6.5 to 7.5 metre performance/cruise multihulls

Once more----everything old is new again.
That is just a re-vamped version of the Crowther International 23.
It was a great racing one. Sliding beams which actually worked.
We used to keep one in the warehouse where "Baggers" was built. We would tow it to Sandringham YC, launch it, then open it up on the water, Very easy to do as the hulls were supported by the water.

 

OS7;
Looks more like a small GBE to me, and it has daggers.
My mate had an International 23, massively overweight solid glass hulls, hopeless on the wind with fixed keels.

 

Not my experience RR. We won races with our IT23.

 

Turned the 650 over and glassed it. That's the bow in second shot.

 

looking like a boat! so thats what, 5 sheets so far?



Barry

 

Filling and fairing, quite boring but there you are.
Yes, 5 sheets - a few offcuts left over.

 

The ray is coming together now Gary

 

Eagle or stingray? Maybe I should call it the latter?

 

how bout the humBOLT! looks a bit like the squid of same name.


Barry

 

About taking up the shroud loads... I'm looking at this as if the crossbeam structure extends continuously from ama to ama. Somewhat as a ladder-like for thinking about it.
Rig loads are mostly generated from the leeward ama - across the boat - to the windward ama - to the chainplate - and up the w/w shroud to the mast hounds.

In your case "across the boat" is (can be) continuous withing the crossbeam itself. no transverse rig loading at the aka/vaka join at all (Yes, rig loads from the headstay need to be handled at the Hull/aka join but those are longitudinal - so a different prospect all together, maybe done with some type of fore and aft truss, maybe even with synthetic line instead of fixed).

No need to even fasten the shrouds to the ama, just draw out your shroud angle (mine was prox 23 degs of sweepback for the 44' Gougeon B-section on the Twiggy) on the crossbeam and mount the attachment point more inboard.

Size the rear (athwartships) bulkhead of the aka, making it deep enough toward the middle to take the loads to eliminate waterstay hassles. Replacing the waterstay load path with a fixed path at the bottom edge of the aka might be done with as little depth as 18 inches (50mm) considering that you can bump the strength in tension with c/f

The boat already looks great - a speeding bullet we'd call it over here - congrats on your amazing and apparently completely ad hoc building approach.

Cheers,
TomH

 

Looks a nice boat. Although I don't like the daggers offset to the outside of the hulls - safer on the inside if you have to try and operate one by hand. The price kind of counts against us ever seeing any down in this neck of the woods. Including a few options like a trailer freight and GST could easily go over 60k $NZ. As OS7 (Hi Paddy) points out an I 23 would function similarly, and be somewhat more affordable.

Shame we never had a (commercially successful) Strider or International 23 equivalent in NZ. 7 x 4-ish metre cats are a useful size for daysailing and camp cruising.

 

For redreuben,

Pic of the real Mini GBE - the Cresta Chevron 7 metre telescoping cat. Molds were evidently fashioned over a cut down GBE hull, instead of following Malcolm's original (and IMO rather unusually for him, awkward looking) plans. I understand a few of these were sold to Aus, but they never took off here.

 

Thanks Tom, good suggestions coming from your experience with the Twiggy.
Because Sid 8.5 and whatever the 6.5 is named, has/have long beams I don't need to take the shrouds right out to the beam/float junction but take them to the halfway mark - and that still gives me a wide staying base, reduced compression loads and good staying angles - and less slackness in the leeward stay when the boat is under sail loads. I have the same setup on 11x11m Groucho, which like Sid 8.5, has hollow water stays/struts.
But unlike your clever Crowther tri with two beams, I have a single airfoil shaped cross section beam ... so the stays have to be attached to the trailing edge (to get a decent and safe angle). If I took the stays to the I beam mid section of said beam, then the angle of the mast stays would be close to in-line, not quite, but close (because of mast rake) and therefore a little concerning if the boat flexed enough going over waves ... and the whole lot embarrassingly falls over sideways. Another reason why I have runners too. See Sid 8.5 photograph.
Agreed, if I could get away from not having water stays on the smaller boat, then that would be a neater solution. The 6.5 has proportionately wider flare than Sid.
If I take the stays to the widest point of the stingray, then instead of water stays, could just triangulate the hull interior with uni-carbon - or even timber laminates going to main bulkhead. With the smaller wing mast of 9 metres or so, maybe I could get away with this?

 

Finished fairing, now cutting, fitting rudder.

 

Beautiful build

Hi Gary,

Firstly wish I had your abilities.

Love your build approach, we call it bubblegum and stickytape in my shed.

Jumping back a bit in one post you said you generate the hull form based on the wood / resistance.

I have done a very similar thing but only on a small model at this time using paulownia sheets.

Started off with a couple of sheets of wood and some left over closed cell foam from other models and put together a similar shaped hull as yours but with the idea of a mono hull family day sailor / power boat in mind.

Form fairly much dictated it's own shape based on the constraints I had set in my mind, I just kept adding and removing foam and reshaping until I was happy with the look and the way it sat and performed in the water.

Ended up with excellent stability and resistance to heel without giving having too much waterline.

Did some very rough pool testing (rod, reel and strain gauge) and was very happy with the outcome.

Had very little resistance, on par with other models of tri's and cat's I have built. Also has very good stability for an unleaded mono, toying with the idea of water ballast if I make a full size version.

Seems to jump up onto a plane very quickly and has very little wake or bow wave.

Did a bit of a heel test with makeshift mast and weights and again was impressed with the stability.

Couple of images of the model to help visualise as attached.

Seems so often in these forums that science and proof are the only consideration given to design, as an engineer I'm glad to see that intuition and instinct still have a place in modern design.

I firmly believe that science when used to help drive intuition and instinct can lead to many new and varied designs and approaches.

Having shaped surfboards in the past also helps

All up keep up the great building work and designs, love to see how people build reality out of concepts.

Hydromann