| Boat | Engine | Reduction | Propeller | Engine RPM | Shaft RPM | Speed Kts | SlipA | |
|---|---|---|---|---|---|---|---|---|
| Laughing Gull #197 | Yanmar 2GM20F | 2.62:1 | 2 blade 13"x12" | 2800 | 1069 | 5.8 | 0.058 | |
| Rinn Duin #272 | Beta Marine 16 | 2:1 | 2 blade 13"x7" | 1200 | 600 | 3.3 | 0.045 | |
| 2000 | 1000 | 4.8 | 0.167 | |||||
| 2500 | 1250 | 5.3 | 0.264 | |||||
| 3000 | 1500 | 6.1 | 0.294 | |||||
| 3500 | 1750 | 6.2 | 0.385 | |||||
| Boat | Engine | Reduction | Propeller | Engine RPM | Shaft RPM | Speed Kts | Slip? | 400 low |
| Constance #262 | Yanmar 2GM20F | 2.62:1 | 2 blade 13RH12 | 2000 | 763 | ?.? | 0.??? | 0.??? |
| 2500 | 954 | ?.? | 0.??? | 0.??? | ||||
| 3000 | 1145 | ?.? | 0.??? | 0.??? | ||||
| Boat | Engine | Reduction | Propeller | Engine RPM | Shaft RPM | Speed Kts | Slip? | 400 low |
| Constance #262 | Yanmar 2GM20F | 2.62:1 | 3 blade 12RH14 | 2000 | 763 | 4.8 | -0.092 | 0.090 |
| 3000 | 1145 | 6.0 | 0.090 | 0.197 | ||||
| Boat | Engine | Reduction | Propeller | Engine RPM | Shaft RPM | Speed Kts | Slip? | 400 low |
| Constance #262 | Yanmar 2GM20F | 2.62:1 | 2 blade 12RH7 | 2000 | 763 | 3.5 | 0.204 | 0.337 |
| 2500 | 954 | 4.0 | 0.272 | 0.373 | ||||
| 3000 | 1145 | 4.5 | 0.318 | 0.398 |
A different view of the matter:
Your equations are industry standard. The latest recommendations on LifePO 4 batteries are to keep the depth of charge between 30-90% optimally to insure the rated life expectancy. You also left out the thermal degradation factors. They must be stored at 72-74 degrees F to insure the life expectancy out of them needed for reasonable per AH storage value (vs lead acid batteries (LABs)). At a storage temperature of 92 degrees F half their life is cooked away. These are a few of the reason why SMG Multihull gave up on their incredible product and folded. Their CEO stated that electric propulsion for this application simply was not ready for prime time yet. So optimistically you will have 4.5 hours of run time or so at 50 amps. That's 22.5 nm at what to me is a very hopeful 5 kts. 5 kts for that size boat in my observations has taken closer to 3.5 KW not the 2.2 (50 AH) you stated.
I cut through the superfluous and just use KW in my equations, it's simpler.
The lithium technology emerging is dry cell technology which they are saying will quadruple the current AHs by weight. However without a real price reduction pr AH, few will be drawn to it. Here I think it safe to assume that this expenditure was around 17K in batteries and support gear (not including propulsion unit) to have the capacity to run 22.5 nm at a very optimistic 5 kts.
The three pros about LifePo 4 batteries for marine propulsion are:
The cons are:
There are rumors now circulating that catamaran manufacturers are getting out of the electric propulsion business as well. So SMG will not be the only one to ditch the electric propulsion. In essence, these systems with today's technologies simply are not living up the their billing in the real world of sailing. In fact, electric propulsion is now being seen as less than a desirable attribute for boat resale because of the issues. They can be great for day sailing though!
And oh yeah, the prop regeneration figures are never published. This is because power production from a prop designed to provide forward propulsion is nothing of that gotten from electric stern water turbines for the hull drag induced. In fact you get many times the power from a wind turbine than you do from dragging the prop of an electrically driven drive train. So here the loss of hull speed and the added stress to the rigging become factors. In short the prop regeneration is a non-starter IMHO. If you have a reversible pitch prop system things get a bit better. So, the capabilities for electric propulsion are still largely limited to near coastal navigation. Very near, and boating paradigms where daily berthing is provisioned with shore power for charging. Short haul water taxis in sheltered waters are a good example where electric propulsion could provide value greater than diesel drive systems. Unless huge money is spent (more than most have for such things) electric boats provide a similar utility on the water as golf carts do on land. And if you are traveling in the ICW, you are doing little more than trading a more efficient diesel for a far less efficient gasoline generator in most instances unless you have the money left over for a diesel generator. Money, money, money, money, money with very little offset in blue water cruising cost "savings". -- spencer marshall comment at https://www.youtube.com/watch?v=vY_-Wfyx0_A&t=464s And
When you run the numbers you find that electric propulsion is the most expensive option out there for a sailing auxiliary propulsion unit.
By the numbers I have from other sailing boats this system won't even allow for a 34' boat weighing 11,200 lbs with a 4' draft to run at hull speed for 100 AMPs. In that case the actual power usage to reach it's hull speed was 5.7KW which running the LOW amp numbers @ 48V comes out to like 118.75 AMPS. If you run the equation at a 50V average which is a truer average value you get 114 AMPS. So Uma weighs 13,500 lbs and we are just going to assume a like payload value for both. Highly doubtful Uma is reaching hull speed on any 100 amps or 4.8-5.2 KW. Yes, some hulls are more easily driven than others despite weighing in over what their slower counterparts weigh in at. However the curve there is razor grade thin when put on top of the basically water pumping figures which are the primary consideration with actual hull line dynamics playing a rather distant 2nd place.
Reaching hull speed is not any big deal at all though to me, nor should it be for any electric cruising sailor. Many sailboats are powered with engines that do not deliver hull speed. High performance competition boats never have an engine capable of that. That same model 34' boat with an A-4 that was not running up to snuff made 1 kt against a 35 kt head wind in a tight channel for a few hours despite only being able to muster up a 6 kt hull speed on flat, windless waters. Her STANDING hull speed was 6.6-6.7 kts. The reason that is a "standing" hull speed is because when these boats heel over the water line increases thus the "hull speed" more appropriately the hull barrier speed, increases too.
I myself would not care about hull speed and would not be inclined to push that motor @ even 80% unless it were a very pressing situation. The bottom line here is that even some really good skippers have lost their boats to decisions that turned out to be incorrect. The same can be said for far more bad skippers. All bets are off when the sea and wind conspire against you no matter what you have for an auxiliary propulsion unit. It is dealing with the limitations placed before us at many levels which is very large part of sailing.
There are in fact limitations everywhere one looks in the sailing paradigm. Limits to heel angle, limits to wind pressure on the sails, limits to motoring range no matter the auxiliary propulsion method. There are limits to anchoring depth, limits to breaking wave height, limits to electrical power, limits to an acceptable seaway distance to land when hove to. There are limits to wind and water current speeds in any direction and unlimited combinations thereof. It isn't the limits which define good sailing skills, it is how the crew handles those limits that does that. Knowing the limits and knowing where not be and when is the key to staying safe, not the auxiliary propulsion method employed.
For ME the benefits of electric auxiliary propulsion at THIS time simply do not outweigh the negatives presented. By my estimates it is by far the most expensive auxiliary propulsion methodology on the table today for the way most sail. Looking at the miles per dollar one can expect from an electric propulsion system is the perspective I view the issue from. And from that perspective there is nothing that can justify it for me today. The range limits and TRUE battery life expectancy (not those sales brochure figures) are just the icing on the cake. Savings in fuel costs are most ALWAYS used to help sell these systems. It does not matter that AND the maintenance AND the initial costs to procure a gasoline or diesel and install it. The costs of these electric systems TODAY will never on a mile by motored mile compete unless you are driving a short haul water taxi around with one OR you are a day sailor who can tie up each evening and are only a few miles from your sailing grounds and using flooded lead acid (FLA) batteries. Even with that scenario however it would be very hard to reach any savings unless you are in and out on the order of a few hundred times or more per year AND able to maintain a depth of charge level conducive to achieving the rated battery life IMHO. However doing even that mandates a slip to plug into. No mooring ball pricing for those people to birth at. Electric propulsion presents nothing but money spent to impart more limitations with the only benefit being very limited silent propulsion, until its generator time.
A diesel to drive Uma would run 8k or so. It is doubtful over that entire 3000 mile trip she would have even burned 15 gallons of fuel. Keeping this apples to apples means the diesel install would have been a DYI adventure too. If one draws the line at her motored miles on that trip on out to the rest of your time sailing her, do you really think you are going to see 1500 gallons of fuel usage? We're talking 7.5 gallons pr yr here! What was that, a two year trip? Leaving the propulsion unit out and just looking at the battery array and support gear costs of 6k BEYOND what a diesel would have run here ok? You are looking at 533 years just to pay for the batteries through fuel "savings". The truth is though that there are NO "fuel savings" because most all of Uma's reusable resource power generation is and will continue to be used for her hotel systems and that electricity for propulsion WILL come from fossil fuel sources 99.999999% of the time. That entails slip services AND/OR a (most likely) gas guzzling portable generator on deck here. It would be nice to see a diesel there though. Nearly twice the power pr gallon as gasoline. So the engine noise will be back. If you add on for what a top shelf propulsion module would have cost it's more like over 800 years of service at least to break "even" running off strictly solar and wind generation, which is never going to happen. Even a maintenance cost evaluation does little to offset the numbers here. Do you really believe those batteries are going run for 800 years without replacement? I mean let's be honest here, you have a battery array with support gear that would run what, 14K soup to nuts DIY?
Do you really think those batteries will even go 10 years without rating any degradation figures worked in? Even the Chevy Volt batteries are a pro rated warranty period. You return them and they replace the bad cells with healthy used ones from returns to get past the warranty period. What happens when the warranty period expires? I have noticed that some of these lifepo sellers pro rate the product and if one goes down will charge you like 800 USD to replace a 1000 USD battery after 5 years of service life. If one looks at the higher end propulsion unit providers like Elco and Thoosa just the motor assemblies can run more than the 8K one would spend for the diesel. And if you do the smart thing and pack a generator along, well then everything on that now fossil fuel burner drives the costs of an "electric" system further through the roof. Let there be no mistake on that last either, as MOST who get these systems will indeed be packing generators. Please note here I did not throw in the costs and issues entailed with pushing Uma around with the dink either.
The bottom line here is that electric auxiliary propulsion is a HUGELY expensive proposition from start to finish. And with swapping out batteries even at 20 year periods it only gets more expensive with time. THIS is why these systems degrade the value of cruising sailboats. But for those with money to burn its really a novelty that only put in place more limitations, especially to one's cruising budget. I myself will gladly clatter my way in and out of a slip before I ever blow money that could be going to wind vane steering units, good navionics, shapely performance sails and easy to handle ground tackle on such a limiting auxiliary drive system. But then, I am in this arena to SAIL, and that is where my money will go. -- spencer marshall at https://www.youtube.com/watch?v=IJN_7Ior16M
