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  #1  
Old 08-25-2012
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Is 6Hp 4-Stroke Enough?

Hi,

I am trying to get a backup outboard but want the bare minimum to save on money. I have a 30' islander. Is a 6hp 4-stroke Tahatsu enough to move it?

Any help appreciated.
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Old 08-25-2012
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Is 6Hp 4-Stroke Enough?

I can understand the temptation but wouldn't it be better to invest in you diesel? The engine already is a backup - you've got two sails.
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Old 08-25-2012
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Re: Is 6Hp 4-Stroke Enough?

6 hp wouldn't be enough if there was any wind or current. even with the 6 hp you will probally move around .5- 1knot if that. I have a 8 hp on my sailboat it's 7 thousand lbs and the 8 hp makes it go about 5mph max at full throttle maybe 6
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Old 08-25-2012
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Re: Is 6Hp 4-Stroke Enough?

If you go with the currents it will be enough ... I use a 8hp 4 stroke honda on a 40 ft boat and it works well enough for me ... yes i did get rid of the diesel ... one of the best things I ever did!!!!
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Old 08-26-2012
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Re: Is 6Hp 4-Stroke Enough?

If the engine isn't sufficient to move the boat at hull speed, you will loose steerage when you go with a strong tidal current. Keep in mind that the conditions for which you need to use an engine are not usually dead calm. High winds, waves, strong tidal currents, docking, tight inlets, holding the boat into the tide while awaiting a draw bridge raising, etc..., all require full control of the boat. That tiny, 6-HP outboard just wouldn't cut it. A lot of smaller, inboard diesels won't cut it either. There are lots of times I see some fairly large sailboats struggling against the tidal currents with their tiny, 1 or 2 cylinder diesel inboards.

I'm not a big proponent of diesel engines. The only two sailboats I've owned during the past 7 years have been powered with 30-HP Universal Atomic-4 gasoline engines. These engines were built to take lots of abuse, they're extremely reliable, and I think you would have to shoot one with a high-powered rifle to kill it, and that might not do the job.

If you're intent on using an outboard, look for something in the 25 HP range - it's not much heavier, doesn't burn a lot of gas, and you'll have no trouble moving the boat in marginal weather conditions.

Good Luck,

Gary
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Old 08-26-2012
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Re: Is 6Hp 4-Stroke Enough?

Quote:
Originally Posted by travlineasy View Post
If the engine isn't sufficient to move the boat at hull speed, you will loose steerage when you go with a strong tidal current. ...
Huh? The only thing one needs to worry about for steerage is the boat's speed through the water. The speed over the ground is irrelevant for steerage. Imagine going with a current of 3 kts and having an engine capable of pushing the boat at 3 kts, the SOG would be 6 kts. It doesn't matter what hull speed is, the boat and its rudder are still "feeling" 3 kts of flow. Turn around, so that the boat is now going against the current, and guess what? the boat and its rudder are still "feeling" 3 kts of flow, even though the SOG is zero.

Quote:
Originally Posted by travlineasy View Post
...Keep in mind that the conditions for which you need to use an engine are not usually dead calm. High winds, waves, strong tidal currents, docking, tight inlets, holding the boat into the tide while awaiting a draw bridge raising, etc..., all require full control of the boat. That tiny, 6-HP outboard just wouldn't cut it. A lot of smaller, inboard diesels won't cut it either. There are lots of times I see some fairly large sailboats struggling against the tidal currents with their tiny, 1 or 2 cylinder diesel inboards....
A 6 hp outboard will probably push an I-30 at about 3/4 of hull speed. A few things to consider -- First, the relationship between power and boat speed isn't linear. The closer the boat is to hull speed, the steeper the curve. In other words, far less than half the power necessary to achieve hull speed is necessary to achieve half of hull speed. Second, the power of an outboard is measured at the prop, while the power of an inboard is measured at the flywheel. So just about all the rated power of an outboard goes toward pushing the boat, while much of the rated power of an inboard is soaked up by the transmission, accessories, shaft bearings, et cetera. Most sailboats in the US are over-powered relative to sailboats in Europe. Somehow they get by just fine.

Quote:
Originally Posted by travlineasy View Post
...I'm not a big proponent of diesel engines. The only two sailboats I've owned during the past 7 years have been powered with 30-HP Universal Atomic-4 gasoline engines. These engines were built to take lots of abuse, they're extremely reliable, and I think you would have to shoot one with a high-powered rifle to kill it, and that might not do the job....
If you're gonna put an engine in the bilge, make it a diesel. A gas engine is fine if you hang it off of the transom, and keep the fuel tank out of the bilge. But, you're far better off to keep the gasoline in your car and power the boat with diesel. Modern diesels will last virtually forever if properly maintained, and are far safer than gas engines.

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Originally Posted by travlineasy View Post
If you're intent on using an outboard, look for something in the 25 HP range - it's not much heavier, doesn't burn a lot of gas, and you'll have no trouble moving the boat in marginal weather conditions...
WAY more power than you need for an I-30. A 9.9 or 12 hp outboard (properly propped) is plenty of power to get that boat to hull speed in calm conditions, and a significant fraction of hull speed in all but the nastiest conditions. Even the 6 hp outboard would be fine, IMHO. However, a 15 to 20 hp inboard diesel would probably be more dependable, and would preform much better in nasty conditions since you're much more sure that the prop would stay in the water.
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Last edited by SlowButSteady; 08-26-2012 at 02:38 AM.
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Old 08-26-2012
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Re: Is 6Hp 4-Stroke Enough?

Yes a 6hp would be too small for your boat. It might get close to hull speed in flat water with no wind but throw some waves or headwind into the mix and you will go nowhere!

Quote:
There are lots of times I see some fairly large sailboats struggling against the tidal currents with their tiny, 1 or 2 cylinder diesel inboards.
Any boat will struggle against currents that approach their cruising speed! If your engine can propel you at 5kts then it stands to reason that you can't go against 5kts of current! My "tiny" 12hp diesel propels my 8600lb boat at hull speed. More power isn't going to improve it much! That's why we have to pay attention to currents!

Quote:
If the engine isn't sufficient to move the boat at hull speed, you will loose steerage when you go with a strong tidal current.
That doesn't make any sense at all! If you are going 4kts and you have current going with you at 6kts, you are still going 4kts through the water! The only difference is that the trees are going by at 10kts!

Quote:
I'm not a big proponent of diesel engines. The only two sailboats I've owned during the past 7 years have been powered with 30-HP Universal Atomic-4 gasoline engines.
I'm glad you like your Atomic Bomb, but did you ever wonder why no manufacturer puts gasoline inboards into sailboats anymore? It is because diesel is vastly superior in every way!
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Old 08-26-2012
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Re: Is 6Hp 4-Stroke Enough?

I have a 1967 Islander 29 with the original Atomic 4 ( that i rebuilt ). It gets me up to 7 mph.

In my opinion, I would like to go faster than that sometimes (most of the times).

Recently I saw a sailboat with a huge outboard that was speeding through the water at ungodly speeds. I was jealous!

Get as big an engine as you can afford. Bigger is better!

Want versus need? To me, 6hp is not enough. If it's cheap and you find it isn't enough, you can always use it on your dingy or inflatable.
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Re: Is 6Hp 4-Stroke Enough?

Quote:
Originally Posted by jpian0923 View Post
I have a 1967 Islander 29 with the original Atomic 4 ( that i rebuilt ). It gets me up to 7 mph.

In my opinion, I would like to go faster than that sometimes (most of the times).

Recently I saw a sailboat with a huge outboard that was speeding through the water at ungodly speeds. I was jealous!

Get as big an engine as you can afford. Bigger is better!

Want versus need? To me, 6hp is not enough. If it's cheap and you find it isn't enough, you can always use it on your dingy or inflatable.
Are you sure that wasn't a MacGregor you saw? Those things are more powerboat than sailboat!

Most of the time with sailboats, if you want to power faster, get a bigger boat! Your Atomic has more than enough power to push your boat. A bigger engine isn't going to do anything more than burn more fuel!
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Old 08-26-2012
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Re: Is 6Hp 4-Stroke Enough?

get a 15hp and call it a day. 8hp and under is far to little. my 27' has a 8 hp and i make it through the current at .2-.5 mphs when its rippins it's a bit scary not sure if youll get one wave that wipes out my motor and it stops running. But the thing is your going to small with your motor your better off useing a troling motor in that case. But if you wanna have a safe back up motor have a motor that can move you through tough currents, decent waves, with a head wind. and then you will have a safe back up motor.
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