# Semi displacement hull

Discussion in 'Powerboats' started by Steph357, Nov 15, 2009.

1. Joined: Oct 2009
Posts: 920
Likes: 46, Points: 0, Legacy Rep: 732
Location: NW Washington State USA

### Easy RiderSenior Member

LL,
"when the water breaks free of the transom"
I thought that for a long time. At another time I thought that planing commenced when the bow started to rise. That's WAY less than hull speed though. Can't remember what's wrong w the "breaks free of the transom" theory but at one point I decided that it was so. Tom says "It's a transition range" and it seems it cannot be pinned down to a specific point in the series of evolving events that spread out over the entire range of boat speeds. So many different shapes to act on water in different ways. Tom's right too in that there never will be a black and white ..now you are and now you aren't.

2. Joined: Jun 2007
Posts: 2,640
Likes: 125, Points: 0, Legacy Rep: 1802
Location: Brisbane

### LandlubberSenior Member

ER..yeah, it does seem that is the case, watching the transom carefully and havintg the speed read out (from the GPS), it also depends on water flow under the boat, as in current moving the boat, funny to watch it.

3. Joined: Dec 2001
Posts: 2,474
Likes: 117, Points: 63, Legacy Rep: 1728
Location: Oriental, NC

### tom28571Senior Member

Make a hole in the water and the natural tendency is for the water to either fill the hole or push up on what is making the hole. In the case of a boat moving through the surface, the water wants to fill the void left as the transom passes.

Landlubber talked about the flow of water under the boat. A better understanding takes place when we realize that the water is not actually flowing under the boat. It may be mathematically accurate, but it impedes understanding. The water is reacting to the boat passage by being forced down. If the boat is slow and/or the transom immersion is deep, much or all of the water has time to fill the void behind the transom. Make the boat go fast enough to be fully planing and the downward impulse given to the water by the hull is still taking place, the water separates at the transom and even continues its downward path after the boat has passed.

It is just Newton's laws at work. In between displacement and full planing, the picture is somewhere between these extremes. Giving a displacement or semi hull some rocker does not impart as much down impulse to the water and allows the water to make an easier path back toward the surface. That takes less energy than driving the deep immersed transom of a heavy planing boat at slow speed. Try to run a rockered hull fast and the forward part of the bottom imparts a large down impulse to the water and it just doesn't want to come back to the surface as soon as the after part of the hull demands. This creates suction or negative lift and sinks the hull further into the water. this is where the "hull speed" limitation comes in.

Flattening out some of that rocker will cause a decrease in that suction and the boat can go a bit faster. Flatten the aft buttocks into straight parallel lines and the hull looses most or all of that negative lift and becomes a fully planing boat. Of course there is a lot more to the story but this may help in the basic understanding of what is happening under the boat.

1 person likes this.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.