a dream of a long narrow planing hull

[Sean Duval]

1st thank you for welcoming me to this forum! I will try not to ramble too much as I describe my overall plan, and the reasons for the compromises that must be made to enable this idea/proposal to go from pipe dream to possibility. Additionally I realize that my goal and my ideas regarding this run very much counter current to todays norm, I am always open to suggestions and will attempt to understand and fully appreciate the responses that I receive. Oh...yes I intent to build from scratch the largest boat that can be trailered without restrictions or oversize permits...and yup I'm in the middle of the Mojave/Sonoran desert...so any real use of the creation will require about 3 hours travel by road...however I can plop it in a local lake to do testing and such should I get to the point where it can float.

Ok enough preamble...

What I am proposing to build is a 38ft long 8.5ft wide hull. Maximum beam at the waterline would be in the 7ft range (have not solidified this measurement yet). Yes that is long and skinny, actual intended ratio of 5:1 is very different for todays standards but harkens to a time gone by of big heavy low powered inefficient engines and the only way the designers then could make speed with what they had.

The intended mission is comfortable ride while underway in the coastal swell of southern California, ability for up to 4 to sleep aboard but packed like sardines, long range at relatively low speeds let's call it 20 knots top speed (most time to be spent at tuna trolling speed so 5-8 knots or so), a nice tidy and simple pilot house with full standing room. and a whole lot of low slung self bailing deck behind. The ability to leave port and comfortably fish with 4 people on board for 3 days (my weekends are 3 days long).
Freshwater cooled, plate coolers, diesel powered, straight shaft (no v drives, no IOs), aluminum construction...I don't want to see a speck of wood on her. At this point basic shape is figured out... deadrise angle is still questionable....and bow shape/form is still questionable as I would prefer to cut waves than ride over them.

The intended planing action would be very different as I do not wish to do the typical stand on stern bow high and apply more power to get over the hump...my intent is to generate a smaller bow wave and be able to ride over it with little trim change at all. I'm pretty sure this takes less power overall...if I keep her light I'm thinking I can do this with around 200 or so hp and not use most of them ponies most of the time.

A link to the thread that gave me hope.

Long Narrow Planing Hulls https://www.boatdesign.net/threads/long-narrow-planing-hulls.13688/

So to preemptively hit some points,

yes the hull would roll a bit more on a beam sea, I think however that provided the Center of gravity is low, and the center of buoyancy is ok then she wouldn't be worse than a shorter boat of the same beam (am I correct here?)

No there won't be a lot of room for accommodations...practicality not plushness is the intent.

Ok...
Please feel free to cut my idea to ribbons, or provide helpful insight where possible. If my idea is rock stupid please let me know why and where, but let's try to be positive when trashing my idea.

Respectfully submitted for thought

Sean

 

[philSweet]

Sorry, Sean, but your performance request and hull geometry are almost perfectly noncompatible. There are perfectly normal 27'ers cabin cruisers that can sleep four and plane at 20 knots. They tend to have about 400-500 hp of inboard power. Planing requires a wide hull. You basically need a hole in the water behind the transom that is the same size as the hull's normal displacement. In order to get that in a long skinny hull, you need to be going very fast to lengthen the depression, and you need a deeply submerged transom to give it volume. So you are looking at very high speeds and power to get the sort of hull you describe to plane. It will have far too much wetted surface to be efficient.

There is also a thread called "the most that can be had" in the "option one" subforum about the largest practical trailerable boats.

Restart by giving us the actual performance requirements, build budget, operating budget, and list the waters you want to operate on (Lake Meade, Lake Havasu, Sea of Cortez, etc.) There are quite a few Sharpie-like hulls that are suitable for home build and efficient travel on protected waters at 7-8 knots. These can run on 50 - 75 hp diesels and have decent comfort for camping and vacations. Members PAR (died recently) and Tanton both have several you can find here and on their websites.

Also, the 8.6" limit can be gotten around with an exemption. You can normally get one for boats up to 10' beam, especially out west where you are. In the Florida Keys, 10' beam boats are trailered in by the thousands each weekend from Miami.

 

[Sean Duval]

I like the 10ft exemption part! That would make some off the shelf designs much more practical I'll have to contact Az DOT and inquire then CA DOT as Arizonais lacking in the seashore access dept.

I very much appreciate your directness, and I am thinking best to start a different thread for the list you requested.

The thing is I don't want to be in protected waters, regularly going out 25miles off the San Diego coast, and looking to do it in more comfort and for extended duration...and have economical Operation cost. The west coast swell means either very big boat...or bouncy ride pretty much anytime you head out...its tall but smooth and long period.

If I moved down to a lower speed would that work out. Thinking semi displacement might be the answer.

I very much like the idea of smaller diesel engines .

Well I'm gonna sit down and pout for a bit....and read some more books and research a bit more before I give up entirely on this idea....would building scale models operating at scale speeds with scaled power show these issues?

 

[Dejay]

I have zero experience but if you need both 20 knot speeds and a "comfortable ride while underway in the coastal swell" I think planing is the wrong concept anyways? Isn't planing always rough on the back? And besides the ride itself, I think planing hulls also don't have ideal motions when drifting in the sea for fishing.

I previously looked around and a design like the ArrowCat 320 sounds pretty good. It makes sense to get a trailer with exception for a 3m / 10' wide boat. Not sure what cheap DIY plans for aluminium are available and cats are generally more complex to build. But you get lots of sleeping room in the hulls.

Of course catamarans aren't "really" planing but they can get over the hull speed and generate at least some dynamic lift.

Or look into power trimarans. I've only found the Kurt Hughes 38' power trimaran. ("6.5 hp will push this tri at hull speed, the specified twin 15 hp outboards will push it along at over 12 knots. See data on vessel range.")

From what I've learned (again I'm a newbie) trimarans are more energy efficient and lighter than catamarans and can be trailered more easily with foldable or demountable amas. I think they have even better motions than catamarans at sea, but less space.

 

[Squidly-Diddly]

push ratio to 6.5/1.
"no wood" check.
efficient at several power levels and seaworthy and soft riding at high speed. check.
classic styling, considered one of the most beautiful boats ever built. check.
scaled down to 40' could sleep 4 "'packed like sardines". check.
design and hull lines now in "public domain" after WW2? priceless.

 

[Dejay]

Or maybe a hydrofoil to lift the front of boat out of the water and use the stern to plane.

 

[Sean Duval]

I really like the lines of the top picture...very like what I had in mind...

Engines back then were way heavier per hp so likely hull redesign wouldn't be too drastic I would guess. Of course....every time I assume something....

But yeah that's a pretty boat on top.... very pretty.

Thank you very much gonna go look drool and research.

 

[Sean Duval]

What make or model is that top one......really sexy.

 

[JSL]

Not a bad idea- perhaps a bit narrow - prepare to give up some convenience- especially if more than 2 people.
Our 3rd family boat (about 1955) was 40' x 10'. Originally built as a commuter with some living accommodations for 2. Cruised comfortably at 12 - 14 knots with a pair of 200hp engines. A terrific sea boat - we used to leave many production boats behind and watched them pound, slam, and wallow in the waves.
Yes, it did plane but there was only about 10*-12* deadrise.

 

[Mr Efficiency]

Three days of trolling around is a lot of fuel. That means weight. The trolling speed requirement is critical, 5 knots can be obtained fairly easily and economically, the 8 knots mentioned in the OP, much less so, it is well into the "hump" range, I think that has to be nailed-down, as to what the acceptable trolling speed is. If at the higher end, it becomes a task to accommodate in any kind of boat that planes.

 

[Squidly-Diddly]

What make or model is that top one......really sexy.

you talking to me?
Oheka II - Wikipedia https://en.wikipedia.org/wiki/Oheka_II

lürssen effect - Google Search https://www.google.com/search?client=firefox-b-1-d&q=l%C3%BCrssen+effect

 

[Sean Duval]

Thank you squidly... the lines are gorgeous on that boat.... and the Lursen effect is fascinating.

Please correct me if I'm wrong but long slender hulls (from my understanding) do not squat nearly as much as they transition to planing because they produce a much less substantial bow wave and trough effect and tend to rise up more parallel with the water surface. I keep thinking of Panga type hulls and their low power planing abilities. Thinking this could be applied to a larger craft as well at least in some part.

Thank you all...and thank you for helping to educate me further on this issue.

 

[DCockey]

Planing requires a wide hull.

I disagree. Planing does not require a wide hull. For example the Series 62 and 65 series of planning hulls include a models with chine length to beam ratio of over 9, much larger than the original posters proposed ratio of 5. See Appendix 4 in Performance by Design, Donald L. Blount. The large hump at around "hull speed" typical of low length to displacement planning craft does vanish as the length to displacement ratio increases.

You basically need a hole in the water behind the transom that is the same size as the hull's normal displacement. In order to get that in a long skinny hull, you need to be going very fast to lengthen the depression, and you need a deeply submerged transom to give it volume.

Can you provide a reference for this concept? I have not encountered it in my studies of the technical literature about planing craft.

 

[DCockey]

Please correct me if I'm wrong but long slender hulls (from my understanding) do not squat nearly as much as they transition to planing because they produce a much less substantial bow wave and trough effect and tend to rise up more parallel with the water surface. I keep thinking of Panga type hulls and their low power planing abilities. Thinking this could be applied to a larger craft as well at least in some part.

That is also my understanding.

 

[Squidly-Diddly]

Please correct me if I'm wrong but long slender hulls (from my understanding) do not squat nearly as much as they transition to planing

Thats a semi-displacement hull, and IIRC that hull form is the least energy efficient at speed, but the S-Boat was known for being able to operate at high-speeds in seas that would force a regular planing hull PT-boat to crawl at full displacement speed.