Twin Jet Boat Build

Discussion in 'Metal Boat Building' started by Sms986, Oct 15, 2020.

  1. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    Hello everyone, new here.

    I'm working on a twin jet boat, powered by two yamaha GP1200 engines. I'm intending on it being a 2272 8 degree vee hull. I am currently in the process of designing the water intake system and wanted some opinions, as I'm new to jet boats.

    The main thing I'm trying to decide is the intake duct mounting. I could mount them flush with the hull on either side, which would put each one at an 8 degree angle. I'm not sure how the performance would be with that orientation-especially on the high side when turning.
    However, this would be the easiest way to mount them. I would only have to weld a 1/4" aluminum ring around the discharge side of the intake duct so that I can clock the rest of each assembly 8 degrees.
    I've seen a lot of people talking about spoons, pitch, etc. and I'm not entirely sure whether I should take one of these routes or simply mount the ducts flush with the hull.

    Any opinions?
     
  2. baeckmo
    Joined: Jun 2009
    Posts: 1,174
    Likes: 138, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    The propulsion performance of waterjets strongly depends on the shape and placement of the inlet. Unless you are trying to design your own jet units, my recommendation is that you follow the installation manuals from the jet suppliers. If you neglect their specification (which is all too common), you are left without assistance when/if your design is in trouble.
     
  3. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    Yeah these units are from jet skis. Not much of an installation manual here.
     
  4. BlueBell
    Joined: May 2017
    Posts: 1,261
    Likes: 242, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    I'm a sucker for easy.
    If it also most replicates the jet layout, then that's where I'd start.

    Baekmo is by far your best reference here on this topic.

    Pictures?

    Sketches?

    Winning lottery numbers for next week?

    Bueller, Bueller... anyone, anyone...

    OR, talk to the jet manufacturer... or both.
     
  5. baeckmo
    Joined: Jun 2009
    Posts: 1,174
    Likes: 138, Points: 63, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Ok, then you have to align the inlets with the streamlines along the aft part of the bottom, which in reality means longitudinally and flush with the bottom. The question is then: what is the shaft angle in relation to the inlet center line? If it is inclined vertically, the engine shafts must have a "toe-in" attitude, ie shafts pointing outwards aft.

    Ooops, BluBell hit it first.......
     
  6. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    I would like to run some CFD on it, but I think there are too many unknowns to get good solid results. Best I would be able to do is get close with streamlines.
    I am working like a dog to finish my thesis this week, but I intend to do some work on the boat this weekend. First up will be a CAD drawing that I will share. Just wanted to get a topic rolling to see what people think.
    The yamaha gp1200 setup is easy as dirt to work with. I'll post some pictures of the engine and pump unit all together this weekend, but as far as installation and such it looks so easy a monkey could do it.
     
  7. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    Speaking of streamlines...

    Wouldn't they kind of shed from the center of the hull out in a v? That's my main thing with a vee hull and two jets.
    With a strakes flat bottom it's pretty straightforward-channel/BL flow straight out the back in between the strakes. The vee hull is a bit more complicated in the way it sheds water. Much more dependant on a lot of other things.
     
  8. Barry
    Joined: Mar 2002
    Posts: 1,313
    Likes: 149, Points: 63, Legacy Rep: 158

    Barry Senior Member

    You should be able to find the installation instructions from the Hamilton Jet company. While even their smallest is much larger than what you are planning, I remember they had detailed instructions for twin installations.

    There is movement from the keel to the chine but with 8 degrees of V it will be minimal and my recollection is that the intakes were parallel with the keel. There were other details about strakes etc ahead of the intake and the distance of
    the intakes from the keel.

    From one of their installation manuals, twins
    Mono hulled vessel Aerated water generated by the vessel’s bow wave must not pass directly aft to the jet unit intake(s).  A vee'd bow stem in conjunction with 10 degree minimum deadrise angle is recommended.  Mount multiple jet units as close to the keel line as possible (staggering the engines can allow closer centres).  Planing strakes, keelsons, plank keels and any other hull appendages that may create turbulent water flow into the jet unit(s) must not be fitted in front of or adjacent to the jet unit intakes. Refer to figure: Mono hull design (► page 3-4). For directional stability at speeds over 30knots, use monohedron (constant deadrise) hulls without appendages. Displacement speed and warped plane (reducing deadrise going aft) hulls may need additional directional stability. Twin bilge keels are normally sufficient as these do not increase draft or interfere with water flow into the jet intake. The jet unit must be immersed with the water line at least up to the underside of the mainshaft (at the impeller) in order to prime the jet unit when the engine is started.. For applications using twin or triple jet units, refer to the installation drawings for jet spacing dimensions. For applications using more than three jets, contact HamiltonJet for jet unit spacing. Mono hull design 1 Strakes inside width of intake should stop 0.5 metres aft of dynamic air/water interface. 2 Keep the shaded areas free of any appendages including water pickups. 3 Air/water interface. 4 Strake. 5 Jet unit in


    And Seadoo made twin lightweight boats with their smaller jet pumps. Crawl underneath one to have a look

    Scarab may offer a twin rotax BRP boat and if you google Twin Sportjet boats, there are many references available
     
  9. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    Thanks for the help guys.

    I can mount the jets pretty close together. The gp1200s are pretty slender inline 3's. The only constraint I have is that I would like to put a 9.9 motor in-between them for smaller lakes.

    I'm essentially building this boat because it's easier for me to invest than to save. Putting a bit of money toward a physical project here and there is much easier than trying to save that money for months/years.
    I want an everything I need type of boat. I only want to do this once. I've got a cabin cruiser for "formal events" but it's not practical to fish out of. My fishing boat is a duracraft jon and it's way too small. I want two engines so that if one goes down I can get back with the other one. The "these new x y and z motors last for lever" pitch only works until you're in the middle of nowhere somewhere on the french river in Canada and your last forever motor sucks something into the water pump or something stupid. Then you're adrift. I love yamaha two strokes. So simple I could literally rebuild one at sea if I had to (I don't intend to). Jets are a must so that I can fish the rivers, and a 9.9 is a must because a lot of lakes have hp restrictions. I fish all up and down the east coast and I just want a boat that doesn't give me a headache and has enough versatility to do just about anything I want.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,954
    Likes: 605, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I'm not that au fait with jet-skis and the like, but adapting the propulsion system out of them to a boat, as they are, would appear to need a bit of luck to be a good result, or better still an experienced hand who knows the requirements, you don't want over-stressed engines, they won't last. Are you confident it is within their comfort zone ?
     
  11. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    So here's how I came up with the engines for this boat. One single GP1200 engine provides 135 hp and a yamaha f115 o/b provides 115 hp (obviously lol). I can't find torque curves for either to compare any better than this, but a lot of 2272 slight vee hull boats are powered by 115's or less. With a 30% power loss estimate for the jet drive, I'm around 95 hp. Double that, and I get 190. The gp1200 engines are also lighter than two comparable outboards.

    In regards to the engines lasting, I used to rebuild my yamaha 2 stroke bikes every 2 years or so. Very, very easy. Not a big deal. I love yamaha two strokes, so simple. Much much easier than outboards in my opinion.

    The price is what really got me. My fab quote for the simple design I described above as well as both engines and pump assemblies is still less than a single 100 hp outboard dating back any less than 20 years. Good deal.

    With all due respect, and I mean that, there are thousands of people who have hacked a jet ski literally in half and just bolted it to the bottom of a jon boat with great results. I do not think it is as technical and complex as it sounds. If someone in high school can do this in his backyard, I think I can give it a whack.

    I'll post some pictures and stuff tomorrow. I'm working on it a little bit tonight.
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,954
    Likes: 605, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Going on that, sounds like more than enough power. You might have got away with one, going on your figures ?
     
  13. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    Well I could go with one and it would be much easier, but I have had engine trouble at some point or another with everything I've ever owned. Like I said, I plan on using this boat up in Canada a lot. If you get stuck up there with engine troubles, a trolling motor or a 9.9 won't get you back. That's the biggest reason. I've learned that you can't walk home if your boat breaks down.

    Also I would like this boat to be relatively fast. I don't plan on cruising around at 70+ mph and will only use all the power to get up rapids or on big open lakes to get where I want to go, but 40-50 mph would be fantastic. I currently have a 40 on the back of my jon boat and my local lake that I fish multiple days a week is a 6 mile ride from the ramp to my spot. Takes a half hour one way.

    Third, I won't lie it will be pretty cool to say I have two engines...
     
  14. Sms986
    Joined: Oct 2020
    Posts: 14
    Likes: 1, Points: 3
    Location: NC

    Sms986 Junior Member

    Also I want to point out that with the design I'm working on, I can always just bolt a solid aluminum block off plate in place of the intake ducts if I ever want to just get an outboard or if this doesn't work out like I'm planning, so no harm no foul I guess.
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,954
    Likes: 605, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    You must have "hot hands", if all engines play up on you ! :eek:
     
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.