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 Boat Design Forums Catamaran dinghy 10-11' long

#46
05-16-2015, 08:30 PM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
2 horsepower, that's what I'm using and its a weak 2 at best. I will use a 4-6 hp on the 11' version.

it also wasn't going to carry the load either now has done that. It will have 2x the wetted surface at 11' but I'll have 2.5x the horsepower and the longer waterline. Care to share your calculations? It would seem that relative hull speed must be linear?
#47
05-16-2015, 08:52 PM
 Mr Efficiency Senior Member Join Date: Oct 2010 Rep: 677 Posts: 5,371 Location: Australia
How accurate is the 5.5 knots, as in, how did you measure it ? You wanted 8-9 knots with a 5hp motor, loaded with 500 lbs "or so", you will get my vote if you manage that. Why restrict it to 5 hp, you should allow some latitude to increase that to at least 8hp. Also, have you experimented to see what happens as you move weight forward or aft, rather than carefully positioning yourself toward the centre ? I would expect a dramatic effect ! You will have a boat where you will need to be careful where the weight is placed, fore and aft, the opposite of a traditional dinghy, where care needs to taken to avoid shifting too much weight to one side.
#48
05-16-2015, 11:51 PM
 Mr Efficiency Senior Member Join Date: Oct 2010 Rep: 677 Posts: 5,371 Location: Australia
Quote:
 Originally Posted by Boatguy30 2 horsepower, that's what I'm using and its a weak 2 at best. I will use a 4-6 hp on the 11' version. it also wasn't going to carry the load either now has done that. It will have 2x the wetted surface at 11' but I'll have 2.5x the horsepower and the longer waterline. Care to share your calculations? It would seem that relative hull speed must be linear?
Your frictional resistance, if we can assume that will be the greater part of resitance at your target speed, will increase considerably, given that a doubling of speed increases it four-fold, and your wetted area will also increase in the same way. The extra length isn't going to help unless you make the hulls slimmer than scaled-up size. You might get what you want with 10hp, given that you don't want to be flogging engines at full throttle all the time. But it will be very sensitive to fore and aft weight shifts.
#49
05-17-2015, 06:31 AM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
It does go a bit faster with the bows pushed down via GPS, but the model bows re so low 7.5" that the bow wave starts to flood the hulls! The actual 11' dinghy will have 12" more freeboard and the front 30" of the hulls decked over.

The 11' will be strong enough to take an 8 hp or 9.9 but you really don't gain anything over a RIB of the same lenght then. I think it will go well, maybe only 7 knots with the 5 hp. And have tons of space for lounging and fishing on the bridge deck.

I don't plan to start the full sized model till August.
#50
05-17-2015, 08:37 PM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
I added this foam fairing to flatten the run a bit aft. Amazingly it is now 10% slower so I guess ill go with the original hull shape. Wakes, etc seemed to be about the same.
Attached Thumbnails

#51
05-20-2015, 04:58 AM
 Manfred.pech Senior Member Join Date: Apr 2010 Rep: 319 Posts: 483 Location: EU
Quote:
 Originally Posted by Boatguy30 I added this foam fairing to flatten the run a bit aft. Amazingly it is now 10% slower so I guess ill go with the original hull shape. Wakes, etc seemed to be about the same.
Your experiments are very interesting. May be the flattning is working better with higher speeds and sharper chines to reflect water down and to prevent it climbing up the sides for more wetted surface and thus more frictional resistance.

Or you can modify the bottom to get it a bit concave as the photo shows on the hulls and the mould and gain some lift ? The boat on the picture is 3,30m.

Discussion has been here:
New Hobie Design?
Team Lalou's new Multi 50 trimaran
Stern Extension to Help Rowing

Last edited by Manfred.pech : 05-21-2015 at 08:48 AM.
#52
05-21-2015, 10:44 AM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
Working on getting my neighbors 4 hp running to test hull with larger motor and foam fairings and knock them off and try to without as well.
#53
05-22-2015, 07:25 AM
 Manfred.pech Senior Member Join Date: Apr 2010 Rep: 319 Posts: 483 Location: EU
Quote:
 Originally Posted by Boatguy30 Working on getting my neighbors 4 hp running to test hull with larger motor and foam fairings and knock them off and try to without as well.
Good luck for your Tests. Nothing in my life with boats has been so exciting as testing hulls in a testank or in reality, trying to understand the results and then to optimize them as far as possible.
#54
05-23-2015, 04:07 AM
 Manfred.pech Senior Member Join Date: Apr 2010 Rep: 319 Posts: 483 Location: EU
Catamaran CFD

May be you are interested in computer fluid dynamics for catamaran hulls (Wigley): http://www.panix.com/~brosen/catamarans.html and http://www.google.de/imgres?imgurl=h...B8F4WRlWqlHNnM, https://www.youtube.com/watch?v=nSvsYgV_0Yw
#55
05-26-2015, 06:59 AM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
Don't think I'm up to figuring out the data on those links.

I did get the 4ish horse running. It's about a 1969 motor that had been sitting for abut 20 years. Full power we achieved rarely as constant attention was needed to the high speed mixture needle.

But I did manage 7.2 knots with the foam removed with little more wake than at 5.5 knots. So I'm going to go with the 11' model. Will start building in August or Sept, unless something else comes along.
#56
05-28-2015, 07:46 AM
 Manfred.pech Senior Member Join Date: Apr 2010 Rep: 319 Posts: 483 Location: EU
Quote:
 Originally Posted by Boatguy30 Don't think I'm up to figuring out the data on those links. I did get the 4ish horse running. It's about a 1969 motor that had been sitting for abut 20 years. Full power we achieved rarely as constant attention was needed to the high speed mixture needle. But I did manage 7.2 knots with the foam removed with little more wake than at 5.5 knots. So I'm going to go with the 11' model. Will start building in August or Sept, unless something else comes along.
Thank you for sharing the results of your tests.
#57
06-04-2015, 12:56 PM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
http://wavewalk.com/blog/2014/11/06/...utboard-motor/

I came across this side via a local dealer selling them on Craigslist. I don't quite see it doing 13mph AT 1/3 throttle. Roughly the same hull shape as mine but a flatter/narrower run aft.

Maybe this guys are interesting in helping build the ultimate cruising dinghy?
#58
06-05-2015, 05:21 AM
 Manfred.pech Senior Member Join Date: Apr 2010 Rep: 319 Posts: 483 Location: EU
Quote:
 Originally Posted by Boatguy30 http://wavewalk.com/blog/2014/11/06/...utboard-motor/ I came across this side via a local dealer selling them on Craigslist. I don't quite see it doing 13mph AT 1/3 throttle. Roughly the same hull shape as mine but a flatter/narrower run aft. Maybe this guys are interesting in helping build the ultimate cruising dinghy?
Wavewalk is a nice little multipurpose boat -- but with such a weighty, noisy bulk of motor ??? There are smaller and lighter four/two stroke motors on the market http://www.ebay.de/itm/Ausenbordmoto...3D261895212073 and http://www.ebay.de/itm/Ausenborder-A...92661482&rt=nc .

5 hp: http://www.ebay.de/itm/Ausenbordmoto...92661482&rt=nc

#59
06-06-2015, 11:13 AM
 Manfred.pech Senior Member Join Date: Apr 2010 Rep: 319 Posts: 483 Location: EU
Quote:
 Originally Posted by Boatguy30 http://wavewalk.com/blog/2014/11/06/...utboard-motor/ Maybe this guys are interesting in helping build the ultimate cruising dinghy?

These guys should help you. You will have enough payload und you will be really fast. With four people on board you will get on the plane with 25hp.
#60
07-23-2015, 04:27 PM
 Boatguy30 Senior Member Join Date: Dec 2011 Rep: 13 Posts: 111 Location: St Augustine, FL
To update this thread, I just picked up 6 sheets or 1088 okoume some for the proposed dinghy and some for a few other interior details in my Vardo. I also have a direct drive 1967 Evinrude 5 hp which weighs only about 18kg. I think I will try the model one more time with the 5hp before going further. I'm sure I can find something else to built with 4 and 6mm okoume.

Will post of pic of Homestead Hardwoods in NW Ohio. They have 2 horse in a pen at the back of the plywood warehouse!

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