Prop pitch question.

[Hoffer]

Ok...so heres a prop pitch question.

I am thinking I may need to know the RPM's of my engine at top speed to get an answer for sure..but here goes anyway.

I brought my 75HP merc into get a few things fixed on the lower unit - among them was a new prop. I just got it home and everything looks great - but I noticed they put on a new prop with a 17 pitch.

I had 2 different props on there before. The original one when I bought the boat was a 21 pitch. I then wrecked that prop and my spare was an 18 inch. I thought I noticed a slight difference in reduced top end speed with the 18 vs. the 21.

So, now that the pitch is even lower at 17 - I am wondering will I lose even more top end speed. I tried to call the shop to ask them about it - but they were already closed for the day - and now its the weekend. I am wondering why they put on a lower pitch prop - maybe they thought that was the correct size overall - but I dont think I want to lose any more top end speed?

Thanks for any advice!

Hoffer

[Bigtom]

yes you will loose speed with a smaller pitch but have more hole shot, handleing

[Bigtom]

I was also told today at my local prop shop that at WOT they like to keep the RPMs aroung 5000.

[Triple_D]

I think you will lose maybe 1-2 mph, not much but some. I bought my prop on iboats.com, they have a good prop explanation section on there covering this and more topics, check it out.

[boatfixer]

Hoffer I'm curious as to what boat you have. Seems to me that a 21 pitch on a 75 HP is a bit high. You should have a prop on there that will keep your engine in the recommended rpm range at WOT. Going from a 21 down to 17 will affect your top speed but more importantly it will also substantially raise the rpms. You are correct that it would be helpful to know what rpms you got with the 18 and 21

[Hoffer]

boatfixer-

I have a 2003 crestliner 1650 fishhawk.

I bought the boat used and it came with the 21 pitch. I wrecked that one and put on the 18 that also came with the boat. So, do you think that 17 pitch is fitted better for my boat or will it raise my rpms too much at WOT?

That is what I was taking from your reply? Would the increase in rpms be bad?

[boatfixer]

Do you have a tachometer?

The right prop for your boat is one that is gonna keep your engine in the recomended rpm range at wot.

Going over that range can cause damage, going under and making the engine lug is not the best either. I would think that the 17 or 18 is what you need. Without knowing your RPMs at WOT, its hard to say for sure.

[Ed Carlson]

What is the general use the motor will see, fishing, fishing and ski?

As was previously mentioned, just fishing use you will likely like the 17 as the hole shot will increase as will power for maneuverability and load. A bit slower trolling speed as well.

The RPMs should be regulated to peak at where it is set. If they tank tested it when they worked on it there is nothing to worry about.

[boatfixer]

You can't tank test a 75hp. the best way to properly prop a boat motor package is on the water with an average load.

[Hoffer]

Boat is used almost 100% for fishing. Average load = 2 adult guys (average size) and tackle etc.

If I lose a few mph - I guess that is OK - but I am also concerned with the rpm issue. I am going to take it out next week and I will repost here what my rpms are.

I DO like the possibility of it trolling down even a little at low speeds. I do a LOT of "speed trolling" and even at the slowest speed sometimes its a little fast - so just getting down even .5 to 1 mph less at lowest troll speed would probably be an overall benefit for me - and would more than offset the 2 or 3 mph I would lose at top speed.

[Hoffer]

alright so finally got the boat out this afternoon.

At WOT the rpms would hit at max right around 5200.

The 17 pitch prop they put on appears to have reduced my top speed by around 3 or so mph. However, the hole shot is very good. The boat doesnt even rise out of the water with the front end up at all - its just like it simply stays flat on plane and is up to 30mph in no time flat.

I also do think it may troll down a little slower. That part was hard to tell becuase it was windy.

If that is the case - i will take the loss of a few mph because I do alot of trolling with the engine and the option of trolling down even a little sloer if needed is good.

With all that said I still want to make sure I am using the correct prop for the boat... do the experts here think at 5200 rpms at WOT for a 2003 merc 75HP - the 17 pitch prop is OK?

[Triple_D]

Pretty similar WOT compared to my Honda 90 and I run a 17 pitch too, which is the original prop supplied from Honda.

Did you think about contacting Merc, they might be able to tell you what they recommend for that motor.

[Tator2k]

I was told up to 5200-5700 was normal for a 17 pitch prop.

[Hoffer]

So Triple D-

I went to the mercury HSOforum.

They have a very nice link where you can enter your boat type, engine model and what you want the prop to do. There are like 5 choices. I just chose "overall performance".

You can then enter your WOT rpm speed and a few other things and it leads you down a path to the right prop.

The first choice it gave me was the exact same prop they put on. 13.75 inch diameter and 17 inch pitch. So, I should be good to go. If anyone else is wondering you can just go to the mercury HSOforum and find "prop selector" under one of the tabs on the top - its real easy to do!

[Triple_D]

Good to hear Hoffer, Honda does something similar where they compare different boat, motor, and pitches for performance comparison.

I'm thinking of buying another 15 or 16 pitch prop for watersports, better holeshot.

Glad you got it figured out, now you can rest easy.

[marine_man]

Is your motor a 2 stroke or a 4 stroke?

If it's a 2 stroke, your motor is rated for 5250 at WOT.. sounds like you've got it right where you need it.

If it's a 4 stroke the RPM range is different.

The RPM range is not set by the prop.. but by the motor.

marine_man