Prop Question

[Prairie Buoy]

Have picked up a spare prop for my 150 main and decided to try it out. Must be slightly different pitch than my old one. Anyway, my old one gave me 25 knots at 3900rpm and new one needs 4400 rpm to get 25 knots. Was told that better to run 4400 as faster speed will be easier on drive so keep that prop on.. doesn't make sense to me??? Any opinions or reasons?

Thanks
PB

 

[wildthing]

check your RPM for WOT with factory spec

2 degree pitch change gives a 400 rpm difference approx

 

[vanboozin]

http://www.veradoclub.com/index.php?topic=137.0

 

[Sharphooks]

Have picked up a spare prop for my 150 main and decided to try it out. Must be slightly different pitch than my old one. Anyway, my old one gave me 25 knots at 3900rpm and new one needs 4400 rpm to get 25 knots. Was told that better to run 4400 as faster speed will be easier on drive so keep that prop on.. doesn't make sense to me??? Any opinions or reasons?

Thanks
PB

These day, that's the conventional wisdom with the newer 4 strokes: they like to be operated in the upper RPM band, and whatever prop you're using, you should be able to achieve WOT under "normal" boat loading conditions without requiring perfect sea conditions to achieve it

For several years I ran a 15.5 x 19P 3 blade aluminum prop on my Honda bf150. WOT for the BF150 is approx. 5600 but I was getting 5,300 to 5,400 with that prop so I thought all was good because that was "close enough". I typically operated the motor at 3,700 to 4,100 for best gas burn

I then stepped up for a new Honda BF150---the seller's mechanic said: ..."you need to hit a WOT of 6,000 with this motor and you need to run it at least in the low to mid 4,000 range on a day to day basis..."

So with a new motor I wanted to protect my investment---I stepped up for a 15 x 17p 4 blade aluminum prop----my RPM's instantly increased, achieving WOT of 6,000 was no problem (if I paid attention to trim) , and during a long trip I just took in which I covered 700 nautical miles, I ran it at 4,500 using that new 15 x 17P 4 blade

The engine seemed to like more RPM's but here's the downside--- RPM's go up, fuel efficiency goes down. Instead of the normal 19 liters per hour I got with the 15 x 19P 3 blade, I was now burning almost 24 liters per hour with the smaller pitche4 blade

So that's the price you pay for running your motor at the upper end of the RPM band, but they say the motor will last longer....

I do know that when I changed out my oil yesterday after 110 hours of straight run time during that trip, the oil was black. I think that was the result of a severely over-loaded boat (weight) and sometimes lugging at 1,500 - 2,000 RPMS for extended periods of time when stuck in BIG waves and crumby sea conditions

 

[profisher]

I would rather burn a bit more fuel and be turning more rpms knowing the engine is not labouring than to lug it down with a bigger prop just to save fuel. Lugging an engine puts strain on the wrist pins and the rest of the rotating assembly. The equivalent of taking off in second gear in your car and driving around town in overdrive verses 1st gear standing starts and 4th gear around town when doing 50k's per hour.

 

[Filletandrelease]

It is impossible to find a prop that will do it all. And fuel economy may or may not decrease due to running at higher rpm. I just finished trying different props on my boat. Went from a 14-19 4 blade to a 15-17 3 blade to a 15.5-15 3 blade. I have gained lots of upper rpm, cruise at 500 rpm higher than before and picked up significant fuel economy. Plus the boat just planes and respond way better. I did all this while using an on line prop slip calculator. I strongly reccomend looking one up and see how your boat and motor are actually performing.