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Old 01-04-2010, 13:59   #1
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Outboard for Large Displacement Sailboat

I have a 37 ft full-keel sailboat which displaces approx 20,000 lbs. The diesel was removed some time ago and the boat is my current resotration project. Be ready for launch - Spring 2011. Thinking in terms of a 'High' thrust model 4 stroke 40 - 60 hp outboard on a specifically designed 'moveable' mount. I have read all the pros and cons of this and would like some info on someone with personal experience. The inboard diesel at this time is cost prohibitive. I sail on Lake superior. Any thoughts out there? Not too concerned about the weight issue either due to the 'solid' nature of the design.

thanks
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Old 01-04-2010, 15:45   #2
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Expensive diesel engines...

I cannot remember hearing of a boat being powered by an outboard larger than 18hp.

The smaller outboards are much cheaper than their diesel cousins, but as the power goes up the difference gets less and less. Installation costs for the diesel will be higher.

Petrol carries the inherent problems of a highly inflammable fuel and lower overall reliability. It would be difficult to match the propeller on the outboard with a large, heavy yacht as outboards are normally designed to drive light boats at high speed.

Having said that I really can't imagine how a 40-60hp outboard could be used to propel a boat, particularly a 37', 20,000lb one. Can we have some more details of how you propose to do this?
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Old 01-04-2010, 15:57   #3
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I have never heard of anyone trying this.
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Old 01-04-2010, 16:14   #4
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Maisso,

Why not??

You are limited only by your ability to build it, and your engineering knowledge.
Saildrives stick thru the hull, barges are built with unitized 300hp diesels driving outboard- legs with 48" props.
Not everbodies cup of tea--but its your boat
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Old 01-04-2010, 16:56   #5
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May be difficult to accomodate a 20" prop on a 50 hp outboard. As well as finding a prop with the right pitch. I would look at Mercury Big Foot for a possibility.
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Old 02-04-2010, 05:56   #6
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Outboard Stuff

Yes, having had the diesel (perkins 4-107) for many years I do realize the difference in torque, prop size, etc. My thoughts of mounting were two Trackes with 4 cars on which an aluminum plate would be attached. The ob would mount to the plate-which is mounted at four points to the cars which would ride the tracks. These would be gussetted (stainless ) with 4 mounting pads and thru bolted to the transom. The entire bracket would be mounted at the appropriate angle to accommodate the tansom. A stern rail (arch) would be mounted at the stern on which a small 12v winch would be attched and cabled to the track system - this would lower and raise the OB - completely out of the water & and back to the sufficient depth to prevent the prop from coming out of the water in heavy seas. I am concerned mostly with performance. The current prop on the shaft for the diesel was 3 blade 12 inch dia. (not sure of pitch) and definately had a bite. The high thrust model 4 stroke ob's out there can accommodate up to a 14 inch prop. I really wanted to discuss performance of this arrangement from someone that has tried. I've read some stuff on the internet but most of it is old technology (ob's) with mixed results.
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Old 02-04-2010, 06:42   #7
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Much larger boats than yours are powered by outboards but they are mostly multihulls,some with as much displacement as yours,of course cats can have the advantage of two but the tris only use one. Is this a steel boat by chance? since you are undergoing a refit maybe you can mount it in a well between the hull and cockpit sole with a blanking off plate when its retracted,the Hobie 33 (monohull)had a very slick arrangement,the advantages are that you get the prop in front of the rudder where you need it,the weight is off the transom and you have a smooth bottom when sailing.while this may sound difficult it may not be as long as you have the location with enough vertical height to raise the motor,the ideal location would put the prop close to the rudder for good manouvering in reverse.We did this on an old trimaran 40yrs ago. You just build a box that goes from the hull to the cockpit sole or aft deck, cut the holes in the hull and deck,(save the hull section for the plug) and cut off the box and then do all the engineering out of the boat on the bench,then glass it into the boat (or weld). This is of course an over simplication but you get the idea. Where do you have the boat? i also sail Lake Superior out of Duluth.
Steve.
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Old 02-04-2010, 06:51   #8
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Quote:
Originally Posted by clockwork orange View Post
Much larger boats than yours are powered by outboards but they are mostly multihulls,some with as much displacement as yours,of course cats can have the advantage of two but the tris only use one. Is this a steel boat by chance? since you are undergoing a refit maybe you can mount it in a well between the hull and cockpit sole with a blanking off plate when its retracted,the Hobie 33 (monohull)had a very slick arrangement,the advantages are that you get the prop in front of the rudder where you need it,the weight is off the transom and you have a smooth bottom when sailing.while this may sound difficult it may not be as long as you have the location with enough vertical height to raise the motor,the ideal location would put the prop close to the rudder for good manouvering in reverse.We did this on an old trimaran 40yrs ago. You just build a box that goes from the hull to the cockpit sole or aft deck, cut the holes in the hull and deck,(save the hull section for the plug) and cut off the box and then do all the engineering out of the boat on the bench,then glass it into the boat (or weld). This is of course an over simplication but you get the idea. Where do you have the boat? i also sail Lake Superior out of Duluth.
Steve.
X 2 My thoughts exactly
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Old 02-04-2010, 07:05   #9
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Re: Clockwork Orange

It is a Ferro boat and which was built by my father (now passed) and we had it in Bayfield for years and now its on the hard in Ashland. We sailed for over 12 years to most destinations on the lake. The hull was professionally done and has great lines - however there is much to do topsides (cockpit rebuild - reglassing etc). I have not considered - a 'well' concept. In order to do this it would have to be off-set from the centerline.
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Old 02-04-2010, 07:21   #10
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You're considering this idea because a new diesel is too expensive? What are the real cost savings over a good rebuilt Perkins? You have, I assume, the engine beds and most of the cabling and controls still in place for your diesel. Though the initial cost of a new 40-60hp outboard would surely be less than a new diesel. I doubt by the time you finished fabricating mounts, stern re-inforcement, ran new controls, ran new wiring, convert diesel fuel lines, filters etc to gas and figured out solutions to all the hidden issues that will surely arise, that the cost of a good rebuilt perkins would not be a better financial choice. Secondly if you ever decided to sell this boat (and some day you will) with it's ahhh, custom power plant, that you'd lose more value in resale value than you ever saved.
Torrenson Marine sells rebuilt perkins 4-107 for about 5K.
Perkins TMI-U2015 4-107 – Used Ships Store Torresen A new 4 stroke outboard runs about 6+K, so where are the savings you think you might realize?
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Old 02-04-2010, 07:30   #11
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The closest thing I have seen to a large(r) sailboat using an outboard was a 35 footer on the Chesapeake. He had put an outboard bracket on the stern that served 2 purposes. 1, he stored his inflatable on the bow, so davits were not an issue, but he kept his 15hp outboard on the bracket and transfered it back and forth to the dingy when he needed it. It was close to the water and it made it very easy for him to do. 2nd, when his diesel would fail, which it did often, he would swing the bracket down, drop the engine, hook up the dingy gas tank and off he went. Granted, he wasn't doing hull speed or doing blue water cruising, but he always got back to the dock.
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Old 02-04-2010, 07:32   #12
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Re: Tellie

I guess the reason is not really financial. Like I said there are many pro's and con's and cost being the same....I believe (in my mind) that the pro's outweigh the cons some of which are: (smell - vibration - maint - noise - shaft - thru hulls - stuffing box) so...that is why I am inquiring about performance. If the performance diesel inboard vs 4stroke outboard greatly favors the diesel...I will redirect my efforts in that direction.
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Old 02-04-2010, 09:24   #13
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Don't waste your money on a big outboard. They will cavitate/ventilate at anymore than 1/3 throttle for several reasons: none are made for displacement speeds: their props are too small in diameter, and the gear ratio is too low. At best you will get 10 hp from a 60 hp engine.

Beware marketing hype: "big foot" is little more than a different decal. There are only two outboard engines geared and propped for your purpose. The have near 3:1 gear ratios and are propped for less than about 15 knots, so you will get full use of the money you spend. They are the 8 and 9.9 High Thrust engines from Yamaha. Note that their 25 hp high thrust does not have the same gear ratio.

As much as I like anything from Honda, Yamaha is the only choice for you or me. Mercury is imported from Tohatsu.

Put one 9.9 off center on the transom. Make it a long shaft and mount it as close to the water as possible. Try if for a while, and if you can't get to hull speed with it, mount another one. You will have invested less than the price of an overhauled diesel, and have the use of the space in the hull. Two High Thrust Yamahas will probably push your Grande Dame into a good bit of wind and waves. Make them steerable from the cockpit and you will be thrilled with what you can do with the boat. Put a short-shaft 9.9 on your dinghy and you will have a lot of parts commonality.

Having said all that, I think you should stick with diesel. It works for ever, and doesn't look like a joke.
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Old 02-04-2010, 09:37   #14
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what he said .. go with a diesel.
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Old 02-04-2010, 09:41   #15
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9.9 horse Yamaha Long Shaft

I use a long shaft, 9.9 horse Yamaha for my dingy outboard and it doubles as a backup auxiliary should my main engine fail (which it has). Because my boat has a transom-hung rudder, I had to mount the retractable bracket off to one side. This didn't to effect the performance very much and I used the rudder to steer with, not the outboard.

I found that the outboard could move my boat slowly in calm air without foul current. No way could it go to weather, but I could definitely dock my boat using the outboard.

I've got a 34 foot steel cutter weighing about 18,000 pounds. Its diesel is a 27 horse Yanmar.
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