Pram Perimeters

original parent hull

designed hull

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” of 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

 

Rules of thumb are only for determining if the vessel is anywhere close to correct box, never for design (i.e. the USCG rule of thumb gives ~10 sqft waterplane per person which is ~320 lbs of flotation per person for a 6 in draft). You have only done half of the design problem, now you need to do your curves of area & form, weights, arrangements, and finally stability. Only then will you if you have a viable design. Where people sit is going to drive this design. Remember, a sitting person is 3 ft long x 2 ft wide (i.e. 6 sqft), whether the second person sits at the transom facing forward or in the bows facing aft will have a big effect.

 

Doing a pram at less than 50 pounds is optimistic. If you use Okumee ply, and are very conservative with the framing, you might be able to build at less than 50 pounds. Best you stick to proven designs that have a lot more forward bearing than the one you have shown. There are a plethora of proven pram designs that are at least marginally equal to the requirement that you have laid out. Many of those designs furnish plans for free. If you need a lot of load capacity, then the ugly but functional PDR will fulfill your SOR.

 

What messabout said. For a 500# on odd occasions making a short jaunt in protected water to the dinghy dock, An 8'5" pram will do that. If it's your daily driver 3 miles by river in all kinds of weather to where you work, then you'll want the 10' version. Weight- wise, you can't get a 10:1 cargo capacity out of a pram dinghy. With flotation and hardware and paint, you are looking at a minimum of 75# for the 8'5" version. That's using 4mm fir ply and a 4 oz drape of glass inside and out and 1x1 for the rails and framing. Make the transoms at least 8mm, 10 would be better. If you plan to row, the oarlock area will need considerable reinforcing. So plan on a full frame at that location. Motor mount needs to be 1" thick and 10" x 10". You can build a scrap sandwich of 4mm ply and 4mm 5lb foam and a 12" x 12" patch of 1708 to get there (1708, ply, foam, transom, foam, ply, 1708)

The bottom panel really begs to be 6mm ply. For the 10' version, the entire boat needs to be 6mm ply. That one will weigh about 100 pounds and need substantial framing. Also check the pickup truck wheel well clearance. Most run about 49" at about 10 inches high. Being able to slide it into a truck is very handy. The 10' version will row easier if it has another inch of freeboard. It will use longer oars, and the oarlocks will be uncomfortably low if you put them at the shear. Note how the one in the photo has blocked up the tholes.

 

Having seen attempts to sail a pram with a leeboard and cat lug rig we concluded prams make awful sail boats. Might just as well try to sail a beach ball with the directional stability the pram hullform gives.

 

Prams can and do sail decently but not spectacularly. The Optimist class has numbers in the gazillions, the El Toro sails pretty well, the Sabot is not a bad one either.

 

Note the difference here, leeboards vs centerboards and I will bet hard chine (optimist, el toro, sabot) vs round chine as the OP shows. A hard chined hull can generate lift fairly well.

 

If you want to comply with the USCG regulations on capacity of small boats the best source of information is probably the USCG Boat Builders Handbook which can be downloaded at Boatbuilder's Handbook Downloads http://www.uscgboating.org/regulations/builders-handbook-downloads.php The Safe Capacity document explains how to determine the "safe capacity" in accordance with the regulations.

 

All great points, thanks.

 

Great Link, Thanks...

 

Hey All,

I found the parent hull through another site. I had been trying to find it for months. Here it is... It's a French designer with the pram "Laita".

Laita | François Vivier, naval architect http://www.vivierboats.com/en/product/laita/



I'm now going to build a balsa wood model with the data that I have found online, and compare it to my latest hull design changes.
Now that I have an idea of the length and width of the parent hull it was a bit easier to tweak my design.

I am still not sure about the size of the little pram. Now that I see the length of these boats, I'm thinking that the extra length could be a good thing.

My last model appears to be too short and stumpy as I have built a 1-1/2"=1'-0" cardboard hull of the original design that was posted in the first post above.

I finished the cardboard model just after I had already decided to adjust the hull.

I'm uploading the original Model although this will not be built, it was my first attempt at the proportions.







I'll update the design below...

I want to make the hull closer to the parent hull proportions. This was revision TWO below. Much closer to the parent hull.






I found the original (reproduction) parent hull below:

This is an idea of what I am kind of working toward.



Below are two designs overlaid on top of the parent hull.



Above is the 10 foot version and below is the 12 foot version so that you can see my design compared to the parent hull.




The Red Design is what I have come up with and the gray is the Parent Hull.

I decided to keep the little boat down at 10 feet for many good reasons...

Here is the latest design revision Three:

In the "Latest Version" of the design... I shortened it up as to keep below 10 feet. I cannot deal with the weight and length of the 12 foot version so this decision is done.

Some other things to think about is how to deal with the transom. I have been kicking around the idea of making it more modern in the stern. I have started that direction in this new design. I also added some width in the back (made the transom area wider) and shortened up the bow and made the bow transom even smaller, like the parent hull. Let me know what you think of the direction.

Thanks,






I like the fact that the strakes are less curvy... Anyway this is where I'm at in the spiral.

Getting ready to build the next model out of bass wood when it dawned on me that I would like to incorporate a few construction/design concepts during this build. Here is where I am at prior to changing the build method.



I was going to just build a center-line strong-back to bend the bottom plank over, and build the hull upside down using a male mold. In the last model, I built a female mold and bending the bottom plank into the form was a bit iffy.

So here is my latest design/construction process issue. Instead of using a strong back (for the model, and real world boat) and, wasting material on something that will not end up into the boat, why not build a backbone for the little boat into the design of the structure. I have seen this in many cases. Here are a few:





ABOVE IS THE OWL: Hermit Cove Boats https://www.hermitcoveboats.com/index.html

With this thought in place, I was thinking that, like the owl pictured here above, I would use two main longitudinal backbones spaced far enough apart to provide spanning space for the sole (transverse planking between the backbones) for the boat and then using the backbones as the inward edge/support of longitudinal seats and flotation chambers along the sides of the boat. With the thought of a knock-down I would like to install flotation chambers close to the mid-point of the boat where it would lie in the water on it's side. (any thoughts on this?)

I was also thinking about using two dagger boards (lee boards) or somewhat twin keels of a layout attached to these same longitudinal backbones. Another thought was (hold on to your hats) making a system to be able to change the bottom of the dagger boards to incorporate wheels for using the boat as a wagon and also adding foils for some fun sailing... keeping my options opens. Having a port and starboard dagger board does mean more work, but then the center of the boat is open for coolers, dive gear and hauling materials/provisions etc.

This type of wheel (10" x 3" with nylon bearing) on a fork like a bicycle attached to a cap / plug from below?

I like to owl system shown above, but I think I want to reverse the seat idea from the center to the sides, so that I can add the dual dagger boards/wheel concepts. (any thoughts there?)

Many issues here to play with and I am thinking about getting deep into this final design now that I have the hull approximately where I want it...

Any input at this stage would be great,

Thanks,

(worlds longest post?)

 

Some have commented above that the round bilge is an issue and that this was too complicated a design.
Therefore: I have designed the worse case model to see if there are any improvements:

I decided to try a single panel square bilge just for kicks. There are better initial stability and more interior volume, however that comes with some costs. Mainly in weight. The area of the hull is 50% LARGER than the last design, making a 100 lb build into a 150 lb build, assuming that the original would be that heavy.

Here is the easy boxy design:



This is the same size and general shape of the last design as shown below. My only issue is that I just don't like it nor do I feel that I could make a strong skin on frame build with this shape.

Here is the last design as above:


So you can see that there is not a big difference. I do like the added stability and the volume, therefore, I decided to plump out this design a bit adding in some volume down low. I used a true radius along the chine making the bottom and the sheer 90 degrees and this did add to the volume and the stability.

Here is the latest design:

I also added to the bulwarks to increase the free board... I know that this is not the pretty little pram that I started with... but it's still moving forward... I want to add a small deck and locker on the bow so this will help with the aesthetics of the pram. I still like that last version better, however, I'm looking at more than just a pretty picture... What do you think?

Any feedback?

Thanks for the comments above...

BJD

 

Lordy, I thought that old rule of thumb had died and gone away 40 years ago. First it was never intended for small dinghies, it was intended primarily for outboard runabouts from 12 to 20 feet. The best way to find maximum weight capacity and persons capacity for a dinghy is to use the Coast Guards rules for boats rated for 2 hp or less or manually propelled. (same rule for ISO, ABYC and just about everywhere)

It works like this. First determine the displacement weight; displacement weight is the weight it would take to sink the boat, or put a better way, the weight of water displaced if the boat were pushed down until water starts to come in ; in a dinghy usually somewhere midpoint along the gunwale. That computer CAD design should be able to tell you that. Just redefine the waterline at what would appear to be the lowest point along the gunwale. It should be able to give you a displacement volume in cubic feet. For salt water multiply the volume by 64 lb. For fresh 62.4
Take the result and subtract the boat weight. Then multiply by 3/10. That is the boats maximum safe load. Maximum persons weight is 9/10 of the maximum safe load. Maximum persons is the (Persons weight + 32 )/ 141. If the result is, say 2.4 then it's 2. If it's 2.5 or more the round up to 3. In other words round up or down.

If you plan on using a small outboard 2 hp or less then subtract 30 lbs from the maximum safe load. If you plan on using a larger outboard then you have to rate it the same way a larger outboard boat would be rated and you would lose a lot of safe load weight because for outboards under 20 ft the safe load is the displacement weight Less boat weight divided by five, to get safe load, Then you have to subtract the weight of the engine, battery, full fuel tank, etc. So using the rules for 2 hp or less gives you a much higher persons capacity.

 

Nice post, thanks... there is much to do if I wanted to create a nameplate for the 10 foot pram, if I wanted to manufacture and sell this boat, however, I'm not doing any of that. This is a one off personal flotation device... LOL The Rule of thumb that I mentioned above is what the "local Coast Guard" told me, specifically for small "without a capacity plate" boats. If and when, the dream police, see five people in a 10 foot boat, "without an capacity plate" they can quickly ask or measure the length and width of the small boat and determine if the boat is overloaded or not. This has nothing to do with reality or being in compliance with manufacturing regs, but a more "seat of your pants" rule because I don't want to get stupid fines from "Big Brother". Thanks again for your help, but my boat will easily be able to carry 150% of what I am requiring of it. Refer to below...

https://i.ibb.co/ts80bMZ/s-l1600.jpg

Thanks again...