Namjet, Traktor Jet

Can anyone comment on this company's products used in yachts?
At first glance they seem quite crude work boat pumps, with, admittedly high bollard pull because of a huge diameter impeller.

 

Afraid I have no "hands on" knowledge from those jets, but I have plenty of experience (both designwise and from testing and operation) from "ordinary designs". If we calculate the possible performance from the scarce info in the NAMjet sales info for the smallest jet (HH381) and compare with a slightly smaller "normal" jet, the difference may become clearer. Of course the precision in these calcs are not overwhelming, but the tendence is shown.

With 150 kW input, the NAMjet will operate with something like 1 m3/sec, while a 290 mm jet will pump about 0,6 m3/s. The N-jet will produce roughly 7800 N and the smaller jet 6800 to 7000 N at 20 knots.

Now, the NAMjet is some 250 to 300 kg heavier (including water and the reduction gear necessary) than the "ordinary" jet. In the low-to-medium speed range the resistance-to-weight ratio of planing hulls is about 0,2, which means that there is a drag penalty of 200 N for every 100 kg weight increase.

Ergo, some 500 N are chewed away to cover for the heavy weight, leaving about 400 N margin, ie 5 % of the total thrust. These 5% are in the same ballpark as the transmission losses in the gearbox required to come down to the 1050 rpm that the N-jet needs in order to be free from cavitation. The "ordinary" jet can operate with 3000 to 3500 rpm, which suits most marine diesels fine with a direct drive.

When these trade-offs are taken into consideration (and without going into detail calculation), the difference in performance is thus practically nil at speeds above about 20 knots. If you are operating in the 15 knot region with full power, the N-jet might be the better choice, provided the heavy weight far back is not creating unsuitably high angle of attack, because then you will have a non-acceptable hump resistance.

Personally, I think that it would pay to improve cavitation performance in the more "normal" axial jet designs, rather than to go the low-speed, extra high flow route. From what I can find out in the sales documentation, I would not select the N-jet variant for yacht type vessels with a design speed above 20 knots.

 

Thank you for the thorough response; it is very much in line with my educated guesswork.
"Traktor" seems an appropriate name for them.
I guess at their low flow rates they do not need to pay as much attention to the streamlining of the pump and appendages; they seem rather crudely fashioned.

 

Naah, I do not quite agree to these statements. First they are working with HIGH flowrates but LOW velocity increase, which makes for good efficiency in general (Fluid power is flow times pressure increase), although at the cost of increased weight.

Second, I'd say that the general shape of the pump element as such seems to be ok enough, and that goes for the inlet duct as well. The steering and reversing mechanism though, is big and bulky and may give the impression of "lack of fashion", but to me, that is not the critical factor.

 

Yes, I did not phrase it correctly. I meant low velocity - compared to "small" diameter pumps.
Thank you again for the answer.

 

We have a TJ 610HT mated to a Volvo D13-700 in a commercial fishing boat. We love it, tons of thrust at lower RPMs, gets on plane easily, large tip clearance makes it very robust to wear and damage. Boat weighs 25,000 lbs, 29 knots max speed.

For maximum speed I would look at a conventional jet, if you need something heavy duty with lots of thrust Traktor is the way to go. Where we fish, conventional jets need a lot of wear ring and impeller service due to high silt content in the water.