Determining Deadrise

[ken52]

I've read somewhere to figure your boats deadrise to use a carpenters square with long side (at least 22") on the keel and move it until it touches the transom. Then measure from the back of the keel at the transoms bottom to the inside of the carpenters short side. Then every inch is equal to 2 degress.

Is this true?

 

[4JawChuck]

Re: Determining Deadrise

Most carpenters squares are 16"X24" but there are 18"X24" models also. Which one are you referring to?

I did some calculations for a 16" square and your "rule of thumb" does not hold true.

http://www.1728.com/trig4.htm

 

[Kevin Morin]

Re: Determining Deadrise

Ken52,
I believe you described the rake [angle] of the transom, I think deadrise is measured from centerline to chine, transversely. I'm pretty sure deadrise is the angle that the bottom rises above the horizontal.

Transom rake, usually 10-15deg in outboard boats, and perhaps plumb or vertical [90deg] in some inboards, would be measured between the waterline and transom, in plane of the keel, and the square would be aligned fore and aft.

You can use a 'Speed Square' to mark your framing square for the degrees along the short blade. Then, using a level to create the flat plane, hold the framing square's long blade at the top of the level while it's touching the bottom center and the bubble is centered.

The angle of the deadrise could read on the vertical, short leg of the framing square 2' from the center, after the larger square were marked in degrees from the long tip back upward along the short blade. You'd have to use another straight edge to align the speed block's angles from the long blade's tip to the short blade's outer edge when marking the framing square in degrees.

cheers,

 

[insanity]

Re: Determining Deadrise

To find the degree of the dead rise use a protractor with a string tied at 0 then a weight on the other end. place it up the the bottom of the boat. Your boat will have to be perfectly level to get an accurate reading. You could probably take the degree reading from both sides if they are not the same add them together divide by 2 and you have your angel.

 

[bigbad 4cyl x2]

Re: Determining Deadrise

 

[45Auto]

Re: Determining Deadrise

I knew those geometry classes would come in handy sooner or later! Easiest way to do it is to use the law cosines. Doesn't make any difference if your boat is level or not.

1) Measure up from your keel a distance along the bottom of your boat on each side as shown below and make a mark. I used 10" in the example, the larger your number the more accurate your answer will be.

2) Measure between the marks. The example shows the distance between the marks as 18.67".

3) Go to this website and put your numbers into this law of cosines calculator.

http://www.analyzemath.com/Geometry_calculators/cosine_law_calculator.html

Put the distance along the bottom to the marks in Boxes A and C, put the distance BETWEEN the marks in Box B.

4) Hit enter

5) Your deadrise will magically appear in the boxes shown circled in red.

 

[ken52]

Re: Determining Deadrise

Thanks Guys, I may have gotten confused about what I read. It very well could have been just transom angle? Lost of good stuff though.