mercury fourstroke

i have a pair of merc 115 fourstrokes fitted to my boat.

current props are 12 3/4 x 21.

does anyone know if i can fit 14" diameter props off the 150 fourstroke.

reason im asking is my boat is not heavy so the engines could easily spin a bigger prop.

ideally id like a pair of 13 1/2 or 13 3/4 x 21 props but i can't find any anywhere.

mercury only show 12 3/4 x 21 for the 115 but the 150 has a 14 x 21.

if the 14 x 21 prop fitted would it make a difference?

are the hubs the same size for the 115 and 150?

im no prop expert so any advice will be much appreciated.

 

I'm not up to date on the prop size selection for your engines, but you should be guided by what rpm you are reaching with the ones you have fitted, as to their suitability for the application. Compare your full-throttle rpm with the maximum rpm range specified by the maker, if you're barely entering the max RPM range given, the props are probably more than what it can cope with, allowing that a new engine might still be a little "tight". If it spins easily out into the high end of the range, they should be good. A quick look at some specs shows the gearcase has a 2.07:1 ratio, I'd be surprised if you are underpropped with 21" pitch. But without test RPM data, you can't know.

 

Going from 12 3/4 to 14" is a big step, maybe too big. The increase in blade surface and tip speed reduces rpm at full throttle by at least 800 rpm; the reduction of available engine power (torque times rpm) will be approx. 15%.

I experienced the same when going from 13" to 14.2" on my Yamaha 100.

 

For offshore work it is better to err on the side of under-propping, anyway, when conditions force a back-off, you don't want to be lugging your engines with high-pitch propellors. It may be different in situations where this is an unlikely scenario, but I prefer to use a bit more fuel than be left labouring engines.

 

I disagree, in that if the boat is run like most, you can afford to over prop, with some added benefits, but you also can't run at WOT much. Simply, you can stay on the "power versis efficiency bands" better with an oversize prop, but this assumes you're running at reduced throttle setting most of the time to save fuel. If you run at WOT much of the time, you'll kill the motors pretty quickly with a bigger (or fatter) prop.

There are several things to do, before installing the bigger props. The first is find how close to the recommended WOT, RPM settings you're running. The next is to see if you have the space available below the ventilation plate, which you likely don't jumping up that much in diameter.

 

been out twice now with the 12 3/4 21p props.

first time out it revved to 6000rpm and was doing 31 knots and still picking up speed but had to slow down as there was not a lot of room to go flat out.

engine power band is between 5000 and 6000 rpm.

today went out again and now the engines are revving to 6200 with the 21p props.

speed still the same and i reckon would do 35 knots if i had the room maybe a bit more.

trim position was just under 1/4 on the trim gauge but i barely look at the trim gauge as you can feel when its right.

it pulls the revs very easy but at around 15 knots is when the props start slipping as the boat is coming up on to the plane.

forgot to mention that there is around 1 1/2" clearance above the prop to the cavitation plate.

 

I'm confused, you are getting only 31 knots @ 6000 rpm, with 21" props and a 2.07 to 1 box. The zero slip speed, by my calculations, would be 5072 feet per minute, you are doing just 3141 feet per minute. At least one of the above figures is wrong. Does not happen in the real world. Your engine simply couldn't supply the power to cope with it. I'm tipping you have props with a much lower pitch. Or a non-standard gear ratio. Or the speed reading is out. Or......

 

We can agree to disagree then. The other thing here is, he has a twin-engine boat, and it is nice to have the ability to plane on one engine, if only briefly, when power is lost on one enine, as in flogging it to get some speed through a bar crossing, e.g., higher pitch props makes that less likely to be achievable.

 

31 knots at 6000rpm with 12 3/4 x 21p props.

that was on gps plotter so must be accurate.

it was still accelerating but i had to slow down due to being in a confined area so it would of probably get up to 35 knots if i had enough water to test it.

2.07 gear ratio is written on the side of the gearboxes.

at 15 knots there is a lot of prop slip as the boat is coming up on the plane.

 

Check the propellor size, simply cannot be 21p and be going so slow at 6000 rpm, or I will give up !

 

You may have 15p or 17p props, can't see any other explanation, unless somehow the engines are too high and the the prop tips are running well clear of the water, that can happen without runaway ventilation with some stainless cupped props. You do report "slippage", which arouses suspicions of it. I recall raising an engine experimentally to see how high it would go without "letting go", it picked up at least 500 rpm at full throttle, and it was obvious there was a good gap between the cav plate and the water, with only the occasional loss of traction, but that was in flat water. I doubt the boat was much faster, and did not leave it raised. Having part of the prop out of the water takes work off the engine, but there is such a wide difference in the numbers here, it is hard to see that accounting for it.