What it took for me to add a second motor

[DVW]

A outboard motor is bolted on with only four bolts. I found there is more to the task than four bolts-much much more. It starts with more buoyancy to handle the weight of the extra motor. I choose to replace the two tubes with jumbo tubes rather than having the drag associated with three tubes. A two key switch panel replaced the original single switch. The single control lever got replaced by a dual lever control with new cables. Trim control got moved to the steering wheel because my dual levers lacked switches. Extra gauges went with the extra motor. In my case I cut or enlarged 8 extra holes in the console. A console wire harness had to be created to interface with an additional engine harness. My project demanded a new fuel tank in a new spot. Steering the extra motor required hydraulic steering with the steer cylinders plumbed in series with each other. I choose to link the steer cylinders hydraulically. This choice required a realignment valve. Everything had to be plumbed with many hoses. The choice of props is still being refined. In the end it was much more than four bolts.

 

[CLDave]

This is the best DIY post of all time. Mad "props" to you sir! Would love to see more details and video on the water.

 

[Bill N]

Very interesting.
Nice clean professional job.
Why the jack plates?

 

[DVW]

Very interesting.
Nice clean professional job.
Why the jack plates?

Thank you for the compliment.
What appears to be a jack plate is actually the transom mounted 10 inches back from the end of the tube. This offset has to do with the way boats respond with more speed when the motor is trimmed up. This happens despite the fact that the thrust angle wants to push the boat deeper in the water. This tradeoff is reduced when the motor is mounted back from the end of the tube. Another benefit has to do to with motor height. As you can see in the attached photo my motor height is high. This height reduces lower unit drag but would not be possible without the motor offset.

 

[Dunn]

Sweet project, How much speed did you pick up before and after, cruise and top end?

 

[Shmily]

Interesting. Sorry about fuel prices. The "drag" of three pontoons is more than offset by the benefit of increased waterline...

It would be interesting to know before and after performance numbers.

 

[DVW]

I almost doubled the speed of the boat mostly thru drag reduction. The top speed of 50 mph is limited by porpoising. There is more speed to be had if I could deal with the porpoising. I am not a speed freak but what I got is a boat that moves thru the water with less effort at normal cruise speeds. Often times in a pontoon a normal cruise speed is 5 mph. A brisk pace is effortless and does not interrupt conversation.

 

[DVW]

Interesting… The "drag" of three pontoons is more than offset by the benefit of increased waterline...

It would be interesting to know before and after performance numbers.

I am confused by the statement above. As you can see by the attached photo the narrow space between a three tube pontoon causes the waterline to rise up to the under deck. Tri-toons require under deck sheeting for that reason. My top speed was achieved without under deck sheeting because the water between the tubes was relatively undisturbed. At rest the displacement of my jumbo tubes offset their reduced quantity.

 

[Bill N]

50 MPH with two 115's is amazing!

 

[mtudb24]

That is a fantastic project. Well done and nicely laid out. Maybe I missed it somewhere, but what props did you settle on?

 

[DVW]

That is a fantastic project. Well done and nicely laid out. Maybe I missed it somewhere, but what props did you settle on?

I have not settled on a prop because I have as be not solved the porpoising problem. When (if) I solve that problem my cheap aluminum props with 19 pitch will probably limit top speed. For now I get up to porpoising speed quickly. Any ideas on how to how to eliminate porpoising without a significant drag penalty would be appreciated.

 

[Vikingstaff]

Any ideas on how to how to eliminate porpoising without a significant drag penalty would be appreciated.

Sandbags up front!

Sorry…inside joke in the forum. Couldn’t resist. And to be clear…not a real suggestion!

 

[Timdelta]

Maybe shims under motors for more trim or the fins that can be put on the motors?

 

[DVW]

Maybe shims under motors for more trim or the fins that can be put on the motors?

More trim is key but unfortunately my boat cannot tolerate the trim range that it has now. My steady state top speed is achieved with relatively modest trim. If I quickly increase trim I am rewarded with a increase in speed until purposing shuts down the party. My current thought is along the lines of your second suggestion. Since my so called cavitation plate is out of the water at speed I am thinking of extending it backwards (not wider) about 2 feet. The trailing edge would drag in the water and maybe counter the instability. Increased drag could also offset any benefit. That is the problem with most common solutions for purposing.

 

[Bill N]

Are you saying that trimming down does not eliminate the porpoising?

 

[DVW]

Are you saying that trimming down does not eliminate the porpoising?

Trimming down does eliminate porpoising but it also adds drag and slows the boat. My goal is to find the maximum speed potential with higher trim angles. That potential is not being reached because even at modest trim angles the boat starts to porpoise. With sorted out planing hulls you will see increased speed with increased trim until a point of diminishing return. Instead of that my boat porpoises. With full throttle if I trim up to a speed right below porpoising speed I will go 49.5 mph. If I quickly trim up at that point I can feel acceleration and see increased speed until porpoising stops the speed increase. In that moment I have observed 53 mph. The porpoising starts with mild oscillation but will grow wild if I let it. For a while I had turn signal style trim controls. With these controls I had to have both hands on the wheel. When the oscillating got wild I had to grip the wheel with both hands. When I let go to throttle back my right arm went up and down like a rodeo bull rider. As a result I initially missed the throttle. When I did eventually throttle back that action created a reaction so violent that I almost got bucked off. It was my eight seconds. All that in a pontoon with modest power and speed.

 

[Bill N]

My initial thought is that you have reached the potential of your particular hull/boat/motor combo. But You clearly believe that you are simply against a barrier that can be overcome. With all of your experience in this, I am betting your instinct is better than mine. Please share as you learn more. I am looking to learn more too!
When younger I had a 15' boat rated for an 85 hp that I hung a 100 on. It would begin chine walking at about 49 mph, and I always felt that if I had just a little more power I could get beyond that and it would smooth out. I think that is why I am so intrigued by your project.

 

[mtudb24]

Trimming down does eliminate porpoising but it also adds drag and slows the boat. My goal is to find the maximum speed potential with higher trim angles. That potential is not being reached because even at modest trim angles the boat starts to porpoise. With sorted out planing hulls you will see increased speed with increased trim until a point of diminishing return. Instead of that my boat porpoises. With full throttle if I trim up to a speed right below porpoising speed I will go 49.5 mph. If I quickly trim up at that point I can feel acceleration and see increased speed until porpoising stops the speed increase. In that moment I have observed 53 mph. The porpoising starts with mild oscillation but will grow wild if I let it. For a while I had turn signal style trim controls. With these controls I had to have both hands on the wheel. When the oscillating got wild I had to grip the wheel with both hands. When I let go to throttle back my right arm went up and down like a rodeo bull rider. As a result I initially missed the throttle. When I did eventually throttle back that action created a reaction so violent that I almost got bucked off. It was my eight seconds. All that in a pontoon with modest power and speed.

Any chance that hyd jack plates could help here? You could drop the nosecones deeper in the water but increase trim to offset the extra drag by being deeper in the water? I know on our bay boat with a jack plate, the height of the motor would definitely induce proposing if I went too high. We had low water pickups, so we never had a fear of being too high and starving the motor of water.

You know your build better then anybody else, but maybe at 49.5mph, this is the limits of your hull and motor combination and you just need to enough is enough. I mean 49.5mph in a twin 115ph engine, 8'6 wide boat is absolutely F'in g fantastic. And its all build by scratch by you! That is something to be said for.

Chasing that diminishing return on investment on getting a few more mph (does it really matter between 49.5 and a smooth and comfortable ride vs 53mph and holding on to a bucking bronco as you stated) worth anymore money thrown at the boat? I'd say, enjoy your accomplishment on something very few people have ever done (if ever) by adding 2 motors to a 8'6" boat and enjoy the hell out of it.

Maybe you just try and dial in some SS props now and try to gain a couple MPH that way and be done with it?

Whatever path you choose, be proud of what you have done with this build.
Todd

 

[rbtnt]

Have you tried sharpening the back edge of the toons with something like epoxy? This helps on some smaller (25'-28') fiberglass boats that porpose.

 

[DVW]

I did inspect the bottom trailing edge for flatness and defect but did not uncover anything. In addition I added sharp edge trim tabs to the lifting strakes. The tabs where adjusted with different hieghts and bent with deferent angles. Testing did not reveal a difference.