? Thrust (lbs or newtons) to push a sailing canoe?

[MJC]

So I'm still fooling around with designing "something different" to sail my little solo canoe.

Specs: 185 lbs of sailor, 30 lbs of canoe, 11' 6' asymmetrical canoe hull.

I've run across several interesting formulas that I don't quite understand yet, but need the forum's help getting some fact based opinions on the amount of thrust necessary to push said canoe and me.

Just so y'all know what's up, I'm currently designing an extremely efficient and aerodynamic 50% chord wingmast / 50% chord sail combination built using carbon fiber/BX-4 monofilm. The whole idea is based on analyses by Tom Speers. The airfoil of choice is the ch10sm. I'm hoping to get nearly 2.5 times the lift of a typical high aspect ratio Marconi rig, much reduced drag (therefore substantially less heeling). I don't expect to plane the hull or anything, just scoot briskly along with a really small, light rig.

But how the heck much push do I need? Thanks in advance for your assistance.

[essaunders]

Personally, that sounds like a complex calculation. Have you though about measuring the force needed? I envision a great big tow tank, towers with weights and pulleys..... oh wait, I suspect that may be harder than necessary. But really, would it be possible to skip the calculation and just do a couple of measurements?

[Aramas]

Without doing a whole bunch of fiddly measurements and calculations from the plans at the given displacement, most of the formulas aren't going to be much help. If you put the lines into a design program then it would be easier.

The easiest 'real world' way is to tow it at the intended displacement and speeds with a fishing scale in the line - the hard part is finding a boat with an accurate enough log. What I would suggest is to spend a day on a lake with a boat calibrating rpm with speed given with a gps.

[Todd Bradshaw]

In terms of thrust, a trolling motor with 18-20 pounds of thrust will usually push a smallish canoe in the 14' range at or close to hull speed with two people aboard. At that point, you can paddle all you want, but the boat isn't going to go much faster if at all. As I remember, they figure that a paddler alone is worth about 1/3 H.P.

For sailing, I've never seen a canoe plane in non-storm winds with less than around 70 sq. ft. of sail area, a large hull (17' or so) riding light and the sailor hiked off of the gunwale to keep it upright. The idea of a boat in the 11.5' range (which pretty much also requires sitting on the bottom) sailing above displacement speeds is most likely not too probable.

There are some crazy guys in Tasmania who sail sea kayaks in big waves, surf and high winds, but as far as I can tell they overpower the boat, cleat off the sail and both balance and steer by hanging a high-brace out to windward with their paddles.

For practical purposes, I suspect that your best bet is to stay in the same 20 sq. ft. to 25 sq. ft. range that would be used for a regular sail on a canoe that size. With the wing mast and hi-aspect airfoil you certainly may wind-up with a more efficient and responsive sailplan than with a soft sail, but I would be surprised to see any major speed gains on a boat where you probably can't overpower it and use body weight to keep it upright.

[skuthorp]

My Macgreggor has a 50sq ft gaff sail, and Iv'e tried it with a sloop rig and a crab claw sail too.
It only draws about 3 or 4ins max so it's hard to tell if your planing or not! Her hull is more or less the same profile as an older wooden rowing shell (16.7ft x 32ins) but deeper of course. At speed and hiked out she lays so far over (it's a decked model) that the 'keel' is on the surface and she skips sideways on the leeboard. Is this planing?
I have built a bent leeboard to try and get a better grip on the water but it may cause more problems than it solves!
I'm still a skiff sailer at heart and my favourite boat was a Cherub, but it's far too shallow where I sail for that.
:D :cool: :D

[Alexander Shramenko]

>Specs: 185 lbs of sailor, 30 lbs of canoe....

Well, total weight 215 lbs

>But how the heck much push do I need?

For hull speed (0.02 - 0.03) X 215 = 4 - 6 lbs

For planing speed (0.3 - 0.4) X 215 = 60 - 80 lbs

>...nearly 2.5 times the lift of a typical high
> aspect ratio Marconi rig, much reduced drag
>(therefore substantially less heeling)...

Run! NASA and DAPRA folks are after you!

[ 12-16-2003, 05:13 AM: Message edited by: Alexander Shramenko ]

[John E Hardiman]

Originally posted by MJC:
I'm hoping to get nearly 2.5 times the lift of a typical high aspect ratio Marconi rig, much reduced drag (therefore substantially less heeling). ....But how the heck much push do I need? Thanks in advance for your assistance."substantially less heeling" :confused:

I think you need a slighty better understanding of how a sail or wingsail works to drive a hull. Get a copy of Aerohydrodynamics of Sailing by C. A. Marchaj ISBN: 1-888671-18-1. He covers this exact topic as well as any I've seen. Has the whole wingsail discussion too.

[MJC]

Chose better terms for my google search and found this interesting calculator page...

Sailboat Calculator (http://www.geocities.com/Yosemite/Forest/2727/flicka_calculator.html)

Ran my numbers, lo and behold, about a third of a horsepower does indeed drive the canoe at or near the hull speed of 4.5 knots, as does something on the order of 25 lbs of thrust.

So, I think I'm getting closer.

Just to amplify my goal - the absolute minimum of mast/sail to push the canoe at hull speed.

I made my comment about heeling based on the assumption that the smaller the sail, the less the heeling moment. Highly efficient sails generate the same thrust with much smaller sail areas. Do I really need to read the book?

Sail when you can, paddle when you must.

[ 12-16-2003, 02:51 PM: Message edited by: MJC ]

[John E Hardiman]

Originally posted by MJC:
I made my comment about heeling based on the assumption that the smaller the sail, the less the heeling moment. Highly efficient sails generate the same thrust with much smaller sail areas. Do I really need to read the book?Yes. More "efficient" sails do not necessarily generate more thrust. DO NOT CONFUSE HIGH "COEFFICIENT OF LIFT" FOR HIGH THRUST IN A SAILING CRAFT!!

Firstly, there is no such thing as "lift", there is only drag in the direction that you want to go :D .
Thrust is the portion of the sail force (i.e. drag) that drives the craft forward. Everything else is heeling moment and side force. The heeling of a sailboat is caused by the sail force & hull/keel drag couple. While sail thrust is mostly a function of sail area, sail efficency, and apparent windspeed and angle; hull speed and heeling moment is only a function of the sail force and its center. Sail area is only one factor in the heeling moment equation and is not a good indicator about how much heeling moment dispariate rig styles generate (and we didn't even touch on the weight aloft stability issue). Get Marchaj and read him. No use spending money and time making the same mistakes that have been made before.