Any Jet Boat experts on the forum ?

[alcesgigas]

I purposely kept about 70% weight aft in my shallow water jetboat so as to haul camp and later, a quartered moose. However, mine is a flat bottom, tunnel hulled aluminum river boat set up to run in 3" of water with a load; otherwise it draws 12" which isn't bad for what it does. I can't say I've ever had any experience with a V bowed jet boat. How's the steerageway?

[scottrods]

ok. I work for HamiltonJet and recently helped a friend rebuild a single stage Colorado with a Ford V6. Forget what brand is spinning the jet.

Your basic problem will be that the single stage Colorado is only good enough for about 100bhp. Even the coarsest single impellor is only capable of running without a lot of cavitation up to 130bhp. Here are some links to the user manual.

http://www.hamjet.co.nz/includes/fil...s%20Manual.pdf

http://www.hamjet.co.nz/includes/fil...s%20Manual.pdf

Page 4 in this one gives you impellor info versus bhp.

http://www.hamjet.co.nz/includes/fil...al%20Guide.pdf


It cavitates simply because you're spinning the impellor too quick to push water and the boat. The impellor is digging a hole in the water - it isn't pushing it. A bit like having 500bhp with the wrong differential and 10inch wheels on a car. Some options - make the jet a multiple stage, try a coarser impellor, change the engine to less BHP or just don't jump on the gas so much and let it take time to get on the plane.

Lightening the boat will change affect the planing speed but won't help with getting on the plane if the impellor doesn't match the bhp.

Plenty of good information over on the nzjetboating forum. nzjetboating.com Web Board. - Index

Jonathan

[Spanners]

I'm not 100% convinced the engine is overpowering the impeller.
The old Buick motors arnt what I'd call a powerhouse.
You really need to pull the pump unit and measure the clearances
On the Seapoos I have played with, a wear ring that looks good means night and day between one with the correct clearances.
Most wear will be in the front end so viewing from the rear prob isn't the best indication.

[Spanners]

Forgot to ask.
Can you get the engine to overev at full pace?

[veitnamcam]

Not comparing apples with apples but my gpr1300r cavitated from brand new. 165 hp single stage.
New intake grate and a progressive pitch solas impeller and it was like it had another 50 horse and actually got traction for lack of a better word.
The extra suck down and power made it corner like a blade in the water with no slide :thumbup:

Sent from my GT-S5360T using Tapatalk 2

[baldbob]

Scottrods teed it i feel.... sounds like 2 things... a is worn impeller and b is you really need more stages... im running same engine on a 17 &19 pitch ss tipped impellers 110mm nozzle and it goes like crazy... 90KMHR goin downstream and straight up on the plane... unit was a 3 stage and wasnt very flash at that as very gutless!!!!

[tui_man2]

Scottrods teed it i feel.... sounds like 2 things... a is worn impeller and b is you really need more stages... im running same engine on a 17 &19 pitch ss tipped impellers 110mm nozzle and it goes like crazy... 90KMHR goin downstream and straight up on the plane... unit was a 3 stage and wasnt very flash at that as very gutless!!!!

Was niddys ship an if built up the impeller n skimmed housing an turned impeller to 'wear it's self in' (very nice fit). A spanners the motor went like fuck in putters car before he crashed it an of put it in the ship. Think it's bit much donk for the unit. The nosel was bored out also to let it flow out easier to

Sent from my GT-I9300T using Tapatalk 2

[Happy]

Forgot to ask.
Can you get the engine to overev at full pace?

Yes... Just jump on it and it flares up. Main problem is not too much the time it takes to get up but the fact the nose is in the clouds. Drops off the plane quite quick though as well.

[Happy]

Im getting a little tapered spacer built to slighty change the nozzle angle as suggested by a few. Yes I reckon it should have been a multi stage unit. Everyone has said that. Ill try to tilt the nozzle starting
at recommended 7.5 degrees in the hope it ll get the nose down a little. Im not going to remove mtr and jet unit as to much other stuff to do , if this dont work Ill maybe relocate battery to up front then maybe fuel tank
and if still no good I ll have no probs selling it. First jet boat I ve owned so picked a cheapy to start with.. I like the sport so Id just save up and buy a grousie and this learning experience is all good as I wont loose any thing. Actually a two person
reliable jet ski would possibly do same things I want to do and could probably launch it by myself where as this needs two due to river style boat ramp. IE 2 Foot drop into the river off the bank so winch it in and out with the truck.
Plate spacerr being made next week so will post once fitted and trialled.

[baldbob]

I doubt changing the nozzle angle will make alot of difference.. to get it on the plane better you need to tilt up but the further you go the more steering you lose!!!

The ass will be boggingdown cos the unit wont have the juice to push it up out of the water... putting weight forward may be the better option over adjusting nozzle angle but it sounds like your trying to make up for lack power here...

Try a coarser impeller!!

[samba]

First prob is get a two stage unit or diff piched impellor check out nzjetboating.com The home of JBNZ Online another forum of good bastards they will have you sorted in no time at all

[Spanners]

Yes... Just jump on it and it flares up. Main problem is not too much the time it takes to get up but the fact the nose is in the clouds. Drops off the plane quite quick though as well.

I mean when you have built up speed and are hammering along (say on a flat lake).. can you overrev the motor or will it hit a wall and not go any further?

[Happy]

I doubt changing the nozzle angle will make alot of difference.. to get it on the plane better you need to tilt up but the further you go the more steering you lose!!!

The ass will be boggingdown cos the unit wont have the juice to push it up out of the water... putting weight forward may be the better option over adjusting nozzle angle but it sounds like your trying to make up for lack power here...

Try a coarser impeller!!

If I do that do you have to remove the whole unit to fit it ?
Which means engine and heaps of other stuff in this case as well ...
Id say the way its all fitted plus being a bastard to work on it would be a painful progress..
I ll get the price of the bits and make a call. Would you go to the special coarse impellor ?
They offer standard, fine and special coarse for reduced revs or high power applications.

The boats worth spending hundreds on but not thousands

[Happy]

I mean when you have built up speed and are hammering along (say on a flat lake).. can you overrev the motor or will it hit a wall and not go any further?

I honestly dont know if it d blow up or hit a brick wall never full throttled it for long enough.
I reckon it would hit a brick wall. Its does get to a point where you can feel its not going to go much faster so I usually drive it just below that point.
It seems happy at 45 kMH 3800 RPM and doesnt feel like you are gonna wreck anything.

[Happy]

Thanks all for all your help. Have had an interesting chat with Rakaia Marine. The guy knows his stuff seemingly. Had same issues before. Has sold me a wedge which wont fix cavitation but will lower the nose. He knows the boat actually has one in his yard. Coarse impellor no good. Fine and standard similar. Some mods to an impellor could be made but even then not a winner. Been there done that and he said waste of good money.
The coarse pitch impellor needs more hp then we have he tried one and it reduced max eng rpm to 3200 where it hit a wall and max speed accordingly.
. They also haven t had any used impellors for a while and Ham Jet said haven't been produced for years so good luck. He could mod impellor to suit but siad no promises and its not cheap enough to make it worthwhile.
Told me he has a client exact same problem and they spent heaps before getting a two stage unit which made it a different boat altogeather and used the horsepower brilliantly.
So option looks like fit the degree plate,Feather throttle to get away, use it like it is until sell it or change to two stage pump or repower with lighter 100 to 120 hp motor which may still not work well.
Will fit degree plate he s sold me one for like 1/8 of price to make one so worth a go and repost then.