I am currently looking at the overall size of the tender/sailing dingy design. I know that I want a usable load capacity of approximately 500 pounds and a hull weight of <50 pounds. I started my design spiral at 9 foot long and 4.5 feet wide with approximately 6” draft and 12” freeboard.

After building a model and looking at capacity, material, internal space, etc. this has led me to experiment with different lengths/widths for this project.

My gut tells me that the smallest dingy would fit a footprint of 8 foot long and 4 foot wide (just to be nominal) which led me to the following. Coast guard rule of thumb for capacity is length times width divide by 15 yields usable capacity in pounds divided by 150 pounds yields number of persons allowed on board. Referenced here:

Calculating Your Boat’s Capacity

On boats less than 20 feet in length with no capacity plate, use the following rule of thumb to calculate the number of persons (weighing 150 lbs. each, on average) the vessel can carry safely in good weather conditions.

Number of people = vessel length (ft.) x vessel width (ft.) ÷ 15

For example, for a vessel 18 feet long by 6 feet wide, the number of persons is 18 times 6 (or 108) divided by 15, which equals seven 150-lb. persons (or a total person weight of 7 x 150, or 1050 lbs.).

I know that this is rough, but since I am in Florida with 100’s of other people on boats who may or may not understand the rules of the road, I would like to stay within the legal limits. Also because I am not a licensed naval architect, I just want to play it safe.

Thus the 4’x8’ dingy would yield the following:Usable capacity of 320 pounds or two people max.

38 pound hull

500 pound displacement would equal 8 inches of draft

My original design was the following:4.57 foot width and 8.85 length with 6.5 inches of draft @ 500 pounds of displacement

This yields the following:

Usable capacity of 400 pounds or two and 1/2 people max.

42 pound hull

My final design is a lengthened hull:4.5 foot width and 10.0 length with 6.0 inches of draft @ 500 pounds of displacement

This yields the following:

Usable capacity of 450 pounds or three people max.

46 pound hull

Reaching my 500 pounds plus or minus would only be slightly overloaded, or just mostly dead!

So therefore the following can be said: the material cost increase from original design to final design is minimal, the usable load is more in aligned with what I wanted, and the design is more closely related to my original idea and parent hull design.

Are there any pros and cons for the design chosen for this type of design/build, which you can chime in on? Any experience with this type of project?

Description: Skin (composite) on plastic frames (composite)… designed as a tender for a larger ship, used to carry supplies to and from, needs to be easy to row (for exercise), needs to be a day-sailor for just the fun of it, and could be needed as emergency flotation life raft if one is not available. Also would like to use a small <5 HP outboard offset on the transom.

Of course, the 500 pound capacity would not be used for sailing or rowing… just as the tender to shuttle back and forth for shore excursions etc…

Thanks for the input.

BJD Bowman