shawn

I am adding 2" shorties. Testing has shown that 15.5 diameter 31"pitch hydromotive Q IVs hit the limiter. Hydromotive offers 16" diameter props as well as higher pitch props. What would the effect be of a larger diameter and what would be the effect of more pitch or both. What happens when the manufacturer adds pitch, does it change the diameter? I need an education on props. What does the diameter vs pitch effect on planing, acceleration top end etc? The shorty added 5 MPH when it hit the limiter so I already know what one prop did. Merc 34s hit the same top end, did not hit the limiter, but planed poorly and did not handle as well. Any advice is appreciated

Thanks
Shawn

cobra marty

Usually the same prop family will have the same diameter regardless of the pitch. When you raise the prop and semi-surface it, there is less prop blade in the water and less bite so you either go to 5 blade or a slightly larger diameter.

Tell us more about your setup- boat, weight, engines, Hp, drives, ratios, propshaft location relative to the bottom, and what you have tested and the results- prop, pitch, WOT mph and rpm, description how it felt(hard to plane, etc).

This is where the fun begins.

Miami

If you are testing this time of year, cold air low humidity, your results will be totally different in your normal summer of high heat & humidity. A prop just hitting the rev limiter now, may not do it in the summer, when the weather, company and fuel slow her down.

Shawn, my memory is not that good. Don't you have a 30' Spectre? If so, a great local source for you would be Dave Govatos of MDG Performance ( 410-956-5381 ). He is extremely close with Jay @ Spectre and has rigged and owned numerous 30's and 36's.

I personally have called Hydromotive and have always found them both knowledgeable and honest about both their props and competing manufactures. Give them a call. Lots of luck.

Paul

shawn

Paul, good memory. I have a 30' spectre with HP 500s. Jay did the testing during October in Florida. I got the test results from him. The Q IV 31" was the best tested, but he only had 2 props to play with and the Merc doesn't seem to do as well on the 30 as the Hydromoives. they were tested with 1.36 and the 29 ran 104, the 31 ran 105 with stock height and with the 2" shorty the 31 ran 110 hitting the rev limiter. the Merc cavitated and had slow acceleration. Hydromotive makes 2 versions with the 16" diameter.
If I start with a 31" pitch and a 16" diameter and need to bump up the pitch by a half to an inch, will that alter the diameter?

cobra marty

What rpm are you hitting the rev limiter at? If 5200 rpm then your slip is 2% and if 5400 rpm then it is 6%. Either on is very impressive and efficient. Your boat is a surprise in that it seems to be one of the few that likes the hydromotive better than the Bravo prop. I think that the hydromotive gives more stern lift and that is what the 30 needs with those 2 heavy big blocks.

Rambunctious

here is an excellent paper on cupping props, a good way to "add effective pitch" to a prop you already own and is close to optimal

you could effectively add pich this way to an existing with no (miniscule) reduction of diameter

www.hydrocompinc.com/pub-bin/PropellerCup.pdf

bottom line. you want to spin the smallest diameter/ least blades you can without significant slip (<10%) to get the best speed by reducing the power losses spinning an excessive amoung of blade through the water.
there will be a cross over point where the excessive high pitch of a given diamer prop results in too much slip and going to a less piched, larger prop performes better. In eaither case you want this at your peak drivetrain power output. Always pitch/diameter to your peak power point.

testing is the only way to know for sure, but Tomcat and i are trying to find propellor efficiency curves that would show these crossover points as a function of thrust for given power inputs and give insite into the best choice before time and money is spent. This would be a powerful tool

shawn

Marty, I only have the info from Jay at Spectre. The boat ran to whatever the rev limiter is set for the HP500 EFI. Is it 5250 or 5300?
The Spectre runs a little bow high, so the stern lift and lower drive height should be a good set up.

From what you are saying, just stay with the 15.50 x 31.

Rambunctious

you may consider chatting with John at York propeller about labbing and/or cupping that prop. I wouldn't want to bounce on the rev limiter.....

tell him your specs like Marty's request, and he will have some good insite. he has received some good press here on the board.

mjuwalters

Shawn: Have you tried cleavers. That will give you stern lift. Don't know about acceleration. most likey handle better. What prop shafts are you running? I;m running 15x34 cleavers on my motion 30 with big shafts. I would be willing to let you try them seeing how you are buddys with the Millers and all Let me know
Michael