Strakes, chine steps and other skegs.

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jacquesmm
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Strakes, chine steps and other skegs.

Post by jacquesmm » Mon Oct 01, 2018 4:13 pm

Questions about strakes, spray rails, step chines and skegs come back regularly. Let's give a summary of my opinion as a designer.

Definitions:
- A strake is a longitudinal step that runs lengthwise on the bottom panel. There can be more than one strake.
- A spray rail runs lengthwise along the bow part of the boat, often but not always along the chine.
- A chine step is a step along the chine. The step is wider at the stern and often becomes a spray rail at the bow.
- A skeg is a small keel usually running from a little forward of the middle of the hull to a few feet before the transom.

What do they do and do we need them?
This depends on the hull shape, intended boat speed and material.
The material is very important. All production fiberglass hulls have strakes and most have a step chine/spray rail. They need them because single skin fiberglass is flexible and need stiffeners. That is why strakes are built in the bottom and the chine has a step, two angles. Those angles work as small beams that stiffen the hull panels.
The skeg would be a nice addition but creates major layup problems and few production boats have them.
I know this because before designing plywood-epoxy boats for amateurs, I worked as an engineer, designer and engineering manager for some major fiberglass boat manufacturers.
Because strakes and step chines exist on almost every boat produced today, we may think that they are absolutely necessary but that is not true.

Should plywood-epoxy boats have those appendages?
We don't need them as stiffeners, our panels are stiffer than single skin fiberglass but, in many cases, they have benefits.
In many cases, I show a skeg. The skeg will help tracking and improve maneuverability. It also protects the hull on the trailer and during beaching. On a planing hull, always end the skeg a few feet before the transom or it will aerate the water that goes to the prop.
Spray rails will deflect sprays and can always be added by epoxy gluing a wide strip on the chine.
Chine steps have another function. They create lift and provide dynamic lateral stability while on plane. They only work if a large part of the stern side of the step stays immersed at high speed. They can be built in or added. In some designs (OB15, CS23 and 25), my plans show a built-in step chine, in others, it can be added to the outside of the chine.
Water must separate quickly from the step chine or it will add drag, we need a sharp edge on those appendages It is impossible to create a sharp corner in a production mold but easy with an added step chine. If you build one of my designs with a built-in step chine, I recommend to add some putty to that corner and build a sharp edge.
Properly done, a step chine will be a spray rail at the bow, work as a stabilizer in the stern and add a little bit of lift.
Strakes along the bottom will help fast boats. Below 25 mph, they don't do much. Strakes improve tracking and maneuverability, just like a skeg. They produce a little bit extra lift (not much) and they separate the water from the hull which reduces drag at planing speeds. This improves speed a little bit and also reduces sprays.

Boat types:
Displacement boats do not need strakes but a skeg is often important on small boats. A spray rail may help, a chine step does not make sense.

On planing boats, those appendages will help. The faster the boat, the more efficient they will be. A moderate speed boat like the Nina only needs a skeg but a CS25 will have a chine step (acting as spray rail) and strakes.

In some flats boats, I do not show appendages to avoid hull slap: spray rails or chine steps. The builder decides if he wants to give priority to stealth or dryness. I always show a skeg for maneuverability. It is underwater and does not make noise.

There is a special case of chine step in boats with tumblehome, we call it a chine fender. Those are visible in the RB18, JA18, RB12/14 /16 and LB26. They protect the round side and located at the correct height, give some lateral stability at planing speeds.

Material and size:

When made from wood, I consider those parts sacrificial, the kind you replace every 5 or 10 years. HD plastic (Aquaplas) last longer too. You can cut them with sharp edges. Some builders want to fiberglass them. In that case, cut round edges and sharpen later with epoxy putty.

Size should be proportional to the boat. A small 14 to 16' boat will be fine with a 1” wide strake, a 25 footer can be fitted with a 2” wide one.

The cross section is a lot less important than some forum discussions may lead to believe. For strakes, I like a plain square beam cut in two along the diagonal with the diagonal (longest side) glued to the hull. Yes, it will provide a little bit less lift but how much? Draw vectors, do the math. Not worth the complication IMHO.


Jacques Mertens - Designer
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Jeff
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Re: Strakes, chine steps and other skegs.

Post by Jeff » Mon Oct 01, 2018 4:38 pm

Excellent summary Jacques!! Thank you, Jeff

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