Melinda Petrol Launch

[Ian GardnerParticipant @iangardner62867]

A friend of mine (really!) is interested in building this launch as he has long wanted to have a go at a beaver stern or drake’s tail launch. This is the only drawing we can find and it looks a bit light on displacement so he thought about enlarging it to 48”.

My question is has anyone built this boat and discovered the displacement as drawn? This will enable us to work out the new weight- long shot I know.

 

Ian

[Ian GardnerParticipant @iangardner62867]

Vic Smeed’s beaver stern launch.

[Colin BishopModerator @colinbishop34627]

Can’ t help you on the specific question Ian, but the description in the plans catalogue does say that it is not suitable for use in windy conditions as drawn so enlargement would seem to be called for.

 

If you have the plan available you can get some idea of the likely displacement by estimating the block coefficient of the underwater body. This involves working out the volume of the submerged section of the hull as a percentage or fraction of the volume of a rectangular box which would fit around it, the ‘displacement’ of the box being easy to calculate.

 

Ideally you might be able to find a value for a hull of a similar type, this one for instance: http://www.steamlaunch.com/hulls/steamlaunch.html

 

Google ‘ hull block coefficient’ or ‘launch block coefficient’ and you will find quite a lot of information which will help.

 

Colin

[Ian GardnerParticipant @iangardner62867]

Thanks Colin,

 

I do have the drawing, which is why I suggested it to my friend, and I understand something of block coefficients. I just thought if someone knew it would save some hassle. I might get some squared paper out and work out a displacement using the volumes of the underwater hull. I will have to dig out a book I have which describes this- can’t remember which one at the moment!

I will certainly have a scrounge round the internet to hone up my knowledge.

One irony; this particular friend of mine always laughs at me because one of the first questions I ask about any new boat is ‘How much does it weigh?’

 

All the best, 

Ian

[Colin BishopModerator @colinbishop34627]

Ian,

 I didn’t mean to tell you how to suck eggs! Block coefficients can be unfamiliar to many modellers these days as there is rarely a need to calculate them.

 The launch in the link I gave above has a block coefficient of 0.2879 which seems surprisingly fine but I bet the hull is pretty similar to Melinda.

 The other approach is to find something around the same size for which a model displacement is given in the catalogue. I have listed three below which would suggest that Melinda as drawn would have a displacement of around 4lb as it is shallow draught.

Swift
http://www.myhobbystore.co.uk/search.asp?k=MM1421

 

Victoria
http://www.myhobbystore.co.uk/search.asp?k=MM1368

 

Thames Penny Steamer
http://www.myhobbystore.co.uk/product/16803/thames-penny-steamer-mm1143

Not knowing how much it’s going to weigh has been the death knell of many a project!

Colin

[Ian GardnerParticipant @iangardner62867]

Thanks again Colin, for going to such trouble. I agree that it’s important to have good idea of the weight of a model before construction. I’m always a bit taken aback when I ask the displacement of a model at the waterside and the modeller doesn’t know, but then I’m probably a bit odd in that respect.

Since my last post I have dug out a couple of books on boat design, Chapelle and Teal, and have just about got a handle on working out the undewater sections and converting it to a weight using the Simpsons Multiplier. I will do this for Melinda and if it comes anywhere around three or four poundsI know I’ll be on the right track. I guessed three or four myself and I reckon it would be difficult to build the boat down to that weight.

Taking it up to 48”, which is an increase of  x 1.37 would give a weight of between 7.7 and 10.28 lb assuming the 3 or 4lb to be right- so it will be interesting to see if all this works out.

I want to be able to give my friend a reasonably accurate idea of the weight so he knows what he’s in for!   It’ll still be light for the size but will have a bit more heft.

I’ll let you know how I get on just for the interest!

 

Ian

[Ian GardnerParticipant @iangardner62867]

Well Colin, I reckon that your block coefficient (Elliott Bay launch-0.2879) was pretty accurate!  When applied to Melinda it gave a weight of just over 2lb which is horribly light. I then used Vic Smeed’s method ( Radio Control Model Boats) to calculate the underwater volume of the hull, measuring the area of each section using graph paper and multiplying the average of each end of a section by the length of the section. I did it in metric and got an underwater volume of 942cc; 942g -about 2lb 1oz. Taking this up to the 48” size still only gives about 5lb and it would have to be enlarged to 1:8- 60” to get a weight of 10lb. This is all assuming I have done my sums correctly but it seems about right.

It was an interesting exercise and I secretly hope my friend will build the boat so that I can check whether my weight calculations have been correct!  I haven’t told him the news yet.

Thanks again for the help and suggestions.

 

Ian

Edited By Ian Gardner on 27/06/2010 20:15:57

[Colin BishopModerator @colinbishop34627]

That’s interesting Ian, I wouldn’t have thought it would be that low, must be the combination of fine lines and shallow draught. Just shows the value of doing your homework I guess.

 

There is one other possible check. If you can find a full size launch of approximately the same size and shape you may find that it’s tonnage is given in which case you can scale that down to the length of Melinda and see what it comes out at.

 

Colin

Edited By Colin Bishop, Website Editor on 27/06/2010 20:53:10

[Ian GardnerParticipant @iangardner62867]

Yes, the maximum draught is 1” and it feathers away to nothing approaching the stern- shame really as it’s a georgeous boat- but I think the light displacement will put my pal off.

I shall certainly keep looking out for something else though.

 

Ian

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