For multihulls, there are no simple universal formulas that reliably predict “pounding once per hour/day” from only bridge deck height and width. In professional design, the risk of slamming/pounding is usually estimated from:
That said, there are practical rules of thumb and a useful probabilistic framework that can give a first estimate. Below I’ll give:
“Bridge deck pounding” or “slamming” happens when the water surface rises enough, relative to the structure, that the underside of the deck or connecting structure impacts the water. For a multihull, the key quantity is:
relative clearance = static underside clearance
- instantaneous wave elevation
- vessel motion contribution
If that becomes zero or negative, contact is likely. If the relative vertical impact velocity is large, the slam can be severe.
There are a few widely used empirical guidelines:
| Rule of thumb | Typical value | Comments |
|---|---|---|
| Bridge deck clearance as fraction of LOA | ~4% to 6% of LOA for cruising cats | For an 80 ft vessel, about 3.2 to 4.8 ft. Fine for ordinary cruising cats, but often not enough for very low slam risk in rougher seas. |
| Clearance as fraction of beam | ~6% to 10% of overall beam | Very rough heuristic only. |
| Clearance relative to design wave height | Bottom of deck often at least 0.3 to 0.5 of Hs above mean water | Again, only a rough starting point. For very low slam frequency, more is needed. |
| Centerbridge fairing / nacelle shaping | Strongly recommended | Can reduce severity of impact but not eliminate insufficient clearance. |
For your platform, these ordinary catamaran numbers are probably too low if your target is:
A workable first-cut method is to treat the relative vertical motion between underside and sea surface as a random Gaussian process. Then the probability that the sea touches the underside is approximated from the ratio:
z = C / σr
where:
Then the probability that a given wave encounter exceeds the clearance is approximately:
P(contact per cycle) ≈ exp( - C² / (2σr²) )
This is not exact, but it is often used as a useful first estimate for rare exceedances. If the average encountered wave period is Te, then the number of cycles in time t is:
N ≈ t / Te
and the expected number of contacts in that time is:
E[contacts] ≈ N · exp( - C² / (2σr²) )
If you want less than 1 contact per day, then for a day:
Nday · exp( - C² / (2σr²) ) < 1
so:
C > σr · sqrt( 2 ln(Nday) )
If the encounter period is about 7 to 9 seconds, then:
So the practical criterion becomes:
C ≳ 4.3 σr
This is the hard part. For a normal catamaran, σr can be large because the vessel follows the waves more. For your concept, with three slender submerged wing-like columns supporting a large platform, the behavior may be better:
But there are competing effects:
For a first estimate, write:
σr² = σwave,local² + σmotion² - 2ρ σwave,local σmotion
where:
For rough first-cut design, people often just use:
σr ≈ k · Hs
with k somewhere around:
| Craft type / motion quality | Very rough k estimate |
|---|---|
| Ordinary cruising catamaran | 0.20 to 0.35 |
| Well-optimized large catamaran | 0.15 to 0.25 |
| Good semi-SWATH / column-stabilized concept | 0.10 to 0.20 |
These are not code-approved numbers—just engineering screening values. For your concept, if it is successful as a low-motion platform, I would tentatively look at:
σr ≈ 0.12 to 0.18 · Hs
Assume Hs = 7 ft. Then:
σr = 0.12 × 7 = 0.84 ft
Required for less than once/day:
C ≳ 4.3 × 0.84 = 3.6 ft
σr = 0.15 × 7 = 1.05 ft
C ≳ 4.3 × 1.05 = 4.5 ft
σr = 0.18 × 7 = 1.26 ft
C ≳ 4.3 × 1.26 = 5.4 ft
σr = 0.25 × 7 = 1.75 ft
C ≳ 4.3 × 1.75 = 7.5 ft
So your required clearance depends massively on how effective the column-stabilized geometry is at suppressing relative motion.
Even if the statistical estimate says 3.5–4.5 ft might work for an excellent low-motion platform, several real effects push the needed number upward:
A seastead intended as a home should generally be designed with a generous clearance margin, because even infrequent slams are unpleasant and structurally costly over time.
Your planform is an equilateral triangle, 80 ft on a side. The distance from centroid to a corner is:
R = a / √3 = 80 / 1.732 ≈ 46.2 ft
That means the corner columns are quite far from the center, which is good for pitch/roll inertia and stability. The big deck area also means:
The best slam-control strategy is usually:
If the underside is just a broad flat plate spanning much of the triangle, pounding risk and impact severity rise sharply. If instead the habitable deck is high and the lower structure consists mainly of slender beams/trusses, slam loads can be much lower.
Using the simplified formula with an 8 second encounter period:
contacts/day ≈ 10800 · exp( - C² / (2σr²) )
Below are rough examples.
| σr | Clearance C | Expected contacts/day | Interpretation |
|---|---|---|---|
| 1.0 ft | 3.0 ft | ~120/day | Way too frequent |
| 1.0 ft | 4.0 ft | ~3.6/day | Still too frequent |
| 1.0 ft | 4.5 ft | ~0.4/day | About once every 2–3 days |
| 1.0 ft | 5.0 ft | ~0.04/day | About once every 25 days |
| 1.25 ft | 5.0 ft | ~3.6/day | Too frequent |
| 1.25 ft | 6.0 ft | ~0.11/day | About once every 9 days |
| 1.25 ft | 6.5 ft | ~0.015/day | About once every 2 months |
These values are screening-level only. Real slam severity also depends on impact velocity and local shape.
Professional multihull and offshore-structure designers usually go beyond simple rules and use:
The more formal version of the probability idea is:
P(R > C) = exceedance probability of relative motion process
where R comes from the wave spectrum filtered by the vessel’s relative-motion RAO:
SR(ω) = |Hrel(ω)|² · Sη(ω)
Then:
σr² = m0 = ∫ SR(ω) dω
and exceedance rates are obtained from level-crossing theory. That is the right approach if you really want “once per hour/day in sea state X.”
Given your priorities:
I would recommend the following preliminary targets:
| Item | Recommendation |
|---|---|
| Static underside clearance | 6 ft minimum |
| Preferred if geometry is flat/broad underneath | 7 ft or more |
| If lower structure is just narrow beams/trusses | 5–6 ft may be adequate |
| Underside shape | Avoid large flat horizontal panels near water; use camber, arch, or streamlined centerbody |
| Validation method | Do RAO/seakeeping analysis before freezing the structure |
Putting batteries, water, stores, and tanks low in the legs will:
That is generally helpful, but remember that too much added mass increases total displacement and may reduce clearance if the columns are not sized correctly.
This could reduce drag at your low speed and may reduce wave excitation compared with bluff columns. But watch for:
If each leg is about half submerged, then changes in displacement and trim must still be checked carefully. Small-waterplane-area concepts can have low motions, but they also need enough restoring and reserve buoyancy for payload variation and damage cases.
Yes, there are rules of thumb, but there is no simple universal bridge deck formula that gives a trustworthy pounding frequency for all multihulls. A useful first approximation is:
contacts/day ≈ (86400 / Te) · exp( - C² / (2σr²) )
For your 80 ft triangular, three-column, slow seastead concept in 7 ft seas, aiming for less than one underside contact per day, a sensible preliminary design target is:
Bridge deck clearance ≈ 6 ft minimum, preferably 6–7 ft
with the higher end favored if:
I can also make a more detailed estimate if you give me any of these:
If you want, I can next produce: