Jet boat not performing

I have owned and operated the fishing vessel Net Profit for about ten years. It is a 32 foot by 16 foot aluminum planing hull with two 400 HP Cummins C-series engines and Hamilton 321 jets. With its current four blade #17 impellers the 30,000 pound vessel does not get up on step and is essentially an 8 knot boat with poor fuel consumption. Pictures are attached below.

Hamilton Jet was involved when the boat was first built, and has been consulted several times since. After moving some weight aft they say everything is right and the boat should do 30 knots. I sent an email to Hamilton Jet New Zealand, but they referred me back to the local Hamilton Jet engineer.

When I bring the engines up to full RPM, with good wear rings and impellers, there is no delay or bogging down of the engines. They go right up to 2800 RPM, high idle. The bow pushes a lot of water and the boat does about 8 knots. I recently had the opportunity to try some #25 five blade impellers with more thrust. Now the engines would only rev up to 2300 RPM and I was doing about 10 knots. According to people on the beach, it looked like the bow was about to pop up. I did not have a chance to play around with it much, as the guy needed his impellers back.

I suspect the boat is just too heavy and wide to get up on plane, but regardless I am hoping there are opportunities to gain some speed and better fuel economy. Jets have advantages that are very beneficial in the Alaska salmon drift net fishery the boat is used for, but at 8 knots they are not very fuel efficient.

What would be a more appropriate impeller for a displacement craft? Is there a way of using a kaplan-style propeller as a ducted propeller in the jet housing for improved performance at low speeds? Could a modification of the bow make a difference?

I have used the forum as a resource from time to time and know there is a wealth of knowledge and experience here. I would greatly appreciate any input.

Below are pictures of the boat:

 

I'm a novice with jet drives, but I believe the hull shape is awkward at the bow.

Your bottom pic may be your answer - look carefully at the white boat next to yours and see the difference in the bow area. The white boat has a "normal" bow you would find on basically all planing hulls.
Your boat has virtually no deadrise at all and is carried right through to the bow, IOW flattish - ok for aft but not good at the bow. There is basically no / little angle of attack of the bottom at front to get on the plane, hence you pushing away a small mountain of water and staying in displacement mode...

A 50% beam to length ratio is also a bit high (unusual) and you have a lot friction and drag to overcome as well.

My 2 cents worth

 

The impellor pitch would act similar to a propellor pitch gearing and not get to maximum revs until planning at the optimum speed for the weight so that if correctly pitched the maximum engine revs would need to be lower at displacement speeds. With only one gear so to speak it's a compromise between sufficient power and torque to get on the plane and maintaining the best speed.

From what you say the engine revs get to maximum at 8 knots so there's nothing left to get you faster on the plan, a bit like trying to do 50mph in your car in first gear and only doing 30! You'd need second or third gear for your maximum speed, provided you can actually pull away!

It can be difficult getting the right pitch but your experiment seems to indicate you're not far off as the revs did load down, definitely need more thrust at lower revs.

I would look at all the weight on board and dump what you don't need for an experiment, then try those same impellors if you can with a bit of weight re-distribution to trim the nose up sufficiently and see what happens. Do you have any trim controls?

 

Afraid Wynand has pinpointed the hull issues quite well. I don't know how to tell you this in a decent way without anaestecia....... But this is one of the worst hull shapes for waterjet propulsion that I have seen! The designer has no idea of basic hydromechanics and should go back to container designing. With these proportions, it will have a huge hump resistance. I must say I am surprized to see Hamilton involved in this, since they normally are very careful to avoid getting mixed up in "maximum risk" projects.

On top of this, the jets are selected to work close to their maximum rpm's, meaning that when you need maximum thrust to get over the hump, the jets will cavitate heavily! This is why the high-pitch impellers are slightly better, but make no mistake: there are no impellers available that can solve this situation!

And that is not all; if you could persuade her past hump speed, there would be aeration problems, since the bubbly water from the spray root up front will follow the bottom into the jet intakes and cause loss of thrust.

So, as reshaping the bottom probably beeing out of scope, the only relevant way to use those Cummins horses is to get rid of the jets and install straight shaft arrangements with props in "overhung" tunnels. With this arrangement, you would have a 20 knot cruising speed in flat water. Any ondulations on the water would kill your kidneys at planing speed with these lines..... . Guess the designer/builder's name was max profit???

 

A setup like this perhaps, just a bit larger?

 

Thanks for the input guys. I guess the guy that had this boat built wanted as much as possible in a 32 foot package, the size limit for the fishery. The company that actually built the boat, Laconner, went out of business before finishing this boat in the mid eighties. They built a lot of good boats, but this is the only jet boat I know of.

To Hamilton Jet's defence, this was an early jet fishing vessel built around here for operation in Alaska. The jets are # 14 and 15. There have been a bunch built with these same and smaller jets since that look similar and perform well.

I bought the boat knowing I would probably not be able to get it up on step and for less than a tenth of build cost. It had less than 200 hours on the engines after ten years. It is a good big work platform, but costly to operate with the fuel prices these last few years.

I will consider the deep tunnel arrangement. The boat would still be shallow and maneuverable with the twins. 20 knots would be great. I would just have to be careful with nets and fork out about $50-60k for gears, shafts, and some aluminum work.

Thanks again for all the input.

 

Sorry Capt Littlelegs, I forgot to get back to you on your last paragraph. No, I do not have any trim controls. Have wanted to manufacture and install a wedge to try to get some lift in the bow, but have not done so. Considered trim tabs, but will they be able to help lift the bow?

I had the same thoughts regarding dumping weight and messing with weight distribution when testing the other set of impellers. I just ran out of time after spending all night in the mud at low tide to switch impellers. Then had to haul out right after getting water.

Do anyone know of where I could find some used impellers for the HJ321s? I have looked for some with more thrust for a few years, but have not been able to come up with any.

Regards,

Truls

 

Jets simply don't work like that . sorry

 

Please enlighten.

 

Truls,

Here at Explorer Marine - www.explorermarine.co.uk - we build RIBs with props and with waterjets. Our experience is that waterjets can be superb if you have enough thrust to be sure that you can get cleanly over the hump and onto the plane.

The simple relationship between power delivered at the flywheel and the thrust delivered at the jet works when everything is new. However, this depends upon very tight tolerances of the impellor to the housing, and that the engine delivers up to max power, without reaching the governor speed. What is the governed speed of your Cummins, because if it is 2800, then you may have reached max revs without absorbing the horsepower and converting it into thrust.

Another area of concern is the thrust jet output. If you can really only reach 8 knots, is the jet output clear of the water, because if it is still under water, the expected thrust will NOT be delivered because you will not have achieved the necessary change in velocity of the required mass of water.

Thirdly, if the hull drag has increased through damage or through distortion, or through weed/barnacles, then the revs may be reached but there may not be enough thrust to get over the new hump requirements.

Fourthly, we have also had problems with the intake grid on the engine being either the wrong part for the jet, or more frequently being obstructed. If you have very clean waters in your area, you could try a test with the screens removed.

Another interesting test - having investigated the above, is to tow the boat with a faster powerful boat - and a suitable releasable long hawser. If at max throttle you can not get over the hump alone, the additional tow may get you up and over and then see how the boat performs once on the plane.

Good luck
Hugh Mattos
www.explorermarine.co.uk

 

capt littlelegs, your quickest route to enlightenment is through the archives of this forum, it is all explained ad infinitum.

 

It won't give me your specific opinion on my comment. If you think I'm wrong then have the decency to say why!

 

Hugh -

Thanks for your comments and good ideas. The engines are rated at 400HP at 2600 RPMs. Since they rev right up to 2800 I assume that I am not absorbing full power. Since it really does not do any good to keep the RPMs high I never go over 2200 RPM, and usually only use half of the available HP. I would think that the impellers that I have, which are matched to the engines at rated RPM, are not very efficient at half power.

Deeper water is clean where I am at, so giving it a shot without the grates is a good idea, though I would have to haul out again at a cost of $1,500 before I went fishing.

At 8 knots water gets sucked up behind the transom to submerse the jets, so yes, this could be an issue as well. When I first got the boat this was not the case. There was more weight forward and there was a clean break at the stern to where the waterflow from the jets was unobstructed. The boat did 6 knots and was very hard to steer. After shifting weight aft and lightening the boat about 3000 pounds, per Hamilton Jet's instructions, handling and speed improved but water gets sucked up behind the stern.

I have never thought about getting a tow to see how the boat behaves at higher speeds, another good idea. Some of my buddies might have the power to make it happen. It is easy to get another 50 HP out of each engine, and a local prop shop can increase the KW rating on the impellers a fair amount, so if there was hope it would be worth the investment.

Thanks,

Truls

 

Looking at the pictures and your waterline I can't help thinking you might be too overloaded. It doesn't sound as though your engines are matched properly to the impellors.

As I said before, it would be worth stripping everything off to lighten as much as possible, change the impellors for more thrust at maximum power at or just below the rated revs, that extra power available might make the difference, remove the grid as suggested, if possible trim to get the nose up and see how she goes. If she still won't plane then she likely never will! If she does plane then without a deeper vee she will likely be uncomfortable on anything but calm water.

Having hopefully got her to plane you can then consider either further fine tuning or, after looking at all the weight to go back on, resign yourself to a displacement boat! In which case your engines are then far too big for the power needed and the jet drive not the best option.

 

Thanks,

I did not bring the impellers down south with me this year, but my plan is now to rebuild and tweak the impellers for max thrust for the 2011 season. I will leave everything loose on the dock and probable pull the intake grates.

I may look at what I can do to ease the entry of the bow. It is aluminum after all and I could probably tack a piece in place for testing, depending on how complicated the part turns out to be. I would be very interested in design suggestions for a bow piece that does not increase the overall length of the boat. Could maybe be a good senior project for a naval engineer.

I am not sure what it would take to eliminate water getting sucked up at the stern, and how much this affects the jets' performance.

If nothing else works I will give myself some time for testing and see if I can get another boat to give me a tug.

Based on what I learn I should know once and for all if the boat will go fast with jets.

Thanks again,

Truls