Pontoon, one or two engines?

Hello,

I hope I'm posting in the right section, it seems more related to hydrodynamics rather than pure propulsion...

I'm close to have a pontoon boat built in Portugal for tours on a French river (12 passengers + pilot).
The whole boat is custom made, the two fiberglass hulls are designed by the maker.
The boat maker seems quite serious and allows me all options for engine (inboard/outboard/electric).

Electric is out of the question for my use, and I tend to always prefer outboard for maintenance.

Now comes the choice to have either one or two engines.

The boat maker gave me a quote for two 90hp outboards at the end of each hull for €34k (complete with rigging and hydraulic steering).

Now, the question that I have is if I would lose a lot of efficiency with by having a single 200hp outboard in the middle. A single 200hp engine would probably save me 10k.

Also, 90hp outboards don't come with electric command, and that's something I'll really miss.

The boat maker has engineers able to help me on this, but they might not be the most objective when it comes to price comparison, thus my request here for advice...

What would you do?

Thanks

 

There are a few things to consider. Potentially you will go further and possibly faster with the same fuel use, with a single engine, But you are dead in the water if the engine fails. If the designer/builder thinks it feasible, such that you won't experience ventilation problems with aerated water entering the prop, that can be mitigated to a degree with careful propeller selection. If the river contains floating debris, the centre engine is more likely to foul it.

 

Thanks for your reply.
I need two engines anyway by regulation, so if I use a single main engine, I'll have a small 15hp up on one side to crawl back home or go on the side if needed.

So you think efficiency is better with a big main engine rather than two mediums? That definitely would save a lot on engine price and rigging.

Debris and safety come into consideration as well as you noted, that's why I would avoid inboard engine where anything stuck with the propeller would be problematic.

 

Welcome to the Forum Yootani.
With 200 hp your little power cat should be able to go quite fast - is this a necessary requirement?
What speed do you require when you are carrying 12 passengers seated in the forward area?
Do the hulls have enough buoyancy at the forward end for carrying 12 passengers - have the Builders given you an assurance about this?
I presume that you will be operating on the Seine, as you are in Paris?

How long is the boat - I am guessing around 7.5 m?
I would definitely go for twin engines, as you will then have much better manoeuverability when coming alongside docks.
But 2 x 90 hp engines on a boat like this sounds like a lot - unless you do have a requirement for going fast when light, I would be looking at 2 x 40 hp O/B motors instead.

 

Yup, Seine in Paris
Already doing tours with regular smallish 6m boats. Most of the other operators are using 7m pontoons with usually 90-150hp, all tritoons boats with single main engine.

The boat in question is 7.3m

Yes, I can do 150hp, speed during the tours is 8-10 knots. So 200hp is overkill, only used when outside of Paris to gain 10 minutes when going inside Paris and leaving (boats are located on the outskirts).
I'm just worried about noise from smaller engines rather than bigger one running at lower rpm. As the passengers are far from the engine it shouldn't be a problem, but it's quite hard to compare.

 

8-10 knots is an awkward speed, certainly 200 hp is not going to be required, it sounds as if the boat will be running light without passengers when the boat you will be able to use extra speed, is that so ?

 

Yes, good reminder, no passengers when going full speed.
Opposite to what I'm doing now, boarding passengers inside the "high speed" area, going full speed for a few minutes, then slow for the tour inside Paris.

With this boat the passenger will always board inside Paris, max speed is 10knots, but all boats go around 8.

So yes, 200hp is definitely not required, the few seconds gained for the trips leaving and going back to dock aren't worth it. I'm just used to have 200hp on my other boats, but you're right, it's not required here.

 

What bajansailor said about the docking in tight spots, the twins would be far preferable, you can usually spin them around if required by one engine forward, the other is reverse. Gets down to what size engine seemingly.

 

Four stroke engines are very quiet really - and even if you only have 2 x 40 hp engines I doubt that you would ever have to run them at full speed, and certainly not with passengers.
Even when you are commuting in from the outskirts you should still be able to travel at a respectable cruising speed.

 

If it is light enough when empty of passengers, to plane the boat easily enough with 40 hp motors, seems little sense in going much more, the drawing suggests not a lot of weight, if accurate. 60/70 would seem about the maximum required

 

Yootani, is your boat a 'one off' build, or have the Builders built other boats like it before?
If this is a class of boat, can you post some photos of previous boats built please?
Or a link to the Builder's website?

 

Yootani,

I drove an 8 meter triple pontoon tour boat (made in the USA) for five years.
It had a Honda 225Hp long-leg on it.
I would not hesitate to recommend you go with a single.
Running empty, 18 - 20 knots @3500 RPM.
8 - 10 knots loaded @ 3200 RPM.

BB

 

They already built two of this precise model (Aqualounge). It's not clear to me if the models made are already sold, but mine would have to be built anyways.

https://nazarethboats.com/boats/aqualounge/

The company is relatively new, they were making more regular "cheap" boats before, now making pretty big houseboat, and this Aqualounge model that caught my eyes.

They only have 3D renderings on their website but their Instagram has a few pictures of houseboats in construction.

Login • Instagram https://instagram.com/nazarethboats

Base price without engine for this model is around €75k, but as they are making them one by one, customization is fairly easy.

 

Light displacement according to the quote they sent me is 1800kg.
3000kg fully loaded.

 

OK, well I would say you need 2 x 70hp at minimum, to be assured of planing along at say 20 knots lightly loaded. 2 x 90 if loaded. If you are prepared to forsake that ability, (plane) you can go to much smaller engines.