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Old 02-07-2018, 14:03   #46
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Re: Sizing engines for a cruising catamaran

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Originally Posted by Rotten Ricky View Post
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.
Yanmar would disagree with you. To do 7.5 knots on one engine running at 80 percent rpm (Yanmar recommended 2400 rpm on the 57 hp engine for example) you will generate 20 kw of power and burn 6 liters per hour. Start the second engine and throttle the engines back to 1900 you will generate the same 20 kw of power which should push you at the same speed. You will burn 3.2 liters per hour per engine or about 6 percent more fuel. However, my question relates to the inefficiencies of asymmetrical power. For example, do I have to turn the rudder and increase drag running one engines? To me there are advantages to running one engine which have been discussed here. In particular, if you want to slow down you can run the engine closer to the recommended rpm if you run on one engine. If you run on one engine 50 percent of the time, average 200 hours per year underpower, and keep the boat for ten years you will have 1500 hours instead of 2000. That’s ten oil changes between the two engines and several impellers etc.
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Old 02-07-2018, 14:37   #47
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Re: Sizing engines for a cruising catamaran

As to wetted area, the circular hull section has the lowest skin area for a given volume.

It is easier to think of this by starting with each station of the hull (Stations are one of the theoretical spacings that designers use to draw hulls). Each station will have an underwater area. This area multiplied by the station spacing gives you a volume that floats the boat. Designers like to reduce hull surface area to reduce drag so getting more area into a station without increasing skin surface much is usually beneficial. (Not always, chines often help dynamically even though they increase surface area.)

Monohulls have better surface area to volume ratios because two thin hulls have much more surface area than one fatter hull for the same weight. So for very light winds, monos will be faster than multis as skin friction is a major drag component of light wind sailing. A short fat mono may be very fast in a drifter but will be slower once wave drag components become more important at higher speeds.

As most cruising cats motor at very low speeds we can focus the design criteria on sea keeping and better performance further up the speed range - be increasing fullness in the ends and reducing waterline beam and increasing length. All of which increase wetted surface area.
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Old 03-07-2018, 11:29   #48
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Re: Sizing engines for a cruising catamaran

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Originally Posted by Zzmeyer View Post
Yanmar would disagree with you. To do 7.5 knots on one engine running at 80 percent rpm (Yanmar recommended 2400 rpm on the 57 hp engine for example) you will generate 20 kw of power and burn 6 liters per hour. Start the second engine and throttle the engines back to 1900 you will generate the same 20 kw of power which should push you at the same speed. You will burn 3.2 liters per hour per engine or about 6 percent more fuel.
Wow - I am surprised. Perhaps we are thinking of apples and oranges here - different circumstances. I wonder why my tri, with the use of two outboards, used almost double the fuel that one would use. But I am sure that Yanmar are far more knowledgeable about this than I am, or should be. Thanks for bringing this to my attention.

I bow to you, sir! RR.
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Old 03-07-2018, 15:59   #49
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Re: Sizing engines for a cruising catamaran

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Wow - I am surprised. Perhaps we are thinking of apples and oranges here - different circumstances. I wonder why my tri, with the use of two outboards, used almost double the fuel that one would use. But I am sure that Yanmar are far more knowledgeable about this than I am, or should be. Thanks for bringing this to my attention.

I bow to you, sir! RR.
RR. I re read my post and it was a bit combative and condescending. For that I apologize. The purpose of these forums is to share experiences or opinions of which you did a great job. What I am now trying to determine is whether there are any disadvantages of asymmetrical thrust running a single engine. As a monohull owner I have no experience. My Saona will have folding props so no particular drag on the engine not running. Will there be any material rudder correction required? etc.
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Old 03-07-2018, 19:20   #50
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Re: Sizing engines for a cruising catamaran

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RR. I re read my post and it was a bit combative and condescending. For that I apologize. The purpose of these forums is to share experiences or opinions of which you did a great job. What I am now trying to determine is whether there are any disadvantages of asymmetrical thrust running a single engine. As a monohull owner I have no experience. My Saona will have folding props so no particular drag on the engine not running. Will there be any material rudder correction required? etc.
No problem and no offense taken, thank you. Sometimes one needs to be very straightforward about discussions, even pedantic, especially when written.

Regarding asymmetrical thrust I don't know much about it. However, sometimes absolutely symmetrical aspects are not so critical, often allowing better use of the interior. For instance, having a centre-board or dagger-board off-centre by a couple of feet, and even canted to one side, does not make a huge difference in performance when comparing one tack to the other, or even if perfectly on-centre. Of course this would be blasphemy for a racing sailor of any kind! On monos, I have heard of a prop being installed deliberately off-centre and sometimes at a slight angle when viewed from above, to counteract prop-walk etc. I imagine that it would affect the rudder and steerage etc.

Best wishes, RR.
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Old 04-07-2018, 17:14   #51
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Re: Sizing engines for a cruising catamaran

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Originally Posted by Zzmeyer View Post
RR. I re read my post and it was a bit combative and condescending. For that I apologize. The purpose of these forums is to share experiences or opinions of which you did a great job. What I am now trying to determine is whether there are any disadvantages of asymmetrical thrust running a single engine. As a monohull owner I have no experience. My Saona will have folding props so no particular drag on the engine not running. Will there be any material rudder correction required? etc.
Once you're moving at cruising speed there's normally very little rudder correction required. Our A/P usually shows 1 or 2 degrees.

Getting moving with one engine, you need to be prepared for the boat going round maybe 90' before you get rudder authority.

If you're pushing into a stiff headwind you'll see the rudder angle increase to the point it makes sense to run both engines.
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Old 04-07-2018, 17:24   #52
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Re: Sizing engines for a cruising catamaran

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Originally Posted by Zzmeyer View Post
Yanmar would disagree with you. To do 7.5 knots on one engine running at 80 percent rpm (Yanmar recommended 2400 rpm on the 57 hp engine for example) you will generate 20 kw of power and burn 6 liters per hour. Start the second engine and throttle the engines back to 1900 you will generate the same 20 kw of power which should push you at the same speed. You will burn 3.2 liters per hour per engine or about 6 percent more fuel.
It doesn't quite work like that. There is fuel being used just in making the engine run and in turning the prop even if it was providing no propulsion.

While using 2 motors to do the same speed doesn't use double the fuel, it does use a fair bit more. The increase would be more in the order of 30-50%
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Old 05-07-2018, 16:04   #53
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Re: Sizing engines for a cruising catamaran

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“Nothing succeeds like excess”

Get the biggest!!!!!
This!! There is not much weight difference between marine diesels (assuming the same # of cylinders). A 39hp Yanmar is 410 lbs and a 54 is 469lbs. Big difference in the trust.

Get the biggest you can fit/afford ... you can always throttle down.
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Old 05-07-2018, 16:14   #54
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Re: Sizing engines for a cruising catamaran

My experience using 1 vs. two engines is that the 2nd engine doesn't start to make a difference until it gets to about 90% of the rpm's of the first engine. We typically saw about a 1-1.5 kt reduction in speed when shutting down an engine (e.g. 2 engines at 1900 rpm gave us 8.0 kts, one would give us 7.0-7.2 kts in calm seas).

If we were making a long passage in really light winds we would try to motorsail with weather helm and the windward engine on - this was very effective. If that wouldn't work we motor on one engine for say 12 hours and then swap. Lost a knot of speed but a lot less fuel burn and one side of the boat was relatively quiet ..

We would use two when maneuverability was an issue.
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Old 05-07-2018, 20:37   #55
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Re: Sizing engines for a cruising catamaran

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Originally Posted by bouncycastle View Post
SNIP

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


SNIP

I get it that efficiency is not the same thing as thrust but Zzmyer wrote
Quote:
I would note a very good article on Yachting Monthly comparing props that there is more than 30% difference in thrust from the best props to the worst.
The point is getting a motor with ~10% more horsepower, or ~30% more horsepower may well not be as productive as changing props.


As others have noted Factor uses a fixed prop on an outboard which most likely gives the best thrust and efficiency with the advantage that when the outboard is tilted up there is no drag from the prop; something even the best two blade folding prop can not claim.


I have two long shaft high thrust 9.9 Yamaha outboards on my Seawind cat with 12" three blade elephant ear props. In flat water with no wind or current I can motor at around 7.1-7.2 knots. I have never really been caught in a situation where I had to fight high winds but have motored against 15-20 knot winds against a 3-4 knot current in 2-3 foot seas at around 5knots.


So for me the real question is how and where you sail, that is a huge factor in selecting the engine that best meets your needs. For most of the history of sailing there were no motors so as much care as possible was taken to avoid being caught facing bad conditions off a lee shore. Now a days a lot of folks motor a lot more than they sail. Just do a search on the topic of 'why do cats motor so much' if you doubt that. I have seen guys posting about having enough fuel and motors to motor 500-1000 miles in a sail boat. On the other hand there are those who sail on and off the hook. On a recent three month cruise when I returned to port I could not fully empty a 5 gallon jerry can of gas in the tanks on my boat. Plenty of sailboats may use more gas than that in a day.


One thing that struck me as significant is that charter cats may well opt for bigger engines with props to maximize top speed. When you are on a time table that makes sense. But as I have often posted the most dangerous thing you can bring on a boat is a calendar and I would rather be in port and wish I was at sea than be at sea and wish I was in port. Others may sail differently.
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Old 17-11-2018, 16:47   #56
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Re: Sizing engines for a cruising catamaran

No one mentions Glazing - the reason not to upsize a motor - especially when you have 2 on a Cat is because Diesels LOVE load - and when you have to much power and you are not loading it properly your engine is going to glaze the cylinders and that greatly shortens engine life. Properly sized props will keep the proper load to keep this from happening - and have plenty left for the big alternator, watermaker and whatever else you are running on it. There are some great articles on this done by Morgans Cloud/Attainable Adventure Cruising. The bigger motor makes the Dealer $$ but does not necessarily create propulsion which is what OP was concerned about.
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Old 17-11-2018, 18:19   #57
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Sizing engines for a cruising catamaran

Assuming the boat is properly propped with both engines running, running only one will have it overpropped, in that case glazing is unlikely as it will be loaded.
It would be more likely of course running both engines at low speed and therefore lightly loaded.
Glazing really isn’t caused by low RPM, it’s low loading that does it, or of course every generator set that runs at a constant 1500 RPM would glaze.
So in my opinion two big motors if the intent was to most;y motor on one and one use two for maneuvering, wouldn’t be likely to cause glazing, unless of course they were stupid big.
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Old 20-11-2018, 00:28   #58
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Re: Sizing engines for a cruising catamaran

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No one mentions Glazing - the reason not to upsize a motor - especially when you have 2 on a Cat is because Diesels LOVE load - and when you have to much power and you are not loading it properly your engine is going to glaze the cylinders and that greatly shortens engine life. Properly sized props will keep the proper load to keep this from happening - and have plenty left for the big alternator, watermaker and whatever else you are running on it. There are some great articles on this done by Morgans Cloud/Attainable Adventure Cruising. The bigger motor makes the Dealer $$ but does not necessarily create propulsion which is what OP was concerned about.
I noticed a 50ft cat for sale recently (and cats this size normally do with a 55hp in each hull) with 2* 125hp motors. Are you saying this is a bad thing? is there too much of a good thing? Why would someone do this? (I was thinking the more power the better)
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Old 20-11-2018, 11:47   #59
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Re: Sizing engines for a cruising catamaran

A Nordhavn 43 with displacement of 55,000 pounds has a 140 hp engine. A world cruising power boat. At best, too much engine is a waste. My 50 foot monohull will motor at hull speed in moderate conditions on 40 hp. I have 80 hp max. As conditions get nasty I generally use less to avoid pounding and head off with reefed main and staysail. As mentioned engines like to be run under load. While I could run a pair of big engines hard I’d just be burning more fuel. Engine access for bigger engines might be difficult. The naval architect designed the hull around a certain weight engine. If you to ever intend to use the bigger engine power you will need to consider bigger fuel tanks. And the cost would be greater to install. If you compare almost all of the volume manufactures they put roughly 2.5 hp per 1000 pound displacement.
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Old 20-11-2018, 11:51   #60
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Re: Sizing engines for a cruising catamaran

Having said the above I went with a pair of 60 hp Volvo instead of 50 hp standard on my FP 47 foot cat. The selling point was the dealer said since everyone ordered the upgrade I’d be hurting resale value by not doing it. The other considerations were the modest additional cost and identical form factor.
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