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11-01-2025, 12:12
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#1
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Registered User
Join Date: Apr 2019
Posts: 39
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Question on running dive compressor off generator
Hello everyone from Panama!
I purchased a dive compressor for our monohull with gasoline engine (Honda GP 200).
When we bought it we knew we wanted to convert to 110V and were told it was easy to do. It is too heavy to lift up/down companion way to use the gasoline version long-term
After 1.5years of back and forth w/ the manufacturer they sent me the electric motor w/ the following specs:
- 4kw/3HP
- Single Phase 115v/60hz
- 43 Amps
My generator is 7.3Kw 60hz Westerbeke.
My concern is the amperage, especially for startup, do you think it will work?
Second question, if I cannot convert to electric can I get a high-temp exhaust hose and run it about 7 ft out a window and attach to the Honda gasoline exhaust and run the compressor with gasoline from inside the forward berth?
Thanks for any help, this has been 1.5 year ordeal, and over $600 in shipping costs alone to get this motor that I'm not sure will even work. This means I'm hesitant to spend any more money on the electric conversion (i.e. even a soft-start) and hence why I'm considering how to run a gasoline version from below decks.
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11-01-2025, 13:11
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#2
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Writing Full-Time Since 2014
Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 10,281
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Re: Question on running dive compressor off generator
Can you fit the gas version in a deck locker? Build a deck locker?
As for the electric, details matter, so you will need to run it past an electrician. Probably, if you turn most things off, most specifically the battery charger. It is really system-specific.
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11-01-2025, 13:34
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#3
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Registered User
Join Date: Jan 2008
Boat: Teak Yawl, 37'
Posts: 3,017
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Re: Question on running dive compressor off generator
Quote:
Originally Posted by captNordy
Hello everyone from Panama!
I purchased a dive compressor for our monohull with gasoline engine (Honda GP 200).
When we bought it we knew we wanted to convert to 110V and were told it was easy to do. It is too heavy to lift up/down companion way to use the gasoline version long-term
After 1.5years of back and forth w/ the manufacturer they sent me the electric motor w/ the following specs:
- 4kw/3HP
- Single Phase 115v/60hz
- 43 Amps
My generator is 7.3Kw 60hz Westerbeke.
My concern is the amperage, especially for startup, do you think it will work?
Second question, if I cannot convert to electric can I get a high-temp exhaust hose and run it about 7 ft out a window and attach to the Honda gasoline exhaust and run the compressor with gasoline from inside the forward berth?
Thanks for any help, this has been 1.5 year ordeal, and over $600 in shipping costs alone to get this motor that I'm not sure will even work. This means I'm hesitant to spend any more money on the electric conversion (i.e. even a soft-start) and hence why I'm considering how to run a gasoline version from below decks.
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You cannot, I repeat, you cannot, run a small stand-alone gasoline engine inside your forward berth or under deck or anywhere inside your boat. You are risking life and limb. I don't have the patience to type in all the reasons. Maybe someone else will.
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11-01-2025, 13:36
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#4
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Registered User
Join Date: Apr 2021
Location: NJ
Boat: Donelle 35 Cruiser
Posts: 54
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Re: Question on running dive compressor off generator
Quote:
Originally Posted by captNordy
Second question, if I cannot convert to electric can I get a high-temp exhaust hose and run it about 7 ft out a window and attach to the Honda gasoline exhaust and run the compressor with gasoline from inside the forward berth?
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First I may be misunderstanding this question.
I would be hesitant to run any gasoline engine anywhere inside the berth of any boat. Gasoline vapors and or Carbon Monoxide are very real and very dangerous. If you were my buddy I would tell you - Hard Stop No.
I am a volunteer fireman and I have seen enough portable and full home generators do nasty things to homes and inhabitants - I would not risk putting that in a boat in close quarters.
Just bring extra tanks - carry extra tanks and get them filled on land... I am also a scuba diver and have thought about this approach. For me it was too expensive at the start and the quarterly/annual inspections.
Good luck and keep us posted I am curious how this plays out.
__________________
Thomas Jefferson: “I prefer dangerous freedom over peaceful slavery.”
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11-01-2025, 13:52
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#5
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Registered User
Join Date: Oct 2021
Posts: 705
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Re: Question on running dive compressor off generator
Forget running the engine in the forward cabin. Could you successfully get the exhaust out? Maybe. What about the heat? Compressing air is terribly inefficient, if you need a 4kW motor you're probably generating 2kW of heat, plus the gas engine's heat. How are you going to get that out of the cabin? Then there's raw gas fumes/smells...
For the electric motor, too few details to tell (is it split phase? capacitor start? capacitor start/capacitor run?), but do some field engineering. Throw a cord on it and connect it up to your generator. Can you start it with zero load? If yes, do that 4-5 times (the starting torque varies with the angle at which the rotor stopped, so one start does not tell the whole story). If you can start it at zero load, grab a piece of heavy leather and wrap it in a "U" around a smooth portion of the motor shaft. Grip the leather with some Channellocks with light pressure and try to start the motor. Repeat with "moderate" pressure. This will apply some load to the motor to see if it will start under an (undefined) load.
Your compressor should unload when stopped so it starts unloaded. Take the belt off and try to turn the compressor pulley by hand. How hard is it to turn? This is the starting load on the motor (plus a little more for bearings, etc.). Use some field guesstimation to apply the same force to the electric motor with the Channellocks.
Should take less than 15 minutes and then you should have some reasonable idea about whether or not your generator and electrical system can start the motor, without having to first go through the whole conversion process.
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11-01-2025, 14:05
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#6
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Registered User
Join Date: Aug 2010
Location: Arctic Ocean
Boat: Under construction 35' ketch (and +3 smaller)
Posts: 3,006
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Re: Question on running dive compressor off generator
Quote:
Originally Posted by captNordy
snip..
After 1.5years of back and forth w/ the manufacturer they sent me the electric motor w/ the following specs:
- 4kw/3HP
- Single Phase 115v/60hz
- 43 Amps
My generator is 7.3Kw 60hz Westerbeke.
My concern is the amperage, especially for startup, do you think it will work?
..snip
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Yes you can. Dive compressors have manual valves to release cylinder pressure before start and you should not open the connected tank valve before the compressor is running so there's no pressure working against..
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11-01-2025, 14:07
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#7
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Marine Service Provider
Join Date: Jan 2012
Location: Thailand
Boat: Herreshoff Caribbean 50
Posts: 1,127
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Re: Question on running dive compressor off generator
__________________
Steve .. It was the last one that did this !
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11-01-2025, 15:03
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#8
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Registered User
Join Date: Apr 2019
Posts: 39
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Re: Question on running dive compressor off generator
Thanks for the responses everyone! Some quick replies:
Thinwater - I cannot build a locker outside, don't want the extra clutter and windage for ocean passages. I am currently speaking w/ an electrician, but I'm in Panama and they are in the US, so working through that part.
LakeSuperior - I understand. I've seen a motorboat blow up in the marina from a spark setting off gasoline fumes.
Love2Boat - thanks for the hard No, I'll adhere. Regarding the tanks, we will be remote in French Polynesia for several months, so need to be able to handle filling tanks ourselves.
PippaB - Similar advice to what my electrician friend said. My main concern is the amperage tripping the main breaker. The master AC on my electrical panel has two 50amp fuses. I'm not sure if the in-rush of current will trip that or not (I do not have confirmation from the Italian motor manufacturer if the 43 Amps is Max or running).
TeddyDiver - Understand there is no load on the motor when it starts, but see my concern on overall amp that I mentioned to PippaB.
Thanks all!
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11-01-2025, 15:15
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#9
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Registered User
Join Date: Aug 2010
Location: Arctic Ocean
Boat: Under construction 35' ketch (and +3 smaller)
Posts: 3,006
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Re: Question on running dive compressor off generator
Change biggers fuses and wiring if needed. You can also use two 30A fuses combined in parallel..
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11-01-2025, 15:39
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#10
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Registered User
Join Date: Mar 2007
Location: Santa Cruz
Boat: SAnta Cruz 27
Posts: 7,213
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Re: Question on running dive compressor off generator
I had a Bauer compressor with a Honda gas engine. It was easy to pop the belt off and separate the engine from the compressor. The weight of the compressor alone was about 60 lbs, and the engine was about 50.
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11-01-2025, 16:39
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#11
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Registered User
Join Date: Oct 2021
Posts: 705
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Re: Question on running dive compressor off generator
The typical full load current (FLC, or FLA for Amps) of a 115V, 1phase, 3HP high efficiency motor is 34A, so I would guess your 43A is the FLC, it certainly does not match what would be the expected starting current.
The typical breaker for that at a factor of 1.25 would be 54A, so a 60A breaker. It might run OK on a 50A breaker, but you will be running the breaker > 80% of its rating, if the motor takes that full current. Regardless of breaker or fuses, if you are going to be starting the motor the protection device must be of the time delay type. For a breaker this would normally be a "C" or "D" curve breaker to account for the starting current. For fuses there is a lot more variability, so it depends on the exact type and rating of the fuse to evaluate the delay. If you truly have fuses and you are going to test you should make sure you have spares available, and if you blow a set you should probably write off the experiment. If you have a breaker(s) it would be beneficial to get the model number and look up the trip characteristics - if it is B curve then it is not likely to work and will most likely trip on motor startup.
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11-01-2025, 18:13
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#12
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Registered User
Join Date: May 2013
Location: Oregon to Alaska
Boat: Wheeler Shipyard 83' ex USCG
Posts: 3,662
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Re: Question on running dive compressor off generator
7.3kw should be good for 60-66 amps depending on the operating voltage 110v-120v.
You may need a start package.
I've run a few dive compressors. For me what worked the best was to run them off the main or generator engine. But I was running large compressors.
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12-01-2025, 08:16
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#13
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Registered User
Join Date: Apr 2019
Posts: 39
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Re: Question on running dive compressor off generator
PippaB - Thank you for this and it aligns with what I'm getting from others. Basically it seems like it, "will be close". Shipping in the 6awg wire, trip-delay circuit breaker, etc. will cost close to another $200, so I'm still trying to figure out with more certainty if it will work.
My main concern is the breaker for the AC Main on the distribution panel. It is a double pole (two breakers;50amp each when viewed from the back, but from front of panel only one on/off switch). I believe this means I have two AC circuits and they are only good to 50amps. So if I have a trip-delay circuit downstream that is 60amp, won't the 50amp main breaker trip at startup as the in-rush current is requested of the generator downstream?
I also have a Victron Multiplus and 1100ah of LiFePO 4, which has the 'shore assist' feature that can throw in some DC power if an overload on the generator is close...I don't think that helps my 50amp limit issue though.
The generator specs say it support 63.3 amps, so not sure why the breakers on the AC main Dist Panel are 50amp...
Boats seem to be 50amp or 30amp in my experience.
Quote:
Originally Posted by PippaB
The typical full load current (FLC, or FLA for Amps) of a 115V, 1phase, 3HP high efficiency motor is 34A, so I would guess your 43A is the FLC, it certainly does not match what would be the expected starting current.
The typical breaker for that at a factor of 1.25 would be 54A, so a 60A breaker. It might run OK on a 50A breaker, but you will be running the breaker > 80% of its rating, if the motor takes that full current. Regardless of breaker or fuses, if you are going to be starting the motor the protection device must be of the time delay type. For a breaker this would normally be a "C" or "D" curve breaker to account for the starting current. For fuses there is a lot more variability, so it depends on the exact type and rating of the fuse to evaluate the delay. If you truly have fuses and you are going to test you should make sure you have spares available, and if you blow a set you should probably write off the experiment. If you have a breaker(s) it would be beneficial to get the model number and look up the trip characteristics - if it is B curve then it is not likely to work and will most likely trip on motor startup.
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12-01-2025, 10:05
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#14
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Registered User
Join Date: Aug 2010
Location: Arctic Ocean
Boat: Under construction 35' ketch (and +3 smaller)
Posts: 3,006
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Re: Question on running dive compressor off generator
Just a though but why not set a circuit of it's own from the genny directly to compressor with a fuse, switch and else big enough for the loading?
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12-01-2025, 10:14
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#15
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Registered User
Join Date: Mar 2007
Location: Santa Cruz
Boat: SAnta Cruz 27
Posts: 7,213
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Re: Question on running dive compressor off generator
Look into soft start capacitors to reduce the peak starting current. They are in all your AC heat pumps.
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