Do I need to drop pitch to go to a 4 BLADE?

[Fordiesel69]

Its on old 70's 120HP 4CYL merc. Has an old michican wheel 15x17P on it now. Looking to buy a solas 4 blade aluminum in a 14-5/8x17P. Do I need to drop pitch or will the slight diameter decrease compansate. Local marine store says when going to a 4 blade you need to drop pitch or it will be a slug out of the hole. Is this true?

 

[Brewman61]

Re: Do I need to drop pitch to go to a 4 BLADE?

I did, but I went from an Aluminum 3 blade to a Stainless Steel 4 Blade.
Not sure if it was because of the increase in blade count, or the extra weight of the Stainless, a combo. of the two or something entirely else.
The stainless prop is significantly heavier than the AL one was.

 

[Fordiesel69]

Re: Do I need to drop pitch to go to a 4 BLADE?

I would like some more opinions. I also heard that 1970's designs the ears wer more flat and round, where newer designs are of an "elephant ear" design and you cannot simply match the same old michigan wheel 15x17p to a new one of modern design. Is this also true?

 

[steelespike]

Re: Do I need to drop pitch to go to a 4 BLADE?

What is the present rpm and speed?What size and type of boat?There are various prop designs Its probably safe to say that some of todays designs are more sofisticated.And no 2 will react the same on the same application.Diameter doesn't enter directly into prop consideration.Diameter is just a function of the design.The Solas Amita has been very predictable,with plenty of present boat,motor,and prop info.In some applications same pitch improved hole shot,mid range performance and wot speed.

 

[Fordiesel69]

Re: Do I need to drop pitch to go to a 4 BLADE?

RPM and speed is spot on with the old 70's vintage michicgan wheel 15x17p prop. I dont have the exact numbers anymore, its just damaged and time for a replacement. I think RPM is 4200-4600 on the little 4 cylinder which it will hit depending on load / trim setting, and it is only a 23-25 mph wide open being a 19' old heavy construction sea ray. The 14-3/8x17p 4 blade prop was already orderd, just wondering if that was a mistake.

 

[steelespike]

Re: Do I need to drop pitch to go to a 4 BLADE?

I feel your motor should produce speeds closer to 40 with a 19 or 21" pitch.
Something is amiss.Could be as simple as the throttle plates not opening all the way.
Anyway,I would have suggested a 1" reduction in pitch because the motor appears to be tired.With a little luck your hole shot will improve and stay on plane with less throttle, better control at speed and at the dock.Your top end may be down a little.

 

[ExxWhy]

Re: Do I need to drop pitch to go to a 4 BLADE?

This may be of some small help, or maybe not.

Merc 100 on 18' Wellcraft CC. Had a 17P merc 3 blade and turned 4950 WOT. Went to a Rubex 4 blade 15P and get 5100 WOT.

My guess (uneducated aside from reading this forum) is you will lose a little RPM staying with the same pitch, maybe 200 in your case.

Love the prop though, top end stayed the same. Hole shot and general respoinsiveness is a huge improvement. Like you, I am a bit underpowered on a fairly heavy boat.

 

[Fordiesel69]

Re: Do I need to drop pitch to go to a 4 BLADE?

The motor *could* be tired, but the blow by is not excessive indicating a worn engine. I beleive it is just the construction and the fact every system is old. A 120HP engine on a 19' boat is stupid. A 165 I-6 is more suitable. All my friends old 19' four winns and older sea rays are all in the 20 ish MPH. A bayliner I agree would be much faster but it is also a very light boat in comparison.

I guess I am going to forgo the 4 blade, but the question I still need answered is the newer "elephant" ear design vs the older round blade design. Do I still need to drop pitch to get it to perform. I am buy a solas brand to save some money.

 

[steelespike]

Re: Do I need to drop pitch to go to a 4 BLADE?

I think you are kidding youself. A 75 SeaRay srv200 19' weighs 2100 with a 165.
I have looked up several 1800 to 2100 lb 115 hp outboards Granted apples and oranges but 2100lb 115 hp official test 39 mph. the lighter boats were as fast as
44. In my opinion a healthy 120 would easily run in the high 30s.
I've seen modern 19' 3.0 130-140 running 43 to 45 mph with a 23" prop.I bet they don't ski too well but they can go.The Bayliners aren't that much lighter maybe 200 but I think a faster bottom.The Sea Ray is a deeper V thats the only reason I don't think it can do 40+
with a 120.