help please - Page 3 - RCCanada - Canada Radio Controlled Hobby Forum
RCCanada - Canada's Radio Control Hobby Forum
General RC Boat Discussion Discuss RC Boats, submarines & hovercrafts in electric or nitro.

Thread Tools Display Modes
Old 11-01-2013, 04:51 PM   #21
RCC Contributor
I am: John R
Join Date: Jun 2008
Location: Kitchener Ontario
Radio of choice:

Feedback: 8 / 100%
Posts: 344
Total Props: 2
Re: help please

       Remove this ad - become a site supporter!
Originally Posted by Max View Post
OK. you win. I dont get it.

Because I dont get it, I will have to take your word for it that energy draw from any boat is subject to unquantifiable black-magic, hocus-pocus and smoke and mirrors.

I'm sure there are a lot of thermodynamic engineers that are rolling in their graves with the very thought that the laws of conservation of energy do not apply to boats. Cars..YES.. Planes..YES.. Toasters...YES!! (!)

Aside from doby, I will take the time to explain some basics to anyone wishing to increasing the run time of their boats.

As I mentioned earlier, run time can be calculated by the following:

Run time(minutes) = Battery capacity in Amps / Amps being drawn/60

Based on the above, you have 2 options;
  • Get a bigger battery
  • Reduce the power consumption

Getting a bigger battery is easy, provided you have the room for it, and your boat will handle the weight.

But what if you cant get a bigger battery?
If the battery capacity can not change, then the only other option is to reduce the amount of amps being drawn.
How do you do this? First you need a starting point- you need to benchmark your present status.

Here is an example of a run from ezhitz's boat(See page 1 of this thread)

The data shows that on average, his boat pulls about 95A over a 2 minute run.
95A/60(minutes) = 1.58 amps/minute
ezhitz reports he gets 2.5 minutes of run time on a 4500,mah battery. Lets check it out
1.58A/minute x 2.5 minutes = 3.95A or 3950 ma from a 4500mah battery.

Given that we should never run our batteries flat (60-80% of capacity is ideal) this checks out perfect.

As a matter of fact, I would be willing to bet that if ezhitz's battery was charged on an intelligent charger, it would probably take 4000ma to charge it back up.(assuming the battery is not off balance)

Back the the question of run time:
"If the battery capacity can not change, then the only other option is to reduce the amount of amps being drawn.
How do you do this? "

So from above, we have a benchmark of 95A
How do we lessen that?

Amps drawn for a boat is made up of a combination of factors:
Prop Diameter, Prop Pitch, hull/water drag, aerodynamic drag and drive line drag. (there are other sources, such as electronic efficiency, but they are negligible compared to the above factors)

The 95A draw we presently have is based on all of the above.

A sticky or mis-aligned drive-line will cause your boat to pull more current than normal. Lets say in the above example, the drive line was dry and caused a lot of friction. Something as simple as lubricating it and doing a proper alignment to free it up will have a big impact on the current draw.

Lets say by doing that and we take it for another spin in the water and measure the current draw, we find that now it only pulls 90A
Given: 4500mah battery (we will take to 4000mah as we did above)
Current draw 90A
Run time = 4000mah (4A) / 90A/60minutes
Run Time =2.7 minutes. Hmm... Not a lot

But it adds up.

Lets say we sand all the blips and bumps from the hull, then polish it with some turtlewax. This will help reduce the drag of the hull in the water.

Now maybe if we take it for a run, we find we are pulling 85A
again, doing the math, we would get
Given: 4500mah battery (we will take to 4000mah as we did above)
Current draw 85A
Run time = 4000mah (4A) / 85A/60minutes
Run Time =2.82 minutes. we are getting there.


The above was simple stuff. The real meat and potatoes is prop diameter and prop pitch. Changing either one of these will make a HUGE difference in power consumption at the cost of speed. Maybe better. Maybe worse. The only way to know for sure is bolt on a different prop and take it for a run, then check your data.

If we can pull the amps down to say 75 (which is not unreasonable) and still have satisfactory speed, run time could be extended to 3.2 minutes.

All of this would have to be weighed against speed, of course.

If this were a 10 lap race, the difference between a 95A run or a 75A run could mean running out of juice before the last lap (if it takes you more than 3 minutes to complete the 10 laps)

On the other hand, if you could blow through the 10 laps in just 2 minutes at a higher speed, then 95A is what you want.

Bottom line is, you need to try different prop scenarios and measure the current draw each time to determine the optimal prop pitch and diameter that satisfies your needs.

Once you have tested and collected enough data to get a good idea of what your boat is pulling in Amps, you can calculate exactly how long it should run for any given battery size.(Provided you dont change any of the variables)
You win for the most words used in a posting
doby is offline   Quick reply to this message.

Sponsored Links - Subscribe to remove this ad.
Old 11-04-2013, 03:33 PM   #22
RCC Supreme Contributor
I am: eric q
Join Date: Jun 2005
Location: Crawford Lake, Ontario
Radio of choice:
# of RCs: too

Feedback: 9 / 100%
Posts: 7,159
Total Props: 26
Re: help please

Originally Posted by doby View Post
Sorry, nope...air is much less dense than water....apples and oranges. friction on the hull and hardware is much more than a plane going through air.
not when it climbs out of the water

Originally Posted by doby View Post
Constant full power is what we run..
full throttle maybe, power will vary with load
desiderantes meliorem patriam
To err is human, to moo is bovine
moo is offline   Quick reply to this message.

Quick Reply

Register Now

In order to be able to post messages on the RCCanada - Canada Radio Controlled Hobby Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

Member names may only be composed of alpha-numeric characters. (A-Z and 0-9)

!!ATTENTION ADVERTISERS!! If you intend on advertising anything on this forum, whatsoever, you are required to first contact us here . Additionally, we do NOT allow BUSINESS NAMES unless you are an Authorized Vendor. If you own a business, and want to do sales on this site via posting or private message, you will need to follow the rules. Shops, Stores, Distributors, Group Buys without being authorized will see your account terminated.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Your Name
Your first name and last initial is required if you plan on using the forum.
Which city & province you live in. This is mandatory for classified listings.
Radio of choice?
Which radio is your current favorite to use?
Number of RC Vehicles?
How many boats, cars, planes do you own?


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
vBulletin Message

Thread Tools
Display Modes

Forum Jump

All times are GMT -5. The time now is 02:14 PM.

vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.