Hull design for water jet propulsion

Information on what I see is required when selecting a hull design for water jets.

 

Very nice information, of course, well presented and with great colors.
That said, this information is exactly the same as can be said for any planing powerboat. The only thing the water jet needs, which is not seen in this information, is a piece of flat bottom, aft (centered if it is a single water jet), to be able to house the suction of the pump.

 

A quick search revealed an Internet link to a very unique hull bottom design. I have not been able to find any performance results of this boat. The link was found when looking up Roger Mutimer Jet Boat Hull design
I would be thinking that this design with all the air entrainment would not be suitable for a jet
One Man’s Quest for a Fully Aerated Hull by Roger MutimerNauticExpo https://trends.nauticexpo.com/project-31199.html


Some comments on Roger Mutimers paper


Page 2 For and Against Lift Strakes
Normally, but not always, jet boats are used in shallow water applications, therefore a low deadrise monohedron hull is most commonly used. For discussion purposes use 12 degrees. A 12 degree keel will provide some
directional stability but in a turn will not provide enough directional stability to keep the boat from slipping excessively or even flipping around 180 (or more) degrees (Victory Roll)
That being said the attached video shows a few pertinent points. These boats have multiple lift strakes but they should perhaps be called directional enhancers. They run these canyons at high speeds and need some grip. Though,
even with multiple strakes you will notice that on extremely tight turns, you can see the amount of sideways slip that occurs. And with enough speed can even make the boat do a 360.

Shot Over Jet Boat New Zealand

My point is that with a shallow degree hull which provides a higher riding hull than a deep V hull, the biggest advantage of strakes are directional stability.

Certainly they will provide additional lift and you can see the pressure distribution of a reversed chine in the illustrations on the current active thread "Transom Pods Floatation and Extended Planing Surface Post 29"
An additional set of strakes between the keel and the chine will provide some more lift AND much better turning ability

Page 4, He mentions that the strake in figure 2, compresses water (not true or at least so negligible as to be ignored) The concept of a downward turned lift strake is to change the direction of the water as it migrates from the
keel to the chine. The larger the change of direction, the more lift that the strake provides. We used a 12 degree downward strake and chine but in a profile that was similar to the strake shown in Figure 3.

We did not carry the front of the strakes much past the monohedron section of the hull. While a strake in the front 1/3 of the boat could have been installed, we did not want the bow to grip on a tight turn which might
instigate a roll. Running larger lift strakes forward if properly designed can knock down water coming over the bow if you hit either large waves or standing waves on a river. Most west coast aluminum boats, jets or not
have larger spray rails/strakes in the bow area of a boat



Other aspect
As Tansl pointed out, the area inhead of the intake of the jet pump will require a flattened area to allow the mounting of the inlet of a jet pump. The flat part of most pumps are about 12 inch - 14 inches wide so you need to
SMOOTHLY transition a 12 degree V into the 12 inch flat pump intake . I believe our wedge went forward around 24 to 30 inches ahead of the intake lip.

Note that our boats had about an 82 inch chine width at the transom and we used one lift strake per side between the reversed chine and the keel.
In some previous threads, comments have been made that suggest to have the middle strakes end much ahead of the inlet to the pump. As we had lots of width, we ran our middle strakes to the transom with no visible
issues.


To the OP
A much more learned and engineer backed instructions or design considerations can be found on the Hamilton Jet website. It has been accepted that Bill Hamilton from New Zealand fathered the jet pump
propulsion industry and are more than likely the leaders in pump design. I would suspect that the other current pump manufacturers have similar hull design recommendations.

An aside
I was able to take the Shotover Jet boat ride about 8 years ago while on a holiday. I have several thousand hours of driving in shallow narrow rivers up to large rivers with 5 foot standing waves and boulder gardens.
With that came a strong sense of ability in my skill level. The video does not give the viewer the sense of speed that these boats are moving at or the skill level that the drivers have. I spoke with one of them after our
run and the fellow said that they train for 3 months before they are allowed to take passengers. The boats are twin engine so that if one shuts down they still have steering ability, at 60mph/ 100kph, with the canyon walls
literally a meter or less on occasion, this is a great idea. These drivers are second to none and I felt quite humbled by their ability.

 

Jet Mad, welcome to the Forum.

Nice presentation.

It could use some proof-reading.
This paragraph has three errors alone:
"Having the V for more than half is then giving the bow a more bluffer entry to the water and with the correct strakes will stop the bow from bearing it’s self into the back of a wave and then broaching.
Also: "is then giving" could be simply "gives".
Perhaps English is not your first language, I don't know and I don't care, just get someone to proof-read it.

 

The first thing about hull for waterjets: forget about strakes or use them very carefully.
I would not use strakes on boats with waterjets, to avoid jet cavitation.

 

And yet almost all current inboard jet manufacturers use them to attain better corning and spin out. The first Jet Boat that I bought did not have strakes and on more than one occasion ending up swapping ends on extremely tight corners.

PROPERLY PLACED lift strakes, ie not immediately ahead of the pump intake give you better control performance than not having them

 

Fully concur with 'properly placed' statement. We either not use them (on boats with low deadrise) or install them outboard of the jet drives (on deep-V). Never in front of jets.
However I don't see any mention of it in the document attached by topic starter, this is surprising. Instead, they do show massive strakes on illustrations.

 

Tansl, this information was to point out the suitable hull shape for water jets. Yes installing a single jet one would need a taper in front of the jet, ( Information supplied by the jet manufacturer )
Barry. Jets are used in open water, lakes and large rivers. Boat size can very and so can their deadrise, 12 to 22 degrees.
Directional stability but in a turn will not provide enough directional stability to keep the boat from slipping.
Have you seen jet sprint racing.
BlueBell. Sorry about the paragraphs.
Alike. Yes I show strakes. Instillation information provided by the jet manufacturer explains all that.

 

For water jets and for any other speed boat, there is nothing specific in there just for water jets, imo. But if there is, I'd like to know about it. Thanks.

 

Tansl, I have been involved with jet boats since the seventies in Australia and I'm sick of seeing and hearing about jet units been sold and installed in hulls not suitable for jets. Monohedron and with out a fine bow entry is what's needed. I have been asked to check out jet boats only to find a verable deadrise or warped hull .

 

@Jet mad, I neither take you away nor agree with you. I see, from what you tell us, that you have great experience hearing about water jet boats. Perfect. The only thing I am saying, for the third time, is that the text that you have shown us, I don't know with what intention, has nothing specific to water jet propulsion boats, nothing that applies only to them but on the contrary, everything that is said in that article (and in its attached copy), is generally valid for any fast motor boat.

 

Reflects poorly on you, mate.

 

You will note the number of strakes on this sprint boat? At the speeds that they run, it is unlikely that they are looking for more lift but rather directional stability. Can they blow out? certainly
The sprint boats have exceptional turning ability due to the strakes as well as the adjustable nozzle which can force the bow down to permit longer keel and strake interaction with the water

Of course jets can be used in other applications.

 

I have a hard time believing that strakes provide any significant roll stability at rest.

 

Tansl, have you ever been in a jet boat, if so what size and what was your thoughts on steering control at all speeds, slow and flat out.
Barry, I think you need to go for a ride in a jet sprint boat then give me your comment. Experience the real g-force in a tight turn.
ziper, you need to try checking with two boats, one with strakes and one with out. It may be hard to find a boat with a full set of the right strakes.