Considering powering DOWN!!

[angus63]

Re: Considering powering DOWN!!

I used cars as an analogy, but it was a technically poor choice. If the HP to RPM to MPH ratios are truly linear, than you are correct tippy. With different types of hulls (pad bottom, v bottom, pontoon, semi-displacement, displacement, foiled, etcc..) the relationships are not linear, hence the difference in throttle position to achieve the same rpm even though propped for the same WOT. Only at WOT (100% throttle position for all types) would the fuel consumptions be equal for the same rpm.

 

[itstippy]

Re: Considering powering DOWN!!

Yeah, we're oversimplifying things a lot. Danny K's pontoon barge ain't gonna plane out, that's for sure. He's just gonna push water, period, no matter what he powers it with. The harder he pushes the more the water pushes back. Theorhetically he could push it around with an 18HP propped for shoving big boats in and out of slips in a Marina. He'd get good gas burn per hour and be turning most of the fuel he burns into true horsepower (work done). Very efficient, low slip, not pushing water. Plus if after 20 minutes on the water he realized he left his sunglasses in the truck he could just wade ashore and get them!<br />He might borrow a lower pitched prop and try it out for 45 minutes at 3000RPM to see if he still drinks 10 gals of gas. Can't hurt. I doubt it will help too much, though.

 

[umblecumbuz]

Re: Considering powering DOWN!!

Look at it another way.<br /><br />You could tether the boat and run 3,000 in gear. The boat isn't going anywhere, but the prop's turning and pushing water.<br /><br />Now imagine the boat moving through the water at 3,000 revs. You've now got to add the water resistance against the hull and even the air resistance. Those resistances don't come free. You pay for them in gph.<br /><br />The motor's still turning 3,000, but your throttle is open wider to do it. Wider throttle opening equals more fuel burned.<br /><br />Heavier boat means greater resistance, which means wider throttle opening for the same revs.<br /><br />There's no such thing as a free meal.

 

[CFronzek]

Re: Considering powering DOWN!!

Danny, <br />Pontoons are not my strong suit but the ones I've seen are all displacement hulls. That means they are not designed to get up on plane. <br />Every displacement hull has a inherent "hull speed" and trying to go over hull speed means you have to use gobs of additional power for every mile/hour extra you get. <br />A rough idea of hull speed for any particular length displacement boat can be calculated and my numbers for your 28 footer say pushing your boat over 8 miles/hour is going to cost you a lot of gas and you will be working the daylights out of your motor. <br />Time to slow down, enjoy the scenery and stop underwriting the oil industry.<br />Charlie