Might need a new prop?

[Thagan]

My boat takes off incredibly slow and won't reach it's operating rpm range. I think the prop would work better for more speed. I'd like to be able to pull a tube one day.

My boat is an '84 Blue Water 17' with a 120 Mercruiser. Weighs approx 1800lbs. Only carry 3 people max. Engine reaches 3500 rpm WOT. On the engine it says rpm range 3900-4300. I can't remember the actual gps speed but shouldn't matter. I feel like I'm not using the full potential of the boat. Lower unit is a 1.98 ratio. Prop shows 832830A45-19P. I googled it and it's a 14.5" prop. I'm assuming that's a 19 pitch?

No idea brand of prop but it's been beat up.

Any recommendations? Thanks?

 

[alldodge]

I would see if you could find a 17 pitch with the same diameter. This will pull you out of the whole easier and get your WOT a bot closer. Rule of thumb is 200 rpm change for each inch of pitch change.

 

[Thagan]

That's pretty much exactly what I was thinking. I'll look for a 17 pitch and see what happens. Thanks!

 

[Texasmark]

That's pretty much exactly what I was thinking. I'll look for a 17 pitch and see what happens. Thanks!

For expected results, like brand, model number, dia (as close as you can get as dia floats with pitch change for a certain diameter range of props) and pitch on your replacement. Look for the same part number with the last 3 characters 17P rather than 19P. 400 rpm would put you in a good spot and no doubt you'll love your hole shot.

The question is, will it help your top speed. That depends on your boats hull parameters. If it gets better lift at higher speeds then yes. 400/3500 is an increase of 11 %. Going to a 17 from a 19 is a decrease of 11%. Assuming nothing changes with your hull efficiency and prop slip with your replacement prop, the go-fast.com BAM prop slip calculator says your theoretical speed will remain at 32 with both props and the rpm increase. Your actual speed will depend on the prop slip and the effect that has on the hull's performance.

Good news is you shouldn't loose any speed, get a great hole shot, and bring your engine out of the "lugging" mode which, if naturally aspired, will increase your vacuum which will increase your mpg.........how bout that!

 

[Thagan]

For expected results, like brand, model number, dia (as close as you can get as dia floats with pitch change for a certain diameter range of props) and pitch on your replacement. Look for the same part number with the last 3 characters 17P rather than 19P. 400 rpm would put you in a good spot and no doubt you'll love your hole shot.

The question is, will it help your top speed. That depends on your boats hull parameters. If it gets better lift at higher speeds then yes. 400/3500 is an increase of 11 %. Going to a 17 from a 19 is a decrease of 11%. Assuming nothing changes with your hull efficiency and prop slip with your replacement prop, the go-fast.com BAM prop slip calculator says your theoretical speed will remain at 32 with both props and the rpm increase. Your actual speed will depend on the prop slip and the effect that has on the hull's performance.

Good news is you shouldn't loose any speed, get a great hole shot, and bring your engine out of the "lugging" mode which, if naturally aspired, will increase your vacuum which will increase your mpg.........how bout that!

Thank you! I was thinking the top speed would be similar because the engine would be able to spin faster. It's not a ski boat and I don't expect it to be one and top speed isn't important. I'd like it to pull a little harder in case I have to pull a tube. I was thinking I might be able to slalom ski off of it. I slalom skied off my '70's Bayliner with a 75hp Johnson about 4 years ago and it was a deep water start. Felt like I was dragging in the water for a long time LOL.

Thanks again!

 

[Texasmark]

Too soon for any results???

 

[Thagan]

I just went to the lake yesterday. Top speed was 31mph/3500rpm with 3 big adults and 1 smaller adult, 3 skis, 2 anchors, cooler with drinks, food, and a bunch of other crap headed into the wind. The speed was using a GPS app on a phone. I was unable to deep start slalom skiing but I did get up on double skis. It barely pulled my 260lb brother up on double skis but still did.

I ordered my new prop today. I ordered same brand but in 17 pitch and it's also 1/2" larger in diameter. I'm hoping to test it out next weekend if I get my prop in time. If it helps noticeably I'll be happy!

Thanks and I'll report back!

 

[NYBo]

A beaten-up prop will wreck performance, so this may not be a pitch issue.

 

[Texasmark]

Mercruiser 120 isn't much power for what you listed with it being an I/O and you probably have jump seats at the stern, adjacent to the engine cover where folks just love to plop down. Once you get your prop changed and performance checked, next on the agenda might be trim tabs. Then they can occupy those seats all they want.

I had a '65 Starcraft 18' Holiday I/0 with the 110. Don't remember the prop nor the gear ratio but for a light boat with a good planing hull, it was a real disappointment for a Sunday afternoon load up all the folks and gear and get after it and if anybody was plopped down back there, they had to get up and go forward to get any skier up.

 

[Thagan]

A beaten-up prop will wreck performance, so this may not be a pitch issue.

That's good to know. Hopefully I won't be disappointed.

Mercruiser 120 isn't much power for what you listed with it being an I/O and you probably have jump seats at the stern, adjacent to the engine cover where folks just love to plop down. Once you get your prop changed and performance checked, next on the agenda might be trim tabs. Then they can occupy those seats all they want.

I had a '65 Starcraft 18' Holiday I/0 with the 110. Don't remember the prop nor the gear ratio but for a light boat with a good planing hull, it was a real disappointment for a Sunday afternoon load up all the folks and gear and get after it and if anybody was plopped down back there, they had to get up and go forward to get any skier up.

Yes I do have jump seats at the stern. Usually no one sits back there or I have them move forward into the open bow. I don't expect this to have ski boat performance but I'd like to pull a kid on a tube. I will look into trim tabs. I've never had them before.

My boat goes fast enough for me.

 

[Thagan]

Got my prop yesterday so I put it on a took my boat to the lake. Boat feels much better. Feels peppy now. It pulls better from a stop now and gets on plane quicker. Now my boat will do 4300rpm and 33mph. Gained 2mph over last run. Only 2 people in my boat this time vs 4 last time. Either way I'm happy with the improvement!

Thanks for all the help!