a JET (yes I'm using the 'J' word) question

[gohelm]

A question for all you much more experienced 'Jet-set' on this forum:

Our Delta with Hamilton jet tends to have the 'hovering' issues like all jetboats have and reading the earlier post on this forum, there is a certain amount of patience involved on learning the trick how to handle the boat.

Here is the thing: we seem to improve handling skills (and it is really starting a course boathandling 1.01 again), especially on the manoeuvring part but the 'wandering around' doing sub 6 knots keeps on being rather annoying in where we use the boat for (i.e. race management).

In what way and how would fins/a skeg and/or any other type of gizmo improve the handling and what 'wrongs' would i do by adding one? The stern section of the boat is flattened in the building mould to accomodate the jet unit but as you can imagine i have some issues with 'abusing it with a large drill' prior considering the consequences. For example the pivot point of the boat seems to be more forward than I am used to so I can imagine I need to keep in mind the consequences of a unlucky spinout when adding a fin...

Any suggestions/ideas are welcomed!

best regards,
Gabor

[Rokraider]

I am currently completely overhauling a BWM 21 jet rib (21' Sea Explore). I bought a Castoldi unit from a guy in Ireland who supplied it with a couple of aluminium skegs that are about 5' long and start from a point I would say around where the hull starts to flatten out and are tapered to a depth of about 6". These sit either side of the flat plate where the inlet ports are situated on the Castoldi. Apparently these made quite a difference to the handiing and helped stop the stern skidding. I haven't tried them on the BWM yet, I will see how it performs without them first.
I have completely overhauled the Castoldi 238 and am installing it behind a BMW 3.0 turbo diesel making around 270 BHP. Hopefully it should be a nice package when it is finished.

[gohelm]

Rokraider,

Thanks for the reaction and will keep the size you mentioned in mind for ours...

Apparently also the skidding part is something you get used to. Jets apparently at first instance are real confidence breakers but WHEN you get used to it it beats everything in the book of manouverability. Getting more confident by the hour....
A dutch yacht designer Pieter Beeldsnijder is rendering some assistance on the fins, since he had some experience on some jet tenders he designed. Will let you know the outcome on that.

I just realized that the change of impeller in fact also helped. It made the flow of water somehow more stable although we changed in to a more coarse pitch one. Finding the right balance on impeller / rpm / engine power prived to be an issue on our boat. AMW, the agent for hamilton in the netherlands has been very helpful by sharing expertise on it, actually beyond the waterjet unit but also for our engine in relationship with the jet and in what/how much we could use/abuse it.

[Rokraider]

I have been wondering whether it would be feasible to fit a horizontal aileron between the skegs for trimming? If it was mounted at the rear and halfway up from the bottom of the skegs, it would be reasonably well protected. Not sure whether it would work or not?