Jet boat designer needed

Discussion in 'Boat Design' started by Sid, Oct 24, 2020.

  1. gonzo
    Joined: Aug 2002
    Posts: 14,564
    Likes: 685, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

  2. Sid
    Joined: Oct 2020
    Posts: 14
    Likes: 0, Points: 1
    Location: Poland

    Sid Junior Member

    Thanks so much @Mr Efficiency! So many interesting designs out there. Spoke to Glen-L and my boat shop and I'm about to buy one of either the Slither or the Snake Shooter (Snake Shooter-boatdesign https://www.boatdesigns.com/Snake-Shooter/products/762/).

    Both designs can be shortened to 17' and 17'6 respectively. The latter has the chine width of 6' as @Barry suggested. Here's the comparison of the two:
    Slither and Snake Shooter: 17', 18' or 20' inboard or outboard sled boat https://www.glen-l.com/designs/hankinson/slither-snakeshtr.html

    Both designs seem very similar. Would there be significant differences in terms of nautical properties? Which one would you choose?
     
  3. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,061
    Likes: 640, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It is just a question of what the extra weight will do to the performance, I think, planing will be little different, but the extra weight and bottom area will slow the larger boat down with your limited horsepower, probably the best guide is the maker's HP range advice, and that points to the smaller boat as better suited to the 80 hp unit. And if opting for the 12 degree version, which I would, as it be less jarring running over any bumpy water, and turn less flat, so things and people won't get tossed around so easily, the power demand will be a little greater than with 6 degrees. In the end, though, the stipulated range of 75-230 hp for the bigger boat, seems to point to the smaller boat. I would think a 5'6 chine width is enough to accommodate a 115hp outboard, which is probably what the 80 hp jet version is, and especially if a lighter two-stroke.
     
    Last edited: Oct 29, 2020
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,061
    Likes: 640, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You certainly don't want to find yourself having to lean on the throttle, to get reasonable speed, outboard jets are not terribly efficient and running beyond three-quarter throttle sucks fuel badly on any outboard.
     
  5. baeckmo
    Joined: Jun 2009
    Posts: 1,189
    Likes: 143, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    The standard outboard jets are somewhat limited in terms of thrust. Before you get too involved with any of the suggested hulls, you ought to get an idea of what is possible with this engine of yours. The problem is this: the thrust/speed characteristics of those units are quite flat, meaning that there is often little (or no) margin between available thrust and thrust required by the hull at hump speed or accelerating. In fact, quite a number of hulls suitable for normal outboards will not work at all in your case.

    To avoid getting "stuck" at the planing hump speed, it is critical that the laden hull has as low hump drag as possible. Now here comes the tricky part; you want a wide hull for stability, and with the restricted length you want, the hull slenderness ratio (wl length/(displacement^0.33)) will be low, which in turn results in very high hump drag. This is where jet boat design experience is needed or you will have problems.
     
  6. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,061
    Likes: 640, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I would say the "Slither" boat would have little difficulty planing with a 50 hp conventional outboard, so perhaps 80 hp jet outboard, which is 115 hp or thereabouts at the powerhead, should be OK
     
    Sid likes this.
  7. Sid
    Joined: Oct 2020
    Posts: 14
    Likes: 0, Points: 1
    Location: Poland

    Sid Junior Member

    Hello @baeckmo and thanks.
    Both Slither and Snake Shooter are jet sleds - designs specific for jet drives. There is the minimum power requirement of 50 and 75 HP for the Slither and the Snake Shooter respectively. Isn't that the power necessary to get over the hump?

    As @Mr Efficiency rightfully remembered, my outboard jet is 80HP at pump (115HP at powerhouse). Based on your design experience and considering my 80HP jet, which hull would you choose and why?
     
  8. baeckmo
    Joined: Jun 2009
    Posts: 1,189
    Likes: 143, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Holy blue smoke, do you really, I mean really!!, think an engine manufacturer would attach a 115 hp powerhead to a pump housing and then relabel and reprize the thing "80 hp"? It is more likely the USA gets Santa Claus for president next week.....

    What is missing in the discussions so far is a realistic weight estimate. Sid wrote initially about taking his family on a trip; how big a family, what equipment, how long a trip, how many beers, is the mother in law invited etc etc. Until this, call it a basic SOR, is on the table, most of what is said in this thread so far (except for Barry's posting) is pure speculation. The point is that the only fact we have for a start, is that a certain engine/propulsor is specified. In a way this is backwards engineering and you must be careful about detail selection.

    For instance, what engine do you have, is there any referrance to the kind of vessels it is (has been) used for, and do you have any hard facts on the jet dimensions (impeller inlet diameter and nozzle diameter)? This (together with a realistic load estimate) would allow a better performance prediction.
     
    kerosene likes this.
  9. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,061
    Likes: 640, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    No, it has been going on for decades, the things are heavily marked down in output with that centrifugal pump being well short of a standard propellor drive, usually around 30 per cent. You only need to look at the maker's engine ranges, the same powerhead mounted to a jet drive will be rated much lower in output, and of course the engines these days have long not been rated at the crankshaft.
     
  10. baeckmo
    Joined: Jun 2009
    Posts: 1,189
    Likes: 143, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    I just looked up the data for Mercury 80 hp. It shows exactly the same power, engine volume, valve arrangement etc etc for both variants. This goes along similar observations from other manufacturers. Now that is not proof of a general rule per se, but I think you have to come up with better validation than your statements above. This is why I am asking Sid for more info on his engine; I simply don't trust unvalidated statements.
     
  11. Sid
    Joined: Oct 2020
    Posts: 14
    Likes: 0, Points: 1
    Location: Poland

    Sid Junior Member

    Why so distrustful? Here are my engine stats:

    Johnson Outboards 80 (J115JLEUM)
    Parts Group: 1997 Johnson/Evinrude 80
    Brand: Johnson
    Prop Shaft Horsepower: 115hp
    Options: Jet Drive
    Shaft Length: Long 20″
    Model Run: M
     
  12. baeckmo
    Joined: Jun 2009
    Posts: 1,189
    Likes: 143, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Excellent, that makes a difference. And yes I am a grumpy old sceptic; if you look through the propulsion pages in this forum, you will see that a very common problem with jet installations is that with an overweight or short-and-fat boat and a too small jet unit. Most people don't understand the hump-speed difficulties with jets. Now you came here for advice and my take on this is that we should be careful to get it right.

    That said, I went on digging data for the Mercury engines I took as an example. There is some info available on fuel consumption for the 80 hp and 115 hp propeller variants. They both share the same capacity of 2.1 liters. The 80 hp consumes 31.7 l/h at wot 6000 rpm, and the 115 drinks 42.0 l/h (as tested). Now there is one test published for the 80 hp jet, and it comes out with 40.1 l/h. So, for this engine there is now proof that the jet variant runs with the same power head as the prop engine, but with a different fuel mapping; in fact close to the 115 hp propeller version. With the engine code you now presented for your rig, we may assume the same goes for your case.........and ok Mr E, there might be a possibility Santa C will be president.....!

    Just for my curiosity: could you reveal the nozzle and inlet diameters for the pump?
     
  13. Sid
    Joined: Oct 2020
    Posts: 14
    Likes: 0, Points: 1
    Location: Poland

    Sid Junior Member

    @baeckmo, the engine is being serviced, but from what I've heard it's the very same pump as in Mercury 80HP outboard jet. I would assume it's standard nozzle and inlet diameters you'd find in jets of comparable power. Total load in the range 150-300 kg.

    Regarding the designs, both Slither and Snake Shooter are jet sleds. Here's the summary of the two hulls:
    Slither and Snake Shooter: 17', 18' or 20' inboard or outboard sled boat https://www.glen-l.com/designs/hankinson/slither-snakeshtr.html

    There is the minimum power requirement of 50 and 75 HP for the Slither and the Snake Shooter respectively. Based on your design experience and considering my engine, which hull would you choose and why?
     
  14. Barry
    Joined: Mar 2002
    Posts: 1,336
    Likes: 168, Points: 63, Legacy Rep: 158

    Barry Senior Member

    Not that it is very important but it is easy to assume that the impeller in the outboard jet uses a centrifugal impeller due to its somewhat involute/round shape of the housing. Assuming that this is the pump that the OP has was built by by Specialty Marine/Outboard Jets in California and is used by Yamaha and Merc et al. A centrifugal flow impellor has the inlet coming into the center area of the impellor, the water is accelerated radially to the edge of the pump housing then out to an outlet. Generally, centrifugal pumps are high pressure low volume. Mixed flow impellors, Berkley, American Turbine, medium volume medium pressure and then axial pumps, Hamilton, Kodiak .

    The inboard jet then uses an axial flow impellor.




    [​IMG]
    Centrifugal
    [​IMG]
    Axial
    [​IMG]
     
    Sid likes this.

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,061
    Likes: 640, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It certainly wouldn't be desirable to have to flog the engine to keep the boat on plane, but to get them up on plane is OK, I really think that 17 foot boat would plane with a standard 40 hp outboard,, so 80 hp should plane it easily enough, and not be working overtime to keep it running 20-25 mph. Top speed around 30 mph. Most people don't report being overwhelmed with the performance of jet outboards, which are more a compromise than the "ideal" jet drive.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.