What prop should I get?

C2 RIBS · 21 March 2009, 18:43

I have a 25hp Mariner 2 stroke which has been enhanced to 30hp. I have fitted an 11" propguard and have read here that I should get up to a 10.5" prop. What pitch? What type do you suggest ?

Turbodiesel · 22 March 2009, 08:33

Really you need to get a tacho on it and get it to rev upto its max Rpm (5850RPM) to get the pitch right. Are you going to pitch it with the prop guard on or off. If you do it with the guard on you need to be aware that if you take the guard off it will overrev and cause damage. What sort of guard are you using?
You are fairly limited to propeller choice on this engine, Mercury/Mariner Props range from, 8" through to 14" all are 3 blade Ally props.

C2 RIBS · 22 March 2009, 15:50

Using a hard plastic type called propguard which was fitted by a marina dealer but not sure about the prop yet. The craft is used for safety work so will keep it on unless I see a distinct disadvantage having it, just looking to get best out of performance/response/top speed if needed to get to craft or people

gibbo500 · 22 March 2009, 19:00

I have a 9.7/8 x 10 used once if you are interested.

Is yours the Tohatsu rebadged as Mariner?

If using the prop guard it should be ok but as Turbodiesel said you need to know what revs you are getting at WOT.

See:

http://rib.net/forum/showthread.php?t=28584

More experienced members of the forum will be able to state the ideal conditions in which to carry out the test i.e flat water, load fuel etc.

Good luck

Turbodiesel · 22 March 2009, 21:44

If your going for fine tuning you want to prop it with the load you would usually be carrying, and for pitching a prop you want the water flat so you can hang on to wide open throttle without coming out of the water.

When we prop boats at work we test the full range from light load to fully loaded up in 100kg increments. Then you can look and see what pitch suits, one that wont over rev when light and works the best loaded, it can take a long time and alot of propellers!. Once you have pitched it right you can then look at how many blades, what type of prop etc. We have got pretty good at it these days and will usually guess it within one pitch and prop type!. We always have a bet on to see who gets it right. Usually the boss annoyingly!