Hull & Deck Structure

The key to this design goal was the ability to deal with heavy weather with a margin for operator error.”
–Bluewater Sailing Magazine

Engineering a boat is a mix of science, numbers, black art, and experience. You have to first establish a set of guidelines to use.

For sailboats, we’ve used the American Bureau of Shipping rules, and then modified the ABS rule to fit the real world. (If you build a boat strictly to ABS, and run it aground at cruising speed, odds are you will need a trip to the local boat yard. For example, we know from long experience that if you design the keel and related structure to four times the ABS requirements, you can hit things without normally requiring a haul out).

This process is the same for a sailboat as it is for power. The big difference is that in a sailboat the vertical center of gravity is critical to both performance and safety (you want it as low as possible). With a powerboat this is not nearly as big an issue, although VCG still must be watched with care..

There are many classification “rules” which can be used for powerboats. In our case, since we’re building aluminum motor yachts, the best rule is what is known as the Lloyds Special Service Classification or SSC as it is commonly called. Lloyds has available a wonderful engineering software package which allows you to play “what if” games with framing, plating thickness, floors, deck, and other structural members. The software takes into account hull shape, and allows you to change values for wave size, boat speed, and impact (G) loadings. Or, you can let the software input these values.

At the same time as doing this, we also look at the weight impact of changes. With the hull structure portion of the weight control spread sheet we can input different plating and framing values, and get an instant read-out of how overall weight, centers of gravity, and polar moments are affected.

We won’t bore you with the literally hundreds of output pages of spread sheet and SSC rule data in one set of calcs, but a couple of examples from the FPB 83 design process might be of interest.

The first is some of the basic data which we control to look at different scenarios. In this case we’ve combined a 12.5 knot speed of the FPB 83 with 7m (23-foot) waves. This combination of speed and waves is not about to happen in the real world! A more realistic speed in these conditions would be 9 to 10 knots. But let’s see what the values look like.

There are a couple of key numbers to check, the first of which is wave height. The rule calculates this to be 4 meters (13 feet) – which should be conservative. We’ve increased this wave height, as previously indicated, to see how much metal is required to do the job with the larger seas.

The other number is vertical acceleration at the LCG (longitudinal center of gravity – roughly the center of the galley). This shows 1.394Gs. What this means is that if you were standing in the middle of the saloon, and hit this mythical 7m wave at 12.5 knots, your feet would leave the deck as the boat plunged down after the wave impact. We know from lots of experience that this is totally unrealistic. In the bow yes, you’d be flying around, but not amidships. So, this set of parameters are probably excessively conservative.

Still, let’s see what this shows us for hull plating.

In the image (above) we have the results for plating thickness required by this highly unusual set of circumstances. In this case the required value (arrow) is 5.9mm (just under a quarter of an inch).

Now let’s get realistic and drop the speed down to 9.5 knots. This is the maximum we could ever envision doing if conditions were truly this awful upwind and we had an urgent need to get somewhere. The calculations now show that the G-loads have dropped dramatically to .805.

And if we go back and check the bottom plating requirement (above), the thickness has dropped to 5.1mm. As the Lloyds SSC rule is considered very conservative, and this is an all oceans no restriction classification we are after, the 5mm plating figure should be good. But what about collisions with logs, other boats, whales, etc.? Here you have to go with instinct (or experience).

For any given structure, the stiffness goes up with the cube of the increase in thickness. What this means is that small amounts of plating thickness increase can generate huge returns in stiffness. 5mm plate has a stiffness factor of 125 (5 x 5 x 5 = 125), while 6mm plate is 216 (6 x 6 x 6 = 216). This means going up a single mm results in an increase in stiffness of 72%. Sounds like a pretty good tradeoff to us.

But what about the impact on overall weight? We add 476 pounds to the hull by increasing plating thickness to 6mm in the bottom. Doing the same for the topsides adds 549 lbs. Is it worth 1,025 extra pounds of weight for this increased factor of safety? We think the emotional comfort, not to mention safety is worth the extra displacement. And there is another benefit from the stiffer plate. Our bare aluminum topsides are fairer with less welding marks showing.

Multiply the above example by hundreds of structural details and you start to get an idea of the process involved. Each time a decision is made, the revised data is entered into our weight control spread sheet, telling us, and the hull builder, exactly what the finished metal hull should weigh.

Bottom line with all of this is that the FPB designs have heavier scantlings than many larger motor yachts designed to this same rule which weigh multiples of our displacement.

In other words, we’ve built a very tough structure, one which can maintain speed into large, irregular head seas, and we’ don’t have to worry about bending metal.

Because the actual surface areas are relatively modest, we can “afford” weight of the more massive structure while still retaining the target performance characteristics. A separate benefit is that this weight is nicely distributed throughout the hull in such a way as to enhance our polar moments, thereby slowing both rolling and pitching motion.

Real World

Now let’s look at the real world, as in the photo above. We have a pretty good historic handle on sailing loads in heavy going. There is a tendency for the boat to unload itself before things get out of hand. And even more of a tendency for a cruising crew to lower the structural threshold through conservative sailing or outright discomfort. But the FPBs are capable of being pushed harder by their crews, because they are more comfortable in lousy sea states than their sailing cousins.

Obviously we wanted to get a handle on the real world numbers as quickly as possible. We were aided in this endeavor by some significant advances in microelectronics and computer processing power. As a result, Wind Horse was fitted with six accelerometers to measure wave forces, as well as a heel angle and roll rate sensors. There is one of these sensors between the props, which measures stern slamming loads as well as prop cavitation. At the center of buoyancy there are three sensors to measure vertical, longitudinal, and transverse wave loads. Finally, at the forward end of the accommodations, in the area subject to the highest slamming loads, we have two sensors measuring vertical and horizontal impacts.

This data is fed to a PC at a rate of ten readings per second. This works out to roughly a gigabyte of data each 24 hours. We can watch this on the nav computer as we’re cruising along, try different settings of ballast, fuel, and stabilizer controls, and see the results on the PC. We can also come back later and analyze the data.

The next series of images were taken during the worst of our crossing from Hawaii back to California, from the computer data being captured. We were powering along at a speed between 10.5 and 11 knots; waves were directly on the bow, averaging 13 feet (4m) with occasionally much bigger sets. There was also a 90-degree crossing swell system. Periodically these waves would cause a chaotic peak in front of us, often significantly larger than the surrounding seas. The image above is one of these collisions.

This sea state and boat speed is in the worst range of what we would ever expect. The exact acceleration numbers are proprietary, but a basic look shows you some interesting data.

The top three traces represent loads on the middle of the boat. The first is longitudinal (fore-and-aft), next is transverse (side-to-side), and the third is vertical. Notice the deceleration on the top as we slam into this mountain. This represents about a quarter of a G, enough to toss you off your feet if you are not holding on. The second, side-to-side, shows we’ve got a crossing component along with the head sea. This only occurs when we hit one of these mountains (simple head seas do not generate the side impacts).

The fourth and fifth columns have side-to-side and vertical data at the forward end of the accommodations. This data is much more severe, as you would expect. Loads here are higher than the Lloyds rule predicts, but well inside of our conservative scantlings.

The next to the last indicates roll angle. There is a sharp heel first one direction and then the other. That’s the crossing swell.

Notice that there are a series of these events within a few seconds of each other. Normal wave period was in the 12-second range, but occasionally we would get closely spaced sets, perhaps due to the crossing interference pattern. Obviously these are very steep seas. You can see that there are three of these “events” closely spaced, each with somewhat less magnitude.

The data above is the worst we have seen since leaving New Zealand. It is nice to know that the boat will take a lot more than even this.

Now, let’s look at a more normal wave encounter, from a little later. First, this data (above) is of a single large wave. Deceleration is less, and the acceleration on the center of the boat is only half of what the bow feels. We’ve still got those crossing swells, which you can see in the bottom two data sets. If this were the worst the sea was throwing at us, the crew could handle more, and we might be tempted to increase speed.

One last bit of data. The loads on the boat are very much a function of boat speed. While the previous data was accumulated with the engines set to push us around 11 knots, the data above is with the boat slowed down to 8 knots. If you compare the traces, you will see significantly less acceleration on all of them. This is much easier on the boat. But we usually choose to push on at higher speeds and get this over with.

House Engineering

So far we’ve shown you the easy stuff. Now let’s look at a hard question. In the last 250,000 miles of ocean sailing, we’ve never been knocked down by a wave. But knockdowns and rollover structural loads still form the basis of our main engineering concern. We think that given the speed of these boats, modern weather forecasting, and the ability of the FPBs to be controlled in the awful sea states, the risk of a severe knockdown or rollover is remote; in all likelihood even more remote than with our sailboats. Still, if a boat were to somehow have engine(s) disabled, and find itself lying in the troughs of large breaking seas, you have to assume the worst could happen (even though, should this occur, you’d stream a Jordan Series Drogue or GaleRider and keep one end of the boat pointed into the waves). Take a look at some of the sea state photos in our book Surviving the Storm and you’ll see what we’re thinking about.

In a worst case situation the boat is picked up by a breaking crest, and dropped or hurled some distance into the trough below. This means that the house sides are going to see very high loads. And we know from anecdotal evidence in sea stories we’ve gathered (happily, not from any of our own designs) that the house-to-deck and window structure are where problems usually occur.

None of the classification society rules specifically address these issues, so we’ve created our own scenario, then engineered the structure on this basis. Our first assumption is the percentage of the total load of this “event” which the house sides must carry. If you look at a side (profile) view for the FPB 83, the house, topside, coamings, and hull, the house represents 17% of the total profile area. Logic would seem to dictate that whatever the total load, the house would have to carry its “share”. However, to be extra conservative, we’ve said it has to carry 25% of the total rather than just the 17% mentioned above. The next step is to establish a load. We’ve assumed an acceleration of two Gs – very high, but possibly in keeping with such a situation.

From here on it is a question of number crunching. There are a series of “mullions” – vertical structural elements – that have to take the side impact load. These mullions are each connected to a hull frame, and are integrated into the coaming. The side loads accumulate primarily in the windows, and then transfer to the mullions, then via the web frames into the hull. You can see these mullions in the image below.

For the purposes of these calculations we’ve assumed that one side of the house sees 100% of the load. Reality is that the load is shared through the roof by all both sets of mullions. What this means in the end is that the leeward side windows and mullions of the FPB 83, representing less than a fifth of the impact area of the side of the boat, is capable of taking the entire weight at one G acceleration (using both sets of mullions to carry the load).

The photos above will give you an idea of what this looks like in the real world.

For more information on the FPB Series, e-mailĀ Sue Grant: Sue.Grant@Berthon.Co.UK.

Posted by Steve Dashew  (October 14, 2011)

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