+- +-

Upcoming Events

« » May 2019
Sunday Monday Tuesday Wednesday Thursday Friday Saturday
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31

User

Welcome, Guest.
Please login or register.
 
 
 
Forgot your password?

Stats

Members
Total Members: 39
Latest: Alan Mark
New This Month: 1
New This Week: 0
New Today: 0
Stats
Total Posts: 725
Total Topics: 278
Most Online Today: 1
Most Online Ever: 44
(June 21, 2017, 06:46:53 AM)
Users Online
Members: 0
Guests: 1
Total: 1

Author Topic: John Cobbs Crusader jet boat - 48 inch long  (Read 450 times)

Ernie Lazenby

  • Guest
John Cobbs Crusader jet boat - 48 inch long
« on: April 21, 2019, 05:14:42 PM »
I have started this thread for the information of those who would like to get into scratch building a model boat and maybe are too embarrassed to ask.
I will post regular updates explaining what's being done and how.
I will add a picture later today.
I acquired the part started boat via a friend. It was made many years ago and I got it pretty much as you see it.
First problem- Its far too light for the size and quite femur. Using Z poxy finishing epoxy thinned out with meths I have sealed the outer hull basically to hold it together while work is done on all the faults particularly around the air intakes. To fill holes and create the right form I used P38 filler from Halfords (Upol light is another good filler) It sand easily and can be added too until the right shape is formed. Z poxy also sands easily and can be used with glass cloth or carbon fibre cloth to skin the outer shell..


At this stage the method of powering the model must be considered. Quite easy using a brushless motor set up however I have been looking at powering with a miniature gas turbine engine of the type used in model aeroplanes. Model aeroplanes are quite light weight however the effort to get them up and away is considerably less than that that needed to get a planning craft out of the water and onto the plane.

20 lbs of thrust needs to be well secured in the hull and I reservations as to the suitability of this hull to handle the forces involved trying to get the mass moving. One does not want to see the jet engine shooting through the front of the boat. I have obtained a lot of Carbon fibre cloth and carbon fibre tape to strengthen up the hull and it may be okay we shall see.  The sponsons have been made very light weight and are weak. Much work is needed to make them suitable for high speed runs and the pounding they can take.
The picture with the little booklet shows the model as I got it.
« Last Edit: April 21, 2019, 05:21:21 PM by Ernie Lazenby »

Ernie Lazenby

  • Guest
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #1 on: April 22, 2019, 08:26:48 AM »
The construction of the model is typical 'egg box'  stringers attached to frames then balsa wood planks to finish off. The base is one piece of lite plywood. Lite ply is basically a thin layer of balsa wood between two layers of thin plywood. Cuts well and keeps weight down. Egg box construction can result in a strong framework if done correctly. The original builder used balsa wood planks because its easily sanded to the correct shape. The downside is the more material that is sanded off to get the right shape the weaker the wood becomes and needs strengthening up with glass or carbon fibre cloth.

As with the full size boat, the area that take the pounding will need extra support; the front planning wedge in particular and where the sponsons join the sponson arms. In order to ensure the sponsons fit correctly and securely I have needed to remove wood from the bottom of each to expose the inside. After the sponsons have been glued in place and supported with carbon fibre cloth each structure will be glass clothed. Expanding foam will be put into to fill all the spaces/nooks and crannies and will provide flotation.
« Last Edit: April 22, 2019, 08:45:26 AM by Ernie Lazenby »

Eddy Matthews

  • Administrator
  • Full Member
  • *****
  • Posts: 62
    • View Profile
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #2 on: April 22, 2019, 08:30:46 AM »
Looking good Ernie, keep the build pictures coming so we can follow your progress - Oh and you MUST let me know when it will have its first test on the water so I can be there!

Regards
Eddy

Ernie Lazenby

  • Guest
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #3 on: April 22, 2019, 08:36:56 AM »
will do Eddy, good to have you with me on its first outing.

Power may be one of these or similar! The latest ones are full kero start. No gas.  I need to see how the hull develops. One consideration where do I run a gas turbine these days. Herrington is out. Our club is out. Brid may be an option but may be too small. Lake district probably when the parky is not about early morning.
« Last Edit: April 22, 2019, 08:40:19 AM by Ernie Lazenby »

Eddy Matthews

  • Administrator
  • Full Member
  • *****
  • Posts: 62
    • View Profile
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #4 on: April 22, 2019, 09:44:49 AM »
A trip to the lakes sounds good to me! :)

Regards
Eddy

Ernie Lazenby

  • Guest
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #5 on: April 30, 2019, 10:45:19 AM »
Just a quick update on work.  Yesterday I took delivery of a two part polyurethane close cell foam (wont absorb water) this will be applied to the inside of the sponsons later today. It will seal everything up and help keep the sponsons in place. After mixing the two parts together one has about 20 seconds to get it into the cavaties before it starts to foam and then after 30 minutes its hard. Can be cut and sanded. The entire front of the boat and the planning wedge will be filled with it. Its very good for buoyancy.

The next step will be to plate the bottom of the sponsons with thin plywood and very thin aluminium sheet.

Ernie Lazenby

  • Guest
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #6 on: May 06, 2019, 06:46:11 PM »
One sponson filled with the two part foam. Now very rigid and water proof. The cured foam (after about 30 minutes) is easy to cut and sand. Outer skin will be fitted in the morning then the entire sponson will be glass clothed with expoxy resin.
« Last Edit: May 06, 2019, 06:49:10 PM by Ernie Lazenby »

Ernie Lazenby

  • Guest
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #7 on: May 15, 2019, 07:29:44 PM »
work on the sponsons for the most part finished. Later this week I will be glass clothing the main hull.
« Last Edit: May 15, 2019, 07:34:50 PM by Ernie Lazenby »

Eddy Matthews

  • Administrator
  • Full Member
  • *****
  • Posts: 62
    • View Profile
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #8 on: May 15, 2019, 07:36:35 PM »
Looking good Ernie....

Regards
Eddy

Ernie Lazenby

  • Guest
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #9 on: May 16, 2019, 06:25:29 PM »
Managed to get the front of the boat covered in glass cloth and Z poxy finishing epoxy. (Cloth is Delux materials Laminating FABRIC 78G/m2)The cloth strengthens up the entire structure and keeps the water out! As soon as its dry I will trim it with a scalpel.

Eddy Matthews

  • Administrator
  • Full Member
  • *****
  • Posts: 62
    • View Profile
Re: John Cobbs Crusader jet boat - 48 inch long
« Reply #10 on: May 16, 2019, 07:26:56 PM »
A really neat job of the bow Ernie.... Well done that man!

Regards
Eddy

 

Recent Topics

Competition 2019- progressive results by Ernie Lazenby
May 22, 2019, 08:18:53 PM

Competition results 2019 by Ernie Lazenby
May 22, 2019, 08:04:13 PM

New lease. by Ernie Lazenby
May 17, 2019, 03:51:33 PM

John Cobbs Crusader jet boat - 48 inch long by Eddy Matthews
May 16, 2019, 07:26:56 PM

Cancelled racing re-scheduled to Wednesday 15th May !!! by Ernie Lazenby
May 14, 2019, 05:21:54 PM

SUNDAY 12th May. by Ernie Lazenby
May 11, 2019, 10:59:55 AM

MOVED: John Cobbs Crusader jet boat - 48 inch long by Eddy Matthews
May 01, 2019, 07:13:49 PM

Scarecrow day by Geoff Fish
April 29, 2019, 02:08:33 PM

Powered by EzPortal