Modifying a PWC hull....

Discussion in 'Fiberglass and Composite Boat Building' started by DSR, Mar 7, 2018.

  1. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi everyone,

    I got myself into a bit of a side project and I'm thinking that the best way to do it will involve materials that are admittedly a little out of my realm of practical knowledge, so I'd like to get some info from the people here with experience regarding these materials and techniques, and what would be the best approach for this project if I could please.

    A friend and myself have gotten into discussing experimenting with changes to the bottom of his PWC hull to improve performance. I'm working on mapping the changes being discussed and It looks like the changes can be done to the existing GRP hull without "violating" the original hull structure, by epoxying foam to the hull, sculpting it to the desired shape changes, sheathing with fiberglass and epoxy resin and refinishing. The PWC runs in waters of known depth with no unusual hazards, so there's no need to make it bulletproof. I'm more concerned with minimal added weight and performance of the materials that are on par with the original hull, for the cost involved. I'm going to recommend he vacuum-bag, but at this point, I'm going to assume that it'll be a hand layup.

    So with those criteria, what would be the best type and weight of foam for sculpting the changes? And what glass layup would you recommend?

    I know the description is kinda vague, so if you need more info, please feel free to ask for any particulars and I really appreciate the help!!

  2. messabout
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    messabout Senior Member

    I am curious. Are you quite sure that the mods are going to improve the performance of the PWC? Obviously you are. Please describe the anticipated changes and tell us why they might improve the boat. You may be onto something, alternatively you may be shooting yourself in the foot.
  3. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi Messabout,

    I'm not 100% certain as to how much of a difference the changes I'm talking to him about will make, but he's willing to experiment with it. He has done quite a bit of experimenting with the hull already, with less than anticipated results, so I figured that I'd do what I could to try and help.

    The PWC in question was designed with much more emphasis on maneuverability, with the associated tradeoff for top speed (something these particular skis are known for), and with the powertrain upgrades he's done to the ski, he feels that it's come to the point that the hull is holding him back and wants to see what we can do to improve hull efficiency and top speed.

    The hull at the transom is a moderate deadrise V configuration with 2 lift strakes each side and the lift strakes have a slight down-angle. The keel is basically a huge radius between the inner strakes and forms a really bulbous forefoot at the fore end of the keel. The keel is also shaped very oddly at the forward edge of the pump intake. The pump intake is also too large for speed and we'll be addressing that as well.

    My thoughts, and I've confirmed this is very doable using a straight edge on the hull, is to attach lengths of foam (with the hull upside down) from the top of the outer strakes spanning in to the base of the inner strakes, and from the tops of inner strakes spanning to the center of the keel. Then fair the foam using the original strakes and keel as guides, to create flat (non-radiused) running surfaces with less deadrise and a more pronounced V along the keel (I was a little leery of doing this part creating a huge deep forefoot,but it doesn't). Form and attach foam lift strakes on top of the original strakes that the foam covers. Then form a smaller radiused drop keel in front of the pump intake to deal with the water boundary layer that will be present.

    Whew! It's much harder to convey the concept in words than visually, so I can post some sketches if that'd help.

    I'm also gathering data to input into the Aeromarine Research program to see if I can gain any insights on this experiment.

    I told my friend that doing this will sacrifice maneuverability in the focus for more speed and he's more than okay with that.

    With this in mind, and not having done this kind of layup before, I wanted to ask the people that have experience with this process, as to what materials would be appropriate?

    Thanks Messabout and let me know what you think?

    Last edited: Mar 8, 2018
  4. Yellowjacket
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    Yellowjacket Senior Member

    The problem isn't just the hull shape, but it is more one of the location of the CG.

    If you do a Savitsky planing hull analysis and plug in numbers associated with any typical jet ski you'll find that for any kind of high speed capability, the CG should be well far aft of where it can ever be in a jet ski.

    For any planing hull, there is an optimum trim angle that corresponds to the lowest drag for that speed. That is, for a given speed, to minimize drag there is an optimum location of the longitudinal CG. Most folks who have small planing boats know that when they are running at lower power levels it often improves speed to move some weight forward. But as you go faster moving the CG aft increases the trim angle and reduces the wetted surface area and the faster you go. The bottom line is that on a planing hull, the faster you go, the further aft the CG has to be to optimize the trim angle and minimize drag. If you look at outboard racing planing hulls (not hydroplanes with sponsons), runabouts or padded v hull boats you will see a CG that is well aft of where the CG is on a jet ski because the jet ski engine is near amidships as is the operator. The reason your hull is full up forward is because you need area forward to keep the bow up because the CG is so far forward. If you run the numbers you'll find that because of the forward CG location you can add a lot of power and still not go much faster. The faster you go the flatter the hull is running and the drag is going through the roof.

    Also, going faster with a jet ski can also get you into an area where lateral stability can go negative and the boat would spin if you encounter a small wake or wave.

    Things you can do is add strakes to get the hull further up out of the water and reduce the wetted surface area, but you need to do some analysis to get that correct. Too high and the strakes won't do much good, too low and they won't do much good either.

    You could try to add a pad that would provide a narrower planing surface with longitudinal steps and strakes, but now you get into issues with lateral stability at high speeds, but this can be addressed with fins added to the back of the hull.

    The first thing you need to do is get Dingo Tweedy's Savitsky analysis spreadsheet, plug in all the numbers and look at what can be done with different planing surface widths and the CG location and weights that you have and that should tell you a lot about what you can get for each modification. If you don't start there, you're just doing a cut and try in the dark and that's not going to be very productive.

    One last thing is that jet skis are relatively heavy and that also impacts the high power demand and the speed that can be had with a typical jet ski hull, so for that reason you may not get as much gain as you would like.

    Edit... I didn't notice that you were going to use Jim's program, that's much better than Dingo's spreadsheet anyway. Still, as you do your input you're going to find that a big part of the issue is the location of the CG relative to the planing surface. Once you get the program up and running you can move the CG and see what that does for your drag curve.
    Last edited: Mar 8, 2018
  5. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Hi Yellowjacket, thanks for stopping by!

    You bring up several good points that I've been dealing with through the thought process.

    I definitely agree that the CG in the skis is a major issue when searching for more speed, but moving the CG would be more than my friend is willing to do (that was the first item I brought up when we started discussing this). Fortunately, his ski has a better CG than a lot of others (though that statement is relative and still not good by any stretch).
    I myself, just finished designing and started building a mini jet boat using the same powertrain as my friend's ski, and the difference in performance was amazing with a more focused hull design with the CG in the right location.

    I'm currently gathering the data needed to run through my Aeromarine Research program, and from what I've experienced with the it, I have to assume that Savitsky is incorporated into the program, considering the input variables and the information generated from those variables. It was a huge help sorting out my own design.

    I also suggested adding a pad to the keel and it was one of the mods he's already done with little effect.

    My focus on the mods I'm thinking of is 1) reduce the wetted surface area at speed by using flat surfaces to replace the original radiused surfaces, 2) making the lifting surfaces more efficient by reducing the deadrise of the running surfaces, and 3) effectively lower the inner lift strakes in relation to the keel to help with lateral stability at higher speeds (along with the lift component in doing this).
    I'm also trying to incorporate these changes as to be non-intrusive to the original hull so changes (or even complete reversal back to the original hull shape) is relatively easy to do.

    After starting this thought process, I decided to investigate what the really fast skis look like, and my ideas are actually mirrored on a lot of these skis (along with steps, which I could easily include in the mods and would help with the angle of attack at speed .....).

    A 3 to 5 deg. angle of attack was also something I'm focusing on, and it is something that the race skiers have been able to achieve despite the forward CG involved. A lot of it on these seems to be dealt with using nozzle thrust lines / angles and ride plate shapes and angles, which we'll also be messing with....).

    Thanks For the help!!
    Last edited: Mar 9, 2018
    Jimboat likes this.
  6. Barry
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    Barry Senior Member

    I would consider playing with the nozzle. Not sure what type of pump that you have but you probably could get some machining done so that you can
    1) reduce the nozzle size, increasing velocity from the pump, albeit perhaps at a higher hp requirement
    2) figure out how to change the angle of the nozzle similar to a Jetovator,
    3) if the intake has a removable grate, and you may be able to reduce the size of this. I have been told but cannot say for certain, that Hamilton Jet was playing around with an intake that they could vary the length with when operating to find out which size was better at what speed.

    Depending on your pump, there are aftermarket impellors available that can give you better speed though the thrust at lower speeds might be less. Stainless instead of aluminum, tightening up the impeller clearances etc

    While this forum offers good info, there are several sites for your type of application that discusses diffusers within the intake path, ie inside the tube, that will move more of the water
    up towards the high part of the impeller for better performance. I would check there as what you might do to the pump to get more thrust has probably already been done. And some actual
    performance results may be available.

    Re the jetovator, 20 odd years ago we built an 18 foot 12 degree aluminum boat. Pretty basic, the guy wanted to get 60 mph out of it. He supplied a stroked, turbo'd engine, and we installed it with a Berkley pump. We were cruising up the river at about 60 mph, and thing felt pretty good, then started adjusting the nozzle up, from the boat of course, and the speed increased
    to maybe 70 mph plus, we were off the pitot tube speedometer at 70, we came around a bend in the river and hit a gust of wind coming up the river, the boat decided that it was not quite as
    stable as it felt and took off course. As you know, when you are trying to steer a jet, you need the thrust so you cannot just kill the throttle and have steering.
    After many hundred of yards of virtual terror , we got the boat under control.
    In hindsight, but that is usually 20/20, a quick reduction in nozzle angle would have been a better solution but my partner was gripping the windshield to ensure that it was going to stay attached to the boat and between the throttle and wheel, I thought I did pretty good getting the boat off plane.
  7. DSR
    Joined: Mar 2017
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    DSR Junior Member


    Hi Barry,

    Thanks for stopping by. Your boat test reminds me of some of the rides I made it through alive growing up. I ran with a crowd building "California-style" shallow-V and tunnel hull jets with big blocks cracking 500-1000+ hp, so I truly understand what you're talking about. My own project design is loosely based off of the Southwind Tunnel Dragster jet hull if you're familiar with them? It also has adjustable trim on the nozzle, and I'm planning on running a wheel-mounted trim switch for just such occasions...... :eek:

    The pumps we're messing with are the factory Solas 155mm axial-flow. They actually have a venturi section after the pump / stator body and before the steering nozzle and they use a "tail cone" that bolts to the back of the stator and extends into the venturi to displace area inside the venturi, regulating water mass and velocity exiting the pump. My friend machined up a mount base and separate cones that can be shimmed and swapped. Cool little setup for tuning the pump and something I'll also be using in my build. Depending on the model of ski, some pumps, such as mine, are trim adjustable. My friends isn't remotely adjustable, but there are wedge kits of various degrees that mount between the pump body and the venturi section to adjust thrust angle. We'll either use wedges or get a tail housing like mine. His choice. Unfortunely, the intake sections on these skis are incorporated as part of the hull, so modifying it's gonna be fun stuff. But it needs to be done if he wants more speed. The mods will also include a radius drop keel leading to the intake and will help load the pump. The intake grate is also removable and there are aftermarket replacements with a variety of shoes and loaders incorporated into them, along with modifying the original. A performance stainless impeller is also on the to-do list.

    I have seen references to adjustable intakes on the interweb, but I don't think it was Hamilton doing what I saw.

    Unfortunately, even with all the tuning /upgrades, these particular skis only realize rather dismal 2-5 mph increases, which in our minds, circles back to the hull itself needing some attention to decrease drag and increase efficiency, to make the most from not only the present upgrades, but any future upgrades.

    With these thoughts starting the process, I think I have a good idea how this can be done, but I have no experience doing foam and fiberglass and wanted to get some info on what types and weights of materials would do the best job?

    Thanks Barry!
  8. DSR
    Joined: Mar 2017
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    DSR Junior Member

    OK, I've done a bunch of trolling both here and out on the interweb thingy, and I didn't have much luck finding info regarding a situation like this, sooooo.....I'm just gonna throw a SWAG out and see what thoughts are on it?

    Do the sculpting using 2 lb. / cu. ft. polyurethane foam, 2 layers of 10 oz. biaxial (7725) and a top layer of 6 oz. Plain weave (3733) to get a good surface for finishing. All in epoxy.

    What do you all think?


  9. DSR
    Joined: Mar 2017
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    DSR Junior Member

    Well, for what it's worth, I was able to map out the ski hull and run some ititerations through Jim's TBDP and I thought I would share..... :)

    Changing only the hull specs with my proposed modifications, I saw top speed increase by ~4 mph with very similar porpoising regimens to the original hull and a slightly lower hump transition zone speed. Dropping the thrust angle by 2 deg. actually gained another 2 mph (which kinda surprised me), so it is definitely trim-sensitive.

    Pretty happy with the results so far and it looks like it's worth pursuing. Now on to the issue of the BOM for the mods?....... :D

    Last edited: Mar 18, 2018
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