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Old 30-10-2012, 00:51   #46
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Re: Which Outboard

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Originally Posted by manitu View Post
I dont know , my outboard , a kiekhaefer/mercury 7.5hp has been running like clockwork for one third of a decade CENTURY, only changed plugs and one coil..
AND it takes my 10 foot zodiac with inflatable keel up in plane with two people an a 100 pound dog... everytime.

but you now , once american industry was indoustrious.. and american..

.manitu
I should add that I use a 5hp prop. On 2strokes, it's important to get past the "torque-kick" to get a heavy dink up in plane. Minimal loss in top speed on an inflatable bottom anyway.
If your propeller has too high a pich, and you don't reach the exhaust port's powerband, then it does not matter wich number the motor has on the cover.

Another trick is to drill some small holes , just in front of the propeller blades. when going slow , some exhaust will force it's way out these holes and allow the propeller to spin in air/exhaust. your engine reaches the right rev's , and off you go. This is at your own risk , don't come whining to me when you have destroyed your propeller with big holes.


.manitu
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Old 30-10-2012, 17:12   #47
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This is not very good advice. Feeding exhaust ahead of the prop causes cavitation. Cavitation is destructive. Ever wonder why a outboard has less thrust in reverse? Cavitation.
Better to have the right prop.
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Old 30-10-2012, 17:28   #48
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I have had some success with the after market hydrofoil fins on my 2 stroke Yamaha 15 hp They really help get you planing earlier Simple to bolt on and go - maybe they would assist on smaller motors as well
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Old 30-10-2012, 17:52   #49
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Re: Which Outboard

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Originally Posted by Ruffriders View Post
So I think I may go with a 11' caribe rib with a 15hp Suzuki 2 stroke, both only have 10-15hours of use and have been stored inside, $4000

Any thoughts?

Could I use the main halyard to raise the dinghy to to deck? I would take the motor off first. The dinghy weighs 154lb
I have a 10' Caribe RIB with a 15hp Suzuki 4-stroke. It's a great boat, but it's not a boat I'd want to be lifting onto the foredeck with a halyard. Yes, you'd want to remove the outboard first, but even then, that's a lot of boat stow on a foredeck, especially if you're lifting with a halyard.

Bottom line: if I didn't have davits, I wouldn't own that particular boat.
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Old 30-10-2012, 18:06   #50
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Re: Which Outboard

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Originally Posted by manitu View Post
I should add that I use a 5hp prop. On 2strokes, it's important to get past the "torque-kick" to get a heavy dink up in plane. Minimal loss in top speed on an inflatable bottom anyway.
If your propeller has too high a pich, and you don't reach the exhaust port's powerband, then it does not matter wich number the motor has on the cover.

.manitu
I made the over pitch mistake
2 stroke 15hp merc on a 9 ft aluminum rigid hull
I thought that the boat was so light that a higher pitch would be fine
With an over pitched prop I did 21 kn but it was a painfuly slow and dangerious (the bow raise limited visibility)to get on step with a couple of people in the boat
Happy with 18kn and a shorter bow up time
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Old 30-10-2012, 23:53   #51
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Re: Which Outboard

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This is not very good advice. Feeding exhaust ahead of the prop causes cavitation. Cavitation is destructive. Ever wonder why a outboard has less thrust in reverse? Cavitation.
Better to have the right prop.
Yes, I know , that's why I said "at your own risk"
But.. When you are getting up to speed there is no water pressure at the exhaust opening at the propeller centre, the exhaust is actually sucked out.
so the holes should actually suck water into the exhaust stream when you get up to speed. Have used this trick on my speedboat with no negative effects , just the ability to use a higher pitched propeller.

.manitu
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Old 31-10-2012, 18:31   #52
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Exhaust gas going by the pro compresses. Compressing gas takes less energy than pushing water (thrust). This allows the motor to rev faster but at the same energy out put. As the compressed gas passes to the low pressure side of the pro it expands rapidly. This gas expansion will erode any material. (prop) for this reason exhaust outlet is always behind or above.
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Old 31-10-2012, 18:54   #53
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Re: Which outboard

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Most likely the greater torque of a 4 stroke allows for a greater pitched prop which will compensate more of the drag of the boat being pushed.
I'm always amazed when the weight issue comes up. Like a 4hp. weighs hundreds of pounds. My Nissan 6hp., 4 stroke is 49 pounds. Although O have a small crane on my radar tower, I can lift it easily without it. Lifting outboards by hand is more awkward than heavy.
I don't understand why a 4 stroke will have greater torque unless there is higher HP at your operating RPM over a 2 stroke with the same advertised gross HP.

Torque(#ft)=HP*5255/RPM
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Old 31-10-2012, 19:05   #54
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Re: Which Outboard

So you say that no matter what speed you're going , the exhaust gas will still pass trough the tiny holes in front of the propeller blades? I was under the impression that when you got up to speed , there would be enough negative pressure just behind the exhaust opening , at the rear of the prop to keep the exhaust from going trough the small holes I've drilled in front of the blades.

I've used this on my 17ft Shakespeare proV/tuned evinrude175hp w/SS prop speedboat for years. Ok , got som some small cavitation marks on it , but no worse than before drilling holes. But the acceleration.. Oh boy..

If I've dealt bad information , I'm sincerely sorry for that.

edit: @foggysail: The problem with 2strokes is'nt torque at full output , but that a 2stroke has very little power until it reaches a certain RPM.

2strokes dont have valves like a 4stroke , but depends on a exhaust port and exhaust resonance chamber. They are tuned for a specific exhaust gas speed. At lower engine speeds , much power is wasted pushing the exhaust out , and much compression "leaks" out the exhaust port. When you reach the powerband , the exhaust is sucked out by the resonance chamber , and bounces back against the exhaust port, wich increases the dynamic compression. Hence the "torque kick" when you reach the right RPM. If you have tried a motocross bike , you know what I mean.

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Old 31-10-2012, 19:17   #55
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I don't think weight is an issue here. I'm willing to bet the Yamaha is lighter.

4 strokes require no mixing.

They stall much less frequently

much less noise, and much smoother operation, especially a brand new one.
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Old 31-10-2012, 19:19   #56
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Also, Mercs are not great quality. They now sell suzuki 4 strokes under the Mercury brand. I'm not sure who is making merc 2 strokes. I don't thimk they sell them in the states.
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Old 31-10-2012, 20:48   #57
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Re: Which Outboard

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This is not very good advice. Feeding exhaust ahead of the prop causes cavitation. Cavitation is destructive. Ever wonder why a outboard has less thrust in reverse? Cavitation.
Better to have the right prop.
I'm sorry but, your point is incorrect.

Feeding exhaust to the inner portion of the prop blade is called ventilating. It is a normal thing to do, and some props do have ventilating holes built in. Ventilating causes no adverse erosion of a prop. I have a 1965 Mercruiser 60 Hp I/O with a ventilated prop. It was designed that way, and has it's original prop. In fact most of the unit is original.

When the prop starts to move faster through the water, the exhaust is pulled from the center, and doesn't ventilate any more. The design is to let the motor rev up some from low rpm, more into it's power range.

Cavitation is the point, at which the water is being pulling into a lower pressure state causing it to boil (happens at the front of the propeller blade.....where the low pressure is formed). Cavitation erodes just about anything it touches. Hydro-electric turbines suffer from cavitation and require rebuilding in certain time intervals due to this. A prop can only cavitate at high speed and/or with an unreasonable load. It is based on physical laws dealing with vacuum.

Sorry for the thread hijack.......

James L
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Old 31-10-2012, 21:07   #58
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Re: Which Outboard

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Originally Posted by propellanttech View Post
I'm sorry but, your point is incorrect.

Feeding exhaust to the inner portion of the prop blade is called ventilating. It is a normal thing to do, and some props do have ventilating holes built in. Ventilating causes no adverse erosion of a prop. I have a 1965 Mercruiser 60 Hp I/O with a ventilated prop. It was designed that way, and has it's original prop. In fact most of the unit is original.

When the prop starts to move faster through the water, the exhaust is pulled from the center, and doesn't ventilate any more. The design is to let the motor rev up some from low rpm, more into it's power range.

Cavitation is the point, at which the water is being pulling into a lower pressure state causing it to boil (happens at the front of the propeller blade.....where the low pressure is formed). Cavitation erodes just about anything it touches. Hydro-electric turbines suffer from cavitation and require rebuilding in certain time intervals due to this. A prop can only cavitate at high speed and/or with an unreasonable load. It is based on physical laws dealing with vacuum.

Sorry for the thread hijack.......

James L

No, no. That was good.
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Old 01-11-2012, 05:07   #59
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Just a word on hoisting with a halyard. I have decided against davits for the time being. We experimented hoisting onto the deck , with and without the outboard. Both are fine. I rigged up a 6-1 mechanical advantage block and tackle for the on deck manouevering. It works very well just got to secure properly. We use inflatable fenders as rest pads and truck webbing tie downs to secure. Confident of rapid full deployment is a bonus for MOB is a bonus
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Old 01-11-2012, 05:43   #60
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Re: Which Outboard

If you get the Merc, carry the best TCW3 Outboard oil you can buy, spare plugs and hose it down with fresh water regularly including under the cover.
IF you get the Yammie DO run the carb dry before a long period of no use, DO switch off the fuel when not running. The pilot/idle jet is incredibly small (economical) and WILL gunge up with dissolved solids that will block this jet.
IF you have any starting problems, or bad idling, DO NOT adjust the idle speed to compensate, go straight to the small jet and clean it.
(my preference is the Yam)
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