Boat questions

futura97 · 15-04-2009, 11:27 AM

Hye guys

Just wondering if any of you can help me out with a q I have about my boat...

I had a problem over easter with my boat. I was getting excessive cavitation and could not go faster than about 5-8knots without it free reving. I have a suspision it might be the prop, but as luck would have it, I left the spare prop at home while I was down in Mandurah :( . Luckily enough though I was on the esturay and not far from shore, so headed back over and put it on the trailer and came home.

With the spare prop I have (freebie) the size is different and want to make sure I'm not going to do excessive damage before I use it. The old one is 13 1/4 x 17, and the spare is 13 3/4 x 15. I'm wondering if the diameter and pitch would cancel each other out, and also if the diameter is ok being slightly larger. I noticed when I put it on, the tips of the blade are alot closer to the cavitation plate, and that is my only real concern.

The motor is an old johnson 90. I'm pretty sure its early 80's.

Cheers

tex · 15-04-2009, 12:21 PM

Quote:

Originally Posted by futura97

Hye guys

Just wondering if any of you can help me out with a q I have about my boat...

I had a problem over easter with my boat. I was getting excessive cavitation and could not go faster than about 5-8knots without it free reving. I have a suspision it might be the prop, but as luck would have it, I left the spare prop at home while I was down in Mandurah :( . Luckily enough though I was on the esturay and not far from shore, so headed back over and put it on the trailer and came home.

With the spare prop I have (freebie) the size is different and want to make sure I'm not going to do excessive damage before I use it. The old one is 13 1/4 x 17, and the spare is 13 3/4 x 15. I'm wondering if the diameter and pitch would cancel each other out, and also if the diameter is ok being slightly larger. I noticed when I put it on, the tips of the blade are alot closer to the cavitation plate, and that is my only real concern.

The motor is an old johnson 90. I'm pretty sure its early 80's.

Cheers

Hi mate, if it was cavitating that bad then the damage to the original prop will be abvious - it should be missing a blade! Or the tip from several. Was the engine vibrating in gear when you gave it some revs....?

the propellor swap you are doing won't hurt anything - it will just make the boat go slower, lift a greater load and make the engine rev more....

You're keeping the diameter the same - just changing the pitch from 17 to 15 - remeber in lamens terms, pitch = the distance in inches that the propellor will move forward in the water with each revolution of the propellor.

It's like holding a car in a lower gear a pitch swap from 17 to 15, with the same diameter.

futura97 · 15-04-2009, 01:03 PM

There was no obvious damage to the prop, but the bushing has looked like it may have seperated. That is why i think it was slipping. Also the diameter is bigger, gone from 13.25 to 13.75. So its half and inch bigger.

I'll drag it down the river and see how I go.

Cheers

arm79 · 15-04-2009, 02:30 PM

I was going to suggest that maybe the hub bush has separated from the prop itself.

I've had that happen before, and experienced the same symptoms.

It usually happens because of age or because you've hit something. They are only rubber and are designed to let go under extreme pressure. Basically separates to avoid damaging the driveshaft or gearbox.

**ebxr8240** · 15-04-2009, 09:26 PM

The motor is set up the same?? [Long / short shaft etc?]
Cav plate just under keel or there abouts..??
The shear pin or rubber bush in prop o/k??
Sounds like its sheared ???

cycle myth · 15-04-2009, 10:17 PM

Yes - had this issue before - as arm79 said most likely the bush hub (bit like the clutch slipping in a car) - sometimes might be cheaper and better to get a new prop.

In relation to the different prop size - the 15" means it will travel forward 15" in one rotation - therefore it will have a lower top end than your 17" (unless it was over propped to start with). You can test how much of an issue this is by taking to WOT and seeing how many rpm over max you are (watch out you don't want to over rev) - Typically 200 rpm = 1" in pitch ie. if max rec rpm is 6000 and you can do 6200 at WOT then your prop is 1" too small in pitch - but hey top speed isn't everything and you might be happier with the way it gets on plane and handles overall.

Depending upon your boat and loading it may work better (like a gearing ratio issue in a car).

The increased diameter will assist in coming onto and staying on the plane at earlier rpm (generally).

Let us know how you go.

futura97 · 15-04-2009, 10:47 PM

Thanks for the replies guys. Took it for a spin this arvo down the river and all seemed good. Got on the plane alot easier and faster (like you said cycle) and overall top speed was comparable. I didn't go WOT because I don't have a rev gauge and didn't want to over rev it. It was happily sitting on 27knots with more to come. I'm more than happy with that, as its rare to do that on the ocean anyway.

Cheers

EDManual · 16-04-2009, 11:52 AM

Its funny how top speed isnt used that often... even though you are allowed to. We have a brain that lets us know its not safe. Unlike on the roads where we dont use our brains....

I got my new (2nd hand) boat and it took me perhaps 6 or 8 trips before I had the conditions to get floor it. Got it up to 35 knots! About 65km/h. Pretty fast on the water. Finding somewhere in the revs where it cruises best for economy is more useful than topspeed though I think.