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Old 27-06-2018, 20:52   #1
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Sizing engines for a cruising catamaran

Most cruising catamarans offer several engine powers. Dealers Iíve talked to all recommend the upgraded engines and say most buyers opt for them. Do catamarans need more power than a monohull to fight headwinds and head seas? I ask because in almost all cases the upgraded cats have much more power than comparable sized (displacement and length) cruising monohulls.
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Old 27-06-2018, 21:25   #2
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Re: Sizing engines for a cruising catamaran

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Most cruising catamarans offer several engine powers. Dealers Iíve talked to all recommend the upgraded engines and say most buyers opt for them. Do catamarans need more power than a monohull to fight headwinds and head seas? I ask because in almost all cases the upgraded cats have much more power than comparable sized (displacement and length) cruising monohulls.
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 rathe have one and not need it than need it and not have it.
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Old 27-06-2018, 21:29   #3
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Re: Sizing engines for a cruising catamaran

Not sure about your final assertion but seems reasonable. Two possible reasons for higher power overall (i.e. two engines vs. one):
- Multihulls generally have higher wind resistance and some designs also higher wave resistance, so need more power at emergency levels
- Cruising catamarans will often plan to use just one engine on passage but still want 2/3 or so boat speed without running the single engine full out

Or do the manufacturers provide little tables showing 6 knots base power option, 7 knots power option B, and 8 knots power option premium? I guess prospective owners think Ďbugger 6 knots, we want to be having drinks while other underpowered rejects are straggling iní and plump for the highest power option?
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Old 27-06-2018, 22:29   #4
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Re: Sizing engines for a cruising catamaran

I have 75hp yanmar shaft drives and they punch through about everything. Whether you want to actually go fast into it is the question.

For many years I was in the single motor camp for cruising. Lately I've been starting to turn towards using two, especially if there is significant chop. The boat just seems less stressed.
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Old 27-06-2018, 23:45   #5
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Re: Sizing engines for a cruising catamaran

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.
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Old 28-06-2018, 02:05   #6
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Re: Sizing engines for a cruising catamaran

One problem with cats is they often come with 2 blade fixed props to reduce drag under sail which is adequate in calm conditions but the blade area ratio is too low to drive into a chop efficiently hence the speed drops dramatically. The ideal from my view would be to have 3 blade folders or feathering props, expensive though.
We have 2 blade folders which behave similar to the previous 2 blade fixed. In moderate conditions, we motor one one engine, in a chop we use both.
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Old 28-06-2018, 08:31   #7
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Re: Sizing engines for a cruising catamaran

I note, that like the dealers, everyone suggests more power. I'm curious about the power discussion because after 45 years of monohull ownership/cruising I'm going catamaran. While I agree (to a point) that having more power gives you the option it does come at a price ($). Windrider9999 comments give me the most concern. Having enough power in all cases is important. 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. I'm going with flexifold versus my historic maxprop in part because it has 21% more thrust (according to the study). Catamarans do have more windage. I would point out that the Nordhavn power boats power all their 40-46 boats with 2.1-2.7 hp per 1,000 lbs and they have a ton of windage. My new cat standard is 3.3 hp (both engines) and 4.0 with the upgrade. The beneteaus, Jeanneaus and Hanses are generally in the 2.5-2.7 range. Yammer recommends running their engines at 80-85% rated rpm. That pushes my current 50 footer at 8.1 knots (flat sees). As seas build, I tend to throttle back and fall off to avoid pounding. That is at 2.4 hp per thousand pounds. In addition to all the good points above one dealer pointed out that cats have more wetted surface, needing more power.
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Old 28-06-2018, 09:05   #8
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Re: Sizing engines for a cruising catamaran

My 42 cat had two Yanmar 3GM30 engines with Max Props and shaft drive. The trouble is those engines only have their rated HP at something like 3600 rpm. They were ok but I would have opted for more HP for sure. There is a lot of windage in a Cat, especially the more modern ones with vertical cabin windows etc. However cats seem less effected by head seas than a mono.... riding/lifting over the waves rather than plowing into them.
So the 3GM30 engines are what... 24 hp? Less if you dont want your engine to sound like an airplane at 3600 rpm. So 48 hp is not more than say a 42 ft mono might have.
My 47 ft mono had an 80 hp engine. Loaded it was 5" down on the waterline. That gear went directly to the cat when I bought it. The 42 ft cat went down only 2" on the waterline.
More HP run at lower rpm will have the engine last longer too.
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Old 28-06-2018, 09:05   #9
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Re: Sizing engines for a cruising catamaran

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Old 28-06-2018, 10:15   #10
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Re: Sizing engines for a cruising catamaran

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My 42 cat had two Yanmar 3GM30 engines with Max Props and shaft drive. The trouble is those engines only have their rated HP at something like 3600 rpm. They were ok but I would have opted for more HP for sure. There is a lot of windage in a Cat, especially the more modern ones with vertical cabin windows etc. However cats seem less effected by head seas than a mono.... riding/lifting over the waves rather than plowing into them.
So the 3GM30 engines are what... 24 hp? Less if you dont want your engine to sound like an airplane at 3600 rpm. So 48 hp is not more than say a 42 ft mono might have.
My 47 ft mono had an 80 hp engine. Loaded it was 5" down on the waterline. That gear went directly to the cat when I bought it. The 42 ft cat went down only 2" on the waterline.
More HP run at lower rpm will have the engine last longer too.
A couple clarifications from my prior post. All power/weight numbers were consistent. That is, rated HP. A 3GM30 actual power is the same in a cat as a monohull. For example, my 80 HP engine at my cruising 2400 RPM is actually generating about 35 HP. Yanmar doesn't agree running the engine at lower power settings leads to longer life. You state that cats are less effected than head seas than a mono. That would imply you need less power in a cat than a monohull. Also, your 47 loaded was down 5" as compared to 2" on your 42 cat implying a significantly better load carrying capability of a cat to a monohull. I thought you needed to be more careful loading a cat. That is good to know.
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Old 28-06-2018, 10:24   #11
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Re: Sizing engines for a cruising catamaran

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A couple clarifications from my prior post. All power/weight numbers were consistent. That is, rated HP. A 3GM30 actual power is the same in a cat as a monohull. For example, my 80 HP engine at my cruising 2400 RPM is actually generating about 35 HP. Yanmar doesn't agree running the engine at lower power settings leads to longer life. You state that cats are less effected than head seas than a mono. That would imply you need less power in a cat than a monohull. Also, your 47 loaded was down 5" as compared to 2" on your 42 cat implying a significantly better load carrying capability of a cat to a monohull. I thought you needed to be more careful loading a cat. That is good to know.
Yes, I was very surprised with the loading comparison. We literally took the stuff off the mono on one side of a finger pier , onto the cat on the other side. re; Loading cats: I think some of the concern may be the lever arm and stress on a bridge deck with hulls heavily loaded. Fortunately my Lagoon TPI 42 had huge radii connecting the hulls to the bridge deck. Watch out for sharp corners there!
Yeah, I know Yanmar likes their engines to be run at high rpm. They are not a lightweight engine really, they are an over rated engine using high rpm to make the HP. It's very common in commercial marine diesels for the same engine to be rated at different HP or RPM depending on use. For instance, a 500 hp engine may be rated at 2400 rpm for 24/7 use and rated at 3000 rpm for intermittent (recreational) use.
BTW, both my 3GM30's run at high rpm in the cat failed before 2500 hours. One was warranteed replacement. The other I had to repair.
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Old 28-06-2018, 11:45   #12
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Re: Sizing engines for a cruising catamaran

Well it certainly is unanimous. Go for the engine option. More windage, more wetted surface of the cats. The larger Volvoís 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 Iíll save net. Props are key both for sailing and motoring performance so Iíll move on to that next. Thanks for the input, all.
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Old 28-06-2018, 13:56   #13
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Re: Sizing engines for a cruising catamaran

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Well it certainly is unanimous. Go for the engine option. More windage, more wetted surface of the some "accommodation biased" cats. The larger Volvoís 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 Iíll save net. Props are key both for sailing and motoring performance so Iíll move on to that next. Thanks for the input, all.
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Old 28-06-2018, 14:49   #14
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Re: Sizing engines for a cruising catamaran

Factor are you talking about props? That won't happen on my boat. A fixed prop costs at least a half knot in sailing speed. That is 5-10%. Also, and perhaps surprisingly, if you look at the prop comparisons, fixed props don't do all that well in motoring either relative to the best feathering or folding props. This is one of the best articles I've read on the subject. What is your rationale other than cost (and I guess simplicity)? Folding and feathering propeller test To me, the only issue is which one. For example, I've had both Flexifold and Maxprop (I've also had fixed). My experience is consistent with the article. The Maxprop (which is feathering) backs better than any fixed prop or my Flexifold. However, the Maxprop did have quite a bit of prop walk unlike my Flexifold. The Flexifold provides more forward speed and more power than any other tested. The Flexifold, while much more expensive than a fixed prop (but a small fraction of the cost of sails to overcome their disadvantage) is about half the price of a Maxprop. Both have been problem free for me over the past 20 years or so. I'm leaning to another Flexifold primarily due to cost, but am interested in learning about others. I also am only considering three bladed props.
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Old 28-06-2018, 14:52   #15
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Re: Sizing engines for a cruising catamaran

Factor - just noticed you have a corsair with a "fixed" prop. An outboard is "cheating" in this discussion for me. You can sail rings around me with that boat (but I'll have a washer dryer)
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