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Old 14-12-2006, 19:44   #1
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waterproof outboard

I am building a new motorboat 36ft long. The boat will be used for cruising offshore.
I want to use a fourstroke outboard (Honda) as the main power plant with an extra outboard as a spare.
I have had inboard marine diesels on previous boats and have never been happy with the cost, weight or reliability(compared to on land use).It is usually not possible to have a spare diesel and I don't feel comfortable with one motor far offshore.
My experience with the newer outboads has been good so far. However, I was pooped once and water got into the air intake of the outboard.

The only issue I see is the air intake for the outboard which may need to be relocated, with flexible tubing to inside the boat.
(i think that was the approach with the Yanmar Endeavour, which went nonstop from California to hawaii some years ago using Yanmar outboard diesels)
If this is properly done, are there any other problems areas that I should take in consideration

Only sensible comments please
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Old 14-12-2006, 21:21   #2
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If this is properly done, are there any other problems areas that I should take in consideration
How much gasoline storage do you think you will have? if you run the math on the boats range it may not be so great. I'll assume the issue of storing gasoline is not something that you consider a problem. I think many might. The details of storage should be a serious concern. You said it was a power boat but can it sail too?

Keeping the engine out of the salt water may be an issue as well. Outboards in following seas and all that complicates a few details. Size of the engines? The logistics of swapping it out based on size and being off shore? You might need to swap it out in heavy weather. I doubt it would fail on a nice day. Having no power to control the boat would be a problem. Where you might store the spare so you could handle getting it in and out. I assume we are not talking about a Honda 4 stroke air cooled 2 hp here (I have one and it's nice).

I think the negatives only start to come out when you back away from just the little bit info you posted. The bigger picture begs for some details that you have not shared. I don't want to say it can't work, but I think it adds complex issues you'll need to solve. The actual operation of the engine I think I would set aside at this point.
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37 15.7 N 76 28.9 W
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Old 15-12-2006, 05:38   #3

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I agree completely with what Paul says above. The real glitch in the plan is your thinking you will have a backup outboard ready to deploy without some kind of crane to put it in place. How do you plan to handle that in heavy seas without the loose outboard smashing the boat up?

I share your thoughts about outboards - especially diesel ones! My thoughts, just thinking outside the box here would be to get that air intake inside (as you suggested) or even use a snorkel air intake that protrudes very high out of the water.

As Paul also said, you'll have to find a way to get it out of the water when not in use to prolong the life of the lower unit.

Also, what about building some kind of outboard well or enclosure for it? Could help prevent nasty following seas from ending up in the outboard case.

Lastly, what about your boat pitching in rough conditions? Where will the outboard be located such that when you go "bow down and stern up" it won't pop out of the water?
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Old 15-12-2006, 07:51   #4
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Inboard small diesels are pretty reliable engines. Mine has been working for 21 yrs even when it has some "serious" problems like a broken valve spring.

What HP are you thinking about?

How about using an OB for some sort of back up for the inboard diesel?

sv Shiva
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Old 15-12-2006, 13:51   #5
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waterproof outboard

More information:
The main outboard is 15 hp and pushes the 36ft motor sailer at an economical 9 knots. (under power alone)
The spare is actually 65hp. Both are permanently mounted to the stern of the boat and can be dropped down and instantly used.

My last boat had a single 280 hp perkins diesel
(no sails) and I did have the occasional problem, and I just was not happy relying on ONE motor.

The reason for the 65 hp is that this new boat will achieve 20knots with that amount of power but uses a lot more fuel.
The hull design is a fast displacement and does not plane. (With narrow bow entry and wide flat stern)

I do not see why storing petrol on a boat is any different from Diesel if the tanks are properly constructed and plumbed correctly.
Yes I will need more storage capacity than if I went with diesel. But look at all the weight I save (no heavy diesel motor)

I have tried on a previous boat to fit a separate compartment for the outboard. I was not happy with it because it confines the outboard and brings with it the dangers of having petrol fumes in the engine compartment etc and if the outboard leg is not sealed off at the bottom the water level comes up anyway.
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Old 15-12-2006, 23:03   #6

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Beau, I don't mean to discourage you but there's usually a reason why things are done a certain way--and not advisable in the ways they aren't done.

"I do not see why storing petrol on a boat is any different from Diesel "
Well, start with diesel having about twice the energy density of gasoline. For every gallon of gasoline you store, you can motor for 2x-4x more time with the same amount of diesel powering an inboard. If I need to carry 20 gallons of diesel, you'll need to carry 50 gallons of gasoline to make the same distance. Or more.

Then there's outboards on a sailboat. The ocean is not flat, and every time you move a pound of weight from the center of the boat to the far end--you make the boat pitch and rock way way worse. Which also pulls your outboard prop out of the water, further reducing your range and increasing your fuel consumption. That's why offshore sailboats simply DON'T do it this way. Can you do it? Sure. Do it well? No. Competitively? No. Safely or economically? Arguably, again, No.
If conventional logic and use was so wrong, and a better answer was so would probably be out there already. One Yamaha factory-sponsored show boat making one run under ideal conditions with factory support teams does NOT mean outboards are good for offshore sailboats.

Diesel motors used to be much heavier than petrol. That's changed in the last 20 years. And that's still ignoring the inboard/outboard question. If you don't like diesel, by all means put an Atomic4 inboard. Just remember, you're still going to need a big fuel tank.

"The reason for the 65 hp is that this new boat will achieve 20knots "
On a motor sailor? Yes, I'm a skeptic. If you have a radical new design that you KNOW will wouldn't be asking these questions, you'd have the engineering all done. So, you have a motorboat of some kind, with plans for auxiliary sailing. That's very different from a sailboat, which is probably going to make six knots with any conventional displacement hull. There's a reason for conventions: Often, because they're what has worked.

Just what kind of superboat are you trying to build? A multihull?
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Old 16-12-2006, 10:14   #7
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I skimmed this, so I might be off base.
In several trawler mags I've read there is a "new" idea. Use a large main proplution diesel mounted center line with a single screw. Then mount a diesel auxiliary with a feathering prop to out board. The weight can be off set by the generator.
Or, use the generator to drive a hydraulic pump and drive an auxiliary shaft and prop with a hydraulic motor.
P.S. I dislike petrol.
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Old 16-12-2006, 12:46   #8
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On Outboard Engines:
I cannot responsibly encourage the placement of large masses, such as engines, at the ends of an offshore vessel. Aside from an Outboard's general vulnerability (to water, collision, etc), heavy ends will virtually always create trouble in terms of pitching moment.

Most trawlers are powered with a single diesel engine, and have a smaller (combination) generator cum Get-home Engine (or Wing engine) .

Excerpts on ďGet HomeĒ auxiliary engines, from boatbuilders:

Although the reliability of single-engine installations has been proven by scores of successful circumnavigations, some owners may wish to opt for an auxiliary "wing" engine. This "get home" package features a totally separate engine, transmission, shaft and propeller.

A single-screw trawler is a bad idea. Itís a bad idea even with a ďget-home engineĒ because get-home engines donít work. Get-home engines exist because single-screw trawler manufacturers recognize that any engine can fail given the wrong circumstances. A single-screw boat always runs the risk of getting caught dead in remote waters or on a hard-chance lee shore.
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Old 16-12-2006, 15:47   #9
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waterproof outboard

In regard to the comments by "hello sailor" I agree with your comments in regard to the superior energy storage of Diesel/lb.

However the 15hp outboard i plan to use for long distance only uses 3/4 of a US gallon per hour. Plus I do not have the WEIGHT of the diesel motor.

I don't accept the philsophy that just because it has been always done a certain way in the past means that is the only way.

For some years now I have been building prototypes 15ft long using a concept developed by Alvaro Calderon (who worked on the American cup team)
In basic theory, he uses a displacement hull which has a very fine entry angle(6.5 degrees on each side ) which continues (the same angle) to the stern of the boat. A double wedge shape. The hull sides are vertical and the bottom is flat. In pure form his concpt is not very practical.
I have modified the design and tested a number (5 so far) of prototypes (15ft long ) with a 8hp outboard and achieved 15 knots. There was NO bow wave and the hull did NOT plane. The stern wave which was significant at low speed flattened out over 10knots. Load carrying capacity is high and towards the stern which is why I can hang the outboards off the back.

I am still experimenting, which is why I have submitted these posts. I want to hear GOOD reasons why the petrol outboard concept can't work. Outboards today are FAR different and more reliable than twenty years ago. They can very easily be made waterproof by the fitting of a snorkel. I have had diesels and outboards. Diesels are very heavy and costly.

So far, other than a couple of good ideas most replys are just negative without explaining why.
Thank you all for your comments I really appreciate feedback
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Old 16-12-2006, 21:21   #10
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My reading suggests that:

An outboard gets pulled out of the water when the boat is pitching... and plunged under on the downward side of the pitch.

When the boat gets pooped and uses the cockpit drains... the outboard gets burried under the swell... and is now sitting a lot lower in the water, perhaps with the motor held under.

That being said, my plans involve an outboard as the auxillary...
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