dory catamaran hull

Refraining from further Gobbo-esque comments, I'll simply agree with regards to keeping the weight from the ends.

 

It's true that I used old Tornado hulls because they were almost free. I'm tempted to build a pair of dory hulls next year to make a comparison. The Tornado hulls don't have a lot of weight carrying capacity and some extra freeboard would be a good thing when two people are standing aft near the engine.
I did design and build a 30' dory power cat in the 70's. It was designed to haul dive tourists at high speed out to an atoll near Pohnpei in Micronesia. With a pair of 115 outboards it could do 28 knots.

Gary

 

hulls

I don't have a set of hulls- that is why I was interested in building a set. Building a new set of dory type hulls in ply is almost as easy as trying to adapt sailing cat hulls that won't work nearly as well. I do like the deck on the Tornado hulls I want something that will run at low planing speeds so I can't imagine that round bottom hulls would be best. I have engines, fittings, steering and old mast sections for crossbeams. I just need a hull design. Bruce

 

Have you seen the Latest News and New Designs pages of my website where I show photos of my 20ft powercat Skoota? It may be something like what you are after

Sorry for the purely commercial post

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

new cat

Thank you Richard, no I had not seen that, but it is still bigger and slower than I want. Nice boat though I "think" I want a two person planing platform- for all the reasons that you give for "Skoota",- just light enough to plane. Bruce

 

This, just in from multihull designer, Kurt Hughes:

"I visited the construction of the second 65' passenger ferry for Lake Victoria last week. Albert and the crew make it look easy. It's cylinder molded plywood/epoxy. Look at the great hull shape and they haven't even poured the keel yet. Albert you're blowing my mind."

 

Great writeup! Thanks for posting the links to your blog.

 

This hull looks like a flat bottomed dory hull to me

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard
The photo of that hull does not mean much because it is not doing anything hydrodynamically. The only consideration when it is flying is the aerodynamics. You need to look at the amas for what is doing the job in the water.

I have attached another view of what I think is the same boat. I guess if these guys had the money to build more expensive rounded section hulls rather than simple dory style flat panel hull they would do even better!! At least according to some here. Or am I being too cynical?

Rick W

 

It seems I am not alone in my knowledge on the high performance hulls as the photo of BMW Oracle demonstrates. These guys have the budgets to do the research and development that I am able to do with much more modest means but then I have a good deal of knowledge of how to go about R&D economically.

Some important requirements for effective R&D are:
1. An open mind - be prepared to be guided by knowledge, measurement and understanding rather than fads and "common wisdom".
2. Uncompromising approach to discovery - aim to optimise rather than compromise. Optimisation requires exploring the bounds of the design space. Compromise is the lazy mans approach that settles for less than what could be.
3. Develop reliable mathematical models of the process - preferably analytical rather than empirical.
4. Pilot the design at the largest scale that allows good comparison with the full scale - main objective is to validate and refine the models at low cost.
5. Measure accurately enough to identify any anomalies that will help extend the model.
6. Don't leap to full-scale until you have the model giving accurate predictions of the pilot.
7. In business you need a time frame but not a prerequisite for a hobby.

Rick W

 

It would be interesting to ask the designer or race team the thinking behind the flat bottomed main hull. With the budget they have to work with on those boats the hull shapes aren't after thoughts no matter how much time they spend in the air.

 

In lower wind the main hull will not fly. The dory form of the main hull is for the same purpose as it is for the amas. The flat bottom, flat flaring sides with tight chine results in lower drag than a round section for their speed of interest. I am certain they have not gone for flat panels because it was easier to build. Although that might be also true the point is that flat panel dory shape actually performs better at high displacement speed on slender hulls than fully round sections.

I have done extensive comparison of the semi-round and flared rectangular sections with the hulls pictured in the speed range 6 to 9kts. The one with the rectangular section performs marginally better overall.

Rick W

 

Nice boats Rick! When you are providing the test power any improvement shows up.

 

An interesting report. I once tried a single flat-bottomed ama on a fiberglass canoe so it could carry a sail. It was half the length of the canoe with a length/beam ratio of about 8:1. The sailing was disappointing until I raised the ama out of the water when the speed increased ramatically and the noise from the water dropped markedly.

It looks as though you can balance on the main hull will the amas barely skimming the water. Does immersing an ama create extra drag?

 

In the Skene's chapter on planing powerboats (sorry about the p word) they used the example of the German E boats with their round hull being able to out run the faster British v bottoms as soon as the waves/chop came up. How about a wave piercing round skinny ama like the outriggers from many cultures. There the idea was also just for balance but there was very little resistance to the waves, they were providing the power too and had a long time to develop the idea.