Solve debate, displacement or semi?

[NorthernMac]

Thoughts

Is this a displacement or semi displacement hull??

[Westcliffe01]

Semi/planing hull should have straight lines (no rocker) over at least the rear 1/2 of the hull with a squared off transom which is partially submerged when stationary.

[Orion Jim]

Semi-displacement.....

[Chotu]

From pics, appears to have rocker so displacement.

[rslifkin]

You can exceed hull speed with some amount of rocker. So having a little bit of rocker doesn't rule out semi displacement. If it's a lot of rocker then it would be displacement.

[SVHarmonie]

How many angels can dance on that pin?

How exactly do you define “semi-displacement”, and who cares anyway?

[otterhound]

Call Peter Legnos at LBI in Groton, CT. It's his design and he built them as well. BTW cool boat!
Rod

[Westcliffe01]

A semi displacement hull from a distance looks like a planing hull. But on close examination the hull will show some rocker and max speed should be limited to 16kt more or less. Will still have an immersed transom and some dead rise all the way to the stern. Whereas a planing hull should show no rocker at all and completely straight sections all the way to the transom.

[StuM]

Displacement - the aft section if far from flat and wide and there' no way that thing will be able to get above displacement speed.

[Jim Cate]

Originally Posted by StuM

Displacement - the aft section if far from flat and wide and there' no way that thing will be able to get above displacement speed.

^^^^^^^^^^^^^^^

Agreed!

Jim

[Wotname]

Originally Posted by StuM

Displacement - the aft section if far from flat and wide and there' no way that thing will be able to get above displacement speed.

Originally Posted by Jim Cate

^^^^^^^^^^^^^^^

Agreed!

Jim

I'm late to the party but I gotta agree with Stu and Jim .

[cal40john]

One definition of semi-displacement -

https://www.oceansportstuition.co.uk...0stern%20waves.

Semi displacement

Semi displacement hulls are normally a cross between a planning and displacement hull, they will generate an amount of lift but the vessels weight will be supported by buoyancy. A semi displacement hull creates a big whole in the water which generate large bow and stern waves. Semi displacement hulls will typically have a similar hull shape to displacement hull but will flatten off towards the stern.

Others have pointed out the aft body shape is not the right shape.

Look at the aft hull shape on a planing sailboat for comparison -
https://www.jboats.com/j70-one-design-sailboat

Getting a displacement hull above hull speed by surfing down a wave does not make it a semi displacement hull, IMO.

[StuM]

Has anyone ever seen a semi-displacement monohull cruising sailboat?
I don't know of any.

[Chotu]

Originally Posted by StuM

Has anyone ever seen a semi-displacement monohull cruising sailboat?
I don't know of any.

Yes! That little MacGreggor! This is a semi-displacement sailboat. The op’s is not. Notice it has no rocker toward the stern.

https://m.youtube.com/watch?v=K1VunIRO_s4

In general, not speaking to Stu: A semi displacement boat in general has a v shaped forward section, then the rocker flattens out like a power boat aft.

The boat pictured in the first pic basically isn’t flat aft.

[rslifkin]

Keep in mind, there are plenty of boats, like the older Grand Banks hull that people refer to as "semi displacement" that are really just a mediocre planing hull with not enough power. A good SD hull will get at least somewhat above hull speed, but it won't have the huge hump and then climb onto plane like a planing hull. Resistance will rise more linearly with speed.