Keel line of motor proa
Hi all!
First message here on proafile. Since I saw the Russell Brown designed outrigger powerboat in Professional Boatbuilder magazine, Nr 130, 2011 I have tried to come up with a design for myself. But the long and narrow hull is so far from my current experience that I can´t decide on a final design.
I want to 1. drastically increase miles/fuel to about what a (smallish) car does, 2. reduce wave induced motion. Size wise Russell´s power proa is about right. When I try to lurk on anything in these general directions on the net, I find that all designs have a belly, as the designers/builders apparently come from the sailing world. As a (former?!?) brute power planing boat designer, I fear belly bottoms as ebola on any faster boat. Then I find that not even the extreme speed AC72 have straight lines. So I´m lost. Any general thoughts along these lines would be helpful. Why belly/straight, why not belly/straight?
Hi all!
First message here on proafile. Since I saw the Russell Brown designed outrigger powerboat in Professional Boatbuilder magazine, Nr 130, 2011 I have tried to come up with a design for myself. But the long and narrow hull is so far from my current experience that I can´t decide on a final design.
I want to 1. drastically increase miles/fuel to about what a (smallish) car does, 2. reduce wave induced motion. Size wise Russell´s power proa is about right. When I try to lurk on anything in these general directions on the net, I find that all designs have a belly, as the designers/builders apparently come from the sailing world. As a (former?!?) brute power planing boat designer, I fear belly bottoms as ebola on any faster boat. Then I find that not even the extreme speed AC72 have straight lines. So I´m lost. Any general thoughts along these lines would be helpful. Why belly/straight, why not belly/straight?
1: Increased fuel mileage will come with the long and narrow hull - they can be driven faster than traditional displacement hull speed with less power than a wide, flatter planing hull. Very narrow, long hulls break the traditional rules. There is no need to have huge amounts of power to climb over the bow wave to achieve planing. Mr. Brown’s example uses a 20hp motor to achieve respectable speeds and very good fuel mileage.
2: Wave motion is addressed by more than one thing - first, the longer waterline helps. Also, keeping the weight out of the ends of the boat helps very much. The double-ended hull form (in the water) also reduces the amount of waves generated because of the prismatic coefficient of the hull.
3: “Belly” bottoms as you call them indicate there is some degree of rocker in the hull - making the boat more likely to actually turn than a boat with no rocker. Since the boat’s normal operating mode is in displacement, a boat with little to no rocker will not turn as easily as one with.
I’m sure some of the more qualified folks will better explain this than me, but you have to give up your preconceived notions about what makes things fast and efficient.
The other thing to be aware of is that fast and efficient is relative - if you are looking to achieve more than 20 - 25 knots, then you are better to stick with a planing hull. If 15 - 20 knots is acceptable, then a smaller fuel efficient outboard may get you there very cheaply. Richard Woods’ Skoota power cats are reaching 20+ knots on a coastal cruiser design - with two 20hp motors.
You are giving up space and carrying capacity to achieve efficiency. The boat’s weight will really matter as well - so careful building is important.
Why don’t you contact Russell Brown’s company, PT Watercraft and see what he has to say? His PT Skiff is brilliant, and he may already have a design on the books to address your needs.
—
Bill in Ottawa
Good points on the weight distribution effects as well as the double ended hull. Yes, rocker is the correct word for the belly. Thanks. Dictionaries are of little use in boat design discussions. I´m afraid of the suction that a convex shape suffers from when moving against a liquid (tea spoon into tap water).
I´m coming around to see that there are circumstances where the implications of the physics may differ drastically. Moving on from a L/B of around 3 to long narrow hulls with L/B at 10 or more has pushed me outside my box in a very pleasant way. This article was an eye opener: /www.cupinfo.com/en/americas-cup-2013-oracle-design-andrew-mason-13067.php But this is once again sail where a huge turning moment is present that for sure effect the design as much as it is irrelevant for a engine powered vessel.
Of course any design is a balance between shape resistance, wetted area and lift distribution along the hull. A planing hull is an “either or” where the “in between” or “both” condition is unsatisfactory half planing. A powered proa can apparently be seen as a “both” design where the benefits come from optimizing the lift vs slick shape. As a result the speed, as you say, has to be selected. My experience from speeds in excess of 40 knots is that there is no lasting happiness in ever higher speeds, just accelerating costs. So I have decided on a top speed between 10 and 20 knts.
My main reason for a bottom with rocker is to reduce the submerged transom area. But I can easily imagine how the rocker bottom sucks the hull down so the transom is submerged despite my best efforts to avoid it. Too high speed will do it with weight aft. Ok. So there is logic in the rather aft position of the AC72 rocker “hump”. It can glide on the long straight forward section whilst the rocker lifts the transom out of the water plane.
Sorry for my freeflowing thoughts that may be totally senseless. These new views into boat design is like breathing pure and fresh oxygen!!
Good points on the weight distribution effects as well as the double ended hull. Yes, rocker is the correct word for the belly. Thanks. Dictionaries are of little use in boat design discussions. I´m afraid of the suction that a convex shape suffers from when moving against a liquid (tea spoon into tap water).
Your fear is groundless about the convex shape - think knife through water instead of spoon. Optimizing the hull width to be as narrow as practical is the goal. As the hull gets narrower, the relevance of the rocker becomes less.
I´m coming around to see that there are circumstances where the implications of the physics may differ drastically. Moving on from a L/B of around 3 to long narrow hulls with L/B at 10 or more has pushed me outside my box in a very pleasant way. This article was an eye opener: /www.cupinfo.com/en/americas-cup-2013-oracle-design-andrew-mason-13067.php But this is once again sail where a huge turning moment is present that for sure effect the design as much as it is irrelevant for a engine powered vessel.
The America’s Cup boats are fast and efficient, but perhaps a bit off target for your research. The AC72’s raison d’etre was to enable getting up on the foils as fast as possible (once they found out sustained foiling was possible). The AC72s were practically unlimited by expense, their design brief was incredibly narrow and the required lifespan was measured in months and a finite number of usages. The scantlings - materials, layup schedules and process sophistication for the AC72s is almost irrelevant for a efficient, utility boat.
Of course any design is a balance between shape resistance, wetted area and lift distribution along the hull. A planing hull is an “either or” where the “in between” or “both” condition is unsatisfactory half planing. A powered proa can apparently be seen as a “both” design where the benefits come from optimizing the lift vs slick shape. As a result the speed, as you say, has to be selected. My experience from speeds in excess of 40 knots is that there is no lasting happiness in ever higher speeds, just accelerating costs. So I have decided on a top speed between 10 and 20 knts.
Lift really isn’t a major design goal with proas - they go through the water rather than on top of it. Fore - aft trim is important, and making sure there is adequate reserve buoyancy in the ends is also important. Keeping the width to length ratio on target will have more effect on efficiency than worrying about hull lift. You want to be better than 12:1 to get really good results.
My main reason for a bottom with rocker is to reduce the submerged transom area. But I can easily imagine how the rocker bottom sucks the hull down so the transom is submerged despite my best efforts to avoid it. Too high speed will do it with weight aft. Ok. So there is logic in the rather aft position of the AC72 rocker “hump”. It can glide on the long straight forward section whilst the rocker lifts the transom out of the water plane.
In a proa you aren’t dragging a wide rear end in the water developing all the drag and turbulence of a conventional boat. Waves generated by moving the hull through the water are far less significant than in a conventional hull. You might want to review some of the threads here about building and testing hull models. You can answer a lot of questions getting your own first hand data, while also getting a feel for building (at very low cost). Add a data collecting load cell and known propulsion standard (i.e. constant speed winch and tow line) - and you will learn more in shorter time than you think possible. The skinny Moth class boats and Moth foilers did a lot of tank testing (one Moth enthusiast had access to the US Navy’s tank testing facility).
In reality, we all have a pretty good picture of what we want to accomplish with our builds, and a boat that meets our needs is a collection of all the right compromises and choices. I’m a big fan of keeping my eye on the ball where the rubber meets the road - money, build time, build quality.
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Bill in Ottawa
If you really want to optimize the performance of a long slender hull Mitchlet is the tool to use.
http://www.cyberiad.net/michlet.htm
The learning curve’s a little steep, but for the price….
For optimization (minimization really) use the Godzilla module in Mitchlet.
Enjoy,
Skip
I think you’re doing the right thing and investigating all your options, learn all you can. In that vein, try googling phrases like “long narrow powerboat design.” There is a great deal of interest and you’ll find diverse opinions and advice in a dozen or more places. Good luck!
Dave Culp
... (tea spoon into tap water).
Your fear is groundless about the convex shape - think knife through water instead of spoon. Optimizing the hull width to be as narrow as practical is the goal. As the hull gets narrower, the relevance of the rocker becomes less.
I was hoping for this. It´s just so final to start cutting those precious plates. With standard boats, I know the what´s and why´s exactly, it´s all on CAD, tried and true. With proas it´s all new, and wonderful I might add!
... The America’s Cup boats are fast and efficient, but perhaps a bit off target for your research. ...
Yes, though I would not mind getting a hull or two for free just to test. I went to the super extremes as it´s there where the wanted characteristics are most prominent, speed/power. In designs for everyday use compromises are abundant. Having a hunch of where the optimum is helps to tweak the compromises within the allowable range.
... Lift really isn’t a major design goal with proas - they go through the water rather than on top of it. Fore - aft trim is important, and making sure there is adequate reserve buoyancy in the ends is also important. Keeping the width to length ratio on target will have more effect on efficiency than worrying about hull lift. You want to be better than 12:1 to get really good results.
Good stuff, yes, most of my design attempts are looking much like Russell´s power proa except the added buoyancy in the bow. I fear nose diving in our choppy seas. 12:1, I hear you! Just that I´m struggling to get to 10:1.
In a proa you aren’t dragging a wide rear end in the water developing all the drag and turbulence of a conventional boat. Waves generated by moving the hull through the water are far less significant than in a conventional hull.
Ok, yes. I bought a Merc 30 4t two years ago that seems ever bigger for every day. Much too big. Hitherto too much power was impossible. Without buoyancy aft the nose will point at the polar star (63° up…). Yes, I need a smaller engine.
... Moth foilers…
Yes, I have thought about rising the proa on three foils. Should be quite easy, especially on the drawing board.
In reality, we all have a pretty good picture of what we want to accomplish with our builds, and a boat that meets our needs is a collection of all the right compromises and choices. I’m a big fan of keeping my eye on the ball where the rubber meets the road - money, build time, build quality.
My main problem is that I´m playing with two balls: need vs fun. When I build the proa it must provide something that my other boats don´t. But if it isn´t fun, it wont be used after the first steam has blown away. So the result must be fairly good, not just so so.
Thanks, Skip and daveculp. Yes, it´s high time to get some outside input. That software seems pretty interesting!
Perhaps it´s the sailing part that has kept me away from many and most cat and trimaran discussions. Catamarans here around are rather massive work boats with grunting diesels. Not at all the sleek wave piercing stuff. Trims, well, why two support hulls when one is all I need? Make it to port and starboard side is free for work, fishing, timber gathering, whatever life brings on. That´s why the image of Russell Brown´s power proa in Professional Boatbuilder magazine was such an instant eye opener!