My buddy is not a jet boat guy, but was telling me about a 2 speed gear box that goes inline with the driveline to the pump!!! Anyone hear of these and are these worth a shit???
This should play back into the does it take HP or TQ to make a jet boat fast. Obviously you have to have one to get the other. But if it takes TQ to get the pump turning, the reduction in RPMs might put the engine back into the peak of the TQ curve and allow it climb back up to the HP peak. It would be an interesting experiment but the engine would have to be built for the split to capitalize on any benifits available.I had the same idea back when hot boat was around. What they r gonna tell ya is that by shifting and dropping your r's....you are gonna be slower because your motor makes less hp at 4k as opposed to 5k...or what ever. Your mileage may vary.coffee
Youll hear all kinds of theories on that one. Ive heard 7700 max, heard the guys running blown fuel ran 8800 on A impellers also and ran ok.So herein lies a possible answer to a question I had. Do jet pumps have a "sweet spot" for rpm's/thrust. Kinda like if there is a efficiency curve to a jet pump? And if so can it be figured for a specific cut impeller.
They are using gear reduction boxes in off- road. Very small, compact,light weight and very spendy that bolt to the back of the trans. It is along the lines of a planetary. To me it would seem almost impossible to shift a gear and not fall out of the power curve with a jet drive. The use of a V-drive has been done before to lower engine rpm and increase pump rpm and for the few I saw worked for that app. In a top fuel boat why not just adapt a lenco 2 speed as the prop boats do. Props will spin till the blades fly off, pumps ???????? Effectively anyway. Not to mention the effect of shifting a light weight boat that is already aired out. MWhat Widowmaker is talking about is the way the Head Over Heels boat runs the gearbox. I have one of the gearboxes from Paul Prange as we ran it in the Pro Gas boat of Robbie Barrett a few years ago.
With the HP development that has taken place in NHRA Pro Stock, these engines are producing a high horsepower at high RPM. The RPM is many times in excess of 9000 RPM. We cannot get an impeller to have any efficiency when cut to the point it would turn 8500 to 10,000 RPM. However, what can be done is to use an efficient impeller size like an "A" then use an underdrive planetary like an Owens unit. this will allow the engine to be turned in the RPM of the engine peak efficiency, while spinning an impeller in a peak efficiency RPM. For an example, we were running Robbie's engine at approx 8500, but turning the "A" impeller around 7000 RPM.
A single planetary drive, not a gear change operation.