Making a Hull Narrower

[fpjeepy05]

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 from which point the pressure goes to close to atmospheric. (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 this high pressure/line.
I think that you are going in the wrong direction.

How come pangas are more efficient?

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.

Most boats that I like the stem have continuous curvature above and below the chine. But when you cut width out of them at the center, the stem rake angle gets sharper and a discontinuity in the stem appears at the chine. Not terrible, but also not the best. Also the boat would get a little shorter as well.

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.

I don't think it will be a huge improvement. And I don't think it would get enough fuel savings to pay itself off. There would be some savings from purchasing a smaller engine as well, but to your point still probably not enough to offset the work. But if it performs better in chop, that is a harder thing to put a value on. Saving an old hull from the landfill has no value, but in an ideal world, it would.

 

[fallguy]

Cutting the transom out and redoing it seems harder than building a new panga. You certainly are not going to butterfly it??

 

[Barry]

How come pangas are more efficient?

How come pangas are more efficient? than what?

The question should be this:
Why is a narrow panga with the same weight and horsepower use less fuel than the same panga with more width and the same weight and horsepower?
It would appear your answer would be that a narrow panga is more fuel efficient or takes less horsepower to drive it a certain speed than a wide panga. (similar hull shape)

 

[fpjeepy05]

How come pangas are more efficient? than what?

The question should be this:
Why is a narrow panga with the same weight and horsepower use less fuel than the same panga with more width and the same weight and horsepower?
It would appear your answer would be that a narrow panga is more fuel efficient or takes less horsepower to drive it a certain speed than a wide panga. (similar hull shape)

Yes a narrow boat is more efficient than a wider boat. But it also takes more material to make a wider boat. Also scantlings need to be heavier for the higher slamming loads on a wider boat. Both of which make the boat heavier.

Yes the stagnation line has the highest lift, but there are a lot of other variables. A larger transom makes a larger wake which is lost energy.

A short, fat lifting plan is more likely to cause proposing.

I don't make the rules. Dave Gerr's Nature of Boats. Chapter 23 - Speed in Style. He explains better than I can.

 

[fpjeepy05]

Cutting the transom out and redoing it seems harder than building a new panga. You certainly are not going to butterfly it??

You might be right. It's just a mental exercise at the moment. Forest on YouTube makes it look easy though

 

[fallguy]

I think you ought to reflect briefly on Barry’s comments differently, as a mental exercise.

If a narrower boat is always more efficient than at what point is the boat narrow enough?

 

[TANSL]

A narrower boat can be more efficient than a wider one, as long as both are operating in displacement. but, while the second will easily reach the planing regime, with the consequent energy saving, the first will be operating at full power without ever reaching the speed of the other, without ever reaching planing, but spending a lot of power. to increase its speed by half a knot.
At the stern, in planing, a wave with much lower energy is formed than when displacemente regime.
Somewhere in his book, I believe, Dave Gerr will explain something similar.

 

[fpjeepy05]

I think you ought to reflect briefly on Barry’s comments differently, as a mental exercise.

If a narrower boat is always more efficient than at what point is the boat narrow enough?

I think that would come down to boat loading. I don't know this math super well I know Blount has a chart that plots (displacement/loa*boa)(loa/boa)^1/2 vs length to show bottom loading.

I think most center consoles would be considered light-loading. Reducing the beam would transition them to heavy loading, but at a certain point they will become overloaded and there won't be enough surface to keep them on plane.

Where would that occur on this boat I have no clue, but I could take a guess. [scribbles on napkin] I think 27ft x 5.4ft would be the extremely-heavy bottom loading at 7000lbs. Much wider than I expected.

No one wants to see my overly simplified calcs... but here they are.

Planning Area= 0.65*(LOA*BOA) [ ft^2]
Bottom Loading = Displacement/ Planning Area [lb/ft^2]
FPjeepy's loading Factor = (LOA^1/2)/Bottom Loading

I couldn't find any efficient planning boat with FPLF < 0.07

So Calculating BOA with LOA=28ft, Displacement = 7000lb FPLF = 0.07

BOA=5.4

Some factors this doesn't account for... deadrise, warp, speed, LCG, chine shape, and lots of others. I don't expect this to be very accurate. But it was a fun mental exercise.

 

[Barry]

Bottom Loading = Displacement/ Planning Area [lb/ft^2]

An interesting formula and perhaps in NA terms, this is accurate

But bottom loading when planing is not so simple.
The equation does not take into account speed. Remember the dynamic forces on the bottom of the hull is the result of the hull moving the cross section of water
out of the way as it moves. (I am only referring to planing here as I assume no pressure recovery as you would see in a displacement hull)
And then some additional pressure due to the bouyancy effect.

Your bottom loading equation is fine at rest but that is all that it encompasses.

To minimize attracting pushback on the fringes of cruising speed, ie this hull at displacement speed, or "hump speed say 10- 12 knots or in excess of 35 knots
I am referring to a cruising speed of say between 25 to 30 knots.

 

[fallguy]

Not sure I appreciate the boa loa 65% guess. Just a friendly guess critique. My fishing skiff is vertical sides and maybe more like 85%?

 

[fpjeepy05]

Not sure I appreciate the boa loa 65% guess. Just a friendly guess critique. My fishing skiff is vertical sides and maybe more like 85%?

Agreed. I was basing it on my 27 Rambo example. For your boat, maybe the minimum beam would be 4.1ft?

 

[TANSL]

Bottom Loading = Displacement/ Planning Area

An interesting formula and perhaps in NA terms, this is accurate

Not at all, it is not accurate. It does not make any sense.