Prop Selection - 17ft Double Eagle

[cargs]

I have a 17ft double eagle runabout with a 150hp Johnson 2 stroke V6. It currently has am OMC 19" pitch prop. At WOT i am at 38mph and about 4000 RPM. The engine specs say WOT should be around 4500-5500 RPM, this leads me to believe that the prop is over sized. I want to be running the motor in the correct range. I have checked out all the online prop sizing calculators and it brings me three sizes 15, 17, and 19.

Does anyone have a similar setup? what size prop do you have?

Anyone have a 150hp johnson? what RPM range do you run it at?

thanks.

cargs

 

[-steelhead-]

where are you? in nanaimo head over to Victoria Prop down by the water and tell them what you got, they will lend you a couple props to try. this is the best way to do it, and it sounds like you need a more aggressive pitch or smaller blades.

 

[Barnacle Bill]

I would try a 15 " prop first. You should gain about 400 RPMs with that, and increase your speed some what. Good luck,..........BB

 

[TheBigGuy]

Are you sure your tach is correct, has your motor been compression tested and tuned recently, was the speed by gps. Is the prop you are using now a stock OMC aluminum prop. What kind of shape is the current prop in.

You definitely need to raise your RPM's, if your tach, speedo, prop, and motor are all in good working order. Confirm all these are good before buying any new prop.

 

[AlK]

Make sure you haven' got a spun hub.

 

[nedarb2]

it sounds like more than a pitch issue. check out the spun hub that alk mentioned, aswell as for proper mounting height. in general going down a pitch will only gain about 200rpm for a rule of thumb so your still most likely out of the ball park

 

[cargs]

Thanks for all the suggestions.

Speed is by GPS
Checked the hub at home, looks ok. i put the motor in gear and manually moved the prop, it doesn't move around the hub. is there a better way to check it?
Compression is ok.

I had the boat out again on the weekend, it was nice weather so i was able to open it up to double check my numbers.
WOT: 37.5mph @ ~4300 RPM

if i go down to a 17 pitch prop it would bring up the RPM to about 4800rpm using this basic calculator. prop slip is about 0.1
http://www.rbbi.com/folders/prop/propcalc.htm

i found this prop on iboats:
http://boatpropellers.iboats.com/Jo.../6515/?session_id=814665142&cart_id=712951627

would i get any advantages going to a 4 blade? i would like to be able to plane at a slower speed.

thanks

 

[TheBigGuy]

A four blade should allow you to plane at a slower speed. It also may help increase your gas mileage at cruising speed. I tried the four blade Sloas Amita you selected in the link, it is a good prop for the money. 4300RPM is better than the 4000 you quoted originally. If you only step down to a 17P your RPMs will still be to low. You should be looking to turn near 5500 RPM. That is why I was asking if your Tach was correct, 4000 RPM seemed far too low for your motor with a 19P. IF the tach is correct you will more than likely still need to go down to a 15P.

 

[TheBigGuy]

Oh ya, forgot.

If you want to check your prop hub for slippage. Paint or put a couple of matching marks with coloured felt pen on the hub and the prop. Take the boat out for a run, if the marks no longer line up, the hub is slipping

 

[cargs]

Thanks for the help, i'll double check the tach and do the paint marking trick for the hub.