Homebuilt jetboat-project

[Scottintexas]

We love pictures but we are also sometimes very lazy, either in reading all the previous posts or clicking on additional links

We are glad to have you join us and hope we can help get your boat working again!

///edited to add /// I am also very impressed by your initiative to take on such a rebuilding/transformation of a boat!

 

[Bruce]

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

We like pictures. They make it easier to understand what we are discussing.

 

[Englewoodcowboy]

A lot of people have a misconception of how a prop/impeller can cavitate and you have to understand aerodynamics as well as thermodynamics to understand this so I will give a brief synopsis of what cavitation really is. Water at atmospheric pressure will boil at 212 degrees Fahrenheit. When you increase the pressure like you do with a pressure cooker, you raise the boiling point and subsequently when you decrease the pressure you lower it. A prop or impeller it is a spinning air foil like a wing on a plane and creates thrust in the same manner a wing creates lift, as the water passes over it, the curved shape will speed up the water on one side and it decreases the pressure. Add to this the friction generated alone with forcing a prop/impeller through the water and you have created a low pressure boiling point which produces steam which is where this cavitation comes from. This is why submarines have a banana shaped prop and turn very low rpms so they do not create the steam and cause cavitation.

In relation to your project, it could also mean your pitch is too light for your boat with its current power vs. weight. A good prop supplier should be able to help you with this by providing them some basic parameters and to fine tune it will take time and experimenting with it. I know a stainless impeller is pricey but look into aluminum as it should be cheaper and still provide you your weak link. You will also need to measure your wear ring as it too may need replacing. The main reason I suggest a metal impeller is they can be re-pitched as you go without the expense and waste of owning several impellers you can no longer use while you find the proper one for your application. I hope my reply is not too lengthy but will give you understanding of what may be happening. I hope this helps.

 

[Lextacy03]

Nope, When I say we love photos I mean it 100%.

 

[Matz]

A lot of people have a misconception of how a prop/impeller can cavitate and you have to understand aerodynamics as well as thermodynamics to understand this so I will give a brief synopsis of what cavitation really is. Water at atmospheric pressure will boil at 212 degrees Fahrenheit. When you increase the pressure like you do with a pressure cooker, you raise the boiling point and subsequently when you decrease the pressure you lower it. A prop or impeller it is a spinning air foil like a wing on a plane and creates thrust in the same manner a wing creates lift, as the water passes over it, the curved shape will speed up the water on one side and it decreases the pressure. Add to this the friction generated alone with forcing a prop/impeller through the water and you have created a low pressure boiling point which produces steam which is where this cavitation comes from. This is why submarines have a banana shaped prop and turn very low rpms so they do not create the steam and cause cavitation.

In relation to your project, it could also mean your pitch is too light for your boat with its current power vs. weight. A good prop supplier should be able to help you with this by providing them some basic parameters and to fine tune it will take time and experimenting with it. I know a stainless impeller is pricey but look into aluminum as it should be cheaper and still provide you your weak link. You will also need to measure your wear ring as it too may need replacing. The main reason I suggest a metal impeller is they can be re-pitched as you go without the expense and waste of owning several impellers you can no longer use while you find the proper one for your application. I hope my reply is not too lengthy but will give you understanding of what may be happening. I hope this helps.

Hi Engelwoodcowboy

Thanks for the explanation of aerodynamics/thermodynamics/hydrodynamics

I´m aware that a standard propeller and perhaps some impellers made out of aluminium or stainless or even bronze can be altered to change pitch, either with heat and mecanical tretment/force or just plain old brute force, like an hydraulic press with the correct tool for the jobb. ( assuming that you know what your doing)

I however have difficult understanding how a pitch can be altered on a impeller such as mine ( dimension-wise, not material-wise ) i mean, the outer diameter is about 8 inches, and the lenght of the blades/impeller is about 7 inches. on a regular shaped propeller you manipulate the "base" of each blade and twist it to get an increase or decrease of pitch.
How this would work on blades with a base that stretches along the complete impeller hub i´m not sure. i´ve only worked on slightly bigger propellers for tankers. ( 1.5-2 meter in diameter ) at my job. But the principal should be similar... i think.

What do you think about my contact at Castoldi Swedens oppinion, that the real problem could ONLY be the gear ratio? even though the previosly owner with this unit connected to a 305 Volvo Penta engine got it going as it should, would prove that there already is a correct gear ration installed.

I just loves when a "customer-orientated" seller/serviceman/dealership asks for information via email to be able to help me, then once he gets the info, he doesn´t get back to you

Sorry for my horrible typing, as mentioned before english is not my native language and Google translate is just rubbish so i´m trying to remember what i learned in school at english classes

//Matz

 

[Matz]

i thought that i you like pics, then i will upload a couple from last year, when most of the hull work was done.

//Matz

 

[Matz]

 

[Englewoodcowboy]

Looking at the splined shaft I would say it looks like a standard spline as used in most outboard boats. As far as diameter, you are stuck with what your wear ring diameter is minus clearance. The pitch can really be made whatever it is needed by the relation of the blades to the hub as well as by adding a 4th blade etc. I would contact a prop/impeller manufacturer to see what they suggest. My father has built a few inboard boats and he took the boat weight, horsepower, length etc to them and they had a chart to get him at a close starting point and it is usually enough to get you at full rom or just under and you have it adjusted from there to get you motor running at WOT (wide open throttle) at the engine specified rpms etc. You will need to keep in mind that you will have limitations of hull speed that you may never overcome etc. I am curious what the impeller manufacturer will have to say.

 

[Ronnie]

We are glad to have you join us and hope we can help get your boat working again!

///edited to add /// I am also very impressed by your initiative to take on such a rebuilding/transformation of a boat!

X2 it takes a lot of will, skill, patience and money to do what you have done/are doing.

There is another member here maybe more than one, possibly on yamahajetboaters that has experience with non yamaha / seadoo jet boats that may have some thoughts on your project. I will post their names of I can find them.

 

[Kevin Turner]

Keep in mind, the impeller clearance is the first issue to contend with. Although we like smooth blade edges, the nicks are most likely not a big concern.

What material is the wear ring made of? / Do you have access to a plastic welder? It would not take much to rebuild the blade diameter.

Does she blow out from a dead stop? Or, does she start to move forward, then when it feels like she is trying to climb on plane, she blows out? There's cavitation and ventilation. When the back of the craft is static, the pump is surrounded with water. However, as she moves fwd and starts to plane, the stern is exposed to air. It's easier to pull air than water, so air gets drawn in from the stern and the pump unloads.

If possible, place a bright light under the trailered craft (point directly at the intake grate). With the reverse gate open, snap a couple pics through the pump nozzle looking in. Also, plse post pics of the pump mounting to hull.

My time is short, plse forgive missed words, grammar, etc...

 

[Matz]

Keep in mind, the impeller clearance is the first issue to contend with. Although we like smooth blade edges, the nicks are most likely not a big concern.

What material is the wear ring made of? / Do you have access to a plastic welder? It would not take much to rebuild the blade diameter.

Does she blow out from a dead stop? Or, does she start to move forward, then when it feels like she is trying to climb on plane, she blows out? There's cavitation and ventilation. When the back of the craft is static, the pump is surrounded with water. However, as she moves fwd and starts to plane, the stern is exposed to air. It's easier to pull air than water, so air gets drawn in from the stern and the pump unloads.

If possible, place a bright light under the trailered craft (point directly at the intake grate). With the reverse gate open, snap a couple pics through the pump nozzle looking in. Also, plse post pics of the pump mounting to hull.

My time is short, plse forgive missed words, grammar, etc...

Hi Kevin.

The wearring is made of aluminium. No, no plastic welder what i know of.

When in "neutral" and raise the bucket for forward motion, she goes, slowly. Any attemp of giving it more throttle results in a reving engine and "cavitating impeller, espesially when trying to go up on a plane.

 

[Matz]

went down to the marina just now and snapped a couple of pics, hopefully will se what is what

Sometimes it´s quite good to have a nice little camera on the phone

//Matz

 

[Murf'n'surf]

We really need a pic of the impeller installed and the gap between the impeller and housing.

 

[Matz]

 

[Matz]

We really need a pic of the impeller installed and the gap between the impeller and housing.

Hi Murf´n´surf.

You asked about the impeller installed and the gap ... don´t bother, just trust me, the gap is more then 2,0 mm. way over what some manufactures recomend.

Have ordered a new impeller, can give you a before and after-pic once i get the new one in the mail.

//Matz

 

[Matz]

We really need a pic of the impeller installed and the gap between the impeller and housing.

Hi Murf´n´surf.

I went down again couldn´t find any electrical outlet for a worklamp inside the tunnel/ housing for theese pics, but i hope they make sense anyway.

 

[Matz]

I took a picture of each blade, then i pulled the impeller backwards a bit, took a new set of pics, that you can see the edge on the housing more clear as a point of reference.

//Matz

 

[Murf'n'surf]

I think the clearance is too much and causing your problem.

Can you find the manufacturers specs on this pump to see what the allowable gap between impeller and housing should be?

 

[Matz]

I think the clearance is too much and causing your problem.

Can you find the manufacturers specs on this pump to see what the allowable gap between impeller and housing should be?

Well, i´m not sure. but when i spoke with the italian guy here in northern Sweden at Castoldis dealership, he said that as long as it stays under 2 mm it wouldn´t be a problem. He wanted me to change gear ratio instead. ( ?? )

But, i´ve place an order for a new composite impeller, see how that works first, AND i will seal off any gaps on the forward side of the impeller in the housing with marine silicone ( or form-a-gasket of some sort.) to prevent any air beeing sucked in from that way.

Will start with that and see if weather conditions allow me to launch it on a quite afternoon and see if anything of the items mentioned above has made any positive improvments.

If not, well then i must start looking at the wearring and/or the gear ratio.

But i doubt the gear ratio, i mean, it should affect speed and/or power but not in this way. more likely it would affect topspeed, by maybe - / + 4-5 knots, rather then the ability to plane out.

Or am i wrong with this guess?

//Matz

 

[Kevin Turner]

No doubt the imp to ring clearance is a huge part of your problem. It is hard to say for sure, however the wear ring looks to be "OK".

Also, as long as the gear box is spinning the blade, it should have nothing to do with your current issue.

I think you are on the right track; impeller first and seal areas that would allow the pump to ventilate.