Flotation - What's the Load Capacity of a River King Cataraft?
Load capacity is largely dependent on how deep in the water you wish to allow your tubes to sink. Deeply sunk tubes will respond more slowly to the oars or motor. If your tubes are deeply sunk the greatest resistance to the oars will be caused by the momentum of the larger mass of your loaded watercraft not the friction drag of the water. Catarafts have a fairly small wetted surface for the loads they float. This effect can be notice when watching an oarsman hold a loaded cataraft in a fast run with relatively little effort. Driftboats show the same effect. Traditional rafts however do require more effort to hold in fast water as the wetted surface of the raft is greater than driftboats or catarafts for the same load.
Actual load capacity is a judgment call and depends on where you will use your cataraft. In calm ponds or lakes and slow moving rivers you can sink the tubes deeper than in shallow rocky rivers for example, where the tubes would constantly be bumping and dragging the bottom. Note that river rocks won't damage cataraft tubes significantly unless the rocks in your river are particularly sharp. Most river rocks are rounded and smooth and covered with moss and algae. (Rivers running through eastern Oregon's Blue Mountains are an exception.) Of course, running a cataraft through big waves and white water is best done with a lightly loaded cataraft (to reduce momentum) and as shallow a draft as possible. For whitewater you want your boat to dance on the top of the water - not plow through it.
The following chart shows, for each model how much weight you can add to your cataraft that will result in a specific draft (depth of tubes in the water). The weight of the tubes and frame has already been accounted for in the calculations so the weights listed below are how much weight you can put on in addition to the tubes and frame - such as people and gear - that will result in the drafts shown. These predictions are derived using some simplifying assumptions for the tube-end geometry and frame weights so these should be considered as estimates rather than exact numbers.
For an example using the chart, for an 8 inch draft, the Px16K (either the PB16K or PD16K) cataraft package (with 27 inch diameter tubes) can take a load of about 1223 lbs or 555 kg.
As it turns out the Blue Water packages weigh about the same as the Deschutes packages with the same tubes - since the additional weight of the motor transom offsets the lighter weight of the oar system on the Blue Water series.
| Total (lbs / kg) you can add to River King catarafts for various drafts | |||||
| For draft = | 6 in / 15.2 cm | 7 in / 17.8 cm | 8 in / 20.3 cm | 9 in / 22.9 cm | 10 in / 25.4 cm |
| For Px14R | 781 / 355 | 866 / 393 | 945 / 429 | 1021 / 464 | 1094 / 497 |
| For Px16R | 900 / 409 | 996 / 452 | 1087 / 494 | 1174 / 533 | 1257 / 571 |
| For Px16K | 1013 / 460 | 1121 / 509 | 1223 / 555 | 1319 / 599 | 1411 / 641 |
| For Px18K | 1149 / 522 | 1271 / 577 | 1385 / 629 | 1493 / 678 | 1597 / 717 |
More Info:
The top surface of the lower deck is approximately at the midpoint of the tube. So it will be 12.5 inches above the tube bottom for 25 inch diameter tubes (Px14R and Px16R). It will be 13.5 inches from the bottom of the 27 inch diameter tubes (Px16K and Px18K). That means a 10 inch draft is about as deep as you'd ever want to go. Deeper than that and the lower deck will frequently be awash in small waves and chop. 9 inches would be a little deep but not a problem in low river flows for a cataraft with 25 inch diameter tubes. 10 inches would be a little deep but not a problem in low river flows for a cataraft with 27 inch diameter tubes. The cataraft design is quite stable and can be occasionally overloaded.
For outboards you want even a bit more space between the water and lower deck as the outboard puts concentrated weight at the stern and forward motion will tend to pull the stern downward due to the low pressure area created by the tube upsweep. 7 to 8 inches is considered an average draft for these catarafts.
You can add about 2.5% to these weights for salt water.
