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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Outboard
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  #11  
Old 09-19-2007
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Although larger in diameter the blade design is more suitable for a displacement hull application than the props you are using which are more semi planning to planning applications.

The Mercury 6hp is the same outboard as the Tohatsu 6hp and Nissan 6hp so consulting with a local mercury dealer could be helpful, they have been known to let you return a prop that is not suitable after you try it as long as there is no damage to it.

Although your outboard was manufactured with a leg long enough to use on a sailboat, the gear ratio in the lower end is not ideal for swinging a large prop for slow speed operation which is why the Yamaha high thrust outboards have such a low gear ratio.

It would appear that with your 4hp you got lucky with the torque curve, gear ratio, and prop combination for your hull speed, and the 6hp is just behind the power curve for this application.



-Tim
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Old 09-19-2007
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Well, there may be a problem with the engine. We have a Tohatsu 6hp 4 stroke for our 9 ft AB RIB and I always thought it was slow.

A few weeks ago we were using it on long stretches on flat water, versus the usual short hops to the boat in the mooring field, we were going wide open then all of a sudden the RPMs increased and the boat took-off. It has not done that before and not done it since. Why would it do that? Is there an adjustment? Is something clogged?
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Old 09-19-2007
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Well this may have burst my bubble. I want one of these motors for my 9' Avon RIB to replace a 3hp Johnson 2stroke. Sounds as if i should just keep my Johnson or look for a larger 2stroke motor.
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Old 09-19-2007
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I use a Tohatsu 3.5 HP four-stroke for my Achilles inflatable, and it works just fine.
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Old 09-20-2007
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It sounds like you have a heavy boat - what is the Type/Brand? Length? and weight of the hull? If your previous engine was a 2-cylinder engine and you're trying to compare it to a single cylinder that may be where you problem lies. 2-cylinder and 1-cylinder engines will act very differently when trying to push heavy loads.
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Old 09-21-2007
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It's the prop

The real answer to this problem is a 4.5-5" pitcjed 3 bladed prop or a 3.5-4" pitched 4 bladed prop. You would then reach WOT, attaining full HP which would increase HP out put and speed (to a point on displacement boats). Thrust would go way up, and fuel economy only slighty down. Another thing to remember, 4 cycle engines don't like WOT for long periods of time, so you would never cruise at 5-5500 RPMS or you would burn up the engine. But then 2 cycle engines don't like being run at below 1/4 throttle or they foul their plugs after a while. My engine does not twist, I have the bolt that holds the collar where the engine turns tight and I do not lube this point ever. If yours is twisting, clean out the grease here with some brake cleaner and it won't turn on you anymore, nor will it steer by the tiller well unless your a gorilla.
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Old 09-22-2007
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SD - What size and model of Achilles do you have? I am looking at buying one for next year, and may buy now with the sales, and good $ exchange.
I am looking at the 9'6" or 10'4" in either the wood floor, or the inflatable floor. I will be towing most of the time, but will store in / on the boat for some trips. Weight difference is about 30 lbs more for the aluminum roll-up.
re: moters, looking at a honda 2hp (light and internal tank, just to get out to the boat / mooring) or 5hp (heavy but fun, able to cruise around a little faster)
Thoughts?
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Old 09-23-2007
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I've got the little R2U Achilles. It's a two-person boat, which I mainly use for local weekending. For long term cruising, I'm probably going to be using my 12' portabote. Much more durable.
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Old 09-25-2007
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Quote:
Originally Posted by newengsailor View Post
It sounds like you have a heavy boat - what is the Type/Brand? Length? and weight of the hull? If your previous engine was a 2-cylinder engine and you're trying to compare it to a single cylinder that may be where you problem lies. 2-cylinder and 1-cylinder engines will act very differently when trying to push heavy loads.
I have a Beneteau F235... Factory says it is 2300 lbs but most owners that have weighed them would say 3300 lbs or better.


yes 1 cylinder is different than 2 cylinder and two stroke vs 4 stroke is different. I would expect a 6 hp 2 stroke to run stronger than a 6hp 4 stroke by some small margin. but not a 6 hp 4 stroke to run weaker than a 4 hp 2 stroke.
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Old 09-25-2007
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Quote:
Originally Posted by LikiTiki View Post
The real answer to this problem is a 4.5-5" pitcjed 3 bladed prop or a 3.5-4" pitched 4 bladed prop. You would then reach WOT, attaining full HP which would increase HP out put and speed (to a point on displacement boats). Thrust would go way up, and fuel economy only slighty down. Another thing to remember, 4 cycle engines don't like WOT for long periods of time, so you would never cruise at 5-5500 RPMS or you would burn up the engine. But then 2 cycle engines don't like being run at below 1/4 throttle or they foul their plugs after a while. My engine does not twist, I have the bolt that holds the collar where the engine turns tight and I do not lube this point ever. If yours is twisting, clean out the grease here with some brake cleaner and it won't turn on you anymore, nor will it steer by the tiller well unless your a gorilla.
I will try the trick with brake cleaner.. the manual states though that the friction adjuster there was not designed to hold the engine straight when in use.

As to WOT... thats rough.. cause if I can't get about 40% more power out of the engine it will be run at WOT for 98% of the time as that is what it now takes to get the 5 NM against 2+ knots of current each way most of the time to get to where you can sail. Against the current over the ground speed will be 3.3 knots now. hour and a half trip or better one way. With the old 4hp 2 stroke I would be doing right at 4 knots over ground and I had to run it WOT to get that. Getting the new engine I hoped to be able to run it at about 3/4 power/throttle and get the same speed... sucks to be me... WOT and it is slower by a lot. Even when dropped down to the 6 pitch prop it is slower by a lot.
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