Sizing engines for a cruising catamaran

[travellerw]

Originally Posted by daletournier

I've recently travelled approx 1000nm in company with a 37ft Island spirit cat with twin 18hp (I think they are 18hp) yanmars. The little engines made extremely hard work of any sort headwind or adverse current, sometimes to the point of struggling to keep above 2 knots.

We all motor at times, few complain of having to much power.

Jesus.. I don't know of any other ISs with 18hp motors. I would definately say they are too small for that boat. Ours is equipped with 2 30hp motors.

[fxykty]

Factor’s fix, IMO, was to correct your assertion that ‘more windage, more wetted surface’ applies to all catamarans while it really applies more to accommodation biased (nicer terminology than condomaran) catamarans.

Our performance oriented catamaran has two 37hp Nanni engines and that has been plenty of power for our boat.

Regarding MaxProp prop walk, we haven’t noticed. But each engine rotated the opposite direction, so I guess it cancels out?

[Four Coconuts]

Far and away the single greatest reason for large engines is so that you can use just one at a time. This was mentioned previously but it needs to really be emphasized. The part of the compelling reason that you go cruising in a catamaran is for system redundancy and efficiency through the water (I.e. speed) On our Leopard 45 with 54 hp Yanmar and upgraded props, displacing about 28,000 pounds, we could do about 7 kn Under one engine at cruise rpm in light chop. We would see about 8.5 +/- with two engines at cruise.

[Zzmeyer]

Coconuts. I guess it is twice as likely for an engine failure on my cat than my monohull because there are two engines, but when one fails I will still have one as opposed to my monohull. Question. You cruise on one engine at 7.5 knots. What rpm are you running. You mention your cruise speed with two engines at 8.5 knots presumably at about the same rpm but do you know the rpm required with two engines to do 7.5 knots. I ask because I know a lot of cat owners cruise on one engine and am trying to decide if that is the right way. I was told by one dealer when I was looking at cats not to. His logic was that if it took 40 hp to push the boat at the desired speed then you could do it with two engines at 20 hp. Your Yanmar is my favorite engine. They are quite consistent at burning about one gallon per 20 hp so fuel consumption would be about the same. I understand you’d be doing engine maintenance twice as often but I only run about 250 hours per year.

[DotDun]

Originally Posted by Zzmeyer

Well it certainly is unanimous. Go for the engine option. More windage, more wetted surface of the cats. The larger Volvo’s on the Saona 47 are not a big upcharge but I wanted to challenge their usefulness. I had pretty much decided to do it based on the higher demand/ potential resale value. I have actually opted for the significant upcharge for Yanmar but looks like Volvo May have solved their MDI problem so I’ll save net. Props are key both for sailing and motoring performance so I’ll move on to that next. Thanks for the input, all.

If a cat is lighter than a mono, why is there more wetted surface?

[a64pilot]

Reason I believe to run one engine is primarily to cut the time you accumulate on your engines in half. Run one today, the other tomorrow, put 20 hours on the engines having motored for 40.

Twice as many years between overhauls, and Boat is worth more at time of sale.

Running one should be slightly more efficient than running both, reason is friction, there is twice as much with two, but I suspect that the slight increased drag of having to position the rudders to hold a straight course due to asymmetrical thrust washes the increased friction of running both.

So while I do not own a Cat, I’d bet it takes pretty much X amount of fuel to go X number of knots running one or both? Biggest difference of course is you get to cut your hours in half and make your engines last twice as long running only one?

So who would want to cruise on two?
Note, I don’t own a Cat, but cutting running time in half is huge I’d think.

I’d also assume you get one quiet hull too wouldn’t you? Run the engine in the hull your Wife isn’t sleeping in at night?

[a64pilot]

Originally Posted by DotDun

If a cat is lighter than a mono, why is there more wetted surface?

Long thin hulls is why isn’t it? Wouldn’t a sphere have the least wetted area, I understand about displacement, but think hull form is relevant?

[DotDun]

Originally Posted by a64pilot

Long thin hulls is why isn’t it? Wouldn’t a sphere have the least wetted area, I understand about displacement, but think hull form is relevant?

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.

[a64pilot]

See I think displacement is tied to amount of interior volume vs exterior volume.
Of course to float if you weigh a lb, you have to displace an lb of water.

Reason a big ole school fat mono can handle weight so much better is it’s closer to a sphere, which is the shape that has the most interior volume for exterior surface area, so it can displace water more efficiently meaning by using less surface area to do it.

[Cheechako]

Originally Posted by a64pilot

Long thin hulls is why isn’t it? Wouldn’t a sphere have the least wetted area, I understand about displacement, but think hull form is relevant?

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

[a64pilot]

I can’t draw pictures etc.
However think of how many soda straws you would have to have to have the same volume as a ping pong ball, then think of how much surface area would be in those straws.
I may be wrong, just it’s my understanding is all.

Would you agree that two boats identical except for the keel, that the full keel boat has more wetted area as opposed to the fin keel?

[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?

Yes, The full keel would have more wetted area.

[a64pilot]

A fine hulled Cat will have more wetted area for the same reason a full keel Mono will.
I’m sorry but I can’t explain why very well.

[Rotten Ricky]

Originally Posted by Four Coconuts

Far and away the single greatest reason for large engines is so that you can use just one at a time. This was mentioned previously but it needs to really be emphasized. The part of the compelling reason that you go cruising in a catamaran is for system redundancy and efficiency through the water (I.e. speed) On our Leopard 45 with 54 hp Yanmar and upgraded props, displacing about 28,000 pounds, we could do about 7 kn Under one engine at cruise rpm in light chop. We would see about 8.5 +/- with two engines at cruise.

I would argue that the absolute priority in any larger engine is the safety margin it brings, when you trying to get yourself or someone else out of a pickle. The conditions that should determine your choice are the worst conditions, not when theoretically sitting at a dock in flat water. Most designers seem to think only of those flat water conditions!

I re-powered with a slightly larger engine because in 30-odd years of sailing, I have pulled boats and people away from lee shore rocks, towed others back to port, and bashed into waves that would force us otherwise into a dangerous or difficult place. As others say, it gives me great comfort knowing that I could do it again.

As well, the bigger engines run smoother, quieter and last longer, and increase the resale value when it comes to selling the boat. Extra weight - a couple of dozen beer! It is well worth the comparatively few extra dollars in engine cost and fuel used, and as a proportion of the overall cost of the vessel, it is a tiny amount.

From an environmental point of view, I believe that it all works out to be about the same, since you don't have to repair or re-power with new engines as soon, so that manufacturing and delivery energy is saved. That extra money will be meaningless after the first time you are saved from the dreaded pickle!

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 also found that the outboards gave terrible propulsion and so were unreliable when the rough stuff came, as they bounced out of the water. This is why I installed an inboard diesel that has a fixed prop (for now) that costs me only about 1/4 to 1/2 knot under sail.)

Again, what is a life worth?

Cheers, RR.

[rabbi]

Originally Posted by windrider9999

Having gotten caught in a squall a lagoon 420 that had 30's on it with a headwind of 60 plus knots, I can tell you.... we were not holding our own well at all. Granted... one does should not get caught in those, but there were several of us in the St Francis channel Labor Day 16 when it happened. The Rocks of Tortola never looked so close. I hope to never have that happen again but my own boat Im ordering is going to be powered with 55's. I can't imagine any situation where Its good to be underpowered. Besides.. Power is like a plan... Id rather have one and not need it than need it and not have it.

Having been caught in a f12 thunderstorm myself i wonder how you motor into these building seas. Because at least in my case the seas were really frightening short and step (not high as they were just building up). No way we could have made headway in these conditions, regardless of horse power.
At some point around 40+ knots the seas cause too much hobby horsing at least in my cats ( around 40ft and saildrive aft of the rudder). This leads to air at the props and cavitation. More horse power doesn't seem to help at this point. Shaft drive or saildrive in front of rudder may help.
Personally i have the impression that a good sized prop is at least as important to punch into building seas.
Its like a sports car with frontwheel drive. At some point more horse power just doesn't help much as the front wheels are slipping.

So buy the upgrade? It depends on the cat model, the standard power option, and if you need the budget for something else.
If i had to decide between upgraded engines and spinnaker/gennaker i would choose the sails over the engine. For a charter boat I'd choose the upgraded engines as they often hurry back to the base.