30-40% HP loss for a jet drive? Why?

[bruceb58]

Re: 30-40% HP loss for a jet drive? Why?

If you increase the ammount of weight in your vehicle you will decrease the RPM's required and increase the ammount of torque output and this will increase the HP rating of you vehicle.

How did you come up with that?

 

[Tail_Gunner]

Re: 30-40% HP loss for a jet drive? Why?

the output of the drive is less than the input. Its actually a very inefficient design but it works. basically the water has to wait to get compressed and pushed out. you have a 12 inch hole that is reduced to a 6 inch hole.
on jet skis you can widen the outlet but that will give you bottom and you loose top due to the drop ion pressure. ( Im not talking 10-20 MPH here im talking 1-2. I dont know abotu jet boats but the pump is very similar to the jet ski pmps
Rob

Agreed, two 90 degree gear's, compression, and fluid coupling tourqe loss..... they all take there toll.

 

[tommays]

Re: 30-40% HP loss for a jet drive? Why?

How much fuel do equal boats use at given speeds with a JET VS a STERNDRIVE


If you look at a twin jet like the Yamaha it has to spin twin motors at 7000 RPM to move the boat 28 MPH and 10000 RPM to hit 50 MPH with 160 HP twins http://www.boattest.com/boats/1220/test_results.aspx

Yet it still gets very a good 3.7 MPG on a 23 boat in a 3100 # boat

If we go to this 280 HP sterndrive cobalt it takes 3200 RPM to move the boat 28 MPH and 5000 RPM to hit 46 MPH a 280 HP
http://www.boattest.com/boats/1031/test_results.aspx

It is a Mucho heavy boat at 4600# yet still gets 3.58 MPG

Now i like the cobalt better but the modern jet does seem to hang in there
Tommays

 

[QC]

Re: 30-40% HP loss for a jet drive? Why?

Reel Poor has tried to get you guys back on track, but the train just kept on chuggin'. Torque and horsepower are what I do for a living . . . There is a lot of good info here, but there are some inaccuracies that are significant.

1) bruce58 is correct to question Yoda's weight comment. Adding weight to a vehicle has absolutely no affect on the engine's horsepower output. I saw an earlier comment in here that was similar about a heavier vehicle with the same engine on a chassis dyno will have less hp at the wheels. This is, in fact, true and I believe where the confusion lies. When there is more weight on the rear wheels, horsepower is eaten up by squishing the tires (tire deflection). It can be very significant and if you have something like a Bus or Motorhome with a bunch of weight back there you can ruin a set of tires in 5 minutes. The lost horsepower shows up as heat in the tires. Trust me, I've bought several sets when we have destroyed a customer's . . .

2) bjsc, who is usually very reliable, has kinda mangled this one. First, there is no change in horsepower (brake or otherwise) when you change gear ratio . . . Torque yes, but hp is the same. Yes I know that the gear itself eats a little, but leave that for just a minute. Where he got messed up was that his example of a 220 @ 4600 RPM actually develops 251 lb/ft. torque, not 215 . . . So if you take the 1.6 gear ratio (4600/1.6=2875) you must also multiply the torque by the same (251 x 1.6 = 401.6) and you end up with the same horsepower (401.6 x 2875 / 5252 = 219.84). OK the load from the gear has not been taken into account, but in a 100% efficient ratio change (impossible) my numbers would be correct. Yes an I/O eats power from the Flywheel to the prop shaft, but it is not from the ratio change, it is from the gears themselves as Tail_gunner has noted. Usually around 20 hp these days I think although sometimes it is as high as 30. The whole point of my notes here is that the 188 hp is incorrect. Sooooo, with my numbers that engine with 220 at the flywheel puts 220 to the jet's solid coupling and impeller and around 190 - 200 to the I/O's prop . . .

Brake horsepower is another discussion, and since I am from the old school I usually write bhp, the fact is that SAE hp and bhp are very close if you are measuring at the flywheel. The only difference being maybe the water pump and/or alternator being driven. It is not usually used to mean shaft horsepower (shp).

 

[bjcsc]

Re: 30-40% HP loss for a jet drive? Why?

2) bjsc, who is usually very reliable,

Hey, thanks for the compliment!

[bjsc](sic)...has kinda mangled this one. First, there is no change in horsepower (brake or otherwise) when you change gear ratio . . . Torque yes, but hp is the same. Yes I know that the gear itself eats a little, but leave that for just a minute. Where he got messed up was that his example of a 220 @ 4600 RPM actually develops 251 lb/ft. torque, not 215 . . . So if you take the 1.6 gear ratio (4600/1.6=2875) you must also multiply the torque by the same (251 x 1.6 = 401.6) and you end up with the same horsepower (401.6 x 2875 / 5252 = 219.84).

You are correct. I made a typo with "215" and meant to enter "251", and the resulting math is as you say, with the exception of how you got back to horsepower. The 1.6 ratio does not apply to the jet drive (it's 1:1), but you still end up with 219.84. Sorry for the mangle... However, your post has convinced me that the "30-40% HP loss" I read time and time again on this forum is unfounded. And if anything, it may actually go the other way, which would explain why my boat seems so fast compared to other BRs. Interestingly enough, the torque is in the 30-40% less range. Exactly how this ends up making me faster I do not know, but it should make the power loss statements irrelevant to someone considering a jet drive. I'm not sure what TG means by two 90deg. gears as there are none, zero. It is direct drive as you say, and in my case, only two bearings. Thanks for straightening me up on the typo...

 

[QC]

Re: 30-40% HP loss for a jet drive? Why?

bjsc,

Not sure what part I did wrong, I understood that the 1.6 was for the I/O . . . Anyway, I have posted a lot about torque and horsepower. I recommend that you all kinda just forget torque when talking a marine engine except at WOT RPM which is in fact the only place where you operate at the max torque for that RPM. Unless your boat cannot plane or struggles a lot, you never reach peak torque value at all. Don't try and separate them, that's the mistake. You can't have hp without torque, this is true, but you are never at max torque at any RPM unless the throttle is completely mashed. And if the RPM rises at full throttle, you didn't reach max torque because it didn't require it to accelerate . . .

The reason you do not see automatics with a torque converter on heavy-duty trucks is that fluid couplings eat fuel i.e. inefficient . . . There is a whole new range of heavy-duty automatic "manuals" (even 18 speeds) that electronically manipulate the clutch and engine RPM to start and shift, but they do not have a torque converter. Typically you see Allison's (torque converter) in Buses and refuse trucks because of all the starts and stops. Saves the drive line and you can put any monkey on the seat . . .

 

[bjcsc]

Re: 30-40% HP loss for a jet drive? Why?

Roger that on the automatics...That's the way I understand it, which is why I had so many questions regarding the post to the contrary.

So, accepting your line of thinking regarding torque and HP, would you agree then that the "30-40% loss" statements are unfounded?

 

[QC]

Re: 30-40% HP loss for a jet drive? Why?

Yes, totally disagree and the recent test data proves it . . . After years of ridicule I would now consider a Yammy 23 HO thing fo sho. Also, they are making biiiiiiig boats with jets now too, ya know, mega yacht stuff.

Here is a chart that may help some see this torque thing better, happens to be a small Cummins diesel and marketed with Merc.. The straighter, lower lines are a prop's demand curve as it spins faster and faster etc. The humped lines is max output at all RPM (max torque for a given RPM). The difference between the lines is all reserve power . . . with a few exceptions. If you are heavily loaded you will use more of that reserve than when you are light and when you are jumping on plane there is a spike of demand. The engine with a higher peak torque value will plane a boat faster than one with a lower value and the same rated bhp. If they weight the same and all else is exactly perfect, then they will go the same top speed.

I like this one also because it shows the torque and bhp lined up so you can see how torque directly affects horsepower (work) and also if you really study it, you can sorta see why we like to run diesels around peak torque RPM for fuel economy. Can't do this without multiple gear ratios though aka a tranny . . .