zap cat prop problems..

[Vincent Knight]

i have a zapcat with a 40 hp yamma/mariner with the standard prop 10.25x15p, its the one that came from the factory a horrid looking aluminum thing, i'm getting cavitation upwind and down wind in the chop (maybe2-3ft) a friend has given me a yamaha prop off a 55 twin but its 11x13p but the blades look more like they should (rabbit ear rather than mouse ear!) am i right in thinking that the extra diameter will counteract the drop in pitch educate me please!!!!

 

[steelespike]

Re: zap cat prop problems..

Welcome, if you read "read this first" at the top of the page and get us as much info as possible we can probably tell you
if your setup is good and likely results with a prop change.
Cavitation is almost universally used to describd what is actually "ventilation" cavitation is actually something you
don't feel but may cause slight erosion on the prop.Most props have some degree of cavitation and it can be an indication a prop is working really well. Generally as pitch goes down diameter goes up and viceversa.Don't know the years of your motors but I checked 1990 the props are the same sku number and hub adapter in the Hustler prop.So your props should interchange depending on the respective years.
Your rpm will very likely go up with the 13. While diameter can affect rpm it's usually of no consiquence.
With the ventilation issues you also may have high slip mumbers.

 

[Vincent Knight]

Re: zap cat prop problems..

thanks steelespike, the engine is revving to the limiter, the engine is a short shaft and was made in 1998 boat is 4m long and weighs about 90kg its a tunnel hulled inflatable catamaran ( same as a dux hammerhead) by cavitation i mean that its revving out and not going anywhere! speed is about 45mph with me on board prop is a mercury aluminum 10.25x15p with what i call mouse ear type blades.

 

[steelespike]

Re: zap cat prop problems..

Prop calculator says you can't do 45 with a 15" prop.
At 5500 rpm Could do about 40 with a reasonable 6% slip.Rpm and and slip are both guesses.
Really need the rpm and gps speed to tell what a 13 might do. It appears it would overrev badly.
You shouldn't mess with props without a tach and gps to evaluate your setup.
You could try the prop just be aware it will likely overrev and may be slower.

 

[Vincent Knight]

Re: zap cat prop problems..

i'll be out at the weekend so ill get some gps speeds up for you. not 100% sure what revs i'm pulling as i've no tach and not likely to fit one as its a very wet ride...... i'm borrowing a s/s prop to see what happens i'll post the speeds up!

 

[steelespike]

Re: zap cat prop problems..

The tiny tach is water resistant and you can fairly easily install it just for the test.
I think it may actually record specs so you could hide it in your pocket during the test.
You may simply need to lower the motor slightly there may be holes in the mount to allow this.
Also you may be able to have a good prop shop tweak the prop a little.
Look over the back and see if you can see the antiventplate that is just above the prop.
Usually suggested that it be visable at wot.

 

[Vincent Knight]

Re: zap cat prop problems..

well my gps speed today was 46mph... tried a semi cleaver 11x15 and got same speed but no ventilation in a 2-3ft confused chop

 

[steelespike]

Re: zap cat prop problems..

Assuming 6% slip a 15" prop would require 6300 rpm to reach 46.
If we assume the 15" acts like a 16"; rpm would be 5900.
If you intend to run it routinely You really need a tach to confirm the rpm.I'm guessing the slip at 6%
then do some research to find out if its ok to run over the rated max.My info indicates max is 5,500
It does appear you could run a 17.But I wouldn't do anything till you get a tach.

 

[Vincent Knight]

Re: zap cat prop problems..

she was over peak revs by ear anyway, as in she slowed slightly at full throttle, i was not quite at wot to get the 46mph, other issue i have is a little too much positive trim on the motor as she was ventilating slightly at flat out.

 

[steelespike]

Re: zap cat prop problems..

With it being rough a little venting may be normal if its momentary.I guess you could trim in a notch and check the results.
Sometimes you can buy or make wedges that will give you a 1/2 notch setting.
If its venting with that prop the motor may be too high.

 

[Vincent Knight]

Re: zap cat prop problems..

well i hope i dont have to drop the motor as its as low as it will go! the run out today with the semi cleaver and no messing with trim was fast and no venting at all!!! looks like a 15/16p 10.25 semi is the way to go....