Propeller recommendation

[rebars1]

1974 Merc 225 Ford V8 pre-alpha outdrive 18' IMP trihull. What is the recommended propeller for this setup? I cnat read the numbers on the existing prop, but think it is too small. I'm only getting about 20 mph at 3500 rpm. The coupler is new and I don't see any signs of prop/hub slippage.

 

[jerryjerry05]

Re: Propeller recommendation

The #s are under the nut on the prop,Wire brush it.WE need to know what size to make recomendations.With out prop# it could cost a bunch to replace the props.Trial and error will cost a lot.Jerry

 

[chiefalen]

Re: Propeller recommendation

Some are stamped on the side.

 

[chiefalen]

Re: Propeller recommendation

That boat should fly and do better than 50-60.

Water in the hull?

Compression is good?

Timed good?

When tune up done last?

You used a gps?

And rpm gauge correct?

 

[rebars1]

Re: Propeller recommendation

I'll pull the prop to check the numbers. The tachometer is new. I was reading the speed off the fish finder. Next time out I'll take the gps.

In the Merc manual I saw a 15-1/5"x21" listed. Am I reading that correctly?

 

[rebars1]

Re: Propeller recommendation

The propeller on the boat has an 18 on the hub. No brand name. It has slightly squared-off blades, kind of clover leaf shaped. I measures about 14-1/4" diameter.

 

[Randybeall]

Re: Propeller recommendation

Reading your post you say boat goes 20mph at 3500 rpm. Is that at wide open throttle (WOT)? Normal load? If so I would more suspect a running problem than prop problem.

 

[rebars1]

Re: Propeller recommendation

I did not take it to WOT. I'm just getting used to the how the boat handles and was too nervous to take it all the way to WOT .

I thought maybe the prop was spinning too fast and just chopping the water instead of pushing the boat.

I have a 19pitch stainess steel prop I could try to see if it makes a difference.

 

[Randybeall]

Re: Propeller recommendation

It is very hard if not impossible to judge how a prop is doing in mid range of the engine. WOT throttle test is the true final measurement. I would suspect that you can pull a higher pitch than what you have presently. Like others have said that sounds like it ought to fly. Try your 19, perhaps someone damaged the orignal prop before you bought it and put that on for sale. Should take off really quick but might overrev on the top end.

 

[rebars1]

Re: Propeller recommendation

I tried the 19" pitch prop today (the lake was pretty empty, thanks to the Super Bowl). I did not take it to WOT but got to about 3800rpm.

The GPS said 33mph, the old fish finder said about 23mph. How accurate are the GPS speed readings (Garmin hand-held)

The boat felt a lot better with the new prop, overall. However,the engine felt a little rough at the higher speed, so I will get/do a good tune up next, then take it to WOT..