Boat design question, checking my math...

[Brian F.]

I'm making some sketches for a new small duck hunting craft... I've been getting my inspiration from kayaks, pirogues, pole boats, aquapods, etc. I'm wanting to build it with stitch-n-glue construction.
For me part of the design process will include building small models of the hull.

So here is my question; If I build a plastic model hull at 1/4 scale, can I float test it with weights and use 1/4 of the weight to get a true representation of capacity? ie. 10lbs = 40lbs (1/4 scale)
That seems odd though, as the water is at full scale. It would seem that I would need to use a liquid that is four times more dense... or, am I really over thinking this?

Any fluid engineers or boat designers out there that want to chime in on this?

Thanks in advance for any reply.

Best,

Brian F.

 

[Andy Grant]

My guess is you should use weight 1/64th. diplacement is based on volume. H x W x L = V.

Full size: 1 x 1 x 1 = 1

1/4 scale: 1/4 x 1/4 x 1/4 = 1/64

It has been a while since I did any real math though.

 

[Justin K]

My guess is you should use weight 1/64th. diplacement is based on volume. H x W x L = V.

Full size: 1 x 1 x 1 = 1

1/4 scale: 1/4 x 1/4 x 1/4 = 1/64

It has been a while since I did any real math though.

1/4 scale sounds kinda big... i think he's right w/ the 1/64th.

Besides..w/ a pup, baby and the rest of the clan..you don't have time to build a full size boat anyway


ps - house went up for sale on monday... if it ever sells we're moving down to "the rock"...

 

[Wispete]

There is boat design program that you can down load. I believe its called Free Ship. It is also a free progam. It's for the type of question your asking. I think your over thinking it.

 

[Brian F.]

My guess is you should use weight 1/64th. diplacement is based on volume. H x W x L = V.

Full size: 1 x 1 x 1 = 1

1/4 scale: 1/4 x 1/4 x 1/4 = 1/64

It has been a while since I did any real math though.

I think you are right on the money with this Andrew. And I'll use this formula when I get around to the fun task of sinking the model.
Thanks.

There is boat design program that you can down load. I believe its called Free Ship. It is also a free progam. It's for the type of question your asking. I think your over thinking it.

over thinking the water density part but totally missed on the volume equation...
Thanks for the heads up on the design program. I'll have to take a look at that.

Best,

Brian F.

 

[daniejon]

Hey guys I am currently going to school for civil engineering and I have recently taken fluid mechanics class. The law of bouyancy states that the bouyant force is equal to the weight of water displaced. As long as your model displaces 1/4 of the water it should hold 1/4 of the weight. I would overdesign the boat though, in building design we call it the factor of safety.

 

[Ed Askew]

Sorry to post a reply on such an old topic, but I've been without internet access for 3 weeks having just moved. Andrew is right, it's 1/64 or the scale cubed. You've built a model that is 1/4 as long, 1/4 as wide, and 1/4 as high as the full sized boat, so 1/4x1/4x1/4= 1/64 when talking about displacement, weight, speed when testing the model, or power. I made a 1/4 scale model when designing the Peregrine, and assumptions I made from the model's performance were right on. I towed it behind my real boat and used a strain gauge to estimate the amount of horsepower it would take to plane it (22lbs/64= hp) and was pretty close I guess at 10hp.

Ed.

 

[Ed Askew]

Now reading my post I see I made a mistake. It's 1/64th the weight, displacement and power, but not the speed. I'm not sure what the scale speed is but it's closer to 1/4th. There are considerations such as hull speed, the speed of a bow wave that a boat has to push past to plane, sort of like a plane breaking the speed of sound, and that depends on the length of the boat but I don't think it's a linear relationship and don't know the formula.

Ed.

 

[Brian F.]

Thanks Ed! (I too have been away from the computer about a week....)

Displacement (load capacity) is really my #1 concern when I get the model made. I'm not too interested (at this point in time) about the hull speed of the model. (the boat is intended for use with push-pole/paddle/oars) But your point is well made and appreciated.

Thanks.

Best,
Brian F.

 

[Ed Askew]

Brian,
Why don't you post up some of these sketches of yours? Lots of us like to live vicariously through other boat designer/builders. Nothing like seeing a guy's dream come to life.

Ed.