Alternator upgrade on Volvo TMD41A

[isophase]

Hello, first time poster here. I've arrived from Trawler Forum in search of specific info, perhaps even from the legendary @volvopaul that I have read so much about.
Quick intro: long-time cruising sailor and sailing instructor on the Great Lakes but first-time owner. In May we bought a 1987 Hershine 37' Newburyport (think Monk 36 hull) with twin Volvo TMD41A, planning to cruise summers in the Great Lakes.

The question is, how much of an alternator upgrade can I reasonably add to my 1987 TMD41A's?
We are looking at 24V / 165A (4 kWe) with smart regulator. It would come with a serpentine-belt conversion for the crank, water pump, and alternator.
The two concerns I've heard relate to engine loading - first the total power takeoff, and also specifically the increased forces on the relevant pulleys.

Any thoughts would be helpful.

Thanks

Jeff

 

[LynnC]

Short answer - Don't do it.
Your thought about putting a serpentine belt on means you will have to design and manufacture all new pulleys. A big job, and then you need to make brackets.
What you could do is to put a stub shaft on the front of the crankshaft and then run a jack shaft off that. You can then have the appropriate pulley sizes to get the correct speed for the alternator. I can't recall exactly how much power you can take off the front of the engine. But it is considerable (say 20%) However bear in mind that whatever power comes out of the front is lost from coming out the back.
You are proposing to take 5 HP off the front. This will impact how fast your boat will go.

I had a similar enquiry from a chap for his boat used to take out divers to their diving area. The intention was to run a compressor whilst they motored to the diving area. I suspect you have the same idea.
I recommended he used a separately powered compressor. And I recommend you do likewise - use a generator.

 

[boatmike]

I have twin TMD41a engines on my boat. I have retrofitted twin 90amp Prestolite alternators on both engines giving 180amp capacity total at 12v. I have had no problem driving them from the standard belt drive off the engine as fitted, albeit that I do change the drive belts every year regardless of need or not. Do bear in mind that if you are simply charging batteries, 12 or 24v, you will rarely be pulling maximum power. If you ever do need to pull maximum power (like your barreries are flat or want inverter power etc) you might notice a slight drop off in top end power to your drive train but with 300HP total available it isn't going to be significant. You don't get anything for nothing though and the total HP required to run whatever you fit can easily be calculated. If you retrofit bigger belts etc as you suggest you will have no problems at all and should bear in mind that unless you have extraordinary loads (like aircon through an inverter etc) you will probably rarely use maximum output.

 

[isophase]

Short answer - Don't do it.
Your thought about putting a serpentine belt on means you will have to design and manufacture all new pulleys. A big job, and then you need to make brackets.
What you could do is to put a stub shaft on the front of the crankshaft and then run a jack shaft off that. You can then have the appropriate pulley sizes to get the correct speed for the alternator. I can't recall exactly how much power you can take off the front of the engine. But it is considerable (say 20%) However bear in mind that whatever power comes out of the front is lost from coming out the back.
You are proposing to take 5 HP off the front. This will impact how fast your boat will go.

I had a similar enquiry from a chap for his boat used to take out divers to their diving area. The intention was to run a compressor whilst they motored to the diving area. I suspect you have the same idea.
I recommended he used a separately powered compressor. And I recommend you do likewise - use a generator.

Understood about the pulleys. The vendor is taking care of that. Their facility is a 30 minute drive from my place, including on-site CNC mill and lathe. They'll be borrowing my existing pulleys to measure so fairly confident that'll work out.

As far as power take-off, it'll be under 10 hp off an engine rated for 150. it's a displacement trawler (should have made that clear) so we're going slow and only typically running 2200 RPM or so on engines rated at 3500. As long as the engine is OK with it, I'm not going to feel the loss.

 

[isophase]

I have twin TMD41a engines on my boat. I have retrofitted twin 90amp Prestolite alternators on both engines giving 180amp capacity total at 12v. I have had no problem driving them from the standard belt drive off the engine as fitted, albeit that I do change the drive belts every year regardless of need or not. Do bear in mind that if you are simply charging batteries, 12 or 24v, you will rarely be pulling maximum power. If you ever do need to pull maximum power (like your barreries are flat or want inverter power etc) you might notice a slight drop off in top end power to your drive train but with 300HP total available it isn't going to be significant. You don't get anything for nothing though and the total HP required to run whatever you fit can easily be calculated. If you retrofit bigger belts etc as you suggest you will have no problems at all and should bear in mind that unless you have extraordinary loads (like aircon through an inverter etc) you will probably rarely use maximum output.

Thanks @boatmike, good to hear it's worked out for you.
I should have added the house bank will be LFP so it will accept absolutely everything and anything that the alternators can produce. The plan calls for 2x Epoch v2 Elite 24V/230Ah. Each battery is rated to accept up to 230A continuous charge at 24V. I'm going to fit a smart regulator so it won't necessarily put the full load on but it sure could accept all of it if I did. Empty to full could be less than two hours, or I can stretch that out.

I will be curious about how it affects the propeller match, ie whether I can still hit rated RPM at WOT with the alternators going full on. Probably not. Remains to be seen how big a shift. Once the batteries are topped up, it'll be back to normal.

 

[boatmike]

Understood about the pulleys. The vendor is taking care of that. Their facility is a 30 minute drive from my place, including on-site CNC mill and lathe. They'll be borrowing my existing pulleys to measure so fairly confident that'll work out.

As far as power take-off, it'll be under 10 hp off an engine rated for 150. it's a displacement trawler (should have made that clear) so we're going slow and only typically running 2200 RPM or so on engines rated at 3500. As long as the engine is OK with it, I'm not going to feel the loss.

Asked and answered then. No problem at all! As far as the load on the engines are concerned, The only other consideration would be the directional thrust on the bearings. Its a Volvo! well over engineered. Don't concern yourself...... and as far as the prop choice is concerned if you are running at under 3000 RPM anyway it will make no difference

 

[isophase]

Yes, that directional thrust is my main concern.
I'll be increasing electrical load about 7x. Factoring in improved efficiency, it might be 4-5x the torque load. That would need proportionally more belt tension (which would load up the bearings) if I stayed with dual-vee. Serpentine should be better, but I don't have a feel for how much.
Sounds like the advice is that it sounds reasonable and not to worry...

 

[boatmike]

Yes, that directional thrust is my main concern.
I'll be increasing electrical load about 7x. Factoring in improved efficiency, it might be 4-5x the torque load. That would need proportionally more belt tension (which would load up the bearings) if I stayed with dual-vee. Serpentine should be better, but I don't have a feel for how much.
Sounds like the advice is that it sounds reasonable and not to worry...

Absolutely don't worry. The main driving pulley is mounted directly on the end of the crankshaft and carries all the direct power load in addition to side thrust, both of which are distributed over all of your main bearings. The water pump bearing will perhaps have a slightly increased side load but is an idler in terms of the alternator load power transmitted. It is well able to handle this. Not a problem! Probably the highest performance demand is on the bearings of the alternator itself which are designed with the total load in mind. In very extreme cases additional drives have been utilised for hydraulic pumps, AC generators etc that utilise a secondary pulley off the end of the crankshaft leaving the existing alternator/waterpump drive in place. This ensures no additional load on the waterpump of course if you were really concerned but in my opinion in your case totally unnecessary.