Efficiency of Baglietto Ischia 80 or similar Vee hulls at low speeds

Hi there,

I have been obsessing over buying a somewhat cheap but otherwise good-condition boat and converting it into an electrically powered boat by repurposing car parts. This has been done before, and I have done so much research by now that I have most of the plan laid out, as well as the support of skilled engineers and technicians. The boat would be retrofitted with about 240–320 kW of batteries (at around 1.6 metric tons / 3,500 lbs), with two efficient motors offering 320 Nm, 160 kW at a wide RPM range, most likely reduced with a 2:1 Gates Poly Chain GT Carbon to keep the efficiency at a peak around 200-700 propeller rpm and double the torque. They would be paired with two two four DC gensets, effectively forming a serial hybrid, based on partially marinized 1.9 TDI or 1.6 Multijet engines, coupled with the same EV motors (for simplicity and redundancy, as the motor with inverter weighs only ~60 kg and costs ~600–800 euro). All this would be relatively cheap and somewhat easy to do, so that part is not really a concern here.

The concern is which boat to use for this conversion project. A trawler would be ideal, given its very low power demand, but trawlers are too expensive and just not really my style. I do love, however, the design of the 1970s–1980s Vee planing and semi-displacement hulls, especially those made in Italy. Not only are they typically in good condition, they have beautiful interiors, are typically made of aluminum and are cheap — 70–100 k euro, likely even less after negotiation.

Specifically, I have my eye on the 1980s-era Baglietto Ischia 80, three of which are now on the market. They are 55–60 ft, ~20 kts cruising speed, ~30 kts max speed, typically 2 × Detroit Diesel 8V-71 650 hp, 55 k lbs.

The problem is, these are Vee / modified Vee / warping planing hulls, while I would naturally be aiming at cruising at lower speeds most of the time (7–10 kts) to take advantage of those ~240 kW of batteries — planing would likely drain them in a matter of 2–3 hours (a guesstimate).

It’s obvious these boats were not designed for displacement speeds. However, most people I talk to immediately reduce the conversation about efficiency to their big 600+ hp, high-RPM diesel engines, whereas what I am after is understanding and estimating the actual hull behavior and efficiency at those lower speeds. For example, theoretically speaking, if you downgraded the engines and the propeller pitch to aim at these ~8–10 kts cruising speeds and lower RPM, what effect would that have on the fuel consumption and boat behavior? It confuses me even more since I did find other boats with similar specs and hulls with same engines and slightly less hp that quote almost impossible fuel consumption numbers at much lower cruising speeds, like this Chris-Craft here at 3.5 gals/hour and 8mph(!): 1974 Chris-Craft Roamer Flush Deck MY in Englewood, Florida https://www.popsells.com/boat-for-sale/1974-chris-craft-roamer-flush-deck-my-302608?showfull

Which brings me to this set of questions:

1. What would actually be a realistic fuel consumption at those 7–9 kts displacement speeds, assuming the most efficient engine and RPM/propeller pitch for them? What would be its estimated power demand, in kW?
2. (nevermind)
3. How uncomfortable would it be to use such a boat at these speeds most of the time?
4. How do those hulls behave at low speeds, seafaring-wise?
5. Is that Chris-Craft example above a complete outlier, or are the numbers stated actually in the expected range for this boat or similar ones?
6. Would adding an aft extension plug to reduce wave-making resistance and drag help in any significant way?
7. Bonus question: would adding skeg/guard for propeller protection effectively kill planing performance?

I will appreciate your input here — I need someone’s executive knowledge to help me decide whether or not to obsess over this any further

Some photos of the Ischia 80 hull below:

 

Hi! Nice boat .

 

Vee bottom boats like that roll a lot until they get up on plane. Also, you are talking about maybe 1.5 hour range at 8 knots = 12 miles. There is nothing wrong about it if you only want to go for a very short cruise inside the harbor. Also, you don't state why you want electric. A proper installation will be expensive. Automotive parts may be adapted, but need to be properly designed and built; particularly when it comes to safety. The most important issue, is what is your budget. If your plan is to save money, it is a bad one. A pair of small diesel engines, plus the fuel for a decade, would be cheaper.

 

Isn't this typically solved with stabilizers in this type of hulls? And isn't the boxy aft supposed to mitigate the rolling as well?

This would mean the engines would be consuming 160kW combined at those 8 knots, while a typical, efficient 60ft/55k lbs full displacement hull would consume around ~40-45kW, per my findings? So a planing hull at displacement speeds would be over 4 times less efficient?

I would need to be able to cruise at least 4h, maybe even 5h out of this battery storage for this to make sense. This would allow for some island hopping or cover small distance from port to port without using any diesel at all.

I want it specifically because I can make it cheap. There's a lot, A LOT of research done on repurposing the EV components (e.g. openinverter.org wiki http://openinverter.org/) with countless of projects done, while me and my family have the skill to get this done well and safely. So together with ~7-8kW of solar at very low prices (~100 USD at >715W today!), you get free charge per every 4-6 days to cover some mileage — the more efficient the hull, the longer the distance.

Respectfully, I am only trying to find the actual answer on the power demand of those hulls at those lower speeds and, if at all, how much can it be improved with an extra extension. The financial and technical feasibility of this project are already sorted out, I just need to find a suitable boat. My spending 25k euro for this conversion is a realistic estimate.

OK, but based on what you just said above, small (low hp) engines wouldn't meet the power demand of this boat, would they? I am confused, tbh.

 

* The diesels are equivalent to the power you claim is necessary. If the power is too low, you need to get back to the power calculations. It is irrelevant where the power comes from, so 1kW = 1.34 HP.

 

OK, I see the confusion here. The 160kW is the max output the motors I would be using for conversion, I fail to see how what I wrote could be misleading. The power demand at cruising speeds is exactly what I need help with, literally in the subject of this thread.

For the record, these motors are extremely efficient, at 92-96% efficiency in almost all RPM range, so it's safe to assume the power demand at propeller would be close to power drawn from the batteries. Which is why I am trying to figure the hull power demand out, to have a rough idea of what realistic range would I be getting with those 240kW batteries.

That is true, but I would be using the DC gensets with this setup, so the actual range would be way larger. The 240kW is the maxium of the "free" solar energy I would be getting in this setup for short-ish passages: say I start the a with a full bank, then after X hours at 20% charge (48kW left in banks) start the gensets to generate extra 48kW and eventually end up using exactly 48kW extra to finish that trip, I would still have used 240kW of free energy for that passage (and generated 48kW extra).

Besides, during a daytime passage, the solar panels themselves would continue to provide ~5-6kWh of solar energy, so the range would be actually be *higher* than what the bank's 240kW suggests.

No, not really, not in 2025: Most powerful solar panels 2025 — Clean Energy Reviews https://www.cleanenergyreviews.info/blog/most-powerful-solar-panels . 450sq ft is now a bit less than 11W with 26% efficiency. That's 40% more power per sq ft than what you suggest.

Not to mention the ability to use flexible solar panels strategically for the extra 1-2kW.

No, the system I designed sets the constraints, mostly with the current battery density and the solar panel performance. I just need to find a hull that fits the criteria for minimum viability of this project.

Let me put it this way: I spent ~4 weeks going super deep on all components, with diagrams, plans, cost breakdowns, etc. laid out. The technology, the software and hardware is at my disposal, so is the necessary skillset to build and set this up. All I need is a boat with a hull efficient enough to allow cruising those short 50-70 miles effectively for free, every other week or so, which is basically what boating in this part of the world is. And I would still have the capability to do longer passages with DC gensets. This would be a live-aboard, for the record, not a recreational twice a year thing.

So all I am asking is for help in estimating the power demand of a boat like Ischia at ~7-8kts and, possibly, for suggestions for other boats of similar type that could offer a modest 40-50kW power demand and possibly make more sense, e.g. semi-displacements. Think Browards, Chris Craft, etc.

I think you underestimate me a bit here.

 

1 .The power requirements at particular speeds is what you asking for . Is 25t real weight ?
2 . Perfect setup isn't mono or classic monohull.
3.?
4.I don't know.
5.May be little differences making huge changes.
6.Yes.
7.No

 

Probably the cheapest way to answer your question number one , is use of cheap outboards with variety of props , dismantling original props of course.It can be done before you pay for the boat.

 

The hull speed is what you are looking at, the point at which it starts to climb the bow wave and require huge horsepower but it needs significant reserve.
Roughly 10 kwh per litre of fuel,
Lets call the planing hull semi displacement at 8knots and 25t average for a 50ft waterline.
7 to 9knots per hour pushing 25t with quite significant windage so bow thrusters twin props included, 600hp you mentioned /447kw is in reserve. 1 litre 10kwh of energy. A semi displacement 15 litres [3.9gal] of fuel p/h is approx 150kwh so for 4 hours 600kw consider variability of tide, wind, chop.
However 400hp semi displacement 37 to 90+litres per hour, 370kw to 900kw per hour around 8knots.
A 200hp outboard 15 to 75 litres per hour, [marine diesel 20 to 50] 200 hp may not be safe in testing conditions.

160 kw is 217 hp
150kw x 4 hours = 600kw

The bow overhang does nothing particularly useful at 9 knots except swell head on and a dry large foredeck, stabilisers are often used at these speeds under way or at rest. Let me know if this makes sense to you, I'll delete this response if you aren't interested, cheers.

 

I appreciate the response, this is actually sound logic, to reverse-calculate the the power demand based on the fuel consumption. However, I believe you got the diesel energy density wrong: the BSFC of modern, low rpm, efficient diesel engines is ~190-210 g/kWh, i.e. about 0.24l/kWh, so 1l is roughly 4kWh — and that's with most efficient engines available. Or am I missing something?

This is very helpful, I appreciate your input!

 

My idea with outboards wasn't very smart. Find a boat with properly functioning engines, go for a short cruise, and you can tell more about the power requirements.

 

It is, 25-30t, depending on the version (some where slightly longer).

Sorry, I did not express myself clearly. What I meant was how uncomfortable would it be to use such a boat at these speeds most of the time?

I appreciate your responses.

Yup, that's the end goal. I also contacted some of the current and past owners of these, hoping for some input.

 

Found some additional photos of a 1988 hull, this one was made of fiberglass by subcontractor. Doesn't look like a Deep Vee, after all, more like a modified Vee?

 

With my limited understanding of hydrodynamics, deep vee is beneficial for displacement speeds at sea . I will even say that high angle vee would be the best for low resistance on the wavy sea. Look at indigenous people designs.
Due my poor language I misunderstood phrase "aft extension plug " . Those hidden torpedo launchers?

 

You are referring to the efficiency of your genset not your power output which appears to be half what you may need, an underpowered large vessel is an unreasonable risk because it is very weather dependent. The required 600kw/4hours refers to a 200hp/160kw output. It suggests you need to double your battery capacity and use 4 electric motors of the type you describe I think, cheers.