how is my design and SOR

SOR 7m protected waters camp cruiser


Hi forum.
I am contemplating a small powerboat for family use on small river and connected lakes. There is a small river that goes by the farm and connects to a 90km2 lake with further connections. This is Finland so all waters are really broken in shape ie. no big open areas (no big waves). My son is 8 months old now and I have major renovation going on but I am hoping that around when he is 5years old it would be great to have a boat.

The SOR is something like this:

Quite minimal ascetic equipped boat. Somewhat minimal fuel use (ideally at reasonably wide speed range).
General description of use: day trips and camp cruising on nearby river and connected lakes. A few narrow spots on the river and quite shallow too limiting the draft to 1 foot range. The typical trip would be about 20nm round trip to closest town but longer day trips should be possible too.
The second use is short trips to access the lake (less than 1.5 nm), maybe pull a lure there, swim, play etc.

capacity: typical use 2 adults, 2 kids with some camping gear (not American motor home style camping, just tent and cooler + little food etc.). More people for short trips - two families would be great. Preferably decently handles varied displacement (within reason).

weather / seaworthiness: Fair weather boat. Summer has some sudden thunderstorms but in general it will be used only on flat water and fine weather. If taken out in autumn rains rain coat will be the primary cover.

construction: Cost efficient building method. This is relative as building own boat has significant value to me so certain irrational aspect is acceptable (compared to buying used). Still work boat finish is acceptable and cheap solutions are preferred.
I would really like round bilged construction. I also might have decent hook ups for local soft wood so I am dreaming of strip planking with glass in and out. Plywood for the cuddy. Expoxy and glass.

Speed target: anything over 10 is welcome. 12-14 would be great. (with 4 people family on it)

Power: as needed but 15hp suzuki lean burn is my current pick. I don’t know if a high thrust 10hp would be better option. I do not know at what speed range / set of conditions they start making more sense. At least the high thrust ones cost a fair bit more with far fewer choices.

Equipment:
-cuddy cabin (room for naps, according to my better half no place is better for a nap than a boat under way). Barely sitting headroom, minimalist.
-storage space for camping gear (as in a tent), food, PFD and portable gas stove
-some kind of light canopy for light rain (not a rainy day boat) this cover can be more of a sun shade

NO: porta potty or head, sink, anything fancy.

The current plan (I will admit right away that the wooden boat article of Nigel Irens had inspirational impact).

Length over all : 23.063 ft
Beam over all : 5.237 ft
Design draft : 0.783 ft
Midship location : 11.483 ft
Displacement : 0.638 tons

 

Look at the Dory's on the Glen-L marine site. This type of hull runs very shallow but is very stable. From 12' to 30' Finish the inside anyway you want. Very easy build and very little Money. Lots of building pictures. http://www.glen-l.com/

 

If you look more closely at Irens lines, recently published for his low weight for length designs, he's incorporating a hook in the very after sections, to control trim and limit speed potential. Getting this hook the right size, shape and location is serious business and really can't be guessed at. It's mostly experience and the benefit of testing that gets this right. The rest of your SOR looks to be easily attainable with many different designs. Considering your shoal requirement, consider one of the tunnel stern, sea brites or box keel designs from Atkins.

 

Those renderings give meaning to the phrase 'skeleton crew'.....

 

In matching your SOR with the design renderings and specifications, I see some things that look like an incompatibility. The hull sections are optimized for an easily driven shape but combined with the narrow beam and slack bilges, indicate a boat that is not highly stable. That is especially true when the people are aboard and might be moving around. The sections are also not those best suited for shallow water. When an outboard is mounted, it appears that a draft of over 2 feet will result.

With the expected use to be in relatively calm water, the deadrise seems excessive and increases draft. For your SOR, I would expect a L/B of about 3 to be a better fit. Its a nice looking boat but does not seem to fit your needs as well as it might.

 

Might be a little tippy without a keel and a round bilge

For slow speed a beamy , chined , axe bow might be nice....a bit more volume forward and lateral resistance

Plumb stem , fine entry, slab side boats are a bit wet....be sure to deflect the spray off the stem .

10 or 15 hp sounds cool for a cruiser.

As. par observed a bit of hook might better keep the boat on its lines.


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I like your requirements and have often thought of a similar set of requirements for myself on the Chesapeake with my family. I think stability is a concern for your length, beam, and hull shape. Also, sensitivity to performance with added weight such as bodies and stowage will be high on such a small narrow platform.
My initial concept design for similar craft has a length of about 30 feet with an initial deck beam of 7.5 feet. It is a "buoyancy stabilized monohull " (2 small detachable outer hulls for added stability, tri hull configuration) . Narrow main hull driven by twin 15 hp outboard engines. Outer hulls would give a max operating beam of about 12 feet, amas removed for trailering. Target fuel consumption of less than 2 gph at 10 knots.

 

I'd bring the WL Beam out to almost full Beam. This would greatly increase initial stability, which is what you want for a narrow boat with four people walking around in.

Doing such would make the bottom practically flat, but it could have rounded bilges.

I think you might also have to forget about a self draining cockpit, so you can keep the weight of the people lower in the boat. Generous flotation or generous sized water tight compartments could take its place.

I also think your displacement estimation is a bit low. I think allowing for 240 lbs per person, including personal gear, plus maybe 1,000 lbs for the boat, engine, fuel tanks, fenders, and ground tackle, may be closer to the mark.

I really love your concept though, a real 21st century powerboat.

 

I will type a proper reply when I get a little more time. Thanks everyone for the feedback.
the draft requirement of 1 foot is probably an exaggeration. There is one rocky shallow of maybe 20 meters/Yards that is the only bottle neck. I will measure it this coming summer.

I think sharpii's suggestion with reducing the forefoot is my next step and I will see how it looks like then.
The stability I am not too worried about. I am used to a rowboat and the principle of no walking or standing and when necessary one at a time. I am sure its more fun when you don't have to worry about it that much but being quick with little power is more important in the end.

 

I think your concept would work very well in Finland, both on the lakes and around the coast. I too would increase the waterline beam. I would also reduce dead rise to almost zero in the stern and maybe start gradually introducing a chine from about midships aft. Now think along the lines of a Petterson boat for styling and you might have a winner for the Scandinavian market!
I have been dreaming of one day getting a boat along these lines built for myself so I would have accommodation and holiday plans all sorted for the odd times I can make it to visit the old homeland.
Now if we scaled this concept up just a little we could start spending a week or two onboard and even do some exciting long distance travel along the European canal network...

 

Still too busy to take a real stabb at refining this.

Kiwifinn, so you have roots in Finland?
I hear what you are saying and I think the slight added beam will make it more stable. I definitely don't want to scale up my boat. Its so common to see here that people are asking about 20ft boat and people suggest scaling up just a little to 25ft (or from 16 to 19, or 30 to 35). Larger boat IS always nicer. But my core idea is simplicity and affordability (naturally relative) so don't want to start adding real accommodation - almost rather go towards a fully open boat to be honest.

And I don't even want think about making a boat to market. There are plenty of real boatshops here to have the market covered. Not going to think I could beat the competition. I just want to build one boat. And design one vehicle. a boat would do.

 

Hello Rerosene,

Yup, I'm a finn. Have been living in NZ since '97 but I try to visit every few years. I definitely get your SOR of rugged simplicity and size, it is spot on for your intended use. I just started musing over my own little dreams of having a boat over there. I spent the my summers of my youth in the archipelago between Helsinki and Hanko on friend's sailing yachts or at the summer cottage so for me a simple boat that could spend some time semi-comfortably cruising those waters would be ideal.
Few things are as rewarding as designing and building one's own boat. Indeed a boat is probably the last remaining form of motorised transportation that a person can legally design, build and use without lots of red tape. I hope you enjoy the process and I look forward to seeing your refined design.
One more thought... It might be worth looking at the possibility of leaving out the sliding hatch to access the cuddy cabin. In stead just leave the opening in the bulkhead larger. It is much easier not to build a hatch than to build one...