Porta-Bote Woes

jimbob · May 5, 2001

Forgetting my dad's advice to "never believe everything you read" I recently bought a 12-foot folding Porta-bote. They claim speeds of "up to 20 mph" and "jumping on a plane in 5 seconds" using a 4-hp motor. I bought a 6-hp Nissan (which is their recommended brand) 4-stroke. I live (unfortunately) and boat at 5,000-6,000 feet. I can't get this rig to go over (GPS confirmed) 5.6 mph!! Can anyone advise me as to: 1) is it worth the trouble to rejet and re-prop this setup? I do occasionally return to Nebraska (1000 feet) 2) Should I have gone with a 2-stroke instead? 3) Any other engines I should have looked at? Thanks! Jimbob

JB · May 5, 2001

Re: Porta-Bote Woes

I don't think there's much to gain by second guessing you, Jimbob. You've got a boat and a motor. The correct question is: How do you get the most out of it? Does the engine rev up at w.o.t. or does it seem to labor? Theoretically, rejetting and/or a shorter prop should get better results, but I have no idea how your rig would perform at sea level. We need some description of the engine's behavior at w.o.t. Standing by. JB

jimbob · May 5, 2001

Re: Porta-Bote Woes

All seems fine when I start it and hold it wide open---the only problem is, it just doesn't GO very fast! Up here, winds can come up fairly quickly, and I hate to be too far from shore with this slow of a boat (especially with a grandkid on board!). I thought it would be good to measure the RPM's, but apparently there isn't an easy/economical unit available for a single-cylinder motor? The dealer is checking it, but says he can't check it "under load", and these new motors that are environment-friendly have essentially no adjustments you can make other than the jet and prop. Just wish I knew if I were getting the optimum out of this setup, and don't know how to figure that out! Thanks again, Jimbob

JB · May 5, 2001

Re: Porta-Bote Woes

Hi, Jimbob. I wonder if your engine is propped to push a displacement hull or a sailing vessel around at 6 knots. Find out the pitch of your prop and the gear ratio in the lower end. Multiply your engine's operating rpm by the gear ratio to estimate how fast the prop should be turning at w.o.t. Now multiply the prop's pitch (in inches) by the rpm you calculated for it, and multiply that number by 60. The huge number you have now is theoretical inches per hour your boat should go with zero prop slip. Divide that number by 63360 (the number of inches in a statute mile) and you will get theoretical mph with zero prop slip. Real world mph would be about 80% of that number. If the number is close to your 6.7 mph you are getting all that that prop can do for you and you need a longer prop. If you come out with a speed more like 15-20mph it's your engine that is falling short. I would then look into rejetting. Good Luck and let me know what you come up with. JB

jimbob · May 6, 2001

Re: Porta-Bote Woes

Thanks! Finally some parameters I can check! Jimbob

jimbob · May 6, 2001

Re: Porta-Bote Woes

Again, many thanks for giving me something I could actually check! Neither the dealer or factory rep could give me any more than, "Well, it MIGHT help to rejet"! Here are my parameters and figures (in case you want to recheck my math, never a strong point of mine!) RPM's 5,000-6,000 (so I used 6,000) Gear reduction 13:28 Prop pitch 7.9 Using the 80% "real world" correction factor, I still come up with 16.67 mph!!! Assume I should look into rejetting?? They also mentioned getting a "high altitude" prop with (I THINK) a LOWER prop pitch??? One thing--does this equation assume a light load in the boat, an average hull, etc? Again, many many thanks! Jimbob

jimbob · May 6, 2001

Re: Porta-Bote Woes

One more of my endless questions! I have wondered if my motor REALLY gets to full revs with w.o.t. The last inch or two of rotation of the tiller throttle doesn't seem to result in any more noise. When I dropped it at my dealer's to test, he told me he doesn't have a tachometer to test one-cylinder motors, and a cursory look thru the Net didn't turn up any. Are there inexpensive yet accurate tachs for these little engines? Thanks once more, Jimbob

JB · May 6, 2001

Re: Porta-Bote Woes

Hey, Jimbob. I think I misled you when I told you to multiply the engine rpm by the gear ratio. But there is more than that wrong, here. Let's step through it: 1. If your engine is turning 6000rpm and the gear reduction ratio is 13.28, then your propshaft would be turning only 452rpm. 2. At 452rpm your propshaft is turning 27,108 revs per hour. 3. With 7.9" pitch and zero slip, the prop would then be advancing 214,156" per hour, which comes to 3.37mph. 80% of that is 2.7mph. I think the gear ratio is wrong. If I run the same calculations using 1.328 as the ratio, I come up with a real world w.o.t. of 27mph, which makes more sense to me, but seems too fast for a 6hp. 13.28:1 reduction gears would make an awkwardly fat lower end that doesn't show in pictures of the 6hp. I think the gear ratio is wrong in my figures, but yours make sense. You need more torque or a shorter (high altitude?) prop, or both. My formula, by the way, assumes that prop and load allow the engine to reach peak rpm at w.o.t. Your 12' Tote-bote shouldn't overload 6hp. One more question: When you are making good 6.7mph at w.o.t., is the boat bow-high and making a big wake? If so, a shorter prop and rejet will probably fix your problem. Let me know about that gear ratio. Good luck. JB

JB · May 6, 2001

Re: Porta-Bote Woes

I don't know why I always discover my error AFTER I have posted my reply. I misread the 13:28 as 13.28:1. Your numbers are correct. Go for it. JB

JB · May 6, 2001

Re: Porta-Bote Woes

I doubt that you are getting much over 3000rpm, Jimbob. Assuming all of the linkages, etc. are correctly adjusted, your engine just hasn't enough torque to accelerate past 6.7mph with that prop at that altitude. A shorter prop and rejetting for more torque should help, but I just don't know how much. A tach for a 2 cyl 4 stroke can measure your rpm. Just multiply the reading by 2. Keep me advised. JB

jimbob · May 7, 2001

Re: Porta-Bote Woes

Once again, thanks for your help. You've been more helpful than the Nissan Folks! One more thing---when adjusting the prop for high altitude, should the prop pitch go UP or DOWN? Believe it or not, different Nissan folks have told me both ways! Thanks, Jimbob

JB · May 7, 2001

Re: Porta-Bote Woes

Go for a shorter pitch, Jimbob. Your current prop is 7.9", try one between 5 and 6". Good Luck. JB

jimbob · May 7, 2001

Re: Porta-Bote Woes

You've been a lot of help, and I hate to keep bothering you, but no luck from the Nissan reps, so here goes: Is there any intrinsic advantage/disadvantage of 2-stroke vs 4-stroke, other than the obvious weight/pollution issues? i.e., is a 6-horse 2-stroke any more powerful/torque-y than a similar 4-stroke? Thanks once more! Jimbob

sony2001 · May 8, 2001

Re: Porta-Bote Woes

jimbob; Try to get from the national distributor the jet conversion table for changes in altitude. The general rule is .002" per 2500 ft. Then if you start trying loaned props your GPS will tell which one is the best. Your not working with much oxygen at that altitude! ------------------

JB · May 8, 2001

Re: Porta-Bote Woes

Yo, Jimbob. On the 2 vs. 4stroke question: If you set aside the smoother, quieter, cleaner and more efficient operation of the 4 stroke and the lighter weight of the 2 stroke, you are still left with the question: "Is a redhead more attractive than a blonde?" Torque characteristics are very individual to a given engine and blanket statements have too many exceptions. It is true that designers of 2 stroke outboards have many years more experience designing for wide, flat torque curves than do designers of 4 stroke outboards. Historically, 4 strokes either run in a very narrow rpm range under load or they have transmissions. Outboard designers have a new challenge in designing 4 strokes for such wide torque demands. I suspect that Yamaha, at least, have done a good job. The others? I don't know, but I'd bet someone reading this has experience with them. Cheers. JB

Porta-Bote Woes

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