Catamaran Evolution

Here we go again...

I am experiencing meltdown from the design bug so it's time for a break. Lost a lot of sleep.

I took an old design the CR33...worked over from scratch. Eventually the design had to evolve into the RC34 and if that wasn't enough...it evolved once again into SR34.

CR33 is a Cruiser/Racer
RC34 is a Racer/Cruiser
SR34 is a Super Racer than can be cruised in the lower lattitudes.

Let me know what you guys think of the designs up to this point and if there is any in particular I should develop further to completion for a possible model. Neither of the designs are completed but they are at their designed full displacement. Any questions, suggestions and comments are appreciated.

Thanks
J

 

Wow....

So quiet in here you can hear a mouse pissin on cotton.
No critiques? Comments? Suggestions? Nuttin?

Well...okay. Take your time. I'm here all week.

J

 

Catamaran files

You might get more feedback if you post your images in a more easily readable image format like PDF or jpeg.

 

nice free delftship drawing(s) for further dev, i like the hard chine and deckhouse start, not a racer, from what i see would not make a distinct selection yet

 

Hawdy

Well... I wanted to let the community have the entire drawing instead of just the pictures for their perusal before making informed suggestions so that the entire design gets feedback. Everything is roughly where it should be but some may want a different layout etc.

J

 

Thank you Yipster...

The drawings are free as you have observed...but they are in Freeship. I don't mind sharing them in their entirety since most everything learned from here has been free...all I ask is that credit is given and if the design is further developed that it be shared so I can incorporate changes if I like them. Some Pentagon type would call that crazy?

I'm assuming you like the RC34. What kind of changes would you recommend before you would decide on a design to develop to completion?

I know that most like to look at numbers also so I am going to attach them for all the designs once I go through the files and find them. If you have the drawings...you can also get the figures with some manipulation.

Thanks
J

 

Okay...

Here are some of the numbers for the designs.
Comments are appreciated.

The hydrostatics are for the hull only without any other components. Two hulls...free floating. The final hydrostatics can be obtained from the drawings.

J

 

The SR34 seems to have more tumblehome than I expect for a cat designed to get good speed. The underwater sections look identical to the other two.

I did a check with Michlet on the SR34 and wave drag kicks in significantly around 7.5kts.

I have attached a comparison showing the predicted power to achieve a given speed for the SR34 and a Godzilla generated hull of 34ft designed for 10kts. The propulsion efficiency is taken as 70% and hulls are assumed equally loaded.

The optimum boat would achieve around 1.5kts better with the two 9.9HP outboards than the SR34. I would expect sailing performance to be similarly improved.

Looking at it the other way. The optimum would do almost 10.5kts with the two by 9.9HP outboads whereas it would take two 15HP outboards to get that speed on the SR34. Alternatively the optimum would only need 2/3rds the sail area as the SR34 for the same speed.

I suupose my answer to the pole is none. I have a lot of faith in what Godzilla produces for slender hulls and I would want to get the best speed for a given rig or for the installed motors.

Rick W.

 

Rick you are the best, i did not go as far checking in michlett and amazing how much that differs
post your optimum shape and i do, have to do some more michlett exercise and start godzilla one of these day's anyway

i was reading Chris White the sailing multihull chapter 10 on safety and seamanship about drag from drogue lines, sea anchors and seabrakes

i never realised a (one line adjustable) bridle can steer and if they could used as fenders they be on my whish list (better than white painted car tires)
havent been to sea for a long time but at times drag and brakes can be important too

 

Hello Rick...

Great job. I haven't done all of that yet and it definitely sheds light on several things. Thanks. The significant difference in the design is that they are lighter as you progress from the CR33 to the RC34 to the SR34.

Please explain why the tumblehome should be smaller for the SR34 if speed to be attained is better? Tumblehome reduces weight at deck level and rounded sides allow streamlined airflow, increased bouyancy if she tries to go turtle and it gives a feeling of open areas inside. Yes...there is added weight in creating the larger radius. Perhaps the radius for the SR34 should be smaller?

I never considered designing for speed oppossed to displacement...it is definitely something to consider. Can you share some of the basic design ratios on the optimized Godzilla Hull so I can compare to the SR34? LWL, LWL/BWL, SDR, RM, CE, Wetted Area etc? I wish I could have made the LWL/BWL ratio greater than 12 but then cruising any hull would be a little bit "tight" for the length.

Also...if my calculations are correct, comparing the speeds and keeping it to the 1.34 rule...the SR34 has a hull displacement speed of 30.4^.5 x 1.34 = 7.38 Knots. Reverse calculation shows that the optimized hull has a lwl of (10.5Knots/1.34)^2 = 61.4 feet. Knowing that this is not possible for this design and we solve for speed to length ratio to compare both hulls then it would be 10.5Kn/(61.4^.5) = 1.34. This appears to be the same limiting speed for the SR34. I would be interested to know how the optimized design was able to simulate a 61.4 foot lwl. Wait...if we do the same with the SR34lwl then we get 10.5Kn/(30.4^.5) = 1.90. Is that a normal speed to length ratio for catamarans? Any light that can be shed on this paragraph would be appreciated because if these figures are accurate...the optimized design is a very specialized design indeed.

As far as the engines are concerned...well, those are there for maneuverability in docking or shoving off and were never intended to motor-sail...although it would not be above me to motor-sail upwind if needed. In all reality...I think that a pair of 5hp is more than enough to get the hull to speeds needed for the purpose. I had a 20' lwl mono with 4 foot draft displacing about 8K#'s and a 9.9HP had that thing doing 6+ Knots burning about a gallon an hour.

The 2/3's difference for the sail area is definitely significant. Can you share how you arrived at 333 sq.' for the same speed? At that sail area, the SDR is what...15 or 16? That would have to be a very thin hull or a very light hull with little wetted area wouldn't it? Would the windward hull have to get unstuck to get that kind of speed? What would happen in light airs with so small a sail area...sail for sail of course since we can always throw a 1000 sq. ft. screecher in there?

It sounds like you have a lot of insight and can contribute honest and hard opinions. I would still like to receive your input on comparison of the denoted designs oppossed to comparing it to an optimized design. My intention is to establish which 1 of the 3 designs has most merit to design to completion based on an overall design, aethetics, comfort, ease of use etc...and not just how much speed I can get for the power only. If you can spare some time and don't mind doing so...please evaluate them further.

Thanks
J

 

I have attached the lineplan for a slightly faired version of the Godzilla optimum for 10kts. Godzilla only produces the underwater part so I have extended width above the waterline to something that would have walk-through dimensions.

In producing this I realisded I had been working on a hull displacement of 2.96t not the total ship. So I had to rework the whole lot. Power reduces considerably and the SR34 is not as much disadvantaged. The wave drag is no where near as significant when I had the wide portion of the hulls in the water. The performance benefit is now only around 0.7kts. As you load it the benefit will get greater for the optimum hulls.

Now when I referred to tumblehome previously I was referring to the rocker in the stern. The SR34 has severe rocker at the stern and would tend to bog down at speed but not as badly as I thought now that I have the correct waterline. Irrespective you are losing some of the advantage offered by slender hulls in doing this.

For easily driven slender hulls you are not constrained by their longitudinal wave in the same way as wider hulls. The notion of hull speed is really not applicable although you cannot neglect the losses to wave nmaking. If you have a canoe like stern you get good energy recovery while having very little aft rocker. This reduces wave drag compared with a hull with a lot of rocker. I also think your flat sections have too much curve to provide lift for planing. Planing hulls have much less aft rocker.

As far as the sail area goes I did not do any calculations on the sail I simply applied the ratios on the required power. At the correct displacement a single 9.9HP outboard would get the SR34 to almost 10kts and the Opt34 to 10.7kts. (making no allowance for windage) The sail area for the revised weight of Opt34 would be about 80% of that required for the same speed on SR34.

As far as outboards go, I do not like volatile fuels below deck. You could make wells in the sides of the cockpit close to where the bridge connects the hull. The wells could be at water level and towards the middle of the boat. This enables the flat rocker aft without having props adding to draft.

So at the correct waterline I have to review my opinion. The SR34 is not too bad. I do not like the rapid increase in beam of the hulls above the waterline particularly on the outside. I think you could transition into the bridge better to get some useful space there rather than carry the wide beam for most of the hulls bridgeside and outside.


Rick W.

 

Rick, The "canoe" stern you suggest, I thought of the power version image I posted in the member galleries.

In my version, I have what I call a significant chamfer between the inboard side of the hulls and the bridge-deck floor whilst still allowing substantial bridge-deck clearance. I hope to be able to post more images in 2 weeks

 

"Chamberlin C10 Screws & rudder removed" I mean, not easy to see, but showing the biggest image helps a bit with clarity - black on black cleaned with a high pressure water-jet not the clearest.

 

Brian
That is a great example. I was trying to get a picture to show that canoe sterns are not so silly. I like the way the stern above the water line is useful.

Do you know the displacement of the C10. I gather it has two 50HP diesels. Is 16kts full throttle? Do you have any estimate of the power to do 10kts? Maybe fuel rate at that speed.

How is the nodel going - got it powered yet?

Rick W.

 

Hello Rick...

Excellent. I have got to learn to do the Michlet/Godzilla thingie. Thank you for revisiting the design and clearing up many of my concerns. I had really believed that I had applied careful and diligent design practice before the Godzilla thingie came along and blew up my spot. Still...you're design below the waterline is .7 knots better and in a circumnavigation that can get you there 9000 miles sooner.

Since you're way ahead on this...would you mind doing the same comparison of your optimized hull with the CR33 and the RC34? I would really appreciate it and I am quite interested in looking over your results. I know it's my responsibility but I have to ask because I would prefer to "optimize" one design only and need to know which would be best.

I see your point on the rocker. For me, I wanted to make sure that the CB didn't roll too far aft when the hull was pressed and I also wanted to make sure that the drag associated with submerged sterns never happened when there are 3 or 4 persons in the aft cockpit. I could have run the stern rocker further aft and then sweep it up sharp but I'm not sure if there would be enough reserve bouyancy there to dampen the rolling. I need to give this some further thought. The aft cockpit in the SR34 is designed to be structural netting instead of solid. All the designs are set up with keeping as much mass centered longitudinally as possible and long overhangs at the bow and stern to minimize hobbyhorsing.

On the flat panels I never gave any thought to making the hulls plane and just thought that a small v would be good to dampen the pounding as she submerges. I'm not sure how stiff she would be if caught in heavy seas or how much slamming there would be so I gave it a little knife edge there to lessen the loads.

I have to apologize to all for failing to publish the sail area as I thought I had done so somewhere and especially to you Rick if it caused any confusion. The main will be around 375 sq. ft. with 125 sq. ft. on a blade jib for a total of approximately 500 sq. ft. The suit will also include a jib genoa, genoa and storm sails.

The outboards are a product of my feeble attempt to keep the weight down and for maintenance and repair simplicity. I agree 100% regarding volatile fuels down below but it appears that all I will be able to do is reduce a necessary evil at this time. I did give that a lot of thought and the outboards will be completely isolated at the stern from the rest of the vessel in wells that close up when the outboards are raised. Access will be only from outside and good ventilation and bilge blowers will take care of whatever fumes may exist. Even with extreme caution, I am still nervous about it. Diesel inboards can be considered but the weight penalty would mean serious redesign to get acceptable displacement and two props back there will act as serious brakes...even if they are folding, which has been proven to react very poorly to forward and reverse transitions in the lower HP's.

Placing wells, nacelles, pods etc anywhere into the footprint of the waterline was immediately excluded from consideration in the design. I have read about some insane design flaws incorporated into the footprint and slamming and drag is increased significantly. Serious interference occured with pure unhindered flow.

In fact, I had contemplated a thin foil, swinging daggerboard trunk on the centerline beneath the bridgedeck that extended to 3 inches below the waterline instead of dual daggerboards in the hull and immediately rejected it. I'm still hoping that someone can talk me into it and justify it over the hull boards.

I wanted room when I opened up the hull above the waterline and I wanted to streamline air flow to reduce drag. I figured move interior components outboard and have some room in there. I was going to go to 20% of LOA but took it down a little bit to about 18%. I could reduce it further on the outboard sides for the RC/SR 34 designs since the cabinetry intended to further reduce weight was going to be tube and canvas, but if you take a look at the CR33 you will see that I made the chines round instead of hard and then moved the galley and pipe berth into the bridgedeck so it looks more pleasing and softer to the eye. I will definitely consider reducing the radius outboard for aesthetic and weight reasons. Inboard I'm not so sure about since it creates a greater span between supports but then again if I don't, then the design will require 2 molds instead of 1 reversible mold.

Man...this is great. Keep it coming. There are a lot of ideas that are popping into my head right now that may actually take me back to the board. Hmmm...maybe.

Thanks again Rick and thanks for amending your initial findings. Hope is alive again!.

J

P.S. In case if anyone wanted to know...the scantlings for the general design has been called out and received the seal of approval from a NA/NE and the section for beams and mast has also been finalized. It is almost time for the next step.