I’m a contractor working for a maritime-based company that is looking to retrofit an old, combustion-powered vessel with an electric motor. One of my tasks is to find a suitable electric motor that will allow a ~150’ OSV-style vessel to achieve 5-7 kts through the water.

From my research, there doesn’t seem to be a neat-and-tidy way to compare power and speed, so I was curious if there is a good way to estimate the wattage / horsepower needed for the application described above? I understand that it is dependent on weight; what other variables do I need to consider? Is there a rule of thumb for HP needed based on weight of the vessel?

Any and all help is greatly appreciated! I hope everyone had a good holiday weekend.

Are you going battery powered or just re-powering from a propulsion diesel to diesel-electric generator/motor combination?

As far as horesepower appropriate for the vessel size, someone’s already figured that out for you…what is the horsepower of the current propulsion system?

Thanks for the quick reply. We will be primarily battery powered, with a diesel generator on standby for redundancy (tied into the propulsion system to power the EPM).

The company hasn’t purchased the vessel yet, but even when we do the rated horsepower likely won’t reflect our needs. For example, say a vessel that we are looking at is rated for 2100 HP in order to make 21 kts. That’s ~14 kts of speed that we wouldn’t need / associated power that goes along with that. We are trying to minimize the electrical power that the EPM will need / save on cost.

The company is research-focused, so the EPM will only be used intermittently, as the vessel will be on station, DIW for extended periods of time.

Interesting. You’re probably going to be looking for a formula based on tons of displacement and maybe hull form.

For a general baseline though if you are able to get any of the engine performance data and maneuvering/transit data from a proposed existing vessel you should be able to extrapolate the horespower required for the speed range you are looking for as a percent of installed rated HP.

Really searching the memory banks here but I seem to recall “the propeller laws” which said shaft speed was directly proportional to ship’s speed, square proportional to fuel consumption, cubic proportional to engine power.

This article which is about emissions talks about this relationship:

“In the rule of thumb, the engine power output is a is a third power function of speed. When the ship reduces its speed by 10%, engine power will be reduced by 27%. When sailing at a lower speed, at the same distance, the sailing time will be longer and the energy required becomes reduced by 19% (quadratic function).”

Yeah, I figured that a ratio comparison would probably be the answer. One reason I was looking for a formula / rule-of-thumb was to aid in the ship selection process. Thanks again for your help.

Thanks for the article Hawes, I’ll take a look. I have a feeling this is going to be a lot of “back of the envelope” calculation given that we don’t have vessel specs yet.

Lots of factor comes in when deciding on power requirements for a ship/boat, incl. weather and sea conditions were it is intended used.
Formula for sea speed in calm conditions can be calculated, but for more accurate results, usually tank testing is carried out.

Here are some info that MAY do for now:

Professional help from a Naval Architect may be advisable if you have the budget.

I wouldn’t recommend the whole article for your concern, I just posted the link to show where I obtained that quoted portion.

The important part for your inquiry is that quoted section. As it is a rule of thumb, I’m sure it makes certain assumptions and simplifications so take those into account if you use it.

A 150’ OSV should be able to do 8-10 knots at full throttle when fully loaded. You should use an electric motor equivalent to whatever the vessel is already powered with unless your own is to run empty all the time then you can cut back on power some.

Rgr, the 2100 HP / 21 kt was an error, I was taking that from a 100’ vessel we are looking at. Mainly I was just trying to illustrate that the rated horsepower likely won’t reflect our needs. If we find a vessel rated for 8-10 kts, that will make this process easy, as the KW → HP conversion is straightforward. Since we’re so early in the planning process, I am really just going by the given power and speed ratings that are posted by the owner to get an idea.

The vessel is posted here: 29m Crew Supply Research Boat. It’s not exactly an OSV, but a supply / research vessel nonetheless. I suppose it could be classified as a fast supply vessel.

After conversion the company is going to be exploring renewable energy and carbon capture technology at sea. The vessel is going to be DIW for extended periods of time, so a goal is to be exclusively battery powered, as the EPM will only be used intermittently.