Jet DIY

I think a hovercraft approach may be more feasible.
Can you skirt the hull and pop some fans on those engines?
One engine for lift and one for propulsion.
Way easier than trying to effectively fit jet drives.

Once again, all the best.
Post your progress!

 

Thank You. I will try with jets. Will keep you informed. I don’t expect 90% efficiencies, but lets try to load engines for beginning.

 

Some photos of the impeller and the wear ring and nozzle would help,

 

To start with, the boat is grossly underpowered at 160HP. As a comparison, Jetskis have engines that start at 100HP. Secondly, the bottom is very inefficient for the location of the intake. It is a project that will never work properly. With the limited power, an inboard or I/O installation would work moderately well. A jet drive could be more efficient at high speeds and flat water. However, that is not possible with the power to weight ratio.

 

You have mentioned loading in several posts and I am wondering how you are able to determine how much load is on the engines. You mentioned 80 hp at 4200 rpm

Is this the engine manufacturers spec at 4200 rpm? If so, is that the max horsepower that the engine can produce (and not likely fuel efficient at that rpm.)

Have you ran the boat at full throttle?
If not, you will not have any idea how much hp that the engine is producing when it is pushing the boat. A couple of exceptions would be if you put a device on the shaft between the engine and the pump, photo- electric strain gauge, fixed brushed WB strain gauge or other methods to determine the actual engine hp output at various rpm and boat speed. ( ie determine torque at a specific rpm, convert to hp) This is not within the ability of a DIYer.
The other alternative is to take extremely accurate fuel consumption values and you can get an approximate hp based on a volume of fuel burned per hour. Then use the rule of thumb for fuel consumed vs engine output. But the variance can be 20%

If you have run the boat at full throttle, AND it maxes out at say 4200 (only used for discussion purposes) then you know that the hp curve for that engine states that it is able to produce 80 hp at that rpm, then the engine is loaded
to max

If you run the boat at full throttle and it reaches say 3800, you look at the hp curve and see what hp engine is producing and it is obviously overloaded. Ie the pump absorption will not let the engine get up to the max available at 4200

If you run the boat at full throttle and it reaches 4600 rpm ie the pump does not fully load the engine to the max until 4600 so that the engine has never reached its max hp of 80 at 4200.
( another way of looking at this last concept is that IF the pump permits the engine to move past 4200, when the hp will begin to drop off, the pump has not loaded the engine to 80 hp.

And there are experienced diesel contributors on this forum who would never suggest to run an engine at max rpm.

Of all the words presented in this thread, these 6 words say it all. "jets are tricky creatures to design"
Cavitation, inlet velocity, outlet velocity, flow loading profile across the the impeller face, bowl pressure between the impellor and the nozzle, nozzle diameter, impeller downside pressure loading, nozzle size, stator profile, inlet outlet angles of the impeller, inlet outlet angles for the stators, mass flow rate, I am sure that Baeckmo can provide many more parameters than I have stated,

I hope that you can see that it is not just a matter of choosing an impellor diameter which you have asked for.


Quick summary --- until you actually run the engine up to max rpm you will not know how much horsepower that the pump is loading the engine to
---- effective design of a jet requires much more than a simple response that the diameter should be Xmm

 

Barry, from his original post.
Could one assume "achieving RPM" answers your question... I don't know.
Doesn't really matter as the cards are stacked against him.

 

I know for gauge for measuring of torque… but I think that I’m really far from that… subject boat with 160hp has speed of 7kn. For sure should be more, and efficiencies of jets are very low. Engines are with max power 80hp, and max rpm is 4200rpm. I tried as I said and achieved 4200rpm speed 7kn. I think that first of all I should load engines with jets to reduce max rpm on abt 3800 rpm. I know that hull is not ideal (but why?) ok hump in front and heavy boat, but 7kn? Now I’m working to make bigger intakes.

 

If the rated RPM is 4200 and you load them to a maximum of 3800 they will not generated the rated HP and it will shorten their life.

 

Thanks BB missed that line