Making a Hull Narrower

Has anyone ever cut a wedge out of the keel of a boat to make it narrower? I was thinking about about starting the cut at the intersection of the chine and the stem and connecting with a straight line to the widest point on the transom. The bulkheads would need to be removed along the centerline and the stem section would need to flex or be cut as well so it can open up. The transom would have a goofy shape and might need to be redone, but many transoms on older boats need to be redone anyway.

The max beam will get smaller by a less amount than the chine beam but whether the shape would be acceptable would depend on the make/model.

Attached is a sketch. On the bottom is a 27x8 typical center console hull. Say something like a Rambo 27. The middle is a typical 20x8ft center console. (Most boats in this size range don't have the narrower beams that I like that are easier to push in the water and handle chop better. They are more tender, but I am okay with that personally) The chine beam at the transom of the two "available" boats is 6'7.5" The hatched section is the wedge I would want to cut out. The top boat is a 20fter that was chopped and reassembled to make a 20x7'4" boat with 4'9.5" chine beam at the transom. Length to chine beam at transom ratio of the 27x8 is 4.07 and 20x7'4" is 4.17. The plan was to get it the same, but missed by a little bit.

I think the weight would drop slightly, and the higher L/B would help efficiency and maybe a little less pounding in chop. Better efficiency means a smaller engine and less fuel which snowballs better efficiency... But would the juice be worth the squeeze? What are your thoughts?

fpjeepy05 said:

Has anyone ever cut a wedge out of the keel of a boat to make it narrower?

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No.

fpjeepy05 said:

But would the juice be worth the squeeze?

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No.

fpjeepy05 said:

What are your thoughts?

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Terrible idea.
Build it right the first time by following a proven design that meets your needs ( SOR ).

My industry has spent untold millions over the last 20 years sponsoning boats.... never thought of making something narrower.

In almost every circumstances there is an unintended consequence. Unless labor and materials are an unbelievable bargin it seems finding the right hull would be a better way to start.

BlueBell said:

No.

No.

Terrible idea.
Build it right the first time by following a proven design that meets your needs ( SOR ).

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There aren't many designs that meet my needs. One that is close is a Nitra 22. 22' x 5'4". 57mph with a 115hp outboard. They start at $200k used. I don't have that kind of money.

Building with off a jig needs a design, a CNC cut jig, and a lot of expensive materials.

20ft bare hulls like a Mako, Kencraft, Seacraft, Dusky, Palm Beach, and Paramount are cheap. The plywood stringers and deck rot out and people will give them away because it is cheaper than paying for them to be stored or disposed of. Cutting a hull and patching it together don't require much tooling, and the bulk of the material of the boat would be free. Resin and glass aren't cheap. A 1000lb boat is more than $7000 in materials even as a solid laminate no core. A hull built on a jig of this size could be 500 hours or more. Cutting and seaming a hull could be done in a weekend. Downeast boats and canoes are commonly widened in this same manner.

comfisherman said:

My industry has spent untold millions over the last 20 years sponsoning boats.... never thought of making something narrower.

In almost every circumstances there is an unintended consequence. Unless labor and materials are an unbelievable bargin it seems finding the right hull would be a better way to start.

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What industry are you in? I don't know anyone that has ever sponsoned a boat.

If you can find a 18-22ft dead vee hull with 5-6ft of beam built as a center console for <$20k or a bare hull <$5k, please forward it my way.

Put a pencil in the bathtub and spin it. The juice not worth the squeeze.

You need to explain the reason for beam reduction better.

What you want is actually a very famous design with thousands of boats all over the world.

it is called a panga

here is s&g version, can build in a year

https://boatbuildercentral.com/StudyPlans/PG20_STUDY.pdf

fallguy said:

Put a pencil in the bathtub and spin it. The juice not worth the squeeze.

You need to explain the reason for beam reduction better.

What you want is actually a very famous design with thousands of boats all over the world.

it is called a panga

here is s&g version, can build in a year

https://boatbuildercentral.com/StudyPlans/PG20_STUDY.pdf

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Less beam, less HP.

And I like pangas, I just don't like the sheer. Ironically, I wouldn't call the plans you linked a panga. Pangas have a wide duck bill bow with a bouncy collar. They are designed that way because they are made to pull heavy nets on the bow. I don't like that look.

You win.

https://youtube.com/@diywaterworld?si=LmheE2rlx7ofBscI

Forest does a lot of this type stuff. Watching him makes me believe sometimes the juice is worth the squeeze.

fpjeepy05 said:

Less beam, less HP.

And I like pangas, I just don't like the sheer. Ironically, I wouldn't call the plans you linked a panga. Pangas have a wide duck bill bow with a bouncy collar. They are designed that way because they are made to pull heavy nets on the bow. I don't like that look.

You win.

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The sheer in any S&G design is easily changed within the dev surface constraints.

I’m not tryin to win, just help. The beam of the linked boat i 6’1”.

My bias tends to be to more beam because I like drift fishing and abeam in a narrow boat is not ideal.

fallguy said:

The sheer in any S&G design is easily changed within the dev surface constraints.

I’m not tryin to win, just help. The beam of the linked boat i 6’1”.

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I don't mind the sheer of Jaques's version. I don't like the normal panga sheer.

Additionally. 6" of strip planking at the top could add a little flair that might look nice.

fallguy said:

My bias tends to be to more beam because I like drift fishing and abeam in a narrow boat is not ideal.

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$1500 for a GPS trolling motor. If I plan to drift fish I wouldn't have a boat without one.

fpjeepy05 said:

You win.

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The guy is doing you a favor and you respond with snark?
I was going to expand on his post till I saw that.

Milehog said:

The guy is doing you a favor and you respond with snark?
I was going to expand on his post till I saw that.

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I was conceding defeat. Not sure how that is snarky

fpjeepy05 said:

Most boats in this size range don't have the narrower beams that I like that are easier to push in the water.

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fpjeepy05 said:

Better efficiency

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I am curious as to where you developed the idea that pushing a narrower beam at planing speeds ( I am assuming due to the drawings) is more efficient.
The highest pressure area on the bottom of a planing hull is near the stagnation point/line from which point/line the pressure goes to close to atmospheric as you move to the stern or chines. (lift strakes etc ignored)
This is a line from chine to chine normally depending on deadrise, speed etc is maybe 1/4 back from the wetted surface at the bow.
So by making a boat narrower, you reduce the length of this high pressure/line.
I think that you are going in the wrong direction if the goal is to have a more efficient hull.
(efficiency being defined as lower hp for a given speed or less fuel burn per hour at a given speed)

As far as the mechanics of actually performing your proposed modification, I’d prefer to take the width out of the entire length of the hull, to avoid the nasty warps that will occur when you try to bend the hull and sides in.

fpjeepy05 said:

I was conceding defeat. Not sure how that is snarky

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I apologize.

I've widened boats by doing that same cut, but spreading the hull to achieve what I wanted. It was easy and did exactly what I wanted it to do. But my needs and hull design were very different than yours.

I'm not sure you'll really get a huge improvement in the areas of performance you're looking to enhance. At least not enough of an improvement to make all the money and labor worth it.

And these typically turn into multi year projects that people eventually lose interest in.