Jet Pump Angle Question

[Jarek]

Hi there,
I wonder if anyone can shed some light for this issue for me.
In some jet pums, like: DOEN, CASTOLDI, AMERICAN TURBINE, the jet shaft is parallel to the keel.
In others: KAMEWA, HAMILTON, ALAMARIN, the shalft is inclined about 4 to 5 degrees down, aft.
The question is why?...and does it affect the hull's behaviour - I would imagine it might, unless the pump's particular design requires a down angle to achieve the same thrust vector, produced in another pump design with a zero angle.
None of those companies mention any specific requirements as to the hull design, beside the typical.

[speedboats]

American Turbine, Berkeley, Dominator and the like are usually installed at an angle created by their mounting plate. AT make a mount block to be welded into the hull.

The angle is there to lower the thrust line of the pump. Lower thrust line = more lift (gross over-simplification, but will suffice) and more performance.

[Jarek]

Thanks, but what does it do to the trim angle of the boat?
Thrust parallel to the keel compared to the thrust at 5 degrees?
If we equipped two identical hulls, one with Castoldi - parallel, and one with Hamilton - 5 degrees down?

[pistnbroke]

Its just the same as tucking your outboard under ..the one with the 5 deg will hold its nose down and rise onto the plane flatter ... less change of porpoising ...

[CDK]

Shaft parallel to the bottom would be the most logical choice, although the vector distribution for 5 degrees is almost as good.
For practical reasons the ideal position isn't always possible (oil pan height for close mounting) or desirable to prolong the life of universal joints.

Castoldi solved the mechanical problems by incorporating a gearbox: the input shaft is above the pump shaft and accepts any engine with 0 degrees alignment.

[tom kane]

u/v joints run more quiet and last longer at 5 degrees angle, in line u/v joints can vibrate and rattle.

[Jarek]

Thank you guys for the answers; it seems that the room for the engine oil pan is the main reason for the down-angle, rather than any benefits of thrust directed down rather than parallel to keel.
Both Castoldi and Doen do have built in gearboxes that allow them to be parallel.
Ultrajet does not(parallel), but it seems it is not possible to close couple it - it needs a cardan shaft to connect to the engine.

[anthony goodson]

@Jarek, nope, here's a drawing of an ultrajet that is not parallel If you think about it a close coupled engine and jet needs a bit of lift in the stern ,as long as the bow has plenty of hydrodynamic lift ,then it all works very well .Integral gearboxes can be heavy,noisy ,and an uneccessary complication. Thats why they are not universal.

[Jarek]

Anthony,
I could not find a picture like yours on their website - all those I could find are dead parallel, like this one, and they tend to be "far" coupled, i.e. have a connecting shaft.
I am specifically interested in the small ones, like the UJ 251.
I see your point though, a close coupled jet, could benefit from a bit of a down angle to compensate for the CG far aft, unless the hull is designed specifically to provide a lot of lift aft.

[anthony goodson]

I haven't looked on the web, our local ferry ,an aluminium cat has twin 250's I know the operator and 3 or 4 years ago he asked me to work with his engineer ,rebuilding and making a minor modification to them.That's where the picture came from.

[baeckmo]

Naah, guys, it is a question of trade-off between getting the impeller center as low as possible to minimize loss-creating bends in the inlet duct, and an input shaft high enough to meet required installation height for the engine.

Generally speaking the thrust line with jet installations is generating an (unwanted) nose-down trim moment at high speed. To some extent this is compensated by a downforce aft, created when the fluid is deflected upwards along the ramp zone. If the hull design is "asking" for a specific trim characteristics, it can be met either by tilting only the nozzle (=simple), or by changing shaft elevation and the inlet structure (not so simple...). Big jet suppliers like Kamewa, MJP and others, design the inlet specifically for each application, but that is not an option for smaller units in serial production.

[anthony goodson]

Jarek from what you have said ,the configuration of your boat will make the decision for you.The 251 is a small jet ,so I will assume a small planing boat. If you need the jet to be an underdeck installation,to facilitate a forward engine , aft conning position for example then a parallel shaft is what you need C of G further forward but flatter thrustline. If you need a close coupled jet ,aft engine installation then you will have to use an angled shaft, C of G further aft, angled thrustline .It's horses for courses .I always feel that the complications and losses in an integral gearbox outweigh its usefulness ,but I may be wrong, and will no doubt be corrected. What I do know, is make sure your hull is medium to deep V with constant deadrise over the planing area and strong gradual lift in the bow. And make absolutely sure that your jet is a large enough diameter for your application,do not believe the brochures ,ask to speak to existing owners, I cannot stress this enough.

[Jarek]

...the hull will actually be pretty shallow(9 degrees) - shallow draft is the requirement.
Close coupling is also a requirement because it is a small boat - 22ft.
Transmission is of course a loss, but sometimes it is needed to match the engine rpm to the jet rpm.

Another question - how essential is it to have the impeller housing removable from the unit?
In some designs the impeller is housed in the jet body casting, and in some others it has its own, removable, section of the assembly.
Impellers can, of course, be damaged by debris; how common is it that the impellers "tunnel" gets damaged to the point that it needs to be replaced?

[speedboats]

While Hamilton Jet have a 5 degree down angle with the mainshaft, the nozzle has some angle built into it so thrust is parallel tot the keel line. Place Diverter and Jet-o-vator make adjustable trim nozzles for the American Turbine / Berkeley types, and SDM have their own in house item. This can adjust the nozzle in much the same fashion as you would trim an outboard or out-drive. Else you can install a wedge between the tail pipe and steering nozzle to adjust the trim in a fixed manner.

Perhaps the term 'thrust line' was a bit of a mis-nomer. In a real-world performance situation, the lower you can thrust from, the better off you are. While I may only posses rudeimentary physics, if you thrust above the line of friction (in a planning hull it is usually very close to where the hull meets the water), then you drive the nose of the boat into the water, this increases waterline length and therefore friction.
The idea is to lift as much boat out of the water as possible. There are two ways of acheiving this,
1, Use alot of up thrust from the nozzle. While this will surely lift the front of the boat up (leverage from driving the stern down), it also wastes thrust. Draw a simple vector diagram, the hypothenuse is the total thrust from the pump, the opposite side is the thrust driving the stern into the water (and therefore the bow up), and the adjacent side it the forward thrust of the boat. The forward thrust gets smaller with more added angle to lift the nose, so at some point any benifit is negated by lack of forward thrust.
2, Lower the point from where the thrust is applied. This then naturally thrusts along the friction point overcomming the desire to thrust the bow into the water (or substantially decreasing it). Now any added trim will have a much larger effect therefore requiring less trim input / angle.

[anthony goodson]

You should be fine with 9 degrees ,a tad more may be better but you have your reasons. 22ft does not preclude an underfloor jet such as the standard 251 .I have a 17ft workboat ,not with an ultra, but with an aft of engine conning position and it works very well, I'm not recommending it, it just depends on what you use the boat for. As will your choice of jet, comparing a Berkeley and a Hamilton is not apples and apples you need to decide what you want this boat to do before you choose.As for the gearbox I still think you are better off matching your impeller to your engine correctly in the first place and saving the weight drag noise and expense. I hope someone else will comment on this soon. I am flying from the UK, to my home in Murcia this afternoon,where my internet connection varies with the weather {light shower within 20 miles I think Basil Fawlty said}so I may be missing for a while ,but you are very lucky that your post has attracted much of the jet unit talent on here ,you are in very good hands