Proa Hulls

Lightweight cruiser - not too long distance to often. 2 adults and young child - maybe an extra 1 to 2 people but less often. Minimum cruiser- ie wont be loading all sorts - just the necessary.
Dont need to be superquick but dont want to be slow either..
On deck - basic cuddy - just enough to keep out of the weather - sitting rm.

Why do you say rocker doesn't make that much difference - everyone else is adamant that cats must have it?
I am willing to work around it - ie bigger daggerboards and using the sails - whether jib or mizzen - i am favouring a ketch rig.
Thank you for the help/interest.

 

Basic/minimum cruiser - wondering why you say rocker doesn't make that much difference, as others are insisting it must have rocker...?
Not long distances cruising and light for 2 adults and a small child. Maybe more for daysails.
Favouring a ketch rig - so use mizzen and larger boards to tack. Otherwise use/back jib.
Speed not a major issue but dont want to be slow - idea that easy/fast hull goes through the water easier...
Easy to build - for me.

Sorry for double post - thought it didn't go through...... oops =)

 

Yes, you need some rocker especially at stern to tack (turn) the catamaran. On the resistance side, you need minimized wetted surface to keep friction resistance low, longer waterline to increase the speed of wave making begins and the right prismatic coefficient to keep wave resistance low at design speed. The rocker is then a following thing more than a determining one.

 

I am having a few problems with cp.....
The wl is longest that i can get from shape.
design speed - 8knots comfortably?

 

The hull shape is not just "how to twist a panel" thing. If you look my ping-pong proa hulls, at fist sight their underwater shape is just a copy of Wharram hulls, but their:
- panels had to be developable for easy building
- prismatic coefficient was optimised to 8 knot speed
- waterplane coefficient was bigger than wharrams to keep pitching low
- waterline length was maximized to move starting speed of wave making higher.

At simple deep-V bottom wetted surface is quite high and the rocker just came by the process. Well, you can't have it all. Rocker is not a primary thing at proa, no need to turn quickly.

Your catamaran may need different starting point.

Yes, that Cp is normally 0.59 - 0.63

 

When me & a few mates built the Voodo Child back in 1984 she was buit from 3 ex USAF grp drop tanks cut & shut together, we intended to put some rocker in but the tight build schedule (3 weeks) & fact we were young & having far too good a time meant that he ended up hogged by 2 inches over his 36ft.
As it happened he sailed ok but the helmsman tended to get rather wet.
2 years later i cut the joins & veed them out putting the rocker back in i think it was approx 3 inches in total. The difference in handling was marked , the beast handled & steered better & was far drier to sail.

 

ok - after some steam coming out of ears etc -
Cw = 0.51
Cp = 0.60
LOA 9m
LWL 8.2m (corrected from 7.5!!!)

 

The waterplane coefficient should be near 0.7 or even slightly over.

 

I have sailed two proas, one for several months while living on board and one briefly.

You have to remember that a cruising boat isn't just for sailing. It has to be a practical floating cottage as well. And the design of that often over rules otherwise desirable sailing features.

And also remember that boats have to be usable in harbours and marinas. Its not like the "good old days" when Slocum and even the Pardney's first went to sea - with no engines and few marinas or even cruisers.

So all cruising boats MUST maneuver reliably under power and be easy to board from both the dock and from a dinghy.

That is one reason why I don't like canoe sterns. They make boarding so much harder than a boat with transom steps (the acid test I always use - "could my mother get on board?")

Safe maneuvering in a small harbour is another reason I like small boats. I also find a trimaran much harder to handle than a catamaran when coming alongside, as it is so difficult to reach the outrigger bows to fend off, especially when compared to the big wide decks of a catamaran.

Successful designs are ones that work in every situation, not just those that sail or motor fast in a straight line.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks - i figured it was too low.... back to the drawing board.

 

And I'd increase Cp while you're at it

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Alex
To give you a bit more of an idea of the lowest drag hull.

Taking a displacement of 1600kg, lwl constraint of 8.2m and target speed of 8kts the hulls would have the following parameters.
bwl 400mm
draft 350mm
Cb 0.7
Cp .78
no rocker

The power to drive the hulls at 8kts would be 2kW - a force of about 500N.

This analysis is based on equally loaded hulls. Probably not too bad an assumption for most conditions because crew weight could be used to offset the modest heeling forces on a relatively light boat that is easily driven.

This is a starting point. You need to home in what accommodation you want in the hulls as the beam is narrower than you specified. You may also take the lwl longer as you have to ask why you want overhang. You may want rocker to help with turning. You may think my weight guesstimate is too much or too little.

You play with the requirements and constraints until you understand their interaction with the aim of finding the optimum for your objective.

Rick

 

OK - After re-drawing - the Cw is 7.0 BUT the Cp is up to 0.87
Too high? Havent had time to make a model for me to see in 3D - as unfortunately i dont have software or ability to use.....
Rick - wt 2000kg - so draft a bit more - .40+

 

Alex
The attached is the linesplan of what Godzilla produced and I detailed earlier. This has no beam constraint. It has got a constraint on the section to suit flat panel construction although I have not shown it as flat panel.

If you are pushing a hull above hull speed then you find the lowest drag shape tends to get fuller in the ends and straight rocker so the Cb gets quite high.

What I normally start out with is looking at the lowest drag hull for the target displacement and the design speed. Having seen what this looks like and the power required I start adjusting constraints and objectives until I get the optimum result. I do a sensitivity analysis on the shape.

You might now say that 400mm beam is way too small. So you do a sensitivity analysis on the minimum beam. Basically find out how much more power/force is required to drive the hull with the extra constraint.

This hull will still turn but adding some rocker will make it easier to turn.

A feature of Rob Denney's proa is that the lee hull is made for outright speed. It is 16m long, about 0.5m wide and something like 0.7m high. It means he can build it very light. This is the hull that does the job hydrodynamically. The aim is to reduce the load on the stubbier windward hull when sailing so it is very lightly loaded. The lee hull will do 14kts with only 500N force applied carrying the entire displacement. He could reasonably expect to see 25kts which takes about 1500N. Very modest load for that speed on a boat that can carry a few people with basic accommodation.

Rick W

 

So this is why / how 2 different hulls can work together with somewhat different criteria? The lee hull assists the shorter/heavier hull?