What Pitch Prop

[Tinshack]

I have a 17' (5.2m) Southwind boat approx. 700kgs with a 115HP V4 2 stroke Yamaha. The prop I currently have is a stainless steel 19K Pitch x 13" Diameter. My cruise speed offshore & fuel consumption at 35-38km/hr @ 3,600 to 3,800 rpm is approx. 1.2 to 1.4 km's per litre. When calm enough I push revs up to 4,000 to 4,200, speed approx. 40 - 42 km/hr and fuel consumption is much better around 1.6 to 1.8 km's per litre. If I go higher than 4,600 rpm fuel consumption goes back down to 1.2 km's per litre. Current prop reaches WOT of approx. 5,400 no problems with top speed approx. 70km/hr.

My question is if I go to a 17K Pitch x 13 1/4" Yamaha prop will I get the better fuel consumption as the revs will increase up into the optimum rev range & maintain cruise speed of approx. 35-38km/hr. I want to get a spare prop & would like to improve fuel consumption if possible during the process. WOT & top speed doesn't worry me as offshore conditions only allow around 35-40km/hr normally. Would be interested what pitch props other people are running on similar size/weight boats, Haines, Cruisecraft, Seafarer ect....

Thanks.:)

[FNQCairns]

No it will all go to the crapper, your wonderful cruise speed and fuel usage numbers will disappear, still you do need to do something to get the rpm up just a little at wot.

IMO you are a candidate for getting the engine as high as possible + sending the prop away for tipping/work. With luck both may see you with another 250rpm with no impact upon known performance (except it may be a little faster)...life will be good.:)

I badly want to get my hands on what I suspect is your prop in the 13x17 version, I thought I had better say that the diameter is quite low, tipping may not be a viable option due to this and it's pitch.....still if at all possible ask them to as a last resort alter the pitch, taking a little cup out might see you home even?

cheers fnq

[Malcolm W]

I went through all of this with my 115 yamaha as the original 3 blade(13x17) stainless prop ventilated no matter what position it was in. I ended up with a 4 blade(13x17) that got me to figures very similar to yours( not quite as good as my boat is a bit bigger and heavier 5.6 and 750+kg). The motor will only rev out at 5300, but as its maximum is 5500 and as its a detuned 130 I have decided to live with that for now. I feel as if I am slightly over proped @4200 45-46km and may raise it in the future to the last hole.The 4 blade seems to grip very well but appears harder to drive and the only way for me to get to 5300 was to raise the motor 2 notches. Your figures appear to be pretty good for this motor and as FNQ said maybe raise it 1 notch. @ 4200 mine runs approx 1.5km to L and lower in bad seas.

[Tinshack]

Sent an email off to Solas and there response was that I won't save fuel but won't have to "work the throttle" in hard unfavourable offshore conditions if I go to a 17" x 13 1/2" Dia. This makes sense.

Now & again in 2 metre sloppy swell it will bog down & drop off plane & have to nudge the throttle to get back up on plane, but not very often & more so when heavily loaded.

I was thinking and the numbers I am getting re: fuel economy. These numbers are with boat fully loaded, 160L of fuel, 3 men + gear & 160L ice box with 2 x 20kg bags of ice. This is the maximum I would have it loaded when fishing so from what others are getting my fuel economy is ok, so maybe won't get any better at 38-40km/h no matter what the revs or prop pitch are.


Motor height is spot on, trim plate level with planing strake on bottom of boat. Can't move higher as steering tube goes through matching hole in motor well. Would have to go to hydraulic steering if wanted to raise motor. Boat is all factory package from Yamaha, Japan.

The thing is i may be getting better fuel economy at higher speed as the boat is more on the plane, less boat in the water rather than thinking it is at a certain revs.

I will probably just get a second hand 17" pitch Yamaha prop as spare and try it. Kids are getting older so may want to pull skis one day also.

Thanks,:)

[FNQCairns]

Sent an email off to Solas and there response was that I won't save fuel but won't have to "work the throttle" in hard unfavourable offshore conditions if I go to a 17" x 13 1/2" Dia. This makes sense.

Now & again in 2 metre sloppy swell it will bog down & drop off plane & have to nudge the throttle to get back up on plane, but not very often & more so when heavily loaded.

I was thinking and the numbers I am getting re: fuel economy. These numbers are with boat fully loaded, 160L of fuel, 3 men + gear & 160L ice box with 2 x 20kg bags of ice. This is the maximum I would have it loaded when fishing so from what others are getting my fuel economy is ok, so maybe won't get any better at 38-40km/h no matter what the revs or prop pitch are.


Motor height is spot on, trim plate level with planing strake on bottom of boat. Can't move higher as steering tube goes through matching hole in motor well. Would have to go to hydraulic steering if wanted to raise motor. Boat is all factory package from Yamaha, Japan.

The thing is i may be getting better fuel economy at higher speed as the boat is more on the plane, less boat in the water rather than thinking it is at a certain revs.

I will probably just get a second hand 17" pitch Yamaha prop as spare and try it. Kids are getting older so may want to pull skis one day also.

Thanks,:)

Hi Tinshack, ATM you get approx 51km/h doing 4000rpm with the 19P prop, any 17p prop will see you at approx 46km/h for the same rpm.

Fuel costs will skyrocket relatively, the bloke is right it will be better when rough and it will trim better, hold plane slower with the new prop. If the revs top out higher, it should also accelerate better.

It's worth owning both I suspect, if the day is calm go with the 19 for the economy and the speed at cruise and if a little sloppy choose the 17 for the extra diameter.

cheers fnq