Need help sizing a prop

[Onexl85]

the Boat is a 2001 rinker 242 Fiesta vee cruiser, with the merc 5.0 efi/alpha 1

neef some help selecting a new prop....was thinking 4 blade stainless. Current prop is a 15x17p the tach will climb over 5000 rpms. It does plain out ok with 2 50 lb dogs in the cabin and two adults at the helm. Calm seas I’m cruising at 3900 rpm at 25 mph. What should cruising rpm be on that motor? What would be a good prop?

 

[Scott Danforth]

what is your top speed at WOT
what is your RPM at WOT
what prop do you have now
what issue do you want to correct

https://forums.iboats.com/forum/gen...p-questions-and-topics/295058-read-this-first

 

[Onexl85]

Rpm is around 5200 wot and top speed is 25mph. Need to lower that rpm and raise my top speed

 

[Scott Danforth]

so you have no change in speed from 3900 RPM to 5200 RPM? is your speed via GPS? you cant go off your pitot tube speedo.

5200 is a bit high as the motor is listed at 5000, however nothing wrong with 5200 RPM lightly loaded because as soon as you load it up, it will drop down.

i would look at a 15 x 19 3-blade stainless myself.

 

[dingbat]

What problem are you trying to solve by going with a 4 blade?

Simply bumping up to a 19p will solve the over rev problem.

 

[Sea Rider]

+ 200 wot rpm over revving as usually loaded is nothing to worry about. If going for a19 pitch prop while maintaining same number of blades, engine will drop to much wot rpm, will lessen best hole shot and probably engine will rev towards min to middle wot rpm range,

Stay with that prop if max wot rpm range is 5K. Add a case of cold ones and foodie for the dogy and max wot rpm will surely drop to 5K. LOL!!

Happy Boating

 

[QBhoy]

You need to go up to a 19” at least. Similar boats I know of will sit just on the plane at a little over 3000 rpm and at about 20mph. Will top out about 30 or just over. This would be the norm. Stick to a 3 blade. Try a black max 19” ally prop. Likely be just ideal.

 

[QBhoy]

+ 200 wot rpm over revving as usually loaded is nothing to worry about. If going for a19 pitch prop while maintaining same number of blades, engine will drop to much wot rpm, will lessen best hole shot and probably engine will rev towards min to middle wot rpm range,

Stay with that prop if max wot rpm range is 5K. Add a case of cold ones and foodie for the dogy and max wot rpm will surely drop to 5K. LOL!!

Happy Boating

Hi searider.
Would going to a 19” not put him right on the money at 4800 rpm wot, assuming the usual rule of thumb. Would bring the cruising rpm down to a more reasonable and fuel efficient figure too. That’s where I would be at.

 

[Sea Rider]

If OP has a 3 blade 15 x 17 prop and achieves at wot 5 K or slight over. If going for a 19 pitch the issue is that isn't known under practice how much wot revs will engine lose going per one more pitch ? now imagine 2 pitches up. Personally wouldn't put my hands to the fire going for that rule of thumb which has never worked right for my applications.

Happy Boating

 

[QBhoy]

Mmm. Not sure. The 2” up in pitch...should see him right in the sweet spot.

 

[Sea Rider]

The 2” up in pitch...should see him right in the sweet spot.

To make it fair : 50 he will, 50% he won't, will depend entirely on how heavy is that combo. if achieved way too less wot rpm, have it re pitched + 0.5/1.0 pitch at a prop shop.

Happy Boating

 

[Onexl85]

OK fellas, so today I switched to a 14x19 4 blade quicksilver nemisis prop. Holds plane nicely at 3600 rpm 24mph. Wot is 4900 and 36 mph. Tryed to plane out with out trim tabs and it wouldn’t. Had to have tabs down to about half.

I should add add that I wot tested that 15x17 and it his 5400 rpms and 36 mph. Was afraid to climb over 5 but the engine didn’t seem to be screaming and it was a short test.

My question is.......what can I do to improve hole shot with out losing that nice 3600 cruising rpm

thanks for all your input on this. You fellas always come thru!

 

[porscheguy]

The majority of 5.0 mercs with the aloha drive are the 1.62 ratio. That gives you a prop slip of 33% which isn’t surprising due to the weight of the boat + a 14” diameter prop. Even if they put on a 1.81 ratio you’re still at 26% slip.

The only way way to improve your hole shot is to decrease pitch. If you really want to maximize performance, you’ll have to step up to stainless props. There’s really only 2 I’d suggest. The mirage plus or the Revolution4. The mirage in a 17P has a large diameter. The rev4 doesn’t have quite the diameter but it adds a fourth blade to get the surface area. When you’re at 26-33% slip performance gains come from reducing slip. Stainless props eliminate blade flex, and can be cupped more aggressively than aluminum. Both things drastically reduce slip.

I cant say which would work better for you, but if you’re in an area with lots of boating activity, you can find a used mirage plus in a 17P for probably half the cost of new, maybe even less. I’ve got three of them. The most expensive was around $250, the next was $100+$120 for polishing and extra cup at the prop shop. The third was $25, but it needs a new hub. The rev4 doesn’t turn up used very often.

 

[QBhoy]

16x16