Surface drives - less fuel & faster?

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mateyboy

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I keep researching surface drives about how much more efficient they allegedly are, how come we're not awash with them if they can save upto 40% in fuel burn???
Expensive to maintain?
 
They are only more efficient at high speeds, and are less so at lower speeds. They are more difficult to turn, more expensive to fit, harder to set-up, have poor acceleration until they reach higher speeds, and are difficult to reverse. Not great for swimmers as the props stick out the back of the boat, and not everyone likes the rooster tail behind the boat. On the plus side I hear that they are very reliable.

I guess the efficiency curves must cross at about 25-30 knots for the average boat, and it seems most cruisers are happy with that speed.
 
It still surprises me that there aren't more about for sea cruising. I guess they are fairly reliable as they are a straight shaft with U/J's as opposed to the Z of a stern drive. I'd have one as the rooster tail is very show offy!
 
You see quite a lot of them on big sport cruisers in the med. I think that's their real market, as flybridges tend to be cruising boats so aren't bothered about speed, and below 45 feet on a sport cruiser you can get similar efficiency/speed with sterndrives.
 
Nick_H said:
You see quite a lot of them on big sport cruisers in the med. I think that's their real market, as flybridges tend to be cruising boats so aren't bothered about speed, and below 45 feet on a sport cruiser you can get similar efficiency/speed with sterndrives.
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Yes there are a few in Monaco (surprise, surprise!)and there's a Predator with s/drives i've seen for a couple of years now, I think its at the little port between Nice and Monaco, can never remember its name, the port or the boat.
 
whisper said:
Are they any good in rough seas? I have visions of them being totally out of the water sometimes.
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I don't know but year before last there was a big storm coming from a SW ish direction about 4pm so it was like the Whacky Races heading back to port, needless to say there were 2 or 3 rooster tails at the front and first back to port. A similar mad dash back happened last year in Mahon, the weather seems worse there than the SoF.
 
mateyboy said:
Yes there are a few in Monaco (surprise, surprise!)and there's a Predator with s/drives i've seen for a couple of years now, I think its at the little port between Nice and Monaco, can never remember its name, the port or the boat.
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Well there's a few ports between Nice and Monaco - Villefranche, St Jean, Beaulieu, Cap d'Ail, and a little one next to Eze. There are a few surface drive boats on our quay in Antibes, they're not that rare.
 
whisper said:
Are they any good in rough seas? I have visions of them being totally out of the water sometimes.
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They were doing OK during "Round Britain 2008" ... :)

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...anyway... wrote something about them in a thread a couple of years ago !!....

http://www.ybw.com/forums/showthread.php?t=14370
 
mateyboy said:
I really enjoyed that write up Alf, an extended bathing platform would be handy on a sports boat then.
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I have to second that, interesting reading. I must have missed the thread at that time.
Re. the need for extended bathing platform, that's not so critical with surface outdrives, which is indeed the most common transmission for very fast monohulls. Though you wouldn't want to swim around a cleaver prop anyway...
 
MapisM said:
I have to second that, interesting reading. ...
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Thanks ... I had to re-read that one and refer back to my old notes, to get the pictures in my head again... it must be some of my most researched subjects ... after all that, I bought an old boat with shaft drives and Detroit diesels, making a desicion that the science should be left to the designers and that boating was to be enjoyed... (nice to have the knowledge though :) )
 
New generation surface drives have also solved the hole they previously held at 15 to between 25 knots. This hole was normally found on Arnesons and similar drives.
Their turning is better to any shaft boat. May be only a stern drive boat would turn better.
I have seen surface drives on fly yachts, I rememer the Viareggio defunct builder Antango doing a unique one off version with Arnesons of its 21M doing a 40 knots top speed versus a 33/34 with the same engines. As well as Couach yachts doing something similar. Also to remember altough not with Arnesons but with their own Levi designed system where the old Italcraft like the record breaking M78 which did 53 knots with 1650hp back in the eighties.

The real problem why flybridges don't use them is that unlike shafts surface drives are less forbidding to the terrible weight distribution most of todays latest generation flybridge boats have....
 
I recall reading a year or so back on the Arneson web site the use of large perspex looking discs about the same diameter as the prop and fixed at the back of the prop, I'm now going to trawl through it again to see if this is for the prevention of the "cavitating hole" that occurs at some point in the throttle curve as mentioned or whether its a safety thing as also mentioned. The sharp edge Cleaver props look very wicked indeed.
 
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PowerYachtBlog said:
The real problem why flybridges don't use them is that unlike shafts surface drives are less forbidding to the terrible weight distribution most of todays latest generation flybridge boats have....
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Is this why they guzzle so much fuel, as the balance is all wrong let alone FB's with drives? Compensated by more powerful engines but not necessarily any great improvement on performance?
 
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