Bi-rig performance cruiser

I have been working on finalising the design details for my dream boat, and would really appreciate any feedback from all the very knowledgeable people here.

I ran the numbers on Terhohalmes spreadsheet and found that his average speed prediction was around 0.88 of windspeed. Using the formula from Multihull Dynamics, it predicted a "Performance Index" of around 1.0 at half load (1.6 tons) which should mean windspeed sailing on a reach without foresails.

I haven't decided for an asymmetric for light winds yet, we still need to crunch the numbers for the unstayed masts.

A few details:
LOA 14.95 m/49 ft
BOA 8.76 m / 28'9"
Displacement lightship. 6800 kgs
Sail area including masts 162 m2
Bridgedeck clearance 100 cms/39"

3 steering positions: for and aft-cockpits as well as saloon.

Thanks in advance

Regards

Alan

P.S. I have a 3D PDF if anyone is interested I can post it (9.2 MB)

 

nice boat

You,ve obvously put a great deal of thought into your design. Out of interest what made you go with the bi rig and no headsails. Have you much experience with these rigs on catamarans? A cats high windage and inability to carry much way will surely make tacking difficult in fresher breezes and chop without the assistance of the headsail?

Can you carry screechers and spinakers down wind ?

Good luck with it all.

 

Hi Bruce,

I got the same questions on another forum, and being a bit lazy (efficient??) I'm copying the answers here:

I'm glad you like the drawings. I have absolutely no experience with a bi-rig, but have owned cats for the last 15 years or more, and have had my share of tacking problems on the first one.
Derek Kelsall, a designer I respect alot, was kind enough to comment on the bi-rig concept on another thread. He has sailed on both the larger bi-rig cats I have been able to find.

His comments were:


Hi Alan

Since first fitting junk rigs to some monos 35 years ago, I have found conventional rigging a distinct restriction.

Having sailed just about all the options - recently adding "Flying Carpet" to the experience. I can confirm that KSS 54 "Cool Change" and "Flying Carpet", although both using a very different approach, do perform and do so without really noticing any interference. "Cool Change" is noted for being exceptionally close winded and for never ever looking like missing a tack. Our first sail was before the sail maker had finished the second sail - strange to have the one sail almost entirely off the boat on one tack, but barely aware in the performance that one was missing. - other than the considerable change in weather helm.

I believe "Cool Change's" close winded performance comes from all round efficiency, but particularly from the leading edge shape for the wing/sail combination and the low rigging drag. She does not have a headsail and not missed. "Flying Carpet" picks up an extra knot with a headsail.

I believe these rigs should really come into their own on trade wind ocean crossing - to help steering and to avoid reefing in squals etc.

Hopefully, one day they will compete on cost to produce. Will continue to work on this one.

Derek.
Derek Kelsall, FRINA.,
Kelsall Catamarans ltd.
Tel 00 64 7863 3332.
<www.kelsall.com>


"Cool Change" has rotating wingmasts like I am going for. See the pictures below, or even better, go for a sail on her if you are in New Zealand!

http://www.sailcoolchange.co.nz/theboat.html


Regarding the use of a large headsail, I'm still not decided, but will design the necessary fittings into the rig, so my options stay open. An alternative for downwind work is a kitesail. I have a 70m2 Outleader on my present boat, that works a treat.

Regards

Alan

 

Hull shape considerations

Hello Nordic

A few thoughts -

I can't tell the hull shape but it seems as though you are pushing the envelope hard with the above water flare. This seems a lot like the shape John Shuttleworth uses on his Spectrum series.

I used the a similar shape on my little cats I designed. They are tricky to design for increased displacement. With a normal shape increased displacement does not change parameters drastically but of course a heavily flared shape does. I hope you have really got your weights correct. Being heavy is a no-no with normal cats but with this hull shape it is a real bummer. This is especially problematic with small cats and less so with your design.

It looks as though you drop the flare towards the back of the boat. This will tend to shift the CB aft as the hull is depressed. This can be a bad thing as the bows can get depressed more easily by the more powerful sterns. I would recommend reading John Shuttleworth on his design ideas for this shape on his web site.

As to the rig - I like the traditional fore and aft rig. Usually we put up a big sail up the front for downwind sailing and the boat is compliant. The bility to alter the CG fore and aft with different sail combinations allows you to trim the boat for different conditions. The twin masts set ups tried here (Radical Bay 800) have not set the world on fire and seem to go pretty slow when raced.

cheers

Phil Thompson

 

Hi Phil,
Thanks for the great comments. The underwaterlines are not finalised yet, but we will be adressing them soon. As with all design issues, compromises are the order of the day.

I agree with your views regarding weight. I have a spreadsheet with about 1000 items identified,basically everything over about 200 grams, with the LCG and VCG.
She is designed for the maximum displacement, not 50% as many boats seem to be. This gives her a load of 3.2 tons that will set her on the DWL.

Your comment about the CB moving aft is mitigated by having the flares on each side asymmetrical both athwartships and fore/aft to attempt to balance this. Also there are 2 water tanks each side, so water can be moved to balance her if necessary. I am looking at increasing the prismatic coefficient up to about O.62, the version shown is about 0.58.
Maximum waterline beam is at station 5.6 approx. Actually she started of as a 45 footer, and I just increased the length, without increasing Bcl.

I have chosen this rig for a number of reasons, the primary one is for ease of use/handling shorthanded.

I sailed on a 47 ft performance cat last year, and frankly the forces involved were so massive, that I decided ther must be a better way! This left me with 2 options, a Ballestron type rig with balance or a bi-rig. The bi-rig was my choice, as it gives me 7 "gears" with 3 reefing points, and will solve the "sailing at anchor" syndrome if the masts are canted in opposite directions. Also a wingmast should enhance the efficiency of the whole rig.

The Radical Bay has an inefficent rig in my opinion. They are chanaging the rudder design due to issues with tacking in strong winds and waves.


What would you reccomend be done reagrding the underwater lines for my boat?

Regards

Alan

 

Hi Alan,
great looking cat, I can see you have put a lot of thought into this design, I particularly like the 'light bulb' hull shape. I have re-read Shuttleworth papers several times and he makes a lot of sense.

What is the bridgedeck clearance, especially aft?

Two suggestions from my practical experience:
1- on my 12.5m cat, I had overhanging sterns and walk through transoms. The aft sections were quite buoyant, but with a following heavy see, we often had the sterns swamped and water in the cockpit, which at times meant wet socks and gear for some time. Your cockpit seems to extend aft a bit more than usual, which might increase bridgedeck slamming aft and or being swamped by big waves that might be running at 25~30kn.

2- You do not have a catwalk, which make me think you deploy the anchor from the middle of the bridgedeck. While it will be ok on the bridle, you might run into problem when heaving anchor as the rode (especially if the wind is up) will go under one hull. As you get to the chain, it will scrape the paint from the hull. I understand that your rig doesn't need a cross-beam, but a cross beam and catwalk make life easier.
Also consider that if you want/need to get to the anchor, it is under the trampoline, so how do you get to it?

Once you finish the revision you mentioned, can you post the hull cross section as well please?

 

Many thanks for the input, some stuff needs rethinking, that's what's so great about this forum. Great pictures of your boat, I especially like the forward steps that can be lowered. I originally had this in mind, but when I decided for a bi-rig, it was cut off the list of must haves.

Will post again later when things are more finally decided.

Regards

Alan

 

Not any more, I built in 2002 and sold it December 07.
I am now looking for my new, live-aboard cat in which to sail past the horizon....
Design/build or just go out and buy... I am torn apart!
My young brother just bought himself a 2004 40' Lavezzi for 170,000 euros!

 

All very impressive, thanks for sharing

 

second that and leaves a few things to think about

 

If you can live with the roomaran designs, ther are plenty of really good offers in the US/caribbean at the moment, I saw a 10 year old Athena for sale for less than 140 k$ US!!! For 40 k you could give her new engines, controls, rigging, sails, all the inside upholstery, and still have some left for a beer or 2.

WIth the present market conditions, the only reason to do your own thing, is because there is nothing out there you want

Hey, when you get past the horizon, remember to give 3 blasts on your horn, so we all know

Regards

Alan

 

Hull Shape -- Inboard Flare -- Windspeed Yachts

Alan: I think that you might be looking for this reference by the Oz designer and builder Wayne Hill of Windspeed Yachts:

Windspeed Yachts --- Hull Shape: Inboard Flare

Design Innovation 0
www.windspeedyachts.com.au
The Inboard Flare

The Design of Inboard Flare
Having undertaken many main beam and hull to underwing repairs on what we considered well designed and constructed yachts, but not designed or built by Windspeed, we wanted to offer our clients a hull shape that was gentle on the multihull structure.
We focused our design effort on dramatically reducing concentrated loads on the beams, reducing wave pressure in the area around the hull to underwing joint and reducing underwing slamming.
Numerous main beam models were tested to destruction, the strongest beam had a 45 degree gusset from the waterline to the underwing. We had to develop a hull shape that incorporated a 45 deg gusset. Photo of Main Beam
We constructed 2 identical multihull yacht models 1000 mm long 580 mm wide. We also designed and constructed an extremely deep V monohull model, this hullshape being capable of high speed with a smooth motion in rough offshore waters. This mono hull model was split in half down the centre line and the port half was fitted and glued to the starboard inboard multihull model hull and the starboard half was fitted and glued to the port inboard multihull model hull. We now had a multihull model that incorporated an inboard flair.

When the two multihull models, one with the inboard flair and one with a conventional hull shape were placed in waves with a motion comfort metre (a bowl of water), the results were amazing. At all speeds and wave angles the hull with the inboard flair had more water in the bowl after the test, indicating less motion.
The flow of waves under the underwing and around the models were carefully studied. Less waves seemed to impact on the underwing of the model with the inboard flare.
The models were then drag tested. A bridal off the bows was tied to a spreader bar to separate the models. The tow line with a short bridal was attached to the middle of the spreader bar and then towed in waves beside a dinghy. The model with the inboard flare was always ahead indicating less drag. The question is how much less?
The tow line was moved away from the hull with the inboard flair, until both models were in line while being towed, indicating equal drag and a strain gauge (fishing scales) were added to the tow line. The exact percentage less drag on the model with the inboard flair was calculated for different wave angles.
The models were then tested for their ultimate stability, they were thrown into beach surf. The models were then tested for stiffness, then destruction tested. The flared model was stiffer and stronger.
Results
The advantages of the inboard 45 deg flare starting at the bow and ending at the aft beam and on the light ship waterline are:
A stronger lighter beam structure.
A stiffer overall yacht.
Less underwing impact.
Less motion at sea.
Less leeway due to asymmetric hull above static waterline.
The leeward hull has more reserve buoyancy.
Less spray on deck.
The yacht with the inboard flair has more internal volume.
Easier assess to hull from bridge deck and best of all faster.

 

Many thanks,

That is the article. I think there were more photos in the one I remember from a few years ago.

My inboard flare is not a straight line from the waterline and 45 degrees inboard as he does it, but there is a well defined softer flare.

His thinking on mast rake is also quite interesting.

Regards

Alan

 

Hull shapes -- from Kurt Hughes Ranier 47 / Ranier 50

Alan: Kurt Hughes designed a somewhat similar "Lightbulb" shaped hulls on his Rainier 47 / 50 catamarans. Here's what Kurt had to say about his design:

"One of the interesting aspects of this cat is the hull shape. It has a generous topsides flare that provides tremendous amounts of room inside, but allows a slender hull shape in the water. The length:beam ratio of a hull is almost 11:1 which means the hulls will be very easily driven, but still be able to carry the weight of stores for long voyages. I hear that one designer new to doing cats believes that the flare should only be on the inboard side. I heard he thinks the fenders will not protect the hull if there is flare on the outboard side. They sell some big fenders down at Fisheries Supply.

When slamming to windward, the hull flare diverts the surge of water that the hull splits. It gives a far drier ride than a hull with no flare. On deck, the wide hull surface provided by the flare gives a great feeling of safety.
There are many other nuances in the hulls coming from over a decade and a half of doing these cats. The center of buoyancy and the center of lateral flotation are not the same location. That dampens pitching. The water plane is decidedly asymmetric as seen fore and aft, that also dampens pitching. The prismatic coefficient at 0.64 is as high as I can push it to put fullness in the ends, but still be easily driven.

In section, the hull in the water is nearly the round, minimum wetted surface shape, but with just a bit more dead rise angle than true round. That comes from the experience on the earlier cats that when they are leaping fully out of the water at speed, they can land hard if there is no deadrise angle. Just one of those tweakings that comes from doing a few of these.

The displacement is 18,603 lbs. (8438 kg.) The lightship weight will be 12,734 lbs (5,776 kg). That gives a payload of 5,869 lbs (2,662 kg.)."

Regards, Mark

 

Hi Alan,
How is your design development going?
Have you finalised the underwater lines?

I am very interested in see the how you are progressing with this great cat.