How to work out horsepower requirement for a catamaran?

[Kit_L]

The current vessel is a 40' ply and fibreglass power catamaran. Its displacement is unknown. She carries 1,200 litres of fuel in two tanks, and the same amount of water. She is fitted with a generator in the port hull. We assume she's around 12 tonnes, but she could be more.

The boat is powered with a pair of Volvo Penta KAD-300s (285 hp), with stern drives, driving Duo-props. At 1500rpm, she makes 6.5kn using 10l/hr; at 1900 around 8kn, using ~24l/hr. At 2250, she does 9kn, using 31l/hr.

At 3250, she's well on the plane, and making 15–16kn, using 70l/hr.

I would be happy to run her at displacement speeds only, which is around 8kn. My question to the gurus is would a pair of petrol 60hp heavy duty ('hi-thrust') outboards (with the 3:1 reduction gearbox and larger props, used by oyster punts in this area) be enough to push her to these speeds?

This is just a thought experiment at this point, but I'm curious. The modern four-stroke outboards are quiet and very reliable, and being able to lift the legs completely out of the water in this area is a big, big plus. The last time the boat was slipped, I spent two hours on each engine getting all the barnacles out of the 10,000 crevices that legs have!

 

[O C Diver]

Can't answer your question, but of equal importance might be the difference in weight between the inboard and the outboard. For and aft trim is supposed to be quite important on catamarans.

Ted

 

[Pete Meisinger]

Yes, that would be plenty. In fact you could probably get by with less hp.

If you are serious why not start with one outboard and add a second one if you feel the need.

pete

 

[Jeff F]

You can get an approximation of power requirements based on diesel fuel consumption - 24 l/hr @8 knots translates into about 115 HP delivered. So I'd expect that to be about the maximum speed from a couple of 60 HP outboards.

 

[O C Diver]

You can get an approximation of power requirements based on diesel fuel consumption - 24 l/hr @8 knots translates into about 115 HP delivered. So I'd expect that to be about the maximum speed from a couple of 60 HP outboards.

20 HP equates to about 1 GPH for a diesel engine.

Ted

 

[Kit_L]

Thanks for these excellent replies. Re. fore and aft trim: if I ever did this, I'd build pods aft of the transom to mount the engines on, and even though the outboards weigh much less than the diesels, they would be ~3' further back. The boat presently runs level up to 8–9kn (fuel tanks are amidships, too. And I could use the house bank for trim if needed, too, I think.

 

[DennisB1]

If you know some one with the type of engine it would be interesting to see how fast a single engine could tow your boat in flat water. If would give you an idea of what it could do. You would do better with the other drives removed which would reduce drag and improve performance when mounted directly on your boat. Let us know how it works if you try.

 

[Kit_L]

@DennisB1: not practical to do in my situation, and the legs can only be tilted, not lifted out of the water, so both sets of props will be dragging.

More to think about; the high thrust outboards I am considering have 3:1 reduction gearing and significantly larger props that the standard 60-70hp outboards do, as mentioned. This aspect makes manoeuvring the boat easier, and are supposed to be more efficient up to displacement speeds, but top speed is reduced.

 

[O C Diver]

If I were considering doing this, there would be two other points I would consider.

One would be propeller diameter. While you can get away with a smaller prop as you will be traveling at a much slower speed, as your props slip more, you may find the engines running at a higher RPM than expected. You may also find that traveling into strong winds and moderate seas may greatly reduce your speed as the props slip more.

The other consideration would be to examine the HP output curve. While these are newer designed outboards, some of the older designed ones didn’t develop meaningful HP till over 4,000 RPM. The consideration is that you might reach a speed where the engines can't develop enough HP to get beyond that RPM to attain your desired speed. Basically the boat would be over propped. Flattening the pitch might allow you to get past that point but have the engines always running at a very high RPM at desired speed. Understanding the HP curve for these outboards will be very important!

Ted

 

[rslifkin]

I don't think this would save any fuel. You'd be going from diesels to gas, which would likely more than offset any gains from the smaller engines.



Personally, I'd be more inclined to convert to shaft drive than outboard if unhappy with the stern drives.

 

[Kit_L]

@rslifkin: I have tackled some projects in the past, but converting from stern drive to shaft would be the biggest, by far. Shafts and rudders and associated gear would have to be fabricated from scratch.

I want to wait until we slip this year, to see if there's any water in the legs (there was late last year, and that's why we replaced the top and bottom seals). Re. fuel cost: because we harbour hop at present, and because we run at 8kn most of the time, fuel use is modest.

@O C Diver: if I did do this, I would be using the modern "hi-thrust" outboards that are designed to move heavy punts and barges, so should be suited to this application.

Where we are moored (home base) is just a breeding farm for barnacles—this is my main worry. And I can clean the props from the swim platform, and I would have to dive on the boat to do this if I went with smaller inboard diesels (something else I have considered) and shafts.

Thanks for everyone's thoughtful replies.