Re-design of 8 Foot Dinghy . . .

[tpenfield]

Greetings,

As mentioned in another design/build thread I am planning on re-making my 8-foot pram dinghy (fiberglass over foam construction). A few pictures of the original dinghy are below.

This was my first attempt at making a boat from scratch, some 15 years ago, so be kind . . .
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The primary use of the boat has been to go out to my mooring and use my big-boy boat. The attributes of this boat are/have been

-> High buoyancy
-> 20" of freeboard
-> Self bailing
-> Stable
-> 15" transom for small outboard motor (also has oar locks)

General dimensions are 8' 6" LOA, 3' 6" beam, 20" freeboard. It was made of XPS foam boards with epoxy/glass over.

Overall it has been fine, but it is showing its age, so a complete re-make is in order. I will basically scrap the existing boat and make a new one.

My plans for a re-make are to make some improvements.

-> Retain stability and high buoyancy
-> Improve self bailing with a greater depth of flooring
-> Improve the bow section to take small waves better (probably making more pointed)
-> Add some side railings
-> Target weight (dry) 130-150 lbs. (similar to previous boat)

I probably will go with a 21" freeboard, since I will be making the floor deeper to improve the self bailing aspect.

My challenge right now is re-designing the bow section, while maintaining the stability that the stern and mid-section offer.

Just throwing this out there at this stage for ideas and inputs. I am working up a 3D model of the new boat so I can get a good idea of what the boat will look like.

Material-wise . . . I plan on using polyisocyanurate (aka polyiso) foam boards instead of the XPS foam that I used previously. Polyiso is more Poly/VE resin friendly than the XPS foam that I used previously. I'll be using biaxial cloth for the most part, along with VE resin.

My plan is to keep the boat about the same size, but I could stretch the LOA to about 9' in order to achieve the more 'pointed' bow.. The beam will want to be the same as before (3' 6").

Thoughts, ideas on the hull design to achieve the desired improvements?

T.I.A.

 

[Skip Johnson]

I question your choice of foam. One of the biggest mistakes I made over some years of trying new things was using a urethane foam core that the homebuilt aircraft people used. I had used some denser 20 pcf stuff with some success for some composite bike parts. But the 5-6 pcf stuff was 'friable' and over time it worked a little bit started turning to dust. Worst part it was a boat for a client.

Pet based foam is becoming ubiquitous and is compatible with Poly/VE foam and a little less expensive than pvc or san.

Best wishes on the deal, lots of challenge in things that small.

 

[fallguy]

Can you dm me your email? I have something I can share, but not publicly. I will explain privately.

 

[bajansailor]

I like the classy mug holders built in to the forward and aft seats - will you be incorporating these on the new boat design as well?
Re how she is self bailing, do you have a standard sailing dinghy self bailer on the inboard side of the 'stern tube' hole in the hull bottom?

Have you had a look at some of the many dinghy designs available on line, such as from Duckworks?
Plans - Plans by type - Rowboats - Page 1 - Duckworks https://duckworks.com/plans-by-type/rowboats/

 

[tpenfield]

I like the classy mug holders built in to the forward and aft seats - will you be incorporating these on the new boat design as well?

Yes . . .

Re how she is self bailing, do you have a standard sailing dinghy self bailer on the inboard side of the 'stern tube' hole in the hull bottom?

The cockpit floor sits above the static water line, so there is a scupper built into the aft portion of the hull (which you can see in one of the pictures). So, I just leave the drain plug out while not in use and the boat naturally drains . . . while most of the other dinghies at the dock fill with rain water.

With the rather large/heavy Mercury 7.5HP engine, the boat sits a little lower in the water, so it does not drain fully. Some increased height of the floor should remedy that situation.

Have you had a look at some of the many dinghy designs available on line, such as from Duckworks?
Plans - Plans by type - Rowboats - Page 1 - Duckworks https://duckworks.com/plans-by-type/rowboats/

I took a look at those plans . . . I saw nothing that stopped me in my tracks. Quite a few are similar to the many dinghies at the dock . . . most of them are quite tippy and easily submerged. Often I have to walk through one or 2 of the boats to get to my dinghy, since it is overly crowded. Some of them . . . I step into the bow and under they go.

Anyway . . . for the bow section, I am thinking of a combination between the Carolina Skiff curved up hull and the European pointed hulls and a little massaging between the two.

 

[fallguy]

Are you familiar with @Dolfiman and his Gene-Hull program?

He has one done for prams that may be useful for you to modify a d know where to cut.

Perhaps he will reply here.

 

[fallguy]

Probably a bit of a tradeoff to have a seaworthy bow and a step in bow…

Methinks a vee for’d might be a compromise.

 

[Dolfiman]

Are you familiar with @Dolfiman and his Gene-Hull program?

He has one done for prams that may be useful for you to modify a d know where to cut.

Perhaps he will reply here.

Here is this Gene-Hull application more specifically dedicated to small dinghy with chines (Hull with 5 developables panels) and a pointy plumb bow. Seems to be adapted for your project, given examples can help you design yours.

Thread 'Gene-Hull Dinghy Chines 3.1'

May 2, 2025

This was a recurring request I received from Gene-Hull Dinghy users : why not a multi chines version of Gene-Hull Dinghy ?
Well, multi chines hull in the general case with multi developable panels was and still be a bit complex (for me at least), but I was able to focus and develop an application for a simplified case which, I hope, could be attractive enough and helpful for an amateur builder aiming a dinghy easy construction with stich and glue (or equivalent method ) ply panels.
So here attached the « Gene-Hull Dinghy Chines 3.1 » spreadsheet application, based on a simplified hull...

• Dolfiman
• Replies: 0
• Forum: Software

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[BoatBuildLog]

Thats a great project to documents from start to finish. we built BoatBuildLog for exactly this kind of build log would be interesting to see how the new dinghy develops compared with your first one.