Homebuilt jetboat-project

[Matz]

Hi everyone.

First up, I´m completely new to this forum, and english is not my native language so please bare with me.

Problem: I started a small project at home, a 1989 Bayliner 1952 with a "broken" ( weak and lots of small problems...) drivetrain concisting of a 2.3 ford striaght 4, and a classic OMC Cobra sterndrive.

A co worker heard about my problem with my boat, and asked me if i wanted to take over his jetboat project.

Got a fair deal on a completly rebuilt 355 Chevrolet V8, and a Castoldi jet 05 from him.

So under the holidays this summer i´ve mounted everything in my boat, jet, engine, the lot. ( scrapped his old ribb due to punctured "ballons")

Engine runs like a clock, but when i launched it in the sea for the very first time, the boat wouldn´t do more than 8-9 kn at a quarter throttle, any more than that, the engine would just increase revs as the boat slowed down. pulling back on the throttle would make the boat "grip" again and continue to move forward in this low speed of 8-9 kn. You could say that it reminds of having a slipping clutch in a manual transmissioned car.

So we pulled the boat out of the water, took out the impeller from the jet, and the impeller is in composite material, with some damages. ( i noticed this when i mounted the jet pump in the boat a year ago, but didn´t think theese small damages would matter.) It looks like some previosly owner has been sucking up pebbels and sand in to the jet, and leaving the "wear ring/ liner" full of circular scratchmarks, and the impeller edges of the three blades looks like a mix of "melted" and as if someone gave them a workover with a very coarse file.

I´ve heard now later on, that impellers are EXTREMLY sensetive to dings and scratches, and the gap of play between the liner and impeller can´t be to big. ( heard /read on forums everything between 2 millimeters down to 0,65 millimeters)

We checked our gap with a 2 millimeter welding rod ( couldn´t find the feelergauge ) and it went in between liner and blade with ease.

So my questions to you guys is: how big shuold the gap be? ( Liner-impeller)
Could my problems be caused by this damaged impeller?

Just want to check with you guys before ordering a new set impeller and liner from italy.

Posted a couple of pics aswell. hopefully you can see what i mean.

Best regards : Matz

 

[Big Shasta]

Absolutely the gaps in the sidewall and dings in the blades are causing cavitation which kills your thrust. You should first of all research a stainless impeller and a new wear ring. Here's a shot of mine with 300 hours on it just realize singe the outer bearing is off, it isn't centered. It's in pretty good shape but probably a little larger than stock since it's never been changed.

If you can get something that big in there, that seems big....

 

[jetboater4life]

I agree with @Big Shasta . The space between pump and impeller needs to be very close to the specification. I've seen impellers that look worse but I'd bet that is the source of your cavitation and lack of power. Other things to look for are any crevasses or disturbances in the path through the pump to the impeller. Yamaha pumps use silicone to seal up the places where the pumps mount to the hull. If that silicone comes loose, the small space/crevasse can create cavitation. In other words, the water getting to the impeller has to have a nice smooth path.

Your English typing is great! and

 

[GTBRMC]

Welcome to jetboaters.net, Matz!

 

[Matz]

Hi again.

Thanks for lighting fast replies and happy to be here in this forum

Stainless impeller does excist as a optional addition to this jet pump, but they are quite expensive. about 1700 eur.

My fears about a strong stainless impeller is that i would "relocate the weak link" . As it is now, with a plastic impeller, the impeller i most likely to break before anything else in the jetpump and its gearbox. With a stronger stainless one, i fear that the stress that would develope in an bad situation could damage something else instead of the impeller, something even more costly and inacessable...

You are all talking about that the gap should be to specs, does anyone know the specs for my jetpump? cause i have no clue

//Matz

 

[jetboater4life]

@Kevin Turner Do you have knowledge of his jet pump?

 

[Matz]

If anyone is interessed, i´ve uploaded a couple pictures during the project and it´s progress and setbacks.

there are three album at this site: http://www.ladda-upp.se/galleri/prime/

Enjoy.

//Matz

 

[Murf'n'surf]

Even if your gap is correct, you may be spinning too fast and causing cavitation. The boat may be too heavy to run on one engine/impeller of this size and pitch.

 

[Big Shasta]

I wonder if these guys would be willing to help with things like pitch and gearing. They build high performance jetboats with one pump.

http://www.needmore.com/smokymountainjetboats/our-boats

Alos, I'm sure you've been to this site, but it looks like you may be at the upper end of horsepower capability of that unit. Not sure what that 355 puts out but the pump is rated up to 238 HP. Careful on the throttle.

http://www.castoldijet.com.au/Jet-05.html

 

[Matz]

Even if your gap is correct, you may be spinning too fast and causing cavitation. The boat may be too heavy to run on one engine/impeller of this size and pitch.

I´ve looked at the weight issue aswell, and this unit is sopposed to cope with 1500 kg, Funny though, the old boat that this unit was mounted in from the very beginning had a dead weight of 1600kg AND was supposed to carry as many as 6 persons ( Ribb/ M.O.B boat for costal tanker)

So the weight isn´t an issue in this case. You mention the output rpms, well i´ve thought about this aswell, but if it where the wrong gear ratio i would like to think that it would have gone faster in the low rpm of the engine, would probably do a couple of knots at idle...

Pitch, well this i haven´t even thought about, just assumed that the correct picth is standard for this unit, and you just alter the gear ratio if needed.

//Matz

 

[Matz]

I wonder if these guys would be willing to help with things like pitch and gearing. They build high performance jetboats with one pump.

http://www.needmore.com/smokymountainjetboats/our-boats

Alos, I'm sure you've been to this site, but it looks like you may be at the upper end of horsepower capability of that unit. Not sure what that 355 puts out but the pump is rated up to 238 HP. Careful on the throttle.

http://www.castoldijet.com.au/Jet-05.html

I´ve looked at almost all of Castoldis differnt hompages

The max power input that you found is one of three diffrent amounts of power that i´ve found 175 hp , 238 and 400 hp ( at speed ).

I will check that link out, and see if anyone there may have a good idea.

Thanx

//Matz

 

[Murf'n'surf]

Is it possible that this MOB boat was designed to last in rough seas and power through to shore and NOT get up on plane and go fast?

 

[Matz]

I wonder if these guys would be willing to help with things like pitch and gearing. They build high performance jetboats with one pump.

http://www.needmore.com/smokymountainjetboats/our-boats

Alos, I'm sure you've been to this site, but it looks like you may be at the upper end of horsepower capability of that unit. Not sure what that 355 puts out but the pump is rated up to 238 HP. Careful on the throttle.

http://www.castoldijet.com.au/Jet-05.html

http://www.nauticexpo.com/prod/castoldi/boat-water-jet-drives-100-300-hp-25307-118763.html

 

[Matz]

Is it possible that this MOB boat was designed to last in rough seas and power through to shore and NOT get up on plane and go fast?

So true. it could very much be so. But when the previous owner, my co worker, drove this MOB boat before he bought it, it planed out. And he thought it was great response and handling in it

 

[Julian]

I would check your intake tunnel for ANY missing sealant and un-smooth surfaces before I invested in a new impeller. What you describe sounds JUST like cavitation, and this can be caused by a number of things. In the Yamaha boats, cavitation typically comes from 4 sources:

1. Something stuck in the impeller (can be something VERY small- like a twig--small stick)
2. Missing caulk on the leading edge of the intake grate. This creates a bubble stream/disruption of the flow into the pump that increases with the speed of the boat. Thus in your instance--works ok until you reach 8kn but then fails.
3. Impeller Clearance being out of tolerance
4. Damaged Impeller causing cavitation.

You impeller doesn't look very good, and could well be the cause of your problem, but I'd rule out 1-3 before I bought a new one.

Below are the impeller clearance recommendations and limit for the pump on my Yamaha.

 

[txav8r]

It isn't just a perfect engine/pump combo...like the guys are eluding to, your intake has to be smooth with nothing to cause a ripple in intake water, or it will introduce air. Since that is an I/O boat originally, I am wondering how you set up the intake/pump/nozzle, because this could be a severe restriction in your setup. Any disturbance will introduce turbulence and lose the hookup, as cavitation is introduced. It isn't rocket science, but it can't have any cavitation at all. While your blades don't look too bad, they are completely rough, and I suspect that is a big part of your issue, combined with a wear ring that is marked up as well. If SS isn't in budget, what will an new wear ring and plastic impeller cost you? I think before I sank a ton into it, I would see if the setup as a whole is capable. It seems the hull design combined with the intake/engine/pump are very critical to get any performance. As Yamaha has made subtle changes, they have had to add/remove ride plates, change strakes, and govern the engine speed to impeller ratio. Bottom line, it may not just be the impeller/wear ring. What did you use for grates over the intake? Pics?

 

[Kevin Turner]

@Kevin Turner Do you have knowledge of his jet pump?

Sorry, not really. However I'll do a bit of research.

 

[Matz]

It isn't just a perfect engine/pump combo...like the guys are eluding to, your intake has to be smooth with nothing to cause a ripple in intake water, or it will introduce air. Since that is an I/O boat originally, I am wondering how you set up the intake/pump/nozzle, because this could be a severe restriction in your setup. Any disturbance will introduce turbulence and lose the hookup, as cavitation is introduced. It isn't rocket science, but it can't have any cavitation at all. While your blades don't look too bad, they are completely rough, and I suspect that is a big part of your issue, combined with a wear ring that is marked up as well. If SS isn't in budget, what will an new wear ring and plastic impeller cost you? I think before I sank a ton into it, I would see if the setup as a whole is capable. It seems the hull design combined with the intake/engine/pump are very critical to get any performance. As Yamaha has made subtle changes, they have had to add/remove ride plates, change strakes, and govern the engine speed to impeller ratio. Bottom line, it may not just be the impeller/wear ring. What did you use for grates over the intake? Pics?

Hi Txav8r.

I´ve fiberglassed a recess in the hull for the unit to fit in. along the sides there is some original plastic "flatbars" with 45 degrees countersunk holes för the countersunk allen bolts. On the forward side of the unit, i´ve done as others have done in theese projects before me: the v-shaped keel transforms into a flat line usig a "triangel-fiberglass-plate" the lenght of this tri-plate is about 10-12 inches, all openings and joints/seams has been applied with marine grade "silicone" aka Sika-flex.
I seen to be missing a picture of the front end of the unit and its mounting but i will give you a link to a guy in the UK who has made a similar installation of a jet unit.

It seems that his hompage is down at the moment, all i could find where small thumbnails and a video on youtube.

check in out.

I admit that the strain of silicone infront of the unit towards the hull isn´t 100% perfect, could need a bit more to fill it out completly. Price for a standard plastic impeller is around 400-500 eur, what a new wear-ring or impeller-housing costs, haven´t i looked into yet.
About the fact that i´ve sunked a "ton" in to it, i´ve been in contact with Hamilton, Castoldi, Alamarin in diffrent countries, all except the italians said ( they just said that as long as their unit wasn´t installed by their own people it would never work ): go for it, you don´t know if it will work untill you tried.
it was either this project or buying a completly new drivetrain, like a mercruiser 3.0 straight 4 with a bravo 2 sterndrive, where just materials would set me back 8000-9000 eur + installation cost. So i decided to at least try this solution out. in worst case, i´ll ripp engine and pump out and scrap the boat.

For grates, i used the standard weed-rake, it was already installed.


If you would like to see more pics i can go down to the marina during tomorrow and take a couple

//Matz

 

[Lextacy03]

We love pictures on this forum.

 

[Matz]

We love pictures on this forum.

Hi lextacy03.

I'm not sure If you are ironic or not. If you are, Then i appologise and Will cut down of The uploading of pictures. If you are sincer, then i posted a link erlier with three albums with pictures during The build.
//Matz