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Old 26-04-2019, 10:49   #1
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golf cart motor for 34' tartan

I just bought a 1970 tartan 34c and the motor is bad. I am pulling the motor and going to rebuild it at work but might take me a while. A friend has a 36v golf cart that he will give me if I want it. Anyone know how to tell if the golf cart motor would be large enough to get my boat in and out of the marina under calm conditions? I haven't looked at the golf cart yet to see if there are any specs on the motor to see how big it is, or if there are sizes written on them. My boat's displacement is around 11,000 lbs. Thanks
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Old 26-04-2019, 11:54   #2
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golf cart motor for 34' tartan

It’s large enough, however your battery bank may not be. In truth a large trolling motor would work, in calm conditions, which to me mean no current and light winds.
If your overhauling your motor to put back in, don’t go down the electric conversion route, it has to be way more hours than simply overhauling your motor, and even though the motor is free, nothing else will be, you’ll almost certainly need a gear box of some type for example and your prop isn’t likely optimum either.
Stick an outboard on it and enjoy sailing and overhaul your motor at your leisure, a good rainy day or Winter project.
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Old 26-04-2019, 11:58   #3
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Re: golf cart motor for 34' tartan

Amen. You're talking about a very expensive conversion, then a very large battery bank that has to be charged some way, and then very limited range for when it's not dead still at the marina. That's too many times "very."
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Old 26-04-2019, 12:01   #4
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Re: golf cart motor for 34' tartan

Big enough for what is the first question to answer.
If you want to come and go from the marina for daysailing then 5kw is probably enough.
If you need to motor directly into wind and/or current then 15kw.

You need to find out the max amperage for the golf card motor. It should have a serial plate on it with that info.

That said, if you are rebuilding the inboard then A64’s proposal is the best one.
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Old 26-04-2019, 12:06   #5
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Re: golf cart motor for 34' tartan

Hi,
A 36 volt electric motor produces about 8 horse power at 100% load. That is assuming the motor can reach 100%. It is likely that it will run at only 85% to 90%. Being conservative, that leaves you 6.8 hp at Normal Continuous Rating (NCR). That is not much. The best I can tell, your sail boat is fitted with a 30 HP diesel. The hull speed power curve is not linear. Your boat makes about 6 knots at approx. 85% engine load or 25.5 hp @ 2600 rpm with a gear box (1:2.4) shaft speed of 1000-1500 rpm.
The propeller curve is rpm dependent. Your diesel should deliver about 3600 rpm and MCR and maybe 2600 rpm at NCR which turns the propeller at 1000-1500 rpm.
We don't know what the rpm of the electric motor is at full load. Since you most likely do not have a variable frequency drive or gear box for the electric motor it will try to run at full available rpm with the loaded rpm being determined by the resistance of the propeller spinning in the water and the hull friction of the sail boat. The load resistance is set by the pitch and rake of the propeller blades.

Sustained resistance on the electric motor that reduces the rpm through load will cause the electric motor heat up. That is why many marine electric motors are water cooled. Its like trying to push voltage through a wire that is too thin. The wire will get hot due to the resistance.

Based on very limited information. Your electric motor without the gear box, producing 6.8 hp @ approx 500 to 700 rpm will push your boat at maybe 1.0 to 1.6 knots going forward. Add any wind or current to the situation and you are sitting still.
Also note: we do not know what the amp draw of your electric motor is at NCR. You will need to know this in order to provide enough battery storage to push the motor over time.
I hope that helps a little.
All the best,
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Old 26-04-2019, 12:08   #6
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Re: golf cart motor for 34' tartan

Where did you get the 8hp number?
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Old 26-04-2019, 12:30   #7
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Re: golf cart motor for 34' tartan

Stock 36v motors are around 8hp but I easily found 36v golf cart motors in the 10.5-12.0hp range.
https://www.golfcarcatalog.com/produ...47%7D2%29.html
https://www.everythingcarts.com/p-46...-dcs-cars.aspx
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Old 26-04-2019, 12:45   #8
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Re: golf cart motor for 34' tartan

The lightship displacement of the T34c is 11,420 with a 25’ LWL.
Sqrt of LWL is —>5kt

Lets say loaded displacement is 14,560 (6.5long tons).

Let’s say 1hp/long ton.

From Skeene’s speed-thrust graph 1hp/long ton will produce a speed of about 1.2*sqrt(LWL) or 6kt. Let’s be a little conservative and say 1.0*sqrt(LWL) or 5kt.
I’ve checked the curve against my own boat’s performance and it was a good approximation.

My guest is that in flat water a 6hp motor will push the OP’s boat 4.5-6.0kt. Click image for larger version

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Old 27-04-2019, 08:59   #9
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Re: golf cart motor for 34' tartan

That will be the most expensive free motor you ever install.
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Old 27-04-2019, 10:15   #10
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Re: golf cart motor for 34' tartan

So this is a major piece of work to get going. The good news is if you are electrically and metal working savvy it can be done. You will also need to execute some detailed welding and have a decent ability from an engineering perspective to design a coupling. Here is the most detailed explanation with a working setup I have seen. https://www.sailinguma.com/electro-beke
What you should see from the hours of video is not the types and diversity of the jobs performed to make a solution but the immense project this turned out to be as a whole. Also you will need a golf course worth of cart batteries to go more than a half mile. Imo pay 1500 for a used kicker and sell it for about the same when you get your main engine back into the boat.
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Old 27-04-2019, 10:48   #11
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Re: golf cart motor for 34' tartan

Quote:
Originally Posted by tkeithlu View Post
Amen. You're talking about a very expensive conversion, then a very large battery bank that has to be charged some way, and then very limited range for when it's not dead still at the marina. That's too many times "very."
You forgot one "very". VERY reliable. (For those that only daysail I/O harbor, which by what I see around me seems to be most people.)

I have often had people tell me they would never trust an electric motor to power their sailboat. To which I curiously always ask; "how then do you start your diesel?"
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Old 27-04-2019, 15:53   #12
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Re: golf cart motor for 34' tartan

will take alot of cash larger than average battery bank and very heavy . Move the water line up too ! You can day sail only no trips more tan 2 hours away !
Its not a elect car water is very dense. !
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Old 27-04-2019, 18:01   #13
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Re: golf cart motor for 34' tartan

As other posts have said, there is a lot of work to installing an electric motor. I am a big fan, but if its temporary, not worth it.

Another option would be a PAIR of electric trolling motors on the stern. Compared to gas outboards they are lighter, easier to mount, and usually have very long shafts. Add a couple more batteries to your bank. A pair of 55 pound thrust motors would give over 100 pounds thrust, for a short period of time while the batteries quickly fry...might be enough to get in and out of your marina.

When my outboard died, I sailed in/out of my slip for a couple weeks while it was being repaired. I found it made me a better sailor. I anchored more, and appreciated a good dock more.
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Old 27-04-2019, 21:53   #14
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Re: golf cart motor for 34' tartan

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Originally Posted by hamburking View Post
As other posts have said, there is a lot of work to installing an electric motor. I am a big fan, but if its temporary, not worth it.

Another option would be a PAIR of electric trolling motors on the stern. Compared to gas outboards they are lighter, easier to mount, and usually have very long shafts. Add a couple more batteries to your bank. A pair of 55 pound thrust motors would give over 100 pounds thrust, for a short period of time while the batteries quickly fry...might be enough to get in and out of your marina.

When my outboard died, I sailed in/out of my slip for a couple weeks while it was being repaired. I found it made me a better sailor. I anchored more, and appreciated a good dock more.
Installing an outboard is probably a lot more heavy work... The real work is the removal of the diesel and all the associated systems that you do not need any longer if you want to go the electric motor route; crane to remove diesel engine and the associated yard bill, removal of the diesel fuel tank, fill hose to fuel tank, and deck fitting, raw water strainer, raw water thru-hull, raw water intake ball valve, exhaust hose, water-lift exhaust, throttle linkage, gear shift linkage, motor starting battery, and cables, instrument panel, and cables, .... a LOT of work! You can't blame ALL that work on the electric motor.

The motor installation? I carried the 60lb 10Kw PMAC motor onboard in my arms. Fitted it to the existing engine beds. Hooked it up to the existing shaft, ran the wires to the throttle control in the cockpit, installed the new LiFePO4 battery where the diesel tank used to be, (perfect balance point) and that was it. Pretty easy peasy compared to a diesel install.

So as far as work goes; the diesel removal is the real work. The motor installation is a breeze. Oh, and you can have a normal conversation in the cockpit while you are motoring out of the harbor. An interesting phenomena you will notice is that people are yelling in the boats around you because they are trying to be heard above the noise of their diesel. It's pretty funny. We never noticed that before we had an electric motor because everyone is yelling. LOL...
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Old 27-04-2019, 22:06   #15
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Re: golf cart motor for 34' tartan

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will take alot of cash larger than average battery bank and very heavy . Move the water line up too ! You can day sail only no trips more tan 2 hours away !
Its not a elect car water is very dense. !
Please don't make these assertions without any personal experience installing and using electric motor propulsion and installations. I just sold my Freedom 32 with a great electric motor installation. The small LiFePO4 battery pack has a 25 mile range, the boat weighs 700 Lbs LESS without ALL the associated diesel components AND the option to add another LiFePO4 bank is available to increase the range to over 50 miles. (68mi by calculation). And...my waterline went UP, not down...

Now, that is not to say that an electric powered sailboat is for everyone, but it IS to say it probably is better for most that just daysail in and out of the harbor.
Or...for those that cross oceans. It's a sailboat. On a long passage you can regenerate enough electricity to completely recharge your battery pack before you arrive in port. Something to consider.

Watch the YouTube show; "Sailing Uma". 14,000+ miles cruised. Gallons of diesel used; Zero

Good luck with your decision...
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