Deal or no deal

[5150abf]

I have a fairly heavy 16' aluminum V-hull with a 50 rude, I am running a 12 1/4X15 prop now but I found a killer deal on a stainless 11 3/4 X17.
Will the lower diameter and increased pitch make it a wash as far as change in RPM at WOT, it runs @ 28mph @ 4700 now?
If anything I want to pick up some revs, what do we think?
I, having no knowledge whatever about these things, think it will be almost the same, the loss of diameter should be made up by the increased pitch.
Should I keep my day job or am I right?

 

[Campylobacter]

Re: Deal or no deal

You will lose RPM, the increase in pitch is more important than the diameter.

 

[mpsyamaha]

Re: Deal or no deal

agreed.... you will lose a little rpm. not a good idea, because as you mentioned you need to gain some rpms.

 

[steelespike]

Re: Deal or no deal

What year 50?
Your present rpm and speed seems off.Rpm is low in relation to speed.
With an ideal setup you should be in the 5,500 rpm range and 29-35 mph.
possibly with a 17 in pitch.You might start by checking your tach is accurate.
Also would be good check speed with a GPS.
Before looking for a prop you should be sure the motor is sound and in good tune.Be sure the prop is in good shape and fine tune your setup.Vertical height of the motor.Antivent plate should be about even with the bottom of the boat.then raise it one hole at a time until just before venting becomes a problem.Be sure it continues to pump water.

 

[5150abf]

Re: Deal or no deal

the tach I am not completely sure about but speed was GPS and I have set the engine height, carbs are surgically clean and synced, it runs about as well as can be expected for a 27 year old motor, still has the original power head in fact.
Compression on both cylinders is 110 with the engine cold.(65 degrees)
The fastest it has ever gone was 32 normally is between 26-28.
The motor is an 81 and it is on a 1980 Sea-Nymph v hull.
I am not really worried about top speed,I night fish rivers and seldom run more than 15-20 minutes WOT then fish my way back to the launch in the dark, so that is normally 10 mph.
I was looking more for something more durable than aluminum that won't completely kill my top end or hurt my motor, I almost got a prop to last an entire season this year but that is really rare in the river, normally 2-3 months is about it till you whack something.

 

[steelespike]

Re: Deal or no deal

If you think you will be whacking something then stick with the aluminum.
It absorbs impact better than stainless and is easier to repair.
Maybe you could borrow a shop tach to check things out.
Not an expert but I don't think 4,700 and 15 in pitch add up to 28.
However 28 is in the ball park for speed.Its important to check your actual rpm.You could be over reving with the 15 in pitch.I did a quick check and it appears a 50 on 600 to 900 lb boat are running 19 in to 15 in props.
the 15 made 29 mph at 5,600 rpm 900 lb aluminum boat.It may turn out your rpm is closer to 5,500 when you check it out.

 

[Silvertip]

Re: Deal or no deal

The 12-1/4 x 15 is a fairly standard prop for that engine and boat combination and I can relate to it because I had a very similar setup. That prop should yield somewhere beween 5,000 and 5500 RPM so 4700 appears incorrect although speed is spot on with what i experienced. Going to 17P stainless is going to pull off some rpm as is the increased pitch. Don't know what you are trying to accomplish but you need to find out what's incorrect before spending money on props. Verify the tach and experiment with engine height. If rpm is near or at 5500 where it should be, the 17P will likely hurt performance not help it.

 

[5150abf]

Re: Deal or no deal

Thanks guys, looks like a new tach is my next project.