Jon Boat Stability Questions

I am trying to inform myself about the trade-offs between two Jon boat designs, for fishing. Both boats are ~10’ long.


One boat, made by Lifetimer, is 49.5” at the bottom, has a 56” beam, 17” freeboard, and weighs ~ 150 lbs.

The other boat, made by Silver Streak is 40” at the bottom, with a 49” beam, 17” freeboard, and weighs 100 lbs.

I am trading off weight versus stability.

How much more stable will the wider boat be for casting standing up off the centerline?

How much of the stability difference can be mitigated by attaching sponsons or a foam buoyancy collar?

I will be running a low <= 1,000 watt electric motor.

 

Depends on how tall you are and how much you weigh. Empty, the heavier Lifetimer has 20% more GM (about 5 inches), but that has little to do with ultimate styability with you standing in it.

 

150 pounds is a bit much for a ten foot Jon boat. If 49 inches acro9ss the bottom, it will need a lot of frames and stringers. It might be made of thicker gage material or more frames, stringers, or more robust transom. the wider bottomed boat will have a bit more stability but the one with the 40 inch bottom will be entirely satisfactory for a sober fisherman.

 

Some images…(the Lifetimer is .08o, not o.8”)
The blue boat is the Silver Streak.


View attachment 205257 Thank you very much.

 

Lets assume that the weight of the fisherman, engine, fuel, supplies, etc is X, and that this is the same for both boats.
The load displacement of the Lifetimer boat is 150 + X, and the load displacement of the Silver Streak boat is 100 + X.
However this is just a red herring to distract you - you will find out later that we do not need the displacements to make a rough stability calculation.

I assume that the measurements 'at the bottom' are the hull widths?
And the beams are the overall widths at the gunwhales?
If so, then both hulls have some flare - the wider boat has less flare (3.25" difference between the hull bottom width and the gunwhale) while the narrower boat has more flare (4.5").
And lets assume that the draft of each boat is 5" (the actual draft will depend of the weight of the supplies, engine, the fisherman's weight, the weight of his companion etc).
If we take into account the flare, then the Lifetimer has waterline beam of 51.5" at a draft of 5", while the Silver Streak has a waterline beam of 42" at a draft of 5".

Here is a very rough calculation, where I am trying to keep it as simple as possible.

The formula for the distance between the Centre of Buoyancy (B) and the transverse Metacentric Height (M) is BM = I/V

Where I is the second moment of inertia of the waterplane area, and V is the volume of displacement.
For a simple Jon boat hull, lets assume that the waterplane is a rectangle, hence the area will be L x B.
The formula for the transverse I about the boat centreline for a rectangle is :
I = L x B x B x B /12.
And the volume of displacement at any given waterline is then L x B x T where T is the draft.

So BM = (L x B x B x B /12) / (L x B x T) = (B x B) / (12 x T)

Both boats are probably going to have very similar drafts T, and they are both approx 10' long.

Hence in a nutshell the BM is roughly proportional to the square of the beam.

For the Lifetimer, with 51.5" waterline beam, the square is 2,652, and for the Silver Streak with 42" waterline beam the square is 1,764.

Divide 2,652 by 1,764 and we get approx 1.5.
Or in other words, the BM for the wider boat will be approx 50% more than the narrower boat.
This is quite a lot really, for not much increase of beam!

And KM = KG + GM where KM is the height of the Metacentre above the keel, KG is the height of the overall centre of gravity above the keel, and GM is the stability lever (the distance of the Metacentre above the Centre of Gravity) - the greater the GM is, the better the stability.

If we assume that the KG of both boats in the loaded condition with the fisherman standing up is approx the same, then we can assume that the stability of the wider boat will be 50% 'better' than the narrower boat.

You have to decide now if the extra 50 lbs of hull weight is worth having, in order to get a 50% increase in stability.
I think it probably is - if you add up all the other outfit weights, like the estimated list below, then the percentage difference in the loaded displacements of the the two boats is much less than the light displacements.
Fisherman 200 lbs
Engine and fuel 150 lbs
Supplies (anchor and cable, L/J, etc) 50 lbs.
Total crew and outfit weight 400 lbs.

Total displacement of the Lifetimer = 550 lbs, vs 500 lbs for the Silver Streak - the Lifetimer is now only 10% heavier than the Silver Streak.
Yet it has 50% better stability.

If the Silver Streak is fitted with a foam buoyancy collar that is above the loaded waterline, then as the boat heels due to a transfer of weight (such as the fisherman going to the side of the boat to pull a fish onboard) then as soon as the collar comes into contact with the water as the boat heels, the boat will stiffen up, as the I of the new waterplane area has suddenly increased, while the volume V of displacement has stayed the same.

 

Going by BajanSailors post above, the LifeTimer takes it by far.
Is stability your only criteria?

 

I would have never guessed that such a small difference in size would mean so much difference in stability!

Ps. Now I'm left wondering if the calculated percent number for better stability can go over 100%? If the weight, Volume and other significant factors can be made about the same, I am wondering what the percentage numbers would be- for something like a coracle?

 

If at all possible try each boat and make your decision then.

 

Fantastic! Thank you so much for describing the underlying math, then simplifying.

The threshold question I have relates to handling the boat, motor, etc., from the car to the water and back. I want to ‘dolly’ the boat using wheels mounted on the stern, rolling it off and on a utility trailer. The weight of 50 lbs increases, perhaps in a non-linear way as we age, it seems.

As a first approximation, assuming a roughly square cross-section, does the effective beam of the boat increase by the total thickness of the collars?

 

I would like to, but this is not possible. I will have to rely on the math.

 

Why your Jonboat must be so short ?

 

I am not aware of any suitable alternative boats that are longer.

 

Short boats are load distribution fragile . Difficult to trim . I remember my 12" plywood flat bottomed build . Loaded with gear , weak 15hp Evinrude we need to move forward to get out of the hole . Another build long and lean it was pleasure to steer with almost no holeshot . Smooth transition from displacement speed to planning speed . Big cargo weight tolerance without handling differences .

 

As I said in the first post, the motor will be a <= 1,000 watt electric unit. I am not planning on planing. Does this change your advice?

 

Do you have any experience with the boats ?