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Old 03-11-2008, 05:14   #1
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Outboard vs Inboards on a Cat

I recently stumbled upon an older MkII with a small outboard here in Michigan. Now my first thought is, while probably fine for Great Lakes sailing, the thought of taking that engine around the globe is suicide. So, that got me thinking, why not slap a newer 4 stroke on the back and go for it?

Someone with way more knowledge than I have (which is pretty much everyone here) enlighten me on why this is, or isn't a good idea?
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Old 03-11-2008, 09:21   #2
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Well, the answer might depend in part on what a MKII is. A quick Google search reveals "MKII" catamarans made by St. Francis, Simpson, Iroquois, PDQ, Catalac, Endeavour, etc.

It would be much better if the boat was designed for outboard power, and better yet if it had two. Either way you have the disadvantage of gasoline and the need to find some way to charge batteries, etc.
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Old 03-11-2008, 09:26   #3
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The greatest factor is the size of the boat. The larger the boat, the less time the outboards propeller will spend in the water. All you have do is imagine how many more feet a larger boat pitches in chop or swell. Beyond a certain length, probably somewhere in the 30+ ft range and you will want an inboard.
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Old 03-11-2008, 09:43   #4
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Send a message via Skype™ to gosstyla
google: bill mathers shearwater catamaran

A 57ft cat with 2 x 36hp Yanmar outboards.
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Old 03-11-2008, 10:09   #5
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It depends on the placement of the outboards as well, PDQ catamarans have outboards in engine wells inside the cockpit, well forward of the transom so they do not pull out of the water while the boat is pitching. I would say a lot depends on the setup. The gasoline tanks should be in a place where they are isolated from any possible sparks and where they would vent quickly and easily. Again, the PDQ had them under the floor of the cockpit, where any breach would be completely isolated and vent immediately below the boat. All in all I'd give a catamaran with completely retractible outboards a consideration. You can carry a complete spare engine, and you can completely remove them for maintanence. People who know and can fix outboards unlike diesels can be found in just about every village. You also have the advantage of being able to completely retract your most expensive and complicated piece of equipment, the engine, from the water and that makes it's maintanence and inspection far easier. Last many outboard driven catamarans have the engines in their own seperate engine well where any smoke or fuel is completely isolated from the living quarters. In my mind, for someone who wants to travel cheaply, it's a viable solution if done well. Supplying outboards with good fuel by filtering for both water and particulates will also eliminate most of your outboard problems.
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Old 03-11-2008, 10:19   #6
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I had a single ultra long legged diesel outboard on my Catalac 9m.

Why - because it previously had a petrol outboard anf this was the easiest upgrade path

would I do it again. - local cruising and a 4 stroke 30 hp with a long enough leg so it doesnt cavitate - probably

major disadvantages:

1. Lack of electrical power generation (Major problem)
2. Lack of manoeuvrability by comparison to two inboards
3. Uses much more fuel (even the diesel, as it is spinning a small prop very quickly in already disturbed water.)
4. Much more susceptible to corrosion.

Major advantages:

1. Additional space. (one outboard leaves two large engine compartments)
2. Ability to lift prop out of the water + for sailing and + for lobster pots

I beefed up the transom, and then fitted hydrofoil wings on the cavitation plate - made quite a difference to bad weather capability. However, the skeg on my motor was the deepest part of the boat. But I never had a problem with cavitation, and was able to work directly to windward in stronger winds than the 9m catalacs with two normal inboard engines.
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Old 03-11-2008, 11:01   #7
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Originally Posted by schoonerdog View Post
It depends on the placement of the outboards as well, PDQ catamarans have outboards in engine wells inside the cockpit, well forward of the transom so they do not pull out of the water while the boat is pitching. I would say a lot depends on the setup. The gasoline tanks should be in a place where they are isolated from any possible sparks and where they would vent quickly and easily. Again, the PDQ had them under the floor of the cockpit, where any breach would be completely isolated and vent immediately below the boat. All in all I'd give a catamaran with completely retractible outboards a consideration. You can carry a complete spare engine, and you can completely remove them for maintanence. People who know and can fix outboards unlike diesels can be found in just about every village. You also have the advantage of being able to completely retract your most expensive and complicated piece of equipment, the engine, from the water and that makes it's maintanence and inspection far easier. Last many outboard driven catamarans have the engines in their own seperate engine well where any smoke or fuel is completely isolated from the living quarters. In my mind, for someone who wants to travel cheaply, it's a viable solution if done well. Supplying outboards with good fuel by filtering for both water and particulates will also eliminate most of your outboard problems.

I have the PDQ32. The outboard placement is very nice except when they foul from crabpots; half the time that will involve a swim to clear.

But for staying in the water they are great. I exited Ft. Pierce on a falling tide with a strong East wind. Made for steep seas at least 8 feet. I buried both bows at the base of every wave but the engines stayed in the water and pushed us out nicely. Plus, I filled the tank at the beginning of the season, haven't touched it since.

The main benefit of the outboards is maintenance. I can completely repower the boat without pulling it out of the water.

Tom
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Old 03-11-2008, 11:22   #8
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why drag your feet in the water?

1. PDQ 36's with outboards raised are ONE KNOT faster than Diesels owing to the weight of the engines and form drag of the drive leg.

2. Solar panels and wind generators provide battery charging, at much less expense, simpler maintenance, and no fuel to run out of.

3. You can pull an outboard out in the sunshine, fix it with a few hand tools, pat it on its little powerhead and get back under way while enjoying a remote anchorage. Pay yourself whatever you think you're worth, and communicate with the repairman in your own language!

4. Eighty seven percent of all recreational boats in the US are gasoline powered. Insurance companies don't provide discounts for diesel boats. The threat of explosion is overstated, and practical precautions work.


So when do outboards not work for cats?

1. When you can't find them big enough with the features you need, such as shaft length, very low gear ratios, big slow turning props, remote controls, etc.

2. When they cannot be located close to the center of motion.

3. When you need to bolt on extra alternators, hydraulic pumps, refrigeration, etc.

4. When you can't carry enough fuel outside the living compartments to get where you want to go.

That means the practical size limit for outboard powered boats is about forty to forty five feet. "Big Foot" and "high thrust" engines aren't really ideal unless you can get close to a 3:1 gear ratio and have room for 12" or bigger diameter blades. That is why the Yamaha High thrust 9.9s are the almost universal choice. Not many bigger engines have been tried, and while the Yanmar Diesel outboards looked really good on paper and in limited practice, they were heavy, complicated and expensive.
when an outboard ets over about 150# its no longer easy to haul around a moving boat.

I'd love to see some "Down-under" wizzards modify a major manufacturer's 20 to 30 hp outboard to run efficiently at 10 knots, with a max output at 12 knots. That's asking a lot, but its what a light 45' cat would need.
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Old 03-11-2008, 16:53   #9
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Sorry about the lack of detail in the earlier post. I've been pondering it for a few days and decided to say to heck with it and throw it on here before work this morning.

The cat is an Iroquois, I would guess early 70's thirty footer. I'm guessing at MKII because the guy new it was an Iroquois and 30', but not a whole lot else.

I'm working on (and would rather spend the $$$) on a solar and wind powered electrical system where regardless of inboard/outboard, we do not need to fire up the engine to charge our power system. I've got a 60 acre camp now that is off the grid. This type of energy system is something that I need to research anyhow.

My biggest concern is fuel. Not from fire (I crash cars for a hobby, if you have a fire from a marine fuel tank you really screwed something up. Trust me on that one). From what I could see, this specific cat (and maybe others are different) was rigged with a 6 gallon metal tank. Assuming a gallon an hour, you're toast in 6 hours. I would have to figure out a safe location for 20-50 gallons minimum.
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Old 03-11-2008, 19:50   #10
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Originally Posted by sandy daugherty View Post
1. PDQ 36's with outboards raised are ONE KNOT faster than Diesels owing to the weight of the engines and form drag of the drive leg.

2. Solar panels and wind generators provide battery charging, at much less expense, simpler maintenance, and no fuel to run out of.

3. You can pull an outboard out in the sunshine, fix it with a few hand tools, pat it on its little powerhead and get back under way while enjoying a remote anchorage. Pay yourself whatever you think you're worth, and communicate with the repairman in your own language!

4. Eighty seven percent of all recreational boats in the US are gasoline powered. Insurance companies don't provide discounts for diesel boats. The threat of explosion is overstated, and practical precautions work.


So when do outboards not work for cats?

1. When you can't find them big enough with the features you need, such as shaft length, very low gear ratios, big slow turning props, remote controls, etc.

2. When they cannot be located close to the center of motion.

3. When you need to bolt on extra alternators, hydraulic pumps, refrigeration, etc.

4. When you can't carry enough fuel outside the living compartments to get where you want to go.

That means the practical size limit for outboard powered boats is about forty to forty five feet. "Big Foot" and "high thrust" engines aren't really ideal unless you can get close to a 3:1 gear ratio and have room for 12" or bigger diameter blades. That is why the Yamaha High thrust 9.9s are the almost universal choice. Not many bigger engines have been tried, and while the Yanmar Diesel outboards looked really good on paper and in limited practice, they were heavy, complicated and expensive.
when an outboard ets over about 150# its no longer easy to haul around a moving boat.

I'd love to see some "Down-under" wizzards modify a major manufacturer's 20 to 30 hp outboard to run efficiently at 10 knots, with a max output at 12 knots. That's asking a lot, but its what a light 45' cat would need.
Yamaha also make a high thrust 25, which does take a large prop. Honda do a "Powerthrust" 20 hp, which has a longer leg than either and is lighter than the Yam 9.9 HT.

"Out of the bag", an Oram 44C is equipped with Honda "Powerthrust" 20s and motors at 11 knots flat out, 8-9 knots cruise, with the standard props.
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Old 04-11-2008, 08:58   #11
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The cat is an Iroquois, I would guess early 70's thirty footer.
You actually have two problems here.

The first is the amount of fuel you will have to carry. This is a real worry and was the reason I changed to a diesel outboard. It is fine if you store it right aft from a safety viewpoint, however, that is a lot of weight aft. when you add the extra weight of the larger 4 stroke, this starts to really become a concern for an iroquois.

Have you considered fiting a single diesel and a Sonic Sillette leg (like the prouts).
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