The Metcalf P Class Re-Build

[Kazzer]

This might be a question for Kazzer?

I see you sell the same size P Class hull that I am resurrecting (and as we all know, necromancy is a sin and I am sure I will pay for this).

I did pick this up used and in need of a complete rebuild. I have no instructions and I was wondering if you could give me a "quesstimate" of how much lead weight I should start with in the hull.

I have the 3.5” Sub driver that Mr. Merriman made me last year.

No idea, sorry. I haven't even thought of doing that boat. Don't we have some documentation on doing this - somewhere?

[Subculture]

Now I don't know if your 3.5" SD has a variable ballast tank or not (some of the eariler ones had a screw jack didn't they?). The standard way of going about this sort of thing as outlined by Skip Asay is thus-

Build your boat, then place it in a test tank.

Hopefully it's significantly buoyant.

Ballast your boat down with lead until it is as close as you can get to neutrally buoyant.

Now add foam blocks (make sure these are fairly accurate and uniform) below the waterline until your boat reaches the required waterline. You may need to make a cradle to hold the foam, or use some sort of lash-up of string and zip ties etc.

Once that's achieved remove the foam, measure it, and calculate the volume of the blocks. That's the size of ballast tank you need.

Having got that, any additional weight needed to get the boat down to the require waterline is placed in the keel of the boat, as low as you can get it.

If you don't have much reserve buoyancy, you can add foam either side of the SD. This should be placed as high as you can get it, but below the waterline.

Note that although the boats may come from the same moulds, there can be significant differences in displacement from boat to boat. Some are laid up thicker or thinner, so it's difficult to say use 'x' amount of lead. Injection moulded models have a higher tolerence in this respect, so it's a lot easier to predict the weight needed.

[Kazzer]

Ballast your boat down with lead until it is as close as you can get to neutrally buoyant.

Can you just clarify exactly what this means please. I imagine it is that the boat is floating, but all parts of it are underwater, just below the surface?

[Subculture]

Yep you got it. In reality neutral buoyancy is very tricky to achieve, and your boat is always a bit positive or a bit negative, but the hydrovanes can usually take care of a few grams either way.

[jeep4grog]

I have an older Metcalf glass hull that I had to reglass a bit.... so who knows the actual weight. I know the procedure but I was hoping that there might be a starting weight to add to the model in the instructions. Since I dont have the instructions for this one (even after several emails and two letters to Metcalf), I hoped that this info might exist here since Caswell sells a very similar hull.
I see ads here for lead weight sold for several other models and was hoping that a build had been completed and that info was avaiable.
I don't mind trial and error a bit, but maybe a rough starting point would keep the wife from getting too excited if I could reduce the ammount of bath tub testing time! It is about 14 deg (F) here today and everybody has thier pools emptied for the season...
Turns out the wife and some friends are going shopping Sunday so I guess that is the day to test.

[Kazzer]

Metcalfe got out of the business a while ago and doesn't seem to answer emails. Now the Darnell molds belong to Andy Grigg, and his workmanship is far superior. These are the models we sell.

Unfortunately, there are no instructions for these boats other than one set I have of the S Class, originally drawn up by Darnell. The drawings don't have any mention of the amount of lead, so it's suck it and see. However, threads like this will be a great asset in the future, so please post your findings.

[jeep4grog]

I shall do so...

[Subculture]

I don't think Dave Metcalf has any interest in model submarines, it was just an extension to his moulding business. He is a member of a club I used to belong to NLSME, and is heavily involved with O-gauge/Garden railways.

I few years ago I organised a sub day at the NLSME pond at Colney Heath, and although he was a member, I never saw him bring anything along.

Simply put, there is no rule of thumb for this kind of thing, you have to use empirical methods to figure it out.