I want to start by saying I encourage meaningful and technical debate on the proposed drive train. If I made a mistake in assumptions or calculations, the goal is not to shut down discussion.
So most of the proposed electric
drive train threads either focus on use cases that represent tiny percentages of the new boat
market (such as ocean crossings or boats that claim average speeds in the mid teens, etc...) or quickly wander off into magic thinking (5kw of power will get a 50ft cat up to 5kts kind of stuff).
Hybrid drive trains don't work
very well on boats. Base hybrid technology really benefits from a highly variable power demand where the hybrid system evens out the loads on the ICE, allowing it to stay at peak efficiency matched to the average power demand. Perfect for a car in stop and go traffic. Since displacement
cruising boats largely run at a steady power demand, there are no peaks and valleys to take advantage of. They are already running close enough to peak efficiency that there really is no efficiency gains to be had and potential losses due to additional energy conversions.
Battery-Electric drivetrains quickly run into limits on how much power can be stored and how to replace it. Fine if you just want to get in and out of harbor but very limiting if you need to power for any significant distance.
Probably the most common displacement
use case is weekend cruisers who go out many weekends per year with 1 or 2 longer week long trips covering more ground.
- Battery-Electric could typically cover the weekend day trips but on the longer trip, if you need to cover some ground to get back in time for work
on Monday, covering 50-60 miles in 8-10hrs is beyond their capability if the wind
This brings us to a Plug-In-Hybrid option.
- I'm assuming this is a new boat
so no retrofit costs, it's all built into the original design other than the cost difference in the drivetrain components.
- It likely will be a cost increase but probably not a huge cost increase. Though I doubt it will pay off in terms of fuel
I'm looking at the outboard
powered 34ft Gemini
as the base boat
. This is an established model that is a popular cruising boat so it's only the drivetrain that is really in question (if you have a general dislike of Geminis take it to another thread if it's not tied into the propulsion
system). Also, since we had one fitted with a 25hp outboard
(call it 20kw for ease of calculations...I know it's off a bit), I'm familiar with the performance and capabilities.
- 20kw outboard was plenty to maintain a 6.5kt cruise
speed (roughly around half throttle so call it 10kw for cruise
speed) and could push the boat up to full hull speed
, so we aren't under powering the boat to make the calculations look good. So lets assume a 20kw electric
outboard is mounted in place of the gas outboard.
- Ours had 2 - 18 gal built in tanks
plus a 6 gal outboard tank, all plumbed to a 3way valve.
- A Tesla 80kwh battery
bank weighs about 1200lb, so rough estimate if we install a 40kwh battery
bank, probably around 600lb. If we place 300lb of batteries
where the built in gas tanks
go and another 80lb where the 2 - BBQ size propane
tanks go, and 240lb where the 4 - 6 volt golf cart batteries
went...so far we haven't added any weight.
- Modify one of the rear compartments to fit a 5kw generator
(remember, new design so no retrofit costs and it can be done properly). Retain the 6 gal portable tank (could be made built in if desired). This would be a swap out for our 2400w generator
...so a small increase in weight.
- Fridge and stove
are converted to electric operation, so it's fine to replace the propane
tanks with batteries.
Typical Use case which I believe is common for typical new boat buyers:
- Boat lives at the marina plugged in all week.
- When you arrive on Friday, battery is fully charged and until you leave the dock
, it stays that way.
- Day trip, 10kw draw from the 20kw motor
is plenty for normal cruise speed. Even with a couple hours of motoring, you still have 20kwh of battery storage
available for use at anchor
if you choose not to return to the marina for the night.
- 20kwh of remaining battery bank should be plenty for house loads including electric galley
and even air/con operations for a weekend if you wanted to anchor
- Longer trips. 40kwh battery bank drawing at 10kw to maintain 6.5kt will get you 4 hours or 26 nm range under pure battery. Crank up the 5kw generator and that should go up to around 8hrs or 52 nm range. If you have to go beyond that, you can continue on using purely the 5kw generator output but at a decreased speed. In moderate conditions that probably drops you to around 4-5kts until the fuel
runs out (Might want to carry a 5gal fuel jug or two for longer trips...which we did anyway).
- After a long run, we tended to marina hop so usually would have been able to recharge off shore power
. Assuming there is 1.5kw excess capacity for charging
after house loads on a 30amp, 15hr would replace 22.5kwh. If fitted with a 50amp shore power
, you could dump 4kw into the batteries, easily fully recharging overnight. (in a pinch if anchored out, the generator could be used to top up the batteries overnight. It wouldn't be the ideal scenario but if only done rarely, at least you didn't give up capability)
- Even a lot of longer distance coastal marina hopping cruisers (great loop cruisers for example) could get 75% or more of miles from shore power sources. We usually stayed under 30miles per day except for a handful of longer 50-60miles days, so the bulk of the miles could be done under battery power from shore power with only modest generator assistance.
- In an emergency
, you can run full out 20kw output for 2hr under battery alone and 2.5hr if you start the generator immediately and even after the batteries die, you have at least some power (5kw) until you run out of fuel. While it's possible you need max power for longer, that's really moving into a situation where it's so rare I would argue I can live with it. It's certainly not the examples where you have 20-30min at max power and I can see situations we've been in where that isn't enough.
- Of course, this assumes a pure motoring scenario...if you get any miles from sailing, it should reduce the generator run time.
- Even if you are on a mooring
, a 750w solar
array could reasonably be fitted on a Gemini
generating say 3kwh per day or about 21kwh per week. It will be limiting if you motor
a lot but for modest motoring needs, it would be enough.
- It will be more expensive but not drastically. I would estimate probably under $10k more but on a boat with a base price
north of $250k, that's not beyond being reasonable.
- The operator will need to understand the operation to maximize performance...example: For short trips, leave the generator off. For longer trips, they need to start the generator almost immediately after departing. For intermediate distance say 30 nm, the operator will need to estimate when to crank up the generator and how much reserve capacity battery they want to maintain.
- Eco street cred...probably 75% reduction in carbon emissions.
- Ability to run the house loads without cranking up the generator (while not giving up the capability if the situation calls for it)
- Instant power with no need to wait for an engine
to warm up.
- Eliminates concerns of having propane on board.
- Able to boost thru a light wind
tack while sailing by just a quick push of the throttle.
- And the big one...this is a common use case so larger manufacturers would be interested and could offer it as a standard option.
Alternative option: Trawler
cat (still sailboat speeds). You could probably up the solar
to 3kw and bump up to a 60kwh battery bank at net zero cost/weight by eliminating the sailing rig. Now you have a 6hr/39nm range under battery alone and are generating around 12kwh per day from solar. For weekending it would be perfect. Even for longer distance coastal cruising (like great loop), you would probably be pushing upwards of 90% of miles under electric power.