Been researching electric vessels. Found the Silent 55 which is pretty fascinating. I feel the Tesla of boats is not quite viable yet. But I'd like to understand some basics of power costs.
Observing fuel efficiency of motor yachts and catamarans there are so many variables that it is hard to derive the formulas for how much power is required to move a boat that weighs X and is length Y at speed Z.
I have observed that longer and heavier boats are less fuel efficient. (But larger boats would also have more space for solar panels)
Am I correct in assuming that a monohull trawler and a sailing catamaran are both "displacement" vessels? Because neither has a hull that's planing.
I've also observed that planing hulls and hydrofoil boats do not seem to get significant efficiency boosts while up out of the water, though they can go a lot faster.
So there are probably three different possible formulas:
1. Displacement power required
2. Planing power required (lets just assume a basic planing hull as efficient as was common in planing boats 30 years ago.)
3. Foiling power required
I have an engineering background and can teach myself, so feel free to point me to books on the subject. Looking for targetted stuff, though, I don't want to learn how to design hull forms... if I put these ideas into practice, I'll buy an existing boat and be repowering it. That's in my wheelhouse totally.
Thanks!
I am looking for the same thing.
I found this simple basic powerboat formula. from
The Lowdown on Boat Horsepower (And How to Make the Most of It) | BetterBoat Boating Blog https://betterboat.com/boat-horsepower/
You need 1 hp for every 40 pounds of boat to go 20 mph.
so you need 100 hp for a 4000 lb boat to go 20mph
to go 30mph you need more.
you need 160hp for that 4000 lb boat to go 30mph.
so 1.6 per 4000lb to go 30 mph.
very rough estimate but that is what you need right?
for a sailboat they dont give speed but just have a rule of thumb of "An old rule of thumb says that for every 2,200 pounds of weight (AKA every Metric ton), you need four horsepower."
now just convert electric to hp and you get about
1kilowatt (1000watts) = 1.3 hp
or one gas hp = approx .745 kw (745 watts)
lets try that on the silent 55
which is 1.8 tons and goes 5 knots at 2 kw of electric motor power.
first lets use the motorboat formula
2 kw is 2.6820441777 gas hp
5 knots is 5.7 mph
1.8 tons = 3800 pounds
so lets rough our boat to 4000 pounds and say to go 20 mph it would need 100 hp like the formula said
but its going roughly 1/4 that which wont scale linearly but says it should need 1/4 the hp or 25 hp to do that but the silent 55 is only using about 2.68 so clearly that doest work at that high speed.
lets try the sailboat rule of thumb.
for every ton you need 4 hp. well they are almost 2 tons and using about 2.6 hp.
so clearly those rules of thumb really fail for the silent 55 which does much better.
I think the big factor is the MUCH slower speed. if you idle along at a slow speed a boat can use barely any hp at all and 5 kt or 5.7 mph is pretty slow.
now the electric boat xzells mentioned has this infor translated from german on their youtube video
"
In the meantime we have covered over 6000 km and have been totally supported by our new propeller. It runs at almost 15% more effectively. Today's video shows us on the day trip from Hiddensee to Wiek. Here we become aware of a drone whose pilot we discover in a bay. When asked about it, we got the great video. Thanks a lot for this. It shows how we continue from the very shallow bay towards Hiddensee. The transition from shallow to deep water on the stern wave is very nice to see. First bubbling (flat water) then the transition to a smooth longer wave (deep water). From here we are traveling at around 4.5 KW and 14 km / h. Electric boat, solar drive, solar boat, solar yacht, electric drive, 48 volt, electric drive, solar houseboat, the quiet boat, electric boat, solar boat, electric drive, solar yacht, 48 volt, electric drive, Bellmann, Bellmarine, Mastervolt, Kräutler, Kraeutler, Elektroyacht, Torqeedo. The video shows the conversion of our 'boat into a solar boat, assembly of the solar panels, installation of the electric motor and our successful test drive"
so they say they do 14 km / hr at 4.5 kw approx 6 hp at about 8.7 mph
they also say they do
" 4,5 kw beziehen sich auf den Verbrauch bei 14km/h. Meistens fahren wir mit 2 kw Verbrauch bei 10 km/h. Eingebaut ist ein 10kw Motor und die max. Geschwindigkeit liegt bei 20 km/h."
which i guess is 4.5 kw or 6 hp at 14km/hr or 8.7mph
2 kw or 2.68204 hp at 10km/hr or 6.2mph
10kw or 13 hp gives 20 km/hr or 12mph
or 6 hp
