johnyrude 9.9 props?

ive heard that a 4 blade 10"x7 pitch will get a bigger boat out on plane faster as well as having a better top speed.

anyone with actual expirience ???????????????

thanx.

right now ive got the stock 9.25x10. boat is lightweight 16' aluminum lone star.

Re: johnyrude 9.9 props?

Its difficult to know exactly what prop would do well for you without your prersent max rpm and speed.A Tinytach (about $47) will get your rpm and a gps your speed.
Actually your big light weight 16 may do better than a smaller lighter 12
getting up to speed especially if you add a passenger or something.
When by yourself a tiller extension will get you in the middle seat for quicker planing and better speed.Motor trim is important as well.A 9.9 should do about 21 on a light reasonably fast hull.A 4 blade prop should get you on plane quicker and should help with a load. Normally wouldn't improve speed unless the motor is struggling with the present prop.Without your present specs its difficult to select a replacement.9.9 prior to 89 are rated at 5,000 to 6,000,after 89 up 5,500 to 6,500
Typically you would prop for the max rpm.
You need to be sure the motor is in excellent tune and the throttle opens all the way.

Re: johnyrude 9.9 props?

awsome thanx! was wondering about a tach too. have a handheld gps so i can gage things partly with that. havent had it in the water yet, still making it seaworthy.

i'll be tickled pink if it goes 20+mph.

Re: johnyrude 9.9 props?

steelespike said:

A 9.9 should do about 21 on a light reasonably fast hull....... 9.9 prior to 89 are rated at 5,000 to 6,000,after 89 up 5,500 to 6,500

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21 mph with a 9.9? I'm only getting 24 with my 1978 15 - light boat (Starcraft Sealite 14), light driver (145 lbs), trimmed properly. Engine runs well.

What should the top speed of a 15 be? I know the torque curve varied by year, with the 15 getting tuned exhaust in 1981.

Does anyone have experience in either the 9.9 or 15, going from one year to another?

Re: johnyrude 9.9 props?

21 on a" light reasonably fast boat". Thats key The typical aluminum boat doesn't have a particularly fast bottom probably 3 keels and a lot of rivots.May even be a little bent up.A 15 would normally approach about 26-27 mph.So your 24 is pretty close.Your antivent plate(just above the prop) should be visable at speed. Usually ends up up to an inch higher than the bottom of the boat.
I believe the "Standard" prop is 9 1/2 X 10" for both 9.9 & 15. The 15 is rated up to a max. of 7,000 and the 9.9 6,000. other props were 10" X 5, 9 X 10 weedless, 9 1/2 X 8.

Re: johnyrude 9.9 props?

The 9.9/15 have shaft lengths that end up as 16.5" to the anti-vent plate, so the plate is 1.5 inches BELOW the keel (I measured my boat's transom and it is exactly 15"). I suppose OMC had their reasons for doing this, maybe as insurance against impeller damage if the motor was used as a kicker in choppy surf where the lower unit might break free of the surface.

I tried raising it by putting wood strips at the top of the transom, for maybe 5/8ths of an inch, and it put me a shade under 25mph, a slight improvement. Raising it a full inch and a half would not be practical - the clamps would be near the top of the transom.

The aluminum boat does have three keels and many rivets, so this may be a factor, though it is in good shape and not full of dents or algae growth. Still, 24mph is disappointing.

Re: johnyrude 9.9 props?

Under the circumstances You shouldn't be dissapointed Most of these max speed
estimates are pretty optimistic.If you raised your transom you might get close.
Boat probably has rounded chines.a little smoother but slower. A tinytach would get you your rpm.you might be able to fiddle with the prop get more rpm and speed.
Or if rpm is maxed a prop shop might be able to add pitch or cup.

Re: johnyrude 9.9 props?

well with the 10 pitch prop, my lonestar would just push water like a friggin tug boat loaded, with JUST myself in the boat it would plane and do bought 18 mph (gps). the following summer i bought a 10x7 4 blade, thats too much, i could fill the boat with bricks and the engine wouldnt even notice, it seems to over-rev and cavitate like crazy too. im gonna buy a 8", then a 9" to find the perfect pitch for my boat when loaded. i also have a 14' very light aluminum boat that absolutly hauls with that motor and stock 10" prop. it can over-rev and i think it could pull a 11 or 12" pitch.

Re: johnyrude 9.9 props?

ChrisAG said:

21 mph with a 9.9? I'm only getting 24 with my 1978 15 - light boat (Starcraft Sealite 14), light driver (145 lbs), trimmed properly. Engine runs well.

What should the top speed of a 15 be? I know the torque curve varied by year, with the 15 getting tuned exhaust in 1981.

Does anyone have experience in either the 9.9 or 15, going from one year to another?

Click to expand...

You'll note I qualified my speed estimate "light reasonable fast boat" and that would be with a fast prop not likely to do well with a loaded boat.The 15 with the same qualifications would probably approach 25.
A 15 at 7000 and 10% slip calculates to 24.7.6% slip would help that estimate. While aluminum is light it isn''t particularly fast
it would depend on how many keels rivets bottom shape etc.Actually a reasonably glass boat with a nice
fast bottom might do better than an al tinny of about the dame size.