SAR Boat Not Achieving Speed

I have a 28' Hike Metal SAR boat with twin Volvo TAMD31's running Hamilton 211 jets through 1:1 Velvet Drive transmissions. The boat has an aluminum hull and the weight is listed as 3000 kg's. According to the shipper who delivered it the boat tipped 9000 lbs on the trailer so it's close to that weight as equipped.

According to the builder, Hike Metal, the original government design spec's for the boat required it to reach 30 knots for production and the engineer on record confirmed it passed trials. We took the boat out and it only reaches 18.5 knots at 36oo RPM so something is off. The boat was built in '98 so I expect the jets will require maintenance and/or tuning of some sort.

If the consensus is that the boat is under-powered, I would like to know what options we have for pushing more water with tuned impellers etc. One thought would be to gear up the transmission to increase shaft speed if thats an option. The boat is designed for towing and dive support but we plan to use it for commuting.

Photo's attached of impellers. I'm new to jets so any guidance would be greatly appreciated.

 

So, first things first. Ask for the trials report - the paper work - to support this claim.

 

...and then contact Hamilton head office in NZ (not the locals!), and supply them with hull info; weight, beam, bottom rise, spray rail configuration, longitudinal center of mass, keel length.....the lot. In addition a few speeds at reduced rpms, say 3300, 3000, 2700.

I'd like to see photos of the inlets and what the attachements to the hull look like. Your hull is heavy for its length, meaning that there is a considerable hump resistance. Which version of the TAMD 31 is it?

 

looks worn, check out this thread, might have some info you can use - River Jet Boating Forum • View topic - Multi-stage Pump Info http://meanchicken.net/webmain/forum/viewtopic.php?f=36&t=62&hilit=772

 

The Engines are TAMD31P-A's. Hull / Config. drawings Attached.

 

So, allegedly, the boat was fine and making good speed and now it's an absolute dog not achieving spec.
So, what happened?
Follow the history.
It's a Canadian government rescue boat?
There will be documented history.
Just follow the history and you'll have your answers.
Is the boat hot? Is it stolen with no history?
WTF?

 

The boat was taken out of service 4 years ago when their replacement was delivered.

 

Looking at the lower impeller picture, you can see a zone of pittings in the SS liner in the inlet tip region. This tells me that your problem is cavitation, and it is very common in this speed region, due to a combination of high trust requirement, high rpm's and high inlet losses. Which leads back to the first two comments in this thread.

Hamilton may have an inducer, or a special impeller to offer, but the main issue is high weight for the hull length, so get rid of all the unnecessary paraphernalia collected in the dark corners of the boat.

 

Just following up to see if Hilinr resolved the issue.
We are looking at purchasing a identical vessel, but would like 30 knots if possible.
Thanks!