Sizing engines for a cruising catamaran

[StuM]

Originally Posted by Rotten Ricky

Regarding using both instead of one engine, the general rule is that a given vessel will use up approx. 1.8 times the amount of fuel of one engine at a given speed and under given conditions. I was surprised when I discovered it, but it has been proven in trials. Your case of going from 7 to 8.5 knots bears this out. It appeared to be the same as when my tri had two outboards.

I find that it actually varies with % of WOT RPM. Below 70-75% WOT, a single engine will be more fuel efficient for a given speed. Once you need to push your revs beyond that, fuel consumption starts to rise rapidly and switching to both engines will maintain the same speed with less total fuel consumption.

[a64pilot]

Originally Posted by StuM

I find that it actually varies with % of WOT RPM. Below 70-75% WOT, a single engine will be more fuel efficient for a given speed. Once you need to push your revs beyond that, fuel consumption starts to rise rapidly and switching to both engines will maintain the same speed with less total fuel consumption.

That may be a function of how the propellor can absorb and transmit the power, cause I don’t think it’s typical of a Diesels BSFC curve.
Usually about half power output in most efficient, more does go down in efficiency, but not cut in half.
Prop probably can only take so much before it get inefficient?

[Rotten Ricky]

Do you have a choice of engine brands? Go with a Kubota-based engine like Universal, Beta or Westerbeke if you want better availability of parts at a cheaper price. Just ask a Yanmar or especially a Volvo owner for the price of their parts! Same basic Kubota engines are used in industrial plants and tractors etc world-wide. Original engine purchase price should be less too! Also consider going up one size in transmission - don't take one just barely within the top of the range suggested.

RR.

[Zzmeyer]

Volvo and Yanmar are two choices. yanmar is an expensive option. I’ve had yanmar for more than 25 years without a single problem. I don’t know any production builder that offer the engines you mention but I’ve heard they are very good. I ordered with Yanmar but dealer believes Volvo has solved their recent problem and I have some time to wait and see.

[Zzmeyer]

All engine manufacturers publish fuel consumption curves. What causes fuel consumption to go up is the amount of energy needed. You need a lot more energy to go 8.5 knots than 7.5 knots for a 45 to 50 foot boat. To go 7.5 knots requires the same amount of energy whether you use one or ten engines. Therefore the question is whether two engines each generating half the energy are as fuel efficient as one at twice the power. For the Yanmar it is very close. Comparing two engines going 8.5 knots with one engine going 7.5 isn’t fair. It seems to me that the principle reasons to use one is how hard you work the engine. Yanmar recommends 80 percent of max continuous rpm; less total hours on engines; less frequent engine maintenance and being quieter. Compelling. Offsetting is boat torque due to asymmetrical thrust. I’m just concerned whether that is a legit concern.

[bouncycastle]

Most cruising Cats do need more power than a many Cruising mono hulls for many varied reasons .

Bear in mind two engines do not do much to improve speed they mostly impact on thrust

However in mathematical terms a single two bladed prop of correct dimensions will return better fuel consumption and thrust than two smaller 2 bladed or three bladed props .

Simply put a single ~100 BHP engine with a single 2 bladed prop will use less fuel than two ~50 BHP engines connected to two separate smaller props

( Hull interference issues can upset the best props and throw these numbers out the window )


All props will have a ideal speed that returns the best efficiency and the best case
efficiency for props will be ~60%- to ~70 %

Most folding props will return less efficiency than their fixed prop types will


Most All engines will have some sort of ideal RPM per percentage that suits their best fuel consumption and life span ratios .
These numbers tend to be in most engines around the ~60% to ~70% rpm regions

Mono hulls tend to have props much deeper down in the water that can often ensure less issues with waves causing aeration effects to cause props to loose their grip

Cats props tending to be closer to surface tend to be more impacted with waves and aeration issues causing props to lose their grip


On one 37 foot Prout with single on z drive mid ships 35 BHP (old ) Yanmar 3 pot engine 3 blade prop it did best case ~ 4 knots at ~2000 rpm and used 5 liters an hour or 3 knots with 10 to 15 knot head wind with ~2000 RPM in slightly choppy seas Roughly ~4 NMPG in calm or ~3 NMPG in same slightly choppy conditions

Top speed in calm would be less than 7 knots
The propeller was probably sized to operate at its best in the 4 knot region

The lack of depth of prop hammered the prop with choppy seas .
The windage from going into the wind hammered the speed .

That same size modern Cat boat would nowdays normally get ~60 BHP or two 30 BHP engines

A larger engine turning a bigger prop sized to give best thrust at 4 knots would probably be able to drive the boat at ~4 knots better using the same amount of fuel with the choppy sea where the ~35 BHP would lose 25% the speed and add 25% more fuel per nautical mile

The boat would probably require nearly double the fuel consumption to use the two 30 BHP engines to drive it at 4 knots in choppy conditions but would probably perform in calm conditions with one engine doing ~4 knots using less fuel as the props would be in deeper water

However if that extra power was then used to drive boat faster the prop would suffer and increase fuel a lot and speeds would drop when going against the wind in slightly choppy waves as in going from 9 knots in calm dropping to 6 knots in choppy conditions

Often the best for cats is to consider to have two engines of different powers to suit the needs
For example one engine of 40 BHP and another of 60 Bhp can give flexibility

The ~40 BHP engine at 70% power in calm conditions might drive a 40 foot cat at 4 knots and give maxim fuel efficiency for long journeys in calm conditions .
If the conditions were choppy with head wind the switching over to the 60 BHP and turning of the 40 BHP would return the best speed probably 4 knots in choppy seas but increase fuel use

In all out war with nature the two engines together would flat out probably return enough speed to fight the strongest winds such as needing to get into a port in bad conditions


other alternatives can be to drive one single larger prop on one hull deep as possible through the use of hydraulic drive using two engines combined

One 55 mono hull deep long keel with 110BHP motor in the Mediterranean left port in the lee of the island of Sardinia . A 100 MPH mistral suddenly arrived and it took them 24 hour of tacking with storm sails and engine flat out to return back to the marina a few miles away even they were in the lee of the island .
Again the problem for them was the windage made it impossible to motor directly against the wind without to have at least ~200 BHP.
The boat was barely making ~0.5 knots against the wind and the strong gust would stop and lose steerage and lose ground.
Again it would require the prop to be correctly sized to give maxim thrust at three knots to give sufficient steerage with the short choppy seas . .
The deep mounted prop was still prone to aerated water from the choppy seas and made steerage speeds to slow to go against the strong winds using only the engine
They estimated from the GPS they made at best 100 meters per 10 mile tack engine on full tilt and very reefed storm sails up

In the Mediterranean due to the fact the top two feet of the water moves with the wind and the sees are short and choppy you double the BHP again than a boat in the large swells of the Atlantic oceans

So the power equations have to be worked out for the regions for the boat types and the cruising distances

if money is no object its probably best to have two engine which has all power diverted to a electric generator .Then drive three props two in the hulls and third prop in a z drive off the center of the boat .The Z drive prop could be fixed type made to turn best at ~3 knots .The under hull types could be smaller folder props more suitable for marina manover or extra thrust in choppy conditions or for cruising in calm conditions at higher speeds like 6 to 8 knots .
Then a battery bank is also included and it could boost the total power for short duration to all props such as extreme manovers getting into in harbors in extreme strong winds .

Hope that helps all the boaters out there

[mikereed100]

Originally Posted by Cheechako

Why would a sphere have less wetted area? The shortest distance between two points is a straight line.

True, but your drawing is flawed. Sure, if the depth is equal, the vee hull has less wetted surface, but depth is dependent on displacement. If you redo your drawing with equal areas inside the vee and semi-circle, representing equal displacement, the vee will be depressed quite a bit more in the water, and will have a greater surface area. Simply put, for a given displacement, the semi-circle is the form with the least wetted area.

[mikereed100]

My 46' 9 ton cat came with 22hp Mitsubishis. I never really got a chance to motor it before tearing it apart and rebuilding, but the PO used to get 2 knots on one engine. I took the Mitsubishis out and put in 29 hp Nannis which are adequate, but I wish they were bigger. We can get 5 knots on one motor in ideal conditions, but the slightest headwind or chop messes that up. We can do 3 knots into 35 knots of wind with both motors in flat water. On the upside, we only burn 1.75 liters/hour and the engines weigh about 300 lbs each, but if I were to do it again I would go bigger.

[valhalla360]

Originally Posted by DotDun

To be honest, I don't know. A boat floats because it displaces water equal to it's own weight. Hence, more weight equals more hull in the water no matter the shape. More hull in the water would equal more wetted surface, I would think.

I've seen people make reference to a cat having more wetted surface, but it has never made sense to me.

Let's do an extreme example:

- A square hull 10' wide 10' long and 10' draft: That's 5 sides and each side is 100 sft...total 500sft of wetted surface and 1000cubic feet of displacement.
- A 1ft draft hull, 1' wide and 1000' long. Sill 1000 cubic feet of displacement but 3002 sft of wetted surface (more than 6 times as much).

Obviously, no one is going to build a boat like this but it demonstrates that shape can have a significant impact on wetted surface.

A monohull is closer to the first example and a cat is closer to the second (though both are much different from the examples)

[valhalla360]

Originally Posted by Cheechako

A spherical shape has more wetted area.
A spherical shape holds more volume, but at the same loading would ride a bit higher in the water right? Is that = to the straight line wetted area due to riding higher?

Your first statement is wrong assuming the same displacement for both hulls. The picture someone posted showed a v-hull with drastically less displacement.

A sphere is the shape that holds the most volume per unit of surface area of any shape. The complication is when you use it as a hull, you need to define how deeply it sits in the water and take into account that the surface area above the water does not add to the surface area. I'd have to break out the calculus but I believe around 50% submerged would be the ideal spherical hull draft to minimize wetted surface.

[valhalla360]

Another difference...particularly if comparing to a heavy full keel mono is the mono carries a lot more momentum punching thru waves. So every time the mono punches thru a wave it loses much less speed and you don't need as much power to get it back up to speed. A light catamaran doesn't have the same momentum, so you can lose more speed and you need more power to quickly get back up to speed.

Windage is definitely a bigger impact compared to a low slung mono.

[Rotten Ricky]

Originally Posted by Zzmeyer

All engine manufacturers publish fuel consumption curves. What causes fuel consumption to go up is the amount of energy needed. You need a lot more energy to go 8.5 knots than 7.5 knots for a 45 to 50 foot boat. To go 7.5 knots requires the same amount of energy whether you use one or ten engines. Therefore the question is whether two engines each generating half the energy are as fuel efficient as one at twice the power. For the Yanmar it is very close. Comparing two engines going 8.5 knots with one engine going 7.5 isn’t fair. It seems to me that the principle reasons to use one is how hard you work the engine. Yanmar recommends 80 percent of max continuous rpm; less total hours on engines; less frequent engine maintenance and being quieter. Compelling. Offsetting is boat torque due to asymmetrical thrust. I’m just concerned whether that is a legit concern.

I should have understood your info more clearly. Yes, it takes a lot more power to get a boat of appropriate size from 7.5 knots to 8.5 knots as it tries to climb uphill and over its bow wave, than it does gaining that knot at lower speeds. Fuel consumption increases exponentially with speed.

However, I think the case still stands correct that two engines propelling a boat at a given speed, and under the same given conditions, will use 1.8 times the fuel of one of the same engines on its own. It also speaks to the low efficiency of energy conversion of the internal combustion engine with all its losses along the way. Most of the energy (fuel) of the second engine goes into just making the engine itself run.

RR.

[Rotten Ricky]

Originally Posted by Zzmeyer

Volvo and Yanmar are two choices. yanmar is an expensive option. I’ve had yanmar for more than 25 years without a single problem. I don’t know any production builder that offer the engines you mention but I’ve heard they are very good. I ordered with Yanmar but dealer believes Volvo has solved their recent problem and I have some time to wait and see.

I know that for many years Catalina Yachts, one of the biggest and most successful sailboat manufacturers in the world, used Universal (Kubota) diesels in their sailboats, but after taking a quick look at their website just now, it seems they are now using Yanmars. I wonder why the change. Was it the result of performance and reliability issues? Was it the result of a decision made by the accountants cos they could buy Yanmar at lower prices, and make more money on Yanmar parts? Perhaps too many people were finding their equivalent Universal engine parts cheaper at the Kubota tractor dealers! Maybe Stu Jackson could tell us when and why the change was made?

RR. (PS. I used to be a Catalina 30 owner for 22 years.)

[contrail]

I think it's really pretty simple. Decades ago, a sailboat (typically a mono) had a relatively small engine that was generally pretty well loaded - good for the engine, bad for top speed, particularly against the wind. As people became more engine dependent, they demanded larger engines that could "get them home at hull speed against weather". The manufacturers and sales people were happy to oblige, and the buyers were happy with their hull speed against weather. But most of the time, the engines were not loaded sufficiently, so it was bad for their life expectancy.

A big advantage of a cat, with large engines, is that you really can cruise on one engine, rather than two. Apart from stretching maintenance intervals by using one engine only, it means that you can go at a slower speed whilst still loading one engine correctly, or a faster speed with two. So, more efficiency, longer maintenance intervals, longer lifetime and better loading are the benefits.

A big benefit is the redundancy, and the higher total oomph and the safety these provide.

Additionally, the great power available makes maneuvering in tight spots with high winds much easier. I can spin in place with the mainsail up if I have to. This is handy in mooring and anchoring fields, for example.

With regard to actual numbers, my Leopard 45 (1999), with somewhat oversized Flex-o-fold props, will do about 7.1 knots at 2500 RPM. With two engines, at the same speed, I will get around 8.3. To get that 7.1 knots with two engines, I need about 2050 RPM.

With just the port engine running, the prop walk cancels the asymmetry just about perfectly. I can go long distances, dead straight, with no autopilot and no one steering. To my huge surprise, on the starboard prop only, where the asymmetry and prop walk are both working in the same direction, and thus considerable rudder is required, my speed stays the same or drops .1 knot but no more. I would have thought it would be a much larger difference, but it is not. The only liability is that either a human or the autopilot needs to actively steer.

If one engine is out, and it's important to maneuver, the easiest turn is the one toward the powered side, not the reverse. This is done by putting the engine in reverse, which makes a really tight turn, and then going back into forward with the rudder hard over. Do it several times. It's like turning "into" your prop walk, on a monohull, rather than with it. That's how you turn really sharply in a marina fairway, for example. It's intuitive to think that turning in the same direction as the prop walk makes turning easier, but it causes a much wider turn.

In the case of my boat, the Flex-o-fold's save about a quarter knot each, at normal cruising speeds, for a total of half a knot. Your mileage may vary. While I have nothing good to say about the company nor their US representation, I do love the Flex-o-fold props. I have had Max Props, as well, and whilst I did like them, I prefer the folders.

The least surface area for a given volume is, indeed, a sphere. The typical U shaped hulls of a cat are a blend of the sphere, on the lower part, and a box, up above, This compromise keeps surface area down whilst increasing load carrying volume.

In summary, I always recommend the smaller engine for a monohull, and the largest for a cat.

[contrail]

One other significant advantage of Yanmars, compared to some others, is that it is not necessary to use a glow plug to start. They will start immediately. You only have to be in urgent need of an engine, one time, while you are waiting on the glow plug, to appreciate the safety of not needing to do this.