Which Will 'Plane' Sooner?

[capecodphyllis]

Would a longer/bigger RIB get up on a plane sooner than a shorter/lighter one with the same outboard? (Let's say 8' vs. 10' RIB, with a 6hp Tohatsu.)

I'm thinking that a longer, deep-V fiberglass hull would [possibly] pop up sooner rather than later, despite the increased weight of the larger RIB.

I mention this because a salesperson told me that he uses a tiny 7' RIB so that he can get it up on a plane with his 3.5hp Honda. This makes no sense to me at all. A tiny RIB would sit lower in the water - and require more ooomph to pop it up and keep it going. (Note: I know that a longer kayak is much easier to paddle than a shorter, lighter one.)

[Tartan34C]

I wonder how it works out if you divide the weight of the rig by the horsepower to get power per pound and then divide that into the area of the bottom to get realized power per square foot of boat. The boat with the better number would be the winner because it has more power per pound per square foot. To make is simple use the projected area of the bottom such as the length of the water line times the beam on the waterline of each boat because this is just a comparison and the absolute numbers don’t really mean anything.
Just wondering,
Robert Gainer

[sailingfool]

Tartan34C is correct that planning is a question of weight versus HP, so a smaller dingy starts out with an advantage as the salesman says. However, there's real tradeoffs as most of us also want the dingy to carry people and gear. I'd think a 7' all but useless other than for 2 people with little gear. And you don;'t want too big an engine or it'll be a max pain to remove and store it everytime the dingy is towed.
The combo that has worked for us is a 11.5 HB with a 8HP 2 cycle. It'll easily carry 5 people with gear, plane with 2, and the engine isn't to bad for hoisting to the stern rail.

[SVDistantStar]

Ive been useing a 3hp Johnson on all my small boats. I have 3 boats that ive used it on, a Watertender 9.4, a modifyed 14' tri-hull fibreglass, and a 10' Jonboat. The jonboat will plane with 2 people where as neither of the other 2 will. The watertender will only plane with 1 person setting far forward in it. The 14' will not plane at all, but does go faster than the watertender with 2 people. Im not really sure what this means, but from what ive seen the 10' jonboat is the one getting used untill ive got my 15 yamaha running.

[mjname]

planning happens when the wave at the bow meets the wave at the stern, a shorter boat will reach a plane state at lower speed than a longer boat. Because the waves at the stern and the bow have a shorter distance to travel to meet.. in theory. However hull shape makes a big difference in how much power it will take to push the boat to and past hull speed to reach a state of planning. A jon boat will take less power because it's not as deep in the water than a v shape hull.

I wonder if there is any major overall speed increase to justify going with a shorter dinghy just so you can plane?

After flipping my dinghy in the Provincetown Bay I would much rather have a 10 foot RIB than the little 8 footer. Really does it matter if you can plane your boat if it's upside down floating away in the swells? Besides a boat is more fun if you can actually get your buddies out to it... even if slower.

Phylis if you are on the Cape get the 10 footer don't even look at anything less than 10. Unless you are behind a seawall or like to swim around alot screaming for your dinghy.

Matthew

[T37Chef]

"Note: I know that a longer kayak is much easier to paddle than a shorter, lighter one."

Please excuse my ignorance IF I am wrong, but

Because longer kayaks (Sea Kayaks) are of the displacement hull type, and the shorter ones (white water) are of a planning design of sorts I believe is why your statement is true? But I don't see that applying to your question about planning on a RIB?

T34...I think your on to something?

[jmunson2]

This is what I understand about planing craft relevant to this discussion. I could be wrong as I haven't made a full study, but rather a cursory one.

First, planing is a function of hull shape. Flatter hulls plane better than round ones. So, the flatter the hull can be, the better the planing result.

Second, smaller boats will plane quicker than larger boats, if powered appropriately.

Displacement vs. power required plays a large role in getting a boat to plane. I don't remember exactly (and don't have the book in front of me), but I believe there is a minimum kph to achieve planing, something in the teens.

Therefore, you have to calculate the power required to move the craft to at least that speed to achieve plane.

So, if you use the same motor on a larger craft as you do on a smaller, the smaller will achieve plane sooner as it is, well, smaller - less to move = quicker plane time (provided the two boats have the same hull shape).

Hope I'm right, and hope this helps.

Sincerely,

/s/ Jon C. Munson II

[Maine Sail]

I have experience and actual results, of the comparison you are looking for and the posts above are certainly incorrect when discussing inflatables! Without question the longer boat will plane easier. Been there done that.

We had a 10 foot AB Rib Light series and when we went to stern mounted davits sold the ten footer and replaced it with the IIDENTICAL RIB only 2 feet shorter so it would not over hang the hull of the boat on the port and starboard. Our ten footer would plane two people with ease with a Yamaha 8hp two stroke motor. With the same exact motor (we kept that) and the same model RIB just two feet shorter we could not get on plane at all! No plane nada, none, no can do. Longer is better think surface area, think barefoot water skiing vs. using two big skis..... It's easier to pop out with the long boards on your feet than nothing or even a slalom ski despite the fact that no skis weighs considerably less than two skis.

The analogy to kayaks above is correct, that longer is faster, but we're not talking about displacement hulls. A planing hull is different than a displacement hull. A planing hull needs the right mix of hull surface area and HP to get up on a plane and a longer hull will be able to maintain a plane at a lower speed than a shorter hull due to surface area.

Take for instance the Correct Craft ski boats my grand parents owned on the lake. For years they had a 14 foot Ski Tique with an inboard V8 motor. That boat was great and could yank your shoulders out of their sockets but would not stay on plane below 16 -17 mph despite adding trim tabs and changing props. My grandfater sold the Tique and they bought a Ski Nautique at 16 feet long with the same motor but no trim tabs. The longer Ski Nautique could maintain plane speed at 12-13 mph and was better suited to towing the grandchildren around on tubes and such because of the lower plane speed and smaller wake.

If anyone wants to trade me an AB Ventus RIB Light ten footer for my eight footer I'd be more than willing. We no longer have the boat with the davits and getting my eight footer on plane with two people requires a 10hp minimum. Anyone want a short RIB??

[T34C]

There is one thing being left in this comparison, Buoyancy. You are talking about an rib, not a traditional displacement v. planing hull. I would guess the example given by halekai- had more to do with the buoyancy/weight than the hull length. You have to include the weight of the passengers along with the weight of the boat and the buoyancy of each craft not just the hull shape.

[jmunson2]

Thank you T34C, I knew there was something I was missing...