Could it be the prop?

[Long View]

Gentelmen,
Any help would be apprecieated. I have a 1994 Nitro 170, It has a 125hp Merc on it that has recently had carbs cleaned and adjusted. When I bought it it had a 21p Thrush Stainless on it that would push the boat at 56 GPS, BUT, had horrible hole shot. It woud take me forever to get on plain and the boat would overrev on occasion and force me to back off the throttle and ease into it again to get on plain. The stainless was dinged up pretty bad and I originally thaught that was the problem so I bought a new 21p aluminum but it also will not plain out. I talked to a friend of a friend who used to own a prop shop and he gave me two props to try that he had laying around. The first was a 17p 4 blade, great hole shot only 39 mph WOT. The seccond (and the one I am using now) 19p 4 blade, great hole shot but only 42 mph GPS and only turns the engine at 4900 rpm. Any suggestions?

 

[steelespike]

Re: Could it be the prop?

Not an expert but generally a 4 blade will give better hole shot and help
fight ventilation.But will lower top end.Think of your prop like a gear in a transmission. 21 pitch is like high gear 17 is more like low or second gear.
If you want all out top end you will have to give up hole shot.You didn't mention wot rpm with the stainless nor with the new 21p aluminum.If we
knew rpm and speed we could tell better where you are at.
Ventilation is usually because you are set up at the ragged edge of motor height this is best for all out speed.but you may sacrifice a little grip as you have seen.If you had your stainless repaired it may have helped your ventilation.Generally speaking an aluminum will not be quite as efficient as a stainless.Do you trim in for hole shot?

 

[Long View]

Re: Could it be the prop?

Steelspike,
Thanks for your reply. I did find a soloution to not being abel to get on plan. My dad came up and we put a hydrafoil on, put the 21p aluminum on, and hit the water. It would not get out to plane. We beached it, took off the 21 and he noticed that I had the original bearing still on as well as the new bearing. So, I put the 21 back on and tried it again and imagine that it worked. But, I still am only getting 43mph Gps with it and it will only run at 4900 rpm. I do trim down and bring it up to as it comes out of the water onto plane. Any other suggestions?

 

[steelespike]

Re: Could it be the prop?

Usually an aluminum prop won't load the motor as much as the stainless.
So it is a little confusing that it won't turn close to your original rpm.
I would expect a little more rpm from the aluminum but less speed.
Assuming the SS prop was in reasonable shape.
I think I would go back to the SS and see if you still get your original results.56GPS etc.And give us your rpm.If it performs get the SS prop repaired and run it.If you can't return the Aluminum Keep it for a spare.
Usually where you buy your prop will let you test for the right prop.