# transom edge sharp or not

Discussion in 'Hydrodynamics and Aerodynamics' started by fallguy, Jul 9, 2018.

Tags:
1. Joined: Dec 2016
Posts: 7,683
Likes: 1,706, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

Does the transom edge need to be sharp on my boat?

I calculate a Froude number of about 0.9, based on a 9m/s velocity and a 10 meter long hull. If the calculation is bad, forgive me, I can redo it.

Someone told me the boat would go 2-3 mph faster with a sharp edge.

2. Joined: Nov 2008
Posts: 483
Likes: 130, Points: 43, Legacy Rep: 288
Location: 59 45 51 N 019 02 15 E

### HJSMember

This is almost full planing speed, so the transom must be as sharp as possible.
JS

Doug Lord and fallguy like this.
3. Joined: Oct 2010
Posts: 10,386
Likes: 1,045, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

"Sharp" needs to be defined, but losing 2-3 mph would require some kind of crazy radius.

4. Joined: Dec 2016
Posts: 7,683
Likes: 1,706, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

How's this? Sorry bout the rotation.

5. Joined: Oct 2010
Posts: 10,386
Likes: 1,045, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

Nothing to worry about, I'd say.

fallguy likes this.
6. Joined: Jan 2006
Posts: 3,371
Likes: 514, Points: 113, Legacy Rep: 1279
Location: Lakeland Fl USA

Mr E. is right.

That radius will not cost you 2 to 3 MPH. If you make it into a sharp edge the corner will become vulnerable to chipping and breaking which may cost more speed that you might have gained

7. Joined: May 2009
Posts: 436
Likes: 35, Points: 38, Legacy Rep: 138
Location: Texas & Austria

### sandhammaren05Senior Member

The sharp edge helps the boat to develop lift faster, to plane faster, and to avoid the transom running deeper in the water
at high speeds (avoiding increased drag). A rounded trailing edge goes against all that. Race boats never have a rounded
trailing edge unless someone got sloppy. The correct Froude nr. is the depth F-nr. at the transom.

VinTin likes this.
8. Joined: Dec 2016
Posts: 7,683
Likes: 1,706, Points: 123, Legacy Rep: 10
Location: usa

### fallguySenior Member

To all. I did in fact run a bead of cabosil and milled glass there. Both will be finished sharp, but paintable sharp.

Thanks.

9. Joined: May 2009
Posts: 436
Likes: 35, Points: 38, Legacy Rep: 138
Location: Texas & Austria

### sandhammaren05Senior Member

True, but some chipping is better than a rounded edge for performance. All five of my classic runabouts have sharp trailing edges.

DogCavalry and VinTin like this.
10. Joined: Jan 2024
Posts: 2
Likes: 1, Points: 3
Location: Portland, ME

### Michael LambertNew Member

Hi everyone, I have a similar question for a different situation: wing and surf foil boards. Early foils were slower and lower aspect, so boards were designed with sort of displacement tails with cutaways to allow for the rider to pitch the board up and out of the water. Now foils are faster, higher aspect, and less tolerant of higher AOA, so full planning speed at takeoff. But many boards still have rounded tails, and when you touch down I think that makes them stick more than them need to. This is important because in many situations, like light wind winging and downwind SUP foiling, there is so little energy that a little friction fora moment can take you down. So my question is: how much of a difference would sharper release edges make in that situation? I’ll try to post some examples.

fallguy likes this.
11. Joined: Jan 2024
Posts: 2
Likes: 1, Points: 3
Location: Portland, ME

### Michael LambertNew Member

12. Joined: Oct 2008
Posts: 7,807
Likes: 1,703, Points: 113, Legacy Rep: 2488
Location: Japan

As sandhammer notes, sharp is always best.
How much, ... simple trial and error will tell you the absolute amount - ie test with radius and after made sharp.... as too many variables to state categorically.

DogCavalry, BlueBell and jehardiman like this.

13. Joined: Jun 2009
Posts: 1,671
Likes: 678, Points: 113, Legacy Rep: 1165
Location: Sweden

### baeckmoHydrodynamics

In a hydrodynamic sense, a general definition would take the radius in relation to the actual boundary layer thickness.

DogCavalry and Howlandwoodworks like 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.