Ballast Requirements (Boat Weights)

[Colin BishopModerator @colinbishop34627]

Most people automatically go for direct drive setups but the truth is that in most cases gearing down is a win – win as it helps take the load off the motor and prop alike and gives a significant degree of efficiency gain in power consumption. Years ago, before brushless motors appeared, there was a rough rule of thumb for scale models that the propeller diameter should not exceed that of the motor casing when using direct drive.

My fishery cruiser Brenda uses 500 size low drain motors with simple V pulley 2:1 reduction using ‘O’ rings. The boat is fairly large at 47 inches and quite heavy but but draws only a bit over 2 amps at cruising speed.

Colin

 

 

 

[jcbryantParticipant @jcbryant]

I had thought that the motor I selected was pretty large, but when I compare it to the propeller diameter is suddenly looks rather small.  It certainly fails the rule of thumb.  The casing diameter is somewhere around half that of the propeller (which might suggest 2:1 reduction?).

The supplier you suggested offers “MFA” T5x10mm pulleys and belts and regular (no prefix) T5x10mm pulleys and belts.  What is the difference and are the two compatible (i.e. can I put a regular belt on a pair of “MFA” pulleys)?

[Colin BishopModerator @colinbishop34627]

Sorry, I’m not an expert on toothed belts, I have always used plain ones but for a boat your size I would think that you would need the extra grip.

Technobots do offer customer support so probably best to ask them direct, you obviously know the sizes of the pulleys you need.

https://www.technobotsonline.com/support-clone.html

Colin

[jcbryantParticipant @jcbryant]

It appears that MFA is just the name of a particular (slightly upscale?) manufacturer for which Technobots is having an “end of line” sale.  Their pulleys and belts are compatible with those produced by other manufacturers.

I’ve been working on finding a pulley and belt combination that keeps the shafts as close as possible while giving an appropriate reduction ratio, and have settled on 12T and 28T pulleys connected by a 185mm belt.  This will put the shafts about 1.5″ apart and give a reduction ration of 1:2.33.

Unfortunately Technobots’ minimum shipping fee for international orders is pretty prohibitive, but the pulleys and belts I need are also available on eBay.  I very much doubt I’ll be able to put things together before winter sets in, but I should be ready for next spring.

[Colin BishopModerator @colinbishop34627]

Yes, shop around, you may find a supplier with lower costs. There seem to be plenty out there.

Colin

[jcbryantParticipant @jcbryant]

Order placed, and the grand total (parts and shipping) worked out to just over Technobot’s minimum international shipping fee.

Along the way, I switched from T5 x 10mm to HTD-5M x 10mm.  The basic difference is that while the teeth on T5 belts have a trapezoidal profile, HTD-5M teeth have a more rounded profile that is evidently more suited to power transmission (HTD stands for “High Torque Drive”).  T5 and HTD-5M have the same 5mm pitch, and in both cases 10mm wide pulleys and belts are available (as well as wider ones).  The final clincher for me was that the pulley and belt sizes I wanted were easier to find in the HTD-5M format.  Hopefully somebody who really knows what they’re talking about won’t shortly tell me that I’ve just made a terrible mistake.

[jcbryantParticipant @jcbryant]

The parts I needed have arrived, and I’ve mocked up what I hope is a suitable arrangement:

The hardest part will be getting the two pillow bearings for the output shaft aligned with the propeller shaft.  Otherwise putting everything together should be a pretty easy job, and if I wasn’t going away in a week’s time, a test sail before winter sets in wouldn’t be entirely out of the question.  The holes in the brackets which hold the motor will be slotted so as to allow the belt tension to be adjusted.

[Richard SimpsonModerator @richardsimpson88330]

My apologies, I missed this.  The MFA/Como Drills range of products have been bought by Component Shop and so are still available through them, here:

https://www.componentshop.co.uk/catalogsearch/result//?q=mfa

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