prop for yamaha F115 on a MULLET SKIFF?

[jaymz101]

I run several mullet skiffs. I know the boat is sluggish by nature, but the boats with the F115 on them are very sluggish (12-16 kts in deep water, 16-18 on the flats) Theyre 24 and 26 foot and we carry a large net (600' seine) and several smaller ones (70' seine and/or 20'trawl). We currently run 13x17 stainless steel props (3 blade) and hit 5100-5300 RPM. Id like to get the RPM up past 5500 and the speed up around the 20kt range. I know it's possible, Ive seen them go that fast.

Any suggestions of a diameter or pitch change? Any advice at all (besides re-powering them, it's not an option)?

 

[steelespike]

Re: prop for yamaha F115 on a MULLET SKIFF?

I'm a little outside of the prop technology but::
I believe they run these motor at 5800 to 6000.A 17" pitch stainless prop
should put you at "about"5500 to 5700.Whether it will result in more speed probably have wait for a test.If you want to mess with motor height you could gain some speed just by raising the motor.Not to much at your slow speeds but you might gain a little.If yoiur running off shore you'll have to be careful about ventilation.

 

[jaymz101]

Re: prop for yamaha F115 on a MULLET SKIFF?

Thanks for the feedback Spike. We dont do any offshore work so thats not a problem. I dont think we can raise the engine anymore though. We run 20" shafts and if there are any kind of seas (>2 foot) it cavitates already. These are very heavy glass over wood boats, I would say at least 2000 lbs fully loaded, probably closer to 2500 or 3000. 1000 lb boat, 500 - 1000 lbs of nets, 4-5 people, another 100 lbs of gear and 40 gals fuel.

I tried a 13.5x15 and we got a little better speed (from 12 to 14 kts in deep water) but RPM stayed steady at just over 5000.

I was poking around the Yamaha Marine site looking over their props. Maybe a pontoon propeller? or a 4 blade? Maybe if I could find a Dual Thrust style that big it would work?

 

[steelespike]

Re: prop for yamaha F115 on a MULLET SKIFF?

Thats about the same speed you said before.Maybe even a little less.
Its logical you would lose some speed with less pitch unless you got a good rpm increase.
I'm a little mystified with the rpm being the same or lower with a 2" pitch decrease.Perhaps the 1/2 diameter increase.Is the prop same brand and type.
The 4 blade might help the venting in seas making it easier to maintain speed though sometimes the extra blade knocks a little off the top end.Any chance these boats have started to absorb water.Have you tried careful weight placement.I just did a little checking around for something heavy and hard to push.Found found 4 pontoons 22 to 24 ft 2800 to 3200 lbs.total,3 blade props.
Toons don't plane yet here they were running 25 to 30 mph.
I have a feeling your boats are heavier than you think or full displacement type hulls.