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Old 19-04-2013, 04:56   #1
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It seems many multi's today are motor boats with auxiliary sails...

Engines way too powerful (and heavy) to allow multihulls to do what they can do best...sail fast.

Re engines, Eric Hiscock and Larry Pardey state that 1 hp per ton is adequate. Tom Colvin, a well respected naval architect, suggests half a horsepower per long ton (2,240) pounds for an ocean-cruising sailboat and one hp per long ton for a coastwise cruising sailboat.

So, I am repowering, and if I have a 62' 22,000 displacement cf trimaran, the engine size could be 10 (or 20 or 30) hp? Their point is that sailors should be sailors, and not power boaters. Super-size me, or make the engine a true auxiliary? Old school or new school?
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Old 19-04-2013, 05:13   #2
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Re: It seems many multi's today are motor boats with auxiliary sails...

I wonder how many horsepowers this one has (yes, it actually is a cruiser):


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Old 19-04-2013, 05:23   #3
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Re: It seems many multi's today are motor boats with auxiliary sails...

To answer your question, I think it depends more on windage and hull form than weight. Slim and low boats are more easily driven than wide beam tall boats. I'm sure that many boats have oversized engines with more than enough horsepower, but I wouldn't go as far as to say that 1/2 or a "full" horsepower per ton is perfectly adequate. I know the above boat is an extreme example, but it's 63 feet long, wave-wise easily driven, and it weighs somewhere between 7,500kg and 8,200 kg depending on load state. I doubt it would be prudent to have a four (4!) hp engine in that thing. 8hp also seems really low for that boat.

Another important factor to think about is the prop: A modern folding propeller is not as effective as big old school non-folding propeller when motoring, but in a lot of cases, the lesser drag makes up for its shortcomings.
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Old 19-04-2013, 06:16   #4
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Re: It seems many multi's today are motor boats with auxiliary sails...

Most cruising boats today are really motorsailors most of the time. However, I think it is highly dependent on what you want to do with the boat and where you want to go. For example, if you want to do much of the Intracoastal Waterway you will be doing a lot of motoring and a more powerful engine will be very useful. Same if you want to be able to go through the Panama Canal or the Suez Canal. A more powerful engine can be useful on boats that sail most of the time. Hauling anchor in much more than 25 knots of wind can be difficult without a decent amount of power. A large multi will have a lot of windage, and will be difficult to handle in harbor in high winds without decent power.
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Old 19-04-2013, 06:45   #5
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Re: It seems many multi's today are motor boats with auxiliary sails...

One significant consideration is resale value. If your boat has much less horsepower than is considered "standard" then it is going to be harder to sell, and you are going to have to take a big hit on price.

Of course, if you plan to keep it for 10-15 years then the savings from smaller engines will probably pay off. Or maybe not. But it is something to think about.
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Old 19-04-2013, 06:47   #6
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Re: It seems many multi's today are motor boats with auxiliary sails...

Its all about windage and being able to go forward into a head wind and not backwards. When your in a storm and trying to keep off the rocks I am sure you would not be saying, Wow I am glad we only have half a horsepower per ton.

Newer diesels are lighter per HP, so why not go with a safty factor
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Old 19-04-2013, 06:49   #7
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Re: It seems many multi's today are motor boats with auxiliary sails...

Another thing I forgot to mention is that different parts of the world have different wind situations. Maybe where you want to go in the Pacific has strong, steady winds and sailing most of the time is favorable, while other areas you might find yourself with long, slow trips without the ability to motor for longer periods at decent speed. Even when you intend on sailing, sometimes the wind doesn't cooperate. I ended up motoring more than 300 miles from Mexico to the US because it was mostly flat calm, but I wanted to keep going because a severe front was coming and I was glad I did.
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Old 19-04-2013, 06:51   #8
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Re: It seems many multi's today are motor boats with auxiliary sails...

It isnt just the size, its also the placement of the weight, some boats have their motors so far back that props are behind the rudders and the the 50hp 200kg lump that far back needs ridiculously fat hulls. But it makes for lovely aft cabins. To each his own, but I am with you Pipeline, a sail boat should be a sail boat. I have 10hp (9.9 actually - yammie outboards) a side, its plenty and being outboards means I can have big meaty props and not worrying about drag when sailing.

There is no doubt many of the current crop of boats are designed around charter and or carrying unneeded crap. If thats your bag then fine, but i often wonder why some boats bother with a mast.
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Old 19-04-2013, 06:52   #9
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Re: It seems many multi's today are motor boats with auxiliary sails...

If the difference in HP has a weight penalty of only a small percentage, why would anyone opt for less? For instance if a 6HP diesel weighs 280# and a 12HP weighs 380# the penalty on a multi would only be equivalent to ONE extra passenger, yet the boat would make hull speed in any condition. I have a pile of time on underpowered vessels, and they are no fun, and fun is WHY I go sailing! My Mono will barely make way against the 5kt tide around the North end of Flemming Key.... I wouldn't mind 50% more ponies!
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Old 19-04-2013, 07:00   #10
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Re: It seems many multi's today are motor boats with auxiliary sails...

Why pick on multihulls? I've seen plenty of leaners with the same problem
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Old 19-04-2013, 07:22   #11
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Re: It seems many multi's today are motor boats with auxiliary sails...

If the purists could decide what was best for us we would be all be sailing wood hulls with cotton sails and hemp lines. We would also be spending more of our time doing boat maintenance than sailing.

Forget them, get the boat that makes you happiest. If you want a cat that can go 10 knots under power then go for it.
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Old 19-04-2013, 08:24   #12
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Re: It seems many multi's today are motor boats with auxiliary sails...

Quote:
Originally Posted by tamicatana View Post
Why pick on multihulls? I've seen plenty of leaners with the same problem
I dunno maybe because OP has a multihull. and thread is in the Multihull sub forum? But as a Mono owner I have to rely only on the power of reading guessing to puzzle that out......

Be that as it may - in response to OP the days of mainstream recreational sailing yachts having to be auxilliary powered by motor are long gone (smaller sized diesels and boat design changes) and therefore, as said already, most yachts are designed to be motorsailors even though not sold as that for marketing reasons (the term is associated with dumpy yachts that don't sail well - from ye olden days of the 60's and 70's ) - and also mostly not bought as such, even if actually used that way!........and IMO no problem with that approach - having hull speed under power does make life more conveniant at times .

Therefore I would discount the words of wisdom about engines and power from such folks as the Hiscocks and their ilk from that era. and even the Pardeys - as I am sure even they admit are well outside the mainstream when it comes to their attitude to motors, and were even when they started! Whilst I don't subscribe to the theory that everyone needs 24 hours of power under full steam, nonetheless I feel that particularly for docking and even the last 5 miles (plus or minus) some sort of power is pretty much essential in the modern era of harbours, anchorages and approaches no longer being designed around the needs of those only under sail. Not to say that sail alone can't be done (it is ), but even a bit of power opens up options / makes life so much easier.

In regard to OP, with 62' of Trimaran I am guessing she is a fast b#gger under sail! and the hull slippery as a slippery thing on a slippery day - so she likely won't require squillions of HP to move her at hull speed, plus less need to actually do so (and my suspicion is that 60 foot of tri is bought by someone with more of a focus on the sailing side of Multis than the condo end - and is happy to accept the downsides)......and having excess weight onboard from unused HP is mostly only downside given that performance will likely mostly be under sail.

Boats is all about choices - few right or wrong ones, all down to personal needs and desires .........if I had 60 foot of Tri I would likely go for the less (HP) is more approach - but that conclusion having nothing to do with the views of the Hiscocks etc.
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Old 19-04-2013, 08:37   #13
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Re: It seems many multi's today are motor boats with auxiliary sails...

Quote:
Originally Posted by tamicatana View Post
Why pick on multihulls? I've seen plenty of leaners with the same problem
He has a trimaran himself.
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Old 19-04-2013, 08:40   #14
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Re: It seems many multi's today are motor boats with auxiliary sails...

well just don't use a diesel engine, and you'll save considerable amount of weight.
I don't get why you want a stinky thumping thing under your berth.
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Old 19-04-2013, 08:53   #15
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Re: It seems many multi's today are motor boats with auxiliary sails...

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Originally Posted by David_Old_Jersey View Post

[...]
Whilst I don't subscribe to the theory that everyone needs 24 hours of power under full steam, nonetheless I feel that particularly for docking and even the last 5 miles (plus or minus) some sort of power is pretty much essential in the modern era of harbours, anchorages and approaches no longer being designed around the needs of those only under sail. Not to say that sail alone can't be done (it is ), but even a bit of power opens up options / makes life so much easier.

In regard to OP, with 62' of Trimaran I am guessing she is a fast b#gger under sail![...]....and having excess weight onboard from unused HP is mostly only downside given that performance will likely mostly be under sail.

Boats is all about choices - few right or wrong ones, all down to personal needs and desires .........if I had 60 foot of Tri I would likely go for the less (HP) is more approach - but that conclusion having nothing to do with the views of the Hiscocks etc.
Sorry, the rest is also good points, but I wonder if in such a big boat where weight still matters, but you have a very deep centerhull compared to a mono, if it would be an idea to consider not only a small (diesel) generator coupled to a (smallish) LFP battery bank, and coupled to a really miniscule electric motor. I mean, the torque in an electric motor is there from 0 rpm, so you wouldn't need to "oversize" as you would with a combustion engine.

Yes, it's a tad more complex than a direct drive diesel, but there might be a weight savings all up. And the diesel would definately live a bit longer, all things equal.
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