Best Design for Minimalist Trailerable Coastal Cruising

[skyl4rk]

Is that a planing or displacement hull? At the design weight, are the sponsons in the water all the time or just when it heels?

[Claus Riepe]

Coastal dayboat with classic looks, trailerable, waterballasted:

http://www.swallowboats.co.uk/content/view/102/64/

I have the bigger sister, the 22ft. SeaRaider, but that is more in the raid racing line, not exactly for cruising in comfort.

Claus

[Raggi_Thor]

I like that one, the BayRider :-)

[Vega]

Claus, it is a very nice boat, but really minimalist, in what regards cruising

The 22ft is also a good looking one...and the stability is impressive, for a small daysailer.

[Chris Ostlind]

Skyl4rk,

Sponson Contact: The boat has a 6" hull draft when loaded. It has to heel 10 degrees to get the sponson to contact smooth water.

Planing: I'm not really looking for this boat to get on a plane. I was looking for ride comfort, modest cruising speeds and great mileage. This is a pretty light boat at 600 lbs displacement, so I wasn't looking to get it going very fast where the sea could beat the crap out of it on a short cruise down the coast. Hence, the small outboard, the small fuel tank and the easily driven hull shape.

[Willallison]

In my case, I was looking at a displacement hullform, with the inflatable tubes just clear of the surface at the transom, sweeping up to the bow well clear of the water - thus able to provide stability in what would otherwise be a very unstable hullform and giving ever increasing reserve buoyancy should you manage to bury the bow.
I guess a roll of only a couple of degrees before making contact with the water.
Hopefully I'll get a chance to do some modelling soon - I'll post some pics as soon as I have...

[Claus Riepe]

Originally Posted by Vega

The 22ft is also a good looking one...and the stability is impressive, for a small daysailer.

Indeed the stability is crucial for coastal racing, as is the full RCD compliance for a category 'C' certificate through option No. 7.

There, the waterballast system has been a positive surprise, how efficient and versatile that is for such small trailerable boats. Swallowboats have really done a good job designing it. They did a test knockown for the press in Scotland. The boat righted like a cork, and within two minutes the crew was back aboard and she was sailing again in full swing.

Claus

[mattotoole]

Why not just combine the boat and trailer, and go amphibious?

[Willallison]

Early renderings of the displacement monomaran

[Chris Ostlind]

I like it Will. Very sleek. Do you have any figures on the L/B?

Did you look at any of the numbers with the added drag of the sponsons in a chop. That was what drove me with the above-water-flare on my hull. I've done a couple of Sit-On-Top kayaks that had a narrow beam in the water with a more or less, conventional, above-the-line hull flare and in a chop, they just turn to piglets. Though the enhanced stability is nice.

I suppose it's that fine line of compromise that will get you every time. Have you done any RIB's with similar, sponsons down low for "right now" firming-up? I have no experience in that area at all, so I'm doing a bit of speculating on this skinny cruiser idea. I had this fear that the boat would just fall over, get comfy there and not come back up again without a load of merit down low. So, now, that's my next task to address.. the placement of all the works and stuff.

Then, there's always the potential of an aft set of amas and forget about the flare.

Anyway, it looks very clean and efficient.

Chris

[Willallison]

I knocked up the model last night more to give you an idea of the shape I was envisaging than anything else. It needs quite a bit of refining. For instance I would proabably tighten up the radius of the chine a bit towards the transom.
From memory, the L/B was 11.5:1 - a bit or tweaking should increase that to 12:1, which if I recall correctly is the minimum desribale for a displacament catamaran hull
I've ridden in a couple of small (dinghy) RIBS that have tubes that almost touch the water near the transom whilst underway. They were very stable - and quick. Having the tubes toughing the water at rest made them very stable - as all inflatables tend to be. But of course, they were planing hulls, so lifted as they sped up, which mine wouldn't do

[Vega]

Originally Posted by Willallison

Early renderings of the displacement monomaran

It looks good

[skyl4rk]

Is there a way to get high efficiency at displacement speeds while still having a hull shape that will get up on plane?

[Chris Ostlind]

Both of the designs illustrate a slender hull displacment design concept that takes advantage of low power and high fuel efficiency. They would perfom much like a single catamaran hull, allowing much faster displacement speeds from the available power source.

Either of these designs could plane if one were to apply the necessary power, though the weight aft would change considerably from the larger engine, as would the low speed characteristics of the design, the overall balance fore and aft and the fuel consumption. The last reason is the most significant for a boat designed to do minimalist coastal cruising in this mode. The need to carry considerable fuel for the planing vessel changes everything about the design ideas and results in a much different boat.

In general, slender hulls do not plane as easily as do wide examples Will may have differing ideas on that approach, but that's where I'm coming from.

[Willallison]

Hmm - it's an interesting side-topic this one. I think it was covered - at least to some extent - in a thread titled slender planing hulls, or something similar.

I would sort of agree with Chris. A short, fat hull will get onto the plane at a lower speed than a long skinny one. But as speed rises, the drag affects the wider boat to a much greater extent. If you have acopy of Dave Gerr's Nature of Boats he discusses the idea of very skinny high speed boats, though not in any great technical detail.
There was a paper put out just recently - the author escapes me at the minute - about the subject. It was of a more technical nature, but still quite easy to understand without being a mathematical genius. "High speed slender planing hulls', or something like that.... Maybe someone else can recall....
...oh - age - it's a terrible thing - though far better than the alternative!