Boat Hull Design Question

Please note the area of the curve is not even in the water.

 

So the question is why does the topsides of the third boat appear to have continuous curvature, and not straight sides aft ? It avoids a less visually appealing slab-sided look, and also makes for a stiffer panel requiring less framing, is all I can think of. As for the concern expressed in the OP about "unexpected turning results", I would think it would turn fairly flat, but with the low power available, I don't see anything untoward happening. How well it would handle a steep following sea would be a small concern, there is a bit of a forefoot there and a fairly flat stern, which is not ideal in that respect, but unlikely it has too many vices or it would not have become popular. Obviously it is designed to handle mild conditions at moderate speeds reasonably well, with economical operation due to the low power requirements.

 

Thank You very much for the well thought out answer.

 

Okay...

not cool...

Here are some construction pics. Hardly worth mentioning a narrow stern that does NOT exist. Zoom in on the picture; the hull is straight sides aft. The curvature is an illusion.

 

Looks like a beautiful build. I would love to build this boat but I live in Thailand and wood is under constant attack here. Leave a book on the floor here and a week latter it is totally infested with termites. I would love to build that boat in aluminum plate.

 

Afaik, the general rule is any boat built in ply can be built in aluminum. It is a lovely boat indeed.

 

A plate boat is going to finish up heavier, I think I'd like more deadrise and a bigger engine.

 

I had actually considered doing a Marissa out of aluminum and was just starting to accumulate the gear, but then I had a kid and didn't want to commit the funds so I built a S&G of a different design - much cheaper. Still have the Marissa plans. Nice boat.

 

Incidentally here's a similarly sized design from Bruce Cope. Very nice aluminum designs, extremely highly sought after here.

 

That boat would need 90hp+, the marissa is designed for lightness and ease of propulsion, a different proposition.

 

Obviously - but if anybody wanted to see a drawing of one of the boats in the OP that actually shows something, well, there's an actual drawing of a Cope. No idea whether that's the one the rendering in the OP is intending to show, but that's the most marissa-sized Cope I can think of off the top of my head.

 

I would not be copying the lines of the Marissa in alloy, far better to get plans intended for alloy

 

Well, I'm sure nobody was going to try to make you do that so I guess everyone wins!

 

Cope has the "Barkley Sounder 16" with a recommended HP of 40 - 50

I live in Thailand and the 2 stroke motors sold here (Tohatsu and Suzuki) max out at 50 hp. So this plan set would seem to work financially. I would love a nice 4 stroke but the price differential is just insane. A 50 HP two stroke is about 4,000 dollars while the 4 stroke 50hp motor is closer to 8,000 dollars. Sure the 4 stroke may be more efficient but not at that price difference.

I really wish boat designers would focus more one the smallest motors that would feasibly work for their boats.

To make a better choice I reversed engineered most of COPE and SPECKMARS hulls in rhino.
They are all fully developable hull shapes and unroll with zero change in area. The driving force of this work was to figure out what design was the most efficient use of the 5x20 sheets of alloy plate I can get here. No major designer seems to even have this on their radar and it's frustrating. I would love a design that said "Uses 90% of 3 sheets of 5x20" or some efficiency statement.

The Marissa Eco 18 was in response to a challenge in wooden boat magazine on designing a hull that would use less then 2 gallons per hour. I see no such challenges in the metal boat building world and that's a shame.

 

The problem with a small plate boat is that the thinnest practicable plate still makes for a fairly weighty boat, you can't really win, if you go thinner plate, you need more framing tp avoid "dishing", and the weight is still too much for a 50hp outboard in a boat with some bulk and a reasonable deadrise angle. I think the 50 hp is a bit limiting, you will find it won't be a good match.
Edit: I see that is only 10 degrees, you would get away with 50hp probably, you will cruising about 20 mph, which is about as fast as that would be comfortable with in a little joggle.