a dream of a long narrow planing hull

[fallguy]

Start with a 20 foot design and stretch it to 40 feet by scaling by a factor of 2 in the length only, beam and height stay the same. Keep the "density" constant so the weight increases by a factor of 2 and the CG height stays constant. Then the resistance to a heeling moment doubles. The roll period stays the same. The moment induced by waves from abeam would increase by approximately a factor of 2 so the roll magnitude from waves would stay about the same.

So why the common knowledge that narrow boats roll more than wide boats? Probably several factors. One is folks are used to comparing boats of the same length and different beams, not different lengths and the same beam, an for a given length a wider boat will be stiffer. Another is longer boats tend to be taller which may mean the CG is higher and that leads to more rolling.

Very few designers would limit a twice as long hull to the same amount of freeboard. And so the center of gravity almost never stays the same.

Even the low profile cigarette boats seem to grow a bit taller.

I would take a cat; even a folding cat over the narrow long boat any day of the year.

Say you have a 13' beam folding cat...there is no question rolling will be much less in typical seas there.

The long monohull will have more spacious accomodations, but you will be fighting the sea more.

Can it be done? Sure. You just want to avoid the beam sea and make sure you are designed well enough to deal with a head sea returning to port.

Unfortunately, the trailerability side of the equation really makes this ship less than ideal. And you will forever be jealous of the 31' Bertram.

 

[fallguy]

I wonder if you could store a boat on a trailer at a marina in San Diego and get a more appropriate ocean boat and a second rig for lakes? That way you avoid slip fees at least..

Just an idea.

 

[Sean Duval]

I wonder if you could store a boat on a trailer at a marina in San Diego and get a more appropriate ocean boat and a second rig for lakes? That way you avoid slip fees at least..

Just an idea.

Lol I wish it was that simple....life for me rarely is. Currently I believe I have 5 boats in the yard...stored nice and tidy. 2 that are lake or sea ready...one on a slow road to ready for the sea...1 needs nothing more than carb rebuilds fuel lines and a battery, it will sell this summer....one that is rather sad and needs a new transom, and a part or two for Nissan engine.

Honestly my earliest memories are aboard boats...we made it from California to new Zealand via most islands between the marquesas and new Zealand on a 40ft sloop. It may explain why I view boats differently than many, so felt I should share that part of my life.

Ok, summing up what we have established so far as facts or are solid givens.

Beam is beam regardless of length and a given beam will exhibit a given roll rate provided COG and profile are the same.

As length increases fretboard must increase at least to some extent...without careful planning this will certainly raise the COG.

6-7 knots isn't going to cost huge fuel Bill's because your close to hull speed.

Longitudinal stiffness would be very important because you have a long thin hull shape.

It should slice nicely through waves due to the long thin shape vs riding up...but will be a wetter ride.

Trailering would be AZ USA and CA USA so as long as I comply with the most restrictive regulations of the combined state regs I should be good to go there.

Ok...more questions.

With the longer leaner shape I believe I could combat roll a bit by having a reduced v shape and likely make planning easier. No intention of flying over waves and dropping off the back side.

With that in mind and a sharper bow to cut more vs riding up the waves... would that reduce roll and increase efficiency a bit when traveling over hull speed?

I'm aware that sharper entry and greater aft flotation will make a following sea a bit more interesting. This might be where a bit of thinking maybe more flam would be useful at the bow.

Please feel free as always to tell me I'm off my rocker....but please also explain why...or how.

I'll see if I can get some basic drawing put together so you folks can see what I am aiming for...and likely suggest improvements.

As always thank you all, even the ones who I am sure think me to be a bit daft.

Sean

 

[Mr Efficiency]

I am sure think me to be a bit daft.

No, you seem pretty well grounded. And I have never heard anyone use the word "daft" that wasn't from the UK ! I think it would be a good idea to come up with some preliminary drawings, and I agree freeboard will have to increase with increasing length.

 

[Village_Idiot]

Although not directly applicable to your desired design, i run a 25' flat-bottom aluminum with a 5' bottom width. It is built to run in shallow waters in rivers, and also contains a longish pocket tunnel as well as a v-bow. I run it on inland waters, so i might deal with some choppy waters, but no real swells.

The boat does roll with beam waves, but not bad, and you get used to it. I never feel like the boat is going to roll over or throw anyone out. In waves or choppy water, it can pound when running on plane, but usually need three-footers before it gets really jarring. I think the weight of the boat is as much of a player here as the hull design. The boat is similar in design to a Panga.

You might consider some built-in rails up high (even a single rail down the center of the boat) to give folks something to hold onto if their legs are green, or seas are rough. If building in sleeping quarters, consider hammocks, as they will self-regulate with the rolling beam, and can be quickly stowed away to make more useable space.

Give careful consideration to the stern / transom design. This will determine how efficiently the boat planes vs how efficient the hull speed remains. Look to canoes, both powered and unpowered, for inspiration.

Freeboard need not be excessive if you don't mind getting a little wet, and have designed the boat for green water to escape easily without compromising floatation. More freeboard means more weight and more windage. Look to surfboards for inspiration...

 

[troy2000]

Regarding the roll issue, is it worth looking at using a steadying sail while trolling - or maybe two, given the length you're talking about? It shouldn't take much sail area to tame things considerably...

 

[fallguy]

Regarding the roll issue, is it worth looking at using a steadying sail while trolling - or maybe two, given the length you're talking about? It shouldn't take much sail area to tame things considerably...

What is a steadying sail?

 

[Mr Efficiency]

A steadying sail acts like a damper, it absorbs the energy of a rolling boat by imparting motion to the air. It doesn't have much propulsive usefulness. Suffice to say a steadying sail really isn't something that can adorn a planing boat, the snappy pitching motions would give it whiplash.

 

[troy2000]

A steadying sail acts like a damper, it absorbs the energy of a rolling boat by imparting motion to the air. It doesn't have much propulsive usefulness. Suffice to say a steadying sail really isn't something that can adorn a planing boat, the snappy pitching motions would give it whiplash.

Of course, boats don't troll on plane. And since steadying sails don't need to be all that big, it shouldn't be a major problem to stow them, masts and all, when they aren't in use.

 

[Blueknarr]

What is a steadying sail?

A photographic example

 

[Sean Duval]

I'm not opposed to steadying sails...also considering Lee boards as a possibility....
Either could be retracted/rotated whatever out of the way when unneeded or unwanted.

 

[DCockey]

Attempting to use lee boards on a power boat to dampen roll will require either a very strong mounting system or a willingness to tolerate the lee boards slamming into the hull.

Lee boards on most sail boats are mounted so that they can swing away from the hull. If a lee board can't swing away from the hull then it will try to bend what is keeping it from swinging. On a sailboat the lee boards swinging away from the hull is not a problem. The leeward lee board is pressed against the hull and the windward lee board is either raised or swings away from the hull.

An alternative to lee boards for roll dampening would be bilge boards which retract into trunks.

 

[Sean Duval]

Bildge boards may work...
I have seen some designs of yore with long keels like dagger boards...

Bildge boards are definitely something to consider.

I'm still having an issue rendering a decent drawing of what I propose the boat to look like. Something about being a working family man.....things constantly get in the way. It's over 110*F today and the rest of summer will be as hot or hotter. So the weekends should start having some down time again. In the meantime almost done with repairs to my little 22ft offshore boat engine repairs and the tuna are still biting

I'll get some drawing up for critique and then go from there.

 

[tspeer]

I'm not opposed to steadying sails...also considering Lee boards as a possibility....
Either could be retracted/rotated whatever out of the way when unneeded or unwanted.

Leeboards would not be an effective means of roll damping. The reason is they are mounted at almost 90 deg to a line from the roll axis to the board, so their motion is mostly up and down along the span of the board. This produces very little in way of lift on the board and not much damping. This is different from a board on the centerline or a horizontal foil projecting from the bilge, both of which would move essentially perpendicular the plane of the board as the boat rolls. A retractable T or L foil in lieu of a straight leeboard would be more effective.

 

[Rurudyne]

Very few designers would limit a twice as long hull to the same amount of freeboard. And so the center of gravity almost never stays the same.

Even the low profile cigarette boats seem to grow a bit taller.

I guess customers want every last cubic foot of accommodations and that gorgeous sleek low profile gets the heave ho.

Myself, I'd like relatively long and lean ... Bolger's Wyoming seems much prettier than his Illinois if I could afford either.