DE 25

[Cowbro]

The PPI should be based on a flat plane cutting through the boat at the waterline. By increasing 10% linearly, you are increasing the area ~21%. So the area of the cut through the boat at the waterline should increase by 21% and thus the amount of water displaced would be about 21% greater or 1.21 x 600lbs = 726lbs

Also, the volume of the hull is increased 33% (1.1^3) . So for the new larger boat to sit at the same waterline as designed the weight would need to be 33% more, not necessarily 10% more.

Phil

[fallguy1000]

The PPI should be based on a flat plane cutting through the boat at the waterline. By increasing 10% linearly, you are increasing the area ~21%. So the area of the cut through the boat at the waterline should increase by 21% and thus the amount of water displaced would be about 21% greater or 1.21 x 600lbs = 726lbs

Also, the volume of the hull is increased 33% (1.1^3) . So for the new larger boat to sit at the same waterline as designed the weight would need to be 33% more, not necessarily 10% more.

Phil

Darnit. You are right on the ppi. I used my analysis which was for beam only on my boat..

As for waterline, not sure I agree with cubing the hull depth. Take a second look.

[VT_Jeff]

Also, the volume of the hull is increased 33% (1.1^3) . So for the new larger boat to sit at the same waterline as designed the weight would need to be 33% more, not necessarily 10% more.

Phil

This is a mindbender. I like it!

If you scaled up a photo of the boat that had an accurate waterline painted on it, that waterline would also scale. If it was 10" from the lowest draft originally, the scaled waterline would be 11 inches. So is the "waterline as designed" the scaled 11" waterline or the original 10" waterline?

[fallguy1000]

Also, the volume of the hull is increased 33% (1.1^3) . So for the new larger boat to sit at the same waterline as designed the weight would need to be 33% more, not necessarily 10% more.

Phil

This is a mindbender. I like it!

If you scaled up a photo of the boat that had an accurate waterline painted on it, that waterline would also scale. If it was 10" from the lowest draft originally, the scaled waterline would be 11 inches. So is the "waterline as designed" the scaled 11" waterline or the original 10" waterline?

The problem with the waterline is all things equal is impossible.

And, the waterline, in your example, does indeed move by 10%.

But the waterline is displacement and weight dependent. Consider a cube box 2'x2x2'. The displacement is 8 cuft*62.3#/cuft = 498.4#. The ppi is 498.4/24 or 20.85#/inch.

So let's say the box weight is 41.7#. The waterline is up 2".

Now expand the box 10% in all directions. The displacement is 10.648*62.3# or 663.4# and the ppi is now 663.4/26" or 25.5". Checking wsa 20.85*1.21 is 25.2".. Now, assume the box mass has increase in 3 directions; the new mass of the box is cowbros 133%. So the new box is 55.46# and 55.46/25.5 is up to 2.17". How did this happen? The new box sinks further! Why? Because the wsa increased less than the mass.

Irony or not, the waterline moved up by 10% or so..

One or more of you guys can check my math!

[VT_Jeff]

Also, the volume of the hull is increased 33% (1.1^3) . So for the new larger boat to sit at the same waterline as designed the weight would need to be 33% more, not necessarily 10% more.

Phil

This is a mindbender. I like it!

If you scaled up a photo of the boat that had an accurate waterline painted on it, that waterline would also scale. If it was 10" from the lowest draft originally, the scaled waterline would be 11 inches. So is the "waterline as designed" the scaled 11" waterline or the original 10" waterline?

The problem with the waterline is all things equal is impossible.

And, the waterline, in your example, does indeed move by 10%.

But the waterline is displacement and weight dependent. Consider a cube box 2'x2x2'. The displacement is 8 cuft*62.3#/cuft = 498.4#. The ppi is 498.4/24 or 20.85#/inch.

So let's say the box weight is 41.7#. The waterline is up 2".

Now expand the box 10% in all directions. The displacement is 10.648*62.3# or 663.4# and the ppi is now 663.4/26" or 25.5". Checking wsa 20.85*1.21 is 25.2".. Now, assume the box mass has increase in 3 directions; the new mass of the box is cowbros 133%. So the new box is 55.46# and 55.46/25.5 is up to 2.17". How did this happen? The new box sinks further! Why? Because the wsa increased less than the mass.

Irony or not, the waterline moved up by 10% or so..

One or more of you guys can check my math!

Interesting.

Should we note that when actually scaling the boat, the mass does not scale proportionally because the thicknesses are not increasing?

[fallguy1000]

I think Phil actually meant to say exactly what I said. I misread it before coffee this morning.

But the big caution on the boat is building it bigger and heavier in all ways.

[fallguy1000]

As to mass scaling proportionately; it is not safe to assume anything. The OP originally considered adding 1/2" to the hull bottom because 'it is a bigger boat'.

The continual and real challenge in lightweight, fuel efficient boats is keeping them that way. If, for example, the DE25 has a 6'2" or 74" cabin height; it would be foolish to build it 10% higher to 81.4" unless you were to modify the walls and roof to something lighter.

Bigger boats also tend to accumulate more stuff. My Skoota is one such example. I have 4 monitors instead of 1. Autopilot, refridgerator, heating system, etc. (I am really nervous I am way too heavy as well)

A good example here would be making a 10% larger fuel tank. Nothing in my calculations included stuff 'in the box'.

[dennisb]

All,

Thanks for your input.

I went back and did the math for myself. I agree that by increasing all dimensions 10%, the surface area will increase by 21%, and the volume will increase by 33.1%. All things being equal (plywood thickness) I don't think the weight will increase by 33.1%. However it will increase due to larger surface area and doubling up the fiberglass layers on the bottom.

My goal is to keep the weight down so I only have to power it with a 90 or 115 HP motor. The performance I am looking for is 30 MPH top end with a cruse of 20 MPH.

If I take the DWL weight for the 25' version, 2800 Lbs and multiply it by 1.331 I get a 3727 Lbs for a new DWL weight, the boat should have the same draft (scaled). Is this logic correct?

At this point I am assembling the bottom and side panels.

Thanks again for your input

Dennis

[rick berrey]

The DE29 builder went +8% on beam , went to 1/2" ply on bottom , added about 4' in length , more if you count the swim platform . He has posted speed , 28 if memory serves . If you scale a full 10% then you have added 10% both below and above the waterline , so the waterline should end up about the same place as per designed , but you will end up with a deeper draft because you have added 10% below the waterline .

[dennisb]

I looked at the posts for the DE29.

With a 90 HP motor he is getting 23 MPH top end after improving air flow around the motor.

One thing he says is, he does not know the weight of the boat.

Using 23 MPH, backing out the weight I get 6125 Lbs using a Crouch No. of 165. I am still unsure he is getting the maximum out of the 90 HP motor, sounds like power may still be limited due to limited air flow around the motor.

If I use the 28 MPH, I get a weight of 4133 Lbs. This makes more sense to me.

Does anyone know the performance of the DE29 with the operating in the optimum RPM range?

Thanks