Planning Hull at Displacment Speeds

[Bob on This]

Planning hull at displacement speeds

Can any of you Marine Architects explain what, if any, differences between a planning hull traveling at hull speed and a displacement hull at the same speed.
I now the different bow will affect the dreaded vee wonder, but what else is going on?
Can a planning hull be made to operate as efficiently, and have the same manners as a displacement hull.
What about semi displacement?
If a boat has a somewhat flat stern section (shallow vee) and a small keel, is this considered a semi displacement?
Sorry for all the questions.
Thanks for any info.
Bob

[tom28571]

Bob,

Your question is really many questions. The best answers will come from reading one or more books concerning boats and their function. A good and informative read is "Nature of Boats' by Dave Gerr. Another just as good but perhaps less entertaining is "Understanding Boat Design" by Ted Brewer. There are others but these will fully answer all your questions.

Planing boats will indeed run at "hull speed" but will be less efficient in doing so than a boat designed for that purpose. The factors that make the planing boat best at high speed also give it more resistance to motion at low speed than the displacement boat. Likewise, the hull shape that is best at hull speed make this boat impossible rise up on top of the water and plane. Semi planing or semi displacement (same thing) are in the middle. They adopt enough characteristics of the planing hull to allow speed greater than hull speed while retaining some of the characteristics of the displacement hull.

A displacement hull will generally have a deeper body, significant rocker, a narrowing transom out of the water and higher weight. A planing hull will generally have a flatter bottom with no, or very little, rocker, shallow draft, a broader transom that is under water and lighter weight. The semi planing hull will retain weight between the other two, moderate rocker and draft and a transom usually even with the waterline.

I'm not offering any more details here because the books listed will do a much better job than anyone can in the limited scope of this or any forum.

[Knut Sand]

Here's the Norwegian commersial (taaadaa...!):

http://www.marex.no/sider/brochure/21brochure.pdf

Actually it has much of the shape as a "snekke" whitch is a deplacement hull, but an American designer (cough...) did a twist, so much of the submerged hull is pretty similar as a planing hull, but; at slow speed, it's still easy to operate, none of the (sideways) sliding some planing hulls have, and close to zero emission at deplasement speeds....

And it can go pretty fast for a boat with this look.

[Bob on This]

Thanks guys,
I did order the book, should be here next week.
I just want to find a 28' Flybridge that gets better than one mile per gallon!
I'd need to sell the house in order to buy fuel so we can do "the loop". (some day)
Don't really need to go fast, If the wife or kids want to wake board, I'll get a jetflee .......... :-)

90% of all 28 footers I find have twin V8, and I can't even see how you could change the spark plugs moreless adjust a packing nut!

Thanks
Bob

[tom28571]

Bob,

If you don't want to sound too much like a landlubber, don't ever say "knots per gallon" or "knots per hour". A knot is defined as one nautical mile per hour so it's "nautical miles per gallon" etc. One knot = 1.15 statute miles per hour. The term comes from the method sailors used to measure their speed before instrumentation was available. Look it up in the book or on google.

[Bob on This]

Opps :rolleyes:

[eponodyne]

Planing. One 'n.' The hull isn't making lists about what it should be doing later.

[Fanie]

Bob,

Consider the waters you are going to move on.

A displacement hull is slower than a planing hull, but you need less power to achieve some speed and is usually better in rougher water. For sea I'll consider this one from a fuel economy point of view.

Planing hulls are fuel expensive and very slow when not planing. In rough water when you cannot plane safely the trip is going to cost you and is going to be very slow. If the water is mostly flat you could get there quicker, but I doubt cheaper. Planing hulls require relative large engines to get on the plane, and they are always heavier on juice.

[Joakim]

I just want to find a 28' Flybridge that gets better than one mile per gallon!

If you want to save fuel, you need to have diesel engine (or a very small gasoline engine). A 28' Flybridge would easily run far less than 1 l/nm (about 4 nm/gallon), with any diesel engine. To give you an example a Bavaria 38 HT (planning hull) with 2*310 HP Volvo Penta has the following measured consumption:

Speed (kn) consumption (l/nm)
4.6 0.6
5.7 0.7
6.9 1.1
8.0 1.5
8.7 2.0
9.5 2.5
10.9 3.0
13.5 3.5
24.6 2.8
36.0 3.6

And a 25' planning hull with a 260 HP Yanmar
Speed (kn) consumption (l/nm)
3.4 0.32
5.2 0.46
5.7 0.63
6.5 0.83
7.1 1.07
7.7 1.44
8.0 1.9
8.3-11.9 2.5
22-30 1.3-1.4
34 1.6

If you just have the patience to drive clearly under hull speed, even a planning hull with big engine(s) can be quite economical. It has to be a diesel, since big gasoline engines are very inefficient under small partial load.

For comparison I have a 10 HP Yanmar on a sailboat, which is a bit bigger and heavier than this 25". It runs 5.5 kn at 0.2 l/nm and 6.4 kn at 0.35 l/nm. Thus a good displacement hull with a small diesel is clearly more economical. Most displacement motorboats are far less economical and propably not much better than the example planning hulls.

Joakim

[Bob on This]

Now if I can just get a couple of diesels, life would be good.
Or maybe one Caterpillar 3056, 205 BHP at 2500 RPM. but at 1312 Lbs, kinda heavy?:-0
What do you think the two Crusader 305s together are, 800 Lbs?

[PortTacker]

Now if I can just get a couple of diesels, life would be good.
Or maybe one Caterpillar 3056, 205 BHP at 2500 RPM. but at 1312 Lbs, kinda heavy?:-0
What do you think the two Crusader 305s together are, 800 Lbs?

Probably closer to 550 lbs each, depending on accessories.

[FAST FRED]

"I'd need to sell the house in order to buy fuel so we can do "the loop". (some day)"

The longest portions of the loop are done at displacement speeds. esp in th Trent Severn and other portions in Canadian canals or really twisty courses in the North Channel.

Our boat can see 12k at 10gph , but the loop was run at 8K and >3gph, speed is expensive , and the trip is a vacation , not a delivery.

"Now if I can just get a couple of diesels, life would be good."

A pair of Kubotas or Yannmar reefer take outs at about 20hp each, $1000 or so for the pair from the truck reefer folks will cut the fuel bill, and provide displacement speeds easily.

FF

[Fanie]

What about a couple of outboards ? They seem to be more available and if you can get some 4 strokes... free up some hull space too.