Osprey CofG issue

[Old School]

While awaiting arrival of some battery straps for my current Pursuit build to complete it, I decided to investigate my next possible build. This would be a Hobbyking "Osprey" with a definitely unsuitable motor, a Turnigy T20 "Proteus" running 1280kv on 6s. I am aware that the motor is a boat anchor in this hull and a 40 series would be better, however, due a change in plans with motors for a proposed 1300 mm catamaran project, I have T20's coming out my ears and need a project to take at least one of them.

The problem that I have encountered doing a little reconnaissance is that to get a satisfactory CoG at around 30% I need to place the motor further aft then I am comfortable with. Without even the additional weight of a carbon inlay I need to place the motor completely aft of the CoG point. Normally I would like the motor centred around the CoG. With the added weight of the carbon inlay then if components remain in the current location shown on the images then the CoG will move forward as there will be more weight of carbon fibre forward of the CoG than aft.

If I run two 6s batteries then rather than side by side forward of the motor, I may need to locate one battery aft to the port side of the motor. Will this unsettle the lateral balance too much, please? Running dual 6s just forward of the motor location shown does not seem to unsettle the CoG too much as to using just a single battery.

I am surprised that the hull is so nose heavy and concerned with the aft motor location mating to the drive shaft. I really need to inlay the hull to get a truer picture but just wanted to suss things out before getting in too deep.

[Old School]

I did a mockup of the shaft stuffing tube arrangement and to align with the collet, the stuffing tube will need to bend upwards immediately forward of the transom. There is a little clearance under the aft mounting point on the motor so I may be able to drop the motor another 0.125" to aid alignment. Probably getting a little ahead of myself until the inlay is done but I am just trying to pre-empt any problems.

[rol243]

Try abit more angle on the motor.

[Old School]

Try abit more angle on the motor.

Unfortunately, using the OSE Racing/RCBB type aluminium motor mounting, the forward mount hole slot each side is at its upper limit (the slot is machined almost to the top of the side rail) and at the aft end with the motor mounted using the upper mounting hole the motor clears the vee in the hull by approximately 1/8". The only adjustment possible to me is filing the aft holes downwards slightly to let the motor almost sit on the hull bottom. As stated earlier this would only allow me to drop the aft end of the motor less than 1/8".

Edit: I just rechecked the siderails and on the forward mount slot I could file approximately 2 mm upwards. Combined with modifying the rear holes this would give me more angular movement.

Edit 2: I have modified the four mount holes for the motor to give me more adjustment for alignment.

[NativePaul]

Why not swap the position of the motor and battery? That would give you a much bigger radius on the shaft bend.

[Peter A]

Old School, I have an osprey set up for endurance racing. I used a 4060 TP motor and it is mounted as far back as I could get so as to still be able to get the the collet. If I did it again I would probably move it forward an inch or so. As far as weight goes, it makes no difference. I run 3-4kg of batteries and it doesn't make any difference to the speed. The CG however is a little more critical. I have found that at 30% the boat is faster and rides free but is easily upset by waves and other wakes, so will quickly stuff in. No problem on flat water for flat out runs. The CG seems to work best in rough water at 32% where even 4" of chop does not bother it at all. I also run with standards batts for oval sprints and find that a CG of 32% is much more stable in race water. BTW I lifted the stinger height by 3mm or so also.

[Peter A]

Also, you are trying to set up the CG without any hardware on or esc in the boat. Add in the steering servo and any other bits and you will get a truer idea of the CG. I thought that I would be front heavy at first but it worked out to be quite light in the front on CG. I have to put my batts almost right back in the battery tray to get 30% CG.

20180204_152426.jpg20180330_114420.jpg

[Old School]

Many thanks Peter and Paul. In the first two images I do have the stinger and rudder assembly placed in the rear of the hull. As the ESC will most likely mount above the motor and is very light I did not think it would impact on the weight arrangement.

Edit: I roughed out motor placement forward of the batteries and this does allow a CoG less than 30% however, it does not allow any CoG adjustment by way of battery placement to compensate for conditions.

With the motor placed aft with the aft edge of the siderails aligning with the hatch cutout and modifying the motor mount holes to allow stuffing tube alignment, I believe this is the best option as it allows me to move the batteries forward if needed. My minimum CoG though would be around 30%.

Edit: I just noticed the stock boat has the batteries each side and aft of the motor. I would prefer though that the 6s batteries I intend using are lower in the hull and thus forward of the motor.

[Old School]

A little brainstorming. I bent up some brass tubing to represent how the drive cable would lay. The tubing will then be used as a template to bend up the stuffing tube later down the track. What is obvious is that the stuffing tube will be very short, approximately 65 mm in length.

The overhead image shows that the aft end of the siderails aligns with the hatch cutout.

Edit: a little further brainstorming, if I leave the motor as shown and mount the batteries either side of the motor then I move the CoG approximately from 32% with two batteries forward of the motor to 30% with a battery either side of the motor. What I may do during construction is mount two alternative battery locations (forward of motor and either side of motor) and see which position works out best. I cannot see that the weight of two battery trays extra is going to add too much to the overall weight plus it gives me the option of playing with the CoG.