Agni Motor in Boat - posing challenges.

Hi there,

I have recently become the boat manager for Somerset Waterways, Taunton, Somerset, UK. I am looking after a 2.5 ton, 12 passenger trip boat that is powered by an Agni Lynch motor (probably a 95, but I have not found a works plate yet to verify this). The boat is three years old. The battery pack is a Powercell 600Ah 24v set, and is not in very good shape at all, due to very poor battery management.

I have no end of 'challenges' with the technical side of this boat - I feel that some of them might have been avoided with some level of technical understanding by the users earlier in the life of the boat, but whatever, I would welcome some opinions on what I have discovered so far.

The Agni is linked directly to the prop shaft - the boat is a canal and river cruiser, not a speed boat, so I would estimate the prop shaft rpm (and thus the motor rpm) as perhaps 250 rpm cruising, maybe 400 max (rarely used). My concern is that there is an awful lot of heat being chucked out by this motor. Someone has installed a thermometer remote sensor about 2 cm away from the motor casing, which records (during the course of a half hour journey, at an estimated 250 rpm, drawing 35 Amps) a gradually increasing temperature, eventually topping out around 65 degrees Centigrade - the thermometer does not record more than 70, which is not useful.

There is evidence of heat damage to the plastic end cap around the power connections. There has been evidence, in the form of excessive heat and a hot-electric-motor type smell, of overheating when the power is pushed up to 50 or 60 amps for any protracted period.

The motor and prop shaft turn easily and freely, much as they have always done - we are not in seize-up country.

As a design 'feature' the motor has no external air flow at all, with the floorboards in place. One of my first concerns is to remedy this - which I find quite amazing (is the polite way of putting it!).

I would really welcome the opinions of anyone with technical knowledge, on the various matters that this installation raises:

Theoretically the motor is rated at 100 Amps. If I ran it like this for a couple of minutes, I am sure I would burn it. Is maximum current limit actually limited, in real life, by the speed of airflow through the motor, and thus by the rpm? If so, is there something technical I could read somewhere to 'learn the rules'?

I am thinking of cutting (inbound) vent slots in the floorboard panel, installing a 24 volt truck radiator fan, with a thermostat, and providing some form of ducting arrangement to get the heat out of the small, unventilated space that the motor lives in.

Really, the whole installation is far to 'lightweight' for a boat that is allowed out on a river - 2.4Kw supposed maximum power - smaller than the smallest outboard. Unfortunately, upgrading or replacing the drive line components with something meatier is not an option - I have to make do with what I am given.

Any comments anyone?

Ken Bussell

An agni is a pretty badass motor, IIRC it's rated at somewhere around 10hp continuous duty. but you have to get it spinning, gearing it down with a belt or chain drive and running it around 48-60v would really pep it up and probably keep it cooler.

Others may be able to elaborate further, but would a different prop (with a different pitch angle) allow more rpm's with less speed? This would let the motor spin easier (better efficiency) while sacrificing a little top speed.

The easiest fix would be to move some air across the motor as you mentioned. The efficiency plummets at low rpm and the heat build up is a sure sign of this. Making some gear reduction would be logical but could increase the noise created by the drivetrain which may have a negative effect on the boats passengers.

Prop change is the best first try, then ventilation. Look for a variable pitch prop. They DO exist.

A for instance. I was given an old wooden rowboat. Got it repaired. Then, bought a 3 HP outboard, air cooled. Prop had 1 blade. Neighbor gave me a junker 6HP same brand outboard. Put the prop on and could never get any RPM's above about half throttle. Boat moved well.

This is your problem, first off. Then, find a way to run a section of bilge hose or something a bit larger, from above deck, and a blower pushing the air to the motor. Then, run a second tube from just under the deck, above the motor well, to up near as high as the gunwale, fastened to the transom or opposite side from the air intake to the motor. NEVER vent a floor-deck in a boat. Always run the intake-exhaust cooling hoses like I described, or, you will be salvaging the boat off the bottom.

Boat builder-repair for over 55 years.

As a previous poster suggested, your only sensible option is to increase the voltage and gear down with a toothed belt drive or similar in inverse ratio. If you double the voltage and use a 1:2 speed reduction) you will halve the current and heat losses will be 1/4 at the same power. Then you might get away with a fan on the motor shaft.
On my home made electric outboard I use a brushless motor 80mm dia x 100mm long under a passive ventilated plastic cover. at 24v/30A it never gets hot even after an hour so you should not be having any heat issues at all with your Agni at about the same power.