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Old 31 August 2010, 11:05   #1
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Zapcat prop advice

Hi,

I was playing with my zapcat over the weekend in devon, and as soon as i moved off i noticed it was doing something odd. If i throttled up, it would 'slip' - not every time but definitely if i put the power on too fast. The revs would go up, but the speed would come down - as if the prop just couldnt bite. On coming back to shore, found a nice big ol chip out of one of my prop blades - probably about a fifth of the blade surface missing!

I assume this is cavitation due to a knackered prop. Im pretty sure we chipped it during launch - it took a bit of a whack (ooops!) - I certainly didnt hit anything in the water. The shaft doesnt appear to be bent, and the motor is otherwise undamaged.

The upshot is - if i have to replace my standard prop now anyway, what should i put in its place? My motor is a yamaha 40FM - its a mid eighties model but in cracking condition otherwise. What sort of size / pitch should I be looking for for all round best performance from my toy?

Any advice gratefully received. I'll check the size / pitch of the current prop when I get home and post it here, if thats relevent. I'll also be more careful in future!
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Old 01 September 2010, 22:31   #2
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Hi Rik,

It sounds most likely to be a spun prop hub. You can check by making alignment markings on the prop nut and the prop housing, then taking the boat out for a run. If the markings are no longer aligned you know the rubber bushing in the prop hub is toasted.

I'm not sure what prop particulars would be ideal for you, but these guys should be able to give you some good guidance in that respect;


http://www.thundercatinflatable.com....isplay.php?f=1
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Old 03 September 2010, 10:02   #3
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thanks tuber - that forum is spot on.

Ive been gifted a new prop - 11x15 G series same as what was originally on it. I know its probably not optimum, but if its ok all round then itll do for the moment. Ill check the hub when i change the props round, and il do the test you suggested regardless of what i find.

Thanks for the help
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Old 06 September 2010, 06:48   #4
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In most cases, the prop that comes fitted from the manafactuer is usually the optimum for most conditions.

Unless you found that you were under/over propped with your previous prop, then really there isn't any need to change beyond the specifiactions of the prop originally fitted by the manafactuer.

Prop's are a compromise, in order to gain in one area, you must sacrifice in another, so the manafactuer's usually do a pretty good job is selecting a prop that offer's good allround performance in all area's.
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Old 06 September 2010, 07:14   #5
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Quote:
Originally Posted by Bus_Boy View Post
In most cases, the prop that comes fitted from the manafactuer is usually the optimum for most conditions.

Unless you found that you were under/over propped with your previous prop, then really there isn't any need to change beyond the specifiactions of the prop originally fitted by the manafactuer.

Prop's are a compromise, in order to gain in one area, you must sacrifice in another, so the manafactuer's usually do a pretty good job is selecting a prop that offer's good allround performance in all area's.
Are you saying the 'standard' prop, supplied by the outboard manufacturer will suit virtually any boat the customer chooses to fit the motor to?
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Old 06 September 2010, 22:18   #6
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I don't think it's a case of will the factory propped motor be suitable for any boat, rather than will the engine itself, be suitable for the boat it's to be fitted to.

No where in the OP's post did he say that he was unhappy with the perfomance of the motor, and boat, with the supplied manafactuer's prop, prior to it sustaining damage, which is a good indication that the prop fitted form the factory, was a good choice for all round performance.
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Old 07 September 2010, 02:27   #7
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Originally Posted by Bus_Boy View Post
I don't think it's a case of will the factory propped motor be suitable for any boat, rather than will the engine itself, be suitable for the boat it's to be fitted to.
He's already got an outboard that is well suited to his hull, the specific prop is what is in queston. In fact any carbureted 2 stroke in the 40-70 hp range from yamaha, tohatsu, evinrude or mercury, could suit a zapcat nicely, especially once modified. Selecting the best prop for this hull and the engine combo is much more complex. The prop that originally came with Rik's engine might be nowhere close to what is optimal for his hull & engine setup. Alot of zapcat/thundercat racers have several different types of props and select particular ones based on the type of race and surf conditions that they are going to be racing in.
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Old 07 September 2010, 03:01   #8
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He didn't mention that he was racing, or competing in surf contests.

I'm talking about gereral allround performance, where you don't have an obvious compomise in any particular area as a result of fitting a particular prop. Someone who has only has one prop, and is not interersted in owning any more, and someone who dosen't want to complicate their boating by buying additional props, and then having to choose which out of how ever many props they own is best suited to that days conditions/activities.

If he is racing, then yes, a prop that is suited to racing, will be better than a prop that is intended for allround duties, same goes with a prop intended for surf use.

But from what i gathered from the post, we weren't talking about either.

I'm not argueing that there isn't a prop that isn't better suited to a certain condition vs another, which is why it pays to have more than one prop, giving you option to swap your prop, to suit both the conditions and your needs.



In a case such as that, then yes i still beleive that with a motor that is suited to the hull, in both weight, HP, also taking into account it's intended use, is close enough to optimum perfomance, that unless the owner/operator is not satisfied, then there is no reason to go changing to anything other than what is fitted by the manafactuer.
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Old 07 September 2010, 06:54   #9
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Zapcats generally work very well with semi cleaver props, as when on the plane approaching top speed, the prop will come out of the water.
That is with a 50hp motor. I suspect the 40 will not get to the same top speed and may not get the prop up so high.

When we used to use a Gemini GRX with standard Tohatsu prop, it performed very well in most areas. It was actually faster out of the hole than the zapcats with race stainless steel semi cleavers. The Zapcats would soon reel it in though and fly past.

Obviously the heavier stainless versus aluminium prop makes a slight difference on spin up time. Also the zapcats are more aerodynamic, so that helps.

i don't think the OP is racing, as you'd have a 50hp.

The standard prop is most probably a good choice for fun use on a 40hp engine.
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Old 07 September 2010, 07:04   #10
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I posted in another thread about this. http://rib.net/forum/showthread.php?t=37771

Basically im new to zapcatting - i used my boat once with the stock prop with ok results but lots of slip on hole shot - but this was on millpond like water.

The next time i tried to go out I dinged the prop, now replaced with another stock prop. having had worse weather im no longer on totally flat sea, and now the new prop is basically unusable due to the sheer amounts of slip and ventilation. I cant get the power down to get it up onto the plane at all. The hubs are fine and not slipping. Its not as if im taking it out on extreme days either - force three tops and im not hanging about in the surf.

Ive contacted various people and the consensus seems to be an alloy standard prop is rubbish on cat hulls and ventilates lots in anything other than flat calm, and for a bit more usability a semi cleaver is needed. A 14p was suggested as pretty much optimum for general use for this class with a 40hp.

Im not racing this boat for the forseeable future. Its just a toy at the moment!

Thanks for the interesting discussion guys!
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