings on the other thread, although known for his small sailing cruisers his commercial work covers this sort of boat and he's been there and done it when it comes to power multihulls. Another guy who knows his onions in this area is Chris Ostlind of Lunada Design.

Cedric Rhyn

Thanks, but that's the software's fault not mine. I just tell it what to draw and it *usually* cooperates by making my input look nicer than I could ever hope to draw by hand.

Sorry, I forgot to post the hydrostatics, here they are, don't forget to click the image for a detailed view:




To summarize this info, I chose 1000 pounds as the design displacement, which makes this one hull a 500 pound displacement hull at 8 inches draft (that's the 0.666 ft draft line in the chart). Waterlines in the pictures are 6, 9 and 12 inches (0.5, 0.75 and 1.0 feet). Oops, sorry the other pix don't have any waterlines, here's one that does:




Bow height is only 2.0 feet so *if* the boat were driven too fast in a 3 foot chop it would pierce the tops of some of those waves. Fortunately the peaked foredeck will help the boat slice upwards through these pierced waves, thus lowering the boat's resistance to emerging from beneath them, and dampening the vertical accelerations that cause humans such discomfort.

Running the boat at slower speeds will simply allow the bows to ride up and over the same waves they might have pierced at higher speeds. Yes, speed definitely makes a big difference in terms of the boat's performance in such conditions. The slower you can go the easier it is to deal with problems like wave slap and such, because at slow speeds many issues like these don't even exist.

Sure if the waves are big (I should say 'steep') enough and/or if the 'wing deck' (as I refer to it) were built too low to avoid it. That's why we need to discuss and decide upon issues like this:

Do we want two or three beams connecting the hulls or a full deck covering the space in between the hulls? If beams only, do we want them arched up to avoid higher waves, or straight across because it's cheaper?

If straight across, do we want the hulls built taller which makes the boat heavier and gives it more freeboard and less 'wave slap' at the same speeds? Or do we just make the coamings taller and attach the beams (or deck) to the coaming top? Or do we keep the hulls and coamings low-profile and deal with wave slap issue some other way ... or just ignore it because it's not really a problem anyways?

[kenjamin]

Try and think of my power cat hulls as two flat iron stiffs or if you prefer surfboards. There is absolutely nothing that will cause these two mostly flat bottomed hulls to ride nose down once they are on a plane. In fact, as I said, I would add a slight downward turn to the last foot or so of the bottom to help keep their noses down. Do I have to drag out the picture of the upsidedown diner table on a plane???? The hulls are way too wide for a sailing cat but for a light cat with twenty horses at the prop, these hulls can not help but plane. Any sailing it's able to do will be just bonus points.

[Captain Blight]

Kite rig!! Packs up into a package the size of a medium daypack, and I can teach anybody the basics in two hours. Might call for pedal steering with that, though.

Just something to think of.

>>>If you were to raise the coaming another 2" all the say around; and maybe peak at 4" higher at the front of the cockpit; and then carry that arc across to the other hull--ya folla?-- then that would give you the transverse arc for a connecting beam. Coincidentally, probably a reasonable height for a center platform/deck/bathtub/thing.

[kengrome]

Ken, my solution is to take two flat iron skiffs with relatively high sides and such narrow beams that they wouldn't work as monohulls but would work fine if they were strapped together.

Your hulls are much simpler to build than mine. If we are going for building economy then yours are certainly better than mine. I probably should have started from scratch rather than posting a trimaran hull anyways since its features may confuse people ...

The bottoms shown in this overhead view will not be totally flat but have slight rocker in the forward third and maybe a little turn down at the release edge of the hulls like permanent trim tabs.

We might want to skip the downturn and design the aft 1/2 or 2/3 of the hulls perfectly flat. Not only does this make construction easier, but adding a downturn later is easier than removing one that shouldn't have been there in the first place. I tend to think of these things because whenever I design a boat I'm constantly thinking of building it too.

The hulls have no transoms and are completely and rapidly self-bailing with a false bottom 6" to 10" above the true bottom of the boat.

This is a nice feature. One of my monohull concepts for this contest is to build a water ballast chamber into the hull below the cockpit sole. Why would a power boat need such 'ballast'? Well, because lightweight power boats sometimes have problems punching through a chop, and adding weight (but only if necessary in certain conditions) adds momentum to help punch through the chop. It also reduces the vertical accelerations JW was talking about earlier, which makes for a nicer ride ...

All beams that tie the boat together are straight but laminated with many layers of solid wood and lashed to the hulls for flexible strength.

Excellent! It's a shame Americans don't think of 'lashing the beams to the hulls' as being a good practice, but it is, not only in terms of maintaining flexible connections to alleviate destructive stresses but also in being able to fix these connections very easily while at sea. It makes the boat easier to disassemble at home too.

Personally I like your concept a lot, with or without the sail option ...

[kengrome]

... that outer chine line is way too steep, it will force the boat to run bow down which makes it tend to try to pivot around the forefoot.

Just remember, this hull came from a trimaran with two forward mounded amas providing both static flotation and dynamic planing lift forward -- so the problem you identified would never occur in the original boat. We can change the hull shape as we go along so that it actually works well as a catamaran hull, or we can start with simpler hulls like kenjamin's ... but now that you've mentioned it I should probably list the reasons for using this second chine:

1- to create gradual additional lift as the boat increases in speed, thereby improving efficient semi-planing performance

2- to create a full length structural stringer that strengthens the hull and supports the sides without the use of transverse frames

3- to create a built-in spray rail that deflects water outward so the passengers will remain dry as speeds increase

And 2 1/2 ft beam on each hull of a small cat? Way too wide ...

If you look at the stats I posted above, you will see a 20 inch waterline beam. Please don't forget that overall dimensions don't affect performance, it's the waterline dimensions we should take notice of, especially in boats with as much hull side flare as this one.

John Welsford ... he's been there and done it when it comes to power multihulls.

I don't know who you're confusing him with, but JW seems to focus exclusively on monohulls, and he prefers monohull sailboats at that. Have you ever seen a multihull designed by JW? If so I would love to see it, especially a power catamaran. Can you post a link?

[Woxbox]

The wateline beam is the issue, I think. At 20" on the waterline, you're just shy of a 10:1 beam/length ratio. That would be good for around 8 knots discplacement speed. What power will it take to bring it up on plane to acheive 15 knots with four adults? Or, as you state, 500 pounds per hull, which seems a good number to work with to me, too.

I can't say I know how to do that calculation, but I suspect this proposal is too wide on the waterline to achieve the performance as a wave piercer, but possibly not wide enough to lift up and plane reliably. I see a need to commit to one direction or the other.

My other thought is that for rough water, those bows need to be higher. Even if the decks do shed water, who wants a boat that will throw gobs of water in their face? Note how deep/high the power cat hulls manufactured now are. There's a good reason for that.

I was thinking of the Bolger power boats, such as the Sneakeasy, which use an almost dead flat bottom to acheive planing as easily as is possible. Keeping the bottom flat and giving the hulls nearly rectangular sections reduces the draft as much as possible on a given length and beam. And reducing the draft as much as possible makes getting up on plane that much easier.

This suggests a boat that pounds, but if the hulls are very fine, perhaps it's not a real issue? Thoughts?

[Chris Ostlind]

... Just remember, this hull came from a trimaran with two forward mounded amas providing both static flotation and dynamic planing lift forward -- so the problem you identified would never occur in the original boat.

Was this a power trimaran you describe? Could you provide a link to the boat being used as an example in this reference?

If you look at the stats I posted above, you will see a 20 inch waterline beam. Please don't forget that overall dimensions don't affect performance, it's the waterline dimensions we should take notice of, especially in boats with as much hull side flare as this one.

While below the waterline dimensions do create much of the performance potential of any craft, the above the waterline structure is often overlooked as a performance inhibitor. There is a very large wind drag component for any boat that is intended to see 15 knots and making the forms wider, or taller, or slab sided, as presented to the direction of travel, will very much slow the boat compared to a more streamlined design.

When the superstructure component is balanced for intended utility and its lowest drag signature, you have arrived at the technical optimum. This may not always be the most aesthetically pleasing solution, so there is that to consider as well.

[kenjamin]

Catamarans by Chris Ostlind:





It's obvious that you can draw a great looking catamaran for sail. Have you thought about designing a power cat for the contest?

Or would you be willing (for a fee) to work out the required calculations for my double flat iron cat???



I do not have the training to prepare the estimated speeds and fuel consumption calculations, but I'm pretty darn sure that a twenty horse motor will plane both the hulls on my powercat above. When I was a teenager, I had a 13 ft. flat bottomed skiff which only had a little rocker forward and I don't think the bottom was much wider than about 40". I had an old Johnson 10HP motor that would jump that skiff on a plane in a hurry and remember that back in those days horsepower was measured at the crank rather than at the prop as it is today. So with more power available and sleeker hulls (but still with flat bottoms) there's no doubt that these hulls will not plane with ten horses per hull available even with two adults per hull aboard. But how do you prove something like that without building it full sized first?

kengrome, thanks for your positive feedback. If one were careful enough, it seems like marine foam below the false floor would be a better choice than water – much lighter, adds strength and stiffness to the structure and most importantly for most of its life it will float you above cold water or warm water sharks. People moan and groan about marine foam and how difficult it is to use but as and old surfer, I would feel much better knowing those big chunks of foam where contained by my hulls. Sure if there's a crack in your hull you've got to dry it out and fix it right away or it will rot your plywood. But what's more important – your life or the life of your boat?

[kengrome]

If one were careful enough, it seems like marine foam below the false floor would be a better choice than water ... much lighter ...

I guess you missed my point about the water being HEAVY so it will add momentum and help the boat punch through a chop ...

and most importantly for most of its life it will float you above cold water or warm water sharks.

Yes it will, and so will air ... and air is lighter and cheaper than foam, and much easier to install and replace too.

People moan and groan about marine foam and how difficult it is to use

Maybe you hear different people than I do. In my experience the folks I hear talking about using flotation foam in boats never complain about how hard it is to use. What they complain about is that it ALWAYS attracts and traps and holds moisture inside the sealed foam compartment, thus making the boat 'gain weight' and exposing the wood to constant moisture which promotes its rapid destruction via rot.

as and old surfer, I would feel much better knowing those big chunks of foam where contained by my hulls.

Well, as a builder I would feel far better knowing I could empty those compartments and allow fresh, clean, dry air to flow through them when I'm not using the boat on the water. As a boater I also recognize that even if my compartment were holed my boat still wouldn't sink because the cockpit sole still hasn't been holed so the water will remain below the cockpit sole.

By the way, I usually put empty soda bottles, inflated beach balls, and other floating items inside a net bag in this compartment. The net keeps them from escaping even if the chamber gets a huge hole in it, or if I forget to close the access ports and a huge wave fills the cockpit.

Regarding my flood-able water ballast chamber concept ...

Basically I like to use features in my designs that offer more than one advantage. A flood-able and purge-able sealed chamber below the cockpit sole serves two purposes ... or perhaps I should say it gives you "two boats in one":

1- A light weight vessel that can easily run at planing speeds in nice conditions when the sealed chambers are full of air instead of water.

2- When the chambers are filled with water, a heavier vessel that gives its occupants a more comfortable ride, with the momentum needed to punch through a chop and make better forward speeds in conditions that prevent planing.

By installing access ports these sealed chambers can be opened and aired out naturally in between uses, too. If you don't want to bother with access ports you can always force dry air through them with an air compressor, or even a household vacuum cleaner ... but personally I favor the access ports not only for their convenience but also so I can 'take a look inside' and see things today that might create problems tomorrow.

[Cedric Rhyn]

I don't know who you're confusing him with, but JW seems to focus exclusively on monohulls, and he prefers monohull sailboats at that. Have you ever seen a multihull designed by JW? If so I would love to see it, especially a power catamaran. Can you post a link?[/quote]

I’m not mistaking John Welsford for anyone Ken, he is well known for his small monohulls, and that’s what he sells plans for so there is a lot of information around on those boats, but he also does other custom design work both in collaboration with other design companies, with a university that he sometimes lectures at, and on his own account. Some of this work is high tech racers both sail and power, and some have been in the area of power multihulls, the latest of which is a 40 ft power trimaran with a 1500 N Mile range at about 20 knots. There are very few pics of this work about as he prefers to keep that sort of stuff between himself and the customer.

I know also that some of his last years college students built power trimarans that were achieving around 20 knots with only an 8HP Yamaha outboard for power. There is research going on at that University and he has been instrumental in some of the slender hull program there.

Like I said, I am not mistaken at all.

Good to see Chris Ostlind pitching in with his thoughts, another man whos overly modest about what he knows.

Cedric Rhyn.

[kenjamin]

Cedric, which university is it that has a slender hull research program? I'm looking for a graduate student who might be willing to do the necessary calculations for me (for a fee, of course) so that I can enter my Double Flat-Iron Cat in the contest.

kengrome, I hear what you are saying about foam trapping moisture. When we surfers dinged our boards, we would patch the area quickly with duck tape so we could keep surfing without the board taking in much moisture. I understand your priorities. You produce a product that you need to last well so that others will also want your product. My priority with my design is to win the contest. To me the advantages that foam brings to the design outweigh its maintenance disadvantages. Air is good but it can escape while the foam can not. Having no transom on a boat is great for ejecting water out of the hull but only if you have a sure fire way of keeping the false bottom above the waterline. There's also the fact that the foam does have mass and stiffness that will aid the boat in punching through waves. With very slender hulls, they don't need much help especially since there are twenty horses driving the hulls. For a production model of my Double Flat-Iron Cat, I would be curious about doing the hulls in recycled aluminum with laminated wood for the beams and marine ply for decking. That way the moisture problem with the foam would not be as much as an issue and neither would metal fatigue because even if cracks developed with time, the metal would still contain the foam. Of course the worst the cracks get, the wetter the foam gets and eventually dies but that could take twenty years or more if the boat lives under a cover on a trailer in the shade. A simple boat hull that can be quickly bonded together by robots on an assembly line may be cheap enough so that the owner will not mind unlashing the hulls every ten to twenty years or so and replacing them. Plywood hulls as simple as mine are ,on the other hand, could be slapped together by a home builder in one weekend. With the right coverings of cloth and epoxy and careful use of rub rails, the plywood may never fail enough to wet the foam. The application of the foam into a plywood sided space is the trickiest part and that may require very good instructions that are actually read and followed closely. Or maybe, like with surfboards, blanks could be manufactured with molds and made available as part of a kit.

[kengrome]

Having no transom on a boat ...

There's a transom at the aft end of each of your hulls. What boat are you talking about that has no transom?

The foam does have mass and stiffness that will aid the boat in punching through waves.

You're not seriously expecting me to believe that foam weighs enough to produce a noticeable benefit in terms of added momentum, are you?

With the right coverings of cloth and epoxy and careful use of rub rails, the plywood may never fail enough to wet the foam.

That's not how foam gets wet in the first place. It gets wet when water vapor passes through the hull and then condenses inside the compartment ... and never escapes because the foam retains this moisture in its liquid state from then on.

----------------------

kenjamin, I like your simple hull concept but not your ideas about using foam. If anything, your boat would be better if the sides were built as a composite sandwich with foam as the core material. At least then the foam would be higher in the hull where it belongs, and you'd have useful space below the cockpit sole for storage if you didn't want to bother with the water ballast chamber I suggested earlier.

Just my opinions of course ... and although I don't agree with some of your ideas at least you're not afraid to suggest things few others have conceived of or are willing to propose.

Carry on!

[kenjamin]

Yo kengrome,

The no transom boat is not the one that has a side view with a single sail. That one is my Down & Dirty Double Dory designed to sail well and motor with a 4HP long shaft outboard. The boat with no transom is the one with only a top view schematic showing multiple sails. It is a take off on the Down & Dirty Double Dory but it has a much wider bottom especially aft for supporting the heavier 20HP motor and for reaching a plane with four adults aboard as the contest requires. I call my contest boat entry the Double Flat Iron Cat because that's basically what it is. The bottoms are basically flat with maybe a little rocker in the first third of their lengths.

I've enjoyed taking with you about the designs and it really helps me to get another opinion. Since I have not suffered any nightmares and catastrophes associated with marine foam, I am probably naive about its use. If by using waterproof hatches and air compartments instead of foam, I could be considered a more competent boat builder then that alone may justify skipping the foam and reconsidering air. But just for the sake of discussion, as for the mass of foam, it does come in different densities with the 2 lb. per cubic foot being most widely used for flotation but the 4 lb. per cubic foot is also available. This is not much weight difference considering the amounts needed under a false bottom but I suspect the additional structural stiffness that the 4 lb. stuff offers would be substantial. However, I'm sure you could get the same kind of stiffness by using longitudinal 3/16" ply partitions that would stiffen the hulls and provide for separate air chambers so that all your eggs are not in the same basket. It's been said that if the Titanic's bulkheads had gone all the way to the top of each compartment so that the chambers were completely separate, that she would have listed heavily but not sunk. I don't mind having a hull breach that causes my boat to list to one side but I am definitely the "chicken of the sea" when it comes to having a boat that can sink or become a floundering useless mass stuck too deeply in the water.

[Captain Blight]

Cedric, which university is it that has a slender hull research program? I'm looking for a graduate student who might be willing to do the necessary calculations for me (for a fee, of course) so that I can enter my Double Flat-Iron Cat in the contest.

kengrome, I hear what you are saying about foam trapping moisture. When we surfers dinged our boards, we would patch the area quickly with duck tape so we could keep surfing without the board taking in much moisture. I understand your priorities. You produce a product that you need to last well so that others will also want your product. My priority with my design is to win the contest. To me the advantages that foam brings to the design outweigh its maintenance disadvantages. Air is good but it can escape while the foam can not. Having no transom on a boat is great for ejecting water out of the hull but only if you have a sure fire way of keeping the false bottom above the waterline. There's also the fact that the foam does have mass and stiffness that will aid the boat in punching through waves. With very slender hulls, they don't need much help especially since there are twenty horses driving the hulls. For a production model of my Double Flat-Iron Cat, I would be curious about doing the hulls in recycled aluminum with laminated wood for the beams and marine ply for decking. That way the moisture problem with the foam would not be as much as an issue and neither would metal fatigue because even if cracks developed with time, the metal would still contain the foam. Of course the worst the cracks get, the wetter the foam gets and eventually dies but that could take twenty years or more if the boat lives under a cover on a trailer in the shade. A simple boat hull that can be quickly bonded together by robots on an assembly line may be cheap enough so that the owner will not mind unlashing the hulls every ten to twenty years or so and replacing them. Plywood hulls as simple as mine are ,on the other hand, could be slapped together by a home builder in one weekend. With the right coverings of cloth and epoxy and careful use of rub rails, the plywood may never fail enough to wet the foam. The application of the foam into a plywood sided space is the trickiest part and that may require very good instructions that are actually read and followed closely. Or maybe, like with surfboards, blanks could be manufactured with molds and made available as part of a kit.

Kenjamin, if you're not a member of Boatdesign.net, join immediately! It's free and a great resource.

I hear what you're saying about the foam/moisture issue. For the volumes of the flotation tanks you're talking about, though... why not Ping-Pong balls? They would displace enough space to provide some real flotation even if their containing tank were breached, and they wouldn't trap moisture the way foam would. If you lined their containment with a layer of 12-oz glass, there wouldn't be much of a weight penalty and I'd think the peace of mind would be worth it.

[kenjamin]

Thanks Captain, I'll take that advice on Boat Design.net

Ping pong balls provide for zero longitudinal stiffness while marine foam (especially the denser stuff) will add quite a bit. Correct me if I'm wrong but the winning design may end up being a production prototype. The technical expansion and bonding problems associated with marine foam and marine ply may be overcome in a controlled factory environment or a switch could be made to a more compatible hull skin like (dare I say it?) aluminum or foam/composite.

Secondly, if a Great White takes a bite out of your boat and decides he likes the popcorn-like texture of ping pong balls, the ones that don't get eaten will surely escape and and what's to keep him from taking a second bite??? Everyone knows they never take a second bite of surfboard foam (just don't ever let your body parts be any portion of the first bite). OK, I'm just being silly now but the truth is almost anything can happen out there. I once had a giant Manta Ray jump way up in the air and almost come down on me and my 13' skiff. He was a lot bigger and heavier than me and my boat.

[john welsford]

[QUOTE=kenjamin;1888964]Cedric, which university is it that has a slender hull research program? I'm looking for a graduate student who might be willing to do the necessary calculations for me (for a fee, of course) so that I can enter my Double Flat-Iron Cat in the contest.

There are programs running at several Universities, and Massey Universities School of Transport Design where I was working last year had several 3rd year students working on slender hull power multihull theory. But that organisation is about as far from you as its possible to get and still be on the same planet ( do I recall rightly you are in the UK?) and you may be much more likely to get someone with whom you can work by contacting Southampton Uni, or even by ringing the RINA and asking who is using the Denny Ship testing tank. Note though that Universities normally charge serious money, and their grads are usually flat out with regular work.
You might be better off joining the RINA ( Royal Insitute of Naval Architects ) as an amateur member and fossiking though their incredible library of publications, or even the AYRS ( Amateur Yacht Research Society ) who have a very interesting range of booklets some of which might be helpful.

JohnW

[john welsford]

Sorry Kenjamin, Talahassee , but thats still a third of the globe away, and both of those organisations work by post.

JohnW

[QUOTE=john welsford;1889476]

Cedric, which university is it that has a slender hull research program? I'm looking for a graduate student who might be willing to do the necessary calculations for me (for a fee, of course) so that I can enter my Double Flat-Iron Cat in the contest.

There are programs running at several Universities, and Massey Universities School of Transport Design where I was working last year had several 3rd year students working on slender hull power multihull theory. But that organisation is about as far from you as its possible to get and still be on the same planet ( do I recall rightly you are in the UK?) and you may be much more likely to get someone with whom you can work by contacting Southampton Uni, or even by ringing the RINA and asking who is using the Denny Ship testing tank. Note though that Universities normally charge serious money, and their grads are usually flat out with regular work.
You might be better off joining the RINA ( Royal Insitute of Naval Architects ) as an amateur member and fossiking though their incredible library of publications, or even the AYRS ( Amateur Yacht Research Society ) who have a very interesting range of booklets some of which might be helpful.

JohnW

[kenjamin]

Thanks JohnW, for the information. In this day and age I should be able to find someone who will work with me using e-mail and web sites. At least I hope so.

My Double Flat Iron Cat is not much more than a napkin drawing (in Adobe Illustrator) at this point but I'm 99% sure it will work as a power catamaran, I just need someone to run the figures for me. I've recently found a place on campus here at FSU where I can go and get a 3-D scan of my model hulls. From those files I can redraw them to clean lines in Adobe Illustrator and then back to a 3-D drawing program like Vellum. It's a round about way of working but as long as I get there I'll be happy.

Here is what I'm up to:

If you take John Gardner's Flat Iron Skiff from his book "The Dory" and split it in half, stretch out the bottoms, make them symmetrical, separate them, ,then strap them together some distance apart from each other, apply more than twice as much power with the weigh of the motor moved forward for better balance — that's all I'm talking about doing. Sure, initially there will be more wetted surface but after it gets on a plane the difference of wetted surface becomes much smaller. I'm thinking what also becomes smaller is the slap and pounding you normally get from a flat bottomed hull. By separating the flat bottoms, each hull will be encountering wave chop at different times so that the boat can flex and average out the differences. The amount of flex the boat imparts to the hulls can be fine tuned by using different amounts of laminations to the straight beams that hold it together. The stiffer the whole thing is the greater the efficiency but maybe at the cost of comfort so somewhere there should be a balance where the hulls can react independently of each other but with enough stiffness in the boat as a whole so that the hulls can help stabilize each other. At least that's the way I see it. What do you think?