Low horsepower proa or motor canoe hull shape?

Hi folks. My name is Alex and this is my first post on the forum. I have designed and built a houseboat and some rowing boats and have learned lots from my mistakes! My next project is either a skinny monohull or proa to be used as a fishing platform in SE Tasmania.
I would like to build a boat that weighs less that 80 KGS, can handle a short 2 foot chop and can travel at hopefully 8 knots + with a 3.5 HP outboard and a payload of 150 KGS.
My main questions are, assuming a beam/length ratio of roughly 1 to 6, and hard chine plywood construction, what is the generally optimal bottom shape and stern shape for my hull? Thanks for reading and apologies if the question is too broad. Cheers, Alex

 

Shape is not that important as long as you stick with something relatively conventional. You will be going over hull speed though so you don't want a lot of rocker at the back. What is more important is the displacement to length ratio. You said you were thinking of a proa. I think you mean maybe an outrigger as a proa goes in both directions. Usually people building these sorts of boats are shooting for length. Just make the main hull as long as you can within your weight limit. See if you can get the length to beam ratio over 8:1 and you should pass hull speed. A long hull will be better through the chop too. I had a 3.5hp on a 80kg rib and it could semi plane at around 8k with a 95kg payload. It would have bogged down with 150 for sure. How long can you make this boat?

 

Tha

Thanks Dennis. Yes I do mean an outrigger, my terminology was wrong. The water is cold where I live so a super skinny, wet boat is not an option, it also introduces engineering complications. I think 17 or 18 feet would be my max LOA. Surely the stern shape has some effect on speed in the semi displacement realm? Thanks for answering my post.

 

Yeah for sure the stern shape will matter in semi displacement. Which is what I meant when I said you would not want a lot of rocker at the back. Making the boat longer makes the most difference so go as long as you can make it IMO.

 

I think your 5.5m limit might break you out of displacement speeds with one person but probably not 2. Sailhand on the forum designed a power cat dingy which does 12k with a 4hp and one person. Its about 3.5m? long but has 2 narrow hulls.

Here is a vid of someone claiming to be doing 7k in a 16.5 foot canoe with 600lbs and 3.5hp.



Here is another doing 8k with 5hp and 2 people.



Seems you might just get there!

 

90 kg with little shelter: New small Multi from Icarus https://www.boatdesign.net/threads/new-small-multi-from-icarus.55772/



or: Outrigger Sailing Canoes http://homepages.paradise.net.nz/garyd/

or this one:

 

You might try this - what was once called a freighter canoe. If you want more stability you could put an outrigger on it. He doesn't actually say how fast it goes / size of motor.
Robb White, Boatbuilder, Thomasville, Georgia http://robbwhite.com/sportboat.html

Narrower could be faster, but wetter - I'm assuming.

Here is one in ply. Outboard Motor Canoe 15 http://www.duckworksbbs.com/plans/sf/canoe/motor15/index.htm

Another that claims 18.5 mph with 9.9 hp
Lutra Laker & Little Laker http://www.duckworksbbs.com/plans/nichols/laker/index.htm

 

Thanks everyone. I have studied all these options and came to the forum because none of them seemed fast enough, or were fast enough but not seaworthy enough. It sound like sailhand's 12 knot 3.5 m cat thread has just the information I'm looking for but a google search is not revealing it to me. Can anyone please post a link or list some key words to search? Thank again for your replies.

 

Hi Al

It sounds like you are in my part of the world. You project is of real interest to me, so please post as much as you can.

You may get a giggle out of this thread
Historic Home Made Tri https://www.boatdesign.net/threads/historic-home-made-tri.41690/

I posted some info on a small trimaran that I built and sailed. I built and designed and re-designed it over 6 or 7 years, it was all a lot of fun. It ran a 6hp outboard, which I found a great size for performance and ease of handling.

I am not sure if this has been mentioned, but my opinion is that Rob Denney at Harryproa http://harryproa.com/ has the best handle on proven Proa design, if you would like to look over his smaller size designs.

Keep up the good work

 

It was me who started the thread years ago. Then Sailhand chipped in more recently. His posts are towards the end. I took the tender out for spin a few weeks ago! Its great. He says it works amazingly well in choppy conditions too. Its under 80kg and does the speeds you want with a 4hp so does not need to be registered. The build is a little more complex and expensive and uses molds which Sailhand may let you use. The design is optimised for these medium speeds with low power. Adding a large motor only gives moderate increases in speeds.

Catamaran tender to replace rib? https://www.boatdesign.net/threads/catamaran-tender-to-replace-rib.50834/

 

Dennis, thanks for the link. Rwatson, will let you know of any developments. Thanks for the blue tri link. I know all about how it goes with "quick, cheap" boat projects and the struggle of finding real estate for them!

 

My latest thinking is leaning toward a monohull. Here is a model. Length is 4.6 m, beam is 1.2 m.

 

Bottom planking would be 6 mm ply, side planking would be 4 mm. Probably glassed inside and out with 6 ounce cloth in epoxy.
Could anyone please advise as to any speed benefit gained by moving the engine in from the transom to a well? I'm thinking about 50 cm forward of the transom and open at the stern like John Welsford's trover or Clarence river dory.

 

I'm also wondering about creating a spray rail/lifting strake by running a chine log along the outside of the bottom panels at their outer edge, that would follow the twist of the panels, then be bevelled to have the side planking attached to it. Alternatively these could be added afterwards. Would it be worth running them right to the stern or blending them in 2/3rds from the bow?

 

A and C represent midships sections without and with the strake. B and D represent stern tramsom sections without and with the strake carried fully aft.