Scale weight for engine

[Gordon Scrim]

Hi All,

I am building a quarter size model of a Riviera from the old plans on this site : http://www.svensons.com/boat/?p=RunaboutsInboard/riviera . I am planning to modify the plans slightly with a different engine placement and would like to weight the model so to observe its attitide in the water with the engine in the new location. I also want to verify the building method to make sure everything fits.

My question is that if I scale the boat by a quarter what weight would the engine be. In the real boat the engine/transmission combination will be around 400 Kg but I don't think simply scaling this to 100Kg will work as there are more complex things to take into effect.

Is there a simple rule of thumb for weight to use when scaling plans down?

regards
Gordon

[Hwyl]

I think it's the cube root. That's a bit more than 7 KG (in my head)

[P.I. Stazzer-Newt]

HWyl's right - cube of the scale ratio for masses.

I.E. 4 to 1 needs mass of 400/64 6.25 kg.

Somewhere in the back of my head I remember an observation about the weight of the driver, and his placement, being important to the trim and balance of Rivas. - might be complete rubbish.

[Gary E]

Sounds fishy to me and here is why I think so..

Looking at that boat it looks aprox 18 ft long with a 6 cyl engine that you say is 400Kg.. I'll buy that and lets round that off to 800 lbs THat engine looks like aprox 100 HP.

Now if you say make the boat 1/4 that size and use the cube of the engine weight to determine appropriate size motor ammouts to 7 kg or again aprox 16 to 17 lbs... What sorta horsepower will that have?

If that works going smaller, it ought to work going bigger...

So... lets build that 4 times bigger longer and wider and deeper etc etc.....
that makes it a 72 footer..
now cube the engine wgt... 800 lbs cubed is....
512,000,000 lbs.. or 256,000 TONS...

Imagine the horsepower in that !!! WOW

And that's why I think the cube idea is wrong..

Now maybe a real naval expert will straighten all of us out.. but somehow I doubt it..

[ 04-11-2005, 06:03 PM: Message edited by: Gary E ]

[Hwyl]

Brunel, figured it out a long time before we did mate. He realised that bigger ships required proportionaly less driving power to their carrying power, so he built the "Huge" at it's time, SS Great Britain and went onto the even bigger Great Eastern

The original question was to do with weight and weight alone as pertains to a scale model.

Power needed has to do with waterline length and surface area and stuff like that. Surface area increases with the square, and volume increases with the cube. As I said this is a scale model: In real life boats get proportionally skinnier and less freeboard as they get longer.

[ 04-11-2005, 06:17 PM: Message edited by: Hwyl ]

[Gary E]

But I do see a lot of 16 ft wide 50 footers

[P.I. Stazzer-Newt]

You don't cube the mass in that example.

You cube the linear scale ratio and use that as the multiplier.

So for the 72 footer (4 time as long) then you multiply the mass by (4 cubed = 64) so the scale mass for the engine is 64 time 800 = 51,200.

The answer is fairly obvious if you consider the "hole in the water" needed to float the boat.
If this is four times as long and four times as wide and four times as deep, then the mass is 4 times 4 times 4 (64) of the original.

[ 04-11-2005, 06:13 PM: Message edited by: P.I. Stazzer-Newt ]

[Gary E]

ok so 50,000 lbs coud fit a pair of diesels... but please explain what that 17 lbs engine is gona do for the 1/4 length boat.. still seems fishy to me

[Hwyl]

Gary, sorry I went in and edited my post to tidy my autistic rambling. So your second posting refers to something I deleted.

He's building a scale model for design purposes. The engines not going to work a quarter scale 18 footer would be 4'6" long.

[Gary E]

Ok.. so even if the scale model is 4 or 5 ft long.. the 17 lb engine is what you would consider normal?.. Somehow I just dont see that..

[Hwyl]

Yes, it's a 17 lb weight to approximate the load of the engine. Actually a 5 horse inboard engine would weigh about that anyway, you'd have to discount the transmission and shaft, because there's stuff you can't scale down. The 64 modifier on a 100 horse would be 1.5 horses, but as I explained badly earlier, that would not drive it worth a -----. As i see it though it'll be a 17lb peice of concrete to see how the boat floats.

[Gary E]

I see where you get the math, but I still dont "see" it..

For example I am thinking back to 2 dif boats I have owned, one a 16 ft runabout the other a 33 ft sportfisherman. If we are to think one is 1/2 the other or maybe one is double the other, the displacement, horsepower, engine weights, speed are so far from that formula to make it look worthless, but hey, it's a model, and nobody gona fall overboard.

[Gordon Scrim]

Hi All,

Thank you for all your input. Yes it will be just a lump of concrete to test the weight placement and to ensure that it does'nt sink in the back end.

I was never considering actually powering the boat as I cannot get a V drive that small to allow the placement of the engine where I would like it.

If the model turns out OK I may turn it into a R/C model with some small engine centrally mounted but that really is not the aim of the exercise.

Thanks again.

Gordon

[P.I. Stazzer-Newt]

Scaling in this game is fraught.
Masses scale with the cube of the linear dimension.
Planing speed with the square root of the length.
So you never get scale performance and appearance together.
That said a 6 kilo budget for enough power to make a 35lb boat plane sounds easy.
2.2 Horse from 54 ounces. (http://www.horizonhobby.com/Shop/ByCategory/Product/Default.aspx?ProdID=ZENE231M)

or
12.5 Horse from 3 kilos (http://www.justengines.unseen.org/mv700p.htm)

[Lurch]

Originally posted by P.I. Stazzer-Newt:
Scaling in this game is fraught.
Masses scale with the cube of the linear dimension.
Planing speed with the square root of the length.
So you never get scale performance and appearance together.
That said a 6 kilo budget for enough power to make a 35lb boat plane sounds easy.Confusing me is a trivial task, and you've done it here. I thought the whole point of tank testing with scale hull models was to determine their performance before spending the money to build a full scale version? Or am I confusing apples and oranges (see "trivial task," above :confused: )?

Don

[Bruce Hooke]

Don,

Indeed tank testing is to determine how the full scale boat will peform, but these scaling issues are what make it tricky...that doesn't mean it can't be done, it does mean that it takes a bit of work to make sense of the results, or so I understand...

- Bruce

[Bruce Hooke]

Originally posted by Gary E:
I see where you get the math, but I still dont "see" it..

For example I am thinking back to 2 dif boats I have owned, one a 16 ft runabout the other a 33 ft sportfisherman. If we are to think one is 1/2 the other or maybe one is double the other, the displacement, horsepower, engine weights, speed are so far from that formula to make it look worthless, but hey, it's a model, and nobody gona fall overboard.Gary,

You are mixing up two very different issues. Trying to understand the scaling factors involved in a scale model by looking at different full size boats just serves to demonstrate why scaling up and down full size boats is not an easy thing to do and why you SHOULD NOT use these formulas if your goal is to produce a working boat rather than a scale model.

Take a look at your average working model sailboat (one designed for racing not a replica of a full-size boat) and look at how radically different the proportions are: A 3' long model boat could easily have a draft of 1' and a mast height of 6'. That's the same as a 22' boat with an 11' draft and a 44' mast -- clearly quite absurd, but very effective on a model sailboat.

The accuracy of the basic scaling formula is easy to demostrate. Consider a cube that is 10' x 10' x 10' and has a density of 1 pound per cubic foot*. It has a length of 10', one side has an area of 100 square feet, and it's weight is 1000 pounds. Now consider a cube that is 1' x 1' x 1' and the same density as the original. It has a length of 1' (so we would call the scale 10:1; one side has an area of 1 square foot, which is the square root of the original; and it's weight is 1 pound, which is the cube root of the original.

To repeat, these formulas are pretty near useless IF your goal is to produce a working boat, but they are correct if you want to make a scale model. It should maybe be noted that a WORKING scale model sometimes needs to deveat from being strictly to scale if it is to work well. This is especially true of sailboats.

*The units here do not matter as long as they are consistant.

[Killick]

Just weighing in ... no pun intended :)

To summarize what folks are saying here... the original question wasn't "how powerful an engine do I need to power this scale boat?"

It was, "how heavy should the scale motor be to make the scale boat float on its designed waterlines?"

Cheers