Points East pram: Displacement calculations

These sketches show the relationship between length and beam in the different proposed sizes of the pram. Images by Clint Chase

March/April 2022

Editor’s note: Points East magazine and Saco boat designer Clint Chase of Chase Small Craft are working together to design a pram that can be used by cruisers as an affordable and flexible dinghy. We have solicited our readers for ideas about size, design, use and handling requirements. Ultimately, Chase plans to add the “Points East Pram” to the inventory of boat kits his company makes and markets. In this latest installment, Chase has refined his initial design and developed specifications for three different lengths of the pram. Previous articles about this project may be found on our pram project page.

By Clint Chase

For this issue of the Points East Pram design update, we have a finalized hull with three different sizes available. The idea for the sizes is to have lots of options for folks – those with small boats don’t want to pull a large dinghy; some who do not intend to use it as a tender may opt for the larger. It will be interesting to see what people want for size. The difference between the models will most likely be seen in the interior arrangement of seats.

The specifications in the chart show the differences between the different sizes. Note how the displacement changes with each foot of added length. The longer models also come with proportionate increases in beam and depth.

Notice the increase in displacement between the eight- and nine-foot hulls (375lbs and 613lbs respectively). That is a big jump. It has to do with how displacement is figured: It is a volume, so length x width x height; and since it is a cubic function, there is an exponential increase in volume even though the length, beam and depth increase linearly.

Displacement is the volume of the underwater portion of the hull multiplied times the density of sea water. So, the seven-foot model displaces 7,255 cubic inches of water when it is sitting in the water loaded so that the designed waterline (DWL) is level with the water. That translates to 269lbs, which is pretty good for a 6ft nine-inch boat. That displacement more than doubles with only two feet of added length.

Another number here that is important is the PPI (pounds per inch immersion). How much added weight does it take to press the hull into the water an additional one inch past the designed waterline? That is what PPI indicates – you can see that if you add 86lbs of beer to the seven-footer, you will press the boat into the water one inch deeper which means you only lose one inch of freeboard. Not bad!

The eight-footer is designed so that when the top plank is flattened, it can be cut out of a single piece of eight-foot plywood. The seven -footer will fit well within an eight-foot panel, allowing other parts to fit in. The nine-footer will require scarf joints to get the plank length, but in the kit, this is easily accomplished. The seven-footer will likely be popular among folks who want to tow a dinghy with their small daysailers so they can row to the beach from an anchorage. The seven-footer has plenty of capacity, even for a big guy like me. The nine-footer will be able to take a couple big guys – and their beer. It will also offer a stable platform for use as a sailing or rowing recreational dinghy.

In the next article of this series, we will look at some interior arrangements – for instance, the seven-footer will likely have a fore/aft seat arrangement, whereas the nine-footer, will have a typical thwart in the middle from which to row and seats in the ends. All three will have built-in flotation in the ends under plywood, enclosed seats.

Clint is continuing to take suggestions from our readers on this project. Feel free to email suggestions to him at boatkits@gmail.com.

You can follow progress on our project on our Points East Pram page.