My 32 foot powercat design for offshore fishing / day trips in the Gulf

[kengrome]

Here are some preliminary renderings for a 32 foot powercat I'm designing.

The concept is maximum efficiency at 20-25 knots which I presume will be displacement mode (or just beginning to plane). The boat will go on day trips (maximum 200 mile round trips offshore and back) and occasional overnights moored at the drilling rigs out in the Gulf.

In addition to displacement speed efficiency I also want the boat to plane at 50+ knots given enough power and good conditions.

I gave the hulls plenty of rocker to get the transoms almost out of the water while running at displacement speeds, but the rocker is mostly forward; the hulls have a nearly flat run aft. At higher speeds when the boat planes the hulls will pitch up a bit, the bows will come out of the water, and the hulls will meet the waves on their bottoms.

Power will be via two transom mounted outboards for simplicity, redundancy and good maneuverability. The wing deck (not yet shown) will have a center mounted console just forward of the midship line. Fuel tanks, batteries, coolers, etc. will be in the hulls.

I'm looking for thoughtful feedback regarding the design -- good or bad. If you have questions or suggestions please let me know, thanks.

[erik818]

Kenneth,
Can you explain why you have a rocker? From what I understand, you will get a negative "planing lift" where you have a negative angle of attack. As I understand it, when you increase speed the aft will sink and the fore will rise until you get a positive angle of attack also at the transoms. Is this what you want to achieve? Without the rocker the trim will change less as you increase speed.

If you take away the rocker the transoms will be wet at low speed, which will hurt efficiency at low speed because of turbulence. I understand that you intend to travel at 20 knots or so for the long distances, so a loss of efficiency at low speeds can hardly be a problem.

I don't have experience of any similar hull, so I can't give any valuable comments on your design.

Erik

[kengrome]

Kenneth, can you explain why you have a rocker?

The rocker brings the transom bottom up to the surface so it does not "drag" in the water. This improves efficiency at displacement speeds.

As I understand it, when you increase speed the aft will sink and the fore will rise until you get a positive angle of attack also at the transoms. Is this what you want to achieve?

Yes.

Without the rocker the trim will change less as you increase speed.

Possibly, but I don't think trim will change that much on these long slim hulls, or if it does I'm hoping/expecting it will be a gradual change.

If you take away the rocker the transoms will be wet at low speed, which will hurt efficiency at low speed because of turbulence. I understand that you intend to travel at 20 knots or so for the long distances, so a loss of efficiency at low speeds can hardly be a problem.

This is probably true at higher speeds but I may still be in displacement mode at 20 knots -- I just don't know yet -- and if 20 knots is a "displacement speed" in this boat then it would seem to make sense to get the transoms out of the water.

[sabahcat]

Originally Posted by kengrome

and if 20 knots is a "displacement speed" in this boat then it would seem to make sense to get the transoms out of the water.

Can you show any other successful powercats that demonstrate this methodology?

[kengrome]

Can you show any other successful powercats that demonstrate this methodology?

No, but if someone else else knows of one I'm interested in learning more about it.

[sabahcat]

Originally Posted by kengrome

No, but if someone else else knows of one I'm interested in learning more about it.

So you are designing a hullshape

You cant show a successful vessel currently using it

But there are existing hullshapes that do work very well for your intended purpose that are different to yours and have had many thousands of hours of research done to ensure their success

I have to ask, why not just draw up or use one that is successful now?

[kengrome]

So you are designing a hullshape

Correct.

You cant show a successful vessel currently using it

Maybe I could if it were important to me, but it's not. You're the one who asked about this. I am satisfied to discuss the concept as I have proposed it. If there are existing examples we can use for the purposes of this discussion that's fine with me, but I know of none.

But there are existing hullshapes that do work very well for your intended purpose that are different to yours and have had many thousands of hours of research done to ensure their success

You are welcome to post some examples. Perhaps they will provide something useful to this discussion.

Why not just draw up or use one that is successful now?

I don't know of any that can achieve my primary goals, I enjoy discussing design concepts and designing my own boats, when I settle on a final design I enjoy building the boat personally, and when I finish building it I can learn from my own success / failure.

[sabahcat]

Originally Posted by kengrome

I don't know of any that can achieve my primary goals,

All of these designers do that now without the just beginning to plane bit

http://www.powercatsnz.com/public/index.cfm
http://www.argusboats.com/
http://www.tennantdesign.co.nz/index.php?page=10-15m-2
http://www.givencats.com/
http://www.givencats.com.au/articles.asp#articles1
http://www.schionningdesigns.com.au/www/welcome.cfm

[kengrome]

Thanks sabahcat. I've seen most of those before but none appear to accomplish my goals. They seem to come in either displacement or planing styles, and both appear to be too slow for my target goals in the 32 foot length range.

I think I can design a hull for a 32' power catamaran that achieves high efficiency at 20-25 knots as well as a smooth ride planing at 50 knots.

[Fanie]

If I may make a suggestion, find as many pictures of such boats as you can find, look at their hull shapes and find the reason why they are shaped as they are.

It is very doable what you want to do, but you have to remember that a boat needs a surface to plane on. The smaller this surface the more power and the more speed is required to keep it on a plane. You won't always want to run flat out, it can become very unpleasant in rough water. The slower plane you can achieve the better. It won't affect your max speed, this is determined by the size motors and the props you run on it

Once a boat is on the plane, very little power is required to keep it there.

You don't need a rocker bottom, the boat is pushed, not pulled. Once the boat exceeds a relative low speed, the turbulent water at the transom will be left behind and will not make any more drag.


BTW, I like the idea you have. Just hope it's not going to be too narrow.

I cannot see how high those hulls are...

What propulsion have you in mind ?

[Fanie]

One more thing, remember that 10m is a fairly long rig. If it has a rocker and you do get it on the plane, that nose is going sky-high and you will have to look around the hull to see wher you are going.

On a flat bottom hull it is going to plane flat while maintaing good visibility.

I'm going to visit one of my friends this afternoon for a couple of days. He has a 12m x 6m cat with two 140 Toyatsu motors on them. The hulls are 1m x 1m. We're going to play with it a bit. If you're still around in a few day's on this thread I'll let you know how it went.

[kengrome]

If I may make a suggestion, find as many pictures of such boats as you can find, look at their hull shapes and find the reason why they are shaped as they are.

I have tried but I haven't come up with much. It is difficult to find drawings or pictures that clearly illustrate hull shapes of boats designed to be very efficient at displacement speeds and also plane smoothly at high speeds.

I think I know enough about the basics to create my own hullform, so not finding appropriate existing hulls does not bother me too much. Perhaps with the input of people here we can come up with a better hull for my goals than exists in the market.

It is very doable what you want to do, but you have to remember that a boat needs a surface to plane on. The smaller this surface the more power and the more speed is required to keep it on a plane.

I understand that it will take a lot of power to plane at 50+ but fuel efficiency at that speed is not my concern. I just want to be able to plane at that speed if I feel like it. My primary concern in terms of efficiency is that the boat is very efficient at 20-25 knots since that's going to be its typical running speed.

You won't always want to run flat out, it can become very unpleasant in rough water.

I agree, but my goal here is to create a power catamaran with long skinny hulls that interact with the surface and waves smoothly -- so I do not have to "slow down" just to avoid being beaten to death. I may have to slow down because of the swell angle or wave character or other reasons, but I do not want to slow down just because the boat is pounding me to death in a bit of a chop.

It just seems to me that long skinny hulls have the ability to run much smoother than short wide hulls of the same planing surface area, so why not take advantage of this to create a boat that can run "really fast" (relatively speaking of course) and does not have to slow down just because there's a little bit of wind and wave action?

The slower plane you can achieve the better. It won't affect your max speed, this is determined by the size motors and the props you run on it.

Right, and with such long narrow hulls I may not even be planing at 25 knots -- but if I reach 50 knots in those same hulls then I am definitely planing, and because this is what I want to achieve I cannot simply use a displacement hullform. Instead I need a hull that's designed for planing.

You don't need a rocker bottom, the boat is pushed, not pulled.

From my understanding a planing power boat should have a transverse flat bottom at the transom for maximum planing efficiency. This means the transom will be below the waterline in a hull with no rocker.

It is also my understanding that the best way to reduce drag in such a hull is to give it enough rocker to bring the transom out of the water, and that's why I gave this hull the rocker it has now.

Please tell me how you might eliminate transom drag at displacement speeds of 20-25 knots without employing any rocker ... while retaining the hull's ability to plane at higher speeds?

Once the boat exceeds a relative low speed, the turbulent water at the transom will be left behind and will not make any more drag.

I agree, but you have to remember that most of the time I'm going to be using this boat at 20-25 knots which (given the length of the hulls) suggests that I will not be planing at this speed. Instead I will still be in displacement mode or semi-planing mode, either of which will be more efficient if the transom is out of the water ... correct?

What propulsion have you in mind ?

An outboard on each transom.

[kengrome]

One more thing, remember that 10m is a fairly long rig. If it has a rocker and you do get it on the plane, that nose is going sky-high and you will have to look around the hull to see where you are going.

Actually I don't think the nose will go that high, but even if it does this is a catamaran which means visibility forward will still be great between the two hulls.

On a flat bottom hull it is going to plane flat while maintaining good visibility.

This is a good argument for no rocker, but I still don't know how to eliminate the transom drag without any rocker.

I'm going to visit one of my friends this afternoon for a couple of days. He has a 12m x 6m cat with two 140 Toyatsu motors on them. The hulls are 1m x 1m. We're going to play with it a bit. If you're still around in a few day's on this thread I'll let you know how it went.

I'll be around, and it will be interesting to see your performance report. Thanks Fanie, and have a good time.

[erik818]

Kenneth,
I don't think that the rocker will keep the transom out of the water; once you start moving at speeds where efficiency matters the rocker will pull the transom down.

I've done some calculations on a 9 m long hull with L/B = 20 and displacement 1 ton. (2 hulls = 2 tons). The bottom is flat without rocker and the trim angle is 3 degrees. According to Savisky, about 80% of the lift will come from displacement and 20% from planing at 20 knots. Friction drag will clearly dominate over wave drag and induced drag.

As far as I understand, neither Michell nor Savitsky are really applicable for resistance prediction on this hull. The Savitsky model is for planing boats, and most of the lift here comes from boyancy. The Michell model is for long and narrow displacement hulls without flow separation. The resuls above are therefore only indicative, and there is also the very real possiblity that I've not understood how to use the Savitsky Excel sheet correctly.

The wet transom area will still be small at low speed, so maybe the efficiency loss is acceptable. Once the transom is dry, I agree with Fanies guess that the hull efficiency will be quite high. I have no good guess regarding at which speed the transom will be dry. For what it's worth, the Michell model indicates 13 kW per hull for 20 knots and about 1 kW for 7 knots (engine and propeller losses not included). Does it really matter if it turns out that you need 2 kW instead of 1 kW at 7 knots because of turbulence at the transom?

Maybe there is a prediction method other than Savitsky or the 1898 Michell equations that can predict peformance for a long and narrow hull with a transom? Maybe Rick knows, he's much more of an expert on performance prediction than I am. There is also the possibility of model testing, provided the ice situation in the Gulf permits this.... Where I live I expect the skating season to continue for another three months, so boating issues will have to remain theoretical until summer.

Erik

[masalai]

http://www.boatsonline.com.au/boats-...s.php?de=63650 will do 20 to 25+?knots more power goes faster, I have been on one with twin 250hp outboards and will go as fast as you are game to push it, I was not game to go above 40knots in the 3 to 4 ft slop (airborne too much)... The Schionning designed "Vapour trail" uses another system to smooth the ride in high speed displacement mode...

Australia is renown for its powercats - developed for use in the Queensland Gold Coast to service the shark-nets that "exclude" those dangers from the popular beaches thereabouts and needed to be stable when stopped and able to travel at speed in almost any seas... "SharkCat" was one of the first, and other manufacturers brands included "Powercat" "NoosaCat" and a host of others... The trend now is for displacement hull-forms that will happily do 23/27knots max and cruise around 15knots 'forever', burning about 1.2litre per nautical mile or less... I understand "Foreign Affair" a 47? ft, long-distance-cruiser was on the market and its range was 1 to 2 thousand miles but the website featuring this Robin Chamberlin design is closed...