Testing the "doubling HP adds 50% more speed rule"

[rancherlee]

Well I finally got sold the old Johnson 70hp 4 stroke last week and got my newer Suzuki DF140 rigged up! Being the cheapskate I am it took me 3 years to find the "deal" I was looking for on an engine, sold the 70hp for 3200$, got the 140hp for 4000$! I've always heard that boats gain 50% more speed when you double the HP and I was curious to see if I could actually achieve that on my home build tritoon. The old 70 served us well over 8 years of service without a single complaint and would run 26-27mph on smooth water with just the Wife, Daughter and I but speed fell off in a hurry with any more people on board and would drop to 18-19mph with a full load of 10. It was windy yesterday and I didn't really get a chance to max out the speed in the chop but the Suzuki 140 managed to get us to 37.6 mph with 3 people on board and a wrong pitch prop! Seems to fit the rule perfectly 26mph + 50% (13) = 39mph and that should be easy to achieve once the propping is corrected and the lake gets smooth. The motor came with a 14x20 Suzuki stainless prop which was turning 5400rpm @ 37.6mph and I could hear it sucking air at higher trim levels. The calculated slip was 14% and I've achieved 6% slip before on the 70hp with a good prop. I need to find a 14x17 or 14x18 to get this motor up to 6200 rpm as it supposedly doesn't make 138hp until 6000rpm. My new mission, 40+ mph with just me onboard I'm also looking to upgrade my pontoons from 3 old 20" equivalents to 23" outer/25" center combo next spring with new scratch and dent tubes.

[Bamaman]

Your pontoon boat is pretty efficient, as the speeds posted were pretty quick for 70 hp. Congratulations on finding a used 140 Suzuki as they're pretty rare animals on the used market. That engine has a reputation for being very durable and trouble free.
Only problem is that Suzuki propellers are high pitch props due to the lower units' very low gear ratios. They're just not easy to find in every pitch size.
Good luck in getting your boat propped right, and you should get a lot of good use out of your new boat/motor combination.

[ron nh]

Congrats on your patients and outcome. great price on the 140.

[rancherlee]

Found a good deal on a Solas Amita 4 prop and have a 13.75x17 on the way to test out! I had good luck with a Spitfire 4 blade aluminum on my 70 Johnsuki but the spitfire in a 17 pitch is where it drops to a smaller diameter/small blade area and I don't think it would have had enough bite on my pontoon. Yes, that's the downside of Suzuki engines is the steep gearcase and trying to find good heavy load props, at least mines an older DF140 with a 2.38 case instead of 2.59

[rancherlee]

new prop is in, this Solas Amita 4 is one stiff pitched prop. Took the pontoon out today with 6 on board to pull some tubes around and managed 35.8mph @ 5800rpm which is only 10% slip and after I booted everyone off a Solo run netted me 38.2 @ 6100rpm which is ~8% slip. Very good numbers for an aluminum prop. I had some more speed in it but it started to Porpoise past 3/4 trim mark so I had to back off the trim a touch. Now to find a 14x18 stainless and chase the 40mph mark.

[RcgTexas]

Good price and good numbers.

[curtiscapk]

That thing is a comet!!! Great work man!