Center board size

I am building a wooden, flat bottom Motor-sailor 29'8" in length with an 8' beam.

How would I determine the size of the center board I will need?

Comments for Center board size

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Mar 08, 2013
regarding a centre board size
by: douglas hunt

i am also building a 25 ft deep vee motor sailer.

The boat is designed as a fishing type with a long keel of about 100 milimeter deep.

So, what I am doing is extending the keel to a depth of 400m/m deep for about 70% of the boat very similar to a catamaran keel then installing an inboard 21hp 3 cylinder diesel engine.

Inboard for the economical aspect and the reliability aspect.

I have also decided to build a gaff rig with a small genoa sail for sailing when conditions prevail which is most of the time this will give me a longer fishing range at low cost.

My e-mail address is should you require any assistance with your keel for your boat as there are many factors regarding the position of your mast in relation to your keel.

Dec 14, 2012
Center board size
by: Anonymous

Yes a dork style.

I am working off plans but the plans are for a motorboat and I am inverting the design to make it a motor sailor.

I have to add all the stuff you mentioned.

So where do I look for help?

Dec 14, 2012
Centre board for a flat bottom Motor-sailor
by: Mike


You say flat bottomed so is this a dory and if so is there any rocker to the bottom?

And is there any exterior keel?

I'm assuming that this board is there simply to resist leeway and heeling when under sail and not as part of the ballast?

If so then it will need to be related to the size and shape of the sail plan, though the hull shape will also have a bearing.

It’s tempting to think that keel is just a flat slab and the bigger the slab the less leeway.

However, consider it as an underwater sail which, like the one up the mast, produces ‘lift’ as the water moves across it.

And like the sail the medium it is moving through (water in this case) needs to be at a slight angle so, it works best when making some leeway (3 or 4 degrees).

Also the efficiency of a centreboard will increases with its aspect ratio, the longer and thinner it is the more lift.

On the other hand a wide (long keel) keel will give better directional stability, better for long range cruising.

The next thing to think about is how the centreboard case is going to fit in the accommodation.

The case has to be high enough for the top to be well above the waterline but it doesn’t want to be so big that it divides the cabin into two separate parts.

Also consider the amount of leverage that the keel will put on the case, a longer (deeper) board will need a stronger and better braced casing.

You will also need to allow space for the lifting mechanism.

All of which will have a bearing on the shape and size of the board itself.

So, lots of compromises.

Presumably you are working from plans?

What size and type of keel was shown in the original specs?

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