Probably a stupid prop question

SimonStDavids · 30 July 2012, 21:38

This might be a bit of a dull question but here goes anyway

I am wondering whether I can extract any more speed out of my engine so i thought i would start by looking at the prop. Reading up on previous threads, I gather that the 10P stamped on the prop is the pitch. With this prop I am getting 26 to 28 knots.

The engine is late 1980's 40hp Mariner/Yamaha and the boat is a 4m Searider, does this seem like the correct pitch from more rib wise people?

I know I need to the rpm at wot but i don't have a tachometer so i am relying on a bit of guess work, i would like to fit a tachometer but that is probably another thread

The boat takes off fine and at no point does it sound like the engine is labouring but could it go faster...

Festinghouse · 31 July 2012, 05:33

keeping it basic, yes you will go faster with the next prop size up. acceleration will be reduced though. normally a lightly loaded boat will easily get up on the plane with a big prop and benefit from higher speed and use less fuel. loaded up, put the small prop back on then the engine isnt struggling.

Cookee · 31 July 2012, 08:11

Only put a bigger pitch prop on if it's reaching max RPM easily otherwise you'll go slower and for that you need a Tacho!

Cheapest way is ask around and see what best speed other people are getting with the same or very similar rig.

9D280 · 31 July 2012, 08:29

I had a 13" on my Suz DT25 when I had an SR4..... Lethargic to plane but could do 19-20 knots when up there.

Yopu have more Gee Gees.

SimonStDavids · 31 July 2012, 11:12

Quote:

Originally Posted by 9D280

Yopu have more Gee Gees.

I think some of those gee gee's might have left the stable over the years

Would a pitch of 10 to 13 be a big jump to make?

9D280 · 31 July 2012, 11:17

I upgraded tform 25 to 30, and the difference was noticable, so even if it;s only pushing out 38Hp I would think a 13" would be fine.

Kev on here has a 40 on his, and I think he normally runs 13" and has a 15" for flat water / light load conditions...

uncle al · 31 July 2012, 13:43

briefly the black art of propellers is too big a pitch it will be like starting of in your car in third gear,too small a pitch and you could/will over rev the powerhead(engine) both of those puts extra stress on the powerhead (not good)always prop to the makers W.O.T. spec

SimonStDavids · 01 August 2012, 11:14

Thanks everyone much appreciated it is all starting to make a bit more sense.

I have started to have a look around and see what props are available but again have another silly question.

I was under the impression that my Mariner is a Yamaha built one, so I was looking at the G-type yamaha props. But when I type in the current props number "48 73 132 A40L 10P" google takes me to merc/mariner props?

Are they one and the same and would either fit?

HughN · 01 August 2012, 12:24

One of the cheap Chinese TinyTach lookalikes go for about a tenner on eBay. They should last long enough to get an idea of WOT rpm (they don't appear to be very waterproof).

9D280 · 02 August 2012, 08:12

Main difference (I have fitted a Yam prop to my Merc - splines & Dia are thesame) is the locking method.

The Merc / Mariner Porps have two small studs on the back of the hub, and it uses a star type lockwasher that two of the arms engage on the studs, the other two get bent over the nut.

Yam / Mariner uses a standard washer, castellated nut and split pin.

Net result is you could in theory use a Merc prop on a Yam shaft, as long as the small spigots on the prop hub don't foul the nut. Packing washers may be needed.