1:72 Los Angeles

[spankey]

So I did what you suggested, and I've mounted bulkheads (tacked them down) for the WTC. See photos. The center of the ballast tank will be 1" fwd of the geometric center of the boat.
Next thing to work on is getting that CG slightly forward of the center as well, then trimming.

[spankey]

Next Update,

Just soldered up a prop linkage from the shaft on the hull to our Most Esteemed Leader's WTC. As you can see, the WTC is presently positioned aft so that the Center of the Ballast tank is ~1 inch forward of the center of the boat (lengthwise). Thanks Admiral Greenman.

With no battery installed, the CG is somewhere near the aft 1/3 of the boat.

I'm planning on a 6 Volt, 14 AH Werker Gel cell, unless someone wishes to correct the abhorrent error of my ways. This cell weighs ~6.1 lbs, which should drive the dry CG into the forward 1/3 of the boat.

Is there a rule of thumb for where the CG should be in relation to the Center of the Ballast tank, which is positioned as stated above?

[greenman407]

Spankey, 6.1 lbs seems like a lot. Looking at what youve got there, youve got plenty of room for a battery WTC up front. Inside you could put a LIPO or a stick NIMH. Im sure that you could make the 6 pounder work.............
As far as center of gravity in relation to center of ballast tank Id start with 1" forward of that and just see how she handles. On my Albacore I had to play around with that, but thats only because I wanted to go fast.

[greenman407]

Im thinking that if you put that much weight in there that you would have to put a heck of a lot of foam in there as well to help your ballast tank float that much weight.

[He Who Shall Not Be Named]

4Ahr's is plenty, and like Mark said, too much battery!

M

[Slats]

I concur with advice above - big problem I have learnt to avoid from the masters in this game is keeping weight down.

Too much weight =Too much mass momentum

Mass momentum is basically the ability of an object to continue under its own momentum on a straight track. The higher the weight - the greater the mass momentum that needs to be overcome to turn the boat. In short - heavier boats don't turn as well as lighter ones.

My first subs used heavy wet area SLAs - yes your sub will work this way, but a better method in my experience is adopting a dry space for a Nimh or Lipo. You could see if you could buy a simple cylinder section from Caswell's to achieve this to work with that old Subdriver you have there.

A conversion I did is found here: http://forum.sub-driver.com/showthre...=Collins+class


Apart from the higher weight issue that a SLA means, you are also fighting with the COG.
COG should be achieved dead centre under the ballast tank. Here the SLA weight forward of that location makes it a hard task to achieve.

Finally I have had numerous sealing methods of external wires over the years. All work fine but longevity of these connections becomes a problematic issue.

For all these reasons I successfully convinced David to manufacture the 3.5" SD with a conduit through the ballast tank connecting a forward and aft dry space. This keeps battery weight down through use of LIpo / Nimh battery, connections are always dry. As for battery capacity I only use a 3AH nimh 7.2v I find the run time is excellent (about an hour).

Hope that helps

John

[He Who Shall Not Be Named]

And I'm most grateful John finally convinced me to provide a forward dry space in the re-designed 3.5" SD's, in which are now housed the battery -- a much more user friendly and rational design than my initial one which put the battery in the wet.

John Slater is my most reliable and informative Beta-tester! Much of the current crop of SD's feature elements suggested or re-designed by guys like John. The products I produce are very much a collaborative activity between me, Mike, and our experienced customers. The soon to be released line of 3" SD's is an example of such cross pollination.

M

[spankey]

So, I went back to the drawing board with the battery...I dug up an old 12V, 2.5 AH battery I had lying around. It weights 2.5#, so more reasonable than 6 #.

I would prefer to get everything dry in a forward compartment, BUT I've been doing this boat for over 2 years now, and Its the only obstacle in my way of getting another boat.... the 1:72 Skipjack, so I'm going to wait on fixing up a dry compartment until I get that Skipjack.

I did some trimming yesterday in the kiddie pool, and discovered some leaks...

One of them, probably the most concerning was a slow leak from the pushrod seal on the Ballast tank bulkhead. I put some silicone on that pushrod, and we'll see how that works.

The other two leaks were embarrassing...there are four (4) pushrod exits on the 3.5" WTC, and I'm only using two of them. Last time I checked, 4 minus 2 equals 2, which leaves two pushrod exits still open. Yup, there was water in the enginerooom.

I'll deservedly get my pee pee schwacked by the boss for doing that, but I guess live and learn. No real damage done.

Below are pics of the battery location, and the pushrod exits, NOW PLUGGED!

[spankey]

Thank you for saving me from 6 lbs of a battery, I would have needed a lighter than air substitute to trim the boat!

Right now, the CG is within +/- 0.5" of the center of the ballast tank, which is 1" forward of the geometric center of the boat.

And, the boat is wet.....

[Woogey]

Good day sir, I’m throwing this question out all over the place hoping I get an answer sooner than later. What propeller are you using for your LA? Do you have a link to the purchase site by chance?

-Preston

[spankey]

I bought it from Prop Shop UK. The model number is....MS/28/7/RH/BR. https://www.prop-shop.co.uk/

I ordered a prop with a 20mm hub diameter, but I believe if you email them, they also have one in a 15mm hub diameter.

Hope this helps.

[SSBN659]

When we moved to Arizona I gave up my Greg Sharpe LA plans along with several others. Now I'm sorry I did that. The LA plans I found online are suspect. Can someone tell me the following dimensions?
a. Length of bow to fwd parallel mid-body hull
b. Length of parallel mid-body hull
c.Length of after end mid-body to fwd end of prop.

This will help as I start another model using 3-1/2 inch PVC pipe as my parallel hull.
.

SSBN659
Will Rogers

[Das Boot]

I love it when a 9 year old build comes back to life.

[trout]

When we moved to Arizona I gave up my Greg Sharpe LA plans along with several others. Now I'm sorry I did that. The LA plans I found online are suspect. Can someone tell me the following dimensions?
a. Length of bow to fwd parallel mid-body hull
b. Length of parallel mid-body hull
c.Length of after end mid-body to fwd end of prop.

This will help as I start another model using 3-1/2 inch PVC pipe as my parallel hull.
.

SSBN659
Will Rogers

Will,
Did I get those plans from you? If so, I will dig them up and send them to you.
peace,
tom

[SSBN659]

Tom,
I don't think I sent the plans to you but if you are able to loan me yours I will return them after I get the information I need.
Thanks
SSBN659
Will Rogers