mariner 15 2 stroke prop?

[Stubber]

Hi all, I have a late mariner 15 2 stroke with a standard prop on it which i think from memory is a 9 and quarter diameter by 9 inch pitch on the back of a quicksilver 340 airdeck inflatable, am wondering if anybody else has tried different pitch props and what the outcome was with a similar setup, is it worth trying this for more speed and maybe dropping the revs a bit as it gets to WOT pretty quick and cavitates with a fair bit of throttle on acceleration ?

[Locozodiac]

You could be badly trimmed, trim must be set at a hole that gives a perpendicular motor to water level when boat is on plane at sea level, usually 2-3 hole out from transom

Could have tramsom/leg height issues, trimming erroneously out of correct trim to perform better.

Playing with prop pitches is never a wild guess, this could ruin your beloved horse. Must install a induction tach go full wot and check max rpm achieved and from there see if under or over propped. Start checking with your current prop before swapping to other prop pitch.

Happy Boating

[Stubber]

I'm pretty sure I have it trimmed right as it's about perpendicular with the boat on the plane and seems to be the best setting for the trim angle and have tried a few times altering the angle and always come up with the same setting. As for height I always just mount the engine mounting saddle on top of transom board and have not tried to lift it up, still wondering if I should put a higher pitch prop on to get a little more top speed and drop the revs back a tad to keep on the plane as the inflatable doesn't usually carry much weight????

[joey_1987]

Is your motor the Japanese 15hp (Tohatsu) or the USA Merc/Mariner model? The Japanese 15hp is really a restricted 18hp and has more cc/power than all the other 15s which may explain yours zipping around on a lightweight inflatable. 9" pitch is the 'middle' of the range/stock prop for this motor. You could move up a pitch to 10" and even go stainless to really improve performance, WOT RPM figures would be handy though as you don't want to labour the engine/still should reach max RPM's for your engine at WOT. I can tell you though that I just swapped an 11p Aluminium stock prop on a 30hp 2s Mariner (Tohatsu) pushing a 4m tinnie to a 12p SOLAS stainless prop and I gained 2-3 knots at cruise and WOT while also having less vibration, awesome acceleration and planing at a lower speed so really taking advantage of the torque available throughout the rev range but my WOT RPM stayed about the same. An 11p stainless may work best for you/your rig as if you go up 1" pitch from Al to SS then you don't really lose WOT RPM's, going up 2" in pitch you'd drop RPM's off a bit but based on what you've said you'd likely plane at lower RPM/cruise faster and hit a higher top speed. My 2c worth on this. Also is your existing prop in good condition?

[Stubber]

It's the American/Belgium version and the whole unit has only done about 25 max hours in its life and the prop is unmarked, I think what you have said was the way I was thinking. The WOT rpm at present seems about right when hearing the engine at full tilt but when its cavitating badly it's screaming, I will keep my eye out for a 11 pitch prop and see what crops up, should be fun with a bit more speed as its not slow now

[Locozodiac]

Some isues :

If your sib has badly inflated tubes, air deck, keel, will peform badly, will lack sib rigidity, get a pressure gauge if you haven't got one as inflating blindless testing with finger gauge is useless. Inflate all tubes/keel to at least 3.0 PSI and air mat to 12-14 PSI once sib is resting at sea for some minutes for pressure to staibize for a nice performance.

Once done, place a mate up bow to ballance sib well (pic 1) go for a wot spin to fully plane sib, sit on deck near transom, pull your neck out and visually check where is your water flow passing with respect to tail. This test must be done on flat no wind water conditions.

Standard props factory delivered usually gives medium RPM readings, that is readings between min and max rpm stated for that horse, which are probably 5000-5000 RPM at wot for the Mariner 15. So you should be fine with that prop.

When water testing, ideal water flow should pass slight under small upper plate (pic 2) if passing way under will have cavitation, prop aireation, if passing way up will have water splashes in/out transom. What's your transom upper middle height with respect to lower transom in centimeters ?

If wanting boating cruising perfection perform this test, before playing with prop pitches sizes, whether alum or SS without a induction tach installed. By ear it's impossible to know your rpm, that's what inductions tachs are for. Are cheap, go for one, could be installed at front cowl. It's natural for the engine to scream when prop ss cavitating due to rpm going way high...

So to have the idea : (check pics)

2-Transom too high with respect to anticav & upper deflector plate. Gives prop aireation, cavitation on non flat seas, worst at close turns. Can see clearly anticav plate is nearly expossed.

3-Ideal water flow with correct transom/leg height match. Perfect set up for wot cruising, best hole shot and inmaculate close turns, and don't need to change factory delivered prop as the engine will be delivering near it's full wot rpm.

Happy Boating

[Jonny H]

I bought a stainless Solas for mine the same pitch as the standard alloy and it performed a lot better being smoother and was able to run the engine a bit higher which improved things.
If you want to change your pitch you will need a tacho as already mentioned to know your max rpm .
The boat was a Zodiac 310 with solid floor

[Stubber]

That was realy helpful information about height of anti cavitation plate, I will check it next time i'm out,from memory it runs with the plate at waterline so you can see the top of it and have always thought it was to high but the only way of changing would be to cut the transom board. Like the thought of a stainless prop, bet they are not cheap tho

[Locozodiac]

Originally Posted by Jonny H

I bought a stainless Solas for mine the same pitch as the standard alloy and it performed a lot better being smoother and was able to run the engine a bit higher which improved things.

Jonny, Did you install a induction tach to say the engine run at a higher rev ? If not, impossible to tell by ear how much more RPM you have gained. In larger engines there's a noticeable difference between runing alum and SS props mainly because alum props tends to bend at fast RPM/torque when at hole shot loosing grip while SS props doesn't. Two different beasts comparing a 310 solid floor to a 310 air deck floor, rigidity issues.

As an example have in use a 2006, 2 strokes Tohatsu 18 HP, that horse has accompanied me while upgrading from a 320 to a 360-380-420 sib and right now to a 430 light rib. This engine produces at wot 5600 rpm, factory max stated is 5800, never needed to change or play with prop pitches mainly because my rib it's well inflated before each outing and my transom/leg has been perfectly matched under trial and error to produce a undisturbed water flow as seen in pic 3.

Check water flow/transom height as recommended, not saying to chop transom down, at least will know without wild guessing what gives the actual cavitation problem.

Happy Boating

[Stubber]

Thanks for all the replies some really helpful information, will take note of height of cavitation plate, trim angle when I manage to get out if the weather improves?, Will we get a summer this year, last year was terrible