Design idea for small open "displaning" multihull

[huibes]

During a long train ride I had an idea and started drawing...See below.

My boat is restricted in length, but has to have good properties in the full speed range up to 20 knots. I would also like the propellor to be protected while beaching/storing without raising the outboard. So it has two 130mm wide ama's that go as deep as the deepest point of the outboard. These ama's have a wider flat at the level of the cavitation plate. At that same level there's also a 500mm wide planing centre pod/vaka. So low resistance displacement hull for low drag at low speeds and the possibility to go 8 knots efficiently, but also a reasonable flat area to avoid the squat and plane at higher speeds.

The objectives:
Max length 370cm to fit storage
Propellor protected when beached
Max speed at least 20 knots
Able to efficiently cruise a little above the max speed in the channels here, say 8 knots.
Good interior space
Wave piercing
20hp?
Dry ride
Stable
Max speed with four persons still 12 knots.
Suitable for inland water and protected coastal area.
Used for cruising, fishing, people transport.

Potential problems that I can see with my limited knowledge:
I kept the bridge clearance only 140mm above the planing flats, for interior space. Would this give any problems?
Strong contruction of narrow part of ama's might be a challenge
Flow of water between the hulls
Drag of displacing hulls at higher speeds, anybody know the max speed of ama's: length x width x depth - 3600 x 130 x 230 mm??
Would the two deep 'keels' allow cornering?

I'm thinking of building a cheap model soon, but scale 1:1. Just to see if it works at all. I have read many really interesting threads on this forum before, and it would be great if I can get any tips or hints or links to other similar designs. Thanks!

Sorry about the drawings, not at 3d CAD level yet...

[huibes]

Anybody?

Even a 'your an idiot, this will never work' will do...Preferably with reason why of course....

Or 'I'd like to hire you and your refreshing ideas as a naval designer' that's ok too...

Already made a model in fluteboard which was very stable and tracked very wel and had low drag floating on the ama's without the flats touching the water. But didn't tell me much about cornering and forces on the deep, narrow ama's.

Any advice from experience please!!

[marshmat]

That's got to be the smallest SES (surface-effect ship) I've ever seen. (It will likely get a fair bit of lift at top speed from the 'ram effect' of air and water in the tunnels.)

I'm a bit mystified by the forward well-mounted outboard engine. It's probably because of your length restriction (really, only 3.7 m?) but it still looks a bit odd.

Performance predictions on such a craft are a bit tricky, as the standard methods for "normal-looking" boats won't work very well here. Your plan to build and thoroughly test a model is, IMHO, a Very Good Idea.

[Willallison]

Huibes,
Well, it's certainly an interesting idea! The problem with small cats like this is that it will be a very complex build and almost certainly heavier than a boat comprised of simpler shapes. Being a displacement cat means that as the speed increases there will be no appreciable lift generated, so the low bridge deck will certainly slam in any sort of chop, which would somewhat negate the wave-slicing advantagees of those slender hulls.
Constricted length, low bridge-deck height and weight are probably the 3 most important things to try and avoid in a displacement cat....

[huibes]

Matt,
I wasn't planning any SES systems...Would there be much ram effect at only 20 knots? I wasn't targeting at that, because that would give drag at low speeds wouldn't it?

You're right, the outboard is 'inboard' for length restriction, so that there is still usable space next to the outboard and extra length. Also puts weight forward, gives sound insulation and for non-outboard look. Do you expect any problems?

Will,
I understand to get this complex shape strong it will be heavier, but also heavy compared a boat with similar interior space? (Btw, on those drawing the ama's are 100 mm. Should be 130 mm)
Also I was hoping that with the buoyancy and flat of the centre pod, weight would be a less crucial factor than with a normal displacement cat??

The plan is to design the bow sections that they will lift slightly at high speeds, but still have the full length in the water. Just less deep up front, raising the bridge clearance up to about 350 mm. I was also hoping the V of the centre pod to break waves...(slamming can only be less than the rib its replacing!)

What do you guys think will happen in corners??

Any tips for improvement, or suggestions for other hull shapes to get my requirements within the restrictions?

Thanks!

[Easy Rider]

Given enough centrifugal force it could flip. Ama's would say "stay on course" and topsides would say "I want to go sideways". Since the ama's are low and the topsides are well above ..over she goes ..maybe.

Easy Rider

[huibes]

Can you recommend any alterations to the design to make this less? I know that planing cats like a bit of lift sideways, but I'm not sure an angled ama bottom would help in this setup (?).

I did consider tipping, but (rough) calculations show that at 20 knots its very unlikely to tip over this wide vessel with reasonably low centre of gravity. Having a steering wheel instead of a tiller will also help, because of less direct steering.

The same calculations also show that the force needed for tipping it, might be higher than what the submerged part of the ama's can take at the given dimensions. But then I still need to think of a way to mae them stronger anyway. Suggestions?

Am pretty sure though that people shouldn't forget to hold on during cornering...

Thanks for you comment!

[Easy Rider]

Make the amas w 45 degrees deadrise and make the boat wider so the "keel" of the amas is in about the same position. The boat will float higher, plane faster, pound less, have greater stability and greater volume as it will be bigger. And if you ever wind up sideways on the face of a large wave the boat will sorta plane sideways down the face of the wave. The top of a wave rushes fwd and the ability to slide sideways can keep your high side from getting much higher. Been there done that .. as they say.

Easy

[FAST FRED]

http://www.stormportboatworks.com/

This is claimed to run 18K , at an amazingly low fuel burn.

FF

[Easy Rider]

Fred,
My kind of boat mate. I'm a fan of long and narrow too. How's things on TF? Last time I was there there were a few new guys and new messages (of course) but pretty much the same old stuff.
I still want to build a 26' Atkin skiff for 12 knots w 60hp OB.

Easy

[Ricardo Ramalho]

The outboard placed 'inboard' for length restriction is not necessary, if the concern is some law requirements (at least according with european laws), because the outboard motor is not considered part of boat length.

I have been thinking about the same design, but for a self-righting displacement hull, with cabin. But it would not be a classic twin keel (nor a bilge keel), it would be like your drawing, to provide a cabin with a full standing headroom along the sides. There would be no tunnel. The boat needs to be trailerable. I believe this solution would increase stability, in a very narrow boat with cabin.

[F3M4]

Originally Posted by FAST FRED

http://www.stormportboatworks.com/

This is claimed to run 18K , at an amazingly low fuel burn.

FF

Thats a slick looking boat.