Need Help On Correct Props

[mt hook]

I have a 25ft 1997 hydra sports CC with twin 1997 johnson oceanrunners. does anyone know what the max rpm's are. I had 14 1/2 X19 aluminum props ,could only reach 5100 rpm at speeds of 41 mph with engines trimed and good conditions before both engine were rebuilt. after the rebuild I installed 14 3/4 x 17 SS props that were recommended by the manufacture and I still can only reach about 5100 rpm and my speed dropped to around 37 to 39 mph. I under stand how the pitch works with a single outboard, but I read on another site its different with twin outboards -- said to go up in pitch with twin engins. can someone shed some light on this as far as getting my rpm's up but not to overrev my engines -- I think the specs for my engines are 5600 rpm's ??? any help would be great.

 

[steelespike]

Re: Need Help On Correct Props

Did the prop manufacturer, boat manufacturer or outboard manufacturer recommend the props?
I think I would go back to the old props and see how it performs. May give you an idea if its prop or motor related.Of course it is logical that at the same rpm the 17 will produce less speed unless the old 19s were pretty worn out.Generally a stainless prop will load the motors more than an aluminum.
Don't assume all is right with your setup just because of a rebuild.
Throttles may not be opening all the way,timing could be off etc.I'm inclined to think prop selection may be the culprit.
There are props that raise the stern and others that raise the bow.
Did they say why their selection would work?What is the hp of your Johnsons?

 

[mt hook]

Re: Need Help On Correct Props

thanks for replying
I contacted hydra sports, gave them all the info and they recommended 15x17 props. I couldn't find 15x17 so I purchased 14 3/4 x 17 Apollo SS props. engines are 1997 oc runners 150hp. do you think prop pitch is calculated different with twin outboards. one site states for twin engines to increase pitch. I attached the link to it.

http://www.boatus.com/boattech/props.htm

what would be my result, (or should be) if I go with a 21 pitch if its true what is said about twin engine props ?

thanks again

 

[junior1113]

Re: Need Help On Correct Props

all v6 ocean pros will last the longest and run the best with 5500-6000 rpm @wot. the only thing you need to be concerned with is achieving that range. great boats i have a 93 2000cc with 200 ocean pro and love it. my wot rpm is 5800. i like the idea of some good agressive ss 17"'s

 

[Dhadley]

Re: Need Help On Correct Props

Why did your motors need to be rebuilt? What does the rebuilder have to say about the obvious lack of power? They sure won't last long like they are.....

 

[mt hook]

Re: Need Help On Correct Props

One of my motors sucked up some debri and overheated, and developed a knock, also had 600+ hrs, so I decided to get both engine done. I'm getting different answers to my max rpm specs. does antone know where online I
can find specs for my engines ? 1997 johnson oceanrunners 150's.
thanks.

 

[steelespike]

Re: Need Help On Correct Props

thanks for replying
I contacted hydra sports, gave them all the info and they recommended 15x17 props. I couldn't find 15x17 so I purchased 14 3/4 x 17 Apollo SS props. engines are 1997 oc runners 150hp. do you think prop pitch is calculated different with twin outboards. one site states for twin engines to increase pitch. I attached the link to it.

http://www.boatus.com/boattech/props.htm

what would be my result, (or should be) if I go with a 21 pitch if its true what is said about twin engine props ?

thanks again

If you increase pitch you will lose rpm. On a twin I don't know how much.
Rule of thumb on a single is about 200 rpm per inch.
Pitch is not the only consideration when selecting a prop rake and Cup and where cup is located affects prop performance.I'm not the guy for cup and rake help but someone should be along with more detail.
It does seem to me your twins should be doing better with a 17" pitch.
Does the boat respond when you trim out.Are you running trim tabs?
Have you considered going back to the old props for a comparison?