I came across a good deal on a couple of c.1985 Johnson 70-HP outboard engine [for use on a 1973 Boston Whaler OUTRAGE 21 boat which presently has a pair of BEARCAT 55-HP engines]. I decided to give the Bearcat 55-HP engines some time off.
Both of the c.1985 Johnson 70-HP engines have different propeller. I'm going to have to buy at least one new propeller. [Seeks] suggestion as to what pitch and diameter might work well with these motors mounted on a 1973 Outrage 21. Since this might be an experiment that may have to be repeated a few times, I will likely stick with aluminum propellers for the time being due to cost. Of course after running those Beacats for so many miles, I'd like some good top-end speed for a change, but I do a lot of trolling so propellers that will help with that would have to be considered. Any input would be appreciated.
1973 Outrage 21 Twin 70-HP
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Fishing Magician
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Re: Counter Rotating 70-HP
Are both of the 70-HP engines standard rotation? Or, is one counter rotating? When mounting twins [the direction of propeller rotation] matters.
Re: 1973 Outrage 21 Twin 70-HP
I don't think you can find any older two-stroke-cycle outboard engines from c.1985 at 70-HP with an option for counter rotation, and, at that power level, I don't think it really matters. I ran a pair of 70-HP standard-rotation engines on my 20-foot Boston Whaler boat for several years, and I never felt any drawback or deficiency in operation that could be attributed to having both engines turning their propeller shafts in the same direction. Not having counter rotation was really no problem, and in some respects an advantage. I only needed one spare propeller.
Don't worry about having two standard rotation 70-HP engines on the OUTRAGE 21 boat. Would you worry if you had a single 140-HP engine? No, you wouldn't; so forget about counter-rotation for 70-HP engines. It is just not available.
The rule of thumb with twin engines and propeller selection is to increase the pitch 4-inches compared to what you'd run with a single engine of that power.
As I recall, my twin 70-HP engines were turning 17-inch pitch propellers. The engines were made by Yamaha, but they were just about exact copies of the OMC engines. On that basis, I'd start with 17-pitch and see what results you get. The top speed should be about 36-MPH.
Don't worry about having two standard rotation 70-HP engines on the OUTRAGE 21 boat. Would you worry if you had a single 140-HP engine? No, you wouldn't; so forget about counter-rotation for 70-HP engines. It is just not available.
The rule of thumb with twin engines and propeller selection is to increase the pitch 4-inches compared to what you'd run with a single engine of that power.
As I recall, my twin 70-HP engines were turning 17-inch pitch propellers. The engines were made by Yamaha, but they were just about exact copies of the OMC engines. On that basis, I'd start with 17-pitch and see what results you get. The top speed should be about 36-MPH.
Re: 1973 Outrage 21 Twin 70-HP
No counter rotation on these [70-HP outboard engines], nor on the Bearcat 55-HP engines, and I never noticed any problems. I guess based on that rule of thumb, 17-pitch (and 13-inch diameter I'm assuming) will be as good a place to start as any. If the engines run close to the maximum RPM, I'll make adjustments to height, trim, toe-in, etc. A top speed of 36-MPH would be just fine. I'm anxious to know how the fuel efficiency will be, as well as how they will do when idled down for long periods of slow trolling.
One of the major things I need to do first is pull the fuel tank. I've replaced a few of those old Outrage fuel tanks over the years and I'm sure this one will be no different; even if it looks like new, I still have to install a pick-up tube for the second motor.
One of the major things I need to do first is pull the fuel tank. I've replaced a few of those old Outrage fuel tanks over the years and I'm sure this one will be no different; even if it looks like new, I still have to install a pick-up tube for the second motor.
Re: 1973 Outrage 21 Twin 70-HP
I don't understand the need for another pick up tube for the second motor. I believe a single pick up tube would serve both engines without difficulty. A simple tee in the existing fuel line would probably be all that is required.
Unless a second dedicated fuel tank for the second engine is utilized for the safety of redundancy, a dedicated fuel line is unnecessary unless the existing pick up tube lacks the capacity to flow sufficient fuel to both engines at WOT.
Unless a second dedicated fuel tank for the second engine is utilized for the safety of redundancy, a dedicated fuel line is unnecessary unless the existing pick up tube lacks the capacity to flow sufficient fuel to both engines at WOT.
Butch
Re: 1973 Outrage 21 Twin 70-HP
I'm assuming that with the 10% rule, these carbed 70's should burn a maximum of 7 gallons per hour (GPH) each, wide open, or 14-GPH total. The factory installed tanks have a pickup tube that looks to be about the size of a 3/8-inch pipe, feeding into a 3/8-inch hose barbed fitting. Would a single 3/8-inch fuel line with a tee be large enough to carry that volume, without one fuel pump depleting fuel from the other? If it is big enough, it could certainly save me a lot of trouble to use the one tube and put a tee in it.
Re: 1973 Outrage 21 Twin 70-HP
You can make the fuel system for a twin-engine outboard boat as simple or as complicated as you like. It is typical on a Boston Whaler boat to find a single internal fuel tank, with the tank equipped with two fuel pick-up tubes and two outlet fittings. In the era when Boston Whaler just sold boats, not boats and outboard engines pre-rigged, the fuel tank fuel fittings would be fitted with fuel hoses, which would run to a fitting located above deck. From that fitting, the dealer who was rigging the boat with outboard engines would complete the fuel system to the engines as they saw fit. As a result, there is a wide variation in fuel system designed found in older Boston Whaler boats.
Re having two engines connected to a single fuel supply hose with a tee fitting: I suppose it is possible that if one engine is running and the second engine is not, the running engine might be able to suck fuel from the fuel system of the non-running engine, but this generally does not happen because the usual rigging it to have an individual primer ball device for each engine, located beyond (or downstream of) the common point.
A primer bulb typically has two check valves, one at inlet and another at outlet. The insertion of a primer bulb in the fuel hose feeding each engine tends to stop any flow of fuel from the engine back to the source. Flow in that direction is impeded by the closed check valves.
Re having two engines connected to a single fuel supply hose with a tee fitting: I suppose it is possible that if one engine is running and the second engine is not, the running engine might be able to suck fuel from the fuel system of the non-running engine, but this generally does not happen because the usual rigging it to have an individual primer ball device for each engine, located beyond (or downstream of) the common point.
A primer bulb typically has two check valves, one at inlet and another at outlet. The insertion of a primer bulb in the fuel hose feeding each engine tends to stop any flow of fuel from the engine back to the source. Flow in that direction is impeded by the closed check valves.
Re: 1973 Outrage 21 Twin 70-HP
Yes, I forgot about the primer bulb check valve, that would solve the back draw problem. Could two of these carburetor 70-HP engines draw enough fuel through the single 3/8-inch primary line and pickup tube? My guess would be "probably?"
Re: 1973 Outrage 21 Twin 70-HP
I do not see using a 3/8-inch-ID fuel hose as an encumbrance. I ran an E-TEC 250 H.O. (about 275-HP) from a single 3/8-inch-ID fuel hose. The fuel flow rate was probably more than 25-GPH. On that basis I do not think that a fuel flow rate of 14-GPH would create any problem due to restriction of fuel flow in a 3/8-inch fuel line.
If you have any doubts and want to get a better measurement, connect a vacuum gauge to the fuel system and measure the suction pressure. The vacuum or suction pressure on the fuel hose should be kept under about 4-PSI. Very high vacuum or suction pressure means the fuel pump is having a hard time getting fuel to flow into the pump from the tank.
If you have not already read my article on fuel hose primer bulbs in the Reference section, I recommend it. It is a good summary of how primer bulbs work, how to install them, and what kind to get. (Get the OEM bulbs--the aftermarket ones are generally not very good.)
If you have any doubts and want to get a better measurement, connect a vacuum gauge to the fuel system and measure the suction pressure. The vacuum or suction pressure on the fuel hose should be kept under about 4-PSI. Very high vacuum or suction pressure means the fuel pump is having a hard time getting fuel to flow into the pump from the tank.
If you have not already read my article on fuel hose primer bulbs in the Reference section, I recommend it. It is a good summary of how primer bulbs work, how to install them, and what kind to get. (Get the OEM bulbs--the aftermarket ones are generally not very good.)
Re: 1973 Outrage 21 Twin 70-HP
Good article. Wish I read it a few years ago, before I struggled with aftermarket primer bulbs that I bought for the Bearcats. The check valves failed in two expensive, brand new bulbs within a few days. Hard to admit it when a product turns out to be junk out of the box. I removed and disassembled carbs, fuel pumps, vents, etc. for days before I tried some old oem mercury bulbs that I had laying around. The faded, hard, 20 year old oem bulbs worked great, the brand new ones wouldn't work in any position. On the outside chance that the fuel tank looks good when I remove it, I'll coat it with Gluv-It, put a T in the single fuel line and see how they run.
By the way, I had an Evinrude carburetor V6 that wouldn't run wide open with a 5/16-inch fuel line, it had to be 3/8-inch, which was the cause for my concern with the twin 70's.
By the way, I had an Evinrude carburetor V6 that wouldn't run wide open with a 5/16-inch fuel line, it had to be 3/8-inch, which was the cause for my concern with the twin 70's.
Re: 1973 Outrage 21 Twin 70-HP
My c.1992 Evinrude V6 engine had a vacuum switch placed in the fuel hose supplying fuel to the inlet of the fuel pump. If the suction exceeded the threshold of the vacuum switch, a contact closed and the operator got an alarm alert. See my article about warning horn signals and how to interpret them. It mentions the FUEL RESTRICTION alarm:
http://continuouswave.com/whaler/refere ... gnals.html
http://continuouswave.com/whaler/refere ... gnals.html
Re: 1973 Outrage 21 Twin 70-HP
It turns out that one of these Johnson 70-HP engines is a 1985 and the other is a 1986. I've read that the pre-1986 engine has a 49.7-cubic-inch displacement and the post-1986 has a 56.1-cubic-inch displacement. In addition, the red line was raised 6,000-RPM from from 5,500-RPM at some time in the 1986 model year. I haven't figured out if my 1986 actually has the larger displacement and higher redline yet, but (the big question is) can I run these two engines as twin engines? Will the difference in displacement make it a problem synchronizing them even though they're both rated at 70-HP?
Maybe this is a question for another thread, or a vintage OMC expert.
Maybe this is a question for another thread, or a vintage OMC expert.
Re: 1973 Outrage 21 Twin 70-HP
Your question is interesting. I look at it this way:
When operating twin engines, the concern for the engines to be synchronized is that each engine must turn its propeller shaft at the same speed. On that basis, if both engines have the same gear ratio in their gear case, both have the same pitch and same diameter propeller, and in your situation both have actually the same propeller because both propellers are right-hand propellers, then we can assume that when each engine rotates its propeller at the same speed as the other, each engine is producing the same amount of thrust propelling the boat forward. We consider that the propeller shaft rotation is always proportional to engine crankshaft rotation, because the gearing is strictly mechanical and does not have any slipping. Therefore, if you are cruising along at, say, 4,000-RPM on each engine, then each propeller shaft is turning the same speed. And since each propeller shaft has the same propeller, the engines must be doing equal work, or as equal as we can ever know with a tachometer as our only instrument to measure engine load.
What may not be identical for each engine is the position of the throttle that is needed to create this same power output. One engine might need more throttle than the other to develop the same amount of power so it can turn its propeller shaft at the same speed as the other engine is turning the other propeller shaft. You could encounter that with any two engines, even if they were mechanically identical, just due to the variations in the performance of the individual engines. Thus you might see that one engine consumes more fuel than the other in order to turn the propeller at a matching speed. But that's not what we are trying to synchronize. We just want the propeller shaft speed synchronized.
In summary, I think as long as the propellers and the gear ratios are matched, when the engines turn those propellers at the same shaft speed the engine power output will be matched.
When operating twin engines, the concern for the engines to be synchronized is that each engine must turn its propeller shaft at the same speed. On that basis, if both engines have the same gear ratio in their gear case, both have the same pitch and same diameter propeller, and in your situation both have actually the same propeller because both propellers are right-hand propellers, then we can assume that when each engine rotates its propeller at the same speed as the other, each engine is producing the same amount of thrust propelling the boat forward. We consider that the propeller shaft rotation is always proportional to engine crankshaft rotation, because the gearing is strictly mechanical and does not have any slipping. Therefore, if you are cruising along at, say, 4,000-RPM on each engine, then each propeller shaft is turning the same speed. And since each propeller shaft has the same propeller, the engines must be doing equal work, or as equal as we can ever know with a tachometer as our only instrument to measure engine load.
What may not be identical for each engine is the position of the throttle that is needed to create this same power output. One engine might need more throttle than the other to develop the same amount of power so it can turn its propeller shaft at the same speed as the other engine is turning the other propeller shaft. You could encounter that with any two engines, even if they were mechanically identical, just due to the variations in the performance of the individual engines. Thus you might see that one engine consumes more fuel than the other in order to turn the propeller at a matching speed. But that's not what we are trying to synchronize. We just want the propeller shaft speed synchronized.
In summary, I think as long as the propellers and the gear ratios are matched, when the engines turn those propellers at the same shaft speed the engine power output will be matched.
Re: 1973 Outrage 21 Twin 70-HP
That's the kind of logic that I was hoping for, and if one of them burns more fuel than the other it won't really matter; and I would never know, anyway, because I plan on using the one tank and pickup tube. Compared to those Bearcats, I figure my overall fuel consumption will double this season anyway. Thanks for the input.
Re: 1973 Outrage 21 Twin 70-HP
The OMC [in-line three-cylinder two-cycle outboard engines] were changed partially as a response to the rating system change, the two 70-HP engines are not putting out equal power. The earlier one is, at best, putting out closer to 60-HP. If you really are running two different 70-HP engines, with one being the greater displacement, it is hard to imagine that running them as twins will be entirely satisfactory. Even if they are turning the same gear ratio, and turing the same props, they would not be evenly loaded.
Re: 1973 Outrage 21 Twin 70-HP
Could the difference in output between 1985 and 1986 really be that extreme, considering that they were both made by the same manufacturer and both were prop-rated at 70-HP? And may have been assembled within weeks of each other? It seems like, in spite of a possible displacement difference, a little tuning and carburetor work would be able to get them close in performance. And I haven't been able to find any information on the web that could tell me if EVERY 1986 model year had the higher displacement, or exactly when the upgrade went into production. Everything I've read said "before or after 1986" with regard to the change, and nothing said whether all of the 1986's were included or just some of them after a certain serial number.
Re: 1973 Outrage 21 Twin 70-HP
gold stem wrote:Even if they [two engines] are turning the same gear ratio, and turing the same props, they [the two engines] would not be evenly loaded.
I cannot endorse that statement. I do not see any basis for it. The propellers do not know what engine is turning them. If two identical--well, nearly identical--propellers on two adjacent shafts turn at the same speed, the power needed to turn them is the same. How would one propeller know it was supposed to impose a greater load on the engine turning it because there is some minor difference in the engine trying to turn it. The load that is imposed on the engine is a function of how fast the propeller is turning, the nature of the propeller, the weight of the boat, the shape of the hull, the wind, the waves, and countless other factors. But the propeller cannot ever know that it is being turned by a 1986 OMC engine and therefore ought to change the load it is creating on the propeller shaft. That is not possible.
The concept of matching the crankshaft rotation speed of the twin engines (by setting to the same RPM on the tachometer) is based on this notion: the load is proportional to propeller shaft rotational speed, which is strictly proportional to crankshaft rotation speed.
Re: 1973 Outrage 21 Twin 70-HP
That makes a lot of sense to me, Jim. I'm going to locate a couple of 17 x 13 props, set up the motors, run them, and see what happens. I'm betting (hoping) that at the most, I'll have to set different adjustments on my throttle cables and linkages to keep the morse control levers close. Unfortunately, even though they are predicting an unbelievable 65 degrees on Christmas eve here in upstate NY, I still wont be able to report any results until spring. Guys are actually catching Lake Trout from shore on Lake Champlain, like surfcasting for Bluefish; crazy year, usually driving trucks on the lakes by now.
Re: 1973 Outrage 21 Twin 70-HP
The only element of difference between the two engines that would throw a monkey wrench into the works is the gear ratio. As long as the two engines have the same gear ratio, the load of the propeller is being translated to a load on the engine at the same rate through the gearing.
Look at the problem with this analogy: we have two 100-lbs weight attached to two rope-and-pulley hoist systems. The two rope-and-pulley hoists are identical and provide a certain mechanical advantage, say 3:1. Assuming a lossless system, to lift the 100-lbs of weight, we only have to pull the rope with 33-lbs of force.
Now two men each attempt to lift the weights, each pulling on one of the hoists individually. One is a skinny guy with very little musculature and the other has much greater strength. They each pull, one at a time, on their rope and lift their weight. Do you think that the force exerted by the muscle-builder was any greater or less than the force from the skinny guy? No, of course not.
We could make the situation a bit different if we tied the two weights together and tied the two hoists together. Perhaps the skinny guy hardly pulls at all and just takes in slack in his rope and pulley as the the strong man pulls up both weights.
We don't have that sort of hard mechanical linkage between the engines and propellers in a twin engine boat. Each engine turns its own propeller. We can have something like the situation above if we let one engine run at a lower speed. If one propeller is not turning as fast as the other, it gets to go along for the ride in some respect, because the other propeller has taken more load. That is, of course, why you want to keep the propeller shaft speeds matched--to keep the loads equal.
Look at the problem with this analogy: we have two 100-lbs weight attached to two rope-and-pulley hoist systems. The two rope-and-pulley hoists are identical and provide a certain mechanical advantage, say 3:1. Assuming a lossless system, to lift the 100-lbs of weight, we only have to pull the rope with 33-lbs of force.
Now two men each attempt to lift the weights, each pulling on one of the hoists individually. One is a skinny guy with very little musculature and the other has much greater strength. They each pull, one at a time, on their rope and lift their weight. Do you think that the force exerted by the muscle-builder was any greater or less than the force from the skinny guy? No, of course not.
We could make the situation a bit different if we tied the two weights together and tied the two hoists together. Perhaps the skinny guy hardly pulls at all and just takes in slack in his rope and pulley as the the strong man pulls up both weights.
We don't have that sort of hard mechanical linkage between the engines and propellers in a twin engine boat. Each engine turns its own propeller. We can have something like the situation above if we let one engine run at a lower speed. If one propeller is not turning as fast as the other, it gets to go along for the ride in some respect, because the other propeller has taken more load. That is, of course, why you want to keep the propeller shaft speeds matched--to keep the loads equal.
Re: 1973 Outrage 21 Twin 70-HP
Jim Dunlap (Jimd on the old forum) has a 1976 21 Revenge with twin 1976 70hp Johnsons. Both are standard rotation and he has done a lot of work to settle out the right mounting heights and has done a fair bit of propeller testing. I know he recently settled a set of Stiletto Stainless Props, but I can not find what size and pitch they were.
The boat performs very well with these motors and tops out in the high 30's.
The boat performs very well with these motors and tops out in the high 30's.
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Re: 1973 Outrage 21 Twin 70-HP
That is great, I'll will try to pm him if that's possible to get some advise on engine height and spacing. I think I've seen photos of that boat underway before, very nice, and high 30's would be terrific. I had a Revenge 19 with twin Bearcat 55's on it, that boat also only ran about 25 mph WOT. I felt that the 19 Revenge just didn't have enough deck space for fishing, especially with more than 2 people.