prop help, Merc 115 4st, 1750 FH

[boyks]

Hi all ,looking for some suggestions as I am getting confusing results trying to tweak my setup. 2008 1750 Fishhawk (25" transom, single console) with 115 Mercury 4 stroke. Motor WOT rpm range is 5800 -6400 rpm.

First ride, 3 adults 2 children in boat, 3/4 tank of gas, prop was 13.75 x 15p aluminum. Boat was hard to steer, handling sucked, rpm was 5800 - 5900, speed about 36 mph on GPS. Motor was down as low as it could go on the transom.

Second ride, moved motor up on transom one hole to get plate closer to bottom of the boat. Just 2 adults this time, less gas, tried 13 x 19p prop, boat handled alot better, rpm at about 6000 rpm, holeshot OK, speed went down to about 34.5 mph on GPS.

Noticed here that the boat is still breaking water beside me in the driver's seat all trimmed up at WOT.

So, third ride, tried a 13.25 x 17p, rpms went down, speed the same. At this time we raised the motor another hole. We then had a great holeshot, at WOT and trimmed up the boat was breaking water right at the rear of the boat. Got to 39.5 - 40 mph on GPS, but rpm was at 5700 - 5800. By this time the gas tank was nearly empty.

So I have borrowed some props and was hoping to do some final tests here and start to enjoy the boat.

Am I safe to assume that we now have the motor at the right height to get it out of the water?

Here are the props I have to try, unfortunately they are all different diameters:

13.75 x 15
13.25 x 17
13 x 19
12 5/8 x 19
12 5/8 x 21

I want to try the 13 x 19 first. I used this in the second ride, ran at 6000 rpm, handled great, but top speed was down. Now with more boat out of the water by raising the motor this might be the ticket?

How does diameter fit into all this?

BTW the normal load of passangers will be 2 adults, 2 children. Looking for good overall performance with an aluminum prop. The boat will mainly for fishing, but will be used for tubing and waterskiing during our vacation time as well.

Thanks in advance for any input

Bob

[Tim_Kelly]

Probably hard to work out when you change too many things at the same time. Propman might be able to give good advice on a generally good prop for that setup, but otherwise you're going to have to experiment with what you've got and only change one thing at a time.

[boyks]

Just come back from today's tests.

Started with the 12 5/8 x 21. Two adults, full of gas, got aroung 36 mph with WOT @ 6000 rpm.

Changed to a 13 x 19, better holeshot, revs went up to 6200 - 6400 speed still at 36 mph.

changed to the 13.25 x 17, good holeshot, got to 40 mph, revs at 5800.

finally changed to the 13.75 x 15, best holeshot, revs at 6100- 6200, speed still at 40 mph. We then stopped and filled both livewells and topped out at 38.5 mph.

So 2 adults, full tank of gas, both livewells full, 38.5 mph, should I expect much more than this? I was thinking of ordering a 13.25 x 16 and run with that and the 13.75 x 15.

Turns out I didn't have a 12 5/8 x 19, it was another 13 x 19.
Any comments/suggestions welcome. Thanks.
Bob

[propmann]

Looks like the 15 pitch is the over all best prop. I would bother ordering the 16 you've got a 17 that will make a good spare. Fortys not bad for an aluminum prop, ss is the only way your going to get any better.

[Burr]

Your data was not what I expected to see, how the 17 p increased speed at a lower rpm range? I used to have a 1750 fh, with a 115 Merc, but a 2 stroke. My boat was pretty heavy with a kicker and 36 volt system, and the 3 blade alum prop didn't seem to perform quite as good as I felt either a stainless or 4 blade would - due to how heavy I was setup.

40ish was about my max. I agree with propman, your 15 appears to be the best performance, and a stainless would be a difference you could feel.

Typically I don't pay much attention to diameter, but it looks like the extra surface area from a larger diameter is helping you.

[boyks]

Well, went out fishing last night with 4 adults in the boat, about 3/4 tank of gas and rear livewell half full and got about 38.5 mph on GPS. I can live with this as it gets up on step nicely.
I was going to order the 13.25 x 16 as my spare since I was just trying out the 13.25 x 17 and was to give it back to the dealer.
If the 17p ran at 5800 rpm, the 16p (same diameteres) should run at about 6000 rpm right?
Bob