Simple power catamaran?

Great ideas-thanks! I did a Craigslist search and there certainly a lot of tired old sailing cats out there-definitely worth a look. The Cougar looks like exactly what I would want, but they don't seem to be that common around here. I suppose if I keep my eyes open one might turn up.

 

It's not clear to me what your planned usage is for this boat. Are you planning to use it as an actual "party barge" with a bunch of people onboard? If so, you could quickly add 1,000 lbs of payload, depending on how many friends you have and the size of your beer coolers. Payload is the same as boat weight.

Why 20 feet? Is that the length limit for the lake you're on? If so, then you're simply going to be limited on speed by physics, as Ad Hoc has demonstrated. But if you can stretch her, then the length to displacement ratio becomes much more favorable. It's striking what even a couple feet can do for you.

If you want something simple and cheap to get on the water, then pretty much any 20 ft pontoon boat will probably meet your needs, though it certainly won't go a lot over 5 kts with 9.9 hp. If you can go longer then the used cat approach has merit. And if you can go longer and are willing to put in time and $$, this might be the boat for you: http://www.westsystem.com/ss/birth-of-the-gougmarans/

Not sure how far you need to run in a day, but if it's not too far, and your pockets are deep, then this would be a quiet option: http://www.torqeedo.com/en

 

Getting back to this after a busy few weeks at work and looking forward to continuing the discussion for those who are so inclined.

Not exactly-when I said "glorified party barge" essentially I just meant that I'm not looking to build a fancy sailing catamaran or sleek power boat, just a deck boat that's not a total pig with a 10HP outboard (or a 15 if I can sneak one on). Crew weight (two adults and three kids) would be about 600lbs, maybe another 150lbs of gear, plus a boat/motor weight of 1500lbs?

20' was sort of arbitrarily selected, but I probably wouldn't go much more than say 26' for storage space and trailer/launchability reasons.

Thanks for posting that-those are some pretty sweet cats, and kind of along the lines of what I'm looking for. Like the authors, I'm looking for something with little draft, easily beachable, and efficient just for cruising the lakes. No massive amounts of people, no beer (not allowed on the lakes), just some island hopping and enjoying a nice day on the water.

With regard to speed, I don't really need to do much more than 5 knots, I'd just like to be able to do 5 knots without having to have the throttle maxed out the entire time. What about building/modifying two rowing shells and using them for hulls? I found a nice one 26' LOA x 30" beam just now. And then there's this-it seems Bateau has designed a catamaran specifically for electric power use-surely a 10HP would do OK. They list a 2000lb displacement at DWL but no hull weight:

http://bateau.com/studyplans/EC24_study.htm?prod=EC24

 

Your assumption is correct.

Ad Hoc loves espousing the importance of the slenderness ratio, and he's right of course, it's a wonderful indicator of how the boat will perform for a given power, but reality is more nuanced than that.
A "boat shaped" hull with the same slenderness ratio as a pontoon cylinder with a flat end will have a lower resistance, as you suspected.

But in a choice between a pontoon with high slenderness and a "boat shape" with low slenderness, the benefits of slenderness will likely outweigh those of improved hull shape.

 

I know you will have seen my Skoota V pontoon boat comments on my website, but have you seen these? More modern than the GlenL designs

http://www.bateau.com/products.php?cat=4#.VbwPQvkjxbg

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Not really. We have done a lot of tank testing and research and proven the "shape" thing to be a myth - Molland et al confirm this too. Obviously if you have a full bluff front bow, a vertical wall compared to a a Vee'd bow, then it shall make a difference, all else being the same.

BUT..in general...two shapes that are same dimensions, inc displacement and the only difference is the "shape" of the hull, there is very little difference in the resistance. The difference is so minor that unless one is going for world records, it is not worth worrying about, or, unless one wish to compared decimal places as a sign of an "improvement"....

 

Am I correct in assuming that shape significantly influences resistance at sufficiently high speed?

 

By "shape" I am assuming you mean a real hull shape with a purpose. Other than those that seem to take the argument to an extreme, which can only ever be a theoretical argument, since who wants/needs a vertical flat bow?!

Not according to our R&D and also Molland's. (Molland used one of our hulls for his research too, btw).

I should point out that I am referring to LD ratios over 6.5 and displacement type hull forms at high Fn's beyond 0.5 rather than fully-planing hulls.

Since once one draws up a lines plan with an LD ratio over 6.5...it is clear that changing the shape can only affects so much. The only real driver that does influences the resistance is the transom immersion. And on monohulls the amount of hard chine in the overall length on semi-D hulls.

 

Ad Hoc, thanks for the response. I guessed that you were referring to displacement hulls with higher LD ratios.

 

A vertical wall to a typical hull shape IS what we're comparing here.

And as for any other comparison, a small difference is still a difference.

 

We did investigate some lower LD ratio hull forms, but that is generally not our bailiwick. But there is a similar trend, yet it starts to diverge once the LD ratios get low.

And that's my point, QED
Those that wish to try for a world record, or simply compare numbers of decimal places will always try and argue a very very mute point. Which considering the design of a boat, hydrodynamics is just one of many disciplines that must be satisfied, each with their own sets of variables. I really don't understand why people focus on the hull shape alone, and at differences in decimal places as being a Eureka moment!...when in reality nothing could be further from the truth in a naval architecture holistic sense.

 

You're going well off topic. The OP asked about a "boat shape" vs the end of a pontoon drum. Of course the shape has significant influence here.

 

Well, three examples were given by the OP...none of them are box shaped, as you're inferring. Thus:

Please explain what is "obvious" here....??

 

I'll quote the OP for you: 'I was assuming that two (for lack of a better term) "boat shaped" hulls would be better than two aluminum cylinders'

Please don't quote and ask me to defend words I haven't used.

But for someone who has used that word re. the situation in this thread, I'll quote yourself: "Obviously if you have a full bluff front bow, a vertical wall compared to a a Vee'd bow, then it shall make a difference, all else being the same."

 

Yeppers. Nice boats.

The hulls are close enough together that there will be more wave system interference the harder you push them ... but since when has going (relatively) fast not been a handy dandy way to help transfer the contents of your account to the oil companies?