Quick, Easy 3-Man Canoe Plans

To some extent, yes, because you are tending closer towards an even pressure distribution on both sides of the chine. The effect probably wont be noticeable though, unless you go for extreme flare in the ends.

Not that you'd notice at any speed you're likely to reach.

True.

What benefits? Ease of paddling?

My guess is that you wouldn't have a snowball's chance in hell of getting nine knots. You might get six.

One thing you don't seem to be aware of is that taper (as you call it) can reduce the wetted surface for a given displacement.

For a three panel section, the minimum wetted surface for a given cross section is achieved when the immersed widths of all three panels are equal, and the topsides are flared at 30 degrees.

This gives the closest (immersed) approximation to a semi-circular section, but may have other drawbacks for you (primarily initial stability and ease of paddling).

 

Thanks NED!

What do you like to call taper?

The benefit of a narrower beam would be less drag.

So, why does your countryman Rick build taper into his hulls?

 

It's usually called flare when you're talking about topsides panels.

Small changes in beam, all else being equal, wont have a huge effect on wave drag.

You'd have to ask Rick about what Rick does.

 

Oh, anecdote for you, just as a point of reference.

Bloke I know used to race those Herreshoff-Gardner rowboats, which have a 16 foot waterline. What he found was that they went fastest solo*, and slower with two people rowing. The extra wave drag from the added displacement apparently outweighed the extra power from the second person.

Point being that if you want any advantage from having three blokes paddling, you probably ought to make the thing longer than you might think.


*Sprint speed around six knots. Short distance only.

 

What sort of length would you recommend?

 

Rick's no longer available to ask.

Don't you know why he does it?

Clarification: This is not just topside flare, it's from the bottom.

 

Hi Tom.

I think the deck is a good idea, but why worry about getting wet? Adding spray trim will add time. This not a concors d'elegance!

4" freeboard sound fine.

I would suggest nice snug seat backs and good foot braces - you don't want to lose crew!

Manfred's pic looks like an example of the extreme "get every inch of boat you can out of the ply sheet" school of design: it's an excellent idea but takes longer to build, cutting precise curves, relatively high bending forces, S&G and epoxy fillets with glass tape: days not hours, but I'd love to try it as a build sometime.

If the tapering is an attempt to get enough stability to run without outriggers then I think it is doomed to failure. No way a boat that narrow will be stable enough without outriggers especially with the crew riding on top, but again you can try removing the floats on the proto. I say square up the sheers in the proto, simplifies the plank cutting and you'll be able to hear excessive turbulence through the hull. The hull shape looks like a fast cat or tri with enough sail power to plane on one hull, but you're not going to get this boat to plane.

My preference is still for a mono for reasons of building time but If I were going for a multi I'd go with a simple main hull, with width and depth chosen to minimize wetted area, and very skinny floats almost as long as the hull. However, if the "planing" floats turn out to be draggy you can always replace them on the proto.

Minimum wetted area for a "square" boat occurs when the bottom width is double the draft BTW. NEDs formula is correct for totally immersed bodies but gives 12.5% more wetted area for the same displacement of a floating body. If I am right about skin drag predominating that translates into significantly more drag, athough it might increase wave drag.

When testing the proto I suggest you determine the effect of the rather deep draft by timing it with one, two and three crew to see how much the extra power is offset by the extra weight and draft. That will go a long way toward validating your theories.

 

My guess is that he (Rick) probably did it to reduce wetted surface for a given cross section area. IIRC he was making float-stabilised pedal boats, so he wouldn't have been worried about stability.

You could check this by looking at the lines of his boats, if you have access to them. If the topsides are flared at 30 degrees off vertical, and the section is such that the immersed widths of the three panels are even at midships, then he would have chosen the section for minimum wetted surface.

Haven't thought about length. Run some things through Michlet if you're curious. That should give you a ballpark. The optimum length for low drag will probably be too much for manouverability though, so it'll be a trade-off.

 

Just in case the words aren't clear, this is what the minimum wetted surface section for a three panel boat looks like.

 

..............................

 

Thanks NED.

Here are Ricks lines:

 

I suggest you do the maths again. No offence, but you are wrong. The idea is to get the closest approximation to a semi-circular section, which is what gives the minimum wetted surface for floating bodies. You don't get this by using a 2:1 rectangle.

It goes like this:

For a 2:1 rectangle you have a girth of 4 units (one for each topside, 2 for the bottom) and an area of 2 square units.

For the same area, the section I suggested will have a girth of 3.722 units, which is a reduction of just under 7%.

ETA: About the picture in the post directly above, I'm going back to saying you'd have to ask Rick about what Rick did. Although one thing he has tried to do is balance the pressure distribution on both sides of the chine. I suspect this is one reason why the hull is deeper than the optimum for low wetted surface. Not sure if it's a net benefit or not.

 

Half the beam of the draw? You mean a draft of half the beam? That's minimum wetted surface IF you are using vertical sides, but it's not minimum for a three panel section. Like I said, you're carrying a 7% penalty compared to the best section.

I looked at Post #24. Not sure what the point of it is, nor why you think it would be fast. You say it has three times the wetted surface of a monohull (which monohull?), and it only has an 8 foot waterline length. That sounds like a really slow combination. To go fast you need it the other way around: low wetted surface and a long boat.

 

So, what would you suggest?

 

I can't see where you get the second girth value from. A square of area 2 would have sqrt (2) sides and bottom = 1.414 each or 4.242 total girth.

Here's the mathematical proof; sorry about the need for calculus:

Let draft be D, let bottom with be W, let area be A, let girth be G, then
A = DW
G = 2D+W or W = G - 2D
Substituting for W in the first equation: A = D(G-2D) or
2D^2 -GD + A = O
To find the value of D that provides maximum area, differentiate with respect to D:
4D - G = 0 or D = G/4
and substituting D in the second equation: W = G/2
Or in other words W = 2D