New Prop????

nuzzy · #1 29-08-2010, 10:00 PM

Hi Guys,

i installed a small tacho to my outboard on the weekend to see what revs i was pulling WOT, i have a 9.5 x 10 prop on my 15 hp and it was revving to 6050 flat out going 35 km/h GPS speed. im pretty sure i can get up to 38 - 40 if i trimmed it correctly. this was done with only myself and empty livewell, would it be worth increasing the pitch of the prop to lower the rpm or should i leave it where it is to cope with the extra load when the livewell is full and i have an extra person onboard?

any advice would be greatly appreciated.

Emo · #2 29-08-2010, 10:42 PM

You could contact Solas, I believe they have a large database of all common outboards in Australia and information about which prop is best for different applications/set ups.

leachy · #3 08-02-2011, 01:15 AM

I to am thinking about this. Is there an easy way to tell without fitting a tacho, I am not exactly what wire is what on the Yamaha CV25. It appears that I reach 40Km/h at about 70% throtle and then only get another 5km/h at WOT would this suggest that the boat could have a higher top speed if I put a larger / more effecient prop on it.

This was only with myself onboard. It seems that the engine is reaching max revs at only 70% throtle. The hull is a AllyCraft 3.95 Shadow with not alot of extras (not much additional weight) the maximum rating is 25HP. Even with it load with two adults and two kids it is still about the same speed.

nuzzy · #4 08-02-2011, 02:59 AM

what i ended up doing was giving the prop a bit of a bash with a hammer to give it a bit of custom cupping, i thought i would give it a go as i was looking at a new prop anyway

but it worked a treat, my top speed stayed about the same, gained 1 km/h but the revs dropped to 5900. before i did it the prop would just slip, eg would be revving at 6000 irrespective of load or speed, and now it sits 5900 with just me in it doing 19.5 knots and 5400 with 2 guys and a full livewell at 15 knots.

with your situation leachy, i will have to let someone with a bit more knowledge of outboards to comment on it.

leachy · #5 08-02-2011, 04:48 AM

Thanks, I actually think the motor has too much power for the boat / prop combination.

Mandurah Marine · #6 08-02-2011, 09:28 PM

Nuzzy - sounds like you got your cav plate height right and you probably ended up with less slip whilst getting on the plane. It should have lifted the nose a little which will help presuming your livewell is up front. If you need to, you can shim up the trim with different thickness pipe around the tilt pin if the next hole causes slip/porpising.

I just came back yesterday from the Renmark Dinghy Derby in SA racing my 18hp Tohatsu which has been sunk 4 times and with worn crank bearings sitting on 7,200rpm in the open sections (15 odd minutes 4 times a day), although I wouldn't recommend that for a bream boat - you want longivity of motor as money needed on tackle

That said I expect looooots of hours out of my motor.

For those of you running tiller steered boats, you can buy a Tiny Tach (which is waterproof) for around $130 - $140. These are tough trick units which need no tools to install (except to mount the earth wire properly on any engine bolt) - the pickup wire wraps around the spark plug lead and the unit can be cable tied to the tiller handle as I have done. They also log max rpm and monitor engine running hours accurately. They adapt to any petrol motor.

leachy · #7 09-02-2011, 02:03 AM

Thanks for the info on the taco that seems the way to go as I dont really want to go splicing into any wires and introduce any reliability issues. When a boat breaks down it not like you can get out and push.

Mooza81 · #8 09-02-2011, 05:05 AM

Quote:

Originally Posted by leachy

Thanks for the info on the taco that seems the way to go as I dont really want to go splicing into any wires and introduce any reliability issues. When a boat breaks down it not like you can get out and push.

Sure you can.

forsterfisho · #9 09-02-2011, 05:08 AM

How much do you think you will gain from the smaller enginer (15-30hp) with prop changes?

Is it really worth the stuffing around to gain 2-3kms?

On another note, is there an relatively simple and cheap mod to take a 25hp motor to a 30?

leachy · #10 09-02-2011, 05:15 PM

I actually think any gains on a smaller enigine would be more noticable as it is a is a greater percentage on the overall weight of the boat / motor, I guess its not so much about a higher top speed but more about a better effeciency.

In relation to turning at Yamaha CV25 HP into a CV30 I amtrying to find out. I have a CV25 and my brother inlaw has a CV30 I havnt been able to find the difference yet. I think it could be in the carb jet sizes but need a workshop manual to find out.

Regards
Leachy

leachy · #11 01-11-2011, 12:20 AM

Hi All

I finally found out what the difference is between the CV30 and the CV25. It is two things the timing is more advanced on the CV30 and it has different retainers on the reed valves that I think would allow a slightly larger air flow, not alot of difference but more of an inconvenience than just removing a restrictor or screw.

I have also since worked out that the current CV25 I am running is hitting max revs and top speed which is pretty much 24-25 knots with the standard prop.

Regards
Leachy

leachy · #12 01-11-2011, 12:32 AM

I am thinking about a new prop again for my motor. The standard Yamaha is 9 7/8 x 12. Its hitting top RPM at about 3/4 throttle. I have just had a look at the Solas web site and they only have 10 x 13 in that range I was wondering does anyone know if 10" is to big and will hit the cav plate, an 1/8" doesn't seem much or is there a certain amount of clearance that is required.

Regards
Leachy

Backlashed · #13 01-11-2011, 07:13 AM

It would probably best to have a talk to Solas, and ask their opinion..

Are you looking to buy another ally prop, or a stainless one?

I have been told by people that earn their living in boats, that an ally prop will straighten (i.e. lose pitch) as the RPM increases.. If this is true, it does explain (to some extent) why there is very little difference in speed when going from 70% throttle to WOT..

Something to think about though..

leachy · #14 01-11-2011, 05:30 PM

I am not sure I have heard fores and against on stainless for small engines. I dont think the 25Hp would deform an ally prop, but anything over 100Hp maybe. I have heard that if you hit something with an ally it is a little more forgiving. Thanks I will have a chat to them and see what they have to say. The reason I think the prop is too small is I have used the different online prop calculators and basically I am hitting top speed at that rev range for the standard prop and at WOT the engine sounds like it is doing about 500 RPm to much.

tchoklat · #15 01-11-2011, 07:45 PM

try Ken at propgods.com he can help with anything regarding props. You will need your prop pitch, gear ratio, WOT speed and he can tell you if you have slip and what to do about it. Engine height and trim are really important. Also see http://go-fast.com/Prop_Slip_Calculator.htm to do it your self.

Tony