Rocker on flat bottom displacement hull.

[Giuliano Girometta]

Hi, Ia am a member of this forum for about one month and so far I spend most of my time reading numerous interesting and very knowledgeable threads.
Now, I have a question and I am hoping someone may be able to give me some explanation that I was unable to find in any book.

I studied several plans for double ended flat bottom boats and I noticed that there is a large discrepancy between the amount of rocker applied to double ender and dories.

According to what I understand, the rocker on such type of boats is helping turning the boat quickly and with minimum effort as needed by a drift boat on rapids. While less rocker will improve the capabilities to maintain the boat on a steady course.

Looking at older design of double enders, I noticed that in the past centuries, several builders contructed the boats with a straight flat bottom even going through more trouble to adjust the shape of the planking along the cines in the case of heavy flared sides.

My questions are as follow:
1) There are advantages to increase the rocker on a boat that will be used on rivers and inland waters?
2) Are those newer designs may be just made to fit the planking into a 4' standard height for a plywood sheet and therefore the accentuated rocker is added in order to avoid longitudinal scarfs or butt joints?
3) Which disadvances will have a straight flat bottom double ender or dory?
4) Will the straight flat bottom reduce the drag and consequently less engine power will be needed to reach the hull speed?
5) Will a straight flat bottom be more user friendly in order to be pulled on and off the trailer at the ramp.

I am thinking about a 26' with strip planking on saw frames (no plywood).

Thank you in advange for reading this post and any information and suggestion will be very welcome and appreciated.

Giuliano Girometta

[skuthorp]

G'day Guiliano, welcome aboard. There are many others with better knowledge of what you ask but I think you're are just about right. I have a double ended sailing canoe, only 15'7" of mahogany ply. A Macgreggor. It has a lot of rocker and needs a rudder to track properly when rowed, but sailed and paddled this is an advantage I think.
Don't know about drag and a flat bottom, depends if you intend it to plane or not I suspect. There is the thing of 'hull speed' with any design, above which a disproportional amount of power is needed to improve it.
That's about my level of theory done, but you might have to post a bit more info about the proposed craft, or what you desire in one first. Then I think you will get some references to various designs, motors, etc that will get you on your way.
Designs 'from past centuries' were often hit and miss affairs without the advantages of modern design technology and new glues and materials. Best of luck and good to talk to you.
Sku

[Giuliano Girometta]

Hi Sku and Rich,
Thank you for reply to my post.
I am searching some answers about boats like the St.Pierre from Nexus Marine, The Lucky Pierre from Glen L. and teh Labrador Grand Banks from Spira Int.
Therefore my intrest was on displacement hulls with a top speed equal to the hull speed.
Still wandering if someone can explain why various designers are adding a large amount of rocker while others are almost stright.
All large ships are flat bottom and the keel is straight. Sure if there was any advantage on speed increase and/or fuel economy by adding a rocker to a ship they were the first one to consider that. (unless something is changing when the maximum speed of the vessel is much less than the hull speed).
Furthermore, I think that if an outboard propulsion is used with the motor in a well, and with an excessive rocker toward the stern, the ventilation plate of the outboard motor will be at an uneven angle with the hull. Is that the reasom why Glen L on the Lucky Pierre is putting the motor almost at midship?

Giuliano

[Lewisboats]

Adding rocker is a quick way of adding capacity and capacity sells.

[Frank E. Price]

A rockered bottom makes a boat more responsive to steering input, whether the input is from paddle, oars, rudder or outboard motor. A bottom without rocker makes a boat hold a consistent heading better. It's a compromise, and all you can do is give the designer as much information as you can about how you will use the boat and where, then have enough faith in his knowledge and integrity to steer you straight (pun intended). Or trust your own experience and knowledge. And as far as responsiveness and bottoms is concerned, we are talking about bottom profile including everything outside the planking: keel, skeg, rudder, foregripe, etc. Look at the bottom profile as a silhouette; it's not necessarily the bottom itself, but the shape of the silhouette that matters.

The boat before my current sharpie skiff was a light double chine boat with a narrow flat on a bottom with moderate rocker and no keel or skeg at all. It was a pleasure boat, designed for easy rowing, not weight capacity. Seemed strange to me just looking at the plan, but I decided to try it out. That boat would not go straight without both oars in the water, and going straight required a conscious effort. But it would spin on a dime provided you started it spinning in the right direction. The sharpie skiff is a flat bottomed work boat with a huge skeg. The bottom has a lot of rocker aft, but the profile is pretty straight. It will coast along in a straight line all day long if you give it a little boost now and then, but turning it is hard work. Under sail it steers like a bigger full keel boat, but you have to think ahead a ways before you tack. The straighter profile gives heading stability, and the bottom rising out of the water to the bottom of the transom almost a foot above the water gives capacity to add weight (fish, lobsters, oysters, etc.) without dragging the transom through the water.

In a boat that is going to be used off and on beaches, a little rise in the bow makes it easier to get the boat a little farther onto the beach before stepping out, which might be important to some users.

If you put an outboard well (or fish well) near either end of a narrow fine-ended boat, you are removing bearing (or buoyancy) in the place where the boat most needs it. An outboard or fish well is a hole in the boat full of water. The weight of that water (in addition to the weight of the added structure) represents the buoyancy lost in the end of the boat. If the boat was not originally designed with the well, adding it will lower that end of the boat in the water, upsetting the designed trim. Much better to put the new hole in the bottom closer to amidship, where the loss in buoyancy will have less effect on trim.

One thing you can be sure of is that no boat is perfect, each one providing an excuse to try another.

Frank

[Giuliano Girometta]

Thanks Steve and Frank for your reply and valuable info.

Giuliano